Sections of Granite and Basalt seen under polarised, light. 
 ( af ter 'Fouque 8c L e'vy) 
 
 (D Quartz (2) Orthod.ase.f3) Oligoclase (4) Apatite (5jBiotite 
 
 (6) Actmr 'vpidote. 
 
 (8) Olivine (9) Labradorite . flO)Augite (Black) Magnetite 
 
MANUAL OF =EOLOGY . 
 
 i-OR^i/A 
 
 Gbeoretical an& practical 
 
 BY 
 
 JOHN PHILLIPS, LL.D., F.R.S. 
 
 SOMETIME URADER IN GEOLOGY IN THE UNIVERSITY OF OXFORD 
 
 EDITED BY 
 
 ROBERT ETHERIDGE, F.R.S. 
 
 AND 
 
 HARRY GOVIER SEELEY, F.R.S. 
 
 IN TWO PARTS 
 PART I. 
 
 PHYSICAL GEOLOGY AND PALEONTOLOGY 
 
 BY H. G. SEELEY, F.R.S. 
 
 PROFESSOR OK GEOGRAPHY IN KING'S COLLEGE, LONDON 
 
 tl) tables anti 3Illu0trati0ns f 
 
 ^^^ OP* THR XT*" 
 
 'UNIVERSITY 
 
 LONDON 
 CHARLES GRIFFIN AND COMPA" 
 
 EXETER STREET, STRAND 
 
 1885 
 [All Rights Renewed} 
 
. : : ; ; :;: : ;:: i /. "**; 
 
 "Baflantpne 
 
 11AU.ANTYNE. HANSON AND CO. 
 KUINBUR(;n AND LONDON 
 
 3o<r>,c7 
 
PREFACE. 
 
 FBO:.I the days when Geology began to develop into an exact 
 science, the student has been encountered by guides in the form 
 of books, of two kinds ; one promising to " lead him the sweetest 
 and easiest way ; " the other demonstrating that " of things good 
 and beautiful, the Gods give nothing to men without great toil." 
 The earlier writers gathered facts from too wide a field to 
 demonstrate the steps of geological evolution. In the words of 
 Professor Suess : " It is an exceedingly difficult task to teach a 
 science well, which grows as rapidly as ours. The difficulty is 
 not caused by the enormous yearly increase of new observations, 
 because this may be overcome by patience, and by a diligent 
 study of every good memoir. But I find that many of our best 
 men go wrong in hanging to details, and losing sight of the 
 grand features of science, or in proclaiming popular theories, 
 and forgetting the painful arts of observation amidst the 
 applause of a short-sighted crowd." l The difficulty of teaching 
 has also been the difficulty of text-book writing. And among 
 the few works which have aspired to achieve a noble ideal, 
 probably the most honoured place must be given to the Manual 
 of Professor John Phillips. As the nephew of William Smith, 
 he knew the history and growth of geological ideas as well as 
 facts ; and as a public teacher was not unmindful of the aspects of 
 geology which are of vital importance in unfolding thought and 
 imagination for the learner. He was thus eminently fitted to 
 state the principles of the science in their mutual dependence ; 
 
 1 Letter of 1866. 
 
vi PREFACE. 
 
 and to elucidate tliem with the history of British and Foreign 
 strata. His book was in advance of the needs of the time, 
 worthy of the University in which he taught, and no doubt the 
 best Manual of Geology which had been written. The law of 
 science, however, is progress. Since 1855, the date of the last 
 edition, geology has grown in every element, has developed new 
 departments of petrology, and become separated from physical 
 geography ; but the plan of the old book still stands, unaffected 
 and excellent. We therefore accepted the responsibility of revis- 
 ing the Manual of Geology, and thus honour the memory of an 
 eloquent teacher, whose geniality reflected the happy influence 
 of nature, who if not a brilliant discoverer had verified much of 
 what was known, and was a sound geologist of balanced philo- 
 sophical habit. 
 
 In this volume I have preserved every page of the original 
 work that was in any way valuable. But the changes necessary 
 to bring the science of the last generation into harmony with 
 current knowledge and thought, have been more serious than 
 were anticipated when the revision began. I have omitted much, 
 have added more, and modified always ; while from exigencies 
 of space I have elected to omit certain subjects, and not to 
 develop others to the length to which they might profitably be 
 followed. Yet with this literary regeneration, the spirit of the 
 old book has been preserved, and it has been revivified with the 
 spirit of the newer geology which is unfolding. 
 
 In endeavouring to sustain that part of the title-page which 
 describes the Manual as theoretical, I have drawn to some extent 
 upon theoretical views enunciated in my lectures during the ten 
 years from 1860 to 1870, for which Professor Sedgwick, F.R.S., 
 deputed to me the practical teaching of Physical Geology and 
 Palaeontology in the University of Cambridge ; not altogether 
 foregoing a hope that days of requisite leisure may yet come, in 
 which the facts dependent upon those views may be elaborated 
 to their legitimate ends. 
 
 The work will be found practical too ; for it aims through- 
 
PREFACE. vii 
 
 out, by indicating localities where phenomena may be seen, at 
 enabling every one to verify, and study in nature, the state- 
 ments and ideas which are herein set forth. Only in this 
 way can knowledge become valuable, and independence of 
 thought be fostered. 
 
 This elementary study over, I commend the reader to the 
 second volume, in which Mr. Etheridge tells the History of the 
 Strata, with a wealth of fact which has hitherto had no parallel. 
 
 H. G. SEELEY. 
 
 THE VINE, SEVENOAKS, 
 
 September i$th, 1884. 
 
GENERAL CONTENTS, 
 
 PHYSICAL GEOLOGY. 
 
 CHAPTER I. DEFINITION AND ORIGIN OF THE SCIENCE. 
 
 Objects and origin of the science 
 Origin of inductive geology . 
 Basis of Werner's system . 
 
 Inductive geology principally 
 
 founded on organic remains . 5 
 
 Progress of palaeontology in England 6 
 
 William Smith .... 7 
 
 CHAPTER II. MODERN VIEWS OF THE EARTH'S DENSITY, SHAPE, 
 STRUCTURE, AND ORIGIN. 
 
 The figure of the earth . . 10 
 
 The earth's solidity inferred . 1 1 
 Evidence of the spherical form of 
 
 the earth . 12 
 
 Earth's orbit influencing tempe- 
 rature of its surface . 
 
 Speculations on the origin of the 
 earth 
 
 Inferences from meteorites . 
 
 CHAPTER III. THE CHIEF MINERALS WHICH FORM THE EARTH. 
 
 Mineral substances which consti- 
 tute aqueous rocks . . .21 
 The family of felspars . . 22 
 
 Augites and hornblendes . . 24 
 
 Micas and talcs 
 
 Garnets 
 
 Zeolites 
 
 25 
 26 
 
 27 
 
 CHAPTER IV. NATURE AND ORIGIN OF CRYSTALLINE AND IGNEOUS 
 
 ROCKS. 
 
 Foliation 31 
 
 Cleavage 32 
 
 Structure of granite 33 
 
 Relations of crystalline rocks, &c. 35 
 
 Quartz-orthoclase rocks . . 36 
 
 Imperfectly crystallised granitic 
 
 rocks 37 
 
 Quartzless orthoclase rocks . . 38 
 
 Quartz-bearing plagioclase rocks . 39 
 
 Quartzless plagioclase rocks . . 40 
 
 | Joint structure of igneous rocks . 42 
 
 CHAPTER V. NATURE, COMPOSITION, AND ORIGIN OF WATER-FORMED 
 
 ROCKS. 
 
 Kinds of deposit 
 Sand . 
 Clay . 
 Selenite 
 Alum . 
 Septaria 
 Phosphatite . 
 
 44 
 45 
 46 
 
 46 
 46 
 
 47 
 47 
 
 Limestones .... 
 Simultaneous origin of water 
 
 formed rocks 
 
 Horizontal sequence of rocks 
 Vertical sequence of rocks . 
 Relation of deposits to land 
 Table of British strata . 
 
 47 
 
 49 
 5' 
 
 52 
 54 
 55 
 
GENERAL CONTENTS. 
 
 CHAPTER VI. PETROLOGY. 
 
 Stratification 
 
 Internal arrangement of rocks 
 Superposition of strata 
 Inclination of strata 
 Synclinals and anticlinals 
 
 Strike 
 
 Outcrop .... 
 Distinction of stratified and un 
 
 stratified rocks . 
 Definition of strata 
 Their thickness and laminae 
 False bedding 
 
 PAOE 
 
 Terms for groups of strata . . 74 
 
 Disturbed stratification . . 75 
 
 Faults 77 
 
 Overlap 78 
 
 Epochs of convulsion . . -79 
 Analogy of mineral veins and 
 
 dykes 80 
 
 Internal structure of rocks . . 8 1 
 
 Master joints .... 83 
 
 Cleavage 84 
 
 Mechanical strata ... 88 
 
 Alternation and gradation of beds 89 
 
 CHAPTER VII. PHYSICAL AND MINERAL HISTORY OF STRATIFIED 
 
 ROCKS. 
 
 Sands 92 
 
 Cambrian grits .... 94 
 Devonian grits . . . .05 
 
 Carboniferous and Triassic sands . 96 
 
 Wealden sands and Tertiary sands 97 
 
 Glauconite 98 
 
 Clay 99 
 
 Slates 100 
 
 Septaria 1 02 
 
 Phosphatite 103 
 
 Salt and gypsum . . . .105 
 Aragonite organisms and calcite 
 
 organisms 1 06 
 
 Limestones . . . . .107 
 
 Travertine 1 08 
 
 Primary limestones . . .109 
 Secondary limestones . . .no 
 Flints . ill 
 
 CHAPTER VIII. CORAL REEFS. 
 
 Rocks formed of corals 
 Fringing reefs 
 
 114 
 
 Barrier reefs and atolls 
 
 116 
 
 CHAPTER IX. COAST LINES AND THEIR ORIGIN. 
 
 North and West Britain . .119 
 
 Mid Britain 121 
 
 South-east Britain and the Channel 122 
 
 Headlands 123 
 
 The shore 124 
 
 Sand-dunes 125 
 
 Landslips and waste of cliffs . 1 26 
 Islands : Wight, Man, Anglesey, 
 
 Lundy 128 
 
 Inner Hebrides . . . .130 
 
 CHAPTER X. GENERAL FEATURES OF SCENERY AND THEIR RELA- 
 TION TO GEOLOGICAL PHENOMENA. 
 
 Tablelands and low plains . .131 
 Low plains 134 
 
 Lacustrine strata . 
 
 Valleys 
 
 CHAPTER XL SUB^ERIAL DENUDATION AND ITS RESULTS. 
 
 Work of the wind 
 Waste of felspathic rocks 
 Effects of heat and moisture 
 Effects of frost and rain 
 Inundations and glaciers 
 Roches moutonn^es 
 Blocs perches 
 Springs . 
 Excavation of caves 
 
 H3 
 144 
 
 H5 
 146 
 148 
 149 
 
 152 
 
 Excavating action of streams 
 Waterfalls and gorges . 
 Taluses and fans .... 
 Arrangement of materials by 
 
 water 
 
 Rivers with lakes 
 Deposition in lakes 
 Deposits in gulfs and estuaries 
 Rate of subserial denudation 
 
 153 
 155 
 
 157 
 
 159 
 161 
 162 
 
 164 
 165 
 
GENERAL CONTENTS. 
 
 CHAPTER XII. NATURE AND ORIGIN OF VOLCANIC ENERGY. 
 
 Internal heat of the earth . 
 
 Hypothesis of magmas . 
 
 Action of radiated heat on the 
 earth's surface .... 
 
 Mallet's theory of volcanic heat . 
 
 Evidence in support of Mr. Mal- 
 let's views .... 
 
 Temperature and pressure involved 
 in rock construction . 
 
 PAGE 
 
 1 68 
 169 
 
 170 
 171 
 
 172 
 173 
 
 How upheaval facilitates the out- 
 burst of a volcano . . .175 
 
 Linear arrangement of volcanoes . 176 
 Origin of the eruptive power of 
 
 volcanoes 177 
 
 Why eruptions are intermittent . 178 
 
 Extinct volcanoes . . . 179 
 
 CHAPTER XIII. MANIFESTATIONS OF VOLCANIC ACTION. 
 
 Historic records : Graham Island 
 
 arid Jorullo . . . . 180 
 Structure of a volcano and se- 
 quence of events . . .181 
 Steam and ashes . . . .182 
 Red clay in the deep sea . .183 
 Mud streams and bombs . .184 
 Lava streams . . . .185 
 Rate of flow and aspect of lava 
 
 fields 186 
 
 Cones and craters . . .187 
 
 Volcanoes without cones 
 Fissure eruptions 
 Nearness of volcanoes to the sea . 
 Relation of volcanoes to springs, 
 
 and decline of volcanic activity 
 Solfataras and mud springs 
 Petroleum springs 
 Geysers . . . 
 
 Hot springs 
 
 Relation of hot springs to mineral 
 
 veins 
 
 CHAPTER XIV. NATURE AND ORIGIN OF IGNEOUS KOCKS. 
 
 Richthofen's hypothesis 
 
 Hypothesis of igneous evolution . 
 
 Views of Sir John Herschel and 
 Sterry Hunt .... 
 
 Texture of volcanic rocks and tex- 
 tures of granite 
 
 Comparison of granite and Clay . 
 
 Evolution of igneous rocks . 
 
 200 
 20 1 
 
 202 
 
 203 
 
 205 
 
 207 
 
 Clarence King on American vol- 
 canic rocks .... 
 
 Relation of plutonic and volcanic 
 rocks 
 
 Igneous rocks related to sand- 
 stones 
 
 Classification of igneous rocks 
 
 Experimental formation of volca- 
 nic rocks 
 
 1 88 
 189 
 190 
 
 191 
 192 
 
 193 
 194 
 
 197 
 
 198 
 
 208 
 209 
 
 211 
 212 
 
 213 
 
 CHAPTER XV. THE GRANITIC OR PLUTONIC GROUP OF ROCKS. 
 
 Mineral composition of granite . 214 
 
 Variation in chemical composition 215 
 
 Varieties of granite . . 216 
 
 Joints and decomposition . 217 
 Inclusions and concretions in gra 
 
 nite 218 
 
 Granite veins . . . 219 
 
 Modes of occurrence of granite 220 
 
 CHAPTER XVI. HISTORY OF BRITISH PLUTONIC ROCKS. 
 
 Geological age of granite 
 Syenite .... 
 Mica syenite ; augite syenite ; else 
 
 elite syenite 
 
 Zircon syenite ; miascite 
 Diorite .... 
 Gabbro 
 
 221 
 
 222 
 
 223 
 22 4 
 22| 
 226 
 
 Age of igneous rocks . . . 
 Granite of Cornwall 
 
 Elvans 
 
 Groby syenite and Mount Sorel 
 
 granite 
 
 Granite of Anglesey . 
 Granites of the Lake District 
 
 228 
 229 
 231 
 
 233 
 234 
 235 
 
 Minette and pikrite of the Lake 
 
 District 237 
 
 Granite in Scotland . . .238 
 
 Arran . . . . , . 240 
 
 Granites of Ireland . . . 242 
 
 Granite veins .... 248 
 
 Veinstones 250 
 
 Phonolite of the Wolf Rock . 251 
 
XH 
 
 GENERAL CONTENTS. 
 
 CHAPTER XVII. THE HISTORY OF VOLCANIC ROCKS. 
 
 PAOE 
 
 Method of study . . . .252 
 Colours of minerals in polarised 
 
 light 253 
 
 Texture of igneous rocks , . . 254 
 
 Propylite 255 
 
 Quartz propylite . . . .257 
 
 Andesites 258 
 
 Dacite 263 
 
 Trachyte 264 
 
 Phonolite 
 Rhyolite 
 Augite andesite 
 Basalt . 
 Leucite basalt 
 Nepheline basalt 
 Peridotite . 
 Serpentine . 
 
 PAGE 
 
 268 
 269 
 274 
 277 
 283 
 
 2^5 
 
 286 
 288 
 
 CHAPTER XVIII. THE HISTORY OF VOLCANIC ACTIVITY IN BRITAIN. 
 
 Evidence of former existence of 
 volcanoes 290 
 
 Pre-Cambrian volcanoes : St. Da- 
 vid's, Wrekin, &c. . . .291 
 
 Cambrian volcanoes . . . 293 
 
 Devonian and Old Red Sandstone 
 volcanoes 297 
 
 Carboniferous volcanoes 
 Permian volcanoes 
 Serpentine . 
 Secondary volcanic rocks 
 Tertiary volcanic rocks 
 
 303 
 3ii 
 312 
 
 313 
 
 CHAPTER XIX. CONCOMITANTS AND RESULTS OF VOLCANIC ENERGY. 
 
 Earthquakes . . . .321 
 
 Changes of level in land . . 325 
 
 Disturbances of the strata . . 330 
 
 Fractures and dislocations of strata 333 
 
 Dykes 334 
 
 Breaks in succession of the British 
 
 strata 340 
 
 Table of disturbances in the British 
 area 
 
 Table of disturbances in the Euro- 
 pean area 
 
 Collateral effects of upheaval and 
 depression .... 
 
 Mountain ranges .... 
 
 343 
 345 
 
 349 
 351 
 
 CHAPTER XX. METAMORPHISM. 
 
 Effects of internal heat . .356 
 Metamorphism of rocks . -357 
 Mineral constituents of metamor- 
 phic rocks . . . .361 
 
 Gneiss . 
 Mica schist . 
 Quartz rock . 
 Crystalline limestone 
 
 370 
 374 
 376 
 377 
 
 CHAPTER XXI. DISTRIBUTION OF GNEISS AND MICA SCHIST. 
 
 In England 379 
 
 In Anglesey . . . .381 
 
 In the Hebrides, &c. . . . 382 
 
 Along the Grampians . . . 383 
 
 In the North-Western Highlands 385 
 
 Scenery of gneiss and mica schist 387 
 
 Scenery of quartz rock . . 388 
 
 Crystalline limestone in Scotland 
 Metamorphic rocks of Ireland 
 Of the Pyrenees .... 
 Of Central France 
 Fossil-bearing schists of Norway . 
 Metamorphic rocks of America . 
 
 CHAPTER XXII. MINERAL VEINS. 
 
 Mineral veins now forming . 
 Source of metals . 
 Daubree on the origin of ores 
 Modes of occurrence of metals 
 Structure of a lode 
 Classification of veins . 
 Stock works 
 
 397 
 398 
 399 
 400 
 401 
 403 
 405 
 
 389 
 39i 
 392 
 393 
 394 
 395 
 
 Intersection of veins . . . 407 
 Directions of mineral veins . . 408 
 Relation of metals to rocks . .410 
 Werner's systems . . -413 
 Relation of mineral veins to local 
 igneous action .... 414 
 
GENERAL CONTENTS. xiii 
 
 CHAPTER XXIII. CHIEF MINERAL DEPOSITS IN BRITAIN. 
 
 Gold 
 
 Silver 
 
 Tin 
 
 Lead 
 
 Zinc 
 
 PAGE 
 417 
 
 Cumberland haematite . 
 
 and copper . . .418 
 
 Cleveland iron ore 
 
 4IQ 
 
 Iron ores of the oolites 
 
 423 
 
 Wealden iron ore 
 
 42? 
 
 Tertiary iron ores 
 
 ution of chief iron ores in 
 
 European iron ores 
 
 lin 426 
 
 
 PALAEONTOLOGY. 
 CHAPTER XXIY. ELEMENTARY IDEAS IN PALEONTOLOGY. 
 
 PAGE 
 428 
 432 
 
 433 
 434 
 435 
 435 
 
 Origin of species .... 
 
 438 
 
 Persistent types of life 
 
 45 
 
 Genus and species 
 
 439 
 
 Ancient types of echinoderms 
 
 
 Fresh-water and estuarine deposits 
 
 441 
 
 now living .... 
 
 452 
 
 Shallow water and deep-sea depo- 
 
 
 Local persistence of types . 
 
 45 2 
 
 sits 
 
 442 
 
 Climatal conditions of ancient seas 
 
 453 
 
 Extinction of species . 
 
 443 
 
 Existing distribution of life . 
 
 454 
 
 Sudden destruction of marine ani- 
 
 
 Distribution of existing genera . 
 
 454 
 
 
 444 
 
 S. P. \Voodward's provinces of 
 
 
 Paleeontological laws . 
 
 T'T'T' 
 
 445 
 
 marine life .... 
 
 457 
 
 Succession of life in time . 
 
 445 
 
 Relation of living to fossil forms . 
 
 ^ 
 
 460 
 
 Migration of life provinces . 
 
 ^ 
 
 446 
 
 Sclater's natural history provinces 
 
 461 
 
 Origin of faunas .... 
 
 447 
 
 Method of palaeontological work . 
 
 4 6 5 
 
 Identification of strata by fossils . 
 
 448 
 
 Distribution of plants . . . 
 
 468 
 
 Homotaxis 
 
 449 
 
 Succession of plant life 
 
 473 
 
 Colonies 
 
 45 
 
 
 
 CHAPTER XXV. THE SUCCESSION OF ANIMAL LIFE. 
 
 The constituents of faunas . 
 
 476 
 
 Brachiopoda .... 
 
 495 
 
 Spongia 
 Foraminifera .... 
 
 477 
 479 
 
 Lamellibranchiata 
 Gasteropoda .... 
 
 496 
 498 
 
 Hydrozoa 
 
 480 
 
 Cephalopoda .... 
 
 499 
 
 Actinozoa 
 
 481 
 
 Palseichthyes .... 
 
 5 01 
 
 Asteroidea 
 
 484 
 
 Teleostei 
 
 CQ4 
 
 Ophiuroidea .... 
 
 *J***t 
 
 485 
 
 Amphibia 
 
 -> 7 
 
 506 
 
 Crinoidea 
 
 485 
 
 Cainosauria 
 
 511 
 
 Cystoidea ... 
 
 487 
 
 Palaeosauria 
 
 c 12 
 
 Blastoidea 
 
 T" / 
 
 488 
 
 Ornithosauria .... 
 
 j 1 ^ 
 517 
 
 Echinoidea 
 
 488 
 
 Aves 
 
 518 
 
 Crustacea 
 
 AQ1 
 
 Mammalia . 
 
 C2O 
 
 Vermes 
 
 *ry A ... 
 
 494 Summary 
 
 ** 
 
 526 
 
 Bryozoa 
 
 495 1 
 
 
 CONCLUSION 
 
 .... 
 
 528 
 
 INDEX . 
 
 
 511 
 
Knowledge should be practical from the first. The student may 
 form gradually a small collection of specimens, and those will often 
 be most instructive which are collected by himself ; but in cases where 
 collection of specimens is not possible, it may be useful to mention 
 that 
 
 MINERALS AND ROCK SPECIMENS may be obtained from Mr. GREGORY of 
 Charlotte Street, Fitzroy Square. 
 
 COLLECTIONS OP SLICES OF MINERALS, ROCKS, AND FOSSILS for. the Micro- 
 scope are supplied by Messrs. How & Co., of Farringdon Street. 
 
 Mr. CATTELL of Leighton Road, Kentish Town, is skilful in slicing rocks 
 and fossils for microscopic study. 
 
TJHI7EHSIT7 
 
 PHYSICAL GEOLOGY. 
 
 CHAPTEK I. 
 
 DEFINITION AND ORIGIN OF THE SCIENCE. 
 
 Objects and Scope of Geology. The term " science " is understood 
 to express, not only the information or facts collected, and the laws 
 founded on this knowledge, but also the ultimate objects and whole 
 field of research. Thus the science of Geology embraces that depart- 
 ment of the philosophy of nature, which investigates the formation 
 and structure of the earth and its system of development, including the 
 building up of rocks, succession of life, production of minerals, &c., 
 and all the changes which the earth has undergone. Geology, in fact, 
 treats of the earth's history and its constitution as a planet, subject to 
 physical laws that have produced changes in the materials of the earth's 
 crust, and have modified the succession and distribution of life in 
 different regions from age to age. 
 
 It is in conformity with this ordinary language that we shall 
 endeavour to define geology. None of the sciences of observation has 
 made more remarkable progress toward successful generalisation than 
 this, yet the prospect of further discovery is so much richer than the 
 retrospect, and the ability employed in the research is so much more 
 skilled now, that we can hardly offer too expanded an expression for 
 the ultimate aims of geology. 
 
 Geology then, in its fullest extent, is that science which undertakes 
 to investigate the ancient natural history of the earth ; to determine 
 by observation what phenomena of living beings or inorganic matter 
 were formerly manifested on or within the globe, in what order and 
 under what conditions ; to employ the comparative data, which are 
 furnished by investigating the present operations of nature as a means 
 for characterising and measuring the successive revolutions which the 
 earth has undergone before it arrived at its present state ; and thus, 
 finally, to furnish a complete historical view of the conditions which 
 have regulated, and still regulate, its system of mechanical, physical, 
 chemical, and vital phenomena. 
 
 From the terms of this definition we may at once understand why, 
 VOL. i. A 
 
2 ORIGIN OF GEOLOGY. 
 
 in former times, the most able men erred in their attempts to elucidate 
 the history of our globe ; for, while physical geography was imperfectly 
 known, before commerce and the knowledge of languages had made 
 us acquainted with the productions and traditions of every nation and 
 clime, before the birth of most branches of physical science, it was 
 impossible to accumulate the numerous and exact observations from 
 which alone geology takes its origin. And since the general truths of 
 geology are made apparent only by the application of the known laws 
 of modern nature, it is evident that, before the discovery and establish- 
 ment of those laws, the wisest of the old philosophers had nothing to 
 substitute for enlightened theory but arbitrary hypothesis and fanciful 
 conjecture. These are the reasons why the ancient doctrines concern- 
 ing the world are almost without exception bewildered with the 
 impossible problem of the creation of matter, and buried in a chaos of 
 subtle inventions. 
 
 The early writers of Babylon, Phoenicia, Chaldsea, Egypt, and China, 
 were occupied, almost absolutely, with the cosmogony or origin of the 
 world, and neglected its physical history. 
 
 What Herodotus says regarding the sediments deposited by the 
 Kile in the valley of Egypt, and carried out to sea; of the time 
 (10,000 or 20,000 years) which he estimates as sufficient for the Kile, 
 if diverted into the Erythraean Sea, to fill up that long gulf ; x and of 
 the shells found on the hilly borders of Egypt which testify to its 
 former submersion beneath the sea, may, however, be quoted with 
 approbation as a fair specimen of ancient observation and inference. 2 
 
 Origin of Inductive Geology. Four different classes of pheno- 
 mena conducted men of observation to a partial acquaintance with the 
 stratification of the crust of the earth. 
 
 1. The effects of disturbance in countries shaken by earthquakes 
 and marked by periodical volcanic excitement as Asia Minor, Italy, 
 &c. 
 
 2. The arrangement of the various strata in England and elsewhere. 
 
 3. The appearances of regular structure in the mines of England, 
 Germany, Sweden, &c. 
 
 4. The remains of plants and animals which are found entombed 
 almost everywhere, where the water-formed or stratified rocks are 
 examined. 
 
 i. Of the first class of observers, the most distinguished in early 
 times is Strabo, who gives an interesting account of the Katakekau- 
 mene (burnt district), in the valley of the Hermus, in Asia Minor a 
 district which remained almost unvisited till Mr. Hamilton renewed 
 our acquaintance with its remarkable features, so similar to those of 
 Auvergne. Etna has engaged the attention of observers from the 
 time of the Greek poets and philosophers down to the period of Dau- 
 beny, Scrope, and Lyell, all of whom have endeavoured to trace the 
 
 1 Her. ii. II: eycb /m.V ^ATTO/ACU ye ical /mvpiuv evrbs x&crdTJva.i S.v - 
 
 2 Her. ii. 12 : I8wv re TTJV Kiyvirrov TrpoK.eiiJ.6vr}v rrjs ^xo/u.eVr;s 7775, 
 <fia.iv6[jLeva. tirl roi<n o&petrt. The shells were, perhaps, truly "fossil," and belonged 
 to the rocks in the hilly ground. 
 
DEFINITION AND ORIGIN OF THE SCIENCE. 3 
 
 source of its fires, their origin and decay. And the phenomena con- 
 nected with eruptions, which fatally stimulated the curiosity of the 
 elder Pliny, have trained up in modern times philosophic men like 
 Steno 1 and Lazzaro Moro, 2 Sartorius von Waltershausen, 3 Palmieri, 4 
 Abich, 5 Robert Mallet, 6 who not only perceived succession of time 
 in the deposition of the strata, but great physical changes displace- 
 ments of land and sea affecting large areas of country. It is in this 
 school that we find the germs of our modern theories of elevation and 
 depression of land, whether by the sudden violence of volcanic heat, 
 or gradual effort depending on a general change of dimensions due to 
 temperature of the globe this being the Leibnitzian theory. 7 
 
 2. Agriculture. The early advancement of agriculture in a country 
 so populous, and of so diversified an aspect as England, necessarily 
 produced a very intimate knowledge of different soils ; and as these 
 depend on the nature of the structure and composition of the under- 
 lying rocks which range through the country in regular courses, it is 
 not surprising that maps of the soil should have been early proposed 
 and prepared by agriculturists. Dr. Lister, residing in Yorkshire, 
 where the limits of soil are very well defined, was the first to propose 
 to the Royal Society, in 1683, a map of the soils of England. 
 
 3. Mining. Miners in every period must have been generally 
 acquainted with the order of succession of the rocks through which 
 they seek for coal and other minerals; and in tracts consisting of 
 alternating coal-seams, limestones, sandstones, and shales, the range 
 and extent of the different strata must always have been familiar to 
 the workmen, although perhaps in an empirical manner. 
 
 There must, therefore, always have been a MINERALOGICAL SCHOOL 
 OF GEOLOGY in every country in which rich subterranean treasures 
 attracted the attention of mankind. 
 
 Agricola embodied the floating information of the miners of Saxony 
 as early as 1546 (De Naturd Fossilium} ; he was followed by Cordas, 
 Gesner, Kentmann, Fabricius, Encelius, " not unworthy of praise," as 
 we are told by Baier (Oryctograpliia Norica, 1708). Sweden, equally 
 celebrated for its mines, produced, between 1730 and 1762, five com- 
 plete systems of mineralogy, including the methods of Linnaeus, 
 "Wallerius, Swab, and Cronstedt. 
 
 In 1750 Tylas, a Swede, and in 1756 Lehmann, a German, broke 
 through the fetters of a mere mineralogical method, and by proving a 
 regular order of superposition among stratified rocks, opened the way 
 for the sagacious generalisations of "Werner, and the cautious inductions 
 of Saussure. 
 
 Werner. A peculiar set of opinions concerning the formation of 
 the earth has been honoured by the title of the Wernerian theory ; 
 and the pupils of Werner, who had found proof of the truth of his 
 
 1 De Solido Intra Solidum, &c., 1669. 2 De Crostacei, &c., 1740. 
 
 3 Atlas de ^Etna, 1845-61. 4 On Vesuvius. 
 
 5 Vues Illustratives de Phe"nomenes Geologiques Observes, &c, 1837. 
 
 6 Earthquake Catalogue, 1858; Ueber Vulkanische Kraft, 1875. 
 
 7 Protogsea, &c., 1680. 
 
4 WERNER'S GEOLOGICAL SYSTEM. 
 
 practical rules and inferences, may be readily pardoned for the deter- 
 mination which they manifested to uphold Werner's hypothetical 
 notions. But if we wish to ascertain the real value of the benefits 
 which the researches of Werner have conferred upon geology, we must 
 forget his theory, and view only the data which he collected for its 
 foundation. 
 
 Werner was educated amidst the mines, and in the society of the 
 most eminent mineralogists of Saxony; their experience and their 
 opinions became his own, and doubtless swayed and directed the 
 energies of his mind. To judge from his own 
 works, and from the course which his pupils so 
 ^ on S P ur sued, the principal point of view under 
 which Werner contemplated the rocks and metallic 
 veins of Germany, was the relative period of 
 their production. Lehmann had, indeed, taken 
 the same course, and already distinguished Primary and Secondary 
 rocks, the former (a) existing in mountain chains, mostly stratified, 
 at high angles, and devoid of organic remains, the latter (b) dis- 
 posed more horizontally, and stored with the remains of life. But 
 Werner, with characteristic tact and boldness, applied 
 this method of investigation to every case, and took it 
 as the basis of his classification of rocks. 
 
 Basis of his System. " When two veins (a I) 
 cross, and one of them (/;) cuts through the other (a), 
 Fi - 2 - the one which is divided (a) is the more ancient." 
 Among stratified rocks superimposed on one another, the lower 
 members of the series, those which lie nearest the centre of the earth, 
 were deposited first, and the relative antiquity of the different strata 
 is exactly in the order of their position. Thus c is the oldest rock of 
 the series c, d, e, /, g. 
 
 By this manner of proceeding in the instance of the Harz Moun- 
 tains, Werner was enabled to frame a system or classification of rocks 
 iii the order of their respective position as far as could then be ascer- 
 
 tained, and consequently in the order of their conse- 
 
 ? cutive formation. Thus the Brocken Mountain was 
 
 - - described by Werner and his followers as a central 
 r d cone of granite, upon which on all sides round were 
 
 c laid various other rocks in a certain and constant 
 
 FiT order of succession; as granite, clay-slate, limestone, 
 
 greywacke and greywacke-slate, old red sandstone, 
 limestones, gypsums, sandstones, and limestones ; the upper and newer 
 strata having their outgoing or terminal edges lower and lower con- 
 tinually. 
 
 He presumed that the order of succession among these rocks in 
 Germany would be found to prevail in all parts of the world, and thus 
 announced a grand principle in the construction of the earth which 
 was destined to have a most beneficial effect on geological theory and 
 observation. For, on the one hand, it dissipated the chaotic dreams 
 of those who maintained that the whole crust of the earth was to bo 
 
ORIGIN OF STRATIGRAPHICAL GEOLOGY. 5 
 
 viewed as a mass of sediment from the waters of " the deluge " ; and 
 on the other, exhibited the most important subject of inquiry respect- 
 ing the constitution of the earth, and fixed a precise method of inves- 
 tigating it. 
 
 That Werner's classification is partly erroneous in principle, and in 
 all respects incomplete, and inadequate to the rigour of modern inves- 
 tigation, is apparent at a first glance, but it obviously contains the 
 essence of rightly planned arrangements, viz., a determined reference 
 to the relative antiquity of the deposit. Werner is, therefore, entitled 
 to the distinguished praise of clearly announcing and striving earnestly 
 to establish one of the most important general laws yet ascertained 
 respecting the structure of the earth. He proved that in a particular 
 district its stratified rocks are laid one on another in a certain order of 
 succession, and affirmed that the same, or a very similar order, pre- 
 vailed over large parts of the earth's surface. 
 
 Michell. One of the ablest of the natural philosophers of England 
 during the middle of the eighteenth century, who, for a short time, 
 filled the Woodwardian chair of geology at Cambridge, and afterwards 
 resided in Yorkshire, had certainly made himself acquainted with 
 the series of English strata, especially in the northern counties, and 
 had even gone so far as to discover some of the most important 
 general relations between the geological structure and the physical 
 features of the globe, defining with a masterly hand the mutual 
 dependence of mountain ranges and lines of stratified rocks. 
 
 In frequent journeys between Cambridge and Yorkshire, mostly 
 performed on horseback, he composed a useful section of the strata 
 between the chalk hills of Bedfordshire and the coal strata of Not- 
 tinghamshire, but never printed an account of his discoveries. What 
 he stated concerning the succession of strata in Derbyshire and other 
 parts, was chiefly derived from the miners and colliers, who, certainly, 
 for a hundred years before the dawn of sound geology, knew perfectly 
 the almost invariable sequence of strata in their own districts. 
 
 4. Inductive Geology principally founded on the Organic Re- 
 mains. The great fact upon which, in modern times, geological 
 inquiries have hinged, the occurrence of marine animals far from the 
 sea and deep in the solid earth, was so far understood by the ancients, 
 that they had ascertained the general agreement of fossil and recent 
 marine shells. 
 
 But the sixteenth century was wholly wasted among the naturalists 
 of Italy, France, England, and Germany in the ridiculous dispute 
 whether fossil shells were genuine marine exuviae, or mere lusus 
 naturae produced by a plastic power of fermenting fatty earth ? and 
 the inquiry assumed a more difficult character from the addition of 
 the question, whether, if they were genuine petrifactions, they were 
 all deposited by the Noachian deluge 1 
 
 In examining both of these points the Italian philosophers 
 
 were by far the most conspicuous, and it is difficult to understand 
 
 how the sound conclusion of Fracastorio (1517), Scilla 
 
 1 La Vana Speculazione Disingannata, Napoli, 1670. 
 
6 ORIGIN OF PALAEONTOLOGY. 
 
 Ramazzini, 1 and Vallisneri, 2 could fail to become the universal creed 
 of geology. Palissy, the philosophic potter of France, published, in 
 I557, 3 from his own experience, the conclusion that fossil shells and 
 other "figured stones" were the genuine exuviae of ancient marine 
 animals. 
 
 Progress of Palaeontology in England. In England, the great 
 interest which belonged to the thousands of fossil plants and animals, 
 was fully understood by Plot, Llwyd, Ray, Lister, Woodward, and 
 Moreton ; who by their rich collections, and publications, as well as 
 resolute though unsuccessful efforts to deduce the causes which had 
 thus buried and preserved imperishable in the earth organic remains 
 of a former period, undoubtedly kindled that ardent spirit of inquiry 
 respecting the structure of the earth, for which the English philo- 
 sophers of the seventeenth century were so honourably distinguished. 
 
 Nevertheless, the progress of geology in England was* retarded by 
 the fettered condition of other sciences, and by a peculiarly unhappy 
 conjunction of truth and fiction. The correct view of the original 
 nature of " formed stones, or petrifactions," was coupled by Woodward 
 and his numerous followers with the assertion, that all the strata 
 superimposed on one another in the crust of the earth, with all their 
 included myriads of fossil animals and plants, were deposited by one 
 general flood, "the deluge !" 
 
 One great merit, however, strikingly characterises the early English 
 school of geology, even in its greatest aberrations, a thorough con- 
 viction that the organic remains entombed in the earth were the 
 surest evidence of the revolutions which it had undergone. 
 
 Lister. Through the research and learning of this industrious 
 and distinguished man, England was filled with collections of fossils, 
 which were compared with native and exotic living species, and 
 almost every naturalist of note from the time of Lister has contributed 
 something to the stock of information respecting them. Lister was 
 free from theoretical prejudice, had the merit of perceiving and of 
 recording in a single instance the principle of mutual dependence 
 between the strata and their organic remains, which afterwards, 
 generalised and promulgated by Smith, became the most important 
 instrument of investigation which has ever been presented to geology. 
 
 Speaking of a small species of belemnite (B. Listen), which is 
 figured in his Historia Animaliwn Ancjlice, he says it is found in all 
 the cliffs as you ascend the wolds, for above a hundred miles in com- 
 pass, at Speeton, Londesbro', and Caistor, but always in a red, ferru- 
 ginous earth. This correct and remarkable result is a striking 
 example of the possibility of even holding in the hands a brilliant 
 discovery, without knowing its value, or taking any steps to ascertain 
 its importance. 
 
 1 De Font. Mutin. Scat. ; b. 1633, d. 1714. His observations are quoted by 
 Vallisueri, Lazzaro Moro, Linnseus, &c. 
 
 2 De Corp. Mar., and other works ; b. 1 66 1, d. 1730. 
 
 3 Palissy's work was first printed at Lyons in 1557. The edition of 1580 
 usually referred to, was the third. 
 
DISCOVERIES OF WILLIAM SMITH. ^ 
 
 Smith. A century later, the perception of the same truth, in several 
 instances near Bath, and the demonstration of its applicability to the 
 whole Secondary series of the strata of England, enabled Mr. William 
 Smith, by his own unaided efforts, to establish the geology of England 
 on a basis from which it can never be shaken. He made an accurate 
 classification of the stratified rocks in the order of their relative anti- 
 quity, accompanied by catalogues of their organic contents, and a map 
 of their ranges and distribution on the surface of the island in confor- 
 mity with the order of succession of the rocks in the interior. 
 
 To study the laws of nature according to the principles developed 
 by Mr. Smith, to ascertain by the order of succession of deposits and 
 by organic remains, what were the contemporaneous effects of the 
 natural agents employed in the formation of the earth's structure in 
 all parts of the world, is the first great problem of modem geology. 
 By the aid of zoological and botanical researches we determine the 
 relative antiquity of every species of fossil plant and animal, and 
 assign the relative period during which its existence was continued. 
 The fossils called Orthoceratites, Producta,Tiilol>ita,, and many Crinoidea, 
 belong to the older and lower rocks ; certain species of Echini, Ammo- 
 nites, Belemnites, and other shells, mark the Oolitic strata ; while others 
 belong to the Chalk ; and a different series of plants, corals, shells, and 
 remains of vertebrate animals lie above the chalk, to those found below. 
 Such inferences, drawn from observations in Europe, have been found 
 constant in America ; and this powerful instrument of research thus 
 placed in the hands of the observer, having been wielded with the 
 caution requisite in questions of analogy, the principles disclosed by 
 Mr. Smith's researches near Bath and elsewhere, and illustrated by 
 Cuvier's philosophical description of the environs of Paris, are found 
 to be universally applicable ; for the distant slopes of the Himalaya 
 and Andes, and the shores of Australia and Greenland, are united in 
 the mind of the geologist who contemplates the evidences of their 
 coeval stratification. 
 
 Hypotheses. We here close our short account of the growth of 
 geology into a science. The paths of observation, along which alone 
 the foundations of the science are to be sought, were hard and difficult; 
 those hypotheses which they displaced were easy and inviting. The 
 globular figure of our planet, the inequalities of its surface, and the 
 occurrence of marine shells in mountains far from the sea, have been 
 thought sufficient data for rashness and speculation to construct de- 
 tailed theories of the earth, to determine the constitution and condition 
 of its centre, and to describe, as if men had actually beheld them, the 
 successive revolutions which the world had undergone. 
 
 These hypotheses were most numerous and discordant during the 
 period when positive geology had made the least progress ; with the 
 advancement of knowledge they diminished in number and improved 
 in consistency ; and at the present moment, though every theory has 
 lost its power of fettering the mind, there is a tacit but almost uni- 
 versal agreement in those fundamental principles of structure, and 
 circumstances of origin of phenomena, by which alone every passing 
 
8 NATURE OF GEOLOGICAL WORK. 
 
 theory must be judged, and to which also all good observations and 
 sound inductions must be referred. To develop these principles in a 
 settled order, to illustrate by their aid the geological structure of the 
 British isles, and to connect and correlate the geology of Britain with 
 that of Europe and other parts of the globe, and thus rise by a legiti- 
 mate process to those comprehensive inferences which the subject 
 admits of, is the aim of the following pages. 
 
( 9 ) 
 
 CHAPTEE II 
 
 MODERN VIEWS OF THE EARTH* S DENSITY, SHAPE, STRUCTURE, 
 AND ORIGIN. 
 
 THE earth's density, size, and shape, are all closely connected with 
 each other. The earth is five and a half times as heavy as a globe of 
 water of the same size would be. It is twice as heavy as a similar 
 globe of granite, half the weight of a like globe of lead, and about a 
 fourth as dense as though it were made of solid gold. This density 
 of the earth is also called its specific gravity. But specific gravity is 
 the weight of a substance in air divided by the difference between its 
 weight in air and weight in water, thus : 
 
 Weight in air 
 
 = specific gravity. 
 
 Weight in air Weight in water. 
 
 In the case of the earth, the specific gravity is inferred by comparison, 
 not determined by experiment. There are several ways in which the 
 density of the earth has been estimated. The most important of these 
 are known as the Schiehallion method, the Pendulum method, and the 
 method of Cavendish. The Schiehallion method, used by Maskelyne 
 in 1742, is essentially this : If a line with a weight attached to it is 
 suspended, it points towards the centre of the earth. Such a plummet 
 was so placed as to be attracted by the somewhat isolated mountain 
 in Perthshire, named Schiehallion. The size and density of the moun- 
 tain being known from measurement and by weighing samples of the 
 rocks, the amount was calculated by which the line ought to be 
 attracted towards it by the mass of the mountain. And since the line 
 was not drawn towards the mountain nearly so much as it would have 
 been if the earth had been throughout of the same density as the 
 mountain, it follows that the earth as a whole must be much denser ; 
 and, in fact, is found by this evidence to be twice as dense as that 
 mountain, or, in other words, has the density indicated by the num- 
 ber 5 J. This experiment has been repeated at Edinburgh and at Mont 
 Cenis, yielding nearly the same results, though in both these cases the 
 density appeared to be slightly less than at Schiehallion. The Pendu- 
 lum method, invented by Sir G. B. Airy in 1854, consisted in observing 
 the difference between the movements of a pendulum at the bottom 
 of the Harton colliery, near South Shields, and on the earth's surface. 
 
10 
 
 THE INTERIOR OF THE EARTH. 
 
 If the earth had throughout the same density as the rocks at its sur- 
 face, the pendulum would beat slower at the bottom of a mine than 
 at the top ; because the attraction of the earth is then that of a globe 
 lessened in radius by the distance which you descend towards its 
 centre ; and as the force of gravity thus becomes diminished with the 
 decrease of the mass, the pendulum swings more slowly. But at the 
 bottom of the mine the pendulum actually goes much faster than at 
 the top, showing that the interior of the earth is formed of materials 
 much more dense than the surface rocks. But the density found in 
 this way being 6 J, is so much greater than that found by other methods 
 that it is not usually regarded as quite so reliable, and the experiment 
 has not been repeated. The method of Cavendish, who used a torsion 
 balance, is of a different character. It has been repeated twice, and 
 gives the earth a density of 5^. This being the mean of the various 
 observations, is believed to be as near the true density as possible. It 
 however by no means follows that the interior of the earth consists of 
 substances which have a high density at its surface, since the compres- 
 sion due to gravity alone would greatly condense the materials forming 
 the earth, and give to the interior a high density, even though it con- 
 sisted throughout of such minerals as form the surface rocks. But 
 the effects of this pressure in condensing the interior of the globe 
 would be far more considerable than they are, were they not resisted 
 within by some general antagonist force, such as the expansive power 
 of heat, or an extraordinary want of compressibility among the parti- 
 cular substances operated on. It may be useful to compare with the 
 specific gravity of the earth itself the specific gravities of a few of its 
 constituent minerals and metals as shown in this table : 
 
 Sulphur . 
 Dolomite 
 Calcspar 
 Felspar . 
 Quartz . 
 Mica 
 
 Sp. gravity. 
 
 
 2.O 
 
 Iron 
 
 2.8 
 
 Copper 
 
 2-5 
 
 Lead 
 
 2-5 
 
 2.6 
 
 Mercury 
 Platinum 
 
 3.0 
 
 Gold . 
 
 Sp. gravity. 
 
 7-5 
 
 8. 5 
 II.O 
 I 3 .0 
 IQ.O 
 2O.O 
 
 From such a list it is evident that the heavier metals can only form a 
 small portion of the interior of the earth. But if the high density of 
 the earth were supposed to be connected with an original fusion which 
 allowed heavy substances to gravitate towards the centre, then it must 
 be remembered that our only knowledge of the existence of such 
 substances in the earth at all is from their occurrence at the surface, 
 where on such a theory they ought not to be found. It is, therefore, 
 improbable that the earth's density is a result of the arrangement of 
 different mineral constituents in successive layers like the coats of an 
 onion, with the density decreasing towards the surface ; and density 
 probably gives no insight into the interior construction of the earth. 
 
 The Figure of the Earth, as it is commonly named, is such as would 
 be ultimately attained by a rotating body, no matter what its original 
 form may have been. If the earth had ever been a sphere, then the 
 wearing and transporting power of water would gradually cut down the 
 
ORIGIN OF THE FIGURE OF THE EARTH. n 
 
 polar protuberances, and transport the materials towards the equator, 
 owing to the action of centrifugal force. At present the polar diameter 
 of the earth is nearly 26 J miles less than its equatorial diameter. There 
 is no reason, however, for supposing that the earth was ever more 
 nearly spherical than now, while, on the contrary, the shape may not 
 improbably have been approximating towards the sphere from a remote 
 period. The flattening of the earth's poles which gives it the orange- 
 like form called an oblate spheroid, is so small in amount that the eye 
 is quite unable to detect the flattening upon any accurate model of a 
 globe that can be made. This figure may result from many causes. 
 Seeing that the excess of length of the equatorial over the polar 
 radius amounts to but little more than the difference between the 
 greatest mountain height -and greatest ocean depth, that fact is con- 
 clusive proof that the earth is sufficiently elastic to owe its flattened 
 form to rotation alone, especially when it is remembered that the 
 centrifugal force or tendency of things to fly from the earth is at 
 the equator o-J-g-th of the force of gravity which draws all things 
 towards the earth's centre of attraction. Hence the shape of the earth 
 may be due entirely to deformation, or alteration from the spherical 
 form, consequent upon rotation, so that altered position of the earth's 
 axis would explain emergence and submersion of land. If any part 
 of the interior of the earth should be fluid, it is possible that the way 
 in which centrifugal force might influence the tendency of internal 
 heated matter to fly towards the equatorial region, might appreciably 
 affect the expansion of the rocks in the equatorial plane, and thus 
 account for a difference between polar and equatorial diameters. Sir 
 William Thomson has stated that if the earth were wholly composed 
 of glass, its mean expansion for every degree increase of Fahrenheit 
 temperature between 30^ and 212 would be i part in 69,660; if it 
 were of iron the expansion in the same limits would be i part in 
 50,760; while if the earth were all copper, the expansion for each 
 degree would be i part in 34,920. If, for the sake of illustration, 
 we assume the mean temperature of the earth at the equator to be 
 80, and suppose no increase of temperature to take place towards its 
 centre, and further suppose the earth to consist of copper, then there 
 would be an increase of the earth's diameter in the equatorial region 
 of many miles, owing to the expansion of the metal ; and if no corre- 
 sponding or any less elongation took place towards the poles, there 
 would be a considerable equatorial bulge due to this cause. It is 
 therefore probable that the earth's internal heat has to be considered 
 as a factor which has influenced its form. 
 
 The Earth's Solidity inferred from the Stability of its Figure. 
 The difference between the polar and equatorial diameters being -g^th 
 of the earth's diameter, gives the earth's surface a curve which deviates 
 from the circle towards an ellipse by about i in 300. And this 
 fact coupled with some remarkable discoveries in the relative motions 
 of the moon and earth made by Professors Adams and Delaunay in 
 1859 and 1866, led Professor Sir William Thomson to speculate from 
 the earth's form upon the length of time for which the earth may 
 
T2 THE SOLIDITY OF THE EARTH. 
 
 possibly have existed. The argument is as follows : The moon at the 
 end of a century gets to be between five and six seconds of time in 
 advance of her natural place in the heavens with regard to the earth. 
 It was suggested that this difference probably resulted from the extent 
 to which the earth's movement is retarded by the friction against it of 
 tidal waters. And when this idea was worked out it was found that 
 the tides were competent to produce a retardation of the earth four 
 times as great as the difference between the motion of the earth and 
 the moon. Hence it followed, if the data for the calculation are 
 correct, that 10,000,000 years ago the earth must have been rotating 
 ith faster than it rotates now, and therefore that the centrifugal force 
 must have been to the centrifugal force now in the proportion of 64 
 to 49. So that if the earth had consolidated at that remote period, 
 the ellipticity of its upper layers would have been a curve of i in 
 230, instead of a curve of nearly i in 300, which the earth actually has 
 now. Therefore, allowing for all possible errors, Sir William Thomson 
 concludes that the consolidation of the earth from a molten state (if it 
 were ever fluid) must have taken place at some period considerably 
 more recent than 1,000,000,000 years ago ; and since that far-off period 
 when the earth began to consolidate, consolidation has progressed till 
 it is probable that at its centre no part of the earth is now fluid. The 
 evidence on which this conclusion rests is remarkably forcible, for if the 
 earth's interior were to any large extent fluid, our planet would behave 
 as a fluid body, and be drawn out of shape by the attracting forces ex- 
 ternal to it, just as the sun and moon draw the ocean towards them in 
 tidal movement, and tides would not exist. On the contrary, however, 
 the earth is extremely rigid, probably as rigid as though formed through- 
 out of solid cold steel, and would appear only to yield a few inches to 
 the attracting powers which produce tides, so that there can be no 
 ground for assuming a fluid interior, when it must be so far removed 
 from the surface, even if existing, as to be inappreciable by any effects 
 manifested at the surface. 1 
 
 Evidence of the Globular Form of the Earth. The figure of the 
 earth has been determined more accurately by actual measurement ; 
 but its globular form has been long demonstrated by experience and 
 observation of the following facts : 
 
 First, during an eclipse of the moon the shadow of the earth is 
 cast upon the moon so as to exhibit under all circumstances a nearly 
 circular outline ; and since this outline is sometimes cast by different 
 aspects of the earth, the demonstration by this means of the earth's 
 globular form is perfect. The Egyptians, Babylonians, and early 
 Greeks all were aware of this evidence of the earth's shape. Secondly, 
 navigators such as Ferdinando Magellan in 1519, Sir Francis Drake 
 in 1577, Sir Thomas Cavendish in 1586, and many others, sailing con- 
 stantly to the west have returned to the point in Europe from which 
 they started, and thus by sailing round the world have demonstrated 
 the earth's spherical form. Thirdly, by ascending increasing heights 
 above a plain in any part of the world, the distance to which the eye 
 1 Thomson and Tait's "Natural Philosophy.*' 
 
MODERN VIEWS OF THE EARTH'S SHAPE. 13 
 
 can recognise the geographical features of the coimtry is steadily 
 increased, and the circular horizon becomes larger ; and this can only 
 he explained on the hypothesis that the earth is a sphere. Because 
 if a number of tangents be drawn to a circle at equal distances on 
 tfach side of a point, and the lines are prolonged so as to meet each 
 other in pairs above that point, then it will be evident that only on 
 a spheroid can the horizon be a circle, which is enlarged propor- 
 tionately as the eye is elevated above its surface. Fourthly, ships at 
 sea gradually sink farther and farther below the horizon as their 
 distance from the shore increases, until even the tops of the masts 
 disappear. This is regarded as proof of the earth's spherical form 
 because, first, it is exactly analogous to the way in which a man 
 disappears from view by walking over ground which rises in a rounded 
 form; and, secondly, because the object which has disappeared can be 
 seen again if we climb a cliff a sufficient height above the sea. 
 Fifthly, if three rods of equal length are set up on a straight canal 
 free from locks at equal distances from each other, so that the top 
 of the most distant post can be seen through a telescope fixed to the 
 top of the post at the other end of the canal, then the top of the 
 middle post will be seen to rise considerably above the straight line 
 between the two end points ; and this can only be because the surface 
 of the water is not level, but has a spherical curve, and therefore goes 
 to prove that the earth's surface is spherical. 
 
 There was no suspicion of the polar flattening of the earth, so far 
 as is known, till the time of Sir Isaac Newton. In the year 1671, 
 Richer, at Cayenne in South America, found that the pendulum of 
 the clock which he had brought from Europe was no longer of the 
 right length to keep time, vibrating more slowly the nearer it was 
 moved to the equator, so that the clock lost 2\ minutes a day. It 
 thus became known experimentally that the force of gravity diminished 
 towards the equator, because the rapidity of the pendulum swing is 
 in proportion to the intensity of the earth's attraction. 'And so soon 
 as one of the 360 degrees into which a circle passing through the 
 centre of the earth is divided, had been accurately measured, it became 
 possible to determine the size of the earth and to calculate the figure 
 which the earth ought to assume in consequence of its rotation. This 
 rotation diminishes the force of gravity at the equator by -g^th ; and 
 the total force of gravity is now known to be -y-p-^th greater at the 
 pole than at the equator. It was hence calculated that the earth 
 must be flattened towards the poles by an amount nearly equal to 
 that afterwards found by measurement. The exact amount of flatten- 
 ing has been determined by finding the curve of the meridians in 
 different latitudes ; and the degree of a meridian has been repeatedly 
 measured, and is known to increase in length from 362,644 feet at the 
 equator to 366,489 feet at the pole. A degree is always ^-g-^th of a 
 circle, and as the circle becomes larger so the degree obviously becomes 
 longer. The lengthening of the degree then, towards the pole, is 
 evidence that the earth's outline there becomes a part of a larger 
 circle, or in other words, it becomes more flattened and more of an 
 
14 VARIATIONS IN THE EARTH'S TEMPERATURE. 
 
 ellipse, because the larger a circle is the more nearly any part of its 
 circumference approaches to a straight line. This polar compression 
 or flattening makes the polar diameter of the earth almost 26^ miles 
 shorter than it would be if the earth were a sphere. According to 
 the most recent estimate, the measurement of the earth from pole to 
 pole is 7899.2 miles. But the equatorial circumference may also 
 be slightly elliptical, though the compression is estimated to amount 
 to less than 2 miles. It is well known that large masses of land rise 
 above the general level to a height of a mile or two, and it is quite 
 possible that the equatorial irregularity of outline, if it really exists, 
 is a consequence of the contractions of the earth's crust which have 
 changed the form of the globe by elevating the great continents. 
 
 The Earth's Orbit as Influencing the Temperature of its Sur- 
 face. The earth's orbit is an ellipse ; its distance from the sun varies 
 from 89,860,000 miles to 92,950,000 miles. The earth is nearest the 
 sun in our winter about the beginning of January, when it is in the 
 position called perihelion ; its distance then is about three millions 
 of miles less than in summer, when it is in the position called aphe- 
 lion. The amount of heat received from the sun varies inversely as 
 the square of the earth's distance from it, so that at first sight the 
 summer at our antipodes would seem to be warmer than our own 
 summer ; but when the earth is nearest to the sun its rate of motion 
 is most rapid, as may be seen from the circumstance that there are 
 fewer days in winter than in summer, and this rapidity of motion 
 compensates for the reduced distance, and the earth receives about 
 the same amount of heat in each of the seasons. A more remarkable 
 circumstance about the earth's orbit is the fact that the ellipse itself 
 rotates, going forward i in 308 years. And this throws the positions 
 of the equinoctial points backward, so that the seasons change their 
 times with regard to the earth's surface, and of course our summer 
 comes round eventually to happen when the earth is nearest to the 
 sun. It is also calculated that owing to the attractions of the planets 
 the eccentricity of the earth's orbit varies both by the ellipse becom- 
 ing longer and shorter ; this change would probably greatly influence 
 climate, by modifying the amount of heat that the earth would 
 receive from the sun ; because the more elongated the form of tho 
 orbit, the less is the distance of the earth from the sun in perihelion 
 and the greater its distance in aphelion ; though Professor Dove found 
 that the mean temperature of the whole of the earth's surface in June 
 is much higher than in December, owing to the land being chiefly in 
 the Northern Hemisphere. When the eccentricity of the earth's orbit 
 is greatest, this part of the world may come to be eight and a half 
 millions of miles farther from the sun than it is now in winter ; and 
 though at present winter is nearly eight days shorter than summer, 
 it then might be thirty-six days longer than it is now. It is certain 
 that from this cause important changes of climate must occur every 
 10,492 years. At the last of these periods, when the earth was 
 nearest the sun in summer and farthest off in winter, the difference 
 of temperature due to that position was calculated by Sir John Her- 
 
VIEWS OF THE EARTH'S ORIGIN. 15 
 
 sehel to probably amount to 23 F. A maximum eccentricity of the 
 earth's orbit is calculated to have been reached 210,000 years ago, 
 and a still greater eccentricity 850,000 years ago, when the winter 
 would have been forty-four days longer than now, and the meaii 
 temperature of the coldest month is stated at -7 and that of the 
 hottest month at 126. Mr. Croll has appealed to these principles as 
 evidence of a rotation of climates on the earth in past time, and 
 especially in explanation of the general phenomena of cold and glacia- 
 tion to which the northern regions of America and Europe were sub- 
 jected during the time which has been called the glacial period. The 
 hypothesis on astronomical grounds is speculative, but not impossible, 
 and deserves attention. 1 
 
 Modern Speculations concerning the Origin of the Earth. The 
 sun and many of the distant fixed stars are formed of mineral sub- 
 stances, similar to those which make up the earth's surface. This 
 conclusion has been arrived at by studies with the spectroscope. This 
 knowledge being the foundation for all sound ideas concerning the 
 internal state of the earth, it is necessary to explain the evidence on 
 which it rests. When a ray of light enters a dark room through 
 a small slit in the shutter, and falls upon a triangular prism of glass 
 held near the slit, the light which passes through the glass is bent 
 and divided into the colours red, orange, yellow, green, blue, indigo, 
 and violet ; and this band like a rainbow is named the solar spectrum. 
 These colours are divided up into narrow strips by an immense number 
 of parallel delicate dark lines. Thousands of these lines have been 
 accurately drawn, and they are found to be always in the same 
 positions, some in each of the colours. They were first studied by 
 the Bavarian philosopher Fraunhofer, and are hence called Fraun- 
 hofer's lines. These lines indicate the existence of many substances 
 which are burning in the sun's atmosphere. And since the light 
 which those substances give is less brilliant than that of the blazing 
 atmosphere in which they burn, it follows that the rays from them 
 come to the earth relatively dark, and appear as dark lines in the 
 spectrum. The true meaning of Fraunhofer's lines was discovered 
 by experiments with the spectroscope upon minerals and metals which 
 form the earth's crust. The spectroscope, invented by Professors 
 Kirclioff and Bunsen of Heidelberg, is merely a telescope with 
 a triangular prism of glass at the end to break up the light into 
 a spectrum. And when a substance is burned in a flame like that of 
 a Bunsen's burner, and the light from it falls on the prism, the spec- 
 trum becomes more or less altered, and instead of a perfect spectrum 
 certain bright lines are seen, which are different for every metal or sub- 
 stance examined. Thus if a piece of clean platinum wire, which will not 
 burn, be touched with the finger and held in the flame, all the colours 
 of the spectrum disappear except a brilliant yellow line in the part of 
 the spectrum where the yellow colour should have been. This yellow 
 streak is caused by burning sodium, which passed through the skin 
 in the form of sodium chloride or common salt. The bright line thus 
 1 Croll, " Climate and Time." 
 
1 6 DISCOVERIES WITH THE SPECTROSCOPE. 
 
 formed is in exactly the same place as the dark line in the yellow 
 
 Kirt of the solar spectrum, which Fraunhofer indicated by the letter 
 . Hence the conclusion is arrived at, that the light produced by 
 sodium burning in the sun is the cause of the appearance of this line 
 in the solar spectrum. Similarly, potassium gives a dull spectrum 
 with all the colours, but has a brilliant red line in the extreme end of 
 the red portion, which corresponds with the dark line in that position 
 called by Fraunhofer A ; and there is a second line for this substance 
 at the extreme end of the violet part of the spectrum. Iron when 
 burnt in the electric light yields about seventy bright lines. On 
 this kind of evidence about eighteen common metals, metalloids, 
 and gases have been identified as contributing by their incandescent 
 vapours to produce the sun's atmosphere. Among these substances are 
 sodium, calcium, magnesium, iron, nickel, copper, zinc, cobalt, man- 
 ganese, aluminium, barium, chromium, strontium, cadmium, titanium, 
 hydrogen, and oxygen. Hereafter many other elements may be found. 1 
 When a metal is burned on the earth, it enters into combinations 
 with gases and becomes an earthy or mineral substance. Hence, it is 
 probable that the combustion now going on in the sun, which illumi- 
 nates the whole solar system, will form in that star rock-substances 
 not unlike those which constitute the earth ; and we are led to infer 
 that at some far-distant time the earth itself may have been incan 
 descent at its surface, as the sun now is, and that the chemical com 
 binations of elements which form its rocks can thus be accounted for 
 as products of combustion. 
 
 It has also been shown that the vast outer enveloping portion of 
 the sun's atmosphere is formed of the gas hydrogen in a burning state. 
 There is every reason to believe that hydrogen burns there as on the 
 earth, by combining with oxygen gas. The product of this combustion 
 is vapour, which must ultimately condense into water when the heat is 
 less, and, dissolving soluble salts, accumulate eventually in depressions on 
 the sun's surface so as to form oceans and seas. It is at least probable 
 that the earth has passed through a phase of this kind, and that all the 
 water on its surface was produced just as water may now be produced 
 artificially, by the combustion of hydrogen in oxygen gas. This conclu- 
 sion is the more likely since the earth's atmosphere is merely a 
 mechanical mixture of oxygen and nitrogen, as though it were the in- 
 combustible residue of gases left after the combustible materials on its 
 surface had burnt themselves out. The other planets are apparently 
 more or less like the earth in possessing atmospheres and seas ; and 
 Mars so far resembles the earth as to display white snow-capped poles ; 
 and both that planet and Jupiter exhibit changing shapes of clouds, 
 which in Mars frequently assume similar outlines over large portions of 
 its surface, as though land and water were grouped into large masses. 
 If the moon has neither atmosphere nor oceans, it may well be that 
 the atmosphere of gases was small while it was undergoing combustion, 
 and that they were entirely combined with the burning elements. 
 The phenomena produced by combustion must, at the smallest 
 1 Roscoe, "Spectrum Analysis." 
 
MODERN VIEWS OF THE EARTH'S ORIGIN. 17 
 
 estimate, extend far deeper than man can ever penetrate. Some other 
 facts bear upon the origin of the earth itself. 
 
 Observations with the spectroscope prove that many of the fixed 
 stars are suns, more or less like the central luminary of our solar 
 system. Thus in the light from a star Aldebaran, Mr. Huggins found 
 the lines which indicate hydrogen, sodium, magnesium, calcium, 
 iron, bismuth, tellurium, antimony, and mercury. And many of the 
 fixed stars, like Sirius, give the light which is produced by incan- 
 descent hydrogen ; and there are very few of the fixed stars in which 
 hydrogen has not been detected. Thus the matter of the universe 
 appears to be as universal as the laws of force, by which its existence 
 is manifested and controlled. It is certain that each of these vast 
 burning worlds is slowly increasing in size by combining the gases of 
 its atmosphere with the superficial substances which burn ; and that 
 eventually they must all burn out when the atmosphere is exhausted, 
 or when the supply of combustible material comes to an end. And 
 this consideration leads up to what has been called the "nebular 
 hypothesis ; " because, if we suppose the sun, for instance, always to 
 have been about as large as it is now, it is difficult to conceive how 
 chemical combinations like those now occurring on its surface, could 
 have taken place throughout its entire mass from the centre outwards, 
 because the gases which support combustion are usually less dense than 
 the vapours which combustion would produce. And since these vapours 
 when condensed would so accumulate as to form a protecting envelope, 
 it has sometimes and naturally been supposed that the central mass of 
 the sun and of the planets may be less oxidised or earthy than the 
 part which is near the surface. The nebular hypothesis supposes that 
 before the stars existed, the materials of which they consist were diffused 
 in the heavens in a state of vapour. The nebulae in the firmament had 
 been observed to fill enormous areas in space, and to give a dull kind of 
 light which long ago suggested that they might be gaseous matter in 
 process of being condensed into worlds. And Sir William Herschel 
 speculated that if they condensed by their own gravity, they would 
 assume more or less spheroidal forms and be denser towards the centre. 
 Then assuming that local centres of condensation would come into 
 existence and gradually absorb the nebulous matter, the nebulae 
 would become resolved into clusters of stars. 1 There may be some 
 truth in this conception, though the nebulae are now known to be 
 dense masses of stars, and in no respect nebulous. And if the masses 
 of matter already condensed became drawn into contact with each 
 other by force of gravity, sufficient heat might be developed by the 
 concussion to melt or vaporise the whole mass. Such a heated mass 
 might then by rapid rotation throw off rings which would cool, break, 
 and condense into masses like planets, and if the condensation took 
 place with sufficient force, the planet might in its turn throw off a 
 ring which in due course would condense into a satellite. This is 
 
 filiation, it is true ; but it is speculation supported by a number of 
 Miomical facts ; and it is mentioned now because it helps us to 
 1 HerscheFs " Astronomy." 
 VOL. I. B 
 
18 METEORITES ON THE EARTH. 
 
 comprehend, though it may not altogether explain, some views con- 
 cerning the nature of the earth's internal heat, its form, and its 
 relations to the solar system. Even if the hypothesis is true, there 
 is great probability that the final condensation of each globe was 
 gradual ; and that after the main fiery mass of our planet was formed, 
 smaller cooled masses fell into it and bombarded the earth, and fur- 
 nished new fuel for combustion and added to the earth's size, just 
 as the falling-in of such masses appears to furnish the fuel which 
 keeps the sun burning. This probability is great because meteorites 
 from time to time still fall to the earth, and in past time may well 
 have fallen in far greater numbers. Sir William Thomson speaks of 
 Temple's Comet of 1 866 as consisting of minute planets, " of which 
 a few thousands or millions fall towards the earth annually about the 
 1 4th of November, when we cross their track." These minute planets 
 called meteorites vary in weight from a few ounces to a few tons. 
 They have been repeatedly seen to fall, and sometimes explode when 
 near the earth. They have been found on the surface or buried a 
 few feet in the earth, in all parts of the World from one pole to the 
 other. 
 
 Inferences from Meteorites. A large number of the specimens 
 in the British Museum are from America ; a few have been found 
 in our own islands, many in various parts of Europe, and no small 
 number in India. Some of the largest are from Australia and 
 Greenland. These masses are either crystalline compounds of 
 native iron with a moderate percentage of nickel, or else consist of 
 such minerals as form volcanic rocks, often with a little metal 
 scattered in them. A few meteorites contain bituminous sub- 
 stances such as upon the earth are only produced under the 
 influence of plant or animal life ; but it is impossible to say how 
 those chemical compounds originated in meteorites. In the Arctic 
 regions minute spherical particles of iron are sometimes brought down 
 from the air in snow, as though the earth occasionally entered clouds 
 of meteoric dust. A like cause must account for the red rain which 
 fell at Blank enburg in 1819, and owed its colour to cobalt chloride; 
 and in some soils, as at Lahisberg in Austria, nickel and cobalt occur 
 at the surface, though there are no neighbouring mineral veins or 
 rocks from which such elements could be derived. Among the 
 substances which compose meteorites are the minerals which are 
 named Labradorite, anorthite, orthoclase, augite, hypersthene, bronzite, 
 enstatite, olivine, Hauyne, graphite, chromite, magnetic pyrites, all of 
 which may be found in volcanic rocks of a more or less basaltic 
 character ; while in addition there are iron, nickel, cobalt, tin, 
 copper, lead, manganese, sulphur, phosphorus, chlorine, nitrogen, 
 oxygen, and hydrogen. 1 We know of no masses of metal on the 
 earth which correspond in composition or mode of occurrence with 
 these meteoric masses, unless some meteoric iron found in Green- 
 land Basalt is to be regarded as originally terrestrial in origin. The 
 meteorites, however, which now fall belong to an altogether different 
 1 Flight, History of Meteorites, "Geological Magazine," 1875. 
 
VIEWS OF THE EARTH'S ORIGIN. 19 
 
 orbit from the earth, immensely more elongated, and therefore cannot 
 be considered to have built up the mass of the earth; but they 
 demonstrate how such materials might, by the action of gravity, 
 become drawn together into a planet. And if the mineral materials 
 which have fallen to the earth are a fair sample of those which 
 move in the heavens, we obtain from them a useful idea as to 
 what the interior of a globe would be like, formed by such materials 
 being attracted towards each other. If the force which brought the 
 smaller masses together were sufficiently great to develop heat enough 
 to convert them into vapour or to melt them when their motion was 
 stopped, then no doubt the metals, being heavy, would find their way 
 towards the centre, and the rocky substances being lighter would 
 remain at the surface. By help of such considerations, we obtain 
 some clue to the origin of the earth, which, although vague, has an 
 inductive foundation. The meteorites, which Dr. Hahn, however, 
 believes to contain corals, sponges, encrinites, and other fossils, cannot 
 be accepted as evidence that life exists, or has existed, in the distant 
 heavenly bodies. 1 
 
 1 Die Meteorite (Choudrite) und ihre Organismen. Dr. Otto Hahn 
 Tubingen, 1880. 
 
 UNIVERSITY 
 
CHAPTER III. 
 
 THE CHIEF MINEEALS WHICH FORM THE EARTH. 
 
 IT has already been seen that minerals maybe now in process of formation 
 in the sun, and the fixed stars ; that they form meteorites, and compose 
 the earth's surface. It will be a sufficient definition of a mineral to 
 say that it is the natural condition in which the substances that form 
 rocks exist in' the earth. Chemists have classified the constituents of 
 the earth into elementary bodies which no analysis has yet been able 
 to further subdivide. Some of these elements, more or less pure, 
 occasionally constitute minerals, such as sulphur, carbon, copper, 
 silver, gold. But more frequently minerals consist of several ele- 
 mentary bodies chemically combined, and then each compound thus 
 made up, is met with usually in a series of geometrical crystalline 
 shapes, and each has a distinctive hardness, colour, mode of cleavage 
 or crystalline splitting, and other peculiarities, by which it may be 
 more or less easily distinguished from other minerals. Thus the 
 mineral galena, a well-known bluish-grey metallic-looking ore of lead, 
 consists of a chemical compound of lead and sulphur, which crystallises 
 in some modification of the cube or octahedron, and readily cleaves 
 parallel to the faces of the cube. 
 
 The abundant chemical elements in the earth are remarkably few, 
 and may be enumerated as oxygen, silicon, aluminium, calcium, sodium, 
 potassium, iron, manganese, magnesium, lithium, chromium, carbon, 
 barium, sulphur, chlorine, nitrogen, fluorine, and hydrogen, which is 
 usually present in combination with oxygen forming water. These 
 elements, variously combined with each other, constitute the minerals 
 which compose rocks, and though occasionally minute quantities of 
 other elements occur in rocks, yet not more than half of those named 
 will usually be found. The mineral substances constituting aqueous 
 or water-formed rocks, are massive and rarely crystallised, though 
 various crystallised minerals occur in them ; and as the minerals of 
 water-formed rocks are usually different in character and appearance 
 from the same substances when found in igneous rocks, it may bo 
 convenient to enumerate them separately in tabular form. They 
 include : 
 
MINERALS WHICH FORM AQUEOUS ROCKS. 
 Mineral Substances which Constitute the Aqueous Rocks. 
 
 21 
 
 NAME. 
 
 Quartz 
 
 Calcite 
 
 Clay 
 
 Gypsui 
 
 Dolomite 
 
 Uock Salt 
 
 Iron Pyrites 
 
 Phosphatite 
 
 Glauconite 
 
 COMPOSITION. 
 
 Oxide of silicon; when crystal 
 lised is in six-sided prisms 
 terminated by six-sided py 
 ramids. 
 
 Carbonate of lime, also 
 called calcium carbonate ; 
 when crystallised is in scale - 
 nohedrons or three-faced 
 pyramids, rhombohedrons 
 or some form in the Rhom- 
 bohedral system. 
 
 It is not, properly speaking, 
 a mineral, but a rock; when 
 pure it is identical in com- 
 position with decomposed 
 felspar, chiefly silicate of alu- 
 mina 
 
 Hydrated sulphate of lime 
 
 Carbonate of magnesia com- 
 bined with carbonate of 
 lime 
 
 Chloride of sodium 
 
 Sulphide of iron . 
 
 An amorphous variety of the 
 mineral apatite, which is 
 phosphate of lime, other- 
 wise called tribasic calcic 
 phosphate 
 
 In green grains, a hydrous 
 silicate of iron, alumina, 
 magnesia, soda, and potash 
 
 MODE OP OCCURRENCE. 
 
 In grains forming sand and sand- 
 stones, m concretions forming flint 
 and chert in limestone, more rarely 
 in crystals filling cavities in rocks 
 and concretions, and massive in 
 quartz veins and quartz rock. 
 
 Usually amorphous, forming lime- 
 stones, and occasionally crystalline 
 as in the skeletons of echino- 
 derms, especially encrinite lime- 
 stones of the Carboniferous age. 
 It also occurs combined with clay 
 in concretions called septaria. 
 
 In beds often made up of thin 
 layers. Slate is a hardened and 
 altered condition of clay. Shale 
 is clay hardened by pressure and 
 infiltration of mineral substances. 
 
 Chiefly in clays in masses called 
 alabaster, and in transparent crys- 
 tals called selenite. 
 
 Forms the magnesian limestone in 
 the Permian rocks as between 
 Nottingham and Sunderland, &c., 
 in this country ; and other rocks. 
 
 Occurs in beds chiefly in the upper 
 part of the Trias in this country, 
 as at Droitwich, Nantwich, Shir- 
 ley wich, and near Cavrickfergus 
 in Ireland. But isolated cubic 
 crystals are found in other forma- 
 tions like the Purbeck and coal. 
 
 Cubic crystals common in many 
 old slates. Iron pyrites or Mar- 
 casite in all clays and many lime- 
 stones. In the chalk these masses 
 are radiated and popularly called 
 thunderbolts. 
 
 Occurs in beds of irregular con- 
 cretionary nodules in the Red 
 Crag, Coralline Crag, Upper 
 Greensand, Gault, Upper Neoco- 
 mian, Rhsetic, and Bala beds. 
 
 Colours Bracklesham beds, Thanet 
 t*ands, Upper Greensand, Lower 
 Greensand, &c. 
 
22 MINERALS WHICH FORM IGNEOUS ROCKS. 
 
 Among the other minerals which form beds of rock are coal, the 
 various oxides of iron, and carbonate of iron ; which we shall describe 
 in another place. And among minerals found in the strata are 
 alum, barytes, &c. For technical descriptions of these species refer- 
 ence may be made to a treatise on mineralogy such as Dana's, or 
 Rutley's, or Phillips' Mineralogy by Brooke and Miller. And in any 
 case, the best knowledge of them will be obtained by examining the 
 specimens in some public collection, such as the British Museum or 
 Museum of Practical Geology, or the Museums of our Universities. 
 
 The abundant minerals which form igneous rocks are rather more 
 numerous, but comprise chiefly the felspars, augites, hornblendes, 
 micas, talcs, quartz, olivine, garnets, zeolites, and a few others. The 
 more interesting general facts relating to the species of these groups 
 which bear upon rock structure may be thrown into a tabular form 
 for easy reference. Of quartz nothing further need now be said. 
 
 THE MINERALS FORMING IGNEOUS ROCKS. 
 The Family of Felspars, 
 
 Felspars are the most abundant minerals in igneous rocks. They 
 can be just scratched with a knife, being softer than quartz, harder 
 than apatite, and much harder than carbonate of lime. The colour 
 is often milky- white, sometimes bright red owing to the presence of 
 oxide of iron, and occasionally grey or black, or even green. All 
 felspars consist chemically of silicates of alumina combined with 
 some other silicate, which is usually silicate of potash, or soda, or 
 lime, or some combination of lime and soda ; and according to 
 variations in chemical composition, the different varieties or species 
 of felspar are identified and named. With these chemical differences 
 are associated differences of crystalline form. When a typical 
 felspar contains potash, it crystallises in prisms in what is called 
 the oblique or monoclinic system, and is recognised by fracturing at 
 right angles to the side of the prism ; but when the crystal contains 
 soda or lime it crystallises in the doubly oblique or triclinic system, 
 and the cleavage is then at an oblique angle. For most purposes it is 
 sufficient to identify these two groups known as orthoclase and plagio- 
 clase, which can almost always be recognised even in microscopic 
 examples by the different ways in which they affect light, when 
 examined in thin slices, under the microscope, with the aid of the 
 polariscope. There are, however, at least six species of felspar pro- 
 perly so called, which are named orthoclase, oligoclase, albite, labra- 
 dorite, anorthite, and andesine ; the nearly allied minerals which may 
 replace the felspars in igneous rocks are nepheline, leucite, sodalite, 
 haiiyne, and noseau (Cotta's "Rocks Classified and Described;" 
 ZirkeFs "Lehrbuch der Petrographie.") 
 
THE FAMILY OF FELSPARS. 
 
 NAME. 
 
 Orthoclase 
 
 San i dine 
 Adularia 
 Albite 
 
 Oligoclase - 
 
 Labradorite 
 Anorthite 
 
 Andesine 
 Nepheline 
 
 Leucite 
 
 COMPOSITION. 
 
 Double silicate of alumina 
 and potash, with a little 
 soda 
 
 A grey and glassy variety 
 of orthoclase, usually with a 
 little lime and magnesia 
 
 A nearly transparent variety 
 of orthoclase with a little 
 lime 
 
 Double silicate of alumina 
 and soda. Usually asso- 
 ciated with orthoclase 
 
 Double silicate of alumina 
 and soda, in which the 
 soda is partly replaced by 
 lime and potash 
 
 Double silicate of alumina 
 and a compound of soda 
 and lime. Usually with a 
 little iron. Dissolves in hot 
 hydrochloric acid 
 
 Double silicate of alumina 
 and lime, in which the 
 lime is partly replaced by 
 small quantities of soda, 
 potash, or magnesia. Usu- 
 ally a little iron. Dissolves 
 in hydrochloric acid 
 
 A grey, green, or red double 
 silicate of alumina, and a 
 compound of soda and lime, 
 usually with some potash 
 and a little magnesia. 
 
 A white or coloured double 
 silicate of alumina, and a 
 compound of soda and 
 potash. Dissolves in hy- 
 drochloric acid 
 
 Ash-grey colour, in 24-faced 
 trapezohedrons, but belongs 
 to the tetragonal system. 
 Is a double silicate of alu- 
 mina and potash. Dissolves 
 in hydrochloric acid 
 
 ROCKS IN WHICH FOUND. 
 
 Granite, syenite, porphyry. Is 
 green from containing copper in 
 some of the rocks 'of South 
 America and Colorado. 
 
 Trachytes, phonolites, pitchstones, 
 obsidian. 
 
 Granite of St. Gothard. 
 
 Trachyte of Pantellaria, miascite 
 of Miask in Siberia, granite of 
 the Muorne Mountains, and in 
 various granites, greenstones, and 
 gneiss, though not as a chief 
 constituent 
 
 In granite of many places in 
 Sweden and Scotland, in gneiss 
 near Freiberg, in syenite, the 
 Verde - Antico porphyry of 
 the Morea, the trachytes of 
 Teneriffe, and in diabase and 
 diorite. 
 
 In the gabbro of Skye, in all 
 basalts, dolerites, hypersthenite, 
 older lavas of Etna, &c. 
 
 Old lavas of Monte Somma, 
 Thjorsa in Iceland, napoleonite 
 of Corsica, diorite of Harzburg, 
 syenite of Carlingford. 
 
 Occur in gneiss in Scotland, in the 
 Andes combined with hornblende 
 forming andesite, also in syenite 
 of the Vosges. It resembles 
 oligoclase. 
 
 In basalt and dolerite, in mias- 
 
 In basalt and dolerite, in mais- 
 cite and zirconsyenite ; in dole- 
 rite it occurs in short thick 
 hexagonal columns ; at Vesu- 
 vius and near Rome in lavas, in 
 phonolite as in the Wolf Rock 
 on the Cornish coast. 
 
 Unknown in the older rocks. 
 Monte Somma lavas of Vesu : 
 vius. trachyte between An- 
 dernach and Laach, dolerite 
 of the Kaiserstuhl in Baden. 
 At Bohmisch-Wiesenthal in the 
 Erzgebirge leucite crystals are 
 changed into orthoclase. 
 
AUGITIC AND HORNBLENDIC MINERALS. 
 
 The Family of Augites and Hornblendes. 
 
 Augite and hornblende are usually dark-green or black minerals 
 which belong to the monoclinic crystalline system, and are commonly 
 a little more easily scratched than the felspars with which they always 
 occur. There are distinct crystalline forms and cleavages for horn- 
 blende and augite ; but when some varieties of hornblende are 
 melted, they assume, on cooling, the crystalline form of augite. Hence 
 the conclusion has not unnaturally been drawn that they are essentially 
 one mineral which assumes the form of hornblende on cooling slowly 
 under great pressure, and that of augite when cooling in lava streams ; 
 and this view is supported by the fact that the hornblende is chiefly 
 found in syenite and completely crystalline rocks, while augite abounds 
 in basalts and those rocks which have cooled at the surface. But 
 though conditions of cooling may have some influence on the develop- 
 ment of these minerals, their formation is probably more dependent 
 upon the chemical composition of the rock matter in which they occur, 
 since hornblende is associated with the felspars which are rich in 
 silica, and rocks which contain quartz, while augite is met with in 
 association with felspars which contain less silica, and rocks from 
 which quartz is absent. Occasionally they occur together as in 
 trachytes of Etna, and they have been found artificially formed in 
 slags. Chemically there is no great difference between these minerals 
 beyond the fact that hornblende contains more alumina and magnesia, 
 and more iron than augite, while augite contains more lime ; in the 
 pale-coloured varieties of both minerals alumina is almost absent, and 
 the quantity of iron is greatly reduced. The chief varieties of the 
 augite or pyroxene group are augite, hypersthene, diallage, bronzite; 
 the pale-coloured varieties are named diopside, sahlite, and mala- 
 colite ; the chief varieties of amphibole or hornblende are tremolite, 
 actinolite, asbestus, and hornblende. 
 
 NAME. 
 
 COMPOSITION. 
 
 ROCKS IN WHICH FOUND. 
 
 Augite 
 
 Dark green or black variety. 
 
 Basalt and dolerite, diabase and 
 
 
 Silica, lime, magnesia, prot- 
 
 modern lavas. The pale varieties 
 
 
 oxide of iron and alumina. 
 
 are chiefly found in altered lime- 
 
 
 Angle of cleavage planes, 
 
 stones. 
 
 
 87 5' 
 
 
 Hypersthene 
 
 Greenish black, distinguished 
 
 In the gabbro of Skye, Penig in 
 
 
 by cleavage, very tough. 
 
 Saxony, and in the island of St. 
 
 
 Differs from augite in con- 
 
 Paul on the Labrador coast. 
 
 
 taining much less lime, 
 
 Crystals in rhombic system. 
 
 
 hardly any alumina, and 
 
 
 
 much more iron ; as hard 
 
 
 
 as felspar 
 
 
MINERALS WHICH FORM IGNEOUS ROCKS. 
 
 COMPOSITION. 
 
 EOCKS IN WHICH FOUND. 
 
 Diallage 
 
 Bronzite 
 
 Hornblende 
 
 Brownish green or brassy 
 brown ; as hard as fluor 
 spar, being much softer 
 than augite. Contains little 
 alumina, a little water, but 
 otherwise is like augite, has 
 a peculiar cleavage 
 
 Reddish -brown or bronzy ; 
 harder than diallage. Con- 
 tains little or no alumina 
 and lime, but large quan- 
 tities of silioa and magnesia. 
 Crystals in rhombic iorms 
 
 Greenish black. The alu- 
 mina, magnesia, lime, and 
 protoxide of iron are more 
 nearly equal than in augite, 
 and average 14 or 15 per 
 cent. each. Crystals col- 
 umnar, angle of cleavage 
 planes at intersection, 124 
 
 Occurs in gabbros of Cornwall, is 
 associated with hornblende in 
 the euphotide of Harzburg* Fo- 
 rest in the Harz, and occurs in 
 gneiss in the Guadarrama Moun- 
 tains in Spain. 
 
 Found in serpentine at the Lizard, 
 the Bacher Mountain in Lower 
 Styria. Enstatite is a similar 
 but paler mineral found in Iher- 
 zolite near Lake Lherz in the 
 Pyrenees, and in some gabbros. 
 
 The pale-green varieties, tremolite 
 and actinolite, are found in dolo- 
 mite and altered limestone; com- 
 mon hornblende in diorite, syenite, 
 hornblende gneiss, hornblende 
 slate, phonolite, trachyte. Ura- 
 lite appears to be augite partly 
 transmuted into hornblende. 
 
 The Family of Micas and Talcs. 
 
 The talcs and micas include many species which usually agree in 
 dividing into thin laminsB which are sometimes more or less trans- 
 parent. The talcs are softer than the micas, may be bent, but will 
 not spontaneously bend back again, give a more or less greasy sensa- 
 tion when touched, and are hydrous silicates of magnesia where part of 
 the magnesia may be replaced with iron, and are not acted on by acids. 
 The micas are usually in rhombic or hexagonal plates, are both flexible 
 and elastic, give a clean sensation when touched, are double silicates, 
 usually of alumina, magnesia, potash, and iron, and some species are 
 soluble in sulphuric acid. Talcs are often deposited from water as 
 pseudomorphs, in place of other magnesian minerals which originally 
 formed part of the rock ; but they cannot be correctly described as 
 hydrated micas because micas contain alumina, but may be formed 
 in rocks which were previously infiltrated with magnesian silicates 
 derived from decomposed mica, hornblende, augite, and olivine. 
 
 NAME. 
 
 COMPOSITION. 
 
 ROCKS IN WHICH FOUND. 
 
 Talc 
 
 White, or greenish six-sided 
 plates. Hydrous silicate of 
 magnesia, with some prot- 
 oxide of iron, and occa- 
 sionally a little alumina. 
 Insoluble in acids 
 
 In talc slate, talc-schist, in the pro- 
 togine of the Alps, in protogine 
 gneiss ; when compact and amor- 
 phous is called steatite or pot- 
 stone ; crystals in the Zillerthal of 
 the Tyrol, at St. Gothard, and 
 about Sal/burg. 
 
26 
 
 MICACEOUS MINERALS. 
 
 NAME. 
 
 COMPOSITION. 
 
 HOCKS IN WHICH FOUND. 
 
 Chlorite 
 
 Muscovite or 
 Potash-mica 
 
 Lepidolite or 
 Lithia-iriica 
 
 Phlogopite 
 
 Biotite or 
 Magnesian- 
 
 Dark olive green. Hydrous 
 silicate of magnesia and 
 alumina, with 12 per cent, 
 of water. Dissolves in hot 
 sulphuric acid 
 
 White or brown or black. 
 Silicate of alumina with 10 
 per cent, of potash, and 
 rarely more than 5 per 
 cent, of iron oxides, with a 
 little water 
 
 Red or violet. Composition 
 similar to muscovite, but 
 with lithia, hydrofluoric 
 acid, and protoxide of man- 
 
 Brown, or reddish brown. 
 Silicate of alumina, mag- 
 nesia, potash, sometimes 
 with fluorine and a little 
 
 Usually dark green, brown, 
 or black. Silicate of mag- 
 nesia, alumina, potash, and 
 oxide of iron, with some 
 water, dissolves in hot sul- 
 phuric acid 
 
 In chlorite slate, protogine, proto- 
 gine gneiss, diabase, correspond- 
 ing to mica as a rock constituent, 
 closely related to soapstone, found 
 in serpentine at the Lizard. 
 
 Granite, gneiss, mica-schist, some 
 lavas of Vesuvius ; has been found 
 in slags, and has been formed in 
 clayey sandstone walls of iron 
 furnaces. 
 
 In granite in Cornwall, in gneiss, 
 and greisen. 
 
 Chiefly in metamorphic limestone, 
 as in the Vosges j also in ser 
 pentine. 
 
 In granite gneiss, trachyte, basalt, 
 miascite, in lavas of Vesuvius. 
 
 Many other micas are met with. 
 
 The Family of Garnets. 
 
 The garnets are a group of silicates of variable composition. The 
 typical garnets have on this account been grouped under six varieties, 
 as lime garnet, which is a double silicate of lime and alumina ; mag- 
 nesia garnet, which is a silicate of magnesia, iron, and alumina ; iron 
 garnet (or common garnet), which is a silicate of protoxide of iron and 
 alumina ; manganese garnet, which is a silicate of manganese, alumina, 
 and iron ; iron-lime garnet, which is a silicate of iron and lime with a 
 little alumina and manganese ; and lastly lime-chrome garnet, which is 
 emerald green, and is a silicate of lime and chrome with a little alumina 
 and iron. These, however, are mostly of rare occurrence in igneous 
 rocks, and it will be convenient to assume that the common almandine 
 garnet is the kind usually met with. The Vesuvian mineral idocrase 
 is closely allied to the garnets, in being a silicate of lime, alumina, iron, 
 usually with a little magnesia. Here also may be placed, though ii 
 no near association, tourmaline, sphene, and zircon. Garnets oftt 
 occur embedded in talc and mica, and they may be regarded as m< 
 closely allied to that group of minerals. 
 
MINERALS WHICH FORM IGNEOUS ROCKS. 
 
 27 
 
 NAME. 
 
 Garnet 
 
 Idocrase 
 
 Olivine 
 
 Tourmaline 
 
 Sphene 
 
 Zircon 
 
 COMPOSITION. 
 
 Usually in rhombic dodeca- 
 hedrons, red or brownish. 
 A silicate of iron and alu- 
 mina, but in some rocks 
 containing magnesia 
 
 Brown or green. Silicate of 
 lime and alumina, with a 
 little iron, manganese, and 
 magnesia 
 
 Green or brown ; crystals in 
 prismatic system ; harder 
 than felspar, equals quartz ; 
 dissolves in sulphuric acid. 
 Consists of silica, magnesia, 
 and protoxide of iron. The 
 silica always less than the 
 magnesia. Only a trace of 
 alumina ; no lime. When 
 transparent called chryso- 
 lite. 
 
 Usually black. A silicate of 
 alumina and magnesia, with 
 much boracic acid and oxide 
 of iron, and a little soda, 
 lime, and fluorine. This 
 mineral is also called schorl 
 
 Green, brown, or black. Sili- 
 cate of lime and titanium 
 
 Red or brownish. Silicate of 
 zirconia, with usually a 
 little iron, and occasionally 
 a little lime 
 
 EOCKS IN WHICH FOUND. 
 
 In eklogite, which is a compound 
 of green diallage and garnet ; in 
 garnet rock, a compound of gar- 
 net and hornblende ; occurs in 
 mica-schist ; also in granite, gra- 
 nulite, trachyte, perlite, and 
 chlorite-schist. 
 
 In old lavas of Vesuvius ; serpen- 
 tine in Piedmont ; has been found 
 in slags of furnaces. Is not an 
 important rock constituent. 
 
 In many basalts, as in the Eifel, 
 in lavas of Monte Somma, in 
 hypersthenite at Elfdalen in 
 Sweden, talc-schist at Katheri- 
 nenburg, in Lherzolite. Found 
 in slags of iron furnaces. The 
 rock in New Zealand called 
 dunite consists of olivine. 
 
 In luxullianite, granite, mica- 
 schist ; not found in volcanic 
 rocks. 
 
 In granite, syenite, zirconsyenite, 
 phonolite, trachytic rocks of 
 Laach, and in mica-schist. 
 
 In several Scotch granites, in zir- 
 consyenite, in basaltic lavas at 
 Unkel on the Rhine. 
 
 TJie Family of Zeolites. 
 
 The zeolites are essentially felspars, which have been dissolved by 
 water slowly percolating through rocks in which those minerals occur, 
 and then have been redeposited in chemical combination with water, 
 in cavities, in volcanic and crystalline rocks. They are all silicates of 
 alumina, and several zeolites in addition contain lime, often with a 
 little soda or potash ; two others contain soda only, and one has a 
 large percentage of sulphate of baryta. These minerals are most abun- 
 dant in the vesicular basaltic lavas, but are also found in gneiss, syenite, 
 and granite, phonolite, and lavas of Vesuvius, &c. Occasionally they 
 form a large percentage of the rock, and furnish an instructive illus- 
 
28 
 
 ZEOLITES. 
 
 tration of the extent to which, a rock may become altered by infiltra- 
 tion. It is zeolitic substances dissolved out of the basalt and re- 
 deposited in the chalk of Antrim and Argyleshire which have imparted 
 a flinty hardness to that rock in those localities. The following table 
 includes some of the more interesting and important zeolites. 
 
 COMPOSITION. 
 
 LOCALITIES WHEKE FOUND. 
 
 Apophylite 
 
 Prehnite 
 
 Thoinsonite 
 
 Chabasite 
 
 Stilbite 
 
 Laumonite 
 
 Heulandite 
 
 Natrolite 
 
 A red, yellow, or whitish 
 silicate of lime and potash, 
 with 1 6 per cent, of water 
 
 Usually green. Silicate of 
 lime and alumina, with a 
 little iron and water 
 
 White. Silicate of alumina 
 and lime, with a little soda 
 and 13 per cent, of water 
 
 White or reddish. Silicate 
 of alumina and lime, with 
 a little potash and 20 per 
 cent, of water 
 
 White or yellowish. Silicate 
 of alumina and lime, with 
 I per cent, of soda and 17 
 per cent, of water 
 
 White. Silicate of alumina 
 and lime, with 15 per cent, 
 of water. Similar in com- 
 position to stilbite, but 
 while that mineral crys- 
 tallises in the prismatic 
 system this occurs in the 
 oblique system 
 
 White or reddish. Silicate 
 of alumina and lime, with 
 1 5 per cent, of water. Ob- 
 lique system, but form of 
 crystals different to lau- 
 monite, and unlike that 
 species does not fall to 
 powder on exposure to air 
 
 White or reddish. Silicate 
 of alumina and soda, with 
 10 per cent, water. Pris- 
 matic system 
 
 Generally in cavities in amygdaloi- 
 dal rocks, chiefly in Iceland and 
 the Faroe Islands. In Tertiary 
 limestone at Puy de la Piquette 
 in Auvergne, near basalt. 
 
 In cavities in basalt in Mull, Skye, 
 Salisbury Crags, Dumbarton ; 
 and in granite at Botallack, near 
 Land's End. 
 
 In cavities of basalt at Kilpatrick, 
 in Scotland ; in lavas of Vesu- 
 vius ; and in phonolite at Dau- 
 bitz, in Bohemia. 
 
 In cavities in basalt at the Giant's 
 Causeway, in Skye and Mull ; 
 and in phonolite. 
 
 In cavities in basalt in Skye and 
 Arran ; and in granite, gneiss, 
 and schistose rocks in fissures. 
 
 In cavities of basalt in Dumbar- 
 tonshire, and fissures in syenite 
 at Dresden. 
 
 In basalt in Skye, at Campsie in 
 Dumbarton ; also in fissures in 
 gneiss and slates. 
 
 In basalt of Giant's Causeway, 
 Hebrides, Rhine, &,c. 
 
THE CHIEF ZEOLITES. 
 
 29 
 
 NAME. 
 
 COMPOSITION. 
 
 LOCALITIES WHERE FOUND. 
 
 Analcime 
 
 Transparent or white. In 
 
 In cavities of basalt and other 
 
 
 24-faced trapezohedrons. 
 
 amygdaloidal rocks at the 
 
 
 Silicate of alumina and 
 
 Giant's Causeway ; in the inner 
 
 
 soda, with 8 per cent, of 
 
 Hebrides; at Dumbarton, &c. In 
 
 
 water 
 
 the Cyclades two-thirds of a dole- 
 
 
 
 rite has been changed into anal- 
 
 
 
 chne. 
 
 Harmatome 
 
 White. Silicate of alumina 
 
 In amygdaloids at Oberstein, and 
 
 
 and baryta, with 15 per 
 
 in veins at Strontian in Argyle- 
 
 
 cent, of water. Prismatic 
 
 shire. 
 
 
 system. Usually in twin 
 
 
 
 
 crystals, having a section 
 
 
 
 like a cross 
 
 
 Professor Daubree notices tliat not only do many of these zeolites 
 occur in the same mass of rock, but that several occur together in the 
 same cavity. There is every reason to believe that they owe their 
 existence to the infiltration of ordinary water, because they have been 
 found formed in the brickwork of several old Roman baths, as at 
 Plombieres in the Vosges, where the warm water had contained 
 alkaline substances. The nature of the rock in which it is deposited 
 appears to exercise some influence on the kind of zeolite formed, 
 because the species in the mortar are different to those which occur 
 in the bricks. 
 
 It may, perhaps, be imagined that innumerable mineral combina- 
 tions are derived from the sixty-six primary chemical elements. But, 
 as many of them are excessively rare, as the remainder combine only 
 upon certain principles, the number of mineral species really deter- 
 mined is, in fact, not large, perhaps hardly exceeding five hundred. 
 Nor is the geologist often called upon to make himself acquainted 
 with all even of this moderate number. Unless his labours are 
 devoted to the detailed phenomena of volcanic productions or of 
 mineral veins, he will seldom have occasion to observe more than ond- 
 tenth of the number. The reason of this is that a large portion con- 
 sists of rare and local species ; and that, in combining to form rocks, 
 the others are associated in families, and united into specific com- 
 pounds without much permutation. In consequence, there is really 
 less difficulty than might be expected in recognising and discrimi- 
 nating the rocks. To class and to describe them in a true natural 
 order is difficult, to compare and to know them according to their 
 mode of occurrence is easy, and should form part of the practical 
 education of every geologist. The student should lose no time in 
 obtaining slices of the chief rock-forming minerals, and must study 
 'iem under the microscope with and without polarised light, if he 
 mid learn to discriminate the varieties of igneous rocks which they 
 
 ibine to form. 
 
CHAPTEE IY. 
 
 THE NATURE AND ORIGIN OF CRYSTALLINE AND IGNEOUS ROCKS. 
 
 THE newest water-formed rocks are similar in appearance to deposits 
 which, are now being deposited ; but the older strata have often under- 
 gone changes which have obliterated some of their original features 
 which were due to deposition, and have imparted characters which 
 sometimes make it difficult or impossible to discover from observation 
 that they were ever deposited in water at all. These changes aro 
 partly the consequence of the slow infiltration of water, which dis- 
 solves certain mineral constituents from one place or one rock, and 
 deposits them again elsewhere, sometimes as crystalline minerals, but 
 almost always in different mineral combinations ; and when a rock is 
 thus altered by the action of water, it may be said to be transformed. 
 Other changes of a more varied and important character result from 
 the action of pressure, when rocks are forced by folding to occupy less 
 space. And when from this cause the original distinction between 
 minor layers of rock disappears, and is replaced by new planes of divi- 
 sion, and when the original mineral character of the rock disappears 
 to give rise to a crystalline texture, and to minerals which are never 
 found in the strata, the rocks are said to be metamorphosed. After- 
 wards it may be seen that these changes go so far, that lavas and 
 granites appear to be formed out of sands and mud by the action of 
 the heat to which pressure gives rise. 
 
 All the older Primary rocks of this country, and in other countries 
 the neAver rocks of all f geological ages, have been more or less meta- 
 morphosed ; clays are' thus changed into slates, sandy clays into 
 schists, certain sandstones into quartzites, and ordinary limestones 
 into crystalline or statuary marble. When limestone is thus altered, 
 its texture becomes amorphous or granular from the small size of the 
 calcite crystals of which it consists ; all traces of fossils disappear, fre- 
 quently crystals of garnets, augite, or other minerals form ; and there 
 may be irregular films of colour, or a greyish tinge, in place of the 
 varied or diffused colour which the rock originally possessed. 
 
 Carbonate of lime being easily soluble in heated water, a compara- 
 tively low temperature may have been sufficient to bring about these 
 changes. An example of such a limestone in this country is seen in 
 
 
SCHISTS AND FOLIATED ROCKS. 
 
 the Laurentian gneiss rocks of Loch Maree in Ross, and the struc- 
 ture also appears wherever intrusive rocks come in contact with lime- 
 stones ; but the most familiar example is furnished by the Carara marble 
 of Italy, which is probably of Carboniferous age. When sand becomes 
 converted into quartzite, the grains of sand are more or less obliterated 
 and blended together, as grains of sago might blend when cooked. 
 The fracture of the rock, when the sand was fine, often resembles that 
 of horn, hence these altered rocks are often called hornstones, but 
 exposure to the weather usually develops on the surface a laminated 
 structure. Here the temperature may not have been very high, since 
 heated water has considerable power of dissolving silica, and all rocks 
 contain more or less water, the temperature of which may be raised 
 by pressure. Schists are rocks 
 which consist chiefly of fine 
 layers of crystalline quartz, ar- 
 ranged in short irregular parallel 
 films, which are separated by 
 films of some other mineral. Thus 
 mica schist consists of quartz and 
 mica; talc schist is quartz and 
 talc ; chlorite schist is quartz and 
 chlorite ; hornblende schist is 
 quartz and hornblende. There are 
 a few other schistose rocks, the 
 most important being gneiss, 
 which is made up of the same 
 minerals as granite, quartz, felspar, 
 and mica, only the minerals are arranged in parallel layers. All these 
 rocks contain various other minerals in small quantities, but schists 
 especially abound in garnets. The arrangement of the minerals in 
 parallel layers was 
 named by Mr. Dar- 
 win foliation. These 
 rocks were originally ^sT~-< 
 all fine sandstones ^c 
 
 C^WTiC 
 
 ^ ^t\03Sfei^,(Rj-J- 
 
 ^MKJX<n\\ 
 
 Fig. 4. Mode of Occurrence of Schists in 
 Scotland. 
 
 and sandy clays ; and 
 under the influence 
 and action of heat, 
 
 Fig. 5. Contorted Schist. 
 
 pressure, and the 
 water contained in 
 the rock,the chemical 
 substances have re- 
 combined into the minerals already mentioned, being arranged in 
 layers, which do not usually correspond with the layers in which the 
 rock was deposited. The schistose rocks are always crumpled and 
 contorted, and commonly occur on the flanks of the older mountain 
 
 **J 9 
 
 Slates were clays, often more or less sandy and micaceous, which 
 
NATURE OF CLEAVAGE. 
 
 have undergone enormous compression. Often they occupy only a 
 
 third or even a tenth of their 
 original horizontal space, as is shown 
 by the distortion of the fossils they 
 \ contain, and sometimes by the 
 w \ folding of hard sandstone layers 
 which could not yield in the same 
 way to the pressure. This com- 
 pression alters the rock so that 
 it no longer absorbs water freely or 
 decomposes into mud. And while 
 
 Fig. 6. -Verticil Cleavage in Folded Strata. ^ Q or igi na l planes of deposition 
 
 are still well marked by changes in the colour of the layers, the 
 rock no longer splits into the strata or laminae of which it was 
 composed, but has acquired the property of dividing into much 
 more regular and sometimes exceedingly thin films by division 
 planes which cross the rocks in different quarries at every possible 
 angle to the original stratification. This property, to which we 
 owe the slates of commerce, was named by Professor Sedgwick 
 cleavage. This rock cleavage is quite distinct from the cleavage 
 of minerals, for that always depends upon the crystalline form 
 of the mineral, and is parallel to one or more of the faces of 
 the particular crystal system to which the mineral belongs. Thus, 
 what is named the cubic system includes besides the cube many 
 other figures, such as an eight-sided double pyramid called an 
 octahedron, a twelve-sided figure named a dodecahedron, &c. ; and 
 though the mineral called fluor-spar usually crystallises in cubes, its 
 eight corners cleave off and convert the cube into the octahedron. But 
 rock cleavage is a mechanical property which is due to rearrangement 
 of the rock particles and crystals by pressure. This has been proved 
 experimentally by Mr. Sorby and Professor Tyndall, who have pro- 
 duced cleavage experimentally by pressure in clay, wax, and other 
 substances. Professor Ramsay has observed that cleavage is only 
 produced in contorted rocks when they are subject to some weight 
 of rock from above as well as lateral pressure ; since when near the 
 surface the rocks yield and fracture, and the particles are not forced 
 to rearrange themselves at right angles to the direction of the com- 
 pressing force. 
 
 Fig. 7. Clay Splitting Obliquely. 
 
 Fig. 8. Same Rock converted into Slate 
 Splitting Vertically. 
 
 The production of cleavage may perhaps be better understood 
 from this diagram. First, we suppose the clay to be unaltered, 
 
STRUCTURE OF GRANITE. 33 
 
 and to show the actual particles of which it consists here drawn 
 greatly magnified and apart from each other ; and secondly (tig. 8), 
 we suppose the same mass of clay to have been compressed from one 
 end, and to have yielded somewhat above. Here all the particles are 
 seen to have shifted their positions, and to be extended at right angles 
 to the pressure. Formerly the rock would split in the direction in 
 which the particles were deposited, now it splits in the new direction 
 into which the particles have crystallised. In this country slates 
 occur throughout the Cambrian, in the Silurian, and in the Devonian 
 rocks ; and are found in Cornwall, Devon, and West Somerset, in 
 Wales, the Lake Country, Scotland, and Ireland ; but the best known 
 slates are from the Lower Cambrian, at Penrhyn, Llanberis, and 
 Ffestiniog. In other countries slates occur, of all geological ages. 
 They almost always form mountain-masses with well-defined and 
 peculiar contours. 
 
 Mr. Darwin first drew attention to the way in which the rocks 
 showing cleavage lie parallel to those showing foliation, and drew 
 the conclusion that foliation is only an intense form of cleavage ; 
 or is due to the same cause when the forces producing it are more 
 powerful. There are some intermediate kinds of rocks ; and Pro- 
 fessor Sedgwick found slates 1 with the cleavage planes sometimes 
 coated over w r ith chlorite and semi-crystalline matter, which not 
 only defines these planes, but extends in parallel flakes through the 
 whole mass of the rock. And when it was found that the granite 
 masses which often form the central bosses or axes, as they are 
 called, of mountain chains, are parallel to the overlying metamorphic 
 rock, it began to be suspected that granite also might be a product 
 of metamorphism ; especially as it consists of the same minerals as 
 gneiss, and only differs in having them mixed confusedly together, 
 instead of being arranged in parallel layers. Moreover, there are 
 many intermediate varie- 
 ties of rock ; and granite 
 in large masses often 
 shows an approach to a 
 banded structure, owing 
 to the mode in which 
 its minerals are arranged. 
 It will readily be under- 
 stood that if the consti- 
 tuents of gneiss have 
 arranged themselves in Fi - 9- 
 
 crystalline layers at right angles to a single plane of pressure, it will 
 follow that if the different crystals arrange themselves simultaneously 
 at right angles to two or three planes of pressure, their disposition in 
 parallel layers will be interfered with, and the materials must become 
 mixed more or less confusedly together. And this is exactly what 
 happens in the central axis of a mountain, where the pressure is more 
 
 K tense than on the nanks, and comes from both sides of the fold. 
 Sedgwick, " Structure of Mineral Masses," Trans. GeoL Soc., 2d series, vol. 3. 
 VOL. I. C 
 
34 
 
 METAMORPHIC ROCKS. 
 
 Thus there are here (fig. 9) three planes of resistance, A, B, c, marked 
 by layers of crystals parallel to each of them, and the result is a 
 confused appearance, in which no single layer can be detected pre- 
 cisely like the aspect of granite. And although owing to the action 
 of resultant forces the particles might not be arranged exactly in the 
 directions here shown, the result would be the same in producing a 
 confused arrangement. Metamorphic granites, according to Professor 
 Haughton, may be seen in Donegal, some parts of Brittany, the Pyre- 
 nees, and many other localities. 
 
 If now we suppose the rocks forming one of the sides A, B, c, to 
 have fractured, and to have opened in a fissure before the granite was 
 consolidated and cooled, then the pasty granitic rock would penetrate 
 
 into the fissure and form an intru- 
 sive granite vein. Such veins may 
 be seen at Cape Wrath, and in 
 many parts of Scotland, in Corn- 
 wall, and wherever granite occurs. 
 Granite usually presents, where ex- 
 posed, more or less rounded out- 
 lines in the scenery. 
 
 We are now able to advance 
 one step farther and appreciate the 
 close relation between granites and 
 lavas. The central cores of many 
 old volcanoes of the Auvergne in 
 France, and of the Hebrides, are 
 found to be granite ; and when this 
 rock cools more rapidly, as at the 
 earth's surface under the pressure of the atmosphere, the minerals no 
 longer form separately, but constitute rock, consisting more or less 
 obviously of a felspathic matrix in which crystals may occur. W r hen 
 poured out in a lava stream these rocks are called felstones, and when 
 they assume a looser texture become scorise or ashes. If now we 
 suppose the rocks over a central granite mass to become fractured 
 through their thickness so as to allow water to penetrate down to the 
 heated mass, and to form a funnel or vent out of which the heated 
 materials may escape, it is obvious that the central crystalline rocks 
 will throw out lavas and ashes which may build up a volcano. Thus 
 it follows that clay, slate, gneiss, granite, felstone, rhyolite, may all 
 exist simultaneously as different conditions of the same rock, which 
 have been produced in sequence to each other by the pressure which 
 also brings mountains into existence, and changes the outlines of 
 land and water. This ideal section (fig. n) will illustrate the rela- 
 tions of the several kinds of rocks to each other, and show the order 
 which the several classes of rocks may succeed each other on 
 
 Fig. 10. 
 
 in 
 
 the flanks of a mountain range. Formerly all the igneous rocks 
 were classed into two groups first, volcanic, or eruptive, which had 
 burst out from beneath the earth's surface ; and secondly, plutonic 
 or crystalline rocks which cooled at great depths, and under great 
 
 
GROUPS OF VOLCANIC ROCKS. 35 
 
 pressure in the earth ; but no clear line of demarcation can bf 
 drawn between these groups, because the conditions of eruption and 
 consolidation are such that every possible gradation of pressure and 
 slowness of cooling must be presented by varying circumstances. 
 
 It is probable that every kind of plutonic rock has its volcanic 
 
 ^^^ VOLCANO 
 CLEAVAGE FOLIATION 
 
 BEDDING 
 
 CLAY SLATE GNEISS GRANITE 
 
 Fig. ix. 
 
 representative; but the volcanic form which the plutonic rock assumes 
 has not always been identified. Long since it was observed through- 
 out Europe that the older Tertiary volcanic outbursts were of the 
 kind of volcanic rock which is called trachyte, while the later out- 
 bursts are basalts. And this observation led to an attempt to classify 
 the igneous rocks into two classes, of which these rocks are types, 
 according to certain differences in chemical composition which governed 
 the appearance in them or absence of certain materials. The im- 
 portant variable constituent was found to be silica. In the basalt family 
 this substance usually forms 45 to 55 per cent, of the rock ; while 
 in the trachyte family the silica may be 60 to 80 per cent., or more. 
 This is an artificial classification, but is convenient in obtaining a 
 general idea of the relations of the rocks to each other ; because wher- 
 the quantity of silica is small, quartz does not form in separate crystals, 
 and only those varieties of felspar are produced which contain little 
 silica; and augite or hornblende is more frequently formed than 
 mica. This division was made by Bunsen, who named the group 
 which contains but little silica, Basic, and that which contains much 
 silica, Acidic. The association of minerals together is almost always a 
 consequence of the original chemical composition of the mass out of 
 which they were formed ; thus, Cotta remarks, quartz and mica occur 
 together ; orthoclase quartz and mica ; orthoclase or oligoclase and 
 hornblende ; labradorite and augite ; but quartz and augite rarely, if 
 ever, are associated. Many of the volcanic rocks appear to owe their 
 varieties to the influence of heated water penetrating to great depths, 
 which has dissolved the silica and certain other minerals met with in 
 its course, and added these to the heated mass, which was afterwards 
 poured out at the surface. And since such large quantities of salts 
 ire dissolved in the waters of mineral springs, it is impossible to 
 >ver-estimate the changes which may be produced by this means in 
 leep-seated heated rocks, and it may sometimes be the true explanation 
 )f the circumstance that when the two classes of lavas occur together, 
 older series is often the richer in silica. We now propose to give 
 short account of some of the chief varieties of the principal groups 
 )f igneous rocks, enumerating a few localities in which they occur 
 ind may be studied. All igneous rocks are divided first into those 
 rhich con tain orthoclase, and secondly, those formed by plagioclase ; each 
 )f these groups includes two families, quartz-bearing and quartz-free. 
 
GRANITIC ROCKS. 
 
 The Family of Quartz-orthoclase Rod's. 
 
 Typical granitic rocks are all perfectly crystalline. The felspar 
 crystals all touch each other without any intervening nncrystalline 
 material ; they may be minute when cooled rapidly, or some inches in 
 length if cooled slowly, when the rock is said to be porphyritic. As a 
 rule felspar is the only mineral in which the crystals are large. Many 
 of the quartz crystals, as was first discovered by Mr. Sorby, contain 
 cavities with a globule of liquid and a bubble of air ; 1 and on their 
 characters an estimate has been made of the pressure under which 
 granite consolidated, as represented in thousands of feet of overlying 
 rocks which have since been removed. In Cornwall this pressure has 
 been stated at 50,000 feet, in the Grampians at 78,000 feet, while in the 
 Lake Country the pressure is considered to have been less. There are 
 five or six chief varieties of rocks of the granite group which agree in 
 containing quartz, besides many other local varieties, resulting from 
 original differences in the rocks which were thus metamorphosed. 
 
 In this country granite is the only rock of the series that will come 
 under the notice of the physical geologist as forming an important 
 element in scenery. In London, examples of some of the chief 
 varieties may be seen in a worked condition in the following public 
 buildings : Thames Embankment, London Bridge, Waterloo Bridge, 
 Club Houses in Pall Mall, pillars in front of St. Paul's Cathedral, 
 Midland Station and Euston Station. 
 
 NAME. 
 
 COMPOSITION. 
 
 LOCALITIES, BRITISH OR FOREIGN. 
 
 Granite 
 
 Quartz, orthoclase, mica, 
 sometimes oligoclase also. 
 
 Dartmoor, Bodmin Moor, St. Aus- 
 tell, Falmouth, Land's End, 
 Shap, Eskdale, Peterhead, Aber- 
 deen. 
 
 Sy en i t i c 
 granite 
 
 Quartz, felspar, mica, horn- 
 blende. 
 
 Charnwood Forest, Ennerdale and 
 Buttermere. 
 
 Protogine 
 
 Quartz, felspar, mica, talc, 
 or chlorite. 
 
 The central mass of Western Alps. 
 
 Luxulianite 
 or Schorl- 
 granite 
 
 Quartz, felspar, tourmaline, 
 with some mica. 
 
 Erzgebirge, between Schneeberg 
 and Eibenstock, Luxulian in 
 Cornwall, Predazzo in Tyrol, 
 and near Heidelberg. 
 
 Haplite 
 
 Quartz, orthoclase. 
 
 Tharand in Saxony. 
 
 Greisen 
 
 Quartz, mica. 
 
 Zinnwald in the Erzgebirge. 
 
 Quart. Journ. Geol. Soc., vol. xiv. p. 453. 
 
FELSITIC AND RHYOLITIC ROCKS. 
 
 37 
 
 The Felsite and Rhyolite Series oj Imperfectly Crystallised Granitic 
 
 Mocks. 
 
 When the materials which would otherwise have constituted a 
 granitic rock solidify under moderate pressure or quickly, then a 
 large part of the rock remains in the form of a paste, which to the 
 unaided eye appears to be uncrystalline. This paste, in old rocks, is 
 termed felsite. In some felsites crystals of quartz, felspar, and mica, 
 &c., occur, but in others crystals have hardly ever or never been 
 formed. Quartz-felsite is an ancient Rhyolite. 
 
 NAME. 
 
 Granite- 
 porphyry 
 
 Elvanite or 
 quartz - 
 porphyry 
 
 Felstone or 
 felsite-rock 
 Petrosilex 
 or Eurite 
 
 Pitchstone 
 
 Rhyolite, or 
 quartz- 
 trachyte 
 
 Perlite 
 
 Obsidian 
 
 COMPOSITION. 
 
 Compact felsitic matrix with crys- 
 tals of quartz, felspar, mica, or 
 chlorite. 
 
 Compact, felsitic matrix with crys- 
 tals of quartz and felspar. 
 
 Compact felsitic matrix sometimes 
 with crystals of quartz and fel- 
 spar. An ancient rhyolite or 
 trachyte. Silica 70-80 p. c. 
 
 A vitreous condition of any of 
 the preceding rocks, but contains 
 more combined water. Often 
 contain s balls of felsite, and some- 
 times crystals of sanidine, quartz, 
 and mica. Silica, 63-70 p. c. 
 
 A compact felsitic matrix, with 
 crystals of quartz, sanidine, and 
 mica. Hence formerly named 
 trachyte-porphyry. 
 
 An enamel - like grey rhyolitic 
 rock,containing concentric grains 
 like pearls, with occasional crys- 
 tals of quartz, sanidine, and mica. 
 
 Dark volcanic glass, often with 
 crystals of sanidine, and some- 
 times balls of felsite. 
 
 Pumice-stone! Obsidian converted into a froth by 
 the multitude of steam cavities 
 which it contains. It is always 
 white. 
 
 LOCALITIES. 
 
 Drusenthal and Schmiede- 
 feld in Thuringia. 
 
 Cornwall, at Penzance, near 
 Marazion, &c. 
 
 North Wales, &c. ; Skye, 
 Mull. On the flanks of 
 granite it is termed felsite 
 schist or Halleflinta. 
 
 Arran, Rum, Mull ; Mexico, 
 Peru, Iceland, Auvergne. 
 
 Lipari Islands, Ponza Is- 
 lands, Euganean Hills, 
 Schemnitz in Hungary. 
 
 Lipari Islands, Schemnitz 
 in Hungary, Tokay, &c. 
 
 Lipari Islands, Peak of Tene- 
 riffe, Iceland, Mexico. 
 
 Generally with obsidian but 
 sometimes without it, as at 
 the Laacher See, near An- 
 dernach, on the Rhine. 
 
 Like the granites these rocks are of all geological ages. Felstone 
 appears to be a form in which granite consolidates when, having been 
 mred out as a lava- stream, it cooled under water. Felstone rocks 
 )m North Wales are commonly used for paving the roadways in 
 my parts of London. 
 
33 TRACHYTIC ROCKS. 
 
 The Family of Quartz-less Orthoclase Roclcs. 
 
 Syenite might be grouped with the granites which it resembles in 
 external appearance, because the conditions of consolidation were the 
 same. But as it does not contain quartz, it is evident that the strata 
 which were metamorphosed to form it must originally have been dis- 
 similar. Typically, syenite contains hornblende ; but occasionally the 
 hornblende is replaced by augite, or in Calabria by mica. The mica- 
 syenite of Kosenbusch is the minette or mica-trap of authors. Professor 
 Bonney regards mica-trap as a family in which he places minette-ker- 
 santite, minette-felsite, mica-diorite, and kersantite-porphyrite. Augite- 
 syenite, like mica-syenite, is chiefly known in dykes. The volcanic 
 representative of hornblende-syenite is beyond doubt trachyte ; though 
 trachyte, like rhyolite, contains glassy felspar, instead of the opaque 
 orthoclase of the plutonic rocks. 
 
 Trachyte differs from the granite series in the circumstance that 
 most varieties of the rock do not contain crystals of quartz ; or when 
 quartz is present, it is in a form named tridymite. The percentage of 
 silica in the rock does not fall much, if at all, below that of granite ; 
 and the trachytes are closely related to the granites. In North America 
 quartz-trachytes are recognised. Trachytes properly so called are 
 rough crystalline rocks ; they occur as lava streams, but sometimes the 
 crystalline trachytes form mountain masses. 
 
 These rocks are probably of all geological ages, though the true 
 trachytes have hitherto only been met with in the newer rocks, being 
 represented in the older series by quartzless felsites. We group 
 phonolite with syenite, because it bears much the same relation 
 to nepheline-syenite that felsite and rhyolite hold to granite. 
 
 Syenite 
 
 Zircon- 
 syenite 
 
 Porphyrite 
 
 Sanidine 
 trachyte 
 
 COMPOSITION. 
 
 Granular crystalline mixture of 
 orthoclase and some oligoclase, 
 with hornblende and magnetite. 
 Many syenites contain mica, 
 quartz, nepheline, or augite. 
 
 Granular crystalline mixture of 
 orthoclase, nepheline, zircon, and 
 a little hornblende. 
 
 A dark felsitic matrix, chiefly 
 of oligoclase or orthoclase, with 
 crystals of felspar, forming fel- 
 spar porphyry, or mica forming 
 mica porphyry, or hornblende 
 forming hornblende porphyry. 
 
 I Granular or compact matrix of 
 
 j sanidine, with crystals of sanidine, 
 
 hornblende, and magnesia mica. 
 
 i 
 
 LOCALITIES. 
 
 Charnwood Forest, Dresden, 
 Guernsey. 
 
 Laurvig and Brevig in Nor- 
 way. 
 
 Elfdalen in Sweden, Lenne- 
 gebiet in the Harz, often 
 with columnar joints, Thur- 
 ingian Forest, Pentland 
 Hills, near Edinburgh. 
 
 Lavas of Monte Nuovo, Berk- 
 urn, near Bonn; Koberts- 
 hausen, in Hesse Darm- 
 stadt. 
 
PLAGIOCLASE ROCKS. 
 
 COMPOSITION. 
 
 LOCALITIES. 
 
 Drachenfels 
 trachyte 
 
 Granular matrix of sanidine and 
 oligoclase, with magnesia mica 
 and hornblende, and large crys- 
 tals of sanidine. May contain 
 tridymite. 
 
 Minette, or A grey felsitic matrix of ortho- 
 mica syen- clase, with much mica, and 
 ite sometimes crystals of augite or 
 
 hornblende; chiefly in veins and 
 
 dykes. 
 
 Drachenfels, near Kb'nigs- 
 winter, on the Rhine ; and 
 other localities in the Sie- 
 bengebirge. 
 
 In the Channel Islands. Near j 
 Framont in the Vosges, near ! 
 Oederan in Saxony, and in j 
 the Lake district, at Sale | 
 Fell west of Bassenthwaite, 
 and Cross How Beck. 
 
 Miascite 
 
 Phonolite 
 
 Granular crystalline mixture of , Miask, Ditro 
 orthoclase, nepheline, sodalite, yania. 
 and biotite. 
 
 in Transvl- 
 
 Compact rock, rings under the 
 hammer, weathers white like 
 felsite. The matrix consists of 
 sanidine, nepheline, hornblende, 
 titanite. Crystals of sanidine 
 occur more developed. Some- 
 times oligoclase is a constituent. 
 Occasionally vesicular and amyg- 
 daloidal. 
 
 Wolf Rock. Mileschauer in I 
 Bohemia, Aussig in Bohe- i 
 roia. Forms conical isolated 1 
 hills in the Mittelegebirge i 
 of Bohemia ; in the Auvergne, 
 as at the Roche Sanadoire. 
 
 Family of Quartz-bearing Plagiodase Rocks. 
 
 These rocks are as distinct from the ordinary plagioclase rocks, as 
 are quartz orthoclase rocks from the quartzless series, but they are 
 less frequently met with. 
 
 NAME. 
 
 COMPOSITION. 
 
 LOCALITIES. 
 
 Quenastite or j A granular compound of quartz, 
 Quartz-dio- plagioclase and hornblende, with 
 rite orthoclase in some localities. 
 
 Quartz- | A micro-crystalline green paste 
 
 propylite j of quartz, oligoclase and horn- 
 ! blende, with larger crystals of these 
 minerals and titaniferous iron. 
 
 Dacite or Grey or brown granular or com- 
 Quartz-ande- pact matrix of felspar and horn- 
 site blende, with quartz, oligoclase, 
 sanidine, and hornblende in fine 
 grains or crystals, with magne- 
 tite. 
 
 Quartz- j A micro-crystalline compound of 
 
 dolerite j quartz, plagioclase, augite and 
 i olivine. 
 
 Quenast in Belgium, Catau- 
 zara in Calabria. In the 
 Tonale Pass in the Tyrol is 
 a micaceous quartz diorite. 
 
 Many places in the U.S., such 
 as Papoose Peak, Hills of 
 Golconda, &c., and some 
 places in Hungary. 
 
 Named from the ancient 
 Dacia ; found in Transyl- 
 vania and Hungary. 
 
 U.S.A., on the Upper Snake 
 River and Shoshone Mesa. 
 
PLAGIOCLASE ROCKS. 
 
 Family of Quartzless Plagioclase Rocks. 
 
 These rocks were formerly included partly as greystones, partly as 
 greenstones, the oligoclase series, except diorite, belonging to the 
 former ; while the labradorite rocks with diorite formed the green- 
 stones. Diorite is the deep-seated condition of the andesites, just as 
 gabbro is the plutonic condition of dolerite. 
 
 
 COMPOSITION. 
 
 
 NAME. 
 
 OLIGOCLASE PREDOMINANT. 
 
 LOCALITIES. 
 
 Diorite 
 
 Granular compound of oli- 
 
 Brazil Wood in Charnwood Forest, 
 
 
 goclase or labradorite and 
 
 Klumpsen in Oberlausitz, Hulm- 
 
 
 hornblende ; usually dark 
 
 berge in Thuringia. In the Ton- 
 
 
 green. 
 
 ale Pass in the Tyrol is a mica- 
 
 
 
 ceous quartz diorite. 
 
 Domite 
 
 Granular matrix of oligoclase, 
 
 Puy de Dome, in the Auvergne. 
 
 
 no sanidine, with crystals 
 
 Puy de Sarcouy,Puy de Chopine. 
 
 
 of oligoclase, augite, or 
 
 
 
 hornblende, and biotite., 
 
 
 Andesite 
 
 Granular or compact matrix 
 
 Chimborazo and Cotopaxi. Pla- 
 
 
 
 of oligoclase, with crystals 
 
 teau de Durbiz in the Auvergne 
 
 
 of oligoclase and augite or 
 
 and Wokenburg and Stenzelburg 
 
 
 hornblende, and sometimes 
 
 in the Siebengebirge. 
 
 
 biotite and magnetic iron. 
 
 
 Augite ande- 
 
 A grey or brown granular 
 
 Peak of Teneriffe, older lavas of 
 
 site or Tra- 
 
 matrix of oligoclase, with 
 
 Etna, crater of Stromboli. The 
 
 chy-dolerite 
 
 augite or hornblende, and 
 
 L6 wen burg in the Siebengebirge. 
 
 
 some mica. Sometimes con- 
 
 
 
 tains much labradorite. 
 
 
 
 LABRADORITE PREDOMINANT. 
 
 
 Gabbro 
 
 Granular compound of lab- 
 
 Cornwall, Mull, Skye ; near Penig 
 
 
 radorite or saussurite and 
 
 in Saxony. Common in dykes. 
 
 
 diallage or hypersthene. 
 
 
 
 Some gabbros contain oli- 
 
 
 
 vine. 
 
 
 Dolerite and 
 basalt 
 
 Labradorite and augite, usu- 
 ally with olivine and titan- 
 ite, and sometimes nephe- 
 
 In dolerite the grains are distinct ; 
 when the grains are very fine it is 
 anamesite. If the texture is com- 
 
 
 line. The volcanic condi- 
 
 pact the rock is basalt ; when 
 
 
 tion of gabbro. 
 
 glassy it is tachylyte. Well seen 
 
 
 
 in Antrim, Skye, Mull, Eigg, 
 
 
 
 Arthur's Seat. 
 
 Diabase 
 
 A dolerite of Primary age in 
 
 Sweden, Berneck, and Saalburg, 
 
 
 which chlorite or serpen- 
 
 in the Fichtelgebirge ; the Harz. 
 
 
 tine is developed by decom- 
 
 In Britain all the basalts of Cam- 
 
 
 position. 
 
 brian or Silurian age are diabase. 
 
 Nepheline- 
 
 Nepheline and augite with 
 
 At Meiches in Hesse, Katzen- 
 
 dolerite 
 
 titanite, &c. 
 
 buckle in the Odenwald. 
 
 Leucite basalt 
 
 Leucite and augite with 
 
 Lavas of Monte Somma. lavas of 
 
 
 titanite, &c. 
 
 Vesuvius in 1828 and 1832, Bell 
 
 
 
 near Aridernach. 
 
NATURE AND ORIGIN OF VOLCANIC ROCKS. 41 
 
 Most basalts were lava streams. The rocks are now often columnar, 
 or have a spheroidal or tabular structure. They are sometimes full 
 of natural air cavities, and are then said to be vesicular or scoriaceous. 
 When these cavities are infiltrated with minerals, the rocks are 
 termed amygdaloids. Behind Tobermory in Mull, these white infil- 
 trations are so numerous that the rock looks as though splashed over 
 with whitewash. The infiltrated minerals, which may form two-thirds 
 of the rock, are collectively termed zeolites. They nave been formed 
 by the water which percolated through the rock dissolving the felspars, 
 &c., and redepositing them chemically combined with water. Easaltic 
 rocks may contain large crystals ; they also form volcanic ashes. There 
 are many minor varieties of rock chiefly named from the zeolites which 
 enter into their composition. 
 
 The preceding tables of the igneous rocks will be sufficient to 
 enable us to estimate the part which they take in forming the earth's 
 surface, though they give necessarily but an imperfect idea of their 
 varieties. Any classification into which they may be grouped must 
 always be a matter of convenience rather than an expression of the 
 necessary combinations of minerals into rocks, if we believe that the 
 materials of igneous rocks were originally the materials of stratified 
 formations. For the sorting power of water gives a differing mineral 
 composition to almost every mile of a formation as it recedes from 
 shore. If the whole deposit on the sea-bed were afterwards melted 
 up and ejected as lava, it would result that the parts near to land 
 would be rich in silica, and would contain the minerals in which 
 silica abounds, forming the quartz-bearing or so-called acidic rocks ; 
 while the parts more distant from land, such as certain clays, would 
 contain the minerals in which silica is deficient, and form the so-called 
 basic rocks. It may be artificial to classify either by the quartz or 
 the felspar in igneous rocks, but this is a chemical classification also. 
 At present we are unable to discover how those strata were spread 
 and composed, out of which igneous rocks have been reconstructed. 
 Obviously there must be many gradations of mineral character in the 
 igneous rocks which cannot be detected, on account of the fragmentary 
 way in which they burst through the earth's surface or become exposed 
 by the removal of superincumbent rock ; and these transitions would 
 probably be as complete were they known, as are the gradations of 
 texture in the rocks which result from differences in the conditions 
 under which they cooled ; which permit the same original rock sub- 
 stance to become a mass of large crystals, finely crystalline, compact 
 like biscuit china, glassy, vesicular, scoriaceous, or ashy ; or when 
 vesicular to be so altered by infiltration that its whole character is 
 changed. We must bear in mind that while the liquefaction of sand- 
 stones would give certain granitic rocks, and liquefaction of clays 
 would yield the greenstones, these materials may often be so contorted 
 and folded together with limestone, that the igneous rock resulting 
 from their fusion would be intermediate in character, or combine 
 minerals which are usually limited to different groups of rocks. 
 
42 JOINTS IN PLUTONIC ROCKS. 
 
 THE JOINT-STRUCTURE OF IGNEOUS EOCKS. 
 
 When igneous rocks cool they all contract, and thus fissures which 
 are called joints appear in them. These joints run through the rock 
 in different directions, according to its composition and the conditions 
 under which it cooled ; and sometimes the same rock presents two or 
 three kinds of joints, or it shows no joints at all. In granite, the 
 prevalent joints run in straight lines which cross each other at some 
 angle ; and in basalt, phonolite, and some other rocks, the joints often 
 form six-sided columns, which may be straight or curved, and vary 
 from an inch or two in diameter up to a width of many feet. Some 
 of the largest may be seen at the old basalt quarries now used for 
 beer-cellars at Nieder-mendig, not far from the Laacher See, on the 
 Rhine. An excellent account of several kinds of joints in volcanic 
 rocks has been given by Professor Bonney, under the names of columnar, 
 tabular, curvi-tabular, and spheroidal structure. 1 There is no doubt 
 that some joints are a consequence of conditions under which the 
 rock cools, but the forms and directions which they assume have 
 always some predisposing cause, usually pressure or strain. The joints 
 in granite could not be accounted for by cooling alone, unless it were 
 supposed that cooling took place from opposite sides of the mass, so 
 that the shrinkage planes formed on one side have intersected those 
 formed on the other side. And it seems likely that jointing is pri- 
 marily a consequence of the development of shrinkage planes in the 
 direction of the predominant arrangement in the rock of its principal 
 mineral constituent. Thus more than half of granite consists of ortho- 
 clase felspar, and if the majority of the felspar crystals have a preva- 
 lent direction, consequent either upon pressure or contraction, then 
 there must have been a tendency for the rock in cooling to behave as 
 though it consisted entirely of felspar, and to divide by joints which 
 correspond more or less with the cleavage planes of orthoclase or with 
 its crystalline faces. And when we bear in mind the circumstance that 
 in granite the minerals have been arranged in at least two directions, 
 it becomes probable that the felspar crystals should have more than 
 one direction, so that a second set of cleavage planes may be produced 
 running through the other minerals associated with the felspar ; and 
 this may be the explanation of the fact, that in most granite quar- 
 ries the joints which correspond with orthoclase cleavage are crossed 
 by others, which at first sight seem to be inconsistent with it, and 
 correspond better with the angular directions of the crystalline faces. 
 In the same way the other kinds of joints might be regarded as 
 consequences of the influence of the rate of cooling upon the mode of 
 arrangement of the predominant mineral forming the rock. Professor 
 Eonney mentions the occurrence of a kind of columnar structure in ice, 
 in haematite iron-ore, in a large quartz vein at Svolvaer in the Lofoden 
 Islands ; in coal where it is in contact with basalt, in volcanic mud 
 beneath basalt at Tideswell Dale in Derbyshire, in the consolidated 
 pelagonite ash of Iceland ; besides finding it in the trachyte of Mont 
 1 Quarterly Journal of the Geological Society, vol. xxxii. p. 140. 
 
JOINTS IN VOLCANIC ROCKS. 43 
 
 Dore in the Auvergne, the pitchstone of Arran, the felstone of Cader 
 Idris, and phonolite of the Rochte Sanadoire in the Auvergne. Mr. 
 Koch has stated, that when some slags are cooled under water they 
 also assume a columnar structure. The hexagonal structure of ice, 
 haematite, and quartz would seem to be connected with the fact, 
 that those minerals crystallise in the hexagonal system, and circum- 
 stances have favoured their division into hexagonal prisms. 
 
 But the prevalent columnar structure of basalt is of an altogether 
 different nature. The surface or the floor of the lava stream cooled 
 uniformly, and therefore contracted, so that the cracks appeared near 
 the surface or base, and penetrated deeper and deeper as the cooling 
 progressed ; sometimes leaving an undivided portion in the middle of 
 a thick lava-flow. And it is extraordinary that these cracks always 
 form an angle of about 120 with each other, so that the entire mass 
 of rock is split up into six-sided columns. Both the augite and labra- 
 dorite which form basalt, belong to the monoclinic system of crystal- 
 lisation ; and we think it possible that the angles of the skeleton-crystals 
 of the augite and felspar have determined the angle of the division 
 planes splitting basalt, causing the columns to take a six-sided figure, 
 rather than any other form, so that the structure itself is essentially 
 crystalline. 
 
 When basalt has been exposed to the weather, as in the Giant's 
 Causeway and in Staffa, the columns are often found to be divided 
 transversely by joints, which have been compared to the joints in the 
 back-bone of a shark. And sometimes the outer layer of each of these 
 short pieces scales off, showing an internal concentric structure. This 
 is beautifully seen at the grotto called the Kaskeller, near Bertrich, 
 by the Moselle, where the columns look as though built of Gouda 
 cheese. But a similar though irregular joint-structure may be seen 
 almost as well, on a small scale, in the country above Tobermory in 
 Mull, where many thin concentric layers of compact basalt will scale 
 off from the irregular blocks, leaving in the centre a small, rough, 
 more crystalline ball, with the texture of dolerite. Up the Ehine 
 from Bonn to Andernach, basalt columns do not usually show trans- 
 verse divisions, but are often enormously long, and have to be broken 
 into convenient lengths for the various building purposes for which 
 they are quarried. 
 
 V 
 
 UNIVERSITY 
 
( 44 ) 
 
 CHAPTER V. 
 
 THE NATURE, COMPOSITION, AND ORIGIN OF THE COMMON 
 WATER-FORMED ROCKS. 
 
 ALMOST the whole of the land surface of the world consists of 
 rocks which have been accumulated under water. It nowhere shows 
 a trace of such materials as would have resulted from an original 
 igneous fusion ; for such surface rocks would have been uncrystalline 
 and like modern volcanic lavas. Only occasionally are large masses 
 of crystalline rocks seen, and those are only exposed by the water- 
 formed rocks which covered them having been removed by the 
 denuding power of water. Though volcanoes are numerous, the areas 
 of the earth's surface covered with sheets of lava poured out from 
 the earth in a molten state are not very considerable. The whole 
 land surface consists of materials which may be classed as superficial 
 volcanic rocks, deep-seated crystalline rocks, and stratified rocks, which 
 have been spread out under water in layers. The stratified rocks 
 can be accounted for directly or indirectly as products of the wear 
 and tear of the other kinds; and it is almost equally certain that 
 the aqueous rocks may be changed by pressure, and the heat to 
 which pressure gives rise when it is arrested, into the more or less 
 crystalline rocks which are severally named Metamorphic, Plutonic, 
 and Volcanic. We have no means of judging what the earliest- 
 formed rocks were like ; and Professor Huxley, with excellent reasons 
 for the suggestion, has remarked that the oldest rocks now known 
 bear the same relation in point of antiquity to those which must 
 have preceded them, that the newest deposits of the geological series 
 bear to the whole series of strata which have been discovered. 
 
 Kinds of Deposits. The water-formed rocks consist of pebbles, 
 sand, mud, or limestone, which have become hardened by various 
 natural cements into solid beds called strata. The pebbles then 
 become a conglomerate, the sand a sandstone, the mud a clay, and 
 the shells, corals, or foraminifera, or other remains of animals, form lime- 
 stones. These are the chief kinds of water-formed rocks. Pebbles, 
 sand, and clay are worn away by sea, river, or lake water from lands 
 which previously existed, and are spread out parallel to the shore, or 
 at the mouths of rivers. These materials are only held in suspension 
 by the mechanical power of moving water, and hence are often called 
 mechanical deposits ; and since they fall as sediments when the 
 
ORIGIN OF THE COMMON WATER-FORMED ROCKS. 45 
 
 materials can be carried no farther, they are more frequently named 
 sedimentary deposits. Limestone does not accumulate as a sediment. 
 The carbonate of lime can only be taken up into the water when 
 water contains sufficient carbonic acid gas to dissolve it ; and water 
 only parts with it again when the surface layer is evaporated, or when 
 some plant or animal separates the lime from the water to form its 
 skeleton. Deposits due to these causes accumulate over the whole 
 bed of the ocean, though their rate of accumulation is so slow that 
 the lime is usually inappreciable near to shore where other deposits 
 are forming, except as a cement, binding sands or hardening clays. 
 These layers of sand, clay, and limestone extend through the country, 
 sometimes turned up from their original horizontal positions almost 
 on end, sometimes bent into basin- shaped folds, and sometimes folded 
 into saddle-shaped ridges. Pebbles are only formed where the rocks 
 on the shore or a shallow sea-bed are hard, and are worn away by the 
 pieces being slowly ground against each other, and thus become 
 rounded. They may consist of any kind of rock, and are usually 
 evidence of near vicinity to land at the time of their deposition. 
 Mr. Darwin mentions that at Santa Cruz, on the sloping east coast 
 of South America, the pebbles near to shore are very large, while at 
 three or four miles from the shore they are as large as walnuts ; at 
 six or seven miles, as large as hazel nuts ; at ten to eleven miles from 
 shore they were from f^ths to j^ths of an inch; at twelve miles, ^gths 
 of an inch ; and from twenty-two to one hundred and fifty miles, the 
 sediment varied in size from ^th of an inch to the finest sand. Over 
 this distance the depth steadily increases to sixty-five fathoms. Many 
 beds of pebbles occur along our own shores, especially where there are 
 chalk cliffs or granite, or other old and hard rocks to furnish materials 
 out of which they may be formed. According to Colonel Greenwood, 
 pebbles of chalk-flint are carried by the sea as far west as the south 
 of Cornwall, and pebbles of the Cornish rocks are mixed with the 
 flint pebbles of Sussex and Kent. There are many beds of conglo- 
 merate among the British strata, as will be seen by a glance at the 
 table given further on. 
 
 Mr. Darwin's observations, just referred to, show that the pebbles 
 become smaller farther from shore. The small pebbles are chiefly 
 pieces broken from the larger masses, and often are the larger pebbles 
 worn small by constant rubbing against each other. When they are 
 no larger than peas, the rock they form is often called grit. The 
 British strata contain several such beds ; as, for instance, the Millstone 
 Grit, which lies below the formation named Coal Measures. 
 
 Sand. The finer kind of sediment called sand consists of the 
 mineral quartz. This is either the fine dust ground from off flints or 
 old quartz rocks in the process of rounding pebbles, or is made up of 
 grains of quartz, which constitute part of some crystalline rocks such 
 as granite and the schists. These grains are usually bound together 
 with a cement of silica, or carbonate of lime, or oxide of iron, so as 
 to be hardened into stone. The most familiar example of sandstone 
 is seen in the paving-stones which form the footways in London 
 
46 CLAY; SELENITE, ALUM. 
 
 and many large towns. Sands and sandstones occur in each of the 
 great divisions of the geological series in this country. Among 
 them may be named Harlech Grits, Llandovery beds, Downton 
 Sandstone, much of the Old Ked Sandstone formation, much of the 
 Millstone Grit, the Pennant, and many sandstones associated with 
 the CoaL The Permian and Triassic rocks in this country are chiefly 
 sandstones. The Lower Oolites in Yorkshire are largely formed of 
 sandstone. There are also the Portland Sands, sands of the Purbeck 
 and Wealden periods, Lower and Upper Greensand, the Thanet Sands, 
 the Bagshot Sands, and other deposits. Sand and sandstones generally 
 form somewhat elevated and dry country, which is frequently wooded, 
 especially with fir, and is sometimes covered with heather. See p. 92. 
 
 Clay. Clay consists chemically, chiefly of silicate of alumina, and 
 has very nearly the same composition as the mineral felspar, which 
 makes up so large a part of fire-formed rocks. Sometimes when hard- 
 ened by pressure, and by containing other minerals, the clay is called 
 shale ; it then splits into thin layers in the direction in which it 
 was deposited. Clay consists of extremely fine particles which can 
 easily be transported by moving water, as may be seen by the muddy 
 state of rivers after rain in clayey districts. The colour of clay is 
 generally due to some oxide of iron ; it is usually grey or blue, some- 
 times brown, occasionally white, yellow, red, crimson, purple, violet, 
 or black. Clay generally forms valleys and low land ; it does not 
 easily allow water to pass through it, but always holds a good deal of 
 water suspended in its substance ; and when this is evaporated, large 
 and deep surface cracks and fissures are formed, which may be enlarged 
 by rain into gullies. The older British clays have undergone certain 
 changes, so that it is convenient to give them another name, and 
 they are now termed slates and slate rocks. Some of the well- 
 known clays are the Lias, Bradford Clay, Oxford Clay, Ampthill 
 Clay, Kimmeridge Clay, Wadhurst Clay, Weald Clay, Gault, London 
 Clay, Barton Clay, and Boulder Clay. These deposits are often well 
 wooded, especially with oak, beech, and elm. See p. 99. 
 
 Selenite. Clays sometimes contain a large amount of iron pyrites, 
 which is usually a yellow brassy-looking mineral, consisting of sulphur 
 and iron. When it decomposes, the sulphur is set free in the form 
 of sulphuric acid. This acid, taken up by water, usually dissolves the 
 carbonate of lime of shells, and then the new compound crystallises 
 and forms transparent crystals of the mineral selenite, which is a 
 variety of gypsum, and is composed chemically of hydrated sulphate 
 of lime. The water may be driven off from this mineral by heat, and 
 then, when ground to powder, the substance becomes plaster-of-Paris. 
 
 Alum. In other cases the sulphuric acid, liberated from the decay- 
 ing iron pyrites, attacks the clay itself, and then forms crystals of 
 alum, which is a sulphate of alumina. This product of the London 
 Clay has given its name to Alum Bay in the west of the Isle of 
 Wight ; and it originated the industry of alum-making on the York- 
 shire coast, in beds of the Upper Lias, which are hence called alum 
 shales. 
 
SEPT ARIA, PHOSPHATITE, AND LIMESTONE. 47 
 
 Septaria. Clays also contain concretions formed of a mixture of 
 lime and clay. They are often oblate elliptical spheroids, and are 
 named septaria. In new strata they are very small, and may be seen 
 in process of first formation in little nests in the brick-earth of the 
 valley of the Thames, as at Crayford. They are found in nearly all 
 clays, and are larger in the older rocks, showing that they grow gra- 
 dually by gathering to themselves, by the solvent action of water, the 
 lime which the clay contains. In the London Clay they are usually a 
 foot or two in diameter ; and in most of the clays are under six feet 
 across. Brickmakers often call them turtle-stones. In the Ludlow 
 rocks they are called ball-stones, and are sometimes eighty feet in 
 diameter. When burned and ground to powder these concretions form 
 hydraulic cement, which sets under water. Septaria are so named 
 from the partitions or septa by which they are divided. They owe 
 their existence to the fact that while the clay was forming, lime also 
 was being thrown down upon the sea-bed, but in quantity too small to 
 form continuous beds of limestone. And proof of this is seen in the 
 fact that layers of septaria, which cover the clay-floor much as raisins 
 might cover a surface of dough, may sometimes be traced into more 
 or less continuous beds of rock. These concretions are rarely or never 
 met with when the clay is sandy. See p. 102. 
 
 Phosphatite. Occasionally beds of small concretions of phos- 
 phate of lime, sometimes called coprolites, rest on clay surfaces or are 
 scattered in sands or limestones. They are highly valued for the 
 manufacture of an artificial manure for root-crops, which is named 
 superphosphate of lime. The chief phosphatic beds in this country 
 are six in number : in the Bala series of North Wales, in the Upper 
 Neocomian, Gault, Upper Greensand, Coralline Crag, and the Red 
 Crag of the south-east of, England. These deposits appear to have 
 been owing chiefly to the growth and decay of sea-plants for many 
 generations, on fixed spots near to the shore, since those plants all 
 contain a quantity of phosphates, which are capable of combining with 
 lime when liberated by the decay of their organic tissues. These con- 
 cretions rarely assume a septarian structure ; and the mineral often 
 invests or infiltrates animal substances. See p. 103. 
 
 Limestone. Limestones consist of the mineral calcite, though 
 usually in an uncrystallised condition. River waters carry dissolved a 
 good deal of carbonate of lime, and Sir Charles Lyell has recorded that 
 the evaporation of the waters of the Rhone which float over the sea is 
 forming a calcareous rock off its delta, in which sunken cannon had 
 become embedded. At the base of chalk cliffs the chalk is always 
 rounded into large boulders, as may be well observed at the Culver 
 Cliff on the east of the Isle of Wight ; and the peculiar green colour 
 of the sea-water off chalk coasts has been attributed to the particles 
 of abraded chalk which it contains. Such particles may be presumed 
 to form a deposit in which small chalk pebbles occur ; for a recon- 
 structed limestone bed of this kind may be seen in the geological 
 deposit called the Stonesfield slate. 
 
 Oolite Another group of limestones is the oolites, so called from 
 
48 OOLITIC AND ORGANIC LIMESTONES. 
 
 their resemblance in structure to the eggs or hard roe of fish. The 
 grains are rarely -^th of an inch in diameter, are spheroidal, cemented 
 together with carbonate of lime, and when sliced and examined under 
 the microscope, are seen to consist of concentric layers of crystalline 
 calcite, arranged about a nucleus, which is sometimes a grain or two of 
 sand, but more frequently a minute foraminiferous shell. Occasionally 
 the grains are radiate, and sometimes they have recrystallised. This 
 structure is seen in some parts of the Bala limestone, and occasionally in 
 the Plymouth limestone, and parts of the Carboniferous limestone near 
 Bristol, &c. ; it is characteristic of much of the Inferior Oolite, Lincoln- 
 shire limestone, Bath oolite, Coralline oolite, and Portland oolite. Oolitic 
 grains occur in Tertiary freshwater limestones of the Isle of Wight. Von 
 Buch has mentioned a stalactitic layer of limestone, which is some- 
 times oolitic, covering the lavas of the island of Lancerote. It has 
 been suggested that the deposit is due to north-west winds in winter, 
 driving the spray of the sea over the island. A similar rock is de- 
 scribed by Mr. Darwin at St. Helena, where masses of white finely 
 oolitic rock are attached to the outside of some of the incrusted 
 pebbles ; and Mr. Sorby has described oolitic grains in the recent lime- 
 stones from Bahama and Bermudas. Concretions of a concentric 
 nature, but of far larger size and darker colour, are formed at Carlsbad 
 by the waste water of the hot mineral spring. In these recent instances 
 the grains may have formed in a subaerial way ; but in the case of the 
 great Secondary limestones, there can be no doubt that their formation 
 is due to evaporation of the surface of the sea, so that a film was formed 
 around some shell fragment, and continued to increase in size as it fell 
 through the water till it sank to the bottom. This explanation will 
 also account for the uniform size of the grains in the same stratum, 
 Sometimes beds occur, like the Pea Grit,at the base of the Inferior Oolite, 
 in which the grains are as large as peas, and often of irregular shape. 
 
 Foraminiferal Limestone. Many limestones are composed chiefly 
 of the remains of animals, such as corals, foraminifera, shells, encri- 
 nites, &c. The chalk is the best example in this country of a fora- 
 miniferal limestone ; but the limestones formed by the Miliola, and 
 especially by the Nummulina, constitute great beds on the Continent, 
 and the latter ranges east and west through the central region of the 
 Old World. Deposits like the chalk are now forming at the bottom of 
 all the deep oceans, chiefly by the accumulation of foraminifera named 
 Globtgerinct and Orbulina, with a few pteropods which also live in the 
 surface waters, and sink to the bottom after death to become mixed 
 with sponges, sea-urchins, shells, and crustaceans, which live at great 
 depths. 
 
 Coralline Limestone. Among the limestones in this country, 
 largely formed of corals, are the Wenlock limestone of the border coun- 
 ties of Wales, the Plymouth limestone of South Devon, parts of the 
 Carboniferous limestone, especially of Derbyshire, and the Coral 
 Rag. Probably all these corals lived in moderate depths, for there 
 is no evidence that they formed great coral reefs, such as exist at the 
 present day, and extend down to great depths in the ocean. Tho 
 
ORIGIN OF THE COMMON WATER-FORMED ROCKS. 49 
 
 individual coral growths are mostly small, and as in a coral reef are 
 usually mixed with abundant fragments of worn corals and other 
 calcareous masses; all are commonly bound into a solid mass by 
 carbonate of lime deposited from the water soaking through the rock. 
 
 Shell Limestone. One of the best English illustrations of a shell 
 limestone in process of formation in shallow water, is seen at Shell 
 Ness at the east of the Isle of Sheppey, in the Thames. There shells 
 are cast up so as to form a considerable level deposit, which in time 
 may well become cemented into a stratum like many of the shell beds 
 in the Carboniferous limestone and Lower Oolites. In some forma- 
 tions, like the Headon series in the north-west of the Isle of Wight, 
 there are considerable oyster-beds which have continued to grow and 
 accumulate although the mineral character of sediments around them 
 has changed several times ; just as oyster-banks grow at the present day 
 on the floor of the English Channel regardless of changing currents. 
 
 Other limestones are almost entirely of vegetable origin. On certain 
 shores, especially in the tropics, plants called nuUipores grow, which, by 
 absorbing carbonic acid from the water, have the power of precipitating 
 around their tissues a dense coating of carbonate of lime. They are slender, 
 stony-looking, jointed tufts, termed corallines, and abundant on our own 
 shores, but with the joints growing to the size of fingers on coral reefs, 
 to the building up of which they contribute a not inconsiderable fraction. 
 
 Freshwater Limestones, &c. In our own fresh waters there are 
 plants of the genus Chara, which thrive wherever waters contain much 
 lime, and possess a similar property of separating it from the water of 
 the pond, river, or lake, to that exhibited by the NuUipores. The stem 
 of the plant becomes coated with an incrustation, which ultimately 
 bears the plant to the bottom of the lake or stream, and so contributes 
 to build up a bed of limestone. Many of the limestones in the fresh- 
 water strata of the Isle of Wight, have originated in this way. Lime- 
 stones generally form ridges of more or less rounded hills, intersected 
 with deep valleys, which have been dissolved by rain water as it has 
 drained over the surface. The rounded contours of chalk hills are an 
 excellent example of the scenery produced in this way. Usually lime- 
 stone hills have but little wood growing upon them. See p. 105. 
 
 Simultaneous Origin of Water-formed Rocks. These several 
 kinds of water-formed rocks are all forming at the present day in 
 lakes and on different parts of the sea-bed; and in all past ages 
 rocks consisting of these different mineral materials have accumu- 
 lated simultaneously in different regions ; so that a formation which 
 is clay in England may perhaps be sand in France, or a sand- 
 stone in this country may be represented by a limestone in Germany. 
 We shall the more readily understand how this is possible if we 
 imagine a coast or an island in process of being worn away by the sea, 
 and suppose it to be composed entirely of some crystalline rock such 
 as granite. This rock consists of three minerals, named quartz, ortho- 
 clase felspar, and mica, as already described. Speaking roughly, these 
 minerals are combined in granite in the proportions 25 per cent, of 
 
 rtz, 55 per cent, of felspar, and 20 per cent, of mica. The quartz 
 
 dirty-looking half-transparent mineral, in relatively large particles 
 
 D 
 
50 ORIGIN OF STRATA. 
 
 called grains, which, ordinary water does not easily dissolve; and 
 when the granite becomes softened by exposure to water containing 
 carbonic acid, these particles, which are relatively heavy, are washed 
 out of the rock by rain or the waves so as to form grains of sand. 
 Because this is the heaviest material derived from the wasting of 
 granite, it is carried to a less distance from the shore than other 
 substances, and forms a belt of sand or sandstone parallel to the coast. 
 If granite were the only source of water-formed rocks, and only 
 denuded by the sea, one quarter of all known stratified formations 
 would be arenaceous or sandy deposits. And if the mineral mica, 
 which is a glistening flaky substance, is supposed not to be altered in 
 character or decomposed, it will often be carried to the limit of the 
 sand and help to form what is called a micaceous sandstone, causing 
 the rock afterwards to split into thin layers. But sometimes the 
 mica is carried much farther. More than half of the granite consists 
 of felspar, which forms large milky-white or red crystals. As we 
 have already mentioned, this mineral is chiefly composed of a silicate 
 of alumina ; but in granite it also includes some potash, soda, a little 
 lime, iron, and other substances. The carbonic acid, which is always 
 dissolved in water, attacks the felspar by dissolving out from it car- 
 bonates of potash, soda, or lime; and then the crystals lose their 
 hardness and become changed into a paste of impalpable fine particles 
 which forms a mud. This mud is held in suspension longer than the 
 sand, and is therefore carried farther out to sea. When it falls to 
 the bottom and is compressed by the weight of water above, it becomes 
 clay, and margins or surrounds the land as an outer belt probably 
 twice as broad as the sand belt. On some coasts, like the South 
 American coast mentioned by Mr. Darwin, there may be no clay 
 deposited within one hundred and fifty miles of land. There is no 
 clearly defined separation between the limits of clay and sand, for 
 they pass into each other from the materials being mixed, and the 
 clays nearest to shore are often micaceous. 
 
 When this denudation takes place upon land by the agency of 
 rain, as on Dartmoor, which consists of granite, the sand is left on 
 the slopes of the mountains, while the mud is carried down into the 
 valleys, where it forms pipe-clay. Usually coast and surface de- 
 nudation go on together, and much of the surface mud is carried 
 away by the rivers ; so that the result on the sea-bed is, that as the 
 clay-deposit which was derived from shore denudation approaches 
 a river mouth, the mud which the river brings down causes il 
 to extend out to sea in an expanded fan-like form. So that whil 
 the coast clay has its greatest extension in the line of the coast, il 
 follows that the river clay has its greatest extension in the line of tl 
 river, which is usually at right angles to the coast. The river 
 is also much thicker than the shore clay, for it is the mud derive 
 from a large area of land, and hence it sometimes persists unchan< 
 through several geological formations, while the shore-deposits 
 come altered in their mineral materials. There can be no doubt that 
 the rusty colour of many sandstones is due to the decomposition 
 mica after it was deposited, so that the iron was set free as an oxide. 
 
HORIZONTAL SEQUENCE OF STRATA. 51 
 
 Other minerals which occur abundantly in some crystalline rocks, 
 such as hornblende and augite, yield iron ore, and whenever volcanic 
 rocks, especially basalts, decay, a deposit of iron ore is formed from 
 the iron they contained. Professor Kamsay has, in North Wales, 
 shown that the volcanic rocks associated with the Tremadoc beds, 
 on being traced to some distance, pass into a deposit of pisolitic iron 
 ore ; and a relation has been observed between the iron ores and the 
 basalts of County Antrim in Ireland. These groups of minerals, 
 micas, hornblendes, and augites, also contain a large amount of 
 magnesia, and when a rock like the magnesian limestone or 
 dolomite of the Permian series is met with, it is reasonable to 
 suppose that the magnesia was derived originally from the decay 
 of minerals which contained that substance. The crystalline rocks 
 are also the only known source of lime, and it has already been 
 indicated that the decaying vegetation on the sea-shore charges the 
 water with a sufficient amount of carbonic acid to enable it to 
 dissolve the lime and hold it suspended in an invisible form. 
 Where the rivers bring down much lime, or where the coasts are 
 very slowly worn away, or where animal life abounds exuberantly 
 near to the coast, deposits of limestone accumulate close to shore. 
 The Coralline Crag in the east of Suffolk is an organic deposit of 
 this kind, and deposits similar to that one are said to be forming 
 near to Tierra del Fuego. But when the shore limestone results 
 chiefly from evaporation of the sea, the deposit is necessarily largely 
 composed of materials which have no relation to living structures. 
 And it follows from what has already been said about the dis- 
 tribution of sands and clays, that when we go out to sea beyond 
 the limits to which sediments are carried from the shore, the only 
 deposit forming on the bot'tom will be limestones, chiefly constructed 
 from the skeletons of animals which live in the ocean. 
 
 Horizontal Sequence of Rocks. We have here been considering 
 in a general way the order in which deposits become arranged on the 
 sea-bed when the parent rock is granite or some such crystalline sub- 
 stance. Bearing this in mind, it will be evident that as the shore is 
 receded from, there is a necessary horizontal sequence ofrocJcs in the order: 
 sand, clay, limestone. Of course, it is possible that the coast might 
 consist entirely of quartzite, which is a rock formed of quartz, when 
 no clay could possibly be produced by wearing it away ; or the shore 
 rocks might be formed of lavas in which little or no quartz exists in 
 grains, and then of course no sand can be produced, and the whole rock 
 will break up into clay. And irregularities of the sea-bed and animal 
 growth may cause limestones to accumulate which are quite indepen- 
 dent of the positions of clays and sands. Such deposits, however, are 
 rather the exception than the rule. And it may be held as generally 
 true that all crystalline rocks decay into materials which become sand- 
 stones, clays, and limestones. And when a cliff composed of layers of 
 these rocks, like . the Yorkshire coast, comes again to be worn away 
 and separated into the mineral substances of which it consists, the 
 materials will be sorted out as before and rearranged parallel to the 
 coast in the order sand, clay, limestone. There are many examples 
 
52 SEQUENCE OF STRATA. 
 
 of such reconstructed deposits in the British strata, but they may 
 generally be detected by containing fragments of the old rocks and 
 fossils which belong to more ancient periods. Thus the Portland 
 oolite of Dorsetshire includes many fossils derived from the Carboni- 
 ferous limestone ; the ISTeocomian sands of Bedfordshire and Cam- 
 bridge contain some fossils which may have been derived from the 
 destruction of the upper part of the oolitic rocks. The Eed Crag of 
 Suffolk contains fossils derived from many of the Secondary and Ter- 
 tiary strata. While in the Boulder Clay, rock fragments and fossils 
 may often be obtained from an immense variety of formations. 
 
 Vertical Sequence of Rocks. We may carry this generalisation 
 concerning the order of horizontal deposits one step farther by remark- 
 ing that as pebble beds and sand form near to shore, and clays and 
 organic limestones farther out to sea, we have in the mineral character 
 of the deposit a rough means of discovering the general direction in 
 which land existed at the time when a geological formation was accu- 
 mulating, if we can only determine the horizontal sequence of its 
 mineral material It follows from the horizontal order of rocks that 
 there must be a similar vertical order or succession in time, because 
 land is always in process of being upheaved or depressed, and is there- 
 fore enlarged or diminished in area, so that the positions of the coasts 
 
 Fig. I2. 1 Sequence of Deposits on a Sea-bed 
 
 advance out to sea from their former position or retreat in the reverse 
 direction. This may be better understood from a diagram (fig. 12). 
 Here we suppose a coast of crystalline rocks (g) to be destroyed by 
 the sea, and the result is seen in the deposit of (s) sands, (c) clays, 
 and (I) limestones. Then if the land (g) is depressed so that high- 
 water mark stands at x, it follows that the point from which the 
 deposited materials are derived being farther inland (p), the sediment 
 cannot be carried by tidal movement so far out to sea. Hen< 
 a new sand will be deposited which will be continuous with tl 
 old sand (s) but since sand can only be carried a definite distain 
 from shore, it results that after the requisite depression no part 
 the new sand will rest upon the old deposit. And then a nei 
 clay will be deposited on the top of the old sand (*) ; and a new linn 
 stone on the old clay (c). If we suppose, again, the sinking dowi 
 of the land to progress a stage beyond so that the high- water marl 
 extends further inland, then as the deposits always retain the same 
 order and relative distances from the shore, it results that the sane 
 will accumulate still farther away from the area where we saw it origi- 
 nally (fig. 1 3) ; a third clay will be formed upon the second sand, and 
 third limestone upon the second clay. Here, then, is a vertical sequence 
 of sand, clay, limestone, resulting from the removal of the licav: 
 1 Seeley : Annals and Mag. Xat. Hist., December, 1867. 
 
VERTICAL SEQUENCE OF STRATA. 
 
 53 
 
 sediments to a greater distance in consequence of the continuous 
 depression and recession of the land from which they were derived. 
 
 Fig. 13. Deposits formed when the shore is sinking. 
 
 And whenever sediments succeed each other from below upward in 
 this order, it must be regarded as an evidence that the shore-line was 
 receding from the area during the whole period of time which they 
 represent. If, on the other hand, the land which is exposed to 
 destruction by the ocean were to be uplifted, so that the place whence 
 the deposited material originated would go farther out to sea, by denu- 
 dation of the shore, and the sands near shore, sand would be carried 
 farther out to sea so as to be spread over clay, and similarly clay 
 
 Fig. 14. Deposits formed when the shore is rising. 
 
 would be spread over the previously formed limestone. If, then, the 
 vertical sequence of rocks, limestone, clay, sand, is met with, it may 
 be regarded as evidence that during the whole of the geological time 
 which is represented by those deposits, the land was in process of 
 being upheaved, so that the shore-line was approaching the place 
 where the deposits were forming and are now seen in section in that 
 order. Therefore, since there are necessary limits over which a 
 formation preserves the same mineral character, it cannot be identi- 
 fied over very wide areas by this means. But strata can be traced 
 to distant regions by using this kind of evidence to discover the 
 physical conditions which limited, determined, and changed their 
 mineral characters, and influenced the distribution of life over the 
 geographical areas which they occupy. Hereafter we shall see how 
 these principles are practically applied. 
 
 )If a formation consists mainly or even largely of sands, we may 
 ect to find evidences that it was deposited in shallow water, 
 and possibly near to shore. Of this we have familiar examples in 
 the ripple-marked sandstones, footprints, sun-cracks, and such like 
 phenomena, which characterise the New Red Sandstone of Cheshire, 
 and parts of the Hastings Sands in Sussex. In the clays we rarely 
 observe any indications of shallow-water conditions ; and though 
 land animals and plant remains are often found in clays, these 
 rather indicate the influx of rivers into the ocean than relative 
 nearness to land. Hence it may be inferred that if a clay is super- 
 imposed upon a sand, we are entitled to conclude that the coast, which 
 was the source from which the material was derived which accumur 
 lated on the sea-bed to form the sandstone below, became de- 
 pressed in the succeeding age, so that the source of the deposited 
 material was removed farther away, and though sands would have 
 continued to be formed, they were deposited at a distance so far off 
 that the only material which became in the British area super- 
 imposed on the sand was the finer flocculent substance which forms 
 
54 
 
 CONDITIONS OF DEPOSITION OF STRATA. 
 
 clay. If we further suppose such an ancient coast-line to still 
 farther recede from the district when deposits are going on, so that 
 not only the sand does not reach the area, but the distance is too 
 great for even the clay to be transported so far, then none but 
 calcareous deposits can take place, due to evaporation of water or 
 evaporation combined with the agencies of plant and animal life. 
 It does not follow that the limestones were formed in deep water, 
 it is simply necessary that the sea-bed should be free from sediment, 
 or that the sediment should accumulate so slowly that its import- 
 ance is lost in the calcareous features of the deposit. Presuming 
 that these general principles are sound, then the lower Secondary 
 strata in the South of England indicate to us a great oscillation in 
 level of the land which furnished the materials for the strata. This 
 may be perhaps best expressed in a tabular form. 
 
 Diagram showing the Altered Position of Land in the South of England 
 relatively to the Strata during the Secondary Period. 
 
 
 
 Mineral Character 
 
 
 
 
 
 
 which some of the 
 
 
 
 
 
 Theoretical 
 Horizontal 
 Sequence. 
 
 Strata should pre- 
 sent if traced to 
 adjacent areas nearer 
 to the source from 
 which the sedi- 
 
 Prevailing 
 Mineral 
 Character 
 of the 
 Formations. 
 
 Names of 
 Strata in 
 South of 
 England. 
 
 Names of 
 Lithological 
 Groups of 
 Strata. 
 
 
 
 ments were de- 
 
 
 
 
 
 
 rived. 
 
 
 
 
 
 
 
 Limestone 
 
 Chalk ^1 
 
 
 
 
 
 Sand 
 
 Upper 
 
 
 
 
 
 
 
 Green- } 
 
 Cretaceous. 
 
 
 
 
 
 sand 
 
 
 
 
 Sand 
 
 Clay 
 
 Gault J 
 
 
 
 
 f 
 
 Sand 
 
 N.Down^ 
 
 
 
 
 
 
 Sand 1 
 
 
 tD -N 
 
 
 Land 
 
 Sand, &c. 
 
 Wealden V 
 
 Psammolithic 
 
 fl 1 
 '3 
 
 
 
 Sand, &c. 
 
 Purbeck | 
 
 
 S 
 
 
 [ 
 
 Sand, &c. 
 
 Portland ) 
 
 
 J.2 
 
 
 ( 
 
 Clay 
 
 Kimme- "^ 
 
 
 c3 S 
 
 
 
 
 ridge 
 
 
 73 Q 
 
 
 1 
 
 
 Clay 
 
 
 tt) 03 
 
 Land 
 
 Sand 
 
 Clay, &c. 
 
 Ampt- 
 hill 
 
 \ 
 
 Pelolithic. 
 
 ill 
 
 
 
 
 Clay 
 
 
 
 n ^ 
 
 
 I 
 
 Clay 
 
 Oxford 
 
 
 
 I 5 
 
 
 
 
 Clay 
 
 
 
 
 
 
 / 
 
 Corn- 
 
 
 
 i-3 
 
 
 
 
 brash 
 
 
 
 
 
 
 
 Forest 
 
 
 
 
 
 
 
 Marble 
 
 
 
 Land 
 
 Sand 
 
 Clay 
 
 Lime- 
 
 Great 
 
 
 
 
 
 
 stones, -| 
 
 Oolite 
 
 
 Oolitic. 
 
 1* j ' 
 
 
 
 &c. 
 
 Fuller's 
 
 
 
 d "d 
 
 
 
 
 Earth 
 
 
 
 tjD 3 . 
 
 SS 
 
 
 
 
 Infe- 
 
 
 
 
 
 
 
 rior 
 
 
 
 -S 1 1 
 
 
 
 I 
 
 Oolite j 
 
 
 
 irj 
 
 Land 
 
 Sand 
 Land 
 
 i 
 
 Clay 
 Sand 
 
 Lias 
 Trias 
 
 Lias. 
 Trias. 
 
EVIDENCES OF UPHEAVAL AND DEPRESSION. 
 
 55 
 
 On the right of the names of the strata are the lithological 
 names of groups into which they are arranged. On the left of the 
 column of strata are the names of the prevailing mineral substances 
 of which they consist, and in the succeeding columns farther to the 
 left are the theoretical deposits which may be supposed to have 
 taken place in adjacent areas if they were always thrown down 
 in the order here given ; and it may be noticed that the land recedes 
 farther from the vertical section from the Trias up to the middle 
 of the Oolites, and then, by elevation, approaches again the area which 
 is now the south of Britain. The Cretaceous rocks are a group of 
 another kind, and include within themselves subdivisions showing 
 three types of mineral character. 
 
 Throughout the period of the Psammolithic series, the same 
 rocks continued to be denuded, as is proved by some fossils they 
 contain which do not belong to the formation, but have been 
 derived from older strata. This group in Yorkshire becomes re- 
 placed horizontally by a pelolithic representative called the Speeton 
 Clay, which being marine, shows no trace of the minute subdivisions 
 which have been recognised in the South of England, and often 
 made the basis of classification. It is a continuous clay between the 
 top of the Kimmeridge Clay and the lower Cretaceous rocks. Simi- 
 larly the Pelolithic group in Yorkshire shows towards its base a 
 tendency to become psammolithic in the Kelloway rock, and 
 southward in the section, at Boulogne, its upper part is putting on 
 Psammolithic characters. The Oolitic group when traced towards 
 Yorkshire becomes in the main psammolithic, thus demonstrating 
 the direction in which land existed relatively to the British area 
 which was being covered with these deposits, and enabling us to 
 infer, with the aid of larger knowledge of European formations, the 
 essential directions of the ancient coast-lines. 
 
 A TABLE OF THE CHIEF BRITISH STRATA 
 
 Arranged in the order in which they rest upon each other, with an 
 indication of the prevalent mineral character of the beds, and some 
 of their chief variations. 
 
 NAMES. 
 
 DESCRIPTION. 
 
 GEOGRAPHICAL CIRCUMSTANCES OF 
 THE DEPOSIT. 
 
 Peat and Bog 
 
 Black vegetable growth 
 on land 
 
 Still forming, especially in the fens 
 of East of England, in West of 
 Scotland, Wales, and much of 
 Ireland. Contains shell beds. 
 
 Valley Gravels 
 and Brick 
 Earth 
 
 Gravels, sand, and 
 sandy clay 
 
 Formed in present river valleys 
 when they were estuaries, and 
 the level of the land was lower. 
 
 Upper Boulder 
 Clay ' 
 
 Clay, with angular 
 fragments of local 
 rocks 
 
 Formed by action of ice when the 
 climate was colder, and level of 
 land higher than now. 
 
TABLE OF TERTIARY STRATA. 
 
 NAMES. 
 
 DESCRIPTION. 
 
 GEOGRAPHICAL CIRCUMSTANCES OF 
 THE DEPOSIT. 
 
 Middle Glacial 
 
 Chiefly sands 
 
 A marine deposit with Arctic shells, 
 
 Sands 
 
 
 formed when the land was sub- 
 
 
 
 merged 
 
 Lower Boulder 
 
 Clay, with angular 
 
 A formation produced by icebergs 
 
 Clay 
 
 fragments of rocks 
 
 and great glaciers from the north. 
 
 
 from north and 
 
 
 
 north-east 
 
 
 
 Chillesford Beds 
 
 Sand and clay 
 
 Only recognised in Norfolk and 
 
 >> 
 
 
 
 Suffolk. 
 
 L 
 d 
 
 Red Crag 
 
 Sands and shell beds 
 
 A beach, estuarine, and shallow- 
 
 1 
 
 
 
 water deposit ; phosphatic nodules 
 
 EH 
 
 
 
 at its base. 
 
 n 
 
 0) 
 
 Coralline Crag 
 
 Sands below, organic 
 
 Formed in a tranquil depth of sea, 
 
 1 
 
 
 limestone above 
 
 many shells now found in Medi- 
 
 & 
 
 
 
 terranean ; phosphatic nodules at 
 
 
 
 
 its base. 
 
 [Here followed a long period of time during which this part of Europe was 
 
 dry land, and few or no British deposits formed in it are pieserved.] 
 
 
 Hempstead Clays and marls 
 
 Chiefly fresh-water and estuarine, j 
 
 
 Beds 
 
 
 marine above, found only in Isle 
 
 
 
 
 of Wight. 
 
 
 Bembridge Beds 
 
 Limestone below, 
 
 At first lacustrine, then marine, 
 
 
 
 marls above 
 
 afterwards estuarine. 
 
 
 Osborne Series 
 
 Sands and marls 
 
 Chiefly fresh-water and estuarine. 
 
 
 Headon Series 
 
 Alternations of lime- 
 
 Alternations of lacustrine and 
 
 
 
 stones and marls 
 
 fluviomarine conditions, with a 
 
 
 
 
 marine bed in the middle. 
 
 
 Upper Bagshot 
 
 Sand 
 
 A few marine fossils. 
 
 8 
 
 Sand 
 
 
 
 
 
 Barton Clay 
 
 Blue clay 
 
 Profusion of marine fossils, nearly 
 
 w 
 
 
 
 all extinct, in Hampshire and 
 
 b 
 
 
 
 Isle of Wight. 
 
 o 
 f>> 
 
 Bracklesham 
 
 Sand, sandy clay, and 
 
 Fossils marine, but some beds of 
 
 E? 
 g 
 
 Beds 
 
 lignite 
 
 lignite grew where found. Ex- 
 
 'B 
 
 
 
 tends to Sussex. 
 
 EH 
 
 Lower Bagshot 
 
 Sand 
 
 Fossil leaves of land plants in beds 
 
 Si 
 
 Sand 
 
 
 of white pipe - clay ; probably 
 
 1 
 
 
 
 lacustrine. 
 
 | 
 
 London Clay 
 
 Clay, sandy at base 
 
 A marine deposit, near to land at 
 
 HH 
 
 
 in Hampshire and 
 
 base, middle, and top, with life 
 
 
 
 Sussex 
 
 of an Asiatic type. 
 
 
 Oldhaven Beds 
 
 Rounded pebbles, be- 
 
 Marine, with current bedding, only 
 
 
 
 coming sands to east 
 
 found in east of London basin. 
 
 
 Woolwich and Sands in east; sands, 
 
 Marine in east, estuarine under 
 
 
 Beading Beds 
 
 clays, and pebble 
 
 London ; marine at base in Berk- 
 
 
 
 beds in middle, clays 
 
 shire ; Lacustrine above. 
 
 
 
 in south-west 
 
 
 
 Thanet Sands 
 
 Sands 
 
 Thinning to the west, fossils 
 
 
 
 marine. 
 
 [Here follows a period unrepresented by any British deposit, but not neces- 
 
 sarily of immense duration, although the fossils in the deposits entirely 
 
 change.] 
 
TABLE OF SECONDARY STRATA. 
 
 57 
 
 NAMES. 
 
 DESCRIPTION. 
 
 GEOGKAPHICAL CIRCUMSTANCES OF 
 THE DEPOSIT. 
 
 /Upper Chalk 
 
 Soft white limestone, 
 
 Marine organic deposit, some- 
 
 
 
 with bands of con- 
 
 times 1 200 feet thick, formed 
 
 
 
 cretions called flints 
 
 beyond the limits to which sedi- 
 
 
 
 
 ment was carried during a period 
 
 
 
 
 when land was depressed. 
 
 
 Lower Chalk 
 
 Hard white limestone 
 
 Organic deposit, formed chiefly of 
 
 
 
 without flints 
 
 foraminifera, mostly beyond the 
 
 
 
 
 limit of sediment, but with occa- 
 
 03 
 
 2 
 
 
 
 sional seams of clay and beds of 
 
 1 
 
 
 
 fine sand. 
 
 V) 
 
 Chalk Marl 
 
 Clayey chalk 
 
 Organic deposit, just within limits 
 
 1 
 
 
 of sediment. 
 
 o ' 
 
 Chloritic Marl 
 
 Sandy chalk 
 
 Marine deposit, formed not far 
 
 _c3 
 
 
 
 from land. 
 
 s 
 
 Upper Green - 
 
 Sand and calcareous 
 
 Deposit formed near shore ; north 
 
 o 
 
 sand 
 
 sandstone with white 
 
 of King's Lynn merges into the 
 
 
 
 flint called chert, and 
 
 Hunstanton Red Limestone. 
 
 
 
 phosphatic nodules 
 
 
 
 Gault 
 
 Blue clay, often mica- 
 
 Formed at some distance from 
 
 
 
 ceous, with much iron 
 
 shore in the south at Folkestone; 
 
 
 
 pyrites, and phos- 
 
 north of Norfolk probably merges 
 
 
 
 phatic nodules 
 
 in Carstone, which is called 
 
 V 
 
 
 Upper Neocomian. 
 
 /North Down 
 
 Sandstone, with some 
 
 Marine shallow-water formation, 
 
 
 Sands, Lower 
 
 beds of clay in Isle 
 
 which is not separable from the 
 
 
 Greensand or 
 
 of Wight 
 
 marine representatives of the 
 
 
 Upper Neoco- 
 
 
 Wealden and Purbeck in the 
 
 
 mian 
 
 
 middle of England. 
 
 
 Wealden 
 
 Alternations of sands 
 
 Almost entirely fresh - water in 
 
 
 
 and clays, with occa- 
 
 South of England, probably 
 
 0> 
 
 JH 
 
 
 sional fresh - water 
 
 formed in a lake or estuary, but 
 
 H 
 
 <u 
 
 tfl 
 
 
 limestones 
 
 becoming marine sands north of 
 
 
 
 
 the Thames, and marine clay at 
 
 -s / 
 
 
 
 Speeton ; sands show footprints. 
 
 . \ 
 
 >-H 
 
 Purbeck 
 
 Alternations of sands, 
 
 Chiefly lacustrine fresh - water, 
 
 O 
 I 
 
 
 marls, and lime- 
 
 with some marine beds, merging 
 
 
 
 stones 
 
 in the marine Neocomian sands 
 
 OS 
 
 8 
 
 
 
 north of the Thames, and repre- 
 
 PH 
 
 
 
 sented by marine clay at Speeton ; 
 
 
 
 
 fossil forest in Isle of Purbeck. 
 
 
 Portland 
 
 Sand below, oolitic 
 
 Entirely marine ; limestone char- 
 
 
 
 limestone above 
 
 acters disappear to the north of 
 
 
 
 
 Oxford, when it forms the lower 
 
 
 
 
 part of the Neocomian Sands ; is 
 
 
 Kimmeridge 
 
 Dark blue clay with 
 
 clay at Speeton. 
 Marine clay, thickens north and 
 
 
 Clay 
 
 septaria ; is some- 
 
 south from Ely ; is sometimes 
 
 
 
 times bituminous 
 
 sandy at top and bottom. 
 
 ' 
 
 Ampthill Clay 
 
 Ampthill clay is an 
 
 Marine. Ampthill clay occurs be- 
 
 22 
 
 and Coralline 
 
 alternation of clays 
 
 tween Oxfordshire and York- 
 
 
 
 Oolite 
 
 with earthy lime- 
 
 shire ; south and north it is re- 
 
 *< 
 
 
 stones 
 
 placed by the Coralline Oolite. 
 
 a 
 
 Oxford Clay 
 
 Blue clay with lime- 
 
 Marine. The Elsworth Rock is a 
 
 Q 
 
 with Elsworth 
 
 stones near top and 
 
 limestone in Huntingdon and 
 
 3 
 
 |PH 
 
 Rock and 
 Kelloway 
 
 bottom ; sand at base 
 in north 
 
 Cambridgeshire near the top. The 
 Kelloway Rock in Wiltshire is 
 
 
 Rock 
 
 
 a concretionary limestone ; in 
 
 
 \ 
 
 
 Yorkshire it is a calcareous sand- 
 
 1 
 
 
 stone at its base, in places. 
 
TABLE OF LOWER SECONDARY STRATA. 
 
 NAMES. 
 
 DESCRIPTION. 
 
 GEOGRAPHICAL CIRCUMSTANCES OF 
 THE DEPOSIT. 
 
 Cornbrash 
 
 Thin limestones, with 
 
 Marine, sometimes shelly ; the only 
 
 
 
 occasional seams of 
 
 bed of the series which keeps its 
 
 
 
 clay 
 
 limestone character from Dorset- 
 
 
 
 
 shire to Yorkshire. 
 
 
 Forest Marble 
 
 Thin bedded shelly 
 
 Marine, sometimes ripple-marked ; 
 
 
 
 limestone, with 
 
 limited to South of England. Re- 
 
 
 
 seams of clay 
 
 presented in middle of England 
 
 
 
 
 by Blisworth clay. 
 
 
 Bradford Clay 
 
 Brown clay 
 
 Marine ; only in South of England, 
 
 
 
 
 probably represented in North- 
 
 
 
 
 ampton, and north, by the upper 
 
 
 
 
 estuarine sandy clays. 
 
 
 Great Oolite 
 
 White shelly lime- 
 
 Marine ; probably represented in 
 
 <8 
 
 
 stones and oolites 
 
 Yorkshire by the upper shale and 
 
 3 
 
 
 
 sandstone, and in Oxfordshire by 
 
 Q 
 
 
 
 the Stonesfield slate. 
 
 h 
 
 Fuller's Earth 
 
 Brown clay and sand, 
 
 Marine ; probably represented by 
 
 1 
 
 
 with a middle bed of 
 
 the Lincolnshire limestone, and 
 
 ^ 
 
 
 limestone 
 
 grey limestone of Yorkshire 
 
 
 
 
 coast. 
 
 
 Inferior Oolite 
 
 Yellow limestones, 
 
 Marine ; represented by the Colly- 
 
 
 
 sometimes oolitic, 
 
 weston slate, Northampton sands, 
 
 
 
 sometimes marly 
 
 and lower estuarine beds in Nor- 
 
 
 
 
 thampton ; and by the estuarine 
 
 
 
 
 lower sandstone and shales on the 
 
 
 
 
 Yorkshire coast. Worked for 
 
 
 
 
 iron ore and coal. 
 
 
 Midford Sands 
 
 Yellow and brown 
 
 Marine ; perhaps represented in 
 
 
 
 sands 
 
 Yorkshire by the Blue Wick 
 
 
 
 
 sands, and Dogger series. 
 
 
 Upper Lias 
 
 Blue clay, with thin 
 
 Marine ; contains much iron py- 
 
 
 
 beds of earthy lime- 
 
 rites and jet, and yields alum. 
 
 
 
 stone 
 
 
 
 Middle Lias or 
 
 Clayey limestones, 
 
 Marine ; distance from shore in- 
 
 
 Marlstone 
 
 with micaceous 
 
 creased. 
 
 
 
 sands, and clays at 
 
 
 J 
 
 
 the base 
 
 
 J 
 
 Lower Lias 
 
 Thin earthy lime- 
 
 Marine. The whole Lias formation 
 
 
 
 stones, alternating 
 
 extends through England, un- 
 
 
 
 with thick beds of 
 
 changed from Dorsetshire to 
 
 
 
 blue clay 
 
 Yorkshire. 
 
 
 Rhsetic Beds 
 
 Sands below, then 
 
 Marine, very shallow water. Con- 
 
 
 
 shales and lime- 
 
 tains thin bed of fish bones. 
 
 
 
 stones 
 
 
 
 Keuper 
 
 Marls and sand- 
 stones above, sand- 
 
 Probably formed in a salt lake, con- 
 tains rock salt, and shows ripple 
 
 4 
 
 
 stones below 
 
 marks, footprints of animals, false 
 
 'C 
 
 
 
 bedding, and other evidences of 
 
 H 
 
 
 
 shallow water. 
 
 
 Bunter 
 
 Alternations of sand- 
 
 Contains no marine fossils, but 
 
 
 
 stone, and conglo- 
 
 these may have been dissolved 
 
 
 
 merates usually red 
 
 away. 
 
 
 Upper Permian 
 
 Red sandstones 
 
 Probably marine, near shore. 
 
 a" 
 
 Middle Per- 
 
 Magnesian limestone 
 
 Marine. 
 
 
 mian 
 
 and marl slate 
 
 
 <3 
 
 Lower Permian 
 
 Red sandstone, marl, 
 
 Probably marine, near shore. 
 
 <y 
 
 PH 
 
 
 conglomerate, and 
 
 
 
 
 breccia 
 
TABLE OF PRIMARY STRATA. 
 
 59 
 
 NAMES. 
 
 DESCRIPTION. 
 
 GEOGRAPHICAL CIRCUMSTANCES OP 
 THE DEPOSIT. 
 
 /Coal Measures 
 
 Alternations of sand- 
 
 Fresh-water estuary and land con- 
 
 
 
 stone, shale, iron- 
 
 ditions, very rarely marine. 
 
 05 
 
 
 stone, fire-clay, and 
 
 
 
 
 seams of coal 
 
 
 
 
 Middle Carboni- 
 
 Chiefly thick sand- 
 
 Marine, but in shallow water near 
 
 
 
 ferous 
 
 stones and shales, 
 
 to land, including Yoredale beds, 
 
 s 
 
 
 with some coal seams 
 
 Millstone Grit, and Gannister 
 
 c8 
 
 
 
 beds. 
 
 
 
 Carboniferous 
 
 Organic limestones 
 
 Marine ; much of it formed of 
 
 
 Limestone 
 
 with shales, chiefly 
 
 corals, crinoids, and shells living 
 
 
 and Shale 
 
 at base 
 
 beyond the limits of sediment. 
 
 
 'Petherwyn and 
 
 Limestones in South 
 
 Marine ; represented by Upper Old 
 
 
 Pilton Group 
 
 Devon, sandstones, 
 
 Red Sandstone north of the 
 
 
 
 micaceous and cal- 
 
 Severn, and thought to have been 
 
 
 
 careous, with shales 
 
 formed in a great lake. Fresh- 
 
 
 
 
 water shell in Ireland (Anodonta). 
 
 J" 
 
 Plymouth Lime- 
 
 Limestones and shales 
 
 Marine, farther from shore ; repre- 
 
 fl 
 
 stone and 11- 
 
 in the south, chiefly 
 
 sented by Middle Old Red Sand- 
 
 O 
 
 fracombe 
 
 shales in the north 
 
 stone, which contains many fish 
 
 1 
 
 Beds 
 
 
 related to those now living in 
 
 ( i 
 
 
 
 fresh-water. 
 
 
 Fowey and Lyn- 
 
 Sandstones and mica- 
 
 Marine, near to shore ; the Lower 
 
 
 ton Beds 
 
 ceous shales, some- 
 
 Old Red Sandstone includes thick 
 
 
 
 times calcareous 
 
 conglomerates, and like the whole 
 
 
 
 
 formation is thought to have been 
 
 
 \ 
 
 
 formed in a lake. 
 
 
 ^Ledbury Shales 
 
 Shales above, and mi- 
 
 Marine beds, near to land ; chiefly 
 
 
 and Down- 
 
 caceous sandstone 
 
 seen in the border counties of 
 
 
 ton Sandstone 
 
 below 
 
 England and Wales. 
 
 
 Upper Ludlow 
 Shales 
 
 Sandy shales, with 
 thin shelly lime- 
 
 Contains a thin bed formed of fish- 
 bones. Marine, distance from 
 
 
 
 stones 
 
 land varying. 
 
 
 Aymestry Lime- 
 
 A concretionary lime- 
 
 Marine ; formed when sediment 
 
 
 stone 
 
 stone, with some 
 
 was not abundant. 
 
 
 
 beds of shale 
 
 
 
 Lower Ludlow 
 
 Sandy shales and mud 
 
 Marine ; perhaps derived chiefly 
 
 
 Beds 
 
 stones, with a few cal- 
 
 from decomposing lavas and vol- 
 
 
 
 careous concretions. 
 
 canic materials. 
 
 
 
 Wenlock Lime- 
 
 Grey concretionary 
 
 Marine, chiefly beyond the limit 
 
 S 
 
 stone 
 
 limestone, full of 
 
 of sediment ; sometimes oolitic in 
 
 : 
 
 
 corals andencrinites, 
 
 Malvern Hills. 
 
 co 
 
 
 with some shale 
 
 
 
 Wenlock Shale 
 
 Thick shales, with oc- 
 
 Marine, distant from shore, but 
 
 
 
 casional septarian 
 
 perhaps derived from decaying 
 
 
 
 concretions of car- 
 
 volcanic rocks. 
 
 
 
 bonate of lime 
 
 
 
 Woolhope Beds 
 
 Concretionary lime- 
 
 Marine, receding from shore. 
 
 
 
 stones, with shale 
 
 
 
 
 and sandstone 
 
 
 
 May Hill Group, 
 
 Brown and yellow 
 
 Marine ; shore formation. 
 
 
 or Upper 
 
 sandstones and con- 
 
 
 
 Llandovery 
 
 glomerates, with a 
 
 
 
 Beds 
 
 shell limestone called 
 
 
 
 ' 
 
 Pentameras lime- 
 
 
 
 | 
 
 stone 
 
 
6o 
 
 TABLE OF LOWER PRIMARY STRATA. 
 
 NAMES. 
 
 DESCRIPTION. 
 
 GEOGEAPHICAL CIKCUMSTANCES OF 
 THE DEPOSIT. 
 
 
 / 
 Upper Bala 
 
 Conglomerates and 
 
 Marine ; corresponds with Lower 
 
 
 Group 
 
 grit above, slates be- 
 
 Llandovery. 
 
 
 
 low with the Hir- 
 
 
 
 
 nant limestone 
 
 
 | 
 
 Middle Bala 
 
 Earthy slates and 
 
 Marine ; corresponds with Caradoc 
 
 jQ 
 
 Group 
 
 sandstone rocks with 
 
 sandstone. Includes great thick- 
 
 8 
 
 
 Bala limestone near 
 
 nesses of lavas, probably derived 
 
 6' 
 
 
 the top, occasionally 
 
 from small islands not far from 
 
 tj 
 
 
 oolitic 
 
 land. 
 
 1 
 
 Lower Bala 
 
 Dark earthy mica- 
 
 Marine ; corresponds with Upper 
 
 O-i 
 
 ta 
 
 Group 
 
 ceous slates, with 
 
 Llandeilo flags; thick interstrati- 
 
 p' 
 
 
 thin limestones 
 
 fied volcanic rocks. 
 
 
 A r e n i g, or 
 
 Sandstones and slates, 
 
 Marine; corresponds to Lower Llan- 
 
 
 Skiddaw 
 
 with some shales and 
 
 deilo ; thick interstratified vol- 
 
 
 Group 
 
 pisolitic iron 
 
 canic rocks, shallow water. 
 
 \ 
 
 
 
 ^remadoc Slates 
 
 Sandstones, iron- 
 
 Marine, shallow water ; interstra- 
 
 
 
 stained slates, shales, 
 
 tified with volcanic ash beds. 
 
 d 
 
 
 flags, and pisolitic 
 
 
 .3 
 
 
 iron ore 
 
 
 
 
 Ffestiniog 
 
 Black slates, iron- 
 
 Marine ; corresponds to Middle 
 
 1 
 
 Group 
 
 stained above, hard 
 
 and Upper Lingula flags ; inter- 
 
 
 
 
 sandstones below 
 
 stratified are many beds of vol- 
 
 o> 
 
 
 
 canic ash and lava, sometimes 
 
 
 
 
 
 ripple-marked. 
 
 g 
 
 Menevian Group 
 
 Black micaceous slates 
 
 Marine ; formerly called Lower 
 
 
 
 and flags with sand- 
 
 Lingula flags. 
 
 
 
 stone 
 
 
 t 
 
 i 
 
 
 
 /Llanberis Slates 
 
 Purple and green 
 
 Marine ; with clay pebbles in the 
 
 *! 
 
 slates and grits 
 
 slates, derived from an older clay 
 
 i-i 
 
 
 formation. 
 
 Q 1 Harlech Grits 
 
 Grits, sandstones, green 
 
 Marine ; evidence in some places 
 
 at 
 
 and purple slates 
 
 of being dry between tides. 
 
 /'Pebidian 
 
 Altered shales resting 
 
 No fossils ; contains thick volcanic 
 
 i 
 
 on conglomerates 
 
 rocks. 
 
 '5 lArvonian 
 
 Altered rocks, with a 
 
 No fossils ; largely volcanic. 
 
 a' 
 
 felspathic base, con- 
 
 
 i 
 
 
 
 
 taining grains of 
 
 
 1 
 
 
 quartz 
 
 
 PH 
 
 Dhnetian 
 
 Quartzites, and altered 
 
 No fossils yet found ; largely vol- 
 
 
 
 shales and limestones 
 
 canic. 
 

 
 
 CHAPTER VI. 
 
 PETROLOGY. 
 
 Stratification. 
 
 SUPPOSING that the student has made himself acquainted, by exami- 
 nation, with the more common and important rocks, as limestone, 
 sandstone, and clay, various kinds of slates, basaltic, porphyritic, and 
 granite rocks, we proceed to inquire in what manner they are arranged 
 in the earth. 
 
 The best way of prosecuting this inquiry is to examine sections in 
 the field, open railway cuttings, quarries, and natural sections in the 
 cliffs on the seashore; comparing one area with another, so as to 
 class the phenomena and deduce general results. 
 
 The stratification of the aqueous rocks is the basis and foundation 
 of all geological investigation the great problems and deductions of the 
 science are based upon a clear understanding of lamination, bedding, 
 and stratification. 
 
 Arrangement of Rocks on the Surface. It might be very excus- 
 able before countries were cleared and cultivated, and before their 
 various mineral productions were employed and understood, to imagine 
 that the materials of the earth were heaped together in confusion ; but 
 at present such a notion will not stand the test of a moment's reflec- 
 tion. One district has chalk beneath the surface, another limestone, 
 a third coal, and a fourth granite, and these are never mixed or con- 
 founded together ; so that the most careless observer must conclude 
 that the different rocks are arranged after some definite and ascertain- 
 able method. These different rocks are not mere insulated patches 
 irregularly scattered through the country, but generally connected on 
 or beneath the surface in long ranges, which, as in the eastern half of 
 England, have their prevailing direction or strike from north-east to 
 south-west. Thus the chalk of the Yorkshire wolds is prolonged 
 through Lincolnshire, Norfolk, Suffolk, Bedfordshire, and Wiltshire, 
 into Dorsetshire, Sussex, and Kent ; the oolitic limestones range 
 through Lincolnshire, Northamptonshire, Gloucestershire, and Somer- 
 setshire ; and many other limestones, sandstones, and clays hold a 
 parallel direction. Hence it is that in proceeding from London 
 toward the south-west, west, or north-west of England, we cross so 
 great a variety of rocks and formations, and so many ranges of hills. 
 
 On proceeding from London to North Wales, after passing low, 
 
62 STRATA SEEN IN SECTIONS. 
 
 gravelly plains in the drainage of the Thames we climb by a long 
 slope the chalk hills of Oxfordshire, cross vales of clay and sand- 
 stone, ascend a range of oolitic limestone, traverse wide plains of 
 blue and red marl, arrive in districts where coal, iron, and limestone 
 abound, and finally see Snowdon composed in great measure of igneous 
 rock, slates, and sandstones. And if, in proceeding from London to 
 the Cumberland Lakes, we find the same succession of gravelly plains, 
 chalk hills, clay vales, limestone ranges, blue and red clays, coal, iron, 
 and limestone tracts, succeeded by the slate rocks which compose the 
 well-known mountain of Skiddaw, we conclude that something beyond 
 mere chance has brought together these rocks with such perfect 
 sequence and order. May we not reasonably conjecture that also in 
 the interior of the earth regularity of structure must equally prevail ? 
 
 Internal Arrangement of Rocks. This conjecture becomes cer- 
 tainty when we explore the relative position of rocks as displayed in 
 pits, quarries, railway cuttings, mines, and wells, or laid bare in cliffs 
 and ravines by the hand of nature. Here we see the rocks formed in 
 layers, strata, or tabular masses of various thickness, but always of 
 very great superficial or horizontal breadth or extent, and placed 
 parallel to or upon one another like the leaves of a book. These 
 layers are called strata. Along the edges and flanks of hills, in the 
 course of precipitous valleys, and by the margin of the sea, in the form 
 of cliffs, it is not difficult to recognise these facts or truths, it is 
 almost impossible to avoid perceiving them. 
 
 Many parts of the English coast present what is termed a natural 
 section of the rocks, and accordingly whoever visits the shores of 
 Northumberland, Yorkshire, Kent, Hampshire, Dorsetshire, Cornwall, 
 South Wales, or Cumberland, may easily observe for himself the 
 stratification of most of the limestones, sandstones, clays, and slates of 
 the different geological formations exposed. For most of the cliffs 
 are composed of distinct layers of rock, which are placed upon or 
 succeed one another in regular order, preserve a definite thickness, and 
 appear under the same or similar circumstances in many distant 
 places. In the interior of the country the same conclusion is to be 
 drawn from examining precipitous hills and deep valleys ; and even 
 in the flattest country art supplies the means of investigation which 
 nature has denied. The wells, pits, quarries, and mines, which have 
 been constructed, all display the same general truth, and lead us to 
 conclude that the principles and laws of stratification among rocks 
 is confined to no particular country, but all over the world, in 
 continents or in islands, it is certain and constant, so much so, that 
 deep pits are sunk for coal, and miners undertake extensive levels, 
 in full confidence that no exception to the laws of stratification will 
 affect the result of their enterprises. It is not a speculative truth, 
 but a practical law of nature, having the most extensive influence in 
 the whole theory of geology. 
 
 So many important facts respecting stratified rocks bear upon the 
 physical history of a county or district, that it is not easy to analyse 
 or realise them on paper in the exact order of their occurrence. But 
 

 CONDITIONS OF STRATIFICATION. 
 
 every student attentive to the subject cannot fail to discover, even in 
 a very limited district, that the different strata which appear above one 
 another are arranged in a certain constant order of succession. A stra- 
 tum which in any one situation is found beneath another will never, 
 in any other situation, be found above it ; in other words, their order is 
 never inverted unless the whole series has been turned upside down. 
 
 Superposition of Strata. As sometimes we neglect to bind in some 
 book a particular leaf, so Nature sometimes omits a particular rock ; 
 but she never misplaces them. Most stratified rocks, when exposed 
 in a sea cliff, quarry face, or river bank, are seen to be composed of a 
 number of parallel planes, or layers, or flat tabular masses which more 
 or less readily separate from each other. These strata or beds, whether 
 composed of sandstone, limestone, or clays, are superposed one upon 
 the other, and are classed under the group of rocks which are bedded 
 or superposed one upon the other. These beds are never misplaced, 
 whatever omission or non-deposition may have taken place; and 
 observation has determined that strata are arranged with respect 
 to one another in a certain constant order of succession. Strata vary 
 in thickness from fractions of an inch to many feet in thickness, 
 whereas laminae seldom occur an inch in thickness, varying from this 
 to the thinness of paper. 
 
 Inclination of Strata. Pursuing our investigation, we find that 
 the strata are generally so disposed that their planes of bedding or 
 broad surfaces of junction with each other are not exactly level or 
 parallel to the earth's spherical surface, but slope in some one direc- 
 tion, so as, in that direction, to sink deeper and still deeper into the 
 earth, and to be covered by other strata. 
 This slope, or deviation from the horizontal 
 position, is called the dip or inclination of 
 the strata; and the rocks are accordingly 
 said to dip or incline to this or that part of 
 the horizon or point of the compass. Dip 
 is the key to the structure of a country, 
 because it acquaints us with the relative 
 antiquity and lie of the beds. It is esti- 
 mated in degrees, but two observations are 
 generally required to find the direction and amount of the dip. Thus 
 in one side of the figure the beds appear highly inclined, and in the 
 other side but slightly inclined. 
 On one side the dip appears to 
 be to the north, on the other 
 side to the east. From these 
 observations the true dip is seen 
 to be NNE. Dip may be at 
 any angle, but if the angle is 
 more than 90 the beds are 
 overturned, and the dip is said 
 to be reversed. Thus owing to 
 folding, the order of the beds on the two sides of the section is re 
 
 Fig. 16. 
 
6 4 
 
 SYNCLINAL AND ANTICLINAL DIP. 
 
 versed. Horizontal beds necessarily have no dip. Dip is a property 
 of an inclined plane which causes the plane of a stratum to intersect 
 the plane of the horizon. 
 
 The angle of dip is locally often due to the solidity and resistance 
 to flexure of thick deposits. But the direction of dip of one deposit 
 usually governs that of the beds above and below. Changes in the 
 direction of dip are due to the ways in which the strata are folded. 
 In the mountains of Wales the dip constantly changes ; and therefore 
 dip enables us to discover the crumpling and folds of the earth's crust. 
 East of the Pennine chain the rocks all dip to the east ; and west of 
 that range there is in much of the country a corresponding dip to the 
 west. Dip, no matter how simple it may appear in a single section, 
 is always a part of a fold of the earth's crust. These folds are either 
 downward and trough-like, or upward and ridge-like, though the rocks 
 themselves often appear on the surface in forms which may suggest to 
 the eye neither one nor the other. Thus the chalk of the Chiltern 
 Hills dips to the south-east, and passing under the ground reappears 
 in the North Downs which dip to the north. Hence the chalk has 
 there a basin or trough-shaped fold, and this complex dip is termed 
 a synclinal dip. Whenever a stratum is inclined in two opposite 
 directions so that the dips converge or meet downward, it is synclinal. 
 Almost every coalfield exhibits synclinal dip, because a synclinal fold 
 by sinking the strata below the general level preserves them from 
 destruction. Mountains often have a synclinal structure. 
 
 MOP] TTebog. 
 
 Tig. 17. Synclinal Dip of the Bala Hocks in Moel Hebog, near Bedgellert, North Wales. 
 
 Similarly, whenever a stratum is inclined in two opposite direc- 
 tions so that the dips converge upward, the inclination is termed 
 anticlinal. This is well seen in the mountain limestone of Derby- 
 shire, in the Mendips, and in the Wealden district of Kent, Surrey, 
 and Sussex. The anticlinal of Woolhope is a locality where the 
 succession of the Silurian rocks is shown. 
 
 wsw 
 
 ENE 
 
 Fig. 18. Anticlinal Dip of the Silurian Rocks in the Valley of Woolhope. 
 
 Thus it is manifest that where the dip is synclinal the newest 
 beds are in the centre of the fold, while where the dip is anticlinal 
 
the oldest rocks occupy that position. Where these dips 
 frequently in a short distance the deposits are said to be contorted. 
 Contortions of the Carboniferous rocks are well seen in the cliffs near 
 Clovelly in North Devon. 
 
 The different rocks which compose the interior of the earth to a 
 considerable depth, therefore, in consequence of this inclination, crop 
 out, or are exhibited in succession on the surface ; and hence it is that 
 we are furnished with a vast variety of mineral productions which 
 otherwise would be deeply seated or hidden, and are able to predict 
 the nature of the beds which occur in succession beneath our feet. 
 
 Continuity of Strata. Any one thus far initiated will be able 
 to construct a section or scale of the strata which occur in his own 
 neighbourhood, naming them in the exact order of their succession or 
 superposition, and thus will be furnished with the means of comparing 
 his own district with others near and distant. The results of this 
 comparison are very important, for we thus learn that one general 
 order of succession is observed among all the stratified rocks of Eng- 
 land. Certain strata are locally deficient, but all those which do 
 occur together are found invariably in the same relative position. The 
 series of stratified rocks in the North of England, taken in a general 
 way, is expressed by the following names: Chalk, Speeton Clay, 
 Kimmeridge Clay, Coralline Oolite and Calcareous Grit, Oxford Clay 
 and Kelloway rock, Cornbrash and Oolite rocks, Lias shales, Red 
 Marl and Sandstone, Magnesian Limestone, Coal series, Carboniferous 
 Limestone, and Slate. The series in the southern parts of England 
 is precisely accordant, except that the magnesian limestone is there 
 nearly deficient, that the Kimmeridge Clay is covered by some strata 
 which do not pass the river Humber, and the Speeton Clay is 
 replaced by the Neocomian Sands. Besides, we find the strata of the 
 north of England actually connected by mutual extension with those 
 of the same names in the south of England, so that we thus prove 
 their continuity over large tracts, as well as the constancy of the 
 order of their succession. Every student should trace one or two 
 formations through the country at an early stage in his work. 
 
 By means of these comparative observations, begun by Mr. W. 
 Smith in 1790, and continued with unabated zeal by others, the 
 whole series of English stratified rocks has been ascertained, and 
 arranged in tabular order; and the geologists of England have, in 
 consequence of the completeness, development, and succession of strata, 
 furnished to the rest of the world a standard of comparison, by which 
 to determine how far the laws of stratification disclosed in this island 
 are applicable to other countries. 
 
 Strike. The direction in which the plane of a stratum extends 
 through a country is termed its strike. This direction is the inter- 
 section of the plane of a stratum with the plane of the horizon, and 
 is a property of an inclined plane which is determined exclusively 
 by the direction of dip. The direction of the strike is therefore 
 always at right angles to the direction of the dip. Thus if the dip 
 is to the north or south, the strike must be east and iccst. The direc- 
 
 VOL. I. E 
 
 
66 
 
 OUTCROP. 
 
 tion in which the plane of the stratum extends will therefore change 
 only with the direction of upheaval of the beds, or the lines along which 
 they are folded, so as to be brought to the denuded surface. Thus in 
 the Lincolnshire Wolds the strike of the Chalk is south-east, in the 
 Cliiltern Hills it is south-west, in the Forth Downs it is east and 
 west. But the strike is entirely independent of the contour or eleva- 
 tion of the ground, and equally independent of the direction along 
 which the edge of a stratum can be followed over the surface of the 
 ground, for that direction may vary with denudation, but no amount 
 of denudation can affect the direction in which the plane of a stratum 
 is inclined. 
 
 Outcrop. The area occupied by a stratum on the surface of a 
 country is termed its outcrop. The line of outcrop or basset is the 
 line where the bed comes to the surface from beneath an overlying 
 deposit. The line of outcrop of an inferior bed is the denudation 
 line, or limit of the outcrop of the stratum which rests upon it. In 
 level country the outcrop usually runs straight, but every hill and 
 valley, every variation in the texture of the stratum tends to make its 
 direction variable and sinuous, because outcrop lines are determined 
 by the ways in which the overlying strata are removed by the action 
 of frost, rain, and the sea, so as to uncover the layers beneath. The 
 general direction of outcrop follows the direction of strike, but the 
 details are the conseqtien'ees of denudation. The nature of the 
 
 outcrop may be in- 
 fluenced by the 
 mineral character of 
 the deposit. Thus 
 since clays are 
 easily worn away, 
 they form valleys 
 or low level plains. 
 But limestones and 
 sandstones being 
 more durable, often 
 form terraces or 
 ridges of hills which 
 extend in the direc- 
 tion of the outcrop. 
 When a stratum in 
 this way rises up 
 like a sloping cliff, and exposes a large part of the thickness of a 
 stratum along the limiting line of outcrop, such exposure is called an 
 escarpment. These exposures, which often resemble inland cliffs, have 
 a base-line which varies in level, and is therefore regarded as having 
 been determined by frost and rain rather than by the sea. 
 
 Two modifications of outcrop called " outlier " and " inlier " often 
 occur. An outlier is a portion of a stratum which has become sepa- 
 rated from the principal mass by denudation, and remains isolated 
 like an island. 
 
 Map. 
 
 Section. 
 
 Fig. 19. Map showing lines of Outcrop. A P; C D. 
 
 
OUTLIERS AND INLIERS. 67 
 
 In this section (fig. 20) a few hills to the north of London are shown, 
 capped with Bagshot Sand, which alone exist as evidence of a con- 
 tinuous stratum, which has been denuded from the surrounding country. 
 
 HIGHBEACH HAVERINQATE 
 
 Fig. 20. 
 
 Over the chalk of Hertfordshire are scattered numerous outliers of the 
 Woolwich and Reading beds of the London basin, giving some idea 
 of the former great extension of that deposit. 
 
 Outliers are frequently widely separated from the principal mass 
 with which they are connected. Thus an outlier of the Lias in 
 Cheshire finds the nearest mass with which it could have been con- 
 
 MASS OF THE 
 FOR MAT in fj 
 1 
 
 Fig. 21. Section of Outlier. 
 
 MAP. 
 Fig. 22 Map of Outlier. 
 
 tinuous in Leicestershire ; and other Lias outliers in Belfast, and the 
 valley of the Eden, have the regular formation nearest to them at 
 Whitby. An outlier is always newer than the formation around it. 
 
 An irilier is an older 1 deposit which is exposed by the removal of 
 a portion of an overlying stratum, so that it lies within a girdle of 
 the surface rock. The most considerable inliers in this Country are 
 
 CHALK (A). 
 
 Fig. 23. Map of an inlier. 
 
 UPPER GREEN SANt) (B). 
 
 Fig. 24 Section of Inlier. 
 
 the Carboniferous Limestone of Derbyshire, which lies within the Mill- 
 stone Grit ; and the Wealden beds of Kent and Sussex, which are sur- 
 rounded by the Cretaceous strata. At Woolhope there is a Silurian 
 inlier rising through the Old Red Sandstone, and at the Wren's Nest 
 near Dudley, another Silurian inlier rising through the Coal. Some of 
 the simplest are seen at Inkpen and Kiiigsclere, where the Chalk is 
 cut through to expose the Upper Greensand beneath. 
 
 Stratification a General Principle. Considerable labour remains 
 
r;8 NATURE OF STRATIFICATION. 
 
 to be accomplished before even the stratified rocks of Europe can be 
 completely compared with those of England, and the want of evidence 
 is still more severely felt with respect to other quarters of the globe. 
 Nevertheless, the following important general results may be regarded 
 as certain. The principle of stratification is found to be universal ; 
 that is to say, in every country of sufficient extent, various rocks are 
 found to be superimposed on one another in a certain settled order of 
 succession, and these rocks are not found only in isolated patches, 
 but often hold their course across provinces and kingdoms. 
 
 Throughout the whole area of Europe, from the Ural Mountains 
 to the Atlantic, and from Lapland to the Mediterranean, the stratified 
 masses, taken in their generalities, are arranged upon the same princi- 
 ples, follow one another in the same exact order of succession, and, in 
 fact, form parts of one vast system of rocks, once more perfectly 
 connected than at present. 
 
 What is known of the geology of North Africa, Egypt, Syria, the 
 countries bordering on the Caspian, Siberia, and Hindustan, leads to 
 a confident belief that the same general system, modified by local cir- 
 cumstances, will be found also applicable to the greater portion of 
 the surface of the Old Continent. 
 
 Analogy of Distant Deposits. Important agreements between 
 the strata of North America, India, Australia, &c., and those of 
 Europe, have been clearly determined, and the time will probably 
 arrive, when, though it cannot be proved that similar rocks were at 
 the same time deposited in every part of the bed of an ancient sea, at 
 least it will be possible to show, that the same system of natural pro- 
 cesses was everywhere in progress, contemporaneously or successively 
 producing analogous effects ; thus exhibiting in chronological order, 
 through the relative antiquity and accompanying circumstances of even 
 the most distant deposits, a history of all the varied operations by 
 which in regular gradation our globe has arrived at its present state. 
 
 Distinction of Stratified and Unstratified Rocks. 
 
 Relative Situation. Stratification is, therefore, the most general 
 condition or mode of arrangement of the rocks ; and in the wide plains 
 and gently undulated portions of the surface, it is often the only one 
 discoverable. A person of good discernment, who should pass his 
 whole life in investigating the south-eastern part of England, or the 
 northern part of France, might conclude, from every observation he 
 could there make, that the external materials of the earth were uni- 
 versally stratified, this arising from the fact that no unstratified masses, 
 igneous or otherwise, occur in these areas. 
 
 On the other hand, the inhabitant of the mountains sees so many 
 examples of granitic and other rocks, totally devoid of any appearance 
 of stratification, and sometimes finds that structure in the slate rocks 
 so dubious and inconclusive, that he is wholly unable to comprehend 
 the magnificent chain of inductions derived from the study of stratified 
 rocks. Unstratified rocks generally abound along mountain chains and 
 
 i 
 
STRATIFIED AND UNSTRATIFIED ROCKS. 69 
 
 groups, and very often form their axis or nucleus. Stratified rocks fill 
 the plains and form the encircling flanks of the mountains. "When a 
 vast mass of unstratified rock, as granite, forms the nucleus of a 
 mountain group, the stratified materials which surround it generally 
 slope away on all sides, as if the granite had been protruded from 
 below these strata, and, during the act of its uplifting, had broken 
 them and caused them to assume their several inclinations. Other 
 unstratified rocks, as basalt and porphyry, appear amongst the stratified 
 and bedded rocks, sometimes in irregularly lenticular masses, as if 
 they had been spread in a melted state around a common centre, some- 
 times filling long vertical fissures in the strata, as if they had been 
 injected from below. 
 
 Mineral Characters. On comparing together the stratified and 
 unstratified rocks, we find their mineralogical composition extremely 
 different. The stratified rocJcs are earthy aggregates, as sandstones, 
 clays, or limestones; such materials, in fact, as we know to be 
 accumulated in the same mode of arrangement by modern waters ; 
 and in a majority of cases we shall find that most if not all of the 
 stratified rocks are non-crystalline. 
 
 The umstrcdified rocks, on the other hand, are generally and evidently 
 crystallised masses, often analogous to igneous or volcanic products, or 
 compounds containing essentially minerals which are not known to be 
 producible from water, but in several instances are obtainable by arti- 
 ficial heat, or generated in the deep furnaces of which volcanic moun- 
 tains are the vents ; and the greater number of the crystalline rocks 
 are unstratified or have no true bedded structure. These generalisa- 
 tions will have their exceptions, some rocks being bedded and crys- 
 talline as well, their crystalline nature or condition having been 
 subsequently induced, or they were originally non-crystalline. 
 
 Stratified rocks have evidently been deposited successively from 
 above ; the lowest first, the uppermost last, in obedience to the laws 
 of deposition. 
 
 Unstratified rocks, on the other hand, seem to be derived from 
 below or at depths in the earth's interior, and to have been ejected or 
 uplifted from below the superincumbent strata, as volcanic matter is 
 protruded at the present day, or they may have occurred as lava- 
 flows, or as volcanic ashes, terrestrial in origin. 
 
 Contents. Stratified rocks contain very generally the remains of 
 plants and animals which were in existence at the period when the 
 rocks were deposited or accumulated, exactly as remains of the 
 present races of plants and animals are found buried in the modern 
 deposits formed in water. All such remains are termed fossils, hence 
 the stratified rocks are termed fossiliferous and the unstratified rocks 
 unfossiliferous, and in nearly every instance a non-crystalline rock is 
 fossiliferous. 
 
 But unstratified rocks contain no such evidences of aqueous origin 
 or mechanical aggregation, and they rarely possess organic remains 
 except when volcanic ashes or mud have entombed the life of the 
 
70 DIVISIONS OF GEOLOGICAL WORK. 
 
 Petrological investigations lead us to arrange the rock masses of 
 the globe into these two classes : 
 
 ist Class. zd Class. 
 
 Crystalline. Non-crystalline 
 
 Unstratified. Stratified. 
 
 Unfossiliferous. Fossiliferous. 
 
 Origin. By all these characters, separately and comparatively con 
 sidered, the two great divisions of materials which compose the 
 external parts of our glohe are proved to have been produced by 
 entirely opposite causes. Stratified rocks are analogous to the modern 
 products of water, and were therefore called by the older authors 
 Neptunian, while unstratified rocks are analogous to the modern 
 products of volcanoes, and receive the names of Plutonic and Vol- 
 canic, according to the conditions under which they cooled. 
 
 Mode of Study. The distinction now insisted upon between rocks 
 of deposition and rocks of eruption or non-crystalline and crystalline 
 rocks, is of the highest importance, and requires the closest attention 
 at the very commencement of the study of geology. For not only are 
 these different classes of rocks distinguished by most important 
 general characters, but even the methods by which they are to be 
 investigated, and the preliminary knowledge required for this purpose, 
 are entirely distinct. Amongst the stratified rocks a knowledge of 
 zoology and botany or biology is required to understand and develop 
 the past history of the remains of plants and animals, which were 
 buried at successive periods ; on the contrary, among the mountains 
 associated with granite and metamorphic rocks, where minerals of 
 every hue and form appear in ever-different combination, scientific 
 mineralogy is of much higher importance, and study of slices of rock 
 under the microscope is often necessary. 
 
 In consequence, geology divides itself into two branches one, 
 Biological, which links itself with the natural history of modern plants 
 and animals ; and the other, Physical, closely connected with chemistry 
 and natural philosophy. And we have now, and have always had, 
 two distinct groups of geologists, whose progress and discoveries have 
 been as different as the preliminary knowledge which their different 
 spheres of research required. 
 
 A geologist of adequate attainments must now indeed be acquainted, 
 at least generally, with both branches of this wide subject; and there- 
 fore he who is unacquainted with either mineralogy on the one hand, 
 or zoology and botany on the other, must be considered as only 
 half-prepared for original investigation. He must be further instructed 
 in palaeontology and physical science before he can be sent to explore 
 an unknown region, or permitted to give an opinion on the whole 
 theory of geology. 
 
 As much practical knowledge, therefore, as can be easily gained of 
 the minerals which enter most frequently into the composition of 
 rocks and veins, and of the natural history of the plants and animals 
 whose remains lie buried in the strata, is absolutely necessary to 
 the student's progress in this science. 
 
 

 STRATIFICATION. 71 
 
 On Stratification in general. 
 
 Strata, the term defined. Strata, layers, and beds are synonymous 
 terms. "Strata," says Professor Playfair, "can only be formed by- 
 seams which are parallel throughout the entire mass." This defini- 
 tion was founded upon the supposition that loose materials deposited 
 under water must be arranged in layers parallel to the surface of 
 the water ; it undoubtedly contains the general or fundamental 
 idea of stratification, but is often too abstract for practice. It 
 includes too much, for slaty cleavage produces truly parallel laminae ; 
 and it excludes many layers produced under greatly agitated water, 
 on lines of sea-coast, and in the direction of sea-currents, such as 
 conglomerates, false bedding, &c. The most remarkably regular and 
 parallel seams or divisions between strata happen in calcareous and 
 argillaceous rocks ; but the partings in sandstone are much less 
 uniform. A particular shelly bed of stone lies at the top of the 
 coralline oolite of Yorkshire, and may be traced for a great dis- 
 tance ; a red rock, long since noticed by Lister, lies at the base of 
 the chalk of Yorkshire and Lincolnshire, and cut through by the 
 Wash, reappears in the same position in Norfolk, for sixty miles 
 in compass ; the cornbrash limestone, seldom more thaii ten feet in 
 thickness, is continuous from Dorsetshire nearly to the Humber, 
 and reappears in the cliffs at Scarborough. ' In these instances, 
 therefore, Playfair's definition applies very well. On the contrary, 
 the beds of sandstone with coal which are interposed in the Lower 
 Oolite system of Yorkshire, are altogether five hundred feet thick near 
 Robin Hood's Bay, but dwindle toward the south, and are entirely 
 deficient before reaching the Derwent 
 
 Such beds are therefore wedge-shaped ; and cases sometimes occur, 
 as in the Lincolnshire limestone, where, by attenuation in all direc- 
 tions from the centre, they become lenticular. See fig. 25 for these 
 and other appearances. 
 
 Interposed Strata. The strata, therefore, are not all co-extensive. 
 Limestones and thick clays are probably the most persistent and 
 regular, sandstones the most limited and local. Local modifications 
 or interposed beds, due to conditions of the sea-bed, cause the principal 
 differences between distant portions of the same formation. 
 
 The Lias of England rests immediately upon red and bluish 
 marly clays with white gypsum ; at Luxembourg these strata are 
 
 separated by a thick sandstone. 
 
 In the north of England, Mag- 
 nesian Limestone separates the 
 Coal Measures from the New Red 
 Sandstone; but in other parts of 
 the island these two formations are 
 in contact. In the breast of Ingle- 
 borough, the limestone beds are 
 aggregated into one vast mural 
 precipice or scar ; but as we proceed northwards, this mass opens and 
 
7 2 DETAILS OF STRATIFICATION. 
 
 subdivides to admit layers of sandstone, shale, and coal, which 
 gradually increase under Crossfell, and swell out to a vast thickness 
 in Northumberland, so as to contain several valuable seams of coal, 
 thick masses of sandstone, and abundance of shale, between the 
 horizontally separated beds of limestone. 
 
 The Oolitic strata, near Bath, are composed of two portions 
 the Upper or Great Oolite, and the Inferior Oolite and between 
 them is a series of calcareous and argillaceous beds called Fuller's 
 Earth, sometimes one hundred and fifty feet thick. As we proceed 
 northward into Lincolnshire, the Fuller's Earth beds die away, thin 
 out, or are excluded from the series ; still farther north the whole 
 series is changed; so that in Yorkshire it includes thick layers of 
 sandstone, shale, and coal. On a first view the districts of Bath and 
 Yorkshire are very unlike, but the contemporanity of their deposition 
 is certain from the continuation of the same Oolitic beds and organic 
 remains through both of them. 
 
 Thickness. The thickness of the beds or strata varies exceedingly, 
 and seems to have reference to the rapidity, regularity, and continuity 
 of the deposition, and the rate of consolidation of the materials. 
 
 The Chalk is commonly about five hundred feet thick, and in all 
 this great mass we can scarcely trace any decided beds ; though the 
 layers of flint at equal distances, and the difference of the organic 
 remains at different depths, evidently prove a succession of stratified 
 deposits. 
 
 The Great Oolite near Bath is, on the contrary, divided into a 
 certain number of beds, definite in quality, thickness, and order of 
 position. 
 
 Laminae. A stratified rock, therefore, is composed of one or 
 more layers of strata, but this is^by no means the last term of the 
 analysis. Each bed is often composed of many laminae, which are 
 
 Sandstone. 
 Laminated Beds. 
 
 Limestone. 
 
 Laminated Beds. 
 
 Sandstone. 
 
 Fig. 26- 
 
 sometimes parallel to the plane of the bed itself, and sometimes lie 
 in it at different angles. Thus micaceous laminated sandstones, and 
 in particular the best flagstones of the coal districts, are composed of 
 a multitude of thin layers parallel to the plane of the bed, and en- 
 tirely covered by plates of mica, which probably cause the splitting 
 of the stone. This appearance is very analogous to the laminated 
 sand quietly left by the successive floods of a river. 
 
FALSE BEDDING. 
 
 73 
 
 
 False Bedding. But the coarser flagstones of the same coal dis- 
 tricts are often composed of laminae, laid at various angles to the plane 
 of the bed, and in consequence producing a rough, uneven, shattery 
 surface, and a tendency to oblique fractures. 
 
 Such appearances of oblique lamination are occasionally found in 
 the modern sediment of agitated waters, both in the banks of rivers, 
 in estuaries, and on the sea-shore. 
 
 When these oblique laminae extend through thick beds, they some- 
 times cause a slight difficulty in determining the dip of the strata, and 
 are then called false bedding. Some of the coarse upper beds of the 
 Great Oolite of Bath, Gloucestershire, Northamptonshire, and Lincoln- 
 shire, as well as of Normandy, are remarkable for this false bedding. 
 But it is in the coarse sandstones that we see the most remarkable 
 examples of this structure, as in the Oolitic sandstones, &c., on the 
 coast at Scarborough, and in the Trias rocks under Nottingham Castle, 
 and it is generally seen in valley gravels. False bedding, oblique 
 lamination, or current bedding, is indeed one of the most characteristic 
 features of shallow-water deposits, and is never observed in clays. It 
 is due to changes in the directions of the currents which accumulated 
 the deposit, and we can often discover from the inclination of the 
 laminae the directions from which the current flowed which formed 
 each of the successive beds. 
 
 In this diagram the layers i and 2 in the face of the section might 
 supposed to be regularly bedded 
 id upheaved, for colour accumu- 
 ites in the laminae and is often 
 cashed out of the planes of strati- 
 ication, but a glance at the other 
 dde of the section shows at once 
 it we have a case of current 
 Iding. Bed i is seen by the 
 )mpass points to have been formed 
 )y a current flowing from the east ; 
 then the current changed, and 
 owing from the north-east cut off the tops of the first set of laminae, 
 d threw down bed 2. In bed 3 the current comes from the north. 
 The more violent the action of the water, the less regular is the 
 ternal constitution of the layers found beneath it. Let any one with 
 his view compare the effects of the tide beating upon the sand and 
 bbles of the eastern coast, or the tumultuous products of a mountain 
 iver, with the tranquil deposit and sediment on the alluvial lands 
 ear Lynn and near Hull. In the former case the materials are fre- 
 uently found heaped together in laminae, variously and confusedly 
 inclined to one another ; in the latter they are all parallel to the 
 " orizon, and to the general plane of the surfaca The former case, 
 the shore lamination, is analogous to the false bedding mentioned in a 
 preceding section, so general in our sandstones and conglomerates, and 
 in shelly beds of Oolite ; the latter is exactly like the regular lamina- 
 ion of clays and shales. Like effects flow from like causes, and thus we 
 
 Fig. 27- 
 
74 SUBDIVISION OF STRATA. 
 
 are enabled to frame very plausible conjectures concerning the condi- 
 tion of the waters under which the several strata were accumulated. 
 
 General Terms. In the same way as a number of similar laminae 
 are sometimes united into one bed of stone, so several similar beds of 
 stone are sometimes associated into one rock, to which a specific name 
 is applied, as the Oolite, the Lias limestone, &c. 
 
 Sometimes several of these rocks are grouped under the title for- 
 mation, as the Bath Oolite formation. Thus the Lias limestone beds, 
 the Lower Lias clay, Marlstone beds, and Upper Lias clay, are all in- 
 cluded in the Lias formation, which rests upon the New Eed Sandstone 
 formation, and is covered by the Bath Oolite formation. 
 
 The International Geological Congress has recommended that the 
 largest series of Geological Deposits, such as Primary or Secondary, 
 should be termed a Group; the Group should be divided into 
 Systems, such as Cambrian System, Silurian System, &c.; the System 
 is to consist of Series ; the Series is made up of Stages ; and each 
 Stage may be resolved into Beds. These terms have corresponding 
 names to indicate divisions of time; thus 
 
 Sedimentary Terms. Chronological Terms. 
 Group, Era, 
 
 System, Period, 
 
 Series, Epoch, 
 
 Stage. Age. 
 
 From these names the familiar English term Formation is omitted, 
 because it has been used on the Continent to indicate the mode of 
 accumulation of a deposit, instead of the deposit itself ; but it may 
 be long before English writers entirely give up this equivalent for 
 the term Series. 
 
 Groups of British Strata. The whole series of British strata are 
 grouped, according to their relative antiquity, into three leading 
 divisions the Primary, or Palaeozoic; Secondary, or Mesozoic; and 
 Tertiary, or Cainozoic strata ; it being understood that such divisions 
 are chiefly adopted for convenience, as expressing with considerable 
 accuracy certain general analogies of origin, composition, and organic 
 contents, which prevail amongst the members of each division, but 
 yet are not to be considered as exclusively belonging to them. 
 
 Two of these three divisions are again subdivided, upon exactly the 
 same principles, into systems of strata, which are marked by certain 
 recurrent rocks, striking analogies of composition, organic remains of 
 similar types, and positions derived from convulsions of the same 
 geological epoch. 
 
 The systems are again usefully divided into formations or series ; 
 these into their several component stages or rocks; whose ultimate 
 analysis gives the strata, beds, and laminae of composition. The 
 superficial accumulations of gravel, sand, peat, &c., are classed under 
 the head of alluvial deposits. 
 
 The Tertiary or Cainozoic Group of Strata are partly lacustrin 
 
 

 DISTURBED STRATIFICATION. 75 
 
 but principally marine, sandy, and argillaceous, and with some cal- 
 careous deposits, abounding in shells and other organic exuviae, closely 
 analogous to existing species. 
 
 Secondary or Mesozoic Group of Strata are principally of marine 
 origin, with rare and local estuary deposits ; consisting of repeated 
 alternations of limestone, flint, sandstone, sand, clay, iron ore, coals, 
 salt, &c., with organic remains, generally very distinct from existing 
 forms of animals and plants. 
 
 Primary or Palceozoic and Hypozoic Strata. The Paleozoic rocks 
 contain organic remains,- mostly of marine tribes, and the species are 
 all extinct. 
 
 Disturbed Stratification* 
 
 Strata originally Level. All strata, says Cuvier, in his admirable 
 " Discourse on the Revolutions of the Globe," must necessarily have been 
 formed horizontally; and this opinion, founded upon the admission 
 that rocks composed of regular layers, containing rounded pebbles and 
 organic remains of water-animals, can only have been formed under 
 water, is supported by observation. For not only do we see at the 
 present day the deposits from water arranged in planes nearly or exactly 
 horizontal, but we also find the ancient strata of the earth, where un- 
 disturbed by convulsions, very nearly level. In consequence of these 
 disturbances the strata are seldom found to be perfectly horizontal, but 
 are often inclined at high angles, and in a few instances stand directly 
 vertical. Their planes are generally continuous over large spaces, but 
 they are sometimes broken and dislocated by faults or dykes. It is 
 now generally admitted that the usual horizontal disposition of the 
 strata is derived from the action of the supernatant waters which ac- 
 cumulated them; and that the irregular declinations and fractures 
 which we sometimes behold are the effects of subterranean convulsions 
 or changes chiefly occasioned by internal contraction. All strata which 
 were deposited in continuous sequence so as to rest evenly upon each 
 other are said to be conformable, and the succession is termed con- 
 formity. These show no evidence of changes affecting the general 
 directions of the coast lines, or form of the sea-bed at the time of their 
 deposition. There is no evidence of destruction of the beds previously 
 formed, and the interval of time between the beds was probably 
 short 
 
 Subsequently Disturbed. Earthy matter deposited from water by 
 tranquil subsidence, as clay and limestone, or accumulated during 
 periods of moderate agitation, as sand and sandstone, must in general 
 be arranged into layers or strata, proportioned to the intervals of 
 deposition ; and these layers, in consequence of the fluctuation of the 
 water and the influence of gravitation, will especially tend to be hori- 
 zontal Nevertheless they must, in a considerable degree, accommodate 
 themselves to the surface on which they are deposited. If the bottom 
 be level, so will be the deposit ; if sloping, the deposit will be inclined ; 
 but if there be a perpendicular subaqueous cliff, no deposit can fall 
 
CONTORTED STRATA. 
 
 Contorted ftra 
 
 upon its face, nor any transported materials be accumulated parallel to 
 it. An originally perpendicular layer or deposit of earthy materials 
 is obviously impossible. Whenever, therefore, we behold vertical 
 strata, we may be quite sure that they were not deposited in that 
 form, but have been displaced by some internal movements of the 
 earth. 
 
 Vertical Strata. Abundance of instances of this position of 
 strata may be quoted in almost any part of the world. The Isle of 
 Wight gives us, in Alum Bay and Whitecliff Bay, a magnificent series 
 of strata, ITOO feet in thickness, reared into an absolutely vertical 
 position ; and this effect is the more remarkable, because the materials 
 uplifted consist of many strata of loose sands and pebbles, which 
 most certainly have been deposited nearly leveL Similar phenomena 
 are seen in the Isle of Purbeck. In the western borders of York- 
 shire, vertical strata of limestone range for miles parallel to the edge 
 of the Pennine chain, and turn eastward through Craven, below Ingle- 
 borough and Pennyghent, to Settle. Magnificent examples of vertical 
 strata are familiar to those who have visited the mountains of Savoy, 
 or who have read the graphic descriptions of Saussure. 
 
 Contorted Strata. There are some remarkable instances of con- 
 torted stratification very difficult to be explained without supposing 
 the strata to have been soft at the time of the flexure. Not to dwell 
 
 on inferior examples, we shall 
 quote the magnificent phenomena 
 of this kind which are seen in 
 the valleys of Chamouni and Lau- 
 terbrunnen, along the shores of 
 the Lake of Lucerne near Fluellen, 
 and the schistoze rocks of the 
 stack in Anglesea. The stratified 
 limestones and other rocks of these 
 localities are bent with such extra- 
 ordinary retroflexions, as to imply 
 repeated or continuous operations 
 of the most violent mechanical 
 agency, producing displacements in different directions ; and observa- 
 tions along the range of the Alps prove that the whole of this chain 
 has been the theatre of enormous and reiterated convulsions, such 
 as might be anticipated from the amount of compression which must 
 have been necessary to uplift that mountain chain. 
 
 Faults. But the most singular case of disturbance is when strata, 
 either horizontal or inclined, being too rigid to bend under flexure, 
 break, and are displaced, so that on one side of the line of fracture the 
 corresponding rocks are much higher than on the other. This differ- 
 ence of level in places sometimes amounts to hundreds or even thou- 
 sands of yards. The succession of strata is on each side the same, 
 their thickness and qualities are the same, and it seems impossible to 
 doubt that they were once connected in continuous planes, and have 
 been forcibly and violently broken asunder. 
 
 Fig. 28. 
 
FAULTS. 
 
 77 
 
 The plane of separation between the elevated and depressed por- 
 tions of the strata is sometimes vertical, but generally sloping a little. 
 
 The direction of inclination of 
 the plane of a fault is termed its 
 hade. In this case a peculiar 
 general relation is observed be- 
 tween the inclination of this plane 
 and the effect of the dislocation. 
 In fig. 29, for instance, the plane 
 of separation, z z, slopes under the 
 depressed, and over the elevated 
 portions of the disrupted strata, 
 making the alternate outer angles 
 2 z &, z z V acute. In several 
 hundred examples of such disloca- 
 tions which have come under notice an exception to this rule is 
 rarely found. The direction of the hade is almost invariably towards 
 the downthrow. A similar law is found to prevail very generally in 
 the crossing of nearly vertical mineral veins; for instance, in fig. 
 30, a a are two portions of a metallic vein, dislocated by another 
 
 Dlsloeativn of a JVin. 
 
 Fig. 29. 
 
 Fig. 30- 
 
 Fig. 31. 
 
 po 
 
 em, b b. In this case the relation of the line b b to the lines a a, 
 s the same as that of z z to the lines b &', &c. The contrary 
 ppearances, had they occurred, would have been as represented in 
 g. 31, and such occur in the mining district of Cornwall; they 
 re termed upthrow or reversed faults. When faults are parallel to 
 h other, and the throw is always in the same direction, the 
 rata descend like steps, and the faults are called step faults. When 
 ,ults cross each other they produce the phenomena termed trough 
 ults or cross faults. 
 
 The line in which a fault extends is always sinuous, and owing to 
 isplacement faults always include many pockets in which minerals 
 ay accumulate. 
 
 The line of dislocation is generally distinguished by a fissure 
 hich is filled by fragments of the neighbouring rocks or by basalt, 
 d then is called a dyke, or by various sparry and metallic minerals, 
 d is then called a mineral vein. The faulted surfaces which have 
 been compressed against each other are hardened, striated, and often 
 lished, when they are termed slickensides. 
 
RELATIVE AGES OF DISLOCATIONS. 
 
 Unconformity of Stra.tijlca.tion. 
 
 Fig. 3*. 
 
 There is every reason to suppose that many faults are produced 
 slowly, when land is being upheaved, since the rocks which were left 
 elevated on one side of the displacement are invariably cut level by 
 marine denudation. Faults often modify the outcrop of strata owing to 
 this circumstance. Thus, in South Lancashire, a coalfield is divided by 
 a fault ; and since it was in the form of a basin, the part which was 
 thrown down shows on the surface as a large curve, while, the other 
 part owing to denudation remains as only the bottom of the basin, and 
 the curve is proportionately small. Some faults affect the rocks very 
 slightly and over a small area ; others, like the Craven fault, have a 
 downthrow of a thousand yards, and may be traced for seventy miles. 
 Relative Age of the Dislocation. The irregular operations by 
 which these disturbances and dislocations were occasioned seem to 
 
 have happened at various 
 periods during the formation 
 of the strata. We know, for 
 instance, examples of hori- 
 zontal strata, as in figure 32, 
 resting upon other highly in- 
 clined strata, which must 
 have been forced into their 
 unnatural position before the 
 deposit of the level strata 
 upon them. 
 
 Such a case occurs in Somersetshire, where the Coal Measures lie 
 at a steep slope beneath horizontal beds of red marl. These Coal 
 Measures are also greatly broken by faults, which in some cases throw 
 or elevate the beds on one side more than seventy fathoms above those 
 on the other side. But the beds of red marl above are altogether 
 uninfluenced either by the steepness of the dip or the abruptness of 
 the dislocations. Therefore, the convulsions by which the effects 
 were occasioned which are shown in the section happened after the 
 deposit of the coal seams and before the deposit of the red marl. 
 
 At Aberford in Yorkshire, and at many other points along the 
 line of the magnesian limestone between Nottingham and Sunderland, 
 similar examples occur. At Yallais Bottom, near Frome, the mountain 
 limestone is found highly inclined, below level beds of oolite ; and 
 the mollusca (Lithodomi) which lived in the oolitic sea have bored 
 holes into the subjacent limestone. 
 
 In such cases the discordance of inclination between the superior 
 and inferior strata is expressed by the term unconformity, and the 
 upper rock is said to lie unconformably upon the lower. Unconfor- 
 mity always implies an unrepresented interval of time, during which 
 (i) the sea-bed was inclined at a new angle, and (2) the previously 
 formed deposits now upheaved were denuded so as to form a new 
 horizontal surface on which the succeeding or unconformable deposit 
 was accumulated. 
 
 Overlap. Strata are sometimes conformable in one section, and 
 yet when traced to a distance are found to be unconformable to the 
 
 

 OVERLAP. 79 
 
 eposits on which they there rest. This condition is termed overlap, 
 or transgression, because the overlying deposit extending beyond the 
 beds previously deposited overlaps and covers them up. Overlap 
 occurs whenever the level of land is depressed over a wide area, so 
 as to allow the sea to extend inland and throw down a stratum upon 
 ground where the series had necessarily been interrupted. A remark- 
 able overlap of the Chalk is seen in Yorkshire, for in the cliffs at 
 Speeton the conformity is perfect ; but as the Chalk extends inland it 
 rests successively upon all the Secondary strata down to the Trias. 
 Similarly in Dorsetshire and Wiltshire, the Cretaceous rocks are con- 
 formable to the underlying series, but as they extend westward the 
 Upper Greensand rests successively upon all the Secondary strata, till 
 in the Haldon Hills it overlaps Carboniferous rocks. 
 
 UNCONFORMABLE 
 
 Fig- 33- Diagram of Overlap. 
 
 Principal Epochs of Convulsion. By pursuing this investigation 
 in different situations, we find that these internal movements or con- 
 vulsions happened at intervals during the whole period of time occu- 
 pied in the deposition of the strata. Some of the most prevalent and 
 remarkable cases of dislocation and unconformity are, however, obser- 
 vable : (i) immediately after the deposition of the Cambrian series, 
 between the Upper and Lower Llandovery beds ; (2) after the accumu- 
 lation of the Coal Measures in the Carboniferous system ; (3) after the 
 deposition of the oolitic rocks ; (4) after the deposition of the Chalk ; 
 and (5) one of the most recent probably of all, after the completion of 
 all the regular formations above the Chalk. It is not to be supposed 
 that all even of these principal cases of dislocation can be recognised 
 in every country ; on the contrary, the subterranean forces frequently 
 shifted their directions and points of action. There is no difference 
 except in magnitude, and the degree to which it is deep seated between 
 the displacement of surface indicated by an ordinary fault and the 
 submergence or elevation of the largest areas of land. 
 
 We shall have occasion to show, while speaking of the organic 
 remains, that there is sometimes observed a singular harmony between 
 these periods of extraordinary internal disturbance and the several 
 epochs when the different races of animals and plants came into exist- 
 ence ; and it is not unreasonable to suppose, that in this manner we 
 may find it possible to establish such a relation between physical and 
 organic phenomena as to demonstrate the geological dependence of the 
 distribution and mutations of life upon changes taking place in the 
 earth's physical geography in successive ages. 
 
 Proximity of Mountains. At present, restricting ourselves to the 
 phenomena of elevation and disruption of the strata, we shall carry our 
 inductions one step further, for the purpose of proving what was before 
 
So EFFECTS OF UPHEAVAL. 
 
 announced, viz., that these disturbances were probably connected with 
 the effects of internal heat. 
 
 We shall assume, then, that granitic, and basaltic or trappean 
 rocks, and others exhibiting the same phenomena, were crystallised 
 from a state of igneous fusion, and were, sometimes in a fluid, and 
 sometimes in a solid state, impelled upwards towards the surface of the 
 earth, as analogous substances are now ejected or poured forth as fluids 
 through volcanoes, or lifted in a solid state by earthquakes, &c. 
 
 In proportion as we approach the mountains where the greatest 
 violence has been exerted to break up the strata, raise the granite, 
 and inject the basaltic dykes, we find the dislocations increased in 
 number and importance, and the confusion of the stratification more 
 prevalent. 
 
 The central nucleus or axis of many mountain districts is a 
 
 mass, or a series of masses, of 
 granite and other unstratified or 
 metamorphic rocks, from which 
 on all sides the strata are 
 found dipping away at high 
 an gl es - In such cases there 
 can be seldom room to doubt 
 Fig. 34 . i and 2 . Primary strata. that the elevation of the moun- 
 
 3. becondary Strata. , . , . , , . , , 
 
 tain ranges and the disturbance 
 
 of the strata was occasioned by the same violence which uplifted 
 the granite. 
 
 The area of granite disclosed between the opposite slopes of 
 strata is indefinite, sometimes very large, sometimes very small, 
 sometimes it is entirely covered over by the rocks which it has 
 uplifted, but not protruded through or perforated. The general analogy 
 in the composition of mountains, in the strata which surround them, 
 and in the dislocations which abound in their vicinity, prove that 
 one common subterranean force has produced all the phenomena in 
 question. 
 
 Basaltic rocks frequently, perhaps generally, show themselves in 
 situations removed from the granitic regions, on the flanks of moun- 
 tains and often in lower ground. In numerous instances, basalt fills 
 up the fissures between the elevated and depressed portions of dis- 
 located strata, and as it cannot be doubted that such a fissure would 
 soon have been filled up by other substances, it is clear that the 
 melted basalt was injected nearly at the same time as the dislocation 
 was produced; that is, that both were local effects of diminished 
 pressure acting on regions affected by internal heat. How this 
 heat was produced, is a question that will receive consideration 
 subsequently. 
 
 Analogy of Mineral Veins and Trap Dykes. So great a general 
 analogy prevails between some mineral veins and basaltic dykes, that 
 in almost all hypotheses their origin has been assumed to be similar 
 in kind. Both in the same manner divide the strata ; in both the 
 materials are crystalline, often such as are not known to be pro- 
 
JOINTS AS AFFECTING SCENERY. Si 
 
 ducible from water, and arranged according to entirely different laws 
 from those which regulate deposits from cold water. It seems, at 
 first, almost inconceivable that materials of such various specific 
 gravity and chemical affinities should be either soluble at once in 
 heated water or capable of being introduced by this process at 
 different times ; but all the circumstances agree in claiming for 
 mineral veins a different origin from basaltic dykes, the igneous 
 origin of which is supported by the strongest possible arguments. 
 We shall, however, discujs the history and nature of mineral veins 
 more at large in a subsequent chapter, and shall then notice pheno- 
 mena concerning them which can with difficulty be explained in the 
 present state of our knowledge of chemistry. 
 
 Exhibition of Useful Minerals. It is not only in the elevation 
 of continents, the varying height of mountains, the division of the 
 sea, and similar striking effects, that we see the utility of the com- 
 bination of subterranean igneous with superficial aqueous agency. 
 Every coalfield in the known world proves distinctly the utility of 
 even the minor dislocations, which in our imperfect language are 
 called " faults " in the strata. The universal effect of these " faults " 
 is to multiply the visible edges of the strata, by bringing them more 
 frequently to the surface, in consequence of which there is, in the 
 first place, greater chance of discovering useful minerals ; and, secondly, 
 greater facility in working them. 
 
 Internal Structure of Rocfcsi 
 
 Joints in Different Rocks. All rocks, whether stratified or not, 
 are naturally divided by -fissures into masses, which are of different 
 forms in dissimilar rocks, and pass in various directions, independent 
 the strata. The fissures or planes of parting between these 
 isses are called joints. Most frequently their direction ' is nearly 
 it right angles to the planes of stratification or bedding, where such 
 dst, and they divide the rock into cubical, rhomboidal, or prismatic 
 )rtions, blocks, pillars, or columns. It is owing to their various 
 lirection and frequency that different rocks assume such characteristic 
 ippearances, and may thus be often and readily distinguished when 
 3n at a distance. 
 
 Some rocks have very numerous, approximate, and closed joints, 
 shale, some kinds of slate, and laminated sandstones ; in others, as 
 limestones, the joints are less frequent and more open. 
 
 In coarse sandstones they are very irregular, so that quarries of 
 us rock produce blocks of all sizes and forms. From this cause, 
 irse sandstone rocks show themselves against or facing the sea, in 
 >recipitous valleys, or on the brow of hills, in rude and romantic 
 grandeur. The wild scenery of the Peak of Derbyshire, Brimham 
 Crags, and Ingleborough in Yorkshire, derive attractive features from 
 the enormous blocks of Millstone Grit ; and the magnificent rocks 
 which stand upon the hills and overlook the Vale of Wye, are com- 
 posed of a somewhat similar material 
 
 VOL. L F 
 
82 JOINTS IN STRATA. 
 
 In clay, vertical joints are numerous, but small and confused, 
 whereas in indurated shale they are of extraordinary length, very 
 straight, and parallel, dividing the rock into rhomboidal masses. This 
 may be well studied in the shale, which alternates with mountain 
 limestone, at Aldstone Moor in Cumberland. Khomboidal joints are 
 frequent and very regular in coal. 
 
 In limestone the vertical joints are generally regular, and arranged 
 in two sets, which cross at nearly equal distances, and split the beds 
 into equal-sized cuboidal blocks ; and thus the mountain limestone is 
 found to be divided into vast pillars which range in long perpen- 
 dicular scars down the mining dales of the north of England. 
 
 All water-formed rocks, after being upheaved, dry and shrink. 
 The superficial beds in any quarry may be seen to be divided more 
 perfectly and into smaller pieces than the masses which are deeper- 
 seated and moist. This shrinkage is not merely lateral, but to some 
 extent vertical also, and these shrinkage planes are the beginnings of 
 joints. Afterwards, when the strata became strained and bent during 
 the changes of level in land, these planes became extended and sys- 
 tematised in definite and parallel directions. 
 
 In slate districts, the joints, more numerous and more regular 
 than in any other known rock, have almost universally a tendency 
 to intersect one another at acute and obtuse angles, and thus 
 to dissect whole mountains into a multitude of angular solids, 
 with rhomboidal or triangular faces, which strongly impress upon the 
 beholder the notion of an imperfect crystallisation, produced in these 
 argillaceous rocks since their deposition and consolidation by some 
 agency, such as heat or pressure, capable of partially or wholly obliterat- 
 ing the original marks of stratification ; but we may with more proba- 
 bility here also appeal to tension in successively different directions as 
 the true cause of these phenomena. 
 
 Vertical joints are frequent in granite and appear to have definite 
 directions. The trihedral and polyhedral vertical prisms of basalt, 
 and some other igneous rocks, coupled with their regular transverse 
 divisions, seem to give us the extreme effect of regularity in the 
 division of rocks by the process of condensation, from the state of 
 igneous expansion. 
 
 Cause of other Joints and Fissures. That contraction aft 
 partial consolidation of the mass is the general immediate cause o 
 the numerous fissures of rocks, may easily be proved by a variety 
 facts observed in conglomerates, where pebbles, and in other rock 
 organic remains, are split by the joints. According to the circum- 
 stances of the case, this process has produced in basalt, slate, and 
 coal, fissures so regular as to give to the rock a largely crystalline 
 structure, but left in sandstone mere irregular cracks. 
 
 From Mr. Gregory Watt's experiments on fused basalt, and some 
 other notices by different authors, we know that a continued application 
 of even moderate heat to a previously solidified body may be sufficient 
 to develop in it new arrangements of the particles, new crystalline 
 structures, new chemical combinations, and to cause a real transfer of 
 
I 
 
 MASTER-JOINTS. 83 
 
 some of the ingredients from one part of the mass to another. From 
 many independent facts it is inferred, as a matter of certainty, that all 
 the strata have locally, and the lower ones perhaps universally, 
 sustained the action of considerable heat since their first deposition, 
 consequent on folding and pressure : we seem, therefore, to be pos- 
 sessed of the clue which is eventually to conduct us to a knowledge 
 of the cause of the different structures observable in rocks independent 
 of their stratification. 
 
 But, though heat be taken as the leading cause of many of these 
 effects, it is by no means inconsistent to suppose that some other 
 independent agent as, for example, electricity might be concerned 
 in modifying the result. From discoveries in electricity, it appears 
 certain that this universal agent is excited in every case of dis- 
 turbance of the chemical or mechanical equilibrium of natural bodies.; 
 and it is especially and very sensibly excited by unequal distribution 
 of heat. Professor Sedgwick's suggestion with reference to Mr. Fox's 
 electro-magnetic experiments on the mineral veins of Cornwall, that 
 electricity was probably concerned in the original production of those 
 veins along which it now circulates, may be perhaps extended to the 
 contents of the joints of rocks ; in the study of which Professor 
 Phillips found abundant reason to believe that the theory of the 
 production of mineral veins is inseparable from that of the joints 
 and fissures, in some of which the metallic substances are deposited. 
 
 The joints in igneous rocks like those in aqueous rocks are due to 
 contraction, but it is contraction on cooling. In granite, the joints 
 are remarkably regular, and generally correspond with the crystalline 
 angles of the mineral orthoclase which constitutes more than half its 
 bulk ; so that if any considerable portion of the crystals are arranged 
 a definite direction in the rock, we might expect the mass 011 
 rinking to divide by joints in planes defined by crystalline struc- 
 This has yet to be proved ; but it is probable that nearly all 
 oints in igneous rocks are due to the combined influence of these 
 uses. 
 
 Direction of Fissures. In examining with attention a consider- 
 ,ble surface of rock, it w r ill be found that amongst the joints are some 
 ore open, regular, and continuous than the others, which occasionally 
 together stop the cross-joints, themselves ranging uninterruptedly 
 or some hundreds of yards, or even for greater distances. There may 
 more than one such set of long joints, and, indeed, this is corn- 
 only the case ; yet, generally, there is one set more commanding than 
 .e others, more regular and determined in its direction, more com- 
 letely dividing the strata from top to bottom, even through very 
 eat thicknesses and through several alternations of rock. For 
 xample, there is a peculiar character of joints in each of the principal 
 strata of the mountain limestone series, limestone, sandstone, shale, 
 and also in the sandstones shales and coal of a coal district ; yet, 
 throughout the whole of Yorkshire, all these rocks are divided by the 
 master-joints passing downward through them all in nearly the same 
 direction, north by west and south by east. These master-joints, called 
 
CLEAVAGE. 
 
 P<i/fo*on of Strata by Master Joi 
 
 slines, backs, lords, &c., are perfectly well known to the workmen, as 
 well as some other very important yet less certain and continuous 
 fissures passing nearly east-north-east and west-south-west. It is 
 according to such joints that the experienced collier arranges his 
 workings, and the slater and quarryman conduct their excavations. 
 Now, surely nothing can be more certain than the inference that some 
 very general and long-continued agency, such as gradual tension or 
 straining, pervading at once the whole mass of these dissimilar and 
 successively deposited strata, was concerned in producing this remark- 
 able constancy of direction in the fissures which divide them all. The 
 deficiency of recorded observations prevents a general development of 
 this important subject by reference to other districts, but it is obvious 
 that a great principle in the construction of the earth is here indicated, 
 which must eventually have an important influence on geological 
 theory. In the meantime, we may remark, first, that these prevalent 
 directions of north by west and east-north-east are those of the prin- 
 cipal mineral veins in the north of England, and that they are also 
 admitted to be very prevalent in the southern and western mining 
 countries as cross-courses; secondly, that these directions are wholly 
 uninfluenced either by the inclination of the strata or by the numerous 
 dislocations to which they are liable. Whatever be the direction of 
 
 the dip, how frequent soever the 
 faults, the lines of the great 
 joints are the same. These lines 
 are frequently the cause of parti- 
 cular courses in rivers and springs, 
 long scars on mountain-sides, and 
 subterranean channels for water. 
 Faults, and dykes, and mineral 
 veins very frequently pass along 
 them, and there is little doubt 
 that the diligent study of them 
 
 will be found to throw much light on some of the most interesting 
 phenomena of geology. 
 
 Cleavage. There is yet another structure not common to all rocks, 
 nor confined to a given geological age. Though most frequently 
 manifested among the Primary strata, it is sometimes observable in 
 others of a later date. This structure is called cleavage, and it 
 consists in a peculiar fissility of the rocks which are affected by it, 
 parallel to a certain plane, which almost always cuts at a considerable 
 angle the plane or curved surfaces of the stratification. In fig. 36, 
 which represents a mass of rocks in which this definite quality of 
 splitting is developed, B B is the surface (curved in this instance) of 
 one bed of the stratification j j is on the plane, here supposed ver- 
 tical, of a joint ; c is one of the planes of cleavage, cutting the sur- 
 face of stratification B B in s-s. Parallel to this plane c, the mass of 
 rock here represented is cleavable by art, and is often actually cleft by 
 nature, into very thin and numerous plates, which, when of suitable? 
 quality and reduced to proper size, constitute the roofing-slates of our 
 
 
PHENOMENA OF CLEAVAGE. 
 
 European houses. The edges of these plates may be traced with care 
 on the vertical surface of the joint J, and the sloping surface of the 
 bed B, and are represented in the figure by 
 fine lines. 
 
 It will be observed that these lines do 
 not cross the bed marked g (fig. 36). This is 
 supposed to be a hard grit or conglomerate, 
 and such rocks are sometimes only in a slight 
 degree affected by the cleavage, which, how- 
 ever, is perfect above and below them in 
 fine-grained and more argillaceous strata. 
 Certain small joints, however, and numerous 
 cleavage planes, often cross sandstone beds, 
 and then the cleavage and joint planes in 
 these beds are not parallel to the general 
 cleavage, but meet the surfaces of stratifica- 
 tion, as in this figure, at angles more nearly 
 approaching to a right angle. At I the 
 cleavage crosses nodular limestone or ironstone, and in these irregular 
 layers becomes irregular, curved, and confused. 
 
 On the surfaces of stratification the cleavage structure is frequently 
 traced in narrow interrupted hollows and ridges ; these surfaces have 
 in fact been folded, or plaited, or puckered by the force which occa- 
 sioned the cleavage ; and the little folds thus occasioned are traceable 
 across shells, trilobites, &c., which are thus more or less distorted in 
 figure. 
 
 On a careful scrutiny of these shells and trilobites, we find that 
 they have been compressed or elongated in one direction, so that a 
 semicircular shell (Orthis), whose hinge-line lies parallel to the cleav- 
 age edges on B, is found to be altered to the figure a ; another, whose 
 hinge-line lies across these edges, assumes the shape of 6 ; and a third, 
 whose hinge-line is oblique to the edges of cleavage, becomes distorted 
 c. To make this more clear, the letters B B are represented as having 
 idergone the compression in question. 
 
 When, as sometimes happens, there are on these surfaces shells and 
 jbbles, too solid and firmly compacted to yield to the cleavage force, 
 are not altered in figure, but the cleavage laminae in the mass 
 )und them are changed a little in 
 lirection. Some alteration of direction 
 iquently occurs when the cleavage is 
 sing from one bed to another of a 
 iifferent degree of solidity. And in 
 >me tracts of country (e.g., the district 
 )f Cork), the cleavage planes commonly 
 not parallel in contiguous beds, of 
 mlike quality, but appear as in fig. 37. 
 
 le cleavage plane is most oblique to the bedding in the softest and 
 lost argillaceous strata. 
 
 One general relation appears between the stratification and the 
 
 37- 
 
86 CHEMICAL DEPOSITION. 
 
 cleavage a relation arising from the displacement of the strata by 
 axes of elevation and depression. Parallel to these axes is the "strike " 
 or horizontal line on the surface of the strata ; if this be taken on a 
 great scale and the " strike " of the cleavage (similarly denned) be 
 compared with it, the direction of each is found to be the same, or 
 nearly so ; in other words, the cleavage edges on the surface of the 
 strata are horizontal lines (s-s in fig. 36). The direction, then, of 
 the cleavage in a given district is dependent in a general sense on 
 that of the axes of earth-flexure in that district ; but the inclination of 
 
 the cleavage has no necessary 
 known relation to that of the 
 strata (fig. 38) ; beyond this, 
 that the dip of the strata 
 
 Fig. sS.-Paralle^cleavagem^contorted slates of being mo d era te, that of the 
 
 cleavage is usually greater. 
 
 In a country where the strata are much undulated, the cleavage may 
 be and mostly is in parallel planes. 
 
 Local Changes of Internal Structure. We must defer to a later 
 page the theoretical considerations which arise out of these facts, 1 and 
 some other valuable data, collected by Mr. Sharpe, and later still by 
 Mr. Sorby; but though a little out of place, we cannot forbear to 
 add here a short notice of facts known in Switzerland, which dis- 
 tinctly prove one of the effects of heat upon common argillaceous 
 shales, to be the alteration of their structure, so as to give a real ver- 
 tical cleavage to a mass of horizontal laminae of clay, as well as that 
 induration which belongs to slate. The Lias shales of the Alps are so 
 altered by proximity to the igneous rocks of that region, that in several 
 places in and near the Valley of Chamouni they are commonly mis- 
 taken by modern tourists for genuine slates of the Primary system, 
 and were always described as such by the older writers. This demon- 
 strates that cleavages, and other peculiarities of structure, not pro- 
 duced in rocks by water, nor coeval with their deposition, have been 
 occasioned subsequently, chiefly by the agency of pressure or molecular 
 rearrangement. 
 
 Composition of Strata. 
 
 Chemical Deposits. Under different circumstances water, at 
 certain temperatures, and by the help of soluble acids or alkalies, 
 dissolves various mineral substances. When, by evaporation, loss 
 of heat, or a change in the composition of the liquid, these sub- 
 stances are no longer capable of remaining in solution in it, they 
 separate in a more or less crystallised form, and the deposit which 
 they occasion is termed a precipitate. By such processes lime, mag- 
 nesia, and other earths and metallic oxides are first dissolved in 
 water, and afterwards separated from it. In this way calcareous marls 
 and irregular accumulations of limestone, in lakes and in the course 
 
 1 These remarks on cleavage are based on observations by Prof. Phillips, 
 and were mostly published in Encyclo. Metrop., 1833 > Guide to (Geology, 1834-6- 
 51 ; Treatise on Geology, 1853 ; Brit. Assoc. Report, 1843. 
 
MECHANICAL DEPOSITION. 87 
 
 of certain streams and at the mouths of some rivers, are thrown down. 
 In ancient times also, the most abundant chemical deposit from water 
 was limestone. 
 
 The chemical stratified deposits are principally limestones, or com- 
 posed of carbonates of lime and magnesia, or are salt rocks with beds 
 of chloride of sodium. This is not the place to discuss points of 
 theory, and we shall therefore speculate no further at present on the 
 origin of these deposits than to say, that the quantity of lime now 
 held in solution in sea-water is subject to daily diminution through 
 the agency of life, and experiences daily renewal by the inflow of 
 streams from the land. The innumerable tribes of corals, mollusca, 
 and other invertebrata, obtain the carbonate and phosphate of lime 
 necessary for their skeletons, &c., from the salts of lime in the sea, 
 and these salts are supplied by streams from the land, which have 
 derived lime from the old rocks. The calcareous rocks are found to 
 be almost wholly composed of shells, corals, Crustacea, &c., and thus 
 we perceive as a very general fact, that it is less by direct chemical 
 reactions than by vital energy and the decay of organised fabrics, 
 that thick calcareous masses of every geological age have been formed 
 and are still forming in the sea. 
 
 Mechanical Deposits. The mechanical agency of water is manifest 
 in removing materials from one place and depositing them in another. 
 Thus pebbles and sand and clay are transported by the tides and by 
 rivers, and accumulated in low situations in regular layers, miniature 
 representations of those thicker strata of the same ingredients which 
 compose the crust of the earth. And as at the present day some 
 materials are transported farther by water than others, and consequently 
 more rounded by attrition, so the materials of the strata are likewise 
 more or less worn and rounded, in proportion to the distance they 
 have travelled and the friction they have suffered. 
 
 In many situations chemical and mechanical products are thrown 
 down successively by the same waters, just as in the older strata 
 limestones and sandstones occur alternately. We see, therefore, that 
 the ancient deposits from water, which form layers several miles thick 
 around a great part of the globe, are not essentially different, except in 
 degree, from the lesser deposits now formed beneath the sea, and 
 by streams from the land. 
 
 The mechanical deposits or strata composed of earthy materials, 
 are distinguished by the coarseness, or fineness, or nature of their 
 ingredients. The following scale will convey some notion of the 
 gradations of size in the ingredients of mechanical deposits : 
 
 Very fine particles, generally containing ) C1 j shal and slate . 
 
 20 to 30 per cent, of alumina ] J ' 
 
 Mixture of clay and sand Sandy clay. 
 
 Sand with some clay Argillaceous sandstone. 
 
 Small fragments of hard siliceous minerals. Sand, sandstone. 
 
 Sandstone including pebbles Millstone grit. 
 
 Large pebbles united by sandstone or clay... Conglomerate or puddingstone. 
 
 Pebbles disunited Gravel. 
 
 Angular stony fragments reunited Breccia. 
 
88 SIMILARITY OF STRATA OF DIFFERENT AGES. 
 
 Ingredients of Mechanical Strata. Considered with reference to 
 the nature of the ingredients which compose them, mechanical strata 
 form another scale. 
 
 Thus gneiss, one of the oldest of these strata changed by meta- 
 morphism, is a compound of the same ingredients as granite quartz, 
 felspar, and mica ; but these minerals, instead of being amalgamated 
 (so to speak) together by crystallisation, are accumulated in successive 
 laminaB more or less regular, and more or less soldered together. Some 
 varieties of gneiss, therefore, differ from micaceous sandstone less 
 than is commonly imagined, and often other varieties occur which 
 have so slight a lamination and so much of crystallisation as to 
 justly bear the name of granitic gneiss. 
 
 Sandstone sometimes contains rolled and broken pieces of crystal- 
 lised felspar, such as that which occurs in the granite of Cumbria and 
 Scotland. There is, therefore, every reason to conclude that coarse 
 sandstones, like the Millstone Grit, have been derived from the waste 
 of ancient tracts of granite and metamorphosed rocks. Nearly all 
 sandstones contain a small amount of felspar. 
 
 Sandstones sometimes extend over vast districts, and are char- 
 acterised by some remarkable mineral ingredient ; as, for instance, 
 the Green-sand of England, France, and Switzerland, which is dis- 
 tinguished by the presence of a peculiar green mineral, termed Glau- 
 conite or silicate of iron. 
 
 Conglomerates, on the other hand, are generally constituted of 
 fragments from the neighbouring mountains or coast. Thus the red 
 sandstone of the Vosges mountains contains quartz pebbles derived 
 from the slate rocks of the vicinity ; the Old Red conglomerate of 
 England varies in composition according to its locality ; that of 
 Herefordshire contains much quartz. 
 
 Whole Series of Strata. The whole series of stratified rocks, 
 then, consist of alternate deposits of limestone, sandstone, and clay, 
 with a few layers of coal, rock-salt, flint, iron ore, &c. The modes 
 of alternation are different in different parts of the series and 
 in different situations. Thus the Siberian limestones are some- 
 times enclosed between beds of slate, the Carboniferous Limestone 
 alternates with sandstone and shale, the Lias limestone lies in marly 
 clays and shales, the Coralline Oolite alternates with calcareous sand- 
 stone. Generally, the different strata are distinguishable by their 
 mineralogical characters, but not always. When the circumstances 
 of the deposit were nearly similar, as in the accumulation of the 
 Carboniferous Limestone and some of the oolites, the strata are 
 remarkably alike ; and often particular beds of one rock are scarcely 
 to be distinguished from beds of another rock. The Old Red Sand- 
 stone and the New Red Sandstone formations are physically very 
 much alike ; it would be difficult by mere mineralogical methods to 
 discriminate the great clay deposits which separate the oolitic lime- 
 stones, and many sandstones of very different epochs are almost 
 undistinguishable from each other. Hence we may infer that nearly 
 the whole series of strata is the result of many repetitions of similar 
 
CONTINUITY OF DEPOSITION. 
 
 89 
 
 xcnange or alternation of Beds, 
 Coralline Oolite. 
 
 mechanical and chemical agencies operating in waters under similar 
 conditions. 
 
 Alternation of Beds. "When sets of strata are in contact as, for 
 instance, limestone lying upon sandstone it often happens that while 
 the limestone above and the sand- 
 stone below are unmixed with other 
 matter, there is a middle set of beds 
 composed of alternate layers of the 
 sandstone and limestone. Thus, let 
 a be the Coralline Oolite of England, 
 and b calcareous sandstone beneath ; 
 the middle beds a' a" b f b" are alter- 
 nately oolite and sandstone. 
 
 In such a case, therefore, the two strata are said to exchange beds, 
 or to be subject to alternation at their junction, and the phenomenon 
 seems to have been occasioned by temporary cessations of the deposit 
 of sandstone allowing the limestone which would normally have been 
 only a cement to the sand to accumulate and form a limestone deposit. 
 
 Calcareous Grit. 
 
 Fig. 39. 
 
 
 Gradation of 
 into one another 
 ceptible gradation ; as for 
 instance, the Oxford Clay of 
 the Yorkshire coast graduates 
 into the Calcareous Grit above 
 so completely, that the bluish 
 colour of the crumbling shale 
 below is shaded off without 
 any hard line into the yellow 
 solid beds of grit above, See 
 fig. 40. 
 
 Beds. In other instances, the two strata pass 
 by imper- 
 
 Gradation of Strata. 
 
 Oxford Clay. 
 
 Fig. 40. 
 
 In either case it seems quite evident that no considerable break 
 or interval of time happened between the different contiguous de- 
 posits; one bed was no sooner formed than another was laid down 
 and deposited upon it. By careful study of these phenomena it 
 appears that, bed by bed, and rock after rock, the whole series of 
 strata, even to miles in thickness, were successively and almost un- 
 remittingly accumulated, and buried and covered up the shells and 
 other organic beings which were then living in the water, or on the 
 shore, or drifted into it from the land. The strata are, therefore, the 
 best witnesses of the lapse of time, and of the changing conditions of 
 land and water during their deposition. 
 
 Proportions of Chemical, Organic, and Mechanical Deposits. 
 Assuming limestones to be of chemical or vital origin, and sand- 
 stones, clays, &c., to be mechanical deposits, and putting for the 
 present out of consideration the detached organic remains which 
 abound, especially in calcareous strata, we shall be able by comparison 
 of the thickness of the several rocks to present a tolerably accurate 
 notion of the relative proportions of chemical, organic, and mechanical 
 deposits. 
 
90 RELATIONS OF LIMESTONES WITH SEDIMENTS. 
 
 If we take our examples of these strata from Great Britain, it 
 may, perhaps, be found a sufficient approximation to the ratio 
 now sought, to say the mechanical are to the chemical deposits from 
 water : 
 
 In Palaeozoic or Primary strata, . . . 20 to I 
 In Mesozoic or Secondary strata, . . . 4 to r 
 
 In Cainozoic or Tertiary strata, . . . 10 to I 
 
 In these comparisons regard is had to the different proportions 
 which prevail in different districts. They would be very different 
 estimates for the Tertiary series in the Isle of Wight, and that of 
 the London basin ; and for the oolites near Bath, and those near 
 Whitby. 
 
 From this comparison it would appear that the ratio of chemico- 
 vital to mechanical strata is greatest amongst the Secondary deposits, 
 and least amongst those of the Primary periods a circumstance on 
 which depend principally the well-marked general characters of the 
 Secondary series of rocks. It should, besides, be observed, that 
 calcareous matter very finely divided exists in nearly all the sand- 
 stones and shales of that series, and sometimes so abundantly as to 
 change, locally, Lias shale into argillaceous limestone, and Calcareous 
 Grit into arenaceous limestone, or coarse oolite. In Secondary strata, 
 the great and prominent masses of limestone almost invariably 
 attract the attention and direct the classification, and thus it happens 
 that while numerous layers of clay and sand pass nearly unobserved, 
 or are merely noticed as interpolated beds, almost every calcareous 
 bed has its characteristic local name. The almost universal diffusion 
 of calcareous matter through the mechanical strata of this large 
 group, combined with the great regularity and persistence of the 
 limestones, generally suggests theoretical notions as to the cause. The 
 observer soon learns to consider the operations by which sandstones 
 and some clays were rapidly accumulated with intermitting action, 
 like the periodical floods of a river, or some less regular inundations 
 or depressions ; while the production of limestone is regarded as 
 the result of one continuous and almost uninterrupted series of 
 chemical and organic changes. This opinion, strengthened by the 
 gradations between calcareous and sandy or argillaceous laminae, and 
 by frequent alternation amongst e%en their thinnest portions, derives 
 plausible arguments from the distribution of organic remains through 
 the several strata. In some cases they teach us plainly that sand- 
 stones, even of great thickness, were the products of temporary and 
 often of very local floods, which swept down from the land the 
 remains of animals and plants then in existence, or result from cur- 
 rents of water due to tidal action or coast interference ; but, tried by 
 the same tests, the calcareous rocks appear to have been of slower and 
 more equable production, in clearer, and more tranquil, and often 
 deeper waters. This is in harmony with the present system of 
 natural operations. The pebble beaches of our actual shores and the 
 gravel and sand-banks of our shallow seas may be compared with tht> 
 
 

 ANCIENT AND MODERN DEPOSITS. 91 
 
 often thin and irregular sandstones and conglomerates of earlier ages ; 
 the finer clays which fill the broader and deeper hollows of our 
 seas, because such fine sediments are held long in suspension by 
 water, are quite similar in position to the older argillaceous de- 
 posits ; and our modern coral reefs and the shell beds which 
 accompany them, produced in clear pelagic waters, unmixed with 
 sediments from the land, are in many respects exactly the repre- 
 sentations of the old limestones of Wenlock, Bakewell, Calne, and 
 Orford. 
 
CHAPTER VII. 
 
 THE PHYSICAL AND MINERAL HISTORY OF STRATIFIED ROCKS. 
 
 ALTHOUGH the greater changes which have taken place in the history 
 of the earth are only to be discovered by following the several beds of 
 rock through the country, and observing their relations to each other, 
 and alterations in mineral character and in fossils, yet much may be 
 learned concerning the conditions under which each deposit was accu- 
 mulated, and sometimes even the direction and district from which the 
 deposited material was derived, by minute and even microscopic ex- 
 amination of the particles of which a stratum is built up. This kind 
 of research has been rendered possible by the labours of Dr. Sorby l 
 and Mr. John Arthur Phillips, 2 and it is on the basis of their researches 
 that we give an indication of the ways in which sands, clays, and lime- 
 stones may be made to yield evidence of their history and origin. 
 
 Sand. 
 
 By sand we understand the materials constituting the fine-grained 
 silicious rocks called sandstones. This sand has in every case been 
 derived from the destruction of igneous or metamorphic rocks, and in 
 some cases of chert or flints. The quartz from granite consists of 
 separate grains which often have an irregular and complex form, but 
 the quartz from felsite is much more truly crystalline, and the planes of 
 the crystals are frequently perfect though the angles are more rounded 
 than in the quartz from granite. Sometimes the grains are corroded as 
 though partly dissolved by the action of the alkalies liberated when the 
 associated felspar was decomposed. The quartz derived from gneiss 
 and mica-schist, especially when those rocks have a thin foliation, is 
 remarkable for being flattened in the plane of foliation, and consists of 
 numerous small crystals dovetailed together, so that when broken up 
 it gives rise to a fine-grained sand, or a sand containing grains which 
 show a compound structure ; and if the parent rock contained mica, 
 thin plates of mica are found between the parallel grains of quartz. 
 When the grains are observed under the microscope they often show 
 fluid cavities, frequently with bubbles. This character is conclusive 
 
 1 H. C. Sorby, Address Quart. Jour. Geol. Soc., vol. xxxvi. 
 
 2 J. A. Phillips, Quart. Jour. Geol. Soc., vol. xxxvii. p. 6. 
 
FORMS OF SAND GRAINS. 93 
 
 evidence that the quartz was derived from a rock which solidified from 
 a heated condition under great pressure. The fluid cavities are most 
 numerous in the Cornish granites, and there they often contain cubic 
 crystals of alkaline chlorides. The schists of Scotland contain but 
 few fluid cavities, and crystals have never been observed in the cavi- 
 ties of these rocks, or in any Scotch granite. There is a further differ- 
 ence between the Scotch and Cornish granites in the fact, that the 
 former generally contain in the grains of quartz fine hair-like crystals 
 of the mineral rutile, while in the latter the grains abound in small 
 prisms of tourmaline. The quartz crystals of some volcanic rocks, 
 like the rhyolites, sometimes contain six-sided or imperfectly rhombic 
 enclosures of coloured glass with accompanying bubbles. The presence 
 of volcanic glass would always indicate denudation of a volcanic rock. 
 The grains of sand are rarely obtained direct from the rock which yields 
 them without experiencing a large amount of wear. This attrition is 
 due to transport of the material by rivers, and grinding by the waves 
 on the sea-shore. Some ancient sand-beds are made up of grains which 
 are unworn and practically new, while the grains on many a modern 
 sea beach are of vast antiquity, and have formed part of several 
 geological formations, in each of which they have been worn. When 
 we examine some of the modern sands in process of formation, 
 the amount of wear is found to be unexpectedly small ; thus the sand 
 of the river-terraces at Dunkeld is almost entirely angular, and pre- 
 sents the features characteristic of sand derived from schists. The 
 sands of the Arabian, Egyptian, and great African deserts, on the other 
 hand, are exceptionally worn, every grain presenting the characters of 
 a miniature pebble, a feature which results from the agency of wind 
 in triturating the grains against each other. The sand which occurs 
 in the decomposed granite of Cornwall is separated from the rock by 
 artificial washing in the china clay works of that district, and the 
 characters of the grains have been observed in the St. Austell river, 
 which has a fall of from 150 to 470 feet to the sea. Here the 
 grains are seen to consist of a mixture of quartz, felspar, and a little 
 mica. They vary in size from over ^th inch to a fine sand of less 
 than T -g-oth inch. 
 
 The fragments of quartz and tourmaline are sharp and unrounded, 
 but the edges of the mica and felspar are distinctly rounded. The 
 felspar grains always have an external coating of clay due to decom- 
 position. As the sand becomes finer the grains become less worn, and 
 the proportion of mica is larger than in the coarse sand, and there is 
 less felspar. Attempts have been made by Professor Daubre"e to esti- 
 mate the distance that a grain must travel to show a definite amount 
 of wear, and he concludes that before a grain of quartz -g^th inch in 
 diameter can assume the appearance of a miniature pebble it must 
 undergo the same amount of abrasion as would result from travelling 
 three thousand miles along a shore. Grains, however, from being 
 corroded in the manner already indicated, sometimes become modified 
 in outline without much wear, and occasionally they are singularly 
 fractured ; but frequently grains which were rounded by attrition have 
 
94 ATTRITION OF GRAINS IN SANDSTONES. 
 
 quartz deposited upon them from water infiltrating through the rock 
 so as to present the aspect of perfect crystals absolutely free from wear. 
 This deposited quartz appears to be almost always derived from the 
 decomposition of associated felspar in the sand, though in the Yosges 
 district M. Daubree finds reason for attributing it to the local action of 
 heated water ; but since the china clay always develops from its free 
 silica sharp crystals which sometimes reach a length of three inches, we 
 have in this action a sufficient explanation for the crystalline condition 
 observed in so many British sandstones. By placing a number of 
 grains of sand from a given deposit under the microscope, and count- 
 ing the proportion of worn to unworn grains, Dr. Sorby has shown 
 that the sand in several formations is derived from different sources. 
 He observes that the sand from the Boulder Clay at Scarborough is 
 fresh and angular, showing few or no rounded grains, and that the 
 modern beach at Scarborough, which is largely derived from the 
 Boulder Clay, has the grains scarcely more worn. In the Thanet Sands 
 
 Fig. 41. Grains of sand magnified, with quartz crystals upon and around the grains. 
 Alter J. A. Phillips and Sorby. 
 
 at Crossness, and in the Hastings Sands at Hastings, one-half of the 
 grains are worn. In the Upper Greensand the amount of wear in- 
 creases as we pass from Dartmoor to the east, one-tenth of the grains 
 being worn at Haldon Hill, one-fifth in the Isle of Wight, one-third in 
 Sussex, and one-half in Kent, indicating that as the deposit recedes 
 from the region from which it was derived the amount of wear 
 increases. Mr. J. A. Phillips, F.R.S., has examined the chief British 
 sandstones, and it may be useful here to give a brief summary of his 
 conclusions with regard to some of the more important. 
 
 The Barmouth grits in the Lower Cambrian series of North Wales, 
 between Barmouth and Harlech, often enclose angular fragments of 
 quartz fully a quarter of an inch in diameter, ^he deposit is an aggre- 
 gation of quartz and felspar united by a silicious cement tinged greyish 
 green with a mineral which is probably chlorite. The quartz contains 
 a few fluid cavities with moving bubbles. The felspar is of two kinds, 
 orthoclase and probably oligoclase. Some of the quartz contains 
 
LOWER PRIMARY SANDSTONES. 95 
 
 crystals of rutile, and the cement contains calcite, magnatite, iron 
 pyrites, and a few imperfect garnets. Near Harlech, the sands are 
 liner and purpler, but only differ in the cement containing crystals of 
 epidote. When analysed, silica forms 80 per cent, of this rock. The 
 well-known sandstone of the Stiper stones west of the Longmynd, near 
 Shrewsbury, has grains with an average diameter of one-fiftieth of an 
 inch ; some, rounded almost like pebbles, are converted into perfect 
 crystals by a deposit of transparent quartz upon the sand nucleus, and 
 the grains are generally so closely cemented by crystalline silica as to 
 form a quartzite. The few grains which contain fluid cavities usually 
 have the cavities full. Felspar is not abundant. 
 
 The grey grit of Aberystwith consists of nearly equal proportions 
 of quartz grains and felspathic grains cemented by silica. Some of 
 the grains are the -^th of an inch in diameter, generally rounded, 
 though a few are sharp ; the quartz contains very minute fluid cavi- 
 ties which are generally full, though some enclose moving bubbles, 
 and some crystals contain needles of tourmaline. The felspar is partly 
 triclinic, and fragments of a volcanic rock like basalt occur ; in the 
 silicious cement are small flakes of mica and a few crystals of iron 
 pyrites. This rock has been derived in part from the disintegration 
 of quartz felsite. 
 
 The May-Hill sandstone is chiefly composed of angular grains 
 o-J^th of an inch in diameter, united by a turbid silicious cement ; 
 there are a few larger grains, these like the smaller ones contain hair- 
 like crystals of rutile, but fluid cavities with bubbles are rare. In 
 the Lickey Hills, in Worcestershire, the rocks of this age have 
 the grains greatly rounded, the diameter of -g^th of an inch; fluid 
 cavities with bubbles are numerous in some of the grains, absent in 
 others ; rutile and tourmaline both occur in the quartz in minute 
 crystals. 
 
 The Denbigh grit consists chiefly of a fine-grained cement con- 
 taining both minute and larger fragments of quartz with felspar and 
 brown mica ; and the quartz sometimes encloses needles of tourmaline, 
 sometimes crystals of rutile. There are few cavities with bubbles. 
 
 In the Devonian rocks the grits near St. Austell are a mixture of 
 angular pieces of quartz and felspar ; the quartz includes crystals of 
 tourmaline and few fluid cavities. The felspar is partly orthoclase, 
 partly triclinic, and the rock contains a little silvery mica and a few 
 crystals of pyrites. Specimens from Ladock enclose many angular 
 fragments of a greenish slate; these are included among small rounded 
 grains of quartz, felspar, and other substances, among which are pieces 
 of volcanic rocks closely resembling the Cornish greenstones and dun- 
 stones ; but a second specimen from the same locality is chiefly made 
 of angular fragments with more abundant fluid cavities in the quartz, 
 some grains of which contain epidote and flakes of white mica. The 
 felspar is chiefly triclinic, and there are a few minute garnets and 
 some fragments of organic rocks. These data sufficiently indicate the 
 varied nature of the materials from which the Devonian grits of 
 Cornwall were derived. 
 
96 PRIMARY AND SECONDARY SANDSTONES. 
 
 In the Carboniferous system, Mr. Phillips has examined a fine- 
 grained sandstone in Cumberland belonging to the Yoredale series. 
 The quartz is angular and generally free from fluid cavities. When 
 cavities do occur they are full ; the rock also contains a little felspar 
 and kaolin, and some white mica. The Millstone Grit in Cumberland 
 consists almost entirely of grains of quartz T i-th of an inch in 
 diameter, bound together with a silicious cement, and containing a 
 little felspar. The quartz includes occasional fluid cavities and a few 
 needles of tourmaline. The sandstones of the Lower Coal Measures 
 near Bradford consist in the main of fragments of quartz and felspar ; 
 the quartz sometimes encloses tourmaline, but contains few fluid 
 cavities ; the felspar is chiefly triclinic, and is associated with some 
 kaolin, a few garnets, and flakes of dark and colourless mica. The 
 fragments in this rock are -5-50^ of an inch in diameter; but many of 
 the Carboniferous sandstones are chiefly formed of quartz grains which 
 have crystallised in the positions in which they now occur, for they 
 show no sign of abrasion, not having lost a point or an angle. 
 
 The Permian sandstone of Cumberland, which has a reddish tinge 
 owing to the opaque ferric hydrate in the cement, is a mixture of 
 angular fragments and minute crystals of quartz and a little felspar ; 
 the quartz grains contain few fluid cavities, and have a diameter of 
 g-J^th of an inch. The flakes of colourless mica are water-worn. 
 
 The Triassic sands vary considerably. Some of the Eunter sand- 
 stones of Lancashire and Cheshire, known as " Millet-seed Beds," 
 flow between the fingers like shot ; these have most of the grains 
 rounded like miniature pebbles ; their diameter is from -^jth to TrJ^th 
 of an inch ; they are partly quartz, partly felspar. The quartz grains 
 are frequently covered with transparent crystalline silica. Such 
 beds may well be blown sands, like those of existing deserts, 
 united by a ferruginous cement which has invested the grains. 
 In many of the beds the quartz is almost entirely in the form of 
 minute crystals; such a rock is well seen near Ormskirk. The 
 Upper Trias or Keuper generally consists of well-rounded silicious 
 grains. At Dymoke, in Worcestershire, the quartz sometimes encloses 
 crystals of rutile and fluid cavities with bubbles ; part of the 
 felspar in this rock is triclinic, and there are a few flakes of white 
 mica. In the cement are minute garnets and a little kaolin. The 
 beds called water-stones include angular fragments of dark-coloured 
 slate sometimes half an inch in diameter ; the quartz grains are often 
 y^th of an inch in diameter, much rounded, and contain fluid cavities ; 
 there is some felspar. Mr. Phillips believes that the grains in 
 Triassic sandstones which contain fluid cavities and crystals of 
 tourmaline are from a different parent rock to those grains from which 
 the cavities are absent. 
 
 The Upper Lias sand in Gloucestershire has a calcareous cement, 
 the quartz grains in which are generally angular, though some of the 
 angles are a little rounded ; they are -s-^th of an inch in diameter, 
 sometimes enclose fluid cavities without bubbles, and are associated 
 with numerous fragments of tourmaline and garnet, and probably a 
 
/> off THE ^P^ 
 
 'UNIVERSITY 
 
 SECONDARY AND TERTIARY SANDSff,^ 
 
 little felspar. This rock near Whitby also contains 
 garnets, but the quartz has no fluid cavities ; the grains are -Ath of 
 an inch in diameter, and have their angles removed, but are sur- 
 rounded with crystalline quartz which forms the cement. The quartz 
 in this rock is such as might be obtained from the decomposition of 
 clay slates. Sand from the Portland Stone in Wiltshire is rounded, 
 quartzose, and contains few fluid cavities, generally without bubbles ; 
 the grains vary from -^th to ^J 5 th of an inch. They are associated 
 with but not enclosed by ovoid grains of carbonate of lime. 
 
 In the Wealden Sands the quartz consists of slightly rounded 
 colourless grains almost free from fluid cavities, but enclosing some 
 hair-like crystals of rutile. The cement is partly carbonate, of lime, 
 partly flint. 
 
 The Neocomian Sand called Carstone at Hunstanton is mainly 
 composed of rounded grains of quartz containing tourmaline and 
 rutile, with a few fluid cavities generally free from bubbles. This 
 quartz is mixed with small granules of dark-brown pisolite, and 
 contains a few scales of mica and a little felspar. The rock contains 
 less than 50 per cent, of silica and 30 per cent, of oxide of iron, 
 though sometimes the amount of iron is more, and the silica some- 
 what less. The Sevenoaks stone of the Lower Greensand consists 
 of chert, in which there are only occasional grains of quartz, some of 
 which are rounded, while others are angular. 
 
 In the Hertfordshire Puddingstone, which occurs below the 
 London clay, the flint pebbles are united by a concrete, partly 
 formed of flint and partly of fragments of quartz. The quartz is all 
 angular, greatly in excess of the flint, and sometimes contains fluid 
 cavities. 
 
 The brilliantly coloured sands of Alum Bay, of Lower Bagshot 
 age, like the Triassic and Permian sands, lose their colour when 
 boiled in hydrochloric acid. The quartz grains are all worn, though 
 not completely rounded ; occasionally they contain crystals of tour- 
 maline, and sometimes fluid cavities, but rarely enclose bubbles. 
 
 In the Headon sands of Hordwell Cliff, all the quartz is com- 
 pletely rounded, and fluid cavities with bubbles are abundant. The 
 ruck contains no fragments of felspar. 
 
 The marine beds at the top of the Hempstead series in the Isle 
 of Wight include sand composed of quartz grains which are well 
 rounded. 
 
 The Boulder Clay at Holy well in Flintshire yields sand of a 
 varied character, some grains being small quartz pebbles, others 
 rounded particles of felspathic and other rocks. Some unworn 
 quartz grains derived from crystalline sandstones are observed, 
 associated with rounded grains derived from the millet-seed sand- 
 stone. 
 
 This detailed analysis of some of the important beds of sand 
 
 which occur in the British strata may serve to point out not only 
 
 the great field which is open to further investigation by examination 
 
 of the conditions under which the beds were accumulated, but is also 
 
 VOL. i. G 
 
98 OCCURRENCE AND ORIGIN OF GLAUCONITE. 
 
 indispensable to a right understanding of the minerals, which may 
 be produced when such sandstones are metamorphosed by the action 
 of heat and pressure, so as to become reconverted into crystalline 
 rocks, such as schists ; for the presence of felspar, iron, and many 
 other minerals, renders the composition and variety of such schists 
 readily intelligible. 
 
 Glauconite. 
 
 This is one of the most characteristic minerals found in sand. The 
 grains arc usually dark green, amorphous, and on being powdered 
 yield a bright-green colour; frequently they invest the cells of 
 foraminifera. 
 
 Glauconite is a double silicate of iron and alumina, with a certain 
 amount of the alumina replaced by magnesia, soda, and potash. Its 
 composition is nearly identical with seladonite, the green earth found 
 in the vesicular cavities of certain basaltic rocks. It is known to be 
 produced by the alteration of augite and hornblende, but in some of 
 the rocks it appears to have formed by replacing particles of yellow 
 ferruginous mud. Its origin is concretionary, and it is probably 
 formed at the time of deposition. At the present day considerable 
 deposits of glauconite are found off the coast of North America, so 
 that their existence was attributed by Professor Kogers to the influence 
 of an ancient Gulf Stream. Similar deposits have been met with 
 during the " Challenger " exploration off Portugal, off the east coast of 
 Australia, and off the Crozet Islands, at depths of 400 to 600 fathoms ; 
 and it may certainly be inferred that a stream flowing in the ocean from 
 a warmer to a colder region would inevitably have a tendency to pre- 
 cipitate certain of the mineral substances which the water originally 
 held in suspension when its temperature was higher. Glauconite occurs 
 in the green slates of the English Lake district, where it fills the cells 
 in fragments of pumice ; a similar material is found in the slate of 
 Penrhyn. But glauconite is most characteristic in the various Secon- 
 dary and Tertiary sands. In the Lias, sandy beds of the Forest Marble, 
 Calcareous Grit, Portland Rock, ISTeocomian Sand, and Upper Greensand, 
 it often gives a colour to the deposit. It is found at the base of the 
 Thanet Sands, and throughout those beds, and in the Bracklesham 
 Beds, and Barton Clay. It sometimes mineralises fossils. Its abundance 
 and constant repetition in the rocks of the South of England strongly 
 suggest that its existence is due to the same physical causes, such as 
 the denudation of like plutonic rocks. Nevertheless, in chemical 
 composition it is curiously similar to the residue of chalk which would 
 be left after the carbonate of lime has been removed ; and since there 
 is abundant evidence that the carbonate of lime of the Chalk has been 
 removed subsequent to the deposition of the Tertiary beds so as to 
 interpose a stratum of unworn flints on top of the Chalk, Professor 
 M'Kenny Hughes has argued that the glauconite in which those flints 
 are embedded has been formed in situ out of the residue of the Chalk, 
 and may be now forming. 
 
ORIGIN OF CERTAIN CLAYS. 99 
 
 Sandstones sometimes contain a large amount of brown oxide of 
 iron. This is due to infiltration. Occasionally sandstones contain 
 beds of chert, as in the Upper Greensand of Devonshire, Dorsetshire, 
 and the Isle of Wight, and in the Lower Greensand of Sevenoaks ; 
 but since chert is only developed where the sandstone is calcareous, 
 and is nothing but flint more or less porous, which contains some 
 calcareous matter, its origin will be better considered in discussing the 
 flints which are found in limestones. Small masses of phosphate of 
 lime sometimes abound in sands, especially the Upper Greensand of 
 the Isle of Wight and the Neocomian Sands of Bedfordshire and 
 Cambridgeshire, but since they are more characteristic of deposits 
 which have a floor of clay, they will be treated of after discussing the 
 history of clays. 
 
 Clay. 
 
 No such careful and detailed examination has been made of exist- 
 ing mud and clay as of sand or limestone. The subject is much more 
 difficult, and as a rule, nothing can be distinguished by the microscope 
 but more or less irregular granules, minute flakes of mica, and sometimes 
 needle-like prisms, with variable amounts of calcareous granules and 
 sand. There is necessarily every gradation between sands and clays 
 on the one hand, and limestones and clays on the other; and the 
 observations on the deposits now forming are too few to completely 
 demonstrate the conditions under which many of the newer clay beds 
 were formed. It may, however, be regarded as certain, that when the 
 quartz grains in clays are coarse the clays are derived from granite, 
 while when fine they are due to the destruction of schists. The 
 newer clays, as a rule, give no indications of pumice or volcanic dust, 
 but many of the older muds now changed into slate rocks appear to be 
 entirely of volcanic origin. Mr. Sorby has noticed that fine-grained 
 mud obtained in the South Pacific from a depth of 2600 fathoms, 
 possesses the following remarkable property : The grains of sand do 
 not separate from the finer mud and subside, but gather the finer 
 particles about them into a compound granule, and this process rapidly 
 clears the water. But it has been determined experimentally, that the 
 solid matter in such muds only amounts to n per cent., while in 
 shales the solid matter is at least 75 per cent., so that when pressure 
 squeezes the water out of these clays they may be reduced to one-sixth 
 of their original thickness, and this change would tend to develop in 
 the planes of bedding exactly such a fissile structure as is commonly 
 met with. Clay may originate in many ways ; the red earth found in 
 caves, and washed in by the streams flowing through them, is obtained 
 from the destruction of the neighbouring limestone rocks, for after the 
 carbonic acid gas dissolved in water has carried away the whole of 
 the carbonate of lime, there remains an insoluble residue of silicate of 
 alumina and oxide of iron, which, although forming but a small per- 
 
 ftage of the limestone, yet has often contributed to the accumulation 
 small deposits, such as those in caves. In the exploration of the 
 antic by the " Challenger," it was observed that there is much 
 
IOD MICROSCOPIC TEXTURE OF SLATE. 
 
 volcanic matter upon the sea-bed near to volcanic centres. Two such 
 masses occur in the middle part of the Atlantic, one obviously derived 
 from the volcanic islands to the west of Northern Africa, and the other 
 derived from the volcanoes of the West Indies and Central America. 
 These are areas occupied by red clay which Mr. Murray attributes to 
 the decomposition of pumice and other volcanic materials, finding 
 among the clay many fragmentary crystals of felspar, augite, and other 
 of the minerals which occur in volcanic rocks. This material has 
 been transported by the winds, and decomposed on the sea-bed often 
 at a depth of between two and three thousand fathoms. These regions 
 of the ocean abound in concentric nodules of black oxide of manganese. l 
 
 Kaolin, or china clay, is actually manufactured from the granite 
 at St. Austell in Cornwall, and near Plympton in Devonshire, by 
 breaking up the decomposed rock, and washing it on an inclined 
 plane, with pits arranged to catch the quartz, tourmaline, mica, and 
 other minerals, until the purified clay obtained from the felspar is 
 deposited in tanks and dried for export. Geological deposits almost 
 identical in character occur in the Bagshot Sands, especially at Poole 
 and Wareham. It may be interesting to remark that the tesselated 
 pavements in general use are produced by compressing dry clay till 
 it becomes solid, though as a rule heat is subsequently applied to 
 increase the hardness. 
 
 Clays when sandy are termed loam ; and when calcareous are marls. 
 Occasionally, as in the Kimmeridge Clay, bituminous beds of con- 
 siderable extent occur, and impart to the clay a combustible quality, 
 enabling it to be burned as fuel, a totally different property from the 
 spontaneous combustion sometimes set up in clay cliffs by decomposi- 
 tion of the iron pyrites. 
 
 Slate. Many of the so-called clay slates differ from modern de- 
 posits of clay in containing a very small amount of kaolin material 
 and an immense amount of mica, in flakes too small to be visible to 
 the naked eye, but which give a sort of silky lustre to the rock when 
 they lie in the plane of fracture, as in some of the black slates near 
 Llanberis. 2 With the mica are numerous needles or small black cry- 
 stals like hairs, probably of hornblende and magnatite. 
 
 The Devonian Slates are of similar character. When the crystals 
 of mica have formed in situ, they often abound about special centres, 
 but, as a rule, the mica is evenly distributed, and appears to have been 
 derived from the disintegration of an older rock, such as the micaceo 
 
 - 
 
 :he 
 ;he 
 
 mo 
 
 1 Volcanic ashes and lapilli occur in great abundance in the red clay of th 
 Pacific, which has its centre about the Low Archipelago, and extends towards th 
 Sandwich Islands. The lapilli are all of the basaltic type, but often become 
 glassy. Many of the palagonites are identical in character with those of Sicily, 
 Iceland, and the Galapagos Islands. The most abundant minerals are plagioclase, 
 augite, magnetite, a little sanadine, and hornblende. Quartz is absent. Many of 
 the lapilli are found to be cemented together with zeolites like christianite. 
 Minute crystals forming radiating masses of the same zeolite occur in prodigious 
 quantities in the red clay so as to form, according to the estimate of the Abbs' 
 Re'nard, about a third of its bulk. These zeolites are evidently formed in sit 
 Dr. Sorby has detected similar specimens in the Gault. 
 
 - H. G. Sorby, Quart. Jour. Geol. Soc., vol. xxxvi. 
 
 
SLATES OF WALES AND TH J LJK 
 
 felsites, which would have yielded materials exactly similar to the 
 micaceous clay slates, for when the particles of mica are so exceed- 
 ingly fine, they are not separated from the kaolin. 
 
 Cambrian Slates. Associated with the slates of Bethesda, near 
 Bangor, is a bed of felsitic ash. Some of the fragments are an im- 
 perfect pumice. There are quartz grains, which appear to have been 
 derived from a quartz felsite, for they occasionally enclose felsitic 
 material similar to the felsites of the neighbourhood. There are fre- 
 quently grains and needles of magnatite, and sometimes black grains 
 of basalt. Fragments of augite appear to have been altered into 
 chlorite. If, observes Dr. Sorby, the micaceous base of this ash 
 were worn down into dust in a volcanic crater, or more completely de- 
 composed by weathering, and the material afterwards sorted by gentle 
 currents, it would yield a deposit corresponding in all essential parti- 
 culars with the fine-grained slates of Penrhyn and Llanberis. Other 
 beds in the same group of rocks contain little mica and much kaolin, 
 and were evidently derived from an igneous rock which contained 
 relatively more felspar than that which yielded the Llanberis slates. 
 
 When examined under the microscope, it is amazing to see how 
 frequently some beds, like the pencil slate of Shap, consist almost 
 entirely of mica. These Shap slates show the particles disturbed by 
 pressure, and in process of being arranged into the parallel films 
 which constitute cleavage. 
 
 The Green Slates of Cumberland and Westmoreland are well 
 known to be almost entirely derived from volcanic ashes. Specimens 
 from Rydal and Langdale show the original material very little altered. 
 Sometimes the fragments are ^th of an inch in diameter, and are 
 derived from very vesicular lavas or rocks passing from the condition 
 of pumice through perfect glass to a devitrified felsite, owing to the 
 development in it of minute crystals. Quartz is frequently absent, 
 just as in the South Pacific red clay ; and when occasional grains of 
 quartz are found, the cavities in them are free from fluid. At Amble- 
 side large fragments of pumice occur with their cavities infiltrated 
 with calcite exactly as is the case at the present day in the fine-grained 
 mud of the ocean. Frequently, however, the rock is more changed, 
 so that it would be impossible to tell whether the ash was erupted 
 and changed in the water, or whether heat took any part in the pro- 
 cess. Much of the felspar, augite, and garnets may have originally 
 belonged to the ash, but other minerals may have been formed where 
 they are now met with. The Green Slates of the Lake district are 
 formed of fragments, so compressed in the plane of cleavage that it is 
 impossible to tell whether they were derived from felsite or recon- 
 structed out of an older slate. 
 
 The slates of Loch Awe, although presenting an ordinary appear- 
 ance to the eye, show under the microscope all the characters of a 
 very fine-grained schist. The slates of Moffat are of the usual granular 
 type, mixed with others which are highly micaceous, in which are 
 3 or 4 per cent, of glassy fibres like the Pele's hair of the Sandwich 
 1 slands. 
 
IPS .SEPTARIA.W CONCRETIONS IN CLAYS. 
 
 The Chiastolite Slate, on the flanks of Skiddaw, abounds in cry- 
 stals of chiastolite and hornblende, which have been developed in situ, 
 and the development of these and other minerals must always be con- 
 sidered to depend chiefly upon the original composition of the clay ; 
 for even the augite, in the consolidated peperino from Vesuvius, has 
 become changed into a zeolitic substance, which has thoroughly hard- 
 ened the whole deposit. 
 
 The chiastolite in some of the black slate of Ivy Bridge in Devon- 
 shire may be seen in process of being changed into mica, and masses 
 of mica are otherwise developed about special centres. The slate of 
 Liskeard contains scattered concretions, composed of crystals of mica 
 and quartz, which have been formed in situ. These little masses, only 
 T ^th of an inch in diameter, have been termed the very germs of 
 mica schist, and the rock appears to present a first stage of the deve- 
 lopment of that material. In other slates the concretions of quartz 
 and mica have become so abundant as to have coalesced and thrown 
 the residue of the rock into patches, which may possibly be incipient 
 garnets. This slate is seen near to granite near Wicca Pool. From such 
 a rock the transition is easy into mica schist, where the crystals all 
 lie in definite directions, which may be parallel to the plane of strati- 
 fication, parallel to the plane of cleavage, or parallel to joints; but 
 even when the structure is perfectly developed, original grains of sand, 
 milk-white, water-worn, and angular, often occur, associated with fel- 
 spar a good deal decomposed, and felspathic grains, which appear to 
 have been derived from a felsite. It has been estimated that these 
 grains of sand occur in about one-fifth of the slates and one-fifth of the 
 schists. The grains of quartz often contain crystals of rutile and 
 minute granules and fluid cavities, in which sometimes crystals of 
 alkaline chlorides are found. No granite or felsite is known in which 
 the quartz grains show all the characters exhibited by the schists of 
 the central Highlands of Scotland, and hence the parent rock for those 
 ancient muds is supposed to have been either a quartz felsite, or a 
 granite and a felsite denuded together. The proof that the crystals 
 of schists were not deposited where they occur is furnished by the 
 fact that they are fitted and dovetailed together in a complicated and 
 accurate manner, such as always occurs when crystallisation takes 
 place in situ. 
 
 Septaria. 
 
 Septaria occur in flattened ovoid masses in nearly all clays. They 
 usually contain 65 per cent, of carbonate of lime, 18 per cent, of silica, 
 and the remainder is about equal proportions of alumina and protoxide 
 of iron, though the amounts of these ingredients vary. They are often 
 called cement stones, because used for the manufacture of roman 
 or hydraulic cement. The stone is not durable, though it is dense and 
 hard. When these concretions are broken across, they are observed 
 to show numerous cracks, which are widest in the middle of the con- 
 cretion, and radiate in many directions from the centre towards the 
 circumference. When these cracks are filled with crystalline deposits, 
 
NODULE BEDS OF PHOSPHATE OF LIME. 103 
 
 tabulae or septa are produced, which give a name to the concretions as 
 well as manifest their most striking character. The septa originate in 
 nests of minute concretions. If a small concretion only an inch or 
 two long is examined, the internal cracks are seen to be empty, and 
 the external surface is visibly increasing in size. We then learn 
 that, as new matter is aggregated to the outside, it is soft, and 
 consists more of clay than of carbonate of lime. Afterwards the 
 carbonate of lime becomes infiltrated into the outer clayey layer, and 
 enters into crystalline combination, so as to expand the outer layer 
 more than the internal part. This splits the concretion internally but 
 not externally, and thus, as the whole mass enlarges, the system of 
 internal cracks also becomes better developed, forming cavities in 
 which water may accumulate, and deposit various crystalline sub- 
 stances on the margins of the fractures. As already observed, septaria 
 are spread in horizontal layers, and formed out of calcareous material, 
 which was originally spread continuously through the clay, but became 
 aggregated about centres by the solvent action of waters slowly passing 
 through the deposit. 
 
 Phosphatite. 
 
 Phosphate of lime, in the concretionary form known as phosphatitc, 
 is perhaps more typically associated with clays, though it sometimes 
 occurs in both sands and limestones. In internal structure the masses 
 are commonly amorphous, though in some rare cases they are septarian. 
 In external appearance they differ with the several deposits. The 
 oldest known bed is found at the top of the Bala limestone, imme- 
 diately under the shale that covers that rock, and is chiefly seen near 
 the town of Llanfyllin and in the Berwyn mountains to the north of 
 Dinas Mowddwy. 1 The bed varies in thickness from ten to fifteen 
 inches, and consists of concretions which vary in size from that of an 
 egg to a cocoa-nut. They are coated with graphite, more or less polished 
 and cemented in a black matrix. They contain from 40 to 60 per cent, 
 of phosphate of lime. The underlying bed of limestone sometimes 
 contains from 15 to 20 per cent, of phosphate of lime, and when the 
 bed of concretions becomes separated into two or three layers it is 
 parted by thin phosphatic limestone. There appear to be few traces 
 of organisms in the concretions. Perhaps the most extraordinary 
 thing in connection with this deposit is the circumstance that in its 
 western outcrop in the flanks of Aran Mowddwy, sulphur has almost 
 replaced the phosphate of lime. 
 
 Phosphoric acid is found in small quantities in some of the oolitic 
 limestones ; and the Cornbrash, so named from breaking up into a 
 soil which yields abundant crops of corn, contains a very appreciable 
 percentage of phosphate of lime. In the Speeton Cliffs, a phosphatic 
 band occurs on the horizon of the Portland beds. Another important 
 deposit is found in the Neocomian rocks of Bedfordshire and 
 Cambridgeshire. Here the deposit is concretionary, largely mixed 
 with fragments of highly altered metamorphic rocks and fragmentary 
 
 1 D. C. Davies, Quart. Jour. Geol. Soc., vol. xxxi p. 357. 
 
104 CRETACEOUS AND TERTIARY PHOSPHATE BEDS. 
 
 fossils, which, indicate considerable denudation of the underlying 
 Secondary strata and more ancient rocks. The concretions are yellowish 
 brown, of the average size of a chestnut to an apple ; the surface is 
 sometimes smooth and sometimes irregular. The bones of extinct 
 reptiles which occur in the deposit are always mineralised with phos- 
 phate of lime. The bed is rarely more than a foot or two in thickness. 
 
 In the Gault there are several beds which contain concretions of 
 phosphate of lime ; first at the base in the zone of Ammonites inter- 
 ruptus ; secondly, a thin band an inch thick in the lowest. zone of 
 Ammonites auritus ; thirdly, in the zone of Ammonites De-la Ruei. 
 A thicker bed, ten inches thick, occurs in the zone of Ammonites 
 mammilaris at the junction of the Folkstone beds and Gault, and also 
 at the junction of the Upper and Lower Gault, and other bands are 
 found higher up. In some localities, as at Farnham in Surrey, and 
 Puttenham in Buckingham, the bed of phosphatic nodules is sufficiently 
 thick to be worked for the manufacture of super-phosphate of lime. 
 On the opposite coast of France, at Wissant, the nodules of phosphate 
 of lime in the Gault are particularly abundant. 
 
 The middle division of the Hunstanton limestone, in Norfolk, has 
 a concretionary character, but the quantity of phosphate of lime is 
 only about 10 per cent. 
 
 The Upper Greensand is particularly rich in concretions of phos- 
 phate of lime in Cambridgeshire, Bedfordshire, and adjacent districts. 
 The nodules are a very dark brown, sometimes tinged green exter- 
 nally with glauconite, grains of which abound in the marl in which 
 they are embedded. The nodules are extremely irregular in form and 
 rarely more than three or four inches in diameter, though rolled 
 masses are sometimes a foot long. Almost all the organisms are more 
 or less mineralised with phosphate of lime. This deposit rests on the 
 Gault. 
 
 Beds of phosphate of lime occur at or near the base of both the 
 Coralline and Red Crag where those beds rest upon the London Clay. 
 The bed is usually a foot or two in thickness ; the nodules are usually 
 more or less smooth and sometimes polished, of a reddish-brown 
 colour. Among them occur many fossils derived from middle Tertiary 
 beds and London Clay, with numerous contemporary remains of 
 vertebrate animals. There has always been some difficulty in account- 
 ing for the existence of these deposits. It is well known that the 
 island of Sombrero in the West Indies has the limestone rock so 
 mineralised with phosphates as to be quarried and exported for the 
 manufacture of artificial manure. Similar deposits occur at Cura9oa, but 
 we have no evidence as to whether this condition is entirely due to infil- 
 tration from guano deposited on the rocks, or to the former growth and 
 decay of plants like those of the Gulf Weed over a submerged region. 
 Certainly, no such condition could be appealed to in explanation of 
 the origin of the concretionary deposits under consideration, nor would 
 it bring us any nearer to the origin of the material to attribute it to 
 the denudation of deposits in pre-existing Palaeozoic rocks, such as are 
 known to occur in Estremadura. All the phosphoric acid must be 
 
ORIGIN OF LIMESTONES. 105 
 
 supposed to have been derived originally from the destruction of 
 volcanic rocks and then extracted from the water by various organisms 
 in precisely the same way as carbonate of lime and other substances 
 are assimilated. 
 
 What these organisms are, has not, however, been satisfactorily 
 determined. Many animals, no doubt, would yield on decay an appre- 
 ciable amount of phosphates, if the material could accumulate tran- 
 quilly on the sea-bed, and infiltrate into limestone ; but all these beds 
 give more or less complete evidence of being formed in shallow or 
 comparatively shallow water; and it has been urged that these are 
 the positions where marine plants, which contain much phosphoric 
 acid in the ash, most abound. Being rooted to the ground or growing 
 over a definite area, they would tend to accumulate during a long 
 interval of geological time a considerable quantity of phosphates, such 
 as could be derived from no other source. These submarine forests 
 are, moreover, the chosen feeding-grounds alike of herbivorous and 
 carnivorous animals, and the phosphates set free would all be capable 
 of combining with lime, and would thus explain the accumulation 
 over limited areas of such beds of phosphatic nodules. 
 
 Salt and Gypsum. 
 
 The origin of rock salt from evaporation ol the ocean and salt lakes 
 is sufficiently obvious to need no detailed exposition ; but Mr. Darwin 
 and Mr. David Forbes have described in South America many beds 
 in which, owing to the action of vegetation, other substances have been 
 formed. There are great deposits of nitrate of soda, as well as horizontal 
 layers of gypsum. The nitrate beds generally contain salt, and the 
 nitrate forms from 20 to 75 per cent, of the bed. In drought, most 
 of the streams of the Pampas become saline, and at Bahia Blanca the 
 surface is covered for a quarter of an inch with a deposit that consists 
 of 93 per cent, of sulphate of soda, and 7 per cent, of common salt. 
 In many of the natural salt lakes near the Rio Negro, the salt- is asso- 
 ciated with crystals of selenite and sulphate of soda. There is seen 
 to be every phase of change from chloride of sodium into sulphate of 
 soda, and from sulphate of soda into sulphate of lime, the latter sub- 
 stance being developed wherever shells were sufficiently abundant to 
 furnish the requisite lime, and the waters sufficiently sulphurous. 
 There can, however, be no doubt but that the isolated crystals of 
 selenite, common in all our clays, result from the decay by oxidation 
 of iron pyrites, so that the liberated sulphuric acid combines with 
 the lime of shells. But the great deposits in the Trias of Nottingham, 
 Stafford, and Cheshire, which are often associated with rock salt, 
 require such an explanation as is suggested by Mr. Darwin's observa- 
 tions. 
 
 Limestones, 
 
 Though many ancient limestones have obviously been partly re- 
 constructed out of older deposits, yet every limestone deposit must be 
 
io6 ARAGONITE AND CALCITE FOSSILS. 
 
 regarded as separated from the water in which the carbonate of lime 
 was originally dissolved. The distinction into fresh- water and marine 
 limestones is not of much importance, though the fresh-water strata are 
 probably more largely due to the precipitation of lime by the growth 
 of plants, like Chara, than to the agency of mollusca and other kinds 
 of animal life, which form so large a part of marine deposits. The 
 carbonate of lime of limestone sometimes exists in the crystalline form 
 of calcite, sometimes in the form of aragonite, and many shells have one 
 layer of calcite, and the other layer of aragonite. There is no means 
 known by which calcite can be changed into aragonite, the former being 
 a remarkably stable substance, but aragonite is as strikingly unstable. 
 When its temperature is raised it passes into a mass of crystals of cal- 
 cite ; it is also easily dissolved, and since calcite is usually deposited 
 from cold solutions of carbonate of lime, it happens that organisms 
 formed of aragonite are often removed entirely from a deposit, or 
 replaced by structureless calcite. This difference explains not only 
 the circumstance of preservation of many groups of fossils, but also 
 important points in the general structure of limestones. 1 The follow- 
 ing table may be useful in explaining the chemical condition of the 
 shelly skeletons of the chief groups of fossils : 
 
 ARAGONITE ORGANISMS. CALCITE ORGANISMS. 
 
 Cephalopoda are aragonite. I Brachiopoda are all calcite. 
 
 Gasteropoda are mostly aragonite ; but ! Annelida are calcite. 
 
 Patella, fusus, I/itorina, Purpura, 
 have the outer layer calcite. 
 Laniellibranchida are mostly aragonite, 
 but Ostrea and Pecten calcite ; Pinna, 
 MytiLus, Spondylus, outer layer calcite. 
 
 Crustacea are calcite. 
 
 Echinodermata are calcite. 
 
 Polyzoa are a mixture of calcite and 
 
 aragonite. 
 Alcyonaria, calcareous forms are calcite. 
 
 Hydrozoa millepora, mainly aragonite. I Foraminifera, calcite. 
 Actinozoa, almost wholly aragonite. j Corallines, mainly calcite. 
 
 All marine limestones are formed from the remains of these 
 organisms, or by chemical precipitation and deposition. 
 
 Of late years it has been demonstrated that everywhere through- 
 out the deep ocean a white limestone is accumulating which, when 
 dried, presents the consistency and appearance of white chalk. In the 
 Atlantic this white globigerina ooze, as it is termed, consists almost 
 entirely of shells of such Foraminifera as Globigerina, Pulvinulina, and 
 Orbulina, mostly entire, with some otolites of fishes, and the mutilated 
 dead shells of half-a-dozen genera of pteropods. With these are asso- 
 ciated a small proportion of finer material which consists of the struc- 
 tures termed coccoliths and rhabdoliths, 2 with a few spines and 
 skeletons of the silicious radiolarians, and fragments of spiculas of 
 sponges. Besides the surface forms of Foraminifera, Cristellarian and 
 Milioline 3 forms occur which lived among the ooze, together with 
 sponges, corals, starfishes, the higher invertebrata, and a few fishes. 
 Below the surface layer thus composed is a somewhat firmer layer, 
 
 1 Sorby, Brit. Assoc. Reports, 1862, Sect., p. 95. 
 
 2 Wyville Thomson, " Challenger " Voyage in the Atlantic. 
 
 3 Carpenter's Foraminifera, Ray Society, 1862. 
 
MARINE LIMESTONES NOW FORMING. 107 
 
 an inch or two thick, with the shells more or less broken up and 
 cemented into a calcareous paste, while beneath this bed the deposit is 
 a nearly uniform paste, with only a few shells and fragments scattered 
 through it. Thus it is evident that changes obliterating many of the 
 more delicate organisms are rapidly brought about in such a limestone 
 as is now forming on the floor of the Atlantic. Where the deposit 
 becomes impure, as in the Mediterranean, it is yellow. 
 
 In contrast to such a deep ocean deposit is the white granular lime- 
 stone forming the Bermuda Islands. This substance consists of coral 
 sand, often cemented into a rock which can be polished. It is produced 
 entirely by the wind, and may show from wind action only, according 
 to Sir Wyville Thomson, in a short distance, appearances which re- 
 semble all forms of denudation and unconformity, as well as anti- 
 clinal and synclinal folds. These seolian rocks exhibit most regular 
 stratification, and at Elbow Bay there is, what has been termed, a sand 
 glacier, about twenty-five feet thick, which has come in from the 
 beach, filled up a valley, and is steadily progressing inland. This 
 limestone is full of caves, hollowed out by running water, or by the 
 sea. One of these caves, called the Painter's Vale, contains a lake 
 through which stalagmites rise up sometimes in pinnacles, sometimes in 
 fringes, while from the roof innumerable stalactites several yards long 
 descend and taper to points like knitting-needles. 
 
 Coral islands are a well-known example of limestone masses, built 
 up partly by the direct agency of organic growth, and partly by the 
 power of the waves to grind the coral into sand. Mr. Darwin has 
 described the margin of a coral island as largely formed by great 
 masses of the coral Porites, which are irregularly rounded, from four 
 to eight feet broad, and parted from each other by crooked channels 
 about six feet deep. As the coral extends upward it spreads laterally, 
 so that many of the masses terminate upward in broad flat summits, 
 where the corals are dead. Next in importance is the genus Millepora, 
 which grows in thick vertical plates, so intersecting as to form a strong 
 mass, like a honeycomb. In some reefs the brainstone coral, Mean- 
 drina, abounds. Other stony corals live at a depth of a few fathoms, 
 and in the lagoon are thin, branching, and brittle corals of other kinds. 
 The water outside a reef deepens gradually to twenty-five fathoms. At 
 less than ten fathoms the surface is rugged when not covered with sand, 
 but below twenty fathoms, coral sand is always met with. On the 
 margin of the reef are three kinds of nullipores, extending as a fringe 
 about twenty yards wide, and a few feet higher than other parts of the 
 reef. One of these grows in expanded masses, like certain lichens on 
 trees, another has a radiating structure, and is made up of stony joints 
 as thick as a man's finger ; and the third is reticulated, and formed of 
 branches no thicker than crow-quills. These nullipores and the coral 
 sand are cast up and cemented together by the evaporation of the sea- 
 water. The rolled fragments often form islets, and as the channels 
 are filled up, the surface of the reef becomes a smooth hard floor, as 
 though composed of ordinary limestone. It is this flat surface, from 
 one to three hundred yards wide, which the nullipore margins. It is 
 
icS NATURE OF LIMESTONES NOW FORMING. 
 
 exposed to the sun at low water, and thus hardened by chemical 'preci- 
 pitation. Hence coral limestone is far from being formed by corals 
 only, and at some depth the cavities in the corals themselves are 
 always obliterated by carbonate of lime deposited in them by infiltrat- 
 ing waters. 
 
 Remarkable calcareous deposits of small extent sometimes occur on 
 land. Dr. Phene l has described a deposit of carbonate of lime which 
 has buried up the Roman city of Hierapolis in Anatolia, and covered 
 a large extent of country. Under the most eccentric and beautiful 
 forms half the city is submerged by a mass of intensely hard rock 
 which blocks up streets, temples, and arches. After reaching the 
 level of its source it ran over the natural aqueducts which it formed 
 as it went, only to begin and build up newer ones from a lower level. 
 Six or eight of these walls occur, each nearly fifty feet in height, show- 
 ing that many hundred feet of deposit have taken place since the 
 Roman occupation. Part of the rock is perfectly white, having the 
 aspect of drifted snow and frozen cascades. Other parts of it are as 
 perf sctly black. This is an example of the way in which springs such 
 as have formed the deposits of travertine in many parts of Italy and 
 the Auvergne may contribute to form limestones on land ; while if the 
 
 Fig. 42. Deposit of Trave-tin at a Cascade. 
 
 spring had flowed into a lake the deposit might have been more evenly 
 spread, but would probably not have been less in amount. 
 
 On the shores of the Bahamas, the rocks are a hardened deposit of 
 fine calcareous mud, but in some can be detected fragments of coral- 
 lines, corals, mollusca, and foraminifera. In all seas the mollusca 
 contribute largely to the formation of limestones, not merely by the 
 accumulation of shell-beds but by becoming disintegrated and broken 
 up a condition which may result either from the mechanical action 
 of currents or from decomposition when the organic substance which 
 bound their particles together is removed. 
 
 Some shells like Pinna and Inoceramus are chiefly composed of 
 minute prisms, which may be set free as fibres when the organic matter 
 of the shell is lost. Similarly encrinites and all echinoderms fall to 
 
 1 Brit. Assoc. Reports, 1879, Sect., p. 344. 
 
OLDER BRITISH LIMESTONES. 109 
 
 pieces when the animal matter disappears, but the separate joints or 
 plates are rarely minute ; and sometimes, as in the mountain limestone, 
 form a rock in which the eye can easily distinguish the component 
 materials by the calcite cleavage. 
 
 Several recent limestones, like those of Bahama and Bermuda, 
 contain oolitic grains. They have the aspect of having been formed in 
 water containing mud derived from corals and decayed shells, but 
 they are essentially chemical deposits, and show a granular texture in 
 some grains and a crystalline texture in others. Thus, all the types 
 of limestone which are met with in the strata are recognised as being 
 in process of formation at the present day, and the following short 
 account of the limestones as observed under the microscope by Dr. 
 Sorby l will sufficiently demonstrate the conditions under which they 
 severally came into existence. 
 
 The Bala Limestone is one of the oldest of British limestones. As 
 typically seen, it consists almost wholly of entire joints of an encrinite 
 embedded in fine-grained material which is for the most part made up 
 of mica or chlorite, such as is found in the associated slates. Locally, 
 it is oolitic. 
 
 The Wenlock Limestone is chiefly formed of more or less com- 
 minuted encrinites, with fragments of corals, bryozoa, brachiopods, 
 and trilobites. Near Malvern, oolitic grains are sometimes found 
 in this rock ; and in the finer-grained beds Entomostraca, are 
 common. 
 
 The Aymestry Limestone and calcareous beds in the Ludlow 
 rocks are essentially similar in composition, except that fragments of 
 brachiopod shells are more abundant, and there are occasionally por- 
 tions of bone, probably fish. 
 
 The Devonian Limestones, seen at Ilfracombe, Torquay, and 
 Plymouth, are chiefly composed of joints of encrinites and fragments 
 of coral. The fragments of brachiopod and other shells are much rarer. 
 As in the Silurian limestones, foraminifera are not met with. Some- 
 times the Devonian limestones are converted into dolomite. 
 
 The Carboniferous Limestone. In the typical localities the greater 
 part of this rock consists of joints of encrinites, sometimes entire, 
 sometimes broken, associated with fragments of brachiopoda and 
 foraminifera, which are often as abundant as in average specimens of 
 chalk. Recognisable fragments of corals and polyzoa are found, and 
 sometimes shell prisms occur in great quantity. Occasionally copro- 
 lites, teeth, and fragments of bone form not unimportant consti- 
 tuents of the rock. At Bristol and other places some of the beds are 
 oolitic. In Derbyshire and elsewhere some beds are changed into 
 almost pure dolomite, but the organisms give no proof that the rock 
 consisted originally of magnesia to any appreciable extent. 
 
 Magnesian Limestone. Many beds originally contained oolitic 
 
 grains and shell fragments ; and such beds have exactly the structure 
 
 that would be developed in an oolitic rock, if it became crystalline, 
 
 by having half of its lime replaced by magnesia. The bulk of the 
 
 1 Address, Quart. Jour. Geol. Soc., vol. xxxv. p. 77, &c. 
 
I io NEWER BRITISH LIMESTONES. 
 
 rock has the beds consisting of large and small crystals with occasional 
 altered shells of Foraminifera and Entomostraca. It is hence evident 
 that the change has been brought about by infiltrating waters. 
 
 Lias. Lias limestones chiefly consist of joints of Pentacrinus, 
 often with fragments of brachiopods, oysters, and shell prisms, with 
 some Foraininif era and fibres of belemnites. Finer beds are almost free 
 from Pentacrinites, and are made up of fine shell sand and decayed 
 shells. Grains of glauconite occasionally occur. 
 
 Oolites. A large proportion of the oolitic limestones consist of 
 fragments of echinoderms, oysters, brachiopods, and shell prisms ; 
 and, as compared with older rocks, contain an unusual amount of com- 
 minuted aragonite organisms mixed with rounded oolitic pellets, 
 formed by chemical precipitation round nuclei. The ferruginous 
 oolitic grains of Dundry have an unusually concentric structure. 
 They were often broken during their formation, and the fragments 
 became the nuclei for fresh grains. The fissile condition of the 
 Stonesfield slate is to a large extent due to small and thin laminae 
 derived from the shells of oysters and brachiopods. In many cases 
 the grains in the Great Oolite show a structure which suggests that 
 they were originally formed of aragonite in concentric layers, and 
 were afterwards changed into calcite. 
 
 The Forest Marble consists to an unusual extent of fragments of 
 Terebratulae, though larger parts of the rocks are made up of echino- 
 derm fragments. At Bath it is chiefly comminuted oysters, and at 
 Frome triturated corals and polyzoa. In some beds oolitic grains are 
 abundant. The Cornbrash in Yorkshire sometimes includes vast 
 numbers of small flat crystals of carbonate of iron. The oolitic 
 grains of the Kelloway rock have generally recrystallised into fibrous 
 calcite. 
 
 The Coralline Oolite is chiefly formed of broken calcite shells, 
 of echinoderms, Ostrea, Perna, Mytilus, Serpula, and brachiopods. 
 Some beds are chiefly formed of the shells of Renulina, others of 
 aragonite corals. Certain of the oolitic grains have for nuclei grains 
 of quartz. 
 
 In the Portland Oolite, the building stone is a shell sand varying 
 in fineness, and mixed with oolitic pellets formed of fine granules 
 (due to decayed shells) cemented together in grains which have a 
 radiate structure. 
 
 The Purbeck Limestones are sometimes shell sand, chiefly derived 
 from aragonite shells ; often they abound in entomostraca, and fre- 
 quently contain a large number of imperfect oolitic grains mixed with 
 shell mud. 
 
 The Wealden Limestones are chiefly composed of fragments of 
 fresh-water shells with entomostraca, and occasional pieces of bone, 
 and in some beds laminae of oyster shells. 
 
 The Kentish Rag in Kent consists of shell sand and shell mud 
 mixed with quartz sand and other impurities. Glauconite abounds. 
 Foraminifera are found sparingly, and there are some oolitic grains 
 derived without doubt from denudation of oolitic rocks. 
 
SILICIOUS CONCRETIONS IN LIMESTONES. in 
 
 In. the Firestone there is some volcanic ash. 
 
 Chalk. Unbroken foraminifera make up but a small part of 
 the Chalk, and all the fragments of these organisms together would 
 not make up half the bulk of the rock. Prisms from the shells of 
 Inoceramus, portions of Ostrea, Pecten, polyzoa, and echinoderms 
 are abundant, as are spines and spiculse of sponges. Fragments of 
 brachiopods are rare. Coccoliths abound. In some localities there 
 is abundance of fine quartz sand. In the soft Upper Chalk the cells 
 of the Foraminifera are empty, in the hard chalk of Yorkshire they 
 are filled with calcite. 
 
 The Tertiary limestones in this country are mostly fresh-water 
 deposits, and limited to the Isle of Wight ; unless we except the 
 marine bryozoa limestones of the Coralline Crag in Suffolk. 
 
 Flints. 
 
 Flints are concretions of silica which have been accumulated in 
 the strata, after their consolidation, by the solvent action of per- 
 colating waters, which have dissolved the substance of various 
 minute skeletons of silicious organisms, and re-deposited the material. 
 The chief accumulations of flint are met with in the Carboniferous 
 Limestone, in the Portland and Purbeck beds, and in the Chalk. It is 
 probable, in some cases, that no small amount of this silicious material 
 has actually been derived from the solution of overlying sandstones, 
 which have happened to contain sufficient lime to render the silica 
 soluble. In the Carboniferous Limestone, the whole substance of the 
 rock is sometimes removed and replaced by flint or chert, which 
 exhibits cavities formerly occupied by fossils. In the Portland 
 beds, the black flint is sometimes found in horizontal, sometimes 
 in inclined fissures ; and it frequently fills the interior of certain 
 fossils, such as the Cardium dissimile. In the Upper Greensand of 
 Warminster, and tire Greensand of Black Down, the substance of 
 many shells has been completely replaced by silica, and this con- 
 dition is often met with in the Thanet Sands and other deposits. 
 The most typical exhibition of flints, however, is furnished by the 
 Chalk. The amount of flint in the Chalk varies with the locality, 
 and is nowhere more remarkable than in the neighbourhood of 
 Axmouth and Beer Head, though the individual flints are relatively 
 small. Their number is, however, so great as sometimes almost to 
 obliterate the appearance of arrangement in horizontal layers. 
 Though chiefly characteristic of the Upper Chalk, flints are not 
 exclusively limited to it, but they never occur in bands of nodules 
 in the Lower Chalk. The size of the nodule, and the distance apart 
 of the horizontal layers, which mark stratification, are extremely 
 variable. The nodules themselves are of very irregular form, always 
 white on the external or growing surface, where it is in contact with 
 the chalk, and black or blackish in the interior. They are well 
 developed in the Chalk of Norfolk, and the softness of the chalk is 
 usually in proportion to the development of flint nodules. 
 
 Near London the nature of flints may be easily studied in the chalk 
 
Ji2 FLINTS IN THE CHALK. 
 
 pits of Charlton, Grays, Gravesend, and Caterham. Sometimes, as in 
 the neighbourhood of Leatherhead, the nodular flints have become sub- 
 sequently cemented together, and embraced in horizontal tabular masses 
 of flint. Another mode of occurrence of this substance is seen in the 
 chalk of Norfolk and of Antrim, where subcylindrical vertical masses, 
 like sacks of flour or sugar, are piled one above the other. These ver- 
 tical flints are commonly called " pot-stones," because as they stand up 
 on the beach near Cromer, they exhibit a tubular or pot-like appearance. 
 This form is nowhere better seen than at Horstead, near Norwich, 
 where the flints extend all round the pit in vertical columns, which 
 pass through the horizontal layers of nodules. Sometimes the external 
 surface is smooth, sometimes nodular and irregular. Each flint varies 
 in height from one to five feet, and is usually a foot or two in diameter. 
 
 A third mode of occurrence of flint is well seen on the coast of 
 Sussex near Rottingdean, where fissures in the Chalk are filled with 
 tabulae of black flint, which are inclined to the stratification, and some- 
 times pass through layers of nodules. . 
 
 The formation of flint has generally been attributed to the 
 growth, at the time the Chalk was forming, of organisms having 
 a silicious skeleton, such as Diatomaceae, Polycystinse, and sponges. 
 The flint in fissures, like a multitude of other phenomena, demon- 
 strates that its accumulation in the forms now seen is of later date 
 than the consolidation of the Chalk. In fact, flints in chalk have 
 grown like septaria in clays. The material was there from the 
 first, diffused in the rock, but it has taken all subsequent time 
 for it to assume its present conditions of aggregation. Dr. Car- 
 penter, impressed with the way in which sponges, like Holtenia, 
 radiate their silicious roots into the calcareous mud of the ocean in a 
 cylindrical figure, has suggested that the accumulation of silica from 
 the growth of successive generations of sponges, one above another, on 
 the same spot, might account for the nucleus around which the silica 
 of pot-stones became aggregated. At the present day the chalk mud 
 of the deep ocean includes in some localities from twenty to thirty or 
 forty per cent, of silica, chiefly due to the accumulation on the bottom 
 of silicious skeletons which fall from the surface ; and therefore, 
 unless they were especially abundant at particular periods of time, 
 the horizontal layers of nodules would have to be attributed mainly to 
 a similar extraordinary development of sponges on the ocean floor, for 
 unless the silicious matter had been deposited more freely in certain beds 
 of chalk than in others, it would have been impossible for the diffused 
 flint in the Chalk to have gathered about nuclei in the way observed 
 in the horizontal layers. Dr. Wallich believes that the flint was ac- 
 cumulated in the protoplasm of sponges, but there is no evidence 
 to confirm this view. It may, however, be remarked that the Upper 
 Chalk of Yorkshire which contains no flints has in it twice as much 
 silica as the middle or flint-bearing Chalk. Flint may occur sporadi- 
 cally, investing organisms of all kinds, or even in internal cavities of 
 teeth into which no organisms could have penetrated. 
 
CHAPTER VIII. 
 
 CORAL REEFS. 
 
 FROM a very early period in the history of stratified formations, coral 
 reefs have been formed in the old seas ; but there is now no evidence 
 to demonstrate the conditions under which those corals flourished. It 
 is quite possible that the extinct forms may have known no limitation 
 of depth or of temperature such as influence the distribution of 
 existing reef-building corals. It is even possible that they may have 
 been able to withstand the influence of muddy waters, so fatal to the 
 animals forming coral growths at the present day. In any case, it- 
 would be unsafe to infer that the old forms of coral life were 
 governed by laws similar to those which now influence the group, in 
 face of the evidence that the generic types have in the long lapse of 
 geological time undergone considerable modification of structure, as 
 well as change in geographical distribution. The simple form of coral 
 is not limited in its distribution by depth of sea ; and some species 
 live off our own coasts and far north, as well as in the tropics. And 
 since the compound form of coral is a consequence of the fact that 
 the polyps increase by buds which rise from the margin of the parent 
 cup, and thus grow into a dense mass, it must be regarded as a geo- 
 graphical accident resulting from the distribution of land and water 
 that reef-building corals are now almost entirely confined to the tropics. 
 
 In its simplest form, the coral is closely allied to the sea anemone, and 
 in fact differs essentially from the latter in possessing a skeleton 
 within the base and tubular investing substance which usually sends 
 converging plates towards the centre, which correspond to the radiating 
 partitions of the sea anemone called septa. In both groups the 
 tentacles, which when expanded give the flower-like appearance to the 
 polyp, remain un calcified, and form soft fleshy substance. Some of 
 these simple corals, such as the mushroom-like forms of Fungia, attain 
 a considerable size ; and such simple forms, represented by genera like 
 Paleocyclus and Petraia, occur in the Silurian and Cambrian rocks. 
 Considerable beds of limestone formed of corals are found in the 
 Palaeozoic strata. 
 
 Rocks formed of Corals. Corals found fossilised do not appear to 
 have formed accumulations of a thickness to be compared with existing 
 coral reefs, but to have grown more after the manner of corals which 
 occur around the shore of a rising or stationary land. The oldest 
 
 VOL. I. H 
 
H4 ANCIENT AND MODERN CORAL GROWTHS. 
 
 British limestone largely formed of corals is the Wenlock limestone ; 
 and here the formation is composed largely of starlike corals of the 
 genera Heliolites, Favosites, and Coanites, together with several which 
 form more or less expanded masses, such as Syringopora and Halysites, 
 Arachnophyllum and Syringophyllum. Again, in the middle of the 
 Devonian formation, the Plymouth limestone often consists to a large 
 extent of corals, among which are many genera, such as Favosites and 
 Heliolites, which first appeared in the Wenlock limestone, with the 
 addition of new forms, such as Smithia, &c. In the Carboniferous 
 Limestone, especially in parts of Derbyshire, corals have accumulated 
 so as to build up a considerable thickness of rock. Among these are 
 the genera Nematophyllum, Stylaxis, Strombodes, Likoshotia, Cyatho- 
 phyllum, and Michelenia. In the Secondary strata, especially of this 
 country, corals of a compound type are less numerous ; and although 
 they have been met with in isolated masses in the Lias, Inferior and 
 Great Oolite, it is only in the Coral Rag that limestone has been formed 
 by their growth and from material derived from corals worn up by the 
 sea. And similarly in the British Tertiary rocks there seems to have 
 been no period during which corals formed reefs, although in the 
 newer beds of the Lower Tertiary series, several examples of compound 
 corals are found. 
 
 In existing seas, the most northern point at which reef-building 
 corals live is the Bermuda Islands, which lie in the direct course of 
 the Gulf Stream, and, therefore, in water warmer than is usual in 
 that latitude. But, speaking generally, reef-building corals range 
 between nearly 30 north of the equator and about 25 south of it. 
 They are considered by Professor Dana l to die when the temperature 
 of the sea falls below 66, and they are not found alive at a greater 
 depth than twenty-five fathoms. This depth they reach at the equator, 
 probably because the water there at that depth is warmed to the tem- 
 perature necessary to their existence. 
 
 All the coral reefs of the world belong to one type, but they 
 have been classified by Mr. Darwin 2 for convenience in illustrating 
 questions of movements of the earth's crust into three groups, which 
 he names fringing reefs, barrier reefs, and atolls. 
 
 The Fringing Eeef, under certain circumstances of depression of 
 land, may in time become converted first into a barrier reef, and at 
 last may take the form of an atoll ; so that no broad distinction can 
 be drawn between these forms of reefs, which succeed each other in 
 much such a way as childhood passes into youth, and youth into man- 
 hood. The fringing reefs are so named, because they grow as a 
 fringe around the shore, usually in shallow water where the sea- 
 bed slopes at a slight angle. They grow at varying distances from 
 the coast, which depends upon depth and the amount of sediment 
 derived by tidal waters from the shore. The reefs are usually laid 
 more or less bare at low water, when the bright-coloured red or green 
 living animals are seen to rise just out of the water like a mud bank, 
 through which numerous channels of water extend to the ocean. 
 1 Coral Islands. * Coral Reefs. 
 
FRINGING REEFS. 115 
 
 Fringing reefs are formed of exactly the same genera of corals as 
 the barrier reefs and atolls. The reef may be from 250 to 500 
 yards wide. The most common corals forming the reefs belong to the 
 genera Porites, Millipora, Pocillipora, and the brainstone coral Mean- 
 drina. 
 
 Fringing reefs are met with at both ends of the Red Sea, 
 on both the Arabian and African shores. They extend all down 
 the Zanzibar coast and the coast of Mozambique. They surround 
 the Seychelle Islands and the Mauritius, and occur round the north- 
 east and south-west shores of Madagascar. They occur on the 
 steeper shores of Ceylon, and round the Nicobar Islands, and are 
 prolonged on the southern coast of Sumatra, and occur along the 
 whole of the chain of volcanic islands which ends with Timor. 
 Fringing reefs occur between Borneo and Malacca, around the 
 Loochoo Islands, among the Philippines, southward to Ceram. The 
 Mariana Isles have fringing reefs, as have the Solomon Isles, the New 
 Hebrides, the Friendly and Navigator Isles; and the Sandwich 
 Islands are thus margined. Fringing reefs also more or less surround 
 nearly the whole of the West Indian Islands, and form a southern 
 prolongation of Florida. Coral reefs are, however, entirely absent 
 from the Pacific Coast of America, and are rare on the coast 
 of Asia, east of the Persian Gulf. Thus it will be seen that with the 
 exception of the reefs on the African coast, they almost all occur 
 around islands, and especially on eastern coasts of the great masses of 
 land. The fringing reefs appear usually to grow on shores which are 
 either stationary, or which in comparatively recent times have been 
 upheaved. Thus in Florida, reefs are found extending inland, exactly 
 similar to those which are now forming on the shores ; and from these 
 the late Louis Agassiz 1 endeavoured to estimate the period during 
 which the reefs must have been growing from the known rate of in- 
 crease of the corals on the Florida coast. He considered the existence 
 of these inland reefs to represent duration of time, equivalent to many 
 millions of years. But the rate at which corals increase varies with 
 the species and in different seas. 
 
 In the Red Sea, where the temperature is extremely high, 
 never falling below 70, and often rising to 120, the coral grows 
 so slowly, that its rate of increase is almost imperceptible. On 
 the coast of the United States and in other places corals have been 
 taken up from year to year, and measured and been found on an 
 average to grow about half an inch in twelve months. But in one 
 case where coral has grown upon the sunken British ship the 
 " Shannon," it was found that supposing the coral to have commenced 
 growing as soon as the ship reached the bottom, it could not have 
 increased at a slower rate than three inches in a year. This coral 
 belonged to one of the more open branching types of the genus 
 Madripora. In other cases corals were planted on the coast of Mada- 
 gascar, which in six months appear to have reached a height of nearly 
 three feet; and it is recorded that a ship which had been in the 
 1 Agassiz, Natural History Studies. 
 
n6 REEFS ON SINKING LAND. 
 
 Persian Gulf for twenty months had a growth of coral on her copper 
 bottom two feet thick. When living on the reef, the corals nourish 
 best in the waters most exposed to the waves. 
 
 The Barrier Reef only differs from the fringing reef in the cir- 
 cumstance that the land near which it grows has sunk or become 
 depressed to a considerable extent, so that instead of coming within 
 half a mile or a mile or two of the shore, as the rule is with fringing 
 reefs, it is usually at a distance of ten or twenty miles, or in some 
 cases even fifty miles from land. Barrier reefs occur in the middle 
 part of the Red Sea ; and like the fringing reefs on the African and 
 Arabian shores, they surround islands like the Comora Isles in the 
 Mozambique Channel, and the Pelew Isles; they extend over con- 
 siderable areas in the Pacific, notably forming the great barrier reef on 
 the north-east coast of Australia, and extend round New Caledonia, some 
 of the Fiji Islands, and the Society Islands. New Caledonia has one of 
 the most instructive of barrier reefs. It is 400 miles long, and has an 
 average distance from land of about ten miles. The island of New 
 Caledonia is 250 miles long; so that we are led to the conclusion that 
 New Caledonia has become by depression of the sea-bed 150 miles 
 shorter since coral first began to grow around it in the form of fring- 
 ing reef. 
 
 The Australian barrier reef, which extends for uoo miles, is 
 usually about twenty miles from the shore, though sometimes the dis- 
 tance increases to fifty or ninety miles. The depth of this channel 
 varies from as little as fifty or sixty feet to as much as sixty fathoms in 
 its southern part, where it is at the greatest distance from land. 
 
 Atoll is the name given to the reef when the land around which 
 the coral grows has become completely submerged, so that the spot 
 where it existed is only marked by the circle of coral which grew up to 
 the surface of the sea as the depression progressed. This ring is 
 occasionally perfect and frequently broken into segments. These gaps 
 are precisely similar to those observed in fringing, and barrier reefs 
 where rivers flow into the sea and bring down fresh water and mud, 
 which renders it impossible for coral to thrive in such positions ; 
 though gaps are sometimes met with where the cliffs furnish to the sea 
 a large amount of mud. Hence it has been inferred by Mr. Darwin 
 that breaks in the continuity of an atoll reef may generally be taken 
 to indicate the positions at which the streams flowed into the sea 
 which drained lands now entirely submerged in the ocean and buried 
 under a crown of coral. As a rule, the reef grows upwards as fast as 
 the sea-bed is submerged, but occasionally the depression is so com- 
 paratively rapid as to drown the coral Atolls are chiefly met with in 
 the Indian and Pacific Oceans. Among the former are the Laccadive 
 and Maldive Islands, the Chagos Bank, and the Saya de Malha, which 
 form a stretch of submerged land between India and the north of 
 Madagascar. It is impossible not to recognise that such a group of 
 islands before they were submerged may well have been higher, and 
 have formed continuous land from Mozambique to the Malabar coast. 
 Such a land would help materially to account for the African element 
 
INDICATIONS OF UPHEAVAL AND DEPRESSION. 117 
 
 in the fauna of India. Reeling's Island is an atoll in the middle of 
 the Indian Ocean, which may have some connection with the ancient 
 continuity which existed between the Mazarine region of Eastern 
 Africa and the Malay Archipelago. In the Pacific, the chief atolls are 
 the Low Archipelago, Gilbert Islands, Marshall Islands, and Caroline 
 Islands. 1 
 
 It is thus seen that atolls extend in lines like fringing reefs and 
 barrier reefs with which they alternate. If, then, fringing reefs occur 
 chiefly in areas of upheaval and other reefs in areas of depression, the 
 Pacific and Indian Oceans exhibit to us by these means corrugations 
 of the ocean floor which are now in progress, insensible it may be in 
 amount during the periods for which they have been observed, but 
 obviously analogous to the changes of level, of which the strata and 
 existing contours of land are demonstrative evidence. If the atolls 
 exist in great synclinal folds which are in process of depression, the 
 lands bordered by fringing reefs are as certainly in the position of 
 great anticlinal folds undergoing upheaval. To the physical geologist 
 the distribution of coral reefs in the strata presents an important 
 factor in elucidating the relations of Land and Water while de- 
 posits were accumulating, especially in the Tertiary, Cretaceous, and 
 Jurassic periods. 
 
 1 See Map in Danvin's Coral Reefs. 
 
CHAPTER IX. 
 
 COAST-LINES AND THEIR ORIGIN. 
 
 EVERY part of the earth which rises out of the sea is distinguished by 
 its own peculiar outline. This outline, in which the ocean marks a 
 definite level around the land, is the sea-coast. Its fantastic curves 
 on some shores, and scarcely broken straight extent on other lands, 
 are not a matter of accident ; for the causes which raise islands from 
 the sea, also determine the main directions in which the coasts run. 
 Inlets, bays, channels, and headlands may have to be explained by dis- 
 covering the courses of old rivers, or the work of rain, and the kinds 
 of rocks exposed; but the coast line has been produced slowly at 
 successive ages of the earth's history, and parts of it have from time 
 to time been portions of lands of far different outline to those of 
 existing continents and islands, though the ancient lands are now 
 more or less destroyed and submerged. 
 
 Influence of Altitude. iSTothing perhaps will help so well to make 
 intelligible the first and simplest law under which a coast line may 
 change, as to take a map on which are drawn lines showing the 
 course taken over the country by contours indicating levels at ever- 
 increasing heights such as would be marked by the sea, if the land 
 were submerged to that extent. Then the successive steps would be 
 traced by which a large mass of land may become broken into islands, 
 and the reason why the smaller islands are formed would be more or 
 less clear, for the sea necessarily would cover the low land first. 
 Similarly with the sea ; lines which mark depths of increasing 
 amount in hundreds of feet enable us to understand how islands may 
 be enlarged, united together, and into continents, and have the course 
 of their coast line changed, by being merely uplifted so that the sea 
 drains off from regions which it once covered. 
 
 Wherever a coast line remains for some time unchanged in level, 
 the wearing power of the tides will usually convert what had previously 
 been a shelving shore into a sea-cliff. If, then, land is upheaved at in- 
 tervals, with periods of pause during which no upheaval takes place, then 
 inland cliffs will be formed which correspond to these intervals of rest. 
 The position in which cliffs are produced is often governed by the way 
 in which the layers of rock forming the country are arranged. This 
 arrangement of the strata into hard beds and soft beds is accompanied 
 by an inclination of the deposits technically called "dip." The sea 
 acting upon deposits so inclined abrades and wears away the exposed 
 

 COAST-LINES OF THE OLDER ROCKS. 119 
 
 edges so as to undermine the rocks and convert them into precipices 
 on the seashore which are called cliffs. But when the deposits 
 shelve down gently into the water, there are no weak places in the 
 single stratum exposed which make it easy for the sea to cut a way 
 through the formation. Since the whole country, even in recent geo- 
 logical times, has been elevated from out of the ocean, terraces must 
 inevitably have been produced inland in this way at successive heights, 
 though in many cases the rounding influence of the action of rain has 
 more or less modified and obliterated the earlier work of the sea. But, 
 although dip and mineral structure may help to demonstrate the reason 
 why some coasts are worn away so as to be bordered by steep sea-cliffs, 
 such considerations give no insight into the origin of the outline of 
 the country or the way in which a sea-coast became a part of its 
 geological history. 
 
 North and West Britain. In dealing with the British Islands, it 
 is necessary to have before us a map of this country and the adjacent 
 parts of Europe coloured geologically. It will then be seen that the 
 prevalent direction of the land forming the northern part of Britain is 
 not a matter of accident. The Hebrides, Shetland, and Orkney all run 
 in a direction from south-west to north-east. The strata in the part 
 of Scotland north of the Caledonian Canal have a similar direction. 
 The larger mass between the Moray Firth and the Firth of Forth is 
 also extended so as to be parallel to the Outer Hebrides, and this mass 
 has the Grampians for its axis. The southern part of Scotland is 
 similarly traversed by the Carrick, Moorfoot, and the Lammermoor 
 Hills, the rocks forming which extend through the country south- 
 west into Ireland. If now the direction of these strata be com- 
 pared with the rocks forming the Dovrefeldt, which is the mountain 
 axis of Norway, the Grampians will be seen to be but a southern 
 prolongation of that range, and, followed on into the north-west of 
 Ireland, the Mourne Mountains carry the chain still farther to the 
 south-west. No one who compares the west coast of Scotland and 
 the coast of Wales with the opposite coast of Ireland, can fail to see 
 that the strata of the two islands were originally continuous, and that 
 the Irish Sea and channels to the north and south have been hollowed 
 out by some cause which has not interfered with the direction in 
 which the strata extend. From a geological point of view, then, 
 Great Britain and Ireland may be treated as though they were one 
 land. Nor must it be forgotten that the moderate elevation of six 
 hundred feet would unite them together as a tableland, and prolong 
 the coast of Scotland in a north-east direction to within a few miles 
 of the Norwegian coast. Any geological student looking at a map 
 will see that this direction of the old Primary rocks forming these 
 countries is a consequence of the way in which the rocks at an early 
 period of geological history were thrown into folds so as to be elevated 
 from out of the sea. Each of the long strips of land forming Scot- 
 land and the isles west and north occurs as a saddle-like fold of the rocks 
 of the kind termed anticlinal. The narrow strips between the Firth 
 of Forth and the Firth of Clyde, and between the Moray Firth and 
 
120 ORIGIN OF BRITISH ISLES. 
 
 the Firth of Lome, are trough-like folds of the kind named syn- 
 clinal. This is manifest at once from the fact that the regions of 
 the firths are composed of basins of newer Primary rocks, while the 
 intervening mountainous regions consist of the older Primary rocks 
 from which the newer deposits have been worn away, if they were 
 ever deposited upon them. 
 
 Scotland probably originated in the elevation of the Grampians, 
 which were at first, as Professor Judd l has demonstrated, a chain of 
 active volcanoes which poured out enormous quantities of angular 
 fragments and sheets of lava during much of the Old Red Sandstone 
 and Carboniferous periods. But it is unlikely that the folds which are 
 now to be seen determining the direction in which the rocks of Scot- 
 land extend, were fully developed until the close of the Primary 
 period, and all these folds appear to be a consequence of a great com- 
 pressing force acting from the north-west towards the south-east. It 
 would be a mistake, however, to speak of them as having formed 
 Scotland at this early period, unless we remembered that Scotland 
 was merely a portion a south-western prolongation of the Scandi- 
 navian land. JSTor can we consider this folding of the rocks without 
 bearing in mind that it is parallel to other ancient folds forming 
 mountain chains near the eastern coast of North America, and that 
 the bed of the Atlantic itself is thrown into mountain ridges, one of 
 which extending from Rockall on the north-west of Scotland runs 
 south-westward far beyond the Azores to the mouth of the Orinoco. 
 Everything in geological history leads us to believe that the great 
 folds of the earth's crust having once been made, undergo no impor- 
 tant change in direction in the successive geological ages. They 
 may be modified chains which were high may sink beneath the 
 ocean ; they may be broken up by folds which cross them transversely, 
 but the evidence of their existence endures in the contorted rocks, 
 and we are thus aided in determining the relative antiquity of our 
 shores. It is probable that the old land thus indicated was connected 
 on the south with the region which is now Normandy and Brittany. 
 The Cambrian rocks of Wicklow correspond with those of North 
 Wales; the Carboniferous and Old Red Sandstone strata of South 
 Wales, which run nearly east and west, are continuous with the cor- 
 responding rocks in the south of Ireland. There these deposits occupy 
 folds which are alternate saddles and troughs similar to those which 
 are prolonged farther south by the rocks of Somerset, Devon, and 
 Cornwall, continuous with the folds of Belgium, and parallel to 
 those of the North of France. But here the direction of the resistance 
 to the compressing force has changed, and hence the strike of the strata 
 is almost due east and west in the south, although the intermediate 
 angles can be traced in Wales and the adjacent English counties suffi- 
 ciently well to demonstrate that there is no conclusive reason for 
 assigning a much later date to the southern than to the northern com- 
 pressions. If the latter took place during the great elevation of land 
 at the close of the Permian period, the former are not more recent 
 1 Quart. Jour. Geol. Soc., vol. xxx. p. 289. 
 
EAST AND WEST COASTS. 121 
 
 than the close of the Triassic age, when the central European land 
 once more disappeared. 
 
 Mid-Britain. At this period the rocks which form the greater part 
 of South Britain were not even deposited in the ocean. They com- 
 prise the whole of the Secondary series newer than the Trias. These 
 rocks are chiefly alternations of clay and limestone running through 
 the country in a direction from north-east in Yorkshire to south-west 
 in Dorsetshire; superimposed upon them in the south-east are the 
 still newer deposits named Tertiary. If our attention be now turned 
 to the northern part of England, it will be noticed that there is a 
 great central axis called the Pennine Chain, which runs north and 
 south. This, it will be observed, consists of Carboniferous rocks. 
 Kesting upon its western side are the newer Triassic strata, upon which 
 in the south of Cheshire rests an outlying mass of the Lias. This 
 little patch of Lias, like a similar patch in the valley of the Eden, is 
 sufficient to show that that formation was once more widely spread 
 in the West of England ; while it may have been continuous with 
 other patches of Lias in the north of Ireland and the Inner Hebrides. 
 But we are more concerned with it, because it demonstrates that the 
 synclinal fold of South Cheshire was produced after the Lias had been 
 formed. But when we turn to the east of the Pennine Chain, the 
 whole of the Secondary strata, up to the Chalk, will be found in 
 sequence, all of them running north-east and south-west, and all 
 dipping to the east. This eastern dip teaches us that the Pennine 
 Chain, though originated after the Permian and before the Trias, 
 and progressing up to the Cretaceous age, was elevated finally at 
 some period more recent than the deposition of the Chalk. There- 
 fore, after the Secondary rocks had all been deposited, this part of 
 Europe continued to experience a compressing force acting from east 
 and west, which threw the strata into north and south folds. It is easy 
 to see that the outline of the country at that remote time was quite 
 different from what it is now ; indeed, if North Britain had then been 
 formed, an annual waste of the cliffs during the long subsequent ages 
 no more in amount than now goes on would have produced a percep- 
 tible effect on the outlines of the land. But between Elamborough 
 Head and the Tees it may be noticed that all the Secondary rocks, 
 instead of continuing their course to the north, bend round in the 
 Vale of Pickering and the Yorkshire moorlands, striking almost due 
 east. It is possible that this eastern bend is of newer date than the 
 north-to-south extension, but in any case it must be attributed to a resist- 
 ance in the north, such as would be created by a folded prolongation of 
 the Scottish Hills towards the Norwegian country. It might at first 
 sight be supposed that the narrow form of the north of England is due to 
 this east-to-west squeezing, but an examination of a geological map will 
 be enough to show us that although there were formed at this time 
 several subordinate parallel folds in the north of Wales, yet that the adja- 
 cent coasts of Wales and Ireland form an anticlinal, which excludes the 
 possibility of the Irish Sea having originated in a compression which de- 
 pressed that region in the same manner as the North Sea was depressed. 
 
122 THE SOUTH COAST. 
 
 South-East Britain. Finally, in the south- west the Tertiary strata 
 extend chiefly in an east-and-west direction, which the Wealden and 
 Cretaceous rocks also share. The Wealden elevation, which, it will be 
 noticed, extends across into France, is clearly of the same date as the 
 folds termed the London and Hampshire basins. The clue to the period 
 when those synclinal folds were formed is furnished by the newer 
 strata in the Hampshire basin. These are chiefly lacustrine and estua- 
 rine deposits, showing that the land towards the middle of the Tertiary 
 period was becoming more and more upheaved. Hence, since we 
 know that upheaval only takes place as a consequence of the lateral 
 compression of the rocks that is to say, of the formation of folds in 
 them we are led to fix the middle Tertiary or Miocene period, during 
 which much of Europe was in a state of dry land and the Alps were 
 rising, as the age when the north-to-south compressions operated in the 
 south-east of England. It will be observed upon the map that the east- 
 to-west extension of these newer deposits is paralleled by the similar ex- 
 tension from west to east of the Primary deposits of the south of Ireland 
 and west of England. Hence, the direction of the eastern folds seems 
 to be attributable to the persistence of an underlying resisting mass of 
 consolidated rock similar to that which determines the folds on the 
 western side of the island. It is within the limits of possibility that 
 the Dogger Bank and the Yorkshire moorlands may show, by their 
 eastern extension, limits of this Miocene compression on the north. 
 Thus the several portions of Great Britain have been formed gradually, 
 and although we have only considered the main folds which influence 
 the directions in which a coast extends, still the effect of these com- 
 pressions is obvious in the great east-to-west extension of the southern 
 part of the island, and in the manner already indicated in the north. 
 
 The Channel We should seek in vain for any evidence of a 
 convulsion of the kind mentioned, which would account for the 
 separation of Britain from the Continent. That problem is like the 
 separation of Ireland from Britain, and requires to be studied along 
 the coast and on the geological map, which demonstrates that the 
 waters which divide these countries are rather analogous to the Bristol 
 Channel, where it separates Wales from Devonshire, than such waters 
 as the Moray Firth or the Firth of Forth. The difference between these 
 inlets of the sea consists in the fact that the former exists in what was 
 an anticlinal fold, while the latter occupy synclinal folds. This anti- 
 clinal fold between Wales and Ireland, and between the Isle of Wight 
 or Kent and France, means of course that the intervening region was 
 thrown into an elevation, which may never have risen from out of the 
 water, but which was essentially a mountain ; and by a well-known 
 principle, familiar to all who know the shape and structure of hills 
 among the old rocks, it has resulted that just where the deposits 
 were 'most uplifted and stretched and cracked, there they were worn 
 away with the greatest ease, and replaced by depressions. The 
 English and Irish Channels are therefore valleys, which have been 
 scooped out in consequence of being thrust up. The rocks thus 
 exposed to the action, first of the sea and afterwards of the atmosphere, 
 
HEADLANDS AND BAYS. 123 
 
 were more easily worn away because they had been already bent and 
 broken by stretching along the axis of their upward bend. The 
 excavation was a gradual process. In the south-west the English 
 Channel commenced to be formed at an early period. The Channel 
 is deeper in that direction, and therefore presumably older. It is 
 impossible to determine how much of the excavation has been accom- 
 plished by ordinary waste along sea- cliffs, as the land was raised so 
 as to bring every portion of a valley successively under the power of 
 the breakers, and how much may have been worn away during the 
 long ages for which its whole area remained in the condition of dry 
 land, and exposed to the action of rain and winds and frost and rivers. 
 It is probable, however, that these two powers, alternating with each 
 other, have widened the Channel slowly in the way in which we see 
 it widening in parts at the present day, by the waste of cliffs when 
 they are undermined by the sea, and sawn out into gulleys by the 
 land-springs which pour over them. Perhaps the most instructive 
 parts of our cliff scenery are those which show the intimate connec- 
 tion of the island with the continent. All along the south coast from 
 the Straits of Dover to Cornwall, the rocks of South Britain and the 
 opposite coast of France face each other as though cleft asunder. 
 
 But besides its direction, every shore presents the minor features of 
 bays, inlets, cliffs, and capes, whose existence is only intelligible by 
 help of a knowledge of the ways in which the several geological 
 formations which make up the dry land have been accumulated, 
 folded, and upheaved so that the edges of strata are exposed on the 
 shores where land rises out of the sea. 
 
 Headlands. This dependence of headlands upon geological forma- 
 tions is well exemplified in Flamborough Head, in the North and South 
 Foreland, in the promontory of Beachy Head, and in Culver Cliff and 
 the Needles at the east and west ends of the Isle of Wight. All these 
 headlands consist of chalk, and although chalk may be worn away by 
 the sea like any other formation, when acted upon by the grinding 
 power of the breakers, it cannot be disintegrated and washed up into 
 easily-transported sediment like the underlying and overlying sands 
 and clays. Hence, since its removal is largely dependent upon the 
 chemical power of water to dissolve the limestone and take it up into 
 invisible suspension, the rock is more enduring than the associated 
 deposits which rest upon it and which it covers. And being a thick, 
 homogeneous formation, which often has its fore-shore defended with 
 a barrier of flint derived from the waste of the Upper Chalk already 
 destroyed, it happens that this formation juts out into the sea, while 
 on each side of it the strata are excavated by tidal attrition into bays. 
 Of such bays Sandown Bay and Compton Bay are familiar examples, 
 due to the removal of the soft underlying strata below the Chalk. 
 
 But the sea is often admitted into the land without any regard 
 to nature of the strata, simply because they happen to be bent down 
 into a trough, part of which sinks below the sea-level. This is the 
 case with the estuary of the Thames and the Southampton Water, 
 both of which owe their existence chiefly to lying in synclinal folds, 
 
124 SHORES CHANGE WITH LEVEL OF LAND. 
 
 though partly to the ease with which the sea could encroach on the 
 loose clayey and sandy formations, when, owing to a different level of 
 the land, circumstances were more favourable for its work of excava- 
 tion. The most important class of inlets occupies the positions of 
 what were formerly dome-shaped or anticlinal folds. 
 
 The Shore. As a district became depressed and the sea admitted, 
 every portion of the land must in succession have been a shore, and 
 the shore moved gradually with the depression of the land to a level 
 which was progressively higher. When we remember the power 
 which the sea possesses of throwing up around our coast in stormy 
 seasons not merely the spoils of life but masses of rock from great 
 depths, a mechanism becomes discernible which has brought gravel 
 beds and our pebble beaches gradually into their present position in 
 times antecedent to the final shaping of the contours of the coasts. 
 The beach follows the shore, and it may be that much of the material 
 thus brought back again had previously been scoured from the present 
 seaward slopes of the country in an antecedent age, when its level 
 was higher. These materials are ever reinforced with the hard frag- 
 ments worn from the nearest local source, and with pebbles driven 
 along the shore by waves lashed by the wind. A remarkable 
 instance, however, of the mode of origin of such pebble beds is 
 furnished by the Chesil Bank, which stretches for about eleven 
 miles on the Dorset sea-board from Portland to Abbotsbury. Port- 
 land Bill has arrested the movement of pebbles to the east like an 
 artificial groin, but when their nature is examined, and the large 
 size of the pebbles in the eastern end of the bank near Portland is 
 considered, there is no sufficient reason for believing that they travelled 
 out of Cornwall and Devon along the existing shores when the rocks 
 of the floor of the English Channel could so easily have furnished an 
 assemblage of this kind, which would equally have become embedded 
 in the ledge of Kimmeridge Clay which forms the foundation of the 
 bank, and there became heaped up as the land descended to- its present 
 position. 1 The same agencies which have brought the pebble beds to 
 our shores have been chiefly concerned in the production of sea-cliffs. 
 We know the rapid waste of certain parts of the coast, where noble strips 
 of land have in historic times passed, often with towns and villages 
 upon them, back into the sediments of which they were originally com- 
 posed, and have been swept out over the floor of the German Ocean. 
 But all our coasts happily do not crumble away like those of Yorkshire, 
 and though the changes which take place from year to year prove 
 that the existing aspect of many cliffs is of very recent origin, yet 
 their geological structure often makes it probable, even when proof 
 is wanting, that they too have come down to us from an immeasurably 
 distant past. Some coasts are especially favourable to the formation 
 of cliffs, because the rocks are hard and not easily worn away, while 
 the land which they form rises to a fair height from the sea. Sea- 
 side towns generally occur where gaps appear between cliffs, though 
 there are many exceptions. The gap furnishes a ready means of 
 1 Prestwich on the Chesil Bank, Institute of Civil Engineers, vol. xl. 
 
SAND DUNES. 125 
 
 reaching the sea, and often owes its existence to a bed of clay 
 which had been exposed down to a low level on that coast, and 
 eaten back by the sea into a bay. This bay is usually a point from 
 which the adjacent harder rocks may be undermined, for drained 
 of the moisture they contained, owing to the dip of the strata, their 
 substance contracts and becomes divided by innumerable cracks and 
 division planes, separating into blocks which have no support or firm 
 coherence with the mass of the stratum, when the underlying portion 
 between tide-marks has been removed. After falling, these fragments, 
 when hurled back by the tidal waters, become battering-rams for 
 making further inroads into the sea-wall of rock, and thus the process 
 goes on, governed by the direction of the wind and the currents which 
 move the water. The height of a cliff is governed chiefly by the 
 height of the adjacent land. On some parts of the west coast 
 of Scotland, the height of cliffs is immense ; and, as a rule, among 
 the contorted and upheaved Primary formations cliffs are higher 
 than among the newer formations. But the waste is less rapid, 
 and the cliffs often show in their retreat from the shore, in their 
 upper portions, evidence of denudation, and different relative posi- 
 tions of land and water to those which exist now. The Secondary 
 rocks, from their loose texture, have wasted at a more rapid rate, and 
 the cliffs are often high, because easily undermined, and so eaten back 
 that the traces of earlier denudation have become obliterated. The Ter- 
 tiary cliffs of the east and south-east of England are mostly of moderate 
 height, because the level of these deposits rises so little out of the sea, 
 as may be seen in the Crag formation at Felixstow and Aldborough, 
 while on many parts of this coast of Suffolk cliffs have no existence. 
 
 Sand Dunes. Though the sea thus destroys the coast, and 
 forms cliffs, yet the winds often protect the shores where no cliffs exist. 
 On sandy coasts, as the tide runs down, the sand left dry is caught up 
 by the wind and blown inland, only to be arrested by vegetation stop- 
 ping its movement at *a little distance from the water. A mound thus 
 formed becomes permeated and reticulated by the roots of those plants 
 which specially luxuriate in these conditions, and year by year the 
 mounds may grow in height until a ridge of hills, or sand dunes, 
 often of considerable height, is formed guarding the shore. In our own 
 country these sand hills are less developed than on flat continental 
 coasts ; but on some parts of Norfolk, South Wales, and Cornwall, 
 good examples of sea-coast scenery of this kind may be seen. Sir 
 Charles Lyell published a sketch of the church tower of Eccles in 
 Norfolk covered up in a sand dune, as it appeared in 1839 ; but the 
 winds which heap up the sand may reverse their direction, and in 
 1863, when we last visited this part of the British coast after unusually 
 strong equinoctial gales which scoured all the cliffs clean in the north 
 of Norfolk, every trace of the outer range of these sand dunes was 
 gone. The church remained clean and roofless by the shore, the fields 
 which had been newly ploughed before the sand had covered them 
 were laid bare, as fresh as though the furrows were newly turned, even to 
 showing the prints of the hoofs of asses which had drawn the small and 
 primitive plough. Prior to the formation of the sand hills the waste 
 
12* DESTRUCTION OF CLIFFS. 
 
 of the coast had been rapid, and their removal serves to show how 
 potent a factor wind is in transforming coast scenery. Probably its 
 importance inland is even greater. 
 
 Landslips. Another feature of the coast which conspicuously 
 affects the scenery in many parts of the south of England is the occur- 
 rence of landslips. Though not directly the work of the sea, they 
 occur most frequently on the coast. When the strata happen to dip 
 towards the sea, and some water-bearing sandy bed in a clay, or 
 resting upon a clay, thoroughly moistens the underlying deposit so as 
 to cause it to become slippery, then the weight of the superincum- 
 bent rock is often sufficient to break a strip of jointed rock away from 
 the adjacent inland mass of the stratum, and cause it to slide into the sea, 
 there to become broken up by the action of the waves. One of the largest 
 of these landslips is that which from Ventnor to Blackgang forms the 
 peculiar scenery of the Undercliff, caused by the water held by the Upper 
 Greensand moistening the underlying Gault, so that Chalk and Upper 
 Greensand have slipped away to the sea. Similar phenomena, formed 
 in 1839, are to be seen west of Lyme Regis, near Axmouth. 
 
 Waste of Cliffs. On the whole length of Holderness, the waste of 
 the cliffs is not less than two and a quarter yards in breadth annually. 
 The average loss on the coast of Norfolk between Weyburn and Sher- 
 ingharn is about one yard per annum ; on the coast of Thanet two or 
 three feet. But these same coasts likewise exhibit, on an equally grand 
 scale, the formation of new land from the materials thus detached from 
 the old. The materials which fall from the cliffs are sorted by the 
 tide, and according to their bulk and weight are differently disposed 
 of. As in many artificial processes of washing powders, the sedi- 
 ment is divided into parts of different fineness by merely shaking it 
 at different distances or depths in the stream of water, so it is in the 
 great currents of the sea. Large stones remain a long time at the 
 foot of the cliff from which they fell, smaller masses yield something 
 to the impetus of the waters, sand and pebbles are drifted along the 
 shore according to the set of the tide, and collected into bays and 
 hollows of the coast, or deposited in a line of moving beach ; but the 
 finer clays are transported far away in the waters, and allowed to 
 settle only where these rest in land-locked gulfs, stagnate over weedy 
 marshes, or lose their force in contest with the freshes. The breadth 
 of the sandy beaches thus accumulated is often very great, even 
 many miles of slow and regular descent. The sand banks which 
 stretch out so far from the low coasts are often regarded as remains 
 of ancient lands overwhelmed by the sea, but in most cases they are 
 probably recent formations, accumulated by the waves from the 
 spoils of other regions. But what is thus left by the sea under 
 some circumstances, may be again reclaimed by it under others. The 
 once fertile district called North Friesland, most probably accumu- 
 lated by the sea, measuring from nine to eleven geographical miles 
 from north to south, and six to eight from east to west, was in 1240 
 entirely severed from the Continent, and in part overwhelmed. The 
 island of Northstrand, thus formed, was, towards the end of the 
 
ORIGIN OF ISLANDS. 127 
 
 sixteenth century, only four geographical miles in circumference, but 
 still was richly cultivated and populous. At last, in 1634, in one 
 night, the llth of October, a flood passed over the whole island, 
 whereby one thousand three hundred houses, with many churches, 
 were lost, fifty thousand head of cattle and above six thousand men 
 perished. Three small isles alone remain, and they are still further 
 wasting. It may often be remarked that substances thrown into 
 the sea are not carried down at once to its depths, but rejected 
 many times to the shore, in the direction of the tidal currents. This 
 happens especially with all light, small, and easily moved bodies ; 
 but the case is different with the large blocks of stone, which, con- 
 tinually pressing by their weight downwards, are for the most part 
 gradually withdrawn from the base of the cliff, sunk in the beach, 
 and rolled down to the deep. 
 
 Islands. Islands originate in very different ways. Most volcanic 
 islands are, properly speaking, volcanoes, and have been upheaved out 
 of the sea which surrounds them by the same forces as produce the 
 volcanic eruption. Coral islands, or atolls, on the other hand, are 
 rather examples of rock structures built up out of the sea by coral 
 polyps than islands in the ordinary sense of the term. What is 
 an island now, in the last geological age was probably a submarine 
 shoal, and in the next age is likely to be part of a mountain axis. 
 Hence it is that islands often occur in chains, when they are but the 
 peaks of mountains rising from out of the sea. Or, they often ter- 
 minate peninsulas, or lie contiguous to large masses of land, because 
 some low pass is depressed beneath the sea-level, through which 
 the sea flows, and severs the island from the adjacent land. Thus 
 Tierra del Fuego is but a portion of the Andes, which does not run 
 continuous with the rest of the chain, because it has not been up- 
 heaved so high. Even these continental islands, however, may be 
 formed in various manners. It is comparatively rare for them to be 
 the result of simple uplifting or depression of the sea-bed. Usually 
 the rocks have been more or less folded and fractured; not un- 
 frequently channels, deep or shallow, have been scooped in them by 
 tidal action, or by ice, so that the sea has been able to surround the 
 mass of land. The small islands around our own coast afford examples 
 of the ways in which most of the islands of the world have been 
 produced. Perhaps the simplest illustration of an island, which 
 might result from depression on a small scale, is furnished by 
 St. Michael's Mount near Penzance. There, when the tide goes 
 down, a tiny isthmus is seen to connect the mount with the main- 
 land, while at high tide all trace of union disappears. This method 
 of the formation of islands can be studied fully on any map of 
 Britain on which the levels of the high and low land are marked. 
 For by means of the scale of colours or shading used, it may be 
 observed how with varying degrees of depression, the sea would 
 penetrate into the land, breaking it up into islands and islets. 
 
 Isle of Wight. But a more instructive example is furnished by 
 the Isle of Wight. This island has its chief extension from east 
 
128 ORIGIN OF THE ISLE OF WIGHT. 
 
 to west : it consists of the rocks which are called Cretaceous in the 
 southern part, and in the north of those called Lower Tertiary lying 
 upon them ; all these deposits being more or less turned up on end. 
 For the Isle of Wight forms the southern portion of a great trough 
 or synclinal fold of the strata, which is known as the Hampshire 
 Basin. And the middle portion occupied by the newest rocks 
 comes precisely in the line of the sea now known as the Solent 
 and Spithead. Over this depression an excavation has been worn. 
 It is possible that it may have been originated by the River Avon 
 and the Southampton Biver, at a time when the level of the land 
 was somewhat higher; and then the sea, obtaining access to this 
 shallow channel by a subsequent depression, probably widened it 
 year by year at the expense of the adjacent land, until the com- 
 paratively broad water was formed which severs the Isle of Wight 
 from England. On the southern coast at Brook, the Wealden beds are 
 seen which correspond with the Wealden strata of Swanage Bay in 
 Isle of Purbeck ; and there can be no doubt but that the Chalk, and 
 all the strata which lie beneath it at the western end of the Isle of 
 Wight, were continuous with Dorsetshire when the uplifting first took 
 place. It will be observed that, since the strata on the southern 
 shores of the Island all dip to the north, the synclinal fold which 
 forms the island might be prolonged up into the air southward of the 
 Isle of Wight, so as to form an anticlinal fold south of it. Such an 
 anticlinal fold, more or less complicated, inevitably existed when the 
 island first began to be upheaved ; but since rocks, of a brittle 
 nature like chalk, are easily cracked along the direction in which 
 they are bent, it must be concluded that the rocks became fractured 
 along the axis of upheaval, and that the part of the anticlinal fold 
 to the north of this fracture or " fault " was squeezed up out of the 
 ocean, while the other or southern portion was excavated by denuding 
 agents. Not that such fractures could have determined the southern 
 outline of the island entirely, because that is to a large extent 
 obviously attributable to the eroding action of the sea now going 
 on ; but that there are many indications of minor faults in the 
 strata of the Isle of Purbeck ; and it is probably owing to small 
 parallel fractures which run through the island that the Upper 
 Greensand between Ventnor and Blackgang has slipped down 
 bodily towards the sea, so as to form the Undercliff. Thus the Isle 
 of Wight must be considered to owe its existence partly to anti- 
 clinal and synclinal folds of the earth's crust, partly to fractures 
 running through the rocks, and to denudation along these lines. 
 
 Isle of Man. Other conspicuous islands are the Isle of Man and 
 Anglesea. The Isle of Man consists chiefly of Upper Cambrian 
 rocks, with a small mass of Carboniferous limestone in the south. 
 This Upper Cambrian or Lower Silurian is obviously continuous 
 with that of the Lake country of Cumberland and the opposite coast 
 of Balbriggan in Ireland, and the island is an extension of that 
 anticlinal fold towards the south-west, similar to the south-westerly 
 extension of the Silurian rocks of Wigtown and Galloway towards 
 
ORIGIN OF ANGLESEA AND LUNDY ISLAND. 129 
 
 the opposite coast of County Down. Geographically the Isle of Man 
 belongs to Cumberland and Lancashire, because an elevation of not 
 more than a hundred feet would again join it to those counties. The 
 shallow sea which severs it from the North of England has probably 
 been ploughed out in part in comparatively recent times. 
 
 Anglesea. The Isle of Anglesea is divided from Wales by a 
 channel scarcely wider than a river ; and it is not easy ac- 
 curately to determine how the separation was effected ; but 
 Professor Eamsay has remarked that with the exception of Holy- 
 head and Garn, the general level both of the island and of the 
 opposite parts of Caernarvonshire is low, not rising more than 
 two hundred feet above the sea. He observes that the rocks are 
 smoothed with ice-marks, and scored with glacial striae, which run 
 towards the south-west ; and that the country is covered with 
 boulder clay full of angular fragments, such as a glacier would 
 carry or deposit. These glaciers could not have originated in the 
 mountains around Snowdon ; and it is concluded from the rock 
 fragments deposited that the glacier must have come from high land 
 farther north in fact, from Cumberland and the mountains about 
 Criifell in the south of Scotland, probably at a period when the 
 level of Britain was higher. A glacier of this large size would 
 have passed between the coast of Cumberland and the Isle of Man. 
 It not improbably scooped out the shallow sea between them. It 
 received a tributary glacier coming down from Morecambe Bay ; and 
 passing over the whole of the Isle of Anglesea may have excavated 
 the Menai Straits, as Professor Ramsay suggests. But since car- 
 boniferous limestone forms much of the shores -of the Menai Straits, 
 it is impossible not to suspect that, though filled with ice in a glacial 
 period, this channel may have originated in times far more remote 
 as a sort of canon. Thus, both the Isle of Man and the Isle of 
 Anglesea belong to a class of islands, of which the existence is 
 directly attributable to denudation. 
 
 Lundy Isle. A third group of islands may be typified by Lundy 
 Island in the Bristol Channel and the group called the Channel 
 Islands. These appear to owe their existence rather to the durability 
 of the rock of which they consist that is, to their greater power of 
 resisting denudation than to other causes. They are granite bosses 
 similar to those which would remain at the Land's End, about 
 Falmouth, St. Austel, Bodmin Moor, and Dartmoor, if the level of 
 Cornwall and South Devon were to be lowered. They are with- 
 out doubt indications of axes of upheaval. It is even possible, as 
 suggested by Professor Judd in the case of Lundy Isle, that they 
 may have been the central cores of old volcanoes ; but it is also 
 quite possible for granite to be formed by pressure in the axis of 
 an anticlinal fold, and to cool there without ever bursting through 
 the great thickness of over-lying rock which originally covered it. 
 The attempts made by Mr. Sorby to estimate the pressure under 
 which granite consolidated, would lead to the conclusion that de- 
 nudation to an enormous extent, involving the removal of rocks 
 
 VOL. i. I 
 
i3o ORIGIN OF INNER HEBRIDES. 
 
 thicker perhaps than the height of the loftiest mountains of the world, 
 must have taken place in the region of the English Channel before 
 the granite cliffs of the little island of Sark could have been laid bare. 
 
 Inner Hebrides. Finally, there are, in the Inner Hebrides, islands 
 where the surfaces are portions of lava sheets poured out from volcanoes 
 which themselves formed larger adjacent islands, or existed on the 
 mainland. It is difficult to judge of the condition of the west coast 
 of Scotland in those older or middle Tertiary times, when the lava 
 streams were poured out. But not improbably the country was a 
 tableland, and the lava flowed out under the atmosphere. The 
 basalt of the Isle of Mull extends across into the opposite peninsula 
 of Morvern on the one hand, and on the other side forms Ulva and 
 Staffa ; while the Treshnish Isles probably but imperfectly indicate 
 its western extent. The lavas of Eigg may have been emitted from 
 the volcano of Rum. Any one who examines this ancient British 
 volcanic country, and observes the broad and deep channels which 
 the streams excavate for themselves in it, will be at no loss to dis- 
 cover the means by which, aided by depression in level of the country, 
 the sea severed islands like Ulva and Staffa from the adjacent 
 land. A similar valley widened by the sea formed the Sound of 
 Mull. This is almost demonstrated by the fact that the rivers and 
 streamlets all run into the Sound of Mull at right angles to its 
 length, just as though they were tributaries joining a larger stream. 
 Hence these volcanic islands and islets have originated in denudation, 
 and only differ from the other examples mentioned, in the fact that 
 most of the rocks seen on their surface have been poured out in 
 molten streams from cones of volcanoes, instead of being deposited 
 as strata on the floor of the ocean. 
 
 It is instructive to observe that an elevation of fifty fathoms 
 would remove the whole of the Malay Archipelago from the map, and 
 substitute in its place a south-eastern extension of the Asiatic con- 
 tinent. It is even more instructive to remark the vast depths from 
 which atolls in the Indian and Pacific Oceans rise from the sea-bed, 
 since we thereby obtain an idea of the height of mountains which 
 have sunk beneath the sea, and the probable area of lands which 
 have been removed by the depression of islands, since the main out- 
 lines of land and water have been what they are now. 
 
CHAPTEE X. 
 
 THE GENERAL FEATURES OF SCENERY IN THEIR RELATIONS 
 TO GEOLOGICAL PHENOMENA. 
 
 Tablelands and Low Plains. When a submarine ridge is elevated so 
 rapidly as to emerge from the ocean as a small island, tidal waters at 
 
 Fig. 43. Gibraltar. 
 
 once begin to cut back the coasts and form cliffs ; but, as the land rises 
 higher and enlarges, other cliffs are cut below them; so as to present in 
 succession all the steep descending slopes of mountain scenery. Eut 
 when the upheaval is more slow, so that the sea can cut away the 
 rock as fast as it is raised, then it does not rise from the water, but 
 the surface is planed level beneath the water, and when it at last 
 emerges, after losing from the uppermost strata in some cases thou- 
 sands of feet, or it may be miles of thickness, presents itself as a 
 plain of marine denudation. Such a plain is usually an anticlinal 
 fold, or rather a series of folds of which one or more originally rose 
 higher than the rest. Much of the surfaces of England and of Ireland 
 were originally plains formed in this way. When such a plain, how- 
 ever, is uplifted high above the sea, it is termed a tableland, and what 
 was an island is often thus enlarged into a continent. Once above the 
 water, the sea erodes its shores into cliffs, and as these descend lower 
 and lower with the increased elevation of the land, it inevitably 
 
J32 ORIGIN OF TABLELANDS. 
 
 results that the tableland becomes surrounded by mountains. Some- 
 times one side rises faster than the other, and then as in the tableland 
 of Mexico, for instance we find the Pacific side rising rapidly in a 
 series of terraces, which indicate more or less stationary intervals, 
 while the tableland slopes somewhat towards the Atlantic coast. 
 
 It is impossible to go into any part of the mountain scenery of 
 Wales, or the other higher districts of Britain, without realising that 
 the multitudinous peaks which occur at approximately the same level 
 are but the last relics of a plain which was originally continuous 
 between them ; sometimes, indeed, a succession of plains descending 
 one below another, may in this way be traced out by the eye. Yet so 
 ancient was their origin, and so powerful have been the denuding 
 agents acting upon them, that the mountains play the part of gigantic 
 earth-pillars, such as are seen in Colorado or the Tyrol, and are the 
 sole remaining points in tablelands which are now variously sculptured 
 into valleys and hills. 
 
 Similarly the Secondary strata, which are less inclined than the 
 Primary rocks, and but little folded, rest upon each other so as to form 
 surfaces which are approximately plains, though the flocculent particles 
 of the unhardened clays have been swept away so as to give them, a lower 
 level than the more enduring limestones and sandstones. The British 
 Isles need but to be elevated some 600 or more feet to present the 
 essential characteristics of tablelands. It will thus be clear that a 
 tableland is a plain distinguished by its mode of origin, and is quite 
 independent of height ; for even where its level is relatively low, it is 
 in all respects the antithesis of what is termed a low plain. It may 
 be convenient here to put these characteristics in contrast, and say 
 that a tableland is the oldest part of a land, while a low plain is the 
 newest part of a land ; a tableland is formed in a region of predominant 
 anticlinal fold, while a low plain is formed in a synclinal fold; a 
 tableland consists of such rocks as form the fundamental structure of 
 the country, while low plains consist of detritus worn off from the 
 higher ground and deposited at a lower level ; tablelands are usually 
 dry, relatively barren regions carrying their rivers in deep narrow 
 valleys ; low plains are the fruitful populous portions of the earth, 
 carrying their rivers through valleys which are broad and shallow. No 
 region exemplifies the relations of these phenomena better than South 
 America. 
 
 A Tableland in the Andes. High up in the Andes of Quito and 
 of Bolivia we find the peaks of the mountains planed away, and a 
 broad level surface presented ; which, in the cases of Desaguardero, is 
 upwards of 13,000 feet high and over 500 miles long. This tableland 
 was described by Mr. David Forbes, F.R.S. Near Arica the mountains 
 rise abruptly 3000 feet from the water's edge, and everywhere as we 
 ascend there is evidence of emergence from the ocean, and evaporation 
 of the sea, in the existence of deposits of salt. ist. At a height of 2500 
 feet to 3500 feet, beds of nitrate of soda run from ten to forty miles 
 inland. These beds were originally chloride of sodium decomposed 
 by carbonate of lime into chloride of lime and carbonate of soda, 
 
ORIGIN OF HIGH PLAINS. 
 
 133 
 
 which in its turn has been decomposed by vegetation into nitrate of 
 soda. 2d. At a height of 7000 feet to 8000 feet in the desert of 
 Atacama, there are immense salt plains on a grand scale. 3d. At a 
 height of 13,000 feet, on the tableland, white crystalline salt is found 
 on the shores of swamps and lakes. This tableland is intersected by 
 ranges of hills which run north and south, and sometimes rise 2500 
 feet above the plateau. The intervening valleys are nearly level plains 
 often formed of gravel, resulting from the wearing down of the ridges. 
 Volcanic cones occasionally rise 6000 feet above the plain. The area 
 may be separated into west, central, and eastern parts, which are re- 
 spectively of Oolitic, Permian, and Silurian age. 
 
 The Silurian rocks here form the higher chain of the Andes, rising 
 to 25,000 feet, and extending through the ranges which feed by the 
 melting of their snows tributaries of the Amazon and the La Plata. Far- 
 ther south in Chili the structure of the country is somewhat different. 
 In the north, mountains predominate, but in the south the mountains arc 
 
 Fig. 44. Pass (Andes). 
 
 subordinate to the plains. Mr. Darwin mentions that Santiago stands 
 on a plain 15 miles broad, and 1750 feet high, which has the undula- 
 tions of its surface parallel to the main valleys of the Andes, against 
 which chain it abruptly terminates. The surface of the plain is formed 
 of stratified pebbles, volcanic ashes, and clay. Southward it contracts 
 and expands successively three or four times, forming a series of basins 
 connected like a necklace. On the eastern side of the Andes, the 
 mountains are abrupt, and rise out of a slope like a talus which is 
 formed of rounded pebbles. This slope blends into a flat space 2700 
 feet above the sea, which is a few miles wide, and is bounded to the 
 east by an escarpment 80 feet high, running north and south, and 
 formed of rounded pebbles, obtained beyond all question by the sea 
 from the Andes, and rolled and rounded at their base, when the 
 higher tablelands had been so far raised from the sea as to connect 
 these mountains into a long and narrow island. As the South 
 American tablelands are followed northward into North America, their 
 
134 ORIGIN OF LOW PLAINS. 
 
 level becomes lower, and the two parallel mountain-chains which 
 border them diverge from each other so as to make the tableland 
 broader. 
 
 It is almost impossible now to discover what share tablelands may 
 have had in the ancient geological history of the British region ; 
 but it is not difficult to see from the present position of the out- 
 crops, and from the manner in which deposits succeed each other, 
 that the history of British scenery is mainly the history of tableland 
 phenomena. 
 
 Low Plains. There are, however, some formations which especi- 
 ally suggest low plains, and it may conduce to clearness if we briefly 
 notice the typical characters of the low plains of South America. 
 For since these sediments are so obviously the wreck and waste of the 
 Andes, slowly worn into mud and sand as the mountains rose, they are 
 typical of all plains which lie on the flanks of mountains, or in 
 synclinal folds. The whole surface of the pampas still retains salt 
 enough to render the streams saline in time of drought. Sometimes, 
 as at Bahia Blanca, salt covers the country like hoar frost for a thick- 
 ness of a quarter of an inch, and sometimes the sandstones are bound 
 together with salt. The material of the pampas is a red earth or mud, 
 which sometimes becomes compact marly rock. Pebbles are only 
 found at the foot of the mountains, and between the mountains and 
 the pampas mud the subsoil is sandy. Near Buenos Ayres the pampas 
 mud is upwards of 200 feet thick. Such plains may be matched in 
 Europe by the Russian steppes, the plains of Hungary, Lombardy, and 
 in our own country by little areas like the Carse of Stirling. Though 
 the fenland of Cambridgeshire is a plain, the deposits upon it are so 
 thin that it can scarcely claim to be an example of a low plain. When, 
 however, we look back in time, many formations suggest an origin 
 like that of low plains, and among such may be mentioned the Coal 
 Measures, and the various secondary and tertiary clays which yield 
 the remains of land vegetation, or of terrestrial reptiles. It may per- 
 haps be doubtful whether the land surface indicated by the Trias was 
 low or high; but, widely spread over Europe, it must have been a 
 plain, during its period of elevation, as conspicuous as any which 
 marks the existing surface of the earth. 
 
 Lakes. Nearly all the great lakes of the world owe their existence 
 to direct upheaval of the ocean floor. If the English Channel were 
 to be raised two or three hundred feet, a number of small lakes would 
 extend along the deeper part of its bed. If Southern Europe were 
 to be somewhat upheaved, then the Black Sea would be perfectly 
 isolated from the Mediterranean. Similarly the Mediterranean might 
 be separated from the Atlantic, and at the same time the level of the 
 Mediterranean would be so far lowered by the draining off of the 
 waters by the Straits of Gibraltar, that the shallow sea between Tunis 
 and Italy would become dry, and the Adriatic would be surrounded 
 by land. Thus, merely by elevation to the amount of a thousand 
 feet, the Mediterranean might be converted into a chain of lakes. On 
 the other hand, if the south of Europe were to be somewhat depressed, 
 
ORIGIN OF LAKES IN UPHEAVAL. 135 
 
 then the lowland of Southern Russia, peopled by the Kirghis and the 
 Cossacks of the Don, would "be overflowed, and the Caspian Sea would 
 become continuous with the Sea of Azov and the Black Sea. Since 
 depression would cause the Caspian to cease to be a lake, it is an 
 obvious inference that elevation severed it from the sea, of which it 
 formerly formed part, by laying bare the shallow sea-bed, which is 
 now largely occupied by salt marshes, and in part liable to inundations. 
 In the same way it may be noticed that the Baltic is almost enclosed 
 by land, and if elevation instead of depression of the country were 
 now going on in the south of Sweden, the Baltic would inevitably be 
 converted into a lake by a process of change in level, which need not 
 necessarily affect a wide area. The great Bussian lakes Ladoga and 
 Onega are merely prolongations of the Gulf of Finland, leading north- 
 ward to the White Sea, and are remains of a channel partly dried up. 
 
 Lakes Formed by Evaporation and Upheaval. Consideration 
 of the lakes of Central Asia, many of which are salt, and of broad 
 areas occupied in Persia, Turkestan, and Northern Jndia by salt 
 marshes, helps to show how old the main outlines of the rocks are 
 which form the physical features of the surface, relatively tp the lakes 
 which fill depressions among them ', and also demonstrates how recent 
 the last elevation of the country from the sea has been. All over 
 the plains between the Caspian and the Sea of Aral, and between that 
 sea and Lake Balkash, and thence on to Lake Baikal, deposits of shells 
 are found, recent in aspect, some of which are similar to those which 
 now live in the Caspian Sea. Lake Baikal, however, is a freshwater 
 lake, yet it contains a large number of saltwater types of animals. 
 Seals abound there similar to those which live in the Adriatic and 
 the Caspian. Many saltwater types of fishes have representatives 
 which thrive in its waters. Hence it may be concluded that Lake 
 Baikal was originally a portion of the great Central Asian Sea, and 
 was one of the deepest pools in its bed ; and that it became, by eleva- 
 tion of the mountain axis of the old world, converted first into a salt- 
 water lake, and afterwards into a freshwater lake. Some evidence 
 in favour of this view exists in the fact that in the deep waters of Lake 
 Superior some types of animals have been found which are otherwise 
 only known in the sea. The question of a lake being salt or fresh 
 depends entirely on the rainfall. When the amount of rainfall is in 
 excess of the evaporation in the district, then the fresh water drains 
 into the lake basin and dilutes the salt water; this diluted water 
 overflows, and the stream carries some of the salt from the lake down 
 to the sea. Thus, little by little, the salt is entirely removed, 
 and what was originally a saltwater lake becomes a freshwater lake. 
 It is probable that in most cases the change goes on more rapidly 
 than happened in the case of Lake Baikal, and in consequence the 
 marine life, unable to adapt itself to the altered conditions, perished ; 
 and it is only conceivable that the change to freshwater conditions 
 in that lake occupied so many generations as to have had little influ- 
 ence on the lives of individual animals. 
 
 Lakes Formed by Folding of the Rocks. A second group of 
 
136 EXCA VATED LAKES. 
 
 lakes has been produced as a consequence of compressions which have 
 thrown the rocks into parallel folds on a smaller scale. Thus valleys 
 have been formed and closed by tilting so as to have no outlet for the 
 drainage. Lakes of this kind are usually situate among mountains, 
 and probably no better examples could be taken than the lakes on the 
 west coast of Scotland, or those of the Alps, which are ancient fiords 
 closed by upheaval. No broad distinction can be drawn between such 
 lakes as these and those of the former class, for both have originated 
 in compressions of the earth's crust. Lakes like Neufchatel may 
 have been fresh ever since the Alps attained their present elevation, 
 although sea-birds still frequent the higher Alpine waters. 
 
 A third group of lakes is exemplified by some in Scotland, which 
 have been described by the Duke of Argyll and others, in which 
 the waters lie along anticlinal folds or saddles, and must be supposed 
 to owe their existence to the ease with which an anticlinal fold was 
 excavated by the sea when the level of the land was lower. 
 
 Excavated Lakes. A fourth group of lakes would appear to have 
 
 Fig. 45. Helm Grag and Grasmere. 
 
 been excavated by the erosive power of glaciers, towards the close of 
 the glacial period, when glaciers descended from all the principal 
 mountains in our own country and Northern Europe. Some of these 
 lakes, like those at the foot of Snowdon, and elsewhere in North 
 Wales, are found to be dammed up at their lower end by loose un- 
 stratified materials, which must be regarded as a terminal moraine. 
 Others of these lakes lie in true rock basins, or depressions excavated 
 in the solid rock, and appear to exist at such places as would be the 
 point of meeting of a great glacier and one of its tributaries. Here 
 the mere increased weight of the ice may be supposed to augment 
 the excavating power of the morainic stones in its floor. Several of 
 the lakes of Cumberland are susceptible of explanation in this way. 
 ' Crater Lakes. Finally, there are lakes which till the craters of 
 
LACUSTRINE STRATA. 137 
 
 old volcanoes, or occupy central depressions in the area which has 
 been the core of a volcano. In the department of the Puy-de-D6me 
 there are 276 small lakes, formed more or less directly as a consequence 
 of. the accumulation of volcanic ashes or lava streams damming up 
 valleys, or due to changes of level ; but there are eighteen lakes still 
 existing or filled up with sediment which occupy the craters of extinct 
 volcanoes. In Central Italy several of these lakes of large size occupy 
 the sites of old volcanoes. North of Rome one is more than 6 miles, 
 and another 10 miles long. Others are seen in the Eifel. 
 
 Thus lakes are to be attributed chiefly to the action of lateral com- 
 pression, which at once throws the earth's crust into folds, and heaves 
 it out of the sea. They rise with tablelands, like the Great Salt Lake, 
 which is 4000 feet above the sea ; or Lake Titicaca, which occupies 
 a depression in the tableland of Potosi, at a height of 13,000 feet 
 in the Andes ; and other lakes in Thibet rise even higher. When 
 a lake is depressed beneath the sea-level, its position is always a con- 
 sequence of evaporation ; the depression is only some 80 feet in the 
 case of the Caspian, but about 1300 feet in the case of the Dead 
 Sea. The tendency of rain, and rivers, and glaciers is rather to carry 
 material from a higher to a lower level than to excavate hollows, so 
 that no large number of lakes of magnitude is likely to be attributable 
 directly to erosive forces. 
 
 Lacustrine Strata. Several of the British geological formations 
 are inferred to have been deposited in lakes. The oldest of such 
 formations is the Old Red Sandstone ; for throughout the region over 
 which it is spread there are no marine fossils, but plenty of land 
 plants with the remains of fish, some of them allied to types which live 
 at the present day in rivers and lakes ; and at Kiltorcan, in the south 
 of Ireland, a fossil shell has been found and referred to the freshwater 
 genus Anodon. Hence the Old Red Sandstone is inferred to have 
 been formed in great synclinal folds where the waters of the adjacent 
 continent drained down and accumulated. In like manner the Coal 
 Measures often speak incontestably to lacustrine conditions ; and the 
 fact that coal-beds succeed each other in vertical order, like the 
 evidences of old land-surfaces in a delta, strongly enforces the infer- 
 ence that the intervening beds of shale and current-bedded sandstone, 
 free as they usually are from typical marine fossils, were accumulated 
 in waters which may sometimes have been brackish, but which were 
 essentially lacustrine. 
 
 Passing onward in time to the Trias, with its beds of rock salt 
 and gypsum, we have another formation which gives evidence of 
 conditions which certainly included the evaporation^ and drying up 
 of salt lakes; for the footprints, rainprints, and ripple-marks in the 
 Cheshire Sandstones are more likely to belong to an ancient lake 
 margin than to the sea. The freshwater and estuarine beds of the 
 Lower Oolites of Yorkshire and Lincolnshire are quite as likely to 
 have been lacustrine, though the lake certainly was not elevated on a 
 tableland, as may have been the case with some of the salt lakes of the 
 Trias. In the Purbeck formation we have evidence of lakes in which 
 
138 BRITISH LACUSTRINE STRATA. 
 
 calcareous deposits were formed because the sheets of fresh water rested 
 chiefly upon limestones, but the limestones alternate with marls indica- 
 tive of small streams which brought mud from time to time into the 
 waters. Mr. God win -Austen, with the best evidence in support of 
 his view, has taught us to regard the overlying Wealden formation, 
 which is scarcely divided from the Purbeck stratigraphically, as another 
 lacustrine deposit. But the physical geography of Europe had in the 
 meantime somewhat changed, and the Wealden lakes accumulated 
 chiefly, alternations of sandstones and clays with thin beds of fresh- 
 water limestone at intervals, and now and then an oyster shell, show- 
 ing that the waters of the sea may sometimes have communicated 
 with the great inland basin. 
 
 Tertiary deposits in the Woolwich and Reading series certainly 
 make us acquainted with a lacustrine formation, which covered up 
 with freshwater shells the leaves of trees and deciduous vegetation 
 which grew on the lake banks. But this was a lake on much the 
 same level as the sea, with the fresh water passing insensibly through 
 brackish conditions to the sea, and giving evidence that sharks and 
 other marine fish penetrated as close up to the freshwater lake as 
 they do at the present day in some of the inlets on the western side 
 of Scotland. After the elevation towards the close of the London 
 Clay period, which is sufficiently evidenced by the multitude of fruits 
 in which the clay abounds, upheaval culminated in the formation of 
 the Lower Bagshot Sands, which appear to indicate lacustrine condi- 
 tions ; for not only are the sands filled with leaves of plants wherever 
 there is a bed of clay in which these spoils of a luxuriant vegetation 
 could be preserved, but the clay itself is, without exception, beautiful 
 white pipe-clay, which could hardly have been the case had it been 
 deposited from the waters of a great river necessarily charged with 
 iron salts from the decay of the crystalline rocks, and with carbonic 
 acid from the decay of vegetation such as is preserved. Finally, 
 with the so-called freshwater strata of the Isle of Wight we enter 
 on a period of time during which upheaval of the sea-bed took 
 place, and lakes and lacustrine deposits became general in Western 
 Europe, with occasional alternations of sea and land. The Headon 
 beds, Osborne and Bembridge beds, and the Hempstead beds give us a 
 scarcely broken sequence of old land-surfaces in the south of England, 
 on which plants of varying kind succeeded each other, and ever-new 
 types of mammals came down to the waters to die. The lakes on 
 this ancient land, best evidenced by the limestones, persisted, some- 
 times changing their outlines and altering the character of the deposits 
 formed in them, occasionally opening to the sea, and sometimes 
 receiving new elements in their fauna. 
 
 Valleys. A valley is a long depression or hollow on the surface 
 of the earth, margined by ground more or less high. It may be 
 broad and shallow, or narrow and deep. It may be surrounded by 
 hills, or run through a country from sea to sea, entirely unassociated 
 with mountains. On a large scale valleys are exemplified by the great 
 depression which is filled with the Atlantic Ocean ; on a small scale 
 
VALLEYS DUE TO STRATIFICATION. 139 
 
 we have the valley occupied by the English Channel or the Thames, 
 and the still smaller valleys which branch about and f oriii glens in the 
 mountains of Scotland, or cwms in Wales. Almost every country 
 offers examples of valleys through which the streams and rivers find 
 their way to the sea, which differ from each other considerably in 
 their scenery and origin. Since they are all formed by (i) the opera- 
 tion of the forces of compression which bend the rocks into folds, or 
 by (2) the action of rain or ice or the sea which fashion and carve 
 the irregularities of the earth's surface, or are due to (3) the alter- 
 nation of the different kinds of rock which make the geological 
 structure of the country, it is convenient to consider valleys according 
 to the ways in which they have originated. 
 
 Valleys of Stratification. In the middle districts of Britain, 
 running from Yorkshire towards Dorsetshire, the several Secondary 
 strata, which are chiefly alternations of limestones and clays, rest 
 successively upon each other, tilted up at an angle, so that the several 
 beds dip into the ground to the south-east. A clay is usually contained 
 between two beds of limestone. The clay being formed of impalpable 
 mud, has its surface particles loosened year by year under the influence 
 of frost, and day by day by falling rain, which sweeps along the 
 loosened particles, holding them in suspension, and delivers them to 
 a river, which carries the waste material to the sea. Thus, in time, 
 the clay becomes hollowed out into a valley more or less deep and 
 broad, while the limestone, which is less easily broken up by the frost, 
 and has few loose particles which can be carried away by moving 
 water, and is only slowly dissolved by the carbonic acid which rain 
 brings to the earth from the air, wastes less rapidly ; and hence the 
 limestone stands up as a terrace, margining the valley hollowed out 
 in the clay below it. Such valleys are formed between the Chalk 
 range of the North Downs and the parallel ridge of the Lower 
 Greensand which extends from Guildford eastward by Leith Hill. 
 The clay worn away is the Gault ; the valley is the Holmedale. A 
 similar valley lies between the range of the Lower Greensand and the 
 parallel range of the Tunbridge Wells Sand in this Wealden area. 
 It is formed by excavation in the Weald clay. A third valley is 
 excavated parallel to the others between the Tunbridge Wells Sand 
 and the Ashdown Sand, which forms the central boss or saddle of the 
 Wealden anticlinal fold. This series of valleys, more or less distinctly 
 marked, is repeated with a repetition of the strata southward, between 
 the Ashdown Sands and the English Channel. So that in the little 
 area of the Weald of Kent, Surrey, and Sussex there is a system of 
 six parallel valleys which run eastward to the sea. The whole of the 
 geological structure of the island offers repetitions of the same pheno- 
 mena. The Vale of Pickering, in Yorkshire, drained by the river 
 Rye, coming down from the Yorkshire moorlands, and by the Derwent, 
 is hollowed in the Kimmeridge clay, between the terrace or escarp- 
 ment of the Coral Rag and the Chalk. Similarly, in the south of 
 England, in passing from Swindon to Bristol, valleys in the Kimmeridge 
 Clay, Oxford Clay, and the Lias are crossed. 
 
HO CAffONS. 
 
 Some geological formations themselves form valleys or low ground, 
 traversed throughout their extent by rivers. This is especially the case 
 with the Triassic rocks. In North Wales, the Clwyd drains a valley 
 of New Bed Sandstone contained in a fold of older deposits. All the 
 latter part of the course of the Dee and the Mersey is through the New 
 Red Sandstone. A considerable part of the Severn traverses the same 
 formation. The Trent almost wholly drains this formation, though its 
 tributaries, the Dove and the Derwent, come down from escarpments 
 in the Carboniferous rocks of Derbyshire. The Ouse flows through 
 the Triassic rocks of the plain of York. In most of these cases the 
 valley drained by the river is so little depressed that it can scarcely 
 be regarded as a valley in the ordinary sense of the term. 
 
 Valleys in Tablelands. River valleys, or valleys in plains, have 
 long been distinguished from the valleys which occur in tablelands or 
 elevated tracts. As regards their origin, they have nothing uncommon. 
 When portions of the earth's crust are compressed, and a part rises 
 upward, the adjacent part of the fold of necessity sinks downward, the 
 elevated part being a saddle, the depressed part a trough. As the 
 saddle rises in the sea, the waves cut its surface smooth, and as it rises 
 higher, it often becomes fractured through its thickness by lateral com- 
 pression. But at last, rising out of the sea as a plain, it begins to 
 experience the wearing or solvent action of the rain, which, descending 
 upon it, drains into the fissures, and makes them wider and deeper. 
 As elevation progresses, the land rises higher, and the narrow valley is 
 cut deeper and deeper. It happens that many tablelands are com- 
 posed either of limestones or volcanic rocks, and just as a mountain 
 brook, swallowed up by a fissure in the limestone, flows underground 
 and excavates a channel for itself, which eventually becomes a cavern 
 with steep sides, so the solvent power of the drainage-water, increased 
 by carbonic acid liberated from decaying vegetation, has enabled brooks 
 in mountain regions to cut deep and narrow gorges for themselves, 
 which characterise limestone scenery. For these valleys the Spanish 
 name canon has been adopted. Similar valleys, however, are formed 
 in some kinds of igneous rock. Thus, the broad sheets of lava which 
 extend through California and the Rocky Mountain region are traversed 
 by some of the most remarkable canons in the world. Similar valleys of 
 enormous depth extend through the tablelands of India. Though their 
 origin is the same as the valleys in the limestones, there is a slight 
 difference in the cause of their formation. The lava consists to a large 
 extent of felspars and minerals which contain small quantities of lime, 
 or soda, or potash. All these substances are capable of being taken 
 into invisible solution in water which is charged with carbonic acid ; 
 and when the rock parts with any of them, its nature is changed. 
 Previously it may have been hard enough to turn the edge of steel, 
 and not easily affected by the beating of rain, or the pounding of 
 stones which the torrent may have carried ; but no sooner does the 
 felspar part with one of its constituents than the rock becomes mud. 
 And then the water sweeps the impalpable particles away, deepening 
 the river channel, and emerges from the hills loaded with sediment. 
 
RIVER VALLEYS. 141 
 
 Valleys in Low Plains. But while the tableland was in process 
 of being elevated and the mountain-peaks were being denuded 
 from off it, the rolled and worn roek fragments thus formed were 
 removed by tidal waters from the heights into the adjacent syn- 
 clinal depression, as elevation went on. As this detritus increased in 
 amount, the force of the waves carried it lower and lower and spread 
 it out evenly ; and when eventually this low plain rose from the sea, 
 the waters draining down from the higher lands ploughed out a course 
 over it, which is the simplest form of a river valley. Such valleys 
 cut through the pampas-mud are typified by the channels of the La 
 Plata, the Amazon, the Orinoco, and most of the rivers of South 
 America. Something of the same sort of thing may be seen in the 
 rivers of Siberia, which flow through a deposit of mud worn from off 
 the plain of Thibet and the northern mountain chains of Asia, by a 
 sea which has retired. In our own country something of the same 
 kind, but on a small scale, is seen in the plain called the Carse of 
 Stirling, which is a silt washed down from the hills of Stirling and 
 Perth, through which the Forth passes to the sea. 
 
 But river valleys have generally been formed gradually during 
 long periods of time, and have been excavated far more largely by the 
 power of the sea than by the rivers themselves. When once a depres- 
 sion exists by which tidal waters make their way into the land, the 
 rising and falling of the tide acts twice a day like a saw on the shores 
 of the estuary and the river-banks, so as to waste the rocks and widen 
 the channel. And when a land drained by rivers is slowly depressed 
 in level, the sea is given an entrance further into the land, and 
 so the tide widens the river valley at that point in the same way 
 as it was widened at the river's original mouth. Thus by depres- 
 sion estuaries enter the land, and carve out channels which vary 
 in breadth and depth, partly with the nature of the rocks, partly 
 with the angle of tbeir upheaval, and which are considerably influ- 
 enced by the rate at which movements of the earth's surface go 
 on. And at last the sea, covering broad tracts, rounds oif the rough- 
 nesses of its work by tidal movement in waters of moderate depth, 
 so as to take away the abrupt characters of the cliffs in the gorges 
 thus excavated. Upheaval causes the land to emerge again in the 
 same slow way as it was depressed, only with this difference, that 
 much of the fine sediment denuded is carried away to the ocean, and 
 the chief part of the detritus already accumulated will be swept out 
 from the valley, so as to leave, when the waters retire, beds of gravel 
 and of inundation mud upon what had formerly been an estuarine 
 sea-bed. And when the emergence of the land is completed, a long 
 and comparatively broad and shallow valley remains, with broad branch- 
 ing tributaries, at the bottom of which the river runs. Such a 
 channel is the. valley of the Thames, in the lower part of its course. 
 Oftentimes the sediment swept out from a river valley by the sea, 
 when the land was lower, remains at the mouth of the river as a delta, 
 or has constituted the obstacle which caused a delta to be formed 
 
 Synclinal Valleys. There are two other kinds of valleys which, 
 
142 V ALLEY-S IN ANTICLINAL FOLDS. 
 
 unlike these valleys of erosion, owe their existence, directly or in- 
 directly, to elevatory forces ; hence they are called valleys of eleva- 
 tion. When the rocks are compressed so as to be thrown into parallel 
 folds such as form the Jura Mountains and some other chains, then the 
 small synclinal depressions between the mountains constitute valleys. 
 In our own country the rocks have been too long exposed to denud- 
 ing agencies, and too little contorted in times comparatively recent for 
 examples of valleys of this kind to have remained unchanged. But 
 many of the undulations of chalk scenery have been produced as a 
 consequence of small undulations of the underlying rock. Valleys 
 formed by contortion are nowhere more grandly exhibited than in 
 the Western Territories of North America, where the rocks are thrown 
 into multitudinous parallel elevations with intervening valleys. 1 
 
 Anticlinal Valleys. But when folds of this kind are formed 
 beneath the sea, and rise slowly from out of it, the tops of the folds 
 are broken away, because the rocks are stretched and cracked and planed 
 down; and then atmospheric agencies complete the work of hollow- 
 ing out a valley where a hill once had been. Not entirely, perhaps, 
 in one geological age, but during immense periods of time, most of the 
 upward folds in Wales and in many parts of the world have been 
 excavated into deep valleys. The part of the country which had 
 originally been a valley produced by elevation, comes to withstand 
 denudation better, in consequence of the hardness imparted to its con- 
 stituent rocks by compression, often thus forming the loftiest moun- 
 tain peaks. Such synclinal folds are Snowdon and Moel Hebog. 
 The valleys around such mountains, at their first compression, rose 
 to immensely greater heights. It is like an illustration of the Scrip- 
 tural teaching that " Every valley shall be exalted, and every hill 
 made low." The valleys are innumerable which were formed in this 
 way. Some of the broadest are the Bristol Channel, and probably 
 both ends of the English Channel. In countries formed of old and 
 contorted rocks, as many valleys have been produced by elevation, as 
 have been excavated by erosion in countries formed of newer rocks. 
 
 Valleys have existed for all geological time, but it is not often 
 that they have survived the changes of the earth's surface so that we 
 can recognise their former extension. A few such examples, however, 
 will come under our notice of ancient valleys excavated in the moun- 
 tain limestone of the North of England, and afterwards filled up by 
 the Trias, again to be partially cleared out by existing streams. And 
 similarly among the Austrian Alps we shall find at Gosau and near 
 Salzburg many valleys excavated in the Triassic rocks, in which Cre- 
 taceous strata have been deposited, in their turn to be laid bare by 
 ravines cutting through them. 
 
 1 Hay den, Reports U.S. Geog. and Geol. Survey of the Territories. 
 
( H3 ) 
 
 CHAPTER XL 
 
 SUBAERIAL DENUDATION AND ITS RESULTS. 
 
 Wasting Effects of the Atmosphere. 
 
 THE gradual wasting of the surface of the superficial parts of the earth 
 is an important element in geological theory and history. The follow- 
 ing examples of the varied results of atmospheric influences in 
 modifying the surface of the works of nature and of man, form but a 
 small fraction of current information on the subject. 
 
 The wasting effects of the atmosphere are those terrestrial processes 
 by which materials are provided for rivers and the sea to transport and 
 deposit in new situations. These processes are dependent on general 
 humidity, variations of moisture, precipitation of rain, and variations 
 of temperature. 
 
 It is not, however, always possible to distinguish accurately the 
 effects of these several causes. Many natural agencies are often con- 
 cerned in one operation, and the general result is the sum or the 
 difference of their effects. The chemical action of the atmosphere is 
 evident in buildings, and on the surface of certain rocks. The same 
 process which slowly reconverts the mortar of walls into crystalline 
 carbonate of lime frequently causes the pulverisation and bursting of 
 the bricks, in consequence of the expansion of the small masses of 
 lime which they contain. 
 
 The surface of bricks is often covered with a saline efflorescence', 
 which is generally nitrate of lime, but sometimes chloride of sodium. 
 The surface of the yellow limestone near Doncaster is sometimes 
 covered with a nitrous efflorescence, and so is the calcareo-magnesiau 
 mortar made from it. 
 
 The Wind. The agency of the wind as a denuding power is easily 
 underestimated, though the amount of dust deposited from the atmos- 
 phere under ordinary circumstances demonstrates that much matter is 
 carried by the air from a higher to a lower level. The modern inven- 
 tion of the sand-blast, by means of which glass, granite, and. other 
 substances are easily etched, illustrates experimentally the ways in 
 which wind, blowing in prevalent directions^ abrades rocks. And when 
 we remark that the contours of the sandhills of Holland are exactly 
 the contours of mountain chains, it is quite possible that the outlines 
 of mountains are in the main to be attributed to the agency of 
 
144 DISINTEGRATING ACTION OF THE ATMOSPHERE. 
 
 the wind. One of the most interesting examples of wind action is 
 recorded from the harbour of Wellington, on Cook's Strait, in New 
 Zealand, where, in a line of low sand hills, are sand-worn stones in 
 every stage of rounding. The prevailing winds drive a cloud of sili- 
 cious particles from one set of dunes to the other until their angles 
 are entirely removed, and they become rounded like the grains of sand 
 in deserts. In this country, ^Eolian action is admirably seen in the 
 pinnacles and crags on the top of Kinder Scout, a tableland of lower 
 carboniferous rocks, on which pillars of sandstone are left, which often 
 stand up in the shape of gigantic clubs or mushrooms. 
 
 Waste of Felspathic Rocks. The exterior of most uncrystalline 
 rocks and buildings is slowly eaten away by the moisture and carbonic 
 acid of the air ; but the influence of this destructive agent is most 
 remarkable among the felspathic rocks, whether they were origi- 
 nally crystalline, like granite, or compact, like basalt. The felspathic 
 
 Pig. 46. Millstone Grit, Yorkshire. 
 
 portion of the hypersthene rocks of Carrock Fell is so wasted that the 
 crystals of hypersthene and magnetic iron project from the surface 
 considerably. Some greenstone dykes are thus entirely decomposed 
 to great depths from the surface, and whole masses of rotten granite 
 wait only for an earthquake or aqueous action to be entirely reduced 
 to fragments. Those who have seen the crumbled granite of Mun- 
 caster Fell in Cumberland, or Castle Abhol in Arran, surrounded by 
 heaps of its disintegrated ingredients, must have been struck by the 
 importance of this phenomenon in reasonings concerning the origin of 
 many stratified rocks. 
 
 Both carbonic acid and oxygen act very decidedly upon the metaJ - 
 lie, and particularly the ferruginous ingredients of rocks, and thus 
 swell and burst them to pieces. Sometimes, however, this very cause 
 seems to harden and bind the rock together, and to render it more 
 durable. In general there is no certain test of the durability ol 
 
INFLUENCES OF TEMPERATURE AND MOISTURE. 145 
 
 any stone but experience of exposure under definite circumstances. 
 The Bath stone, apparently so permanent amongst its native hills, 
 perishes in the salt air of Norfolk ; and few calcareous freestones of 
 any kind will long resist the carbonated and sulphurous atmosphere 
 of London. 
 
 Preserving Power of the Soil. It is worthy of remark that sculp- 
 tured stones buried under ground are perfectly and even wonderfulb 
 preserved, while their fellows left exposed to the sky have almost 
 crumbled to dust. In the course of excavations for the Yorkshire 
 Museum at York, the statues which once stood between the arches of 
 the nave of St. Mary's Abbey were discovered, some with blue, others 
 with red drapery, one with gilded hair, all retaining the most delicate 
 chisel marks. But at a few yards from them, at the west end of the 
 church which they once adorned, the atmospheric influences have 
 nearly obliterated a beautifully sculptured wreath of leaves round the 
 doorway, so that antiquaries have doubted whether they were meant 
 to represent the vine 01 the ivy. 
 
 Waste from Humidity. Frequently, in looking at buildings com- 
 posed of porous materials, like the Portland stone, or a grit freestone, 
 we observe the parts which are overhung by a ledge, and thus kept 
 in a state of continual shade and dampness, to be more rapidly con- 
 sumed than the projections ; but the parts which hasten soonest to 
 decay are those near the ground where they are most affected by rain, 
 and moisture. The same rules are exemplified in many remarkable 
 rocks, as, for instance, in the quartzose conglomerates of the old red 
 sandstone of Monmouthshire and the millstone-grit of Brimham Crags 
 in Yorkshire. The " Buckstone," near Monmouth, is a huge rock 
 inversely conical, expanded above into a large area, but contracted 
 below by continual waste to a narrow base of attachment. This 
 process, a little further continued, might convert the Buckstone, as 
 probably some of the Atones of Brimham have been converted, into a 
 "rocking stone." 
 
 From Changes of Heat and Moisture. In northern zones of the 
 earth the variations of heat and moisture are greatest on the south and 
 west fronts of buildings, and in consequence those fronts to our 
 cathedrals decay most rapidly. This is remarkably the case with the 
 cathedral of York, built of magnesian limestone, which is in many 
 places quite consumed on these fronts, but comparatively uninjured on 
 the northern face. 
 
 The weathering of the surfaces of buildings by the fluctuations of 
 heat and moisture is partly dependent on the structure and composi- 
 tion of the stone. The flagstone of Yorkshire is in many houses at 
 Bradford gradually decayed grain by grain, so that the surfaces of the 
 stone, continually renewed, and never permitting the growth of lichens, 
 appear always neat and clean. The magnesian limestone of the same 
 county, often traversed by veins of calcareous spar, presents frequently 
 a cellular or honeycomb appearance, in consequence of the projection 
 of these veins above the excavated limestone ; but the coarse shelly 
 beds of the Northamptonshire Oolites, and the irregularly laminated 
 VOL. i. K: 
 
146 DESTROYING INFLUENCES OF FROST AND RAIN. 
 
 millstone-grit, are decomposed in lines corresponding to the differences 
 in the composition of the stone. 
 
 In these cases the stone appears to undergo gradual and continual 
 waste, but sometimes the whole surface exfoliates. Basalt very fre- 
 quently suffers this kind of waste, granite not rarely ; and it has been 
 sometimes supposed in these instances, that the atmospheric action 
 merely discloses the incipient concretionary structure of the rocks. 
 
 From Frost. Frost is likewise an important agent in reducing to 
 smaller masses the materials of the earth. Some stone, if brought to 
 the surface in winter full of its " quarry water," will break in pieces 
 directly. Advantage is taken of this circumstance by the slate-workers 
 of Stonesfield and Collyweston, who quarry their stone in the winter, 
 taking care to shield it from the sun and the wind till the frost has 
 acted upon it, with the aid of water, if necessary, which, by disclosing 
 the natural fissility of the stone, permits the blocks to be cleft into 
 thin, sound roofing - slate. Landslips in mountainous regions are, 
 probably, much accelerated by the power of frosts. In ascending the 
 Righi from Waggis, on the Lake of Lucerne, we are much struck by 
 the extraordinary length and continuity of the joints of the nagelflue. 
 It is from these natural partings that the landslips fall, when repeated 
 rains, snows, and frosts have worn or burst them open, and the water 
 passing down them undermines the foundation of the cliff. Thus 
 huge blocks, liberated from their attachments, roll down the steep 
 descent, or half the summit of a mountain slides upon its argillaceous 
 bed Vast portions have thus slipped from the Righi towards the 
 isthmus which divides the lakes of Zug and Lucerne, and others are 
 preparing to follow. 
 
 Effects of Rain. We come now to the effects of rain, and without 
 dwelling on the general degradation of the softer surfaces of the earth 
 caused by this agent, we shall proceed to show, that within the his- 
 toric era hard and dura'ble stones have been greatly furrowed by the 
 rain ; and that in more ancient periods, the precipitations from the air 
 have carved channels of various kinds, and sometimes formed real 
 though miniature valleys of great length and continuity. 
 
 On Monumental Stones, &c. Many Druidical monuments in the 
 north of England are constructed of coarse millstone - grit, a rock 
 admirably suited for yielding those enormous blocks preferred by the 
 ancient architects. Three huge Druidical stones, now standing near 
 Boroughbridge, called the " Devil's Arrows," present us with an in- 
 structive lesson on the ultimate fate of all human erections exposed 
 to the ravages of time. 
 
 The rain, beating for 2000 years upon these venerable pillars, 
 has cleft their tops, and ploughed deep furrows down their sides. 
 The grooves are deepest at the top, and become wider and less distinct 
 towards the bottom ; they cross indifferently the false-bedded layers 
 of pebbles, and go directly downwards. One of the stones leans re- 
 markably and threatens to fall, but an examination of the furrows 
 shows the inclination to be of most ancient date, for they descend much 
 farther down the pillar on the upper inclined face than on the under. 
 
EARTH-SCULPTURE BY RAIN. 147 
 
 Similar effects of rain are visible to a greater extent on the bold 
 crags, like Almias cliff and Brimham rocks, which crown the summits 
 of so many hills of north-western Yorkshire, from, some of which the 
 Devil's Arrows were obtained. 
 
 In the valleys of Switzerland (Sarnen) blocks of limestone, which 
 have fallen from the mountain sides, have been furrowed in the same 
 way since their descent. 
 
 On Rocky Escarpments and Floors. The carboniferous limestone 
 of England has been little employed in building, except partially in 
 old castles, where it seems durable. But those who know the magni- 
 ficent ranges of scars which gird the hills of Derbyshire and West- 
 moreland, will acknowledge that few rocks seem more likely to endure 
 the rage of the elements. Yet, on close inspection of these giant cliffs, 
 the dry and bleached aspect, and smoothed angles, show plainly wasted 
 surfaces. Those who have stood on Doward Hill, near Monmouth, 
 to contemplate the rain-furrowed white limestone there, will not need 
 another example. In the north of England analogous and more re- 
 markable instances present theniselves in the wide limestone base of 
 Ingleborough, and in Hutton roof crags near Kirkby Lonsdale. 
 
 The vast limestone floor which supports the cone of Ingleborough 
 is marked in all directions by natural fissures, and divided into com- 
 partments like a map. 
 
 If one of these compartments be examined in the western part of 
 the mountain, its surface will be found scooped into little hollows 
 which unite into a common channel, and terminate by indenting the 
 edges and furrowing tlie sides of the fissure. They are, in truth, 
 valleys in miniature, produced separately Ly the drainage of the 
 several blocks. 
 
 The mere decomposing effect of the atmosphere produces on the 
 edges of the stone a different effect, by wearing away the softer 
 laminae, but the smooth surface of the miniature valleys, their regular 
 descent, winding course, and union into a common channel, show that 
 they were fashioned by the repeated fall of rain. 
 
 This scar is nearly level, but in Hutton roof crags we have an 
 opportunity of tracing the rain-channels over an iinmense surface of 
 bare limestone rocks lying nearly level on the hill-top, but sloping 
 rapidly down the sides to the east and south. On the level top of 
 the hill the stones are variously worn in hollows and grooves irregu- 
 larly united and running in different directions, according to little 
 variations of the groun'd ; but on the steep east and south slopes the 
 channels are extended into long furrows, which, uniting at acute angles, 
 enlarge, widen, and descend the hillside in lines following exactly 
 the declination of the rocks, and stopped only by few and distant 
 fissures, beyond which other systems of concurrent grooves begin. 
 
 Rain-Channels like Miniature Valleys. It is impossible by draw- 
 ings or descriptions to convey such an idea of the appearances of the 
 Hutton roof crags, as to awaken in others the impressions which 
 are fixed for ever in the mind of the observer. The astonishing resem- 
 blance which these little rain-channels present to the great system of 
 
i 4 8 LOCAL EFFECTS OF INUNDATIONS. 
 
 valleys which undulate the stratified rocks, seizes upon the imagina- 
 tion, and we re-examine all our notions of the origin of these great 
 surface-undulations. The fissures in the limestone rocks which stop 
 and swallow up the gathered streams, are analogous to those longi- 
 tudinal valleys beneath the escarpments of the Oolites and the Chalk 
 by which the rivers are turned at right angles to their earlier course, 
 while the lower edge of the fissure corresponds to the escarpment 
 itself, with its new system of denudations. 
 
 To see these rain and time-ploughed furrows winding in uncertain 
 directions over the horizontal limestones on the hill-top, like a slow 
 river in a level plain, but running a straight downward course on the 
 slopes, like a stream descending from its parent mountains, is enough 
 to impress on every beholder a secure conviction that the excavation 
 of many valleys must be explained upon similar principles ; that, as the 
 feeble currents of descending rain, aided by long time, have been 
 sufficient to plough their little courses, so the greater action of existing 
 streams has been sufficient to work out their actual channels, though 
 the excavation of the broad valleys in which they run, may have been 
 accomplished by more violent and voluminous waters. 
 
 Effects of Inundations. The slow but incessant action of rain 
 beating perpetually on the hard and the soft surface of the earth, and 
 removing grain by grain the materials loosened by the expansive 
 agency of frost, and by moisture and chemical changes, may, in a long 
 series of years, be more important in its effects than the violent water- 
 spout, or the ravaging inundation of a bursting lake. Yet the effects 
 of water-spouts are tremendous in countries composed of easily de- 
 structible or unequally indurated materials. A waterspout which fell 
 above Kettlewell, in Yorkshire, committed terrible ravages in the 
 narrow valley of the Wharfe, near Kettlewell and Starbottom. On 
 the sides of the mountains in Cumberland, traces of these visitations 
 seem utterly ineffaceable ; and the memory of the sudden bursting of 
 the Peat Bog, above Keighley, will long be preserved in the valley of 
 the Aire. The floods which rushed simultaneously from the Cairn 
 Gorum and other mountains, in August 1829, over 5000 square miles 
 of Aberdeenshire and other counties, were of prodigious fury, removing 
 hundreds of tons of large stones, whole acres of woodland, and almost 
 hills of earth. The desolating effects of the bursting of the ice-dam 
 which had formed the temporary Lake of Bagnes, are matters of 
 history. The moving mass of water, mud, and monstrous rocks, which 
 swept with violence down the valley of the Dranse, carried away 
 forests, houses, bridges, cattle, and men. In six hours and a half it 
 passed through an unequal and irregular course of forty-five miles, till 
 lost in the Lake of Geneva. 
 
 Glaciers. Glaciers are likewise to be enumerated among the 
 powerful agents by which the higher lands are wasted, and materials 
 provided for raising the lower. As the summer heat melts every 
 year the lower portions of these long winding rivers of ice, and the 
 heated ground thaws, the gathering water dissolves their founda- 
 tion, and the whole mighty mass of snowy ice slides slowly down- 
 
SCULPTURING EFFECTS OF GLACIERS. 
 
 149 
 
 ward on its bed, where it ploughs up the stones, breaks up the rocks, 
 and, adding their spoils to the accumulations from avalanches, rinally 
 throws down huge banks of rubbish at the foot of the glacier, which 
 is thus surrounded by an immense mass of loose materials, called the 
 terminal moraine, which is deposited as the ice melts. 
 
 Almost every valley in the Alps is more or less filled with this 
 morainic matter, a mixture of angular stones and mud, which is often 
 cut into by small streams. Every river bed in the dry season is a floor 
 of large stones, more or less rounded, which travelled on the glacier 
 
 Fig. 47. Track of a Glacier, Mer de Glace, showing median moraines. 
 
 before they were driven along by the mountain torrent and worn. 
 The glacier wears its bed smooth, partly by the abrading action of 
 rock fragments, which fall through cracks produced when the ice- 
 stream squeezes through a defile, when they become unbedded in its 
 floor, and rasp, groove, and scratch the subjacent rocks as the ice 
 moves onward and grinds them into mud. Rocks thus worn show that 
 succession of small rounded bosses which, from their resemblance to 
 the backs of a flock of sheep, have been named rovhes moutonnees ; 
 they are well seen in Cwm Arthur, near Ffestiniog. The striated 
 
ICO 
 
 ANCIENT BRITISH GLACIERS. 
 
 surfaces are indubitable evidence of ancient glacial action. They 
 abound in Switzerland and the north of Europe, and in our own 
 country are excellently seen in North Wales about Snowdon, in the 
 valleys of the Lake district, along the west coast of Scotland, and on 
 both flanks of the Grampians. The morainic matter from the old 
 British glaciers is spread along the country, and known to us as 
 Boulder Clay. Where the mountain districts show marks of glacial 
 action, they commonly exhibit vast blocks of stone perched in posi- 
 tions from which tidal waters would at once have swept them down. 
 These masses, well seen in the Pass of Llanberis, are termed blocs 
 perches. They are portions of moraines which were carried by glaciers 
 moving over the places where they occur, and as the ice melted away 
 beneath them, they were deposited on the sides of the valley, on any 
 ledge broad enough to afford them resting-ground. 
 
 Some small lakes are dammed up with terminal moraines. This 
 
 Fig. 48. Conical Boulders (Arran) 
 
 is the case with Llyn Llydaw on Snowdon, and Llyn Idwal in Nant 
 Francon, and many others in the lake district of Cumberland. Numerous 
 mountain tarns which lie in true rock basins thoroughly glaciated, 
 appear to have been excavated entirely by the erosive action of 
 ancient glaciers. 
 
 Professor Eamsay has detected polished and striated boulders in the 
 Permian Breccias of Enville in Worcestershire, the Abberley Hills, 
 Clent and Lickey Hills, and other localities. In all cases the fragments 
 are angular, and have been carried twenty-five to forty-five miles from 
 the parent rock. These deposits testify to ancient glaciation. 
 
 Where a glacier runs into the sea, as in the Arctic Regions, and is 
 buoyed up by the density of the water, the terminal fragment breaks 
 off as an iceberg, loaded with the stones which have fallen upon it 
 from the sides of the valley along which the ice travelled. As these 
 icebergs are carried by currents into warmer water they melt, and 
 
UNI7ERSI 
 
 * r1 
 
 EXCAVATING EFFECTS OF LAND-SPRINt 
 
 deposit upon the sea "bed the stones they carried, or sometimes they 
 are drifted on shore, and the boulders there accumulate. During the 
 glacial period, when the British islands were at one time submerged, 
 multitudes of such erratics became stranded on the higher ground 
 as it emerged from the sea. But there are also evidences that icebergs 
 played a part in the formation of many geological deposits : large 
 blocks of granite have been found in the CJialk of Surrey, and 
 blocks of coal in the Chalk of Kent, and angular fragments abound 
 in the Cambridge Greensand, consisting of granijbe, mica schist, gneiss, 
 basalt, and a variety of felspathic rocks, hard sandstones, conglome- 
 rates, slates, and limestones, occasionally with indications of carbonife- 
 rous limestone fossils. Similar fragments are found in many of the 
 Oolitic rocks, especially the Portland limestone, and large blocks of 
 syenite have been recorded in the Coralline Crag. 
 
 Springs. The precipitation of moisture on the surface of a country 
 is determined chiefly by the configuration of the land and the direc- 
 tion of the prevalent winds ; and hence high ground which rapidly 
 radiates its heat, and thus .condenses moisture out of the air, experi- 
 ences more torrent-like denudation than the lower districts of our 
 eastern and southern counties. But the character of the denudation 
 depends not only on the quantity of rain and inclination of the surface 
 over which it flows after reaching the earth, but also on the capacity 
 of the rocks to absorb the water. Thus Mr. Mellard Reade, treating 
 of the basin of the Thames, has estimated that of the rainfall of 27 
 inches 19 inches are absorbed by the porous rocks of the valley, and 
 only about 8 inches drain directly off the land to the sea; while in 
 regions of the more compact slate rocks of Cumberland and Wales, 
 where the rainfall is far heavier, only about 10 inches are absorbed 
 by the earth. But the water which is absorbed by the rocks passes 
 through them to emerge again at a lower level when it is charged with 
 various solids dissolved out of the porous beds. This underground 
 flow of water gives rise to the phenomena of springs, in which most of 
 our rivers take their rise. A spring may be defined as water flowing 
 from the rocks, which was originally absorbed as rain at a higher 
 level. Springs are classed into two kinds, commonly known as land 
 springs and perennial springs; for thermal springs and the intermittent 
 spouting springs termed geysers are varieties of perennial springs which 
 make a transition towards volcanic phenomena. 
 
 Rain which falls on the surface of land where the subsoil is im- 
 pervious, is partly absorbed by the superficial earth, gravel, or sand, 
 and draining down the incline of the surface of the country, the water 
 flows out from the surface bed as a small stream, which is termed a 
 land spring. Such streams abound throughout the country, and 
 determine the existence of many small sheets of water and the village 
 ponds round which population has gathered. But not unfrequently 
 these streams make their way to the sea, and, where the destruction 
 of the cliffs is slow, produce, by their excavating power on the ground 
 they run over, those miniature valleys which in the Isle of Wight 
 are termed Chines, and are the Bunnys of Hampshire and Coombs of 
 
152 EXCA VA TION OF CA VES. 
 
 other parts of the South of England. Where the cliffs waste more 
 rapidly than the tiny streams can cut them away, waterfalls are shot 
 over them into the sea. 
 
 The perennial springs draw their water from the porous beds 
 among the regular geological formations ; and all the tributaries of the 
 Thames are derived from limestones or sandstones, usually at the 
 point where they rest on clays. The fact that the water comes from 
 limestone prepares us to find that it has dissolved from the rock, and 
 holds in suspension in the river a large amount of solid matter. This 
 solid matter, in the waters of the Thames and its tributaries, varies 
 from 20 to 33 grains in the 100,000 grains of water, the average quan- 
 tity being always upwards of 20 grains to the gallon. This invisible 
 matter consists chiefly of carbonate of lime, but in the tributaries 
 one-fifth of the amount, or more, may consist of sulphate of lime. In 
 the lower part of the course of the river the amount of the solid 
 matter is somewhat less, but every day about 1000 tons of carbonate 
 of lime and 238 tons of sulphate of lime are said to be carried to the 
 sea, with smaller quantities of carbonate of magnesia, chlorides of 
 sodium and potassium, sulphates of soda and potash, some silica and 
 a little iron, alumina and phosphates, making in all about 1500 tons 
 delivered into the ocean daily by the Thames. Large as this amount 
 is, the quantity removed is so small that, according to the estimate 
 of Professor Prestwich, the surface denudation of the Thames basin at 
 this rate by chemical means, would amount to less than one-tenth of 
 an inch in a century, so that in the course of 13,200 years about 
 one foot in thickness would be removed from the Chalk and Oolite 
 districts. 
 
 Excavation of Caves. When, however, the water is condensed 
 into a narrow stream, as it often is in flowing under ground, its 
 solvent action is particularly impressive. Charged with carbonic acid 
 from the air, and further enriched with the same substance from 
 decaying vegetation, the surface waters in the mountain -limestone 
 country often disappear in fissures, called "swallow holes." The 
 water here enlarges the fissure by dissolving the limestone, deepens 
 the bed over which it flows, and gradually eats out the lofty chambers 
 known as caverns. Many caverns have a branching ground-plan like 
 a river with its tributaries, showing that when the infiltrated waters 
 have been liberated in cracks within the rock, they have flowed on, 
 excavating channels for themselves. It is only when the cavern 
 admits of a certain amount of evaporation taking place that the 
 erosive action begins to be counteracted by the deposit of the dis- 
 solved material in pendent masses descending from the roof of the 
 chamber. These stalactites may grow, like columns, until the cavern 
 is filled with them ; for the drippings from the stalactites often form 
 corresponding masses on the floor termed stalagmites, which rise up to 
 meet the descending pillars. Many, however, of the most interesting 
 caverns have been filled up with mechanical deposits before the 
 stalagmitic covering was developed ; and in such gravels or red cave- 
 earth the remains of fossil mammals occur. The chief British caves 
 
EXCAVATION BY MOUNTAIN STREAMS. 153 
 
 are in the Plymouth limestone, as at Brixham and Kent's Hole ; in 
 the Carboniferous limestone, as in the Mendip Hills, Derbyshire, and 
 the district about Settle ; and in the Coralline Oolite of Yorkshire at 
 Kirkdale. 
 
 Streams which flow under ground, like the Mole in Surrey, which 
 flows through the Chalk, traverse chambers which are still concealed. 
 
 - 
 
 Descending Streams and Rivers. 
 
 The wasting effects of the atmosphere are sensible in all regions, 
 and therefore in every country some materials are available for the 
 streams to transport. But the proportion of matter thus prepared in 
 mountainous countries is so vastly greater than elsewhere, that in 
 general, the less conspicuous effects of the same causes are overlooked 
 in lower regions. The common notion respecting the action of alpine 
 streams appears to be, that these are the principal agents of destruction 
 upon the faces of the mountains, and that it is to them that the actual 
 waste of the surface is to be attributed. But though these streams 
 are indeed powerful agents of excavation, their principal influence is of 
 quite another kind, and it is chiefly by the disposition of the materials 
 brought into them by rains and avalanches that they effect such 
 important changes. 
 
 Erosive or Excavating Action of Streams. In considering the 
 action of streams and rivers, we must distinguish between their powers 
 of eroding or excavating, and of transporting solid matter. 
 
 The river works upon the channel and floodway, and its effects 
 have relation to the consolidation of the matter traversed, and to the 
 rapidity and volume of the moving water. About their sources, 
 and for a long part of their early courses, streams continually deepen 
 their channels, and wear away barriers of rock ; but in their broad 
 expansions near the sea, this power of excavation wholly ceases, as 
 a general law, and is only seen in particular cases, as when great bends 
 are cut off or banks undermined. 
 
 We have abundance of examples in all our mountain regions of 
 the actual excavation of their channels by rivulets and rivers. In 
 the district of Aldstone Moor, the south Tyne runs to the north from 
 the side of Cross Fell, for some miles along a slope of shale, over the 
 Tyne bottom limestone. In this shale, which is itself excavated into 
 a broad valley, the river has evidently cut its own narrow yet suffi- 
 cient channel ; and no contrast can be more striking than that here 
 afforded by the mighty valley of Tynedale, 1500 or 2000 feet below 
 its bordering mountains, and the little channel holding the waters of 
 the river Tyne. Every river works out its own channel in elevated 
 regions, and in lower ground the soft clays and sands yield a passage 
 to the feebler currents. In the level regions, along the rivers of 
 Yorkshire, Lincolnshire, and Cambridgeshire, the channels have been 
 many times changed, even by those sluggish streams ; and still more 
 in the deltas of the Rhine, the Nile, and the Mississippi ; and among 
 the Alps, fluctuation of the river courses is excessively irregular. 
 
154 EARTH-SCULPTURE BY WATERFALLS. 
 
 No doubt, then, can remain of the fact that some rivers and running 
 waters excavate and alter their channels ; though the changed course 
 sometimes results from the deposition of sediment in the river-bed. 
 
 Lyell has given a remarkable case of recent excavation in a bed 
 of modern lava of a channel from 50 to several hundred feet wide, 
 and 40 to 50 feet deep, by the river Simeto, flowing from Etna. 
 Scrope has also shown that similar phenomena have happened in the 
 volcanic region of Auvergne. In these cases the action of the river 
 has probably been excited by the flowing of a current of lava across 
 its course, so as to dam up the waters, and give them something of 
 the force of a cataract. 
 
 Waterfalls, &c. The waterfalls and cataracts upon the line of a 
 stream afford some curious points of study. It is especially in these 
 cases that the increase of excavating power, derived by a river from 
 the solid matter which it transports, is most evident. 
 
 Fig. 49. Balkan. 
 
 A cataract is formed upon the river Eden, in Westmoreland, near 
 Kirkby Stephen, by some beds of calcareous red sandstone conglo- 
 merate. The pebbles which the river brings down, contribute with 
 the whirlings of the water to excavate many deep perpendicular 
 pits or potholes, similar on a small scale to swallow holes on the 
 mountain limestone ranges, or those romantic cavities on the Caldew 
 in Cumberland. Below many waterfalls in Wales and Scotland the 
 same effect is produced. Near Christiana are deep pits of this 
 kind, termed " Giants' Kettles," often from 30 to 40 or more feet 
 deep. They have a spiral form like a corkscrew, are about four times 
 as deep as wide, and contain the stones which were rotated to exca- 
 vate them. They occur in groups near the sea, often at a height of 
 1 200 feet; small kettles 18 inches deep have been excavated in eight 
 or nine years by small streams. * 
 
 1 Brogger and Reusch, Quart. Jour. Geol. Soc., vol. xxx. p. 750. 
 
ORIGIN OF A GORGE. 
 
 155 
 
 But the most characteristic effect of a cascade is that ceaseless 
 undermining of its base and sides, and consequent rupture of the 
 spout or edge of the fall, which causes the cascade by slow degrees to 
 retire farther and farther up the mountain-side, and produces those 
 deepening portals of impending rocks which so much augment the 
 sublimity of a waterforce. 
 
 This effect may be excellently observed in the Carboniferous lime- 
 stone district of the north of England, where so many streams leap 
 from beds of limestone over perishing shales and sandstones, and 
 rising in foam, sap and undermine the base of a large semicircular 
 cliff, till at length the solid limestone crown gives way, and the insa- 
 tiable river renews its destroying attacks. The same destroying power 
 is seen in many of the Swiss waterfalls, particularly in the numerous 
 falls of the Giessbach. 
 
 Gor 
 
 River 
 
 Fig. 50. Diagram of a Gorge. 
 
 This diagram explains the mode of formation of a gorge (B) by the 
 recession of a waterfall (A). The hard bed (R) dipping in the oppo- 
 site direction to the course of the river, forms a ledge at A, over which 
 the water falls, undermining it by excavating the soft shales (s) ; so 
 that at length part of the ledge falls, and as the waterfall recedes the 
 
 I'ig. 51. Falls of Niagara. 
 
 orge extends. If the hard beds are thin there is rarely a great fall. 
 If the rocks dip in the same direction as the stream, rapids are often 
 produced. 
 
 Lyell ingeniously applied the acknowledged fact of the recession 
 
156 GORGE OF THE RHINE. 
 
 of the Falls of Niagara, which appear to have been pushed back 
 several miles, at the rate of 40 or 50 yards in fifty years, to the pos- 
 sible discharge hereafter, through the St. Lawrence, of the waters of 
 Lake Erie. Such a discharge, if it were brought about suddenly, 
 would occasion a local deluge ; but the lake is so rapidly filled up by 
 sediment, that it is a question whether it will not become dry ground 
 before the Falls of Niagara shall have been pushed back so far as to 
 be capable of emptying it. Excavation at the rate of a yard a year 
 would require twelve thousand years for the formation of the Niagara 
 gorge, which is seven miles long. The Falls of St. Anthony, near the 
 junction of the Minnesota with the Mississippi, have formed a gorge 
 eight miles long to Fort Snelling. The falls have been receding since 
 1680 at an average rate of five feet a year, so that they may have 
 required between eight and nine thousand years to excavate the valley, 
 according to Prof. Winchell. 1 The Fall of the Rhine at Schaffhausen is 
 a grand exhibition of the erosive power of water, particularly the wear- 
 ing of the base of the island pinnacles of limestone, which now stand 
 proudly in the midst of the currents, but will eventually be hurled 
 down the thundering cataracts. The gorge of the Rhine, sixty miles 
 
 Fig. 52. Gorge of the Rhine. 
 
 long from Eingen to Rolandseck, has been cut by an ancient waterfall, 
 long since passed away, which has lowered the level of the Rhine and 
 its tributaries, and drained lakes in its course, which are now small 
 plains. 2 
 
 Transporting Power of Streams. In considering now the trans- 
 porting action of streams, we may distinguish between such as flow 
 through valleys of uniform declivity without lakes, and such as pass 
 through broad receptacles of water before arriving at the sea, as is the 
 case with several rivers of England, Wales, and Scotland, and streams 
 which flow down from the Alps. 
 
 Elvers without Lakes. A certain velocity of current is requisite 
 for the transport of every kind of earthy matter ; the finer the matter, 
 the less the force required to move it along. Hence in the lower parts 
 of rivers, where their course slackens, and they approach the sea, though 
 they can no longer remove rocks and transport loads of sediment, 
 their waters are muddy, and their channels and sides receive continual 
 augmentation. Such a river as the Yorkshire Ouse is very instruc- 
 
 x Quart. Jour. Geol. Soc., vol. xxxiv. p. 886. 
 2 Ramsay, Quart. Jour. Geol. Soc., vol. xxx. p. Si. 
 
VALLEYS FILLED UP BY RIVERS. 157 
 
 tive. As its branches descend from Shunnor Fell, Cam Fell, and 
 Whernside, they transport daily and hourly from those elevated sites 
 the materials accumulated by atmospheric agencies and mechanical 
 attrition ; the soil, the stones, the loosened rocks, grain by grain, and 
 piece by piece, move onward with the current, and thus the whole 
 mountain region, by a slow yet not imperceptible progress, is lowered 
 in height, and its wasted spoils swept away for ever. But let us follow 
 this process. Wherever the valley originally presented great inequali- 
 ties, these are constantly diminished by the upfilling of the hollows, 
 and at length the originally rugged chasm is changed by additions and 
 upfillings into the smooth, evenly declining hollow, which, because of 
 that smoothness and uniform declination, is supposed by many to be 
 entirely a valley of denudation. In this process, the lateral action of 
 rains and inundations from the sides of the valley is a very important 
 auxiliary. Any one who contemplates the valleys of the Jura, near 
 Schaffhausen, and sees them in many cases rugged on the sides, and 
 evidently traced by nature in contortion, must be struck by the 
 smooth, even, equally declining plane of their bottom, which cuts 
 the rude precipices of the sides, and clearly indicates a subsequent 
 powerful modification of the original roughness of the chasm. Still 
 more abundant is the deposit of sediment as the stream glides into 
 lower ground. There, above its narrow channel, rise the broad 
 meadows, which, with every fresh inundation, receive a new coat of 
 sediment, and above these swell the real boundaries of the valley, 
 often consisting of water-worn materials, gravel and sand, left there 
 by ancient floods of greater power, flowing at a higher level. As we 
 approach the sea, when the tidal currents meet the freshes, the sus- 
 pension of motion permits a great part of what sediment still remains 
 to discolour the water, to drop on the bed of the river and its alluvial 
 banks. Thus the streams become choked, their channels sinuous, 
 their beds elevated, and the banks which confine the river, heightened 
 both by nature and art, look like the ramparts and terraces of a lofty 
 military road rather than the boundaries of a river giving passage to 
 the drainage of the neighbouring country. 
 
 Taluses and Fans. In the upper basin of the Indus, Mr. Drew * 
 has described some remarkable deposits of sediment, due to the 
 circumstance that the accumulation of the material is more rapid 
 than its removal ; though the like conditions may be observed on a 
 smaller scale in our own Lake district and in all hilly countries. 
 Wherever the rocks become disintegrated, they fall and accumulate 
 taluses, such as may be seen on the south-eastern side of Wast Water, 
 in Cumberland, extending for miles. The materials of a talus partly 
 fallen and partly washed down by the rain generally lie at an angle of 
 about 35. Sometimes they expand downward in a fan shape, having 
 its foot in a valley, and its apex in a steeper tributary ravine, giving 
 the talus a vertical height, which is often 1000 or 2000 feet. Some- 
 times such a mass becomes infiltrated and cemented by calcareous 
 
 1 Quart. Jour. Geol. Soc., vol. xxix. p. 441. 
 
i 5 8 
 
 FANS IN KASHMIR. 
 
 matter, so as to form a hard breccia. But often at the mouths of side 
 ravines, where they open into a plain or wide valley, there are broad 
 alluvial fans, well seen in Ladakh, in Kashmir. Such fans have a 
 slope of about 5, and an extent of about a mile, so that the apex 
 rises some 500 feet above the surrounding plain. These depressed 
 cones have been accumulated by the agency of streams bringing down 
 loosened detritus from the higher parts of the mountains. Sometimes 
 a number of fans unite together, as on the left bank of the Indus, 
 opposite Leh, where they form a continuous deposit for thirty miles, 
 which is fully two miles broad. The materials of the fan include more 
 or less rounded blocks of granite, which may be as much as four feet in 
 diameter, with sub-angular pieces of slate and shale, a few inches in 
 diameter, mixed with gravel and sand. Occasionally these fans have 
 been denuded by the Indus so as to form cliffs 50 to 100 feet high ; 
 and cases have occurred where a succession of fans has been formed 
 and denuded one below the other, in the same locality. 1 
 
 Fig. 53. Fan at Tigar in Nttbra, at Ladakh (after Drew). 
 
 The channels and banks at the mouths of rivers often extend out- 
 wards into a cape or headland, and contribute to extend the whole 
 breadth of the bordering coast, so that by the waste of the uplands 
 the low land is filled up, the river channels are raised, the coast is 
 extended into the sea, and the sea filled with shoals and sandbanks. 
 Thus the mouths of the Po, the Rhine, the Nile, the Euphrates, the 
 Ganges, and the Mississippi, have formed for themselves those broad 
 deltas which, within the historic era, have transformed ancient ports 
 into inland towns, and extended fertile pastures into areas where the 
 sea formerly washed. 
 
 1 The fan-shaped masses are also commonly met with on the coasts of Norfolk 
 and Yorkshire, where the finely-divided gravel, mud, and drift descend from the 
 cliffs. 
 
 
SEDIMENT BROUGHT DOWN BY RIVERS. 159 
 
 The substances transported by the stream, and deposited along 
 its sides, are, of course, such as are furnished by the hills around its 
 sources, and above its channel ; and the almost incessant accumula- 
 tions of earthy matter which thus take place, may be varied, according 
 to the nature of the country, by interposed layers of vegetable re- 
 mains. In tropical and warm regions, and in unenclosed countries, 
 this is the case to a far greater extent than an acquaintance with Euro- 
 pean rivers would lead us to expect. The mighty forests of America, 
 untouched by human industry, annually furnish to the great rivers 
 which intersect them an immense spoil of trees, which being easily 
 supported by the current, are carried towards the sea, and deposited 
 at the river mouth, or drifted away on the waves. 
 
 Arrangement of Materials. The arrangement of the materials 
 brought down by streams is in general regulated by a tendency to the 
 production of a level surface, and thus the original inequalities of a 
 valley are continually lessened. In a high region like the Alps, the 
 rough streams leave in the higher levels chiefly a collection of pebbles 
 and sand in local confusion ; but the general effect is a uniformly 
 declining plane, through which the capricious stream finds for itself 
 new channels, and thus continually shifts its deposits over the whole 
 broad pebbly floor of the river valley. Such effects may be well 
 seen on the line of the Arve, as it hurries down from the glaciers of 
 Savoy. On the contrary, in the lower and more level expansions of a 
 valley, where the gentler waters transport only fine sediment and 
 vegetable substances, these materials are arranged in most exact 
 parallelism over a large extent of plane surface, and by counting the 
 laminae of deposition, some notion may be formed of the period occu- 
 pied in the process. On the borders of streams which are periodically 
 swollen by rain, as in the tropical regions, or by the melting of snows, 
 as in those which descend from high mountain countries, this mode of 
 computation of the laminae may even be trusted so far as to determine 
 the number of years employed in producing a given depth of deposit ; 
 and in districts where the rivers swell irregularly at uncertain in- 
 tervals, there might be deduced an average rule as to the rate of 
 deposition. Nor would the accumulation during a short period of 
 time, tried by this test, appear inconsiderable. In a single season, the 
 rivers of Yorkshire, aided by the sea, deposit many inches of rich soil 
 upon the level peat-moors which adjoin their estuary ; and at Ferry- 
 bridge, at the point where the tide, formerly flowing up the river, 
 neutralised the freshes of that river, many of the modern works of 
 man, as oars of a boat, a coin of England, were found buried under 
 the alluvial sediment, which contained petrified hazel branches and 
 nuts, bones of deer, &c. The rivers of the Bedford Level have con- 
 stantly silted up in historic times. The Great Ouse formerly (1292) 
 joined the Nene and flowed out at 'Wisbeach, but as that outfall 
 became silted up owing to the deposit of sediment by the stagnating 
 waters, the whole drainage of the Level was diverted towards the 
 outfall at King's Lynn. The inundations of the Nile raise the land 
 of Egypt 4^ inches in a century. 
 
160 EFFECTS OF RIVERS ON THE EARTH'S SURFACE. 
 
 From what has been said of the action of rivers, it is evident that 
 their effects upon the physical features of a country are varied and 
 interesting. The tendency of all descending streams of water is the 
 same, to equalise the surface of the earth, to remove its ridges and 
 asperities, and smooth its depressions and fissures. 
 
 The degree in which they respectively perform this work depends, 
 first, on the amount of atmospheric and local influences in wasting the 
 surface of the higher ground, and bringing materials for the rivers to 
 act upon. Hence the rapid waste of high Alpine tracts exposed to 
 fluctuating heat and cold, to storms, avalanches, and glaciers. Hence 
 the streams of sand and pebbles, which are carried from the gritstone 
 hills of England ; and, on the contrary, the almost unvaried purity of 
 the springs which break from the Carboniferous limestone. 
 
 The second circumstance which determines the modifying power of 
 the river is its own volume and velocity, and these are principally 
 dependent on the physical geography of the region. The datum of 
 the volume of water flowing in any valley is principally useful for 
 comparison with the observed effects; the kind of effect produced 
 being determined by the velocity of the current. 
 
 If we conceive that in its first fury a river may have power enough 
 to sweep along even large blocks of stone, but that its velocity gra- 
 dually diminishes, there will be a certain point where these large 
 blocks will be left by the enfeebled current, pebbles will roll farther, 
 coarse sand will travel beyond, and the finer sediment will be moved 
 on till the languid waters permit their slow and equal deposition. 
 This gradation of deposits is always observed in examining valleys 
 of sufficient length and elevation. The deposits in the upper 
 parts are tumultuous and confused, in the lower regions level and 
 regular. 
 
 A third circumstance, of still more importance than the others, 
 serves to regulate the action of the river. This is the form and 
 character of the valley itself. However produced, there can be no 
 question that the present aspect of almost every valley in the. world 
 is smoother and more equalised than it was formerly, since we see 
 evidently, and take as a principle, that the characteristic effect of 
 denuding agents in action is to reduce continually the inequality 
 which remains. We may, therefore, easily, for each valley, restore in 
 imagination its ancient condition, remove the sediment from its 
 expanded meadows, and leave, instead of level or gently sloping 
 plains that wind smoothly round the hills, and ascend far up 
 toward the sources of the stream, deep chasms between cliffs rent 
 asunder by convulsion, or eroded by tidal attrition, or the solvent 
 chemical action of carbonated waters. That such has been the origin 
 of many valleys is evident. That many of these may have been 
 partly cleared out, and others wholly excavated by violent floods, 
 sweeping over and denuding the land during its elevation from 
 the sea, may be considered as proved. But we may content ourselves 
 for the present with the datum that the fundamental features of 
 valleys are not always the result of the excavating action of their 
 
FILLING UP OF LAKES. 161 
 
 streams, but that valleys have been in part filled up by the accumu- 
 lations brought by their own rivers ; and that their present smooth- 
 ness and uniformity is really the result of modifying powers of the 
 sea, atmosphere, local influences, and the river combined, exerted 
 through long time upon a ruder channel, left by more violent marine 
 agents. 
 
 Rivers with Lakes. Let us now see what peculiarities in the 
 effects of rivers are occasioned by the circumstance of their traversing 
 quiet lakes. Two things are here to be attended to. First, the lake 
 causes, according to its extent, a more complete stagnation of the river 
 movement, and consequent deposition of the sediment brought by the 
 rivers, than is occasioned by the most level area of a river-valley ; 
 secondly, the materials dropped in the lake are regulated by somewhat 
 different laws from those which direct their accumulation on an 
 ordinary surface. 
 
 When a river charged with sediment expands into the waters of 
 a lake, its motion, communicated to that large area in directions 
 radiating from the place of entr}^ is checked and almost lost, and the 
 sediment which it brought to the lake is gradually, and at last wholly, 
 deposited ; and the purified stream issues from the lower extremity 
 without a taint of its stormy origin, unless it be the colour, due to moun- 
 tain-peat, or some other substance held in chemical solution. Like 
 the lake from which it escapes, or the ocean far from shore, it gene- 
 rally assumes the purest ethereal hue, its native tint of blue or green, 
 but soon in its onward course again becomes turbid with sediment. 
 Every lake in Switzerland exhibits these effects upon the rivers, which 
 commonly enter turbid, and issue of a transparent green, though the 
 waters of the Ehone are pre-eminently blue. These lakes are filling 
 and contracting at their upper ends with the sediment which they 
 gather from the rivers ; and the process, though historically slow, is 
 monumentally impressive, since we find large tracts of level 
 meadows, cultivated, covered with trees, and supporting ancient and 
 modern towns, where formerly flowed the deep waters of the lake. 
 Thus the Roman town Portus Valesise, originally on the water's edge, 
 is now nearly two miles inland, owing to the delta of the Rhone 
 having encroached to that extent on the lake in the last 800 years, 
 and behind Port Vallais the delta extends inland for five or six miles 
 as an alluvial plain, which has displaced the waters of the lake. 
 
 All this new land was formed from the spoils and waste of tho 
 upper countries drained by the river, and is a measure of the effect 
 of atmospheric and local influences in weathering the face of the 
 hills, and of rivers in carrying away the materials thus prepared 
 for them since the earliest period when the streams began to floio 
 down the valley of the Rhone. 
 
 Arrangement of Materials in Lakes. The second thing to be 
 attended to in considering the effects of lakes on the line of rivers, is 
 the arrangement of the materials which they receive. It is known to 
 practical engineers that loose earth will remain at rest if it be placed 
 at an angle not exceeding 45 with the horizon, and when loose, earthy 
 VOL. i. L 
 
162 STRUCTURE OF LACUSTRINE STRATA. 
 
 materials are poured from a height, they usually arrange themselves in 
 a conical heap, whose sides make nearly this angle with the horizon. 
 On the slopes of mountains liable to avalanches or rapid waste, the 
 loose debris is usually found in a plane declining at about this angle. 
 When streams falling over a ledge, transport with their waters a quantity 
 of earthy matter, the conical heap so thrown down is very much more 
 obtuse than when the materials fall dry, because their weight in water 
 is less, so that the fall of the particles is not so direct, and the larger 
 the proportion of water that comes down, and the more forcibly it 
 descends, the flatter is the slope of the cone. This will easily be 
 understood upon the principle that by partial suspension in water 
 each particle is influenced by the tendency of that fluid to become 
 level. 
 
 It is easy to understand from this that the form in which coarse 
 sediment will be deposited by rivers entering a lake, must be in a very 
 obtuse cone, radiating round the point of entrance. As the heap of 
 sediment is advanced into the lake by continual additions, its outline 
 remains circular, with a larger radius, and its section will be nearly 
 level toward the land, but sloping more and more rapidly toward the 
 interior of the lake. Were the particles to be arranged in obedience 
 to the double forces of horizontal movement with the river, and of 
 perpendicular descent from gravitation, the curve of the edge would 
 be parabolic, arid the surface left upon the sediment toward the land, 
 nearly level. 
 
 But the earthy matter being unable to support itself at more than 
 a certain angle of elevation, the lower part of the curve will become 
 less steep, and be reduced to a straight line. Observations on the 
 Swiss lakes assign to the sediment left therein an outline of this 
 kind. 
 
 It is obvious that in these cases the sloping layers nearest the 
 entrance of the stream are of older date than those farther advanced 
 into the lake. It is an interesting subject of inquiry to learn whether, 
 as is most probable, the particles of the sediment which differ in bulk 
 and specific gravity, are arranged according to those qualities so as 
 to constitute horizontal strata, of finer and coarser matter, &c. ; and 
 whether, this being the case, the sloping lines of deposition, &c., are 
 visible or obliterated in the section. In this manner the upper ends 
 of lakes are filled almost to the surface with the deposits from the 
 rivers ; and the dams of the lower ends of the lakes being worn 
 away by the incessant action of the stream, these deposits become 
 visible above the water, and constitute those smoothly declining, often 
 moist surfaces, which usually confine within their indefinite border 
 the shallow and weedy waters, destined in their turn to retreat from 
 the desiccated land. While this process proceeds near the shore with 
 the coarser particles, it is obvious that the finer sediment will be car- 
 ried farther into the lake, and be spread more widely over its general 
 bed. 
 
 These remarks apply only to deep lakes, whose waters rest tran 
 quilly on their beds, and are only agitated at the surface. In shallow 
 
RIVER DEPOSITS IN THE SEA. 163 
 
 lakes, which are agitated to the bottom, the materials must neces- 
 sarily be distributed in planes very nearly horizontal, in consequence 
 of the influence of fluctuations at the surface. 
 
 Lacustrine Limestones. Before we dismiss the subject of lakes, 
 it will be proper to take notice of another process tending also to fill 
 them with new deposits. Many streams which enter lakes carry 
 along, dissolved in their waters, a quantity of carbonate of lime, which 
 may afterwards, by the loss of carbonic acid from the water, fall in 
 calcareous sediment, and constitute beds of marl, or by the slow 
 absorption by mollusca, be converted into shells. In the latter case, 
 beds of LimnseaB, Paludinae, &c., are formed, and as the light argillace- 
 ous sediment entering such lakes is generally pretty equally diffused 
 through the waters, the result is a bed of marly clay full of fresh- 
 water shells. This process is daily going on, and in the course of a 
 few years canals and river courses, as well as ditches and ponds, are 
 choked by the abundant accumulation. In this manner, aided by 
 occasional inundations bringing layers of vegetable matter, or 
 detritus of the neighbouring country, many old lakes have become 
 entirely filled up ; and when the deposits are cut open for any pur- 
 pose, they present layers of peat, clay, shell-marl, and sand, a faith- 
 ful image, on a small scale, of those great fresh- water deposits which 
 mark the force and extent of ancient currents on the surface of the 
 earth. 
 
 Deltas. The delivery of the sediment of rivers into quiet, tide- 
 less, land-locked seas is almost perfectly analogous to what happens 
 in a large lake ; but according to variation of circumstances, as the 
 river flows into the open ocean, and contends with strong tides and 
 sweeping currents, or discharges into a gulf, enters deep or shallow 
 water, the disposition of its sediment is different. The most remark- 
 able deltas at the mouths of rivers are formed round such as empty 
 themselves into tideless seas, as the Mediterranean, Black Sea, Cas- 
 pian, Baltic, &c., or into comparatively quiet bays of the ocean, as the 
 Bay of Bengal, the Gulf of Mexico ; and the least effects of this 
 nature are accumulated on coasts which are subject to be raked by 
 lateral currents of the sea. But it is probable that most deltas 
 originate in the materials scoured off the surface of the land by 
 estuarine waters, when the land last emerged from the ocean, which 
 are swept out from the river valley and remain at its junction with 
 the sea ; so that the existence or absence of a delta may depend upon 
 the rate at which the land is raised, and the angle of upheaval. 
 
 Most of the great rivers which enter the Mediterranean are daily 
 increasing their deposits along the coasts, and spreading a quantity of 
 sediment over the general bed of the sea. The Mediterranean has 
 been proved by a line of soundings on the Skerki Shoal from the 
 African to the Sicilian coast, varying unequally from 7 to 91 fathoms, 
 to be divided into two basins. In the western portion, near Gib- 
 raltar, the bottom, consisting of sand and shells, has been reached at 
 5880 feet, and in the straits at 4200 feet, though between Capes 
 Trafalgar and Spartel the depth is nowhere 1200 feet. Almost under 
 
1 64 VARIETY OF DEPOSITS DUE TO RIVERS. 
 
 the shore at Nice the depth is 2000 feet; but in the Adriatic, where 
 it receives the sediment of the Po and other rivers in the upper part, 
 the greatest depth is 22 fathoms. Yet from the abrupt borders of 
 the hill ground within the area of the sedimentary land, it is inferred 
 that the Adriatic must formerly have been a deep gulf. 
 
 Nature of the Deposits in Gulfs, Estuaries, &c. Farther from 
 the influence of the rivers the depth increases considerably. Donati, 
 on dredging the bottom of the shallow portion of the Adriatic, found it 
 to consist partly of mud, and partly of calcareous rock, enclosing shells. 
 The form of these sedimentary deposits must be what in common lan- 
 guage is called horizontal, the substance of them is fine clay and calcareous 
 matter with shells ; and as the ratio of accumulation is nearly uniform, 
 there will be little appearance of stratification, unless the calcareous 
 deposits be formed at intervals. If by any effort of upheaval this 
 bed of the Adriatic should hereafter be elevated and made dry land, 
 as so many other extensive tracts along the borders of the Mediterranean 
 have been, we should have an argillaceous deposit similar to the 
 London clay, and perhaps identical with the subapennine marls, 
 except for some difference in organic remains, and of such an area as 
 would appear incredible to those who believe in the almost slumbering 
 condition in modern times of the mechanical and chemical forces 
 which modify our globe. The same conclusions might be derived 
 from an examination of the mouths of the Rhone, Volga, Danube, 
 Ganges, Euphrates, &c., which enter the sea under the same favour- 
 able circumstances, and transport enormous quantities of fine sediment 
 into comparatively tranquil and now shallow waters. A river like 
 the Mississippi, which hurries along an enormous volume of deep 
 waters, and preserves its velocity to the edge of the sea, discharges a 
 prodigious quantity of matter, which settles round its many mouths 
 into a vast and growing delta. But the kind of matter here deposited 
 and the mode of its arrangement will be different. Forests matted 
 together by the growth of ages, with all their foundations, their 
 alligators and other inhabitants, are swept down by this mighty 
 stream, and either embedded for a time among its winding and vari- 
 able channels, or hurried into the sea, and there agitated, and partially 
 or completely separated into beds of earthy and of vegetable matter ; 
 and thus the Gulf of Mexico is now filling with deposits, which 
 in no uncertain way simulate carboniferous strata. We are in- 
 formed by Lyell, whose volumes are full of valuable information on 
 all subjects connected with the modern operations of natural agencies, 
 that a great part of the new deposit at the mouth of the Rhone consists 
 of calcareous and arenaceo-calcareous rock, containing broken shells of 
 existing species ; and Admiral Smyth ascertained that over the broad, 
 very gently inclined bed of this growing delta, marine shells were 
 occasionally drifted by a south-west wind. In this way alternations 
 of fresh-water and marine shells may be occasioned, in which the 
 marine portions will predominate towards the sea and the fresh-water 
 part be most decided toward the land. 
 
 The shorter and more rapid the course of a river, the larger and 
 
ACCUMULATIONS AT THE MOUTHS OF RIVERS. 165 
 
 coarser is the sediment which it may be able to transport. While the 
 Po, slackening its velocity, deposits its gravel where it joins the Trebia, 
 west of Piacenza, 130 miles from the sea; and the Ganges 180 miles 
 above the commencement of its delta, and 400 miles above the present 
 line of coast ; the rough bed of the Yorkshire Tees is pebbly quite 
 down to the sea ; and the streams which descend by a short and 
 furious course from the Maritime Alps, bear down pebbles into the 
 Mediterranean. 
 
 From these instructive examples of pebbly, sandy, argillaceous, 
 and calcareous strata, forming at the same era, in different basins of 
 the sea, and even in different parts of the same basin, under similar 
 conditions, enveloping deposits entirely marine, entirely freshwater, 
 or partly marine and partly freshwater, we may turn with advantage 
 to the contemplation of the older strata of conglomerate, sandstone, 
 clay, marl, and limestone ; and by carefully noting points of agree- 
 ment and circumstances of difference, may frame satisfactory notions 
 of the conditions under which they were respectively deposited. 
 Especially we may be guided in our decision concerning the extent 
 and connection or separation of the several basins of the ancient 
 oceans, and the relative influence of ancient and modern rivers. 
 
 Bars at the Mouths of Elvers. Rivers which discharge them- 
 selves into the sea, where tides and currents contend with the freshes, 
 may, like the Rhine, be enabled for a certain time to deposit their sedi- 
 ment in a delta, and to increase this even to a vast degree, in conse- 
 quence of entering the sea at a deep emargination of the coast, or amidst 
 shallow sands which impede the action of the tide. But in such a 
 case the accretion of land will gradually diminish, and at length the 
 movements of the sea must balance the current of the river. In this 
 case a line of sand-banks will be formed varying in position according 
 to the alternate predominance of the contending forces, and the 
 river entrance will have a bar. The Rhine, the Thames, and all the 
 eastern rivers of England, are nearly in the same case. The sea, 
 indeed, has again reclaimed from the Rhine, by most destructive 
 floods, the large space of the Zuider Zee and the Bies-Bosch. 
 
 Thus also the seaward growth of the Nilotic delta, once so rapid, 
 is greatly retarded or almost annihilated by a current of the Medi- 
 terranean ; and the rivers of Western Africa, as well as the mighty 
 Amazon, no longer extend themselves into the sea, but meet its cur- 
 rents in furious strife, drop the sand at their mouths, and resign 
 their finer sediment to its disposal. The distance to which 
 the ocean can waft this sediment on its surface along with 
 the lighter fresh water is very great. General Sabine supposed him- 
 self to have crossed the discoloured waters of the Amazon 300 miles 
 from its mouth, where it still retained its comparative lightness, and 
 kept its place on the surface of the sea. 
 
 Thus may the sediments of distant countries be mixed or alter- 
 nate, in deposits far from shore, and even in the deep sea. The dis- 
 tinctness of currents of water which flow down the same river 
 channel, even with a rapid descent, has often been noticed. Thus 
 
1 66 SIZE AND GROWTH OF DELTAS. 
 
 the Arve and the Rhone flow far without mixing, the Nahe takes one 
 side of the Rhine, and even in the mining districts of England, the 
 discoloured streams from the different valleys can often be distin- 
 guished along considerable lengths of a united river. 
 
 We shall not further extend our remarks on this subject than by 
 mentioning a few instances of the actual extent of the deltas of great 
 rivers. The whole area of the dry delta of the Po and the Adige, 
 and other rivers which contribute to extend the same line of 
 coast, must exceed 2000 square miles, and within the last 2000 years 
 a space of 100 miles in length, and from 2 to 20 miles in breadth, 
 has been added to the land. The area of the Nilotic delta is about 
 12,000 miles, and according to Girard the surface of Upper Egypt has 
 been raised by the Nile sediment 6 feet 4 inches since the Christian era ; 
 the area of the Rhone delta is 1500 square miles ; of the Niger delta 
 is 25,000 square miles. 1 The delta of the Ganges, without reckoning 
 that of the Burarnpootra, which has now become conterminous, is 
 considerably more than double that of the Nile, and its head com- 
 mences at a distance of 220 miles in a direct line from the sea. The 
 base of this magnificent delta is 200 miles in length. 2 
 
 The fen lands of Lincolnshire, Huntingdonshire, and Cambridge- 
 shire occupy 1000 square miles, and the levels in connection with the 
 Humber 300 or 400 square miles. 
 
 It has been attempted to deduce the age of existing continents from 
 the rate of increase of the deltas of rivers within the historic era. 
 Thus the Nile was supposed by Herodotus to have formed Lower 
 Egypt ; and he states that if diverted into the Red Sea, it would fill 
 that gulf with its deposits in less than 20,000, or even 10,000 years. 
 Since the time of Herodotus it is supposed that the increase on the 
 Nilotic delta has been, upon an average, one mile and a quarter. The 
 average annual growth of the delta of the Po, opposite Adria, which 
 was once on the edge of the Adriatic and is now thirteen miles inland, 
 was, from 1200 to 1600 A.D., 25 metres, and from 1600 to 1800, 70 
 metres ; a very rapid increase of rate, probably connected with the 
 increasing shallowness of the sea. 
 
 But all inferences from observations of this nature, and similar 
 ones on the shallowing and conversion into land of |he upper ends of 
 lakes, can only lead to speculative results, without a knowledge 
 of the original depth of the sea or lake at all points over which 
 the river sediment has flowed, a datum very difficult to obtain ; 
 for it is not by the area of the delta, but by the cubic content of the 
 sediment transported that the time occupied in the process is to be 
 ascertained. 
 
 Rate of Sub-^lrial Denudation. The average rate at which sub- 
 ferial denudation, however, goes on, is now determined with some 
 approximation to accuracy. Professor Geikie finds that a large river 
 carries away from its basin a mass of sediment which, if uniformly 
 spread, would amount to ^Vs f a f* ever y y ear ? so that taking the 
 
 Dr. Fitton, " Geology of Hastings." 2 Lyell. 
 
RATE OF SUB-MR1AL DENUDATION. 167 
 
 mean level of Europe at 600 feet, and supposing it everywhere worn 
 clown uniformly at this rate, little more than three and a half millions 
 of years would be required to wear it level with the sea. But de- 
 nudation is far more rapid in valleys and mountain regions ; so that 
 while T V of an inch may be removed in seventy-five years from plains 
 and table-lands, an equal amount of denuded matter is carried away 
 from valleys in eight years and a half, so that a valley 1000 feet 
 deep might be excavated in 1,200,000 years. 1 
 
 1 Geikie, Geographical Evolution. Proc. Roy. Gepg. Society, July 1879. 
 
168 
 
 CHAPTEE XII. 
 
 NATURE AND ORIGIN OF VOLCANIC ENERGY. 
 
 Continuity of Ancient and Modern Phenomena. The resemblance 
 between the stratified rocks now forming, and those which were 
 deposited in ancient waters, has already become so manifest that, in 
 most cases, we are compelled to use the same mineral names to 
 characterise them, no matter what may have been the period at which 
 they were formed. And when we turn our attention from the 
 aqueous to the igneous rocks, exactly the same law will be found to 
 govern their existence. The ancient forms of volcanic rocks consist 
 of the same minerals as those which have been poured out from 
 active or existing volcanoes. And although Ave are able to prove that 
 the cores of ancient volcanoes, in which the rocks have cooled slowly 
 under great pressure, can be so identified as to demonstrate their 
 existence during all periods of past time, yet the variations in mineral 
 composition, and even in chemical composition, of these granites 
 and granitic rocks are small. We learn that the positions in which 
 volcanoes formerly existed have from time to time been changed, and 
 although volcanoes were recurrent in Britain during the Primary 
 Period and the Tertiary Period, the Secondary Ages, except in the 
 trias of East Devon, were as free from such disturbances as are British 
 lands and waters at the present day. We therefore are led to inquire 
 into the nature and origin of volcanic energy. 
 
 Nature of Volcanic Energy. This, though usually manifested in 
 an eruption, may exhibit itself in a multitude of other forms known 
 to the practical geologist. We shall find the most important questions 
 in connection with volcanoes to be, first, an explanation of the heat 
 of the rocks ; secondly, the source of the eruptive power which 
 results in volcanic activity ; and thirdly, we must account for the 
 nature of the materials which are ejected from the volcanic throat, or 
 consolidate beneath a mountain mass. 
 
 Internal Heat. There are many ways of accounting for the heat 
 of the rocks, but unfortunately they are all more or less hypothetical ; 
 and we find it necessary to adduce evidence if any view on this subject 
 is to be accepted. A belief in the original igneous fusion of the 
 earth was long the favourite doctrine. It is, however, quite possible 
 that there may never have been any igneous fusion in the common 
 acceptation of the term ; and although the surface may have been 
 
INCREASE OF THE EARTH'S INTERNAL HEAT. 169 
 
 fused and incandescent, yet the earth may have been built up 
 gradually by the infalling of meteoric matter. Either view gives us 
 a high original temperature, which presumably has been reduced by 
 radiation, so that the interior is now much hotter than the sur- 
 face. It is well known that in mines, and deep wells, and borings, 
 the temperature steadily augments, though the rate of increase varies 
 with the locality; and the conducting power of the rocks is so 
 variable that the increase of temperature beneath the surface is rarely 
 regular. The most celebrated experiment, made at Sperenberg, near 
 Berlin, reaching to a depth of 4172 feet, and passing entirely through 
 rock salt, demonstrates that with increasing depth the distance 
 which has to be passed through to gain an additional degree of 
 temperature, augments. In the first 300 feet the increase was at the 
 rate of i R. for 33 feet and 45 feet; at 400 feet and 900 feet the in- 
 crease was only at the rate of about i R. for 500 feet ; at 2500 feet 
 it was i R. for 166 feet ; and at upwards of 4000 feet 1 R. for 310 
 feet. In the first 1000 feet there was an increase of 11 R., in the 
 second 1000 feet of 7 R., and in the next 2000 feet of 10 R., so that 
 the rate at which the temperature augments steadily diminishes, 1 which 
 may be only another way of saying that radiation is rapid in pro- 
 portion to nearness to the earth's surface. In other localities the 
 temperature at moderate depths increases i F. for 15 feet, and most 
 of the temperatures fall between 30 feet to 60 or 70 feet for a degree. 2 
 In our own country, the average increase is i F. for 51^ feet. 
 Hence we may conclude that an enormous amount of heat has been 
 lost by the earth's surface cooling, that this loss of heat is still in 
 progress, and that there is an immense amount of internal heat un- 
 exhausted. Whether temperature goes on augmenting to the centre 
 of the earth, or whether it soon reaches a constant amount compara- 
 tively near t the surface, is a matter of small moment practically, if 
 ws bear the earth's rigidity in mind ; for then, since the pressure of 
 superincumbent rock increases beneath the surface, side by side with 
 the augmentation of temperature, and since liquefaction of highly 
 heated rock only takes place when the pressure is removed, it is 
 probable that the temperature may never overcome the pressure 
 so as to render the interior liquid. Since we can no longer 
 assume a thin crust for the earth, it is impossible to believe this crust 
 to be blistered by liquid rock boiling beneath it. And we as neces- 
 sarily abandon the ideas that igneous rocks are protrusions of the 
 original fluid substance of the earth, and that volcanoes are chimneys 
 by which such molten rock rises to the earth's surface. Many eminent 
 men of modern times, however, still accept the old idea of internal 
 igneous fusion. 
 
 Hypothesis of Magmas. M. Durocher propounded the hypo- 
 thesis of there being beneath the surface rocks two magmas, from 
 which all erupted rocks were derived. The outermost of these layers 
 was supposed to correspond with the acidic rocks, and to have yielded 
 
 1 See Fisher, "Physics of the Earth's Crust," p. 10. 
 
 2 Mallet, "Report on the Neapolitan Earthquake," vol. ii. p. 310. 
 
i;o CAUSE OF CRUMPLING OF ROCKS. 
 
 the granites, trachytes, and rhyolites ; the deeper-seated layer, less 
 perfectly combined with oxygen and silica, would correspond with 
 the basic rocks, and was supposed to have yielded the syenites, 
 diorites, basalts, and andesites. 1 Professor Judd 2 has gone a step 
 farther, and assumed that beneath these layers are the unoxidised 
 metals, which are occasionally brought to the surface in volcanic erup- 
 tions. Baron Itichthofen, refining on this principle, recognised five of 
 these successive magmas, which have yielded to the surface as many 
 different kinds of volcanic rocks. All these views, however, imply 
 original igneous fusion, increasing oxidation of the rocks towards the 
 surface, increasing specific gravity on descending beneath the surface, 
 and a thin crust through which the materials could reach the surface. 
 8uch views constitute an important school of geological thought, and 
 they deserve careful consideration. But the whole bearing of field 
 observation leads us to assign another interpretation to volcanic 
 phenomena. 
 
 Hypothesis of Metamorpbic Origin of Igneous Rocks. For a long 
 time only certain granites have been claimed as eruptive, while it has 
 been conceded that in many cases, if not in most, they are meta- 
 morphosed slaty rocks, and have been elaborated in the positions in 
 which they occur. We believe that the so-called eruptive granites 
 will prove to be nothing but metamorphic granites, which have been 
 displaced from the positions in which they were formed, by the forces 
 which brought the rock into existence. If granitic rocks can thus 
 originate independently of the supposed magmas, it is manifest that 
 the liquefied forms of those materials, termed 'volcanic rocks, have also 
 originated as products of metamorphism. Therefore we do not believe 
 that the internal heat of the earth necessarily points to any order in the 
 arrangement of igneous materials beneath the surface, or furnishes a 
 logical explanation of the phenomena of volcanic energy. 
 
 Theoretical Influence of Radiated Heat on the Earth's Surface. 
 The heat, however, which is radiated seems more competent to give 
 rise to physical changes in the earth's envelopes, which connect the facts 
 of metamorphism, of rock structure, and igneous phenomena, so as to 
 explain their existence in a systematic way. For since nearly every 
 known substance contracts in dimensions on cooling, it follows that 
 in descending beneath the surface the rocks must be now undergoing 
 a process of contraction. At the surface, where cooling is complete, 
 contraction is complete. But if a warmer layer existed beneath this 
 surface layer and inseparable from it, and cooled subsequently, it must 
 contract, so as to draw the upper layer together laterally and throw it 
 into folds. If we further suppose the cooling to extend deeper and 
 deeper, then each successive addition of a new contracting layer 
 below will increase the number and intensity of the folds and contor- 
 tions of the surface rocks. And since the surface rocks are thus 
 caused to occupy less horizontal space, they necessarily become 
 variously crumpled, elevated, and depressed. This conception of 
 plication of the earth's crust was first enunciated by Constant Prevost. 
 1 See Haughton's " Geology :" Appendix. 2 "Volcanoes." 
 
CAUSE OF UPHEAVAL AND DEPRESSION. 171 
 
 De la Beche and many of the founders of geology either arrived 
 independently at the same idea, or adopted Prevost's views. They 
 thus eliminated the idea that upheavals were due to direct elevation 
 from beneath ; and affirmed that they result from lateral pressures, 
 acting against each other in nearly horizontal directions, and trans- 
 verse to the lines of elevation. The Kev. Osmond Fisher proved 
 that the mechanical force thus originated was ample to account for 
 the elevation of the highest mountains. 1 And this method of upheaval 
 accounts for the diminished density of the earth which has sometimes 
 been detected beneath great mountain-chains. 
 
 Observations which Illustrate the Theory. Such changes in 
 the earth's crust leave effects which are easily recognised; and we 
 find indubitable evidences of changes of level now in progress in 
 great masses of land. The depression of Greenland is well attested 
 by the removal of settlements farther inland, and by the sinking down 
 and submersion of islands off the coast. 2 A corresponding elevation 
 of the north of Norway and Sweden, originally observed by Von Buch, 
 was demonstrated by Lyell. 3 On our own coasts sunken forests, in 
 many places, testify to depression ; and the existence of raised sea- 
 beaches proves recent upheaval. And as we go backward in time, 
 the whole series of strata, with their alternations of deep-sea and 
 shallow-sea deposits, with fresh-water beds and old-land surfaces, 
 superimposed in the same region, afford incontestable evidence that 
 the permutations of level of the earth's surface in past ages were pre- 
 cisely such as are now in progress in almost every sea ; while the very 
 existence of continental lands, chiefly composed of marine strata, 
 demonstrates upheaval on a grand scale. 
 
 If, further, we examine the geological structure of almost any land, 
 its strata are seen to be almost invariably thrown into undulations and 
 grand upward and downward folds, such as have already been indicated 
 in the structure of our own islands, and may be seen on a large scale 
 in a geological map of Europe. And whenever we approach regions 
 which have been greatly elevated, the intensity of the plication of the 
 rocks always bears some relation to the height above the sea which is 
 attained. Not rarely we find the oldest rocks the most disturbed, 
 because they have been longest exposed to disturbing forces, but in 
 many cases newer rocks are almost as much changed. 
 
 Mr. Robert Mallet's Theory of Volcanic Heat. It is well known 
 that all movement or pressure which meets with resistance becomes 
 transformed into heat. 4 And Mr. Robert Mallet, F.RS., 5 developed 
 the remarkable conception that the heat which produces volcanic 
 energy is developed locally within the solid crust by transforming 
 the mechanical work of compression into heat; while these compres- 
 
 1 Trans. Camb. Phil. Soc., vol. xi. pt. 3. 
 
 2 Arctic Papers of the Royal Geographical Society, Proc. Geol. Soc., vol. ii. p. 
 208. Quart. Jour. Geol. Soc., vol. xxvi. p. 690. 
 
 3 Phil. Trans. Royal Soc. 1835. See also ''Lyell's Principles of Geology." 
 
 4 Tyndall, " Heat as a Mode of Motion." 
 
 5 Mallet, " On Volcanic Energy," Phil. Trans. Royal Soc., vol. clxiii. p. 
 147- 
 
T72 ORIGIN OF VOLCANIC HEAT. 
 
 sions are themselves produced along parallel axes, or over definite 
 areas, by the more rapid contraction, from cooling, of hotter material 
 of the earth's mass beneath the crumpled shell. For when the mass 
 of a mountain, already forced up by lateral pressure, presses down- 
 ward upon the resolved vertical force, so as to equal the resistance to 
 crushing of the rocks on either side, then the further force of lateral 
 pressures must be used up in crushing the rock between them to 
 powder, or in developing such heat as shall render the mass plastic 
 and so displace it. The contraction is necessarily greatest along lines 
 or planes of weakness in the crust ; and the heat developed in the 
 rocks thus squeezed is necessarily greatest along those lines or planes 
 or places where movement and pressure are greatest. If one bed of 
 rock is less compressible than another, it will become more heated by 
 compression. Thus quartz would become nearly three times as hot 
 as clay, and communicate its heat to the adjacent beds. In such 
 positions the temperature may rise locally to a red heat, or even to 
 the point of fusing the rocks which are crushed and pressed together. 
 Hence heat is produced beneath the places where it is exhibited in 
 volcanic vents ; and the heat produced locally is consumed locally in 
 originating physical and chemical changes in the rock substance, and 
 in the mechanical work of ejection. 
 
 This being the theory, the question arises, Is the compression 
 competent to produce the results which are inferred ? And upon this 
 point Mr. Osmond Fisher has discussed some theoretical doubts in 
 his " Physics of the Earth's Crust," to which reference may be made. 
 
 Evidence in support of Mr. Mallet's Views. Mr. Mallet observes 
 that under the ordinary conditions of experiment at the earth's 
 surface, such a rock as granite or porphyry crushes under a pressure 
 of 14 tons to the square inch. On the hypothesis of a contracting 
 crust one mile thick, it is calculated that the lateral pressure per 
 square foot would amount to 952,666 tons, or more than 472 times 
 the force necessary to crush granite. The height of a column of rock 
 which would be crushed by its own weight is about 4 miles; but 
 the horizontal force is equal to a column of 2000 miles or one-half 
 of the earth's radius ; so that it is concluded that the resolved forces 
 of gravitation will crush the solid crust if it is left partially 
 unsupported, by the shrinking away of a contracting nucleus beneath. 
 Mr. Mallet conducted a number of experiments to show that a 
 considerable temperature was developed by crushing various rocks, 
 though the heat thus obtained was less than the temperature 
 under which crystalline rocks consolidate. The amount of heat lost 
 from the earth every year by radiation, could be produced by crush- 
 ing 987 cubic miles of rock ; and Mr. Mallet estimated that, if used 
 up in volcanic energy, it would be sufficient to produce all existing 
 volcanic cones in less than eight years ; so that only a minute fraction 
 of the radiated heat of the earth can reappear in the form of volcanic 
 activity. 
 
 Geological Evidence on the Nature of Volcanic Heat. Whatever 
 may be the value of Mr. Mallet's calculations, no matter whether the 
 
EFFECTS OF LATERAL PRESSURE. 173 
 
 force requisite to crush a rock will melt one-tenth or two-tenths, or 
 any greater or less fraction of it, further contraction in deeper layers 
 must develop additional heat in the crushed layers, and hence we 
 may admit the sufficiency of the means to develop the heat which 
 is necessary. But if these views are to be substantiated, they ought 
 to receive elucidation from the structure of the rock in districts 
 which exhibit effects of pressure ; and in the ways in which folded 
 rocks have been modified, should be found proofs that heat augments 
 towards the centre of a region which has been uplifted by lateral 
 pressure. 
 
 Where the rocks have been greatly compressed, as in Wales, the 
 Lake district, or Scotland, the old clays are altered into slates. Their 
 chemical composition is not changed, but they have lost their 
 elasticity as much as clay which has been burned into brick. And 
 Mr. Sorby has demonstrated, by sections of the rocks examined under 
 the microscope, that this change is due to an incipient crystallisation 
 by which a large part of the rock matter has become transformed into 
 a mineral, arranged in minute plates which have the characters of 
 mica or chlorite. 1 The immense compression and contortion which 
 these rocks have undergone is well known, but nowhere perhaps 
 better seen than in the section near Llanbabo in Anglesey. 2 
 
 Whenever mountain- masses exist which show crystalline rocks 
 entering into their structure, those rocks always occupy the central 
 axis of upheaval and have the slates on their flanks. The outer part of 
 the crystalline mass consists of schists, which show a foliated structure 
 with the layers made up of crystals dovetailed and densely packed ; and 
 these rocks often pass by insensible gradations into slates on the outer 
 limit, and into granitic rocks towards the central axis ; while large bosses 
 of indubitable granite are often exposed in the centre of the chain. 
 Here is a sequence of changes, increasing in intensity, giving evidence, 
 from more perfect crystalline texture, of augmenting temperature, as 
 we approach the position in which the lateral pressures most perfectly 
 antagonise each other, and become most intense. So that since the 
 bulk of masses of granite and schists often bears some proportion to 
 the evident elevation of the region in which they occur, as in the 
 Pyrenees and Alps, though the present heights are often diminished 
 by denudation we may recognise in mountain structure and mineral 
 condition, exactly such phenomena as ought theoretically to exist, if 
 the heat which altered the rocks were produced locally by compres- 
 sions. And under no other explanation can we account for the 
 observed physical structure of mountain-chains. 
 
 Temperature and Pressure involved in Rock Construction. 
 There is no means of estimating the extreme slowness with which 
 these changes have been brought about, except such as are suggested 
 by the changes of level in land which have been observed to be now 
 in progress ; and although high temperatures may be necessary to 
 approximate to similar conditions in our experiments, it is probable 
 
 1 Sorby, Quart. Jour. Geol. Soc., vol. xxxvi. Address, p. 73. 
 - Ramsay, Mem. Geol. Surv., vol. iii. p. 246, fig. ico, new ed. 
 
174 ORIGIN OF CRYSTALLINE ROCKS. 
 
 that, in the lengthened periods over which the natural operations 
 extended, the heat involved may have been less than would at first 
 have been expected ; and it is more than probable that the pheno- 
 mena are not to be explained by the action of heat alone. All 
 experiments at the earth's surface are necessarily under a pressure 
 which is infinitesimal in comparison Avith that of the superincumbent 
 rock, which has since been denuded from a granitic district, some- 
 times for a thickness of miles, to say nothing of the force of pressure 
 from lateral contraction which is superadded. And while the water 
 within the rock at once escapes in a furnace experiment, the water 
 is inevitably imprisoned in a metamorphosed rock, so that the con- 
 ditions of the experiments are not the same. The presence of water 
 appears to be necessary to the production of such crystalline forms 
 for minerals as are met with in nature ; for in blast-furnace slags, 
 which are run out at a temperature of about 3700 F., only complex 
 feathery skeletons of crystals are commonly formed, with those 
 minute light and dark needles scattered in the glass, which have been 
 named belonites and trichites. And when igneous rocks, such as 
 basalt, are artificially melted, the augite crystallises in flat feathery 
 plates, like those of furnace slags, which are rarely if ever seen in 
 nature ; and the felspar prisms end in complex fan-shaped brushes, 
 so that the structure of the rock is changed by the conditions of 
 liquefaction and consolidation. 1 Similarly when the Leicestershire 
 syenite is fused and slowly cooled, the solid crystals are lost and 
 replaced by feathery skeleton crystals of magnetite, and flat prisms 
 of triclinic felspar ending in fan-shaped brushes. Dr. Sorby suggests 
 that the difference is due to the presence and absence of water. 2 
 The quartz in schists was found by Sorby to abound in fluid cavities, 
 the fluid being water which usually contains chlorides of potash or 
 soda ; which indicate in the schists of Cornwall a temperature of 
 392 F., and in the schists of the southern Highlands a temperature 
 of 221 F. The quartz of granite also frequently abounds with fluid 
 cavities, so numerous as not to be more than y^TT ^ an ^ ncn a part, 
 so that there may be a thousand millions or more in a cubic inch ; 
 and they constitute 5 per cent, of the volume of the quartz. Similar 
 cavities also exist in the felspar and mica, though they are relatively 
 rare. Crystals of sulphates and chlorides occur in these cavities. 
 Gas cavities and stone cavities both occur. Dr. Sorby remarks, "The 
 proof of the operation of water is quite as strong as that of heat ; and, 
 in fact, I must admit that, in the case of coarse-grained highly 
 quartzose granites, there is so very little evidence of igneous fusion, 
 and such overwhelming proof of the action of water, that it is 
 impossible to draw a line between them and those veins where, 
 in all probability, mica, felspar, and quartz have been deposited from 
 solution in water, without there being any definite genuine igneous 
 fusion, like that in the case of furnace slags or erupted lavas." 
 "While, from the fact that schorl melts readily at a bright red heat, 
 
 1 Sorby, Brit. Association Report, 1880, p. 568. 
 
 2 Sorby, Quart. Jour. Geol. Soc., vol. xiv. 
 
ORIGIN OF VOLCANIC FISSURES. 175 
 
 and multitudes of hair-like crystals of schorl are enclosed in the 
 quartz of Cornwall, it is inferred that the granite did not become 
 finally solid at a temperature much higher than a dull red heat. 
 The temperature inferred for an elvan dyke from the fluid cavities 1 
 is 608 F., which indicates a pressure of 18,100 feet; but most of 
 the observations on Cornish elvans gave Dr. Sorby a pressure of 
 40,300 feet, while the quartz-porphyry dykes of the Highlands 
 indicate, on similar evidence, a pressure of 69,000 feet. The granite 
 of St. Austell in the same way indicates a temperature of 490 F., and 
 a pressure of 32,400 feet, while near Penzance the pressure cor- 
 responds to 63,600 feet; the mean pressure indicated by Cornish 
 granites is 50,000 feet. The mean pressure of the Aberdeen granite 
 is about 76,000 feet, while the centre of the main mass of the granite 
 of Aberdeen requires a pressure of 78,000 feet. Whence we learn 
 that the inferred temperatures, under which these rocks were pro- 
 duced, are scarcely higher than would be reached at corresponding 
 depths beneath the surface by the mere natural augmentation of the 
 earth's heat, so that if anything like 50,000 or 70,000 feet of rock 
 has been denuded to expose the granite, all difficulty as to the 
 temperature vanishes ; and the water, though greatly heated, was 
 in most cases caught up by the crystals in a fluid state, more or less 
 saturated with the alkalies which enter into the composition of the 
 minerals forming the rock. 
 
 How Upheaval Facilitates the Outburst of a Volcano. When a 
 great upward fold of the earth's crust is in process of formation or further 
 elevation beneath the sea, two things inevitably happen : first, the 
 external surface of the rocks is stretched, and therefore fissured; 
 and the greater the elevation, the deeper and more numerous these 
 fissures must become. Down such cracks water would inevitably 
 penetrate, and modify the condition of heated rocks with which it 
 came in contact. Its presence in this form was probably unnecessary, 
 as we may hereafter show, to the production of any crystalline rock ; 
 but such infiltrated water initiated changes which modified a rock, 
 that might have become crystalline, into a fluid and eruptive form. 
 And secondly, as the upheaved part of the earth's crust approached to- 
 wards the surface of the ocean, it became cut down by denudation 
 into a plain, so that an immense thickness of sediments was removed 
 from above the central axis, where heat was already developed by 
 a lateral compression ; and when relieved of this superincumbent 
 pressure, and the weight of the water by emerging from the sea, the 
 pressure above the heated mass is so far reduced, that the expansive 
 force of the steam and liquid rock overcomes resistance, and a volcanic 
 eruption is possible. No small number of volcanoes exists in table- 
 lands, or among mountains near to the sea. And if we find that 
 granite, for instance, has become liquid and is poured out on the surface 
 of the earth as a volcanic rock, and that the parent masses from which 
 
 1 For a discussion of nature of this evidence, see Sorby on the Microscop'cal 
 Structure of Crystals, &c., Q. J. G. S , vol. xiv. p. 453. 
 
1 76 GROUPING OF VOLCANOES ON FISSURES. 
 
 the streams flowed can be identified, another step is made in geological 
 demonstration of the nature of volcanic activity. 
 
 Relation of Felsitic Bosses to Granitic Mountain Chains. It is 
 now well known that when granite is liquefied, and cooled so rapidly 
 as to put on a micro-crystalline texture, it becomes a grey or reddish 
 rock, which, to the eye, may be homogeneous or diversified with 
 crystals of felspar. In this state it is known as felsite, felstone, 
 petrosilex, or eurite ; and when it assumes a foliated schistose form is 
 known as halleflinta. Professor Judd has described on the flanks of 
 the Grampians 1 vast sheets of felsitic lavas of enormous thickness, 
 mixed with ashes, pumice, scoriae, volcanic bombs, and other evidences 
 of ancient volcanoes, the parent materials of which can only be sought 
 in the granite bosses of the Grampian chain. Similarly, in most of 
 the larger islands of the Inner Hebrides, granite peaks occur, which 
 are obviously the solidified cores of ancient and vast volcanoes, from 
 which flowed the surrounding lava streams of felsite or rhyolite, and 
 similar rock-substances. So that, without appealing to other instances, 
 we believe these to sufficiently establish the conclusion that granites 
 produced by metamorphism may be erupted, and pour out the rock- 
 material in all the forms which characterise volcanic eruptions. 
 
 Linear Arrangement of Volcanoes. If we now examine a map of 
 the world, so as to observe the positions of existing volcanoes, they 
 will be found running for the most part in chains or lines, which are 
 regions of conspicuous upheaval, in some cases still undergoing per- 
 ceptible elevation. Thus the chain of the Andes and Central America 
 contains a multitude of well-known volcanoes, such as Corcobado, 
 Aconcagua, Villarica, Osorno in Chili, followed by Yiejo, Cotopaxi, 
 Coseguina, Popocatepetl, and many more, succeeded in the United 
 States by vast lava sheets in a region where volcanic activity is now 
 all but extinct. On the opposite side of the Pacific, lines of volcanoes 
 similarly extend through the Kurile islands southward by the Philip- 
 pines ; through Sumatra, Java, and adjacent regions ; and wherever 
 volcanoes exist they will be found to be in regions in which the force 
 of upheaval is manifest. Or, to take the case of a single volcano, we 
 have in Etna a vast mass where the base of the mountain consists of 
 lava streams and ashes, alternating with marine sediments ; proving 
 that the mountain, even if it did not originate in a submarine eruption, 
 has been greatly elevated during the progress of the eruptions, which 
 have resulted in its present bulk. And if there are no corresponding 
 evidences of changes of level of Vesuvius, the history of the Temple of 
 Jupiter Serapis 2 attests that certain changes have taken place in the 
 neighbourhood during the period of Vesuvian activity. And the 
 elevation of the coast of Chili, recorded by Mr. Darwin, was contem- 
 porary with volcanic activity in the adjacent mountains. So that we 
 have good ground for affirming that lateral pressure, similar to that 
 
 1 Judd, "Secondary Rocks of Scotland," Quart. Jour. Geol. Soc., vol. xxx. 
 p. 220. 
 
 * See Lyell, " Principles of Geology." 
 
ORIGIN OF VOLCANIC MINERALS. 177 
 
 which elevates mountain chains, is one of the conditions of the linear 
 form of volcanic activity. 
 
 Origin of the Eruptive Power of Volcanoes. We next have 
 to inquire into the source of the explosive and eruptive power of 
 volcanoes, for the phenomena do not permit a belief that, the 
 materials are merely extravasated through a crack by the uplifting 
 force of lateral pressure. It has already been seen that volcanoes 
 exist in positions where superincumbent pressure on the mass below 
 must have been relieved by the formation of cracks penetrating from 
 above, and the researches of Mr. Hopkins and Professor James 
 Thomson taught us that, if a mass is greatly heated and kept in the 
 solid state by pressure, it will become liquid as that pressure is 
 removed. But it will not become eruptive ; and the first sound 
 explanation of the eruption was due to Mr. Poulett Scrope, who 
 attributed it to the influence of water. This conclusion is almost 
 inevitable, considering what enormous masses of steam are discharged 
 from volcanic vents during eruptions, and how the vapour given off 
 from Stromboli forms a constant cloud or mist above the island, 
 while it may even give rise in Polar regions to showers of snow. 1 
 It is well known that in a closed vessel water may be made white 
 hot without being converted into vapour; and if we suppose the 
 water from the sea to penetrate down such fissures in the neighbour- 
 hood of volcanoes as have been suggested, then, heated beneath the 
 surface by contact with rocks at a high temperature, it would escape 
 by the path where the pressure was least, flashing into steam with 
 explosive energy as the pressure disappeared. 
 
 Influence of Water on kind of Rock Ejected. We have already 
 seen how thoroughly some granitic rocks have their crystals saturated 
 with water, which was included and caught Up at a high temperature ; 
 and when this is borne in mind, it will be understood that heated rock 
 has the power of dissolving vast quantities of water which produce 
 many changes in its substance. The researches of Daubre"e show that 
 when common glass is raised to a temperature of 400 C., in the presence 
 of its own volume of water, it swells up and changes into a mass of 
 crystals of wollastonite ; while the alkali is separated, and the excess 
 of silica crystallises in the form of quartz. When the glass thus acted 
 on contains oxide of iron, the wollastonite is replaced by diopside. 
 Similarly, it was found that the volcanic glass obsidian, when thus 
 treated, produced crystals of felspar, and was changed into a rock 
 like trachyte. 2 And when kaolin was heated with a soluble alkaline 
 silicate and cooled, the mass was converted into crystalline felspar, 
 and quartz. It is thus seen that water plays a very important part 
 in the formation of volcanic minerals, as well as in the actual 
 eruption. 
 
 Mr. Scrope inferred that the presence of water would render the 
 particles of mineral matter easily movable upon each other, and that 
 the rise of lava in a volcanic vent is occasioned by the expansion of 
 
 1 Scrope's "Volcanoes," 2d edition, 1862, p. 38. 
 
 2 Sterry Hunt, " Chemical and Geological Essays," p. 6. 
 VOL. I. M 
 
178 FAILURE AND RENEWAL OF VOLCANIC ENERGY. 
 
 the highly compressed steam which it contains ; comparing the 
 ebullition to the expansion which takes place when the cork of a 
 soda-water bottle is drawn. 
 
 Depths at which Volcanoes Originate. It is almost impossible 
 to estimate the depth at which volcanic phenomena originate. Mr. 
 Mallet's investigations assigned to earthquakes a depth of from three 
 to ten miles, and although they occasionally reach a depth of 30 miles, 
 according to the late Dr. Oldham, they are obviously phenomena of 
 the superficial portion of the earth's crust. Although these disturb- 
 ances are probably different in kind from volcanic phenomena, yet 
 they must often be consequences of the disruption in which a 
 volcano originates ; and so far may give some idea of the depth to 
 which water must penetrate before it reappears in a volcanic outburst. 
 There is another form of evidence adduced by Mr. Sorby, in the size of 
 the fluid cavities relatively to the fluid in minerals ejected in blocks 
 of volcanic rock from Vesuvius. Some of these in the mineral 
 nepheline indicate a temperature of 706 F., equal to a pressure of 
 3222 feet. 
 
 Why Eruptions are Intermittent. It is a well-known fact that, 
 in the majority of volcanic regions, eruptions are intermittent. This 
 circumstance is well exemplified in the history of Vesuvius ; for 
 although presenting the form of a volcano, there was no record of an 
 eruption prior to the year 79 ; and it was not till the year 203 that a 
 second eruption is described, while the third took place in the year 
 472 ; and frequently intervals of several hundred years have occurred 
 between the eruptions, though for the last two hundred years the 
 intervals of tranquillity have rarely lasted longer than five years. 
 Almost the only exception to this paroxysmal condition among vol- 
 canoes is furnished by Stromboli, which is unceasingly active, and 
 more like a solfatara than a volcano. The reason for this intermission 
 in the outbursts is not far to seek according to the principles which 
 we have so far developed ; for with the progress of an eruption the 
 amount of water which is given off in the form of steam diminishes ; 
 and if the water slowly infiltrates down to great depths, the supply 
 must be more readily exhausted through the wide vent of an eruption 
 than renewed through minute fissures ; and therefore, as the explosive 
 force fails, the eruption fails But we further conceive that the 
 evacuation from beneath the surface of the vast masses of matter 
 which are poured out in volcanic outbursts, takes place more rapidly 
 than the contraction of the rocks can progress, in consequence of the 
 radiation of heat ; therefore, there comes to be a failure of the lateral 
 pressure, which generates the heat that is the primary cause of an 
 eruption. And until a sufficient interval of time has elapsed for the 
 renewal of these elements of volcanic energy, the eruption must be 
 intermitted. But the intermission may to some extent be due to the 
 strength of the cone, and the security with which its throat and 
 various apertures have been plugged by rocky materials ; but, if the 
 force necessary to burst a way to the surface is small, then feeble 
 eruptions may take place frequently, or even in continuity. 
 
RELATION OF VOLCANOES TO STRATIFED ROCKS. 179 
 
 Extinct Volcanoes. Many examples exist of volcanoes which 
 have so long ceased to be active that they are regarded as extinct, 
 and we must attribute the failure of their eruptive power in most 
 cases either to changed conditions in the relations of land and water, 
 which have deprived the countries where they occur of the requisite 
 water supply for the generation of steam, or to the exhaustion of 
 rock-material beneath the surface, which was capable of producing 
 ashes and lava, or else, as is probably frequently the case, to changes 
 in the direction of the grand contractions beneath the surface, by which 
 the subterranean energy became transferred from a region where it was 
 formerly manifested, to a new locality changes which have taken place 
 in all past periods of geological time, with the result of altering the 
 distribution of sediments, and of life, as well as of volcanoes. 
 
 Relation of Volcanoes to Stratified Kocks. Finally, the mate- 
 rials which volcanoes bring to the surface remain to be examined It 
 is well known that though these present in the rock-substance every 
 variety in texture, yet at different times, and in different regions, 
 volcanoes pour out ashes and lava which differ materially in their 
 mineral and chemical composition. As we have already observed, 
 rocks which are poor in silica, like basalt and the leucite basalt of 
 Vesuvius, form a group which has been termed basic, while the 
 trachytes and rhyolites, which are rich in silica, form another series 
 of volcanic rocks. Why these two groups, which were recognised by 
 most of the early masters in geology, should exist and alternate, is a 
 problem that can only be estimated by recognising that, the alterna- 
 tion has existed for all geological time. But we might fail altogether 
 to elucidate this problem unless we observe that, some of the ancient 
 volcanic cones in the Eifel, as remarked by Professor Judd, are largely 
 made up of fragments of slate, which have been ejected from the 
 vents by explosive forces. And it is well known that the surface 
 of Vesuvius is covered with fragments of limestone, ejected from the 
 throat of the volcano ; and many of these blocks are so little altered 
 that Professor Guiscardi has been able to recognise several hundred 
 species of shells in these masses. We thus discover that, deep beneath 
 many volcanoes, stratified rocks exist; and as we are compelled to 
 believe that the plutonic rocks were metamorphosed out of such strati- 
 fied materials, so we find no anomaly in the basis of a volcano being 
 formed by the liquefication of similar strata, nor indeed is there any 
 other probable explanation available for the diversity of lavas ejected. 
 All the phenomena which are connected with the existence of vol- 
 canoes are hence related to each other in a sequence, which owes its 
 existence primarily to the cooling of the earth's crust. 
 
CHAPTEE XIII. 
 
 THE MANIFESTATIONS OF VOLCANIC ACTION. 
 
 Historic Records. The circumstances connected with volcanic erup- 
 tions are either mutually dependent, or naturally connected with each 
 other. For, whether exhibited as outbursts of ashes, lava-flows, mud 
 volcanoes, sulphur springs, geysers or hot springs, they are attributable 
 to the action of heat upon water, beneath the surface of the earth ; 
 and the phenomena only differ with the different conditions under 
 which the heat is developed, and the water gains access to the sub- 
 terranean regions. We obtain the clearest conception of the modes of 
 action of these agents, in their geological relations, by examining the 
 history of the origin of new volcanoes. Some of these are marine, 
 others are on land ; and although these new outbursts appear to differ 
 fundamentally from older volcanoes, in having been in eruption but 
 once, it may well be that there is nothing exceptional in such a 
 circumstance, since the most celebrated volcanoes have had long inter- 
 vals of repose. 
 
 Graham Island. On the shallow sea-bed of the Mediterranean, 
 between Tunis and Sicily, at about sixty miles from Sciacca, and 
 thirty miles from the island of Pantellaria, an eruption took place in 
 1831 which built up a submarine cone. This volcano, when largest, 
 rose above the sea to a height of 200 feet, and attained a circumference 
 of three miles. Formed, however, of loose materials, it rapidly 
 wasted away under marine attrition, and its position is now only 
 marked by a shoal. Such an eruption is peculiarly instructive, as 
 demonstrating how unsubstantial is the fabric of which a volcanic cone 
 consists, and as indicating the nature of the evidence for the former 
 existence of a submarine volcano, which the geologist might expect 
 to find when the consolidated core left at its base by denudation was 
 enveloped by sediments. 
 
 Jorullo. A scarcely less instructive history records the eruption 
 which formed Jorullo in Mexico in 1759. Prior to that date, the 
 farm of Jorullo was laid out in sugar-cane and indigo. It was situate 
 more than 100 miles from the coast, about 2500 feet above the sea, 
 and far away from any active volcano. This farm was bordered by 
 two streams, named Cuitimba and San Pedro. The first indication of 
 disturbance consisted in hollow subterranean sounds, accompanied by 
 constant earthquakes, lasting for about two months ; when, after a 
 

 DIFFERENT TYPES OF VOLCANIC ERUPTIONS. 181 
 
 short interval, on the 2Qth of September, the ground opened, and thick 
 clouds of ashes and rock fragments burst into the air, with the 
 appearance of flames along the fissures. So sudden was the outburst, 
 that it became known to the farm-servants by ashes falling on their 
 hats. The two rivers were swallowed up in the open chasms, and in a 
 short time the eruption built up six volcanic cones which extend in a 
 line. One of these, called Jorullo, rises to a height of 1600 feet above 
 the plain, which was formed around the volcano from ashes and 
 basalt, poured out to a depth of 500 feet, in a deposit which thins 
 away so as to give a convex appearance over an area of about four 
 square miles. 
 
 When Baron Humboldt, twenty years later, crossed this district, 
 termed the Malpais, the heat in a fissure in lava was still sufficient to 
 light his cigar ; and he records that the rivers, which had disappeared 
 on the day of the eruption, flowed out again a mile and a quarter 
 farther west as hot springs, which then had a temperature of 126 F. 
 
 The whole surrounding plain was covered with thousands of small 
 cones, 6 to 9 feet high, which gave off vapour rising to a height of 20 
 to 30 feet; and the ground gave out the peculiar resonant sound 
 when struck which, is known to the Italians as the rimbombo. Since its 
 formation Jorullo has shown no signs of activity, and in this respect 
 is comparable to the Monte Nuovo, produced in the Phlegraean Fields 
 in September 1538. 
 
 Structure of a Volcano. Although it is not possible to examine 
 far into the structure of active volcanoes, yet it is not difficult to 
 conceive of the steps by which a cone is built up. Although dust 
 may sometimes be ejected, according to Mr. Whymper, to a height 
 of 20,000 feet, and carried, as we have already seen, to great distances, 
 it as a rule is simply shot into the air, and falls down again, so that 
 most of the particles descend round the spot from which they were 
 thrown up; and the thickness of the deposit becomes less and less 
 farther out from the eruptive throat. But although the materials of the 
 flanks of the cone may thus come to be arranged in layers, something 
 like coarse gravel, which are inclined outward, some of the material 
 falls within the sloping throat, so that the layers dip towards its 
 centre; and wherever a volcano has been naturally dissected by 
 denuding agencies, as in the Auvergne and Eifel, this condition is 
 well seen. The whole mass then shows an irregular stratified appear- 
 ' ance, crossed and bound together transversely by the walls of lava, 
 termed dykes, which have been injected into fissures, opening 
 downward. 
 
 Sequence of Events in an Eruption. The succession of events 
 which constitutes a volcanic eruption is variable in different volcanoes, 
 and at different times in the same volcano. Professor Dana tells us 
 that in 1789 Kilauea, in the Sandwich Islands, poured out an 
 enormous quantity of light pumice-like scoriae and sand which 
 darkened the air. Whereas the well-known outbursts of that volcano 
 have consisted of fluid lavas, which have sometimes run down to 
 the sea. Similarly, in the grand eruption of Vesuvius in the year 79, 
 
182 ERUPTION OF ASHES FROM COSEGUINA. 
 
 no lava appears to have been produced, but enormous quantities of 
 dust and ashes, which sometimes ran down the mountain in torrents 
 of mud ; while at other times Vesuvius has ejected ashes and lava. 
 
 The differences in these conditions are attributable to the supply of 
 water. Professor Dana has inferred from the fact that borings on a 
 sea-shore will always yield fresh water, that the water is more likely 
 to be fresh than salt ; and that the rains and melting snows, absorbed 
 by the cavernous rock of which a volcano consists, may contribute 
 materially to supplying the explosive force to the fires below. But, 
 on the other hand, many of the springs in Italy contain a perceptible 
 amount of salt; and since the volcanic fires rise from a depth far 
 below the sea level, the pressure of the sea must exert itself in forc- 
 ing water into the rocks. It may be doubtful, perhaps, how far the 
 substances dissolved in sea-water contribute to volcanic phenomena, 
 but we may remember that though the salts consist chiefly of chloride 
 of sodium and magnesia, lime and potash, chiefly in the form of 
 chlorides and sulphateSj Dr. Forschammer detected minute quantities 
 of a large number of elements^ such as from time to time are met 
 with in volcanic and other igneous rocks; 
 
 Steam. If we suppose water ready to take the form of steam, to 
 accumulate beneath the surface until the pressure becomes so great 
 as to burst a way through the rocks, as they become flexured, frac- 
 tured, and heated, and then to expand, it becomes intelligible that the 
 steam will be discharged with such force as to rise to a great height 
 in the air, before it is chilled so as to condense in clouds. The force 
 of the steam accounts for the abrasion and trituration of rock-fragments 
 from the side of the eruptive throat, which, as we have already seen, 
 are sometimes brought to the surface. Eut after a time, especially in 
 the volcanoes of the south of Italy, dust is thrown up with the steam. 
 This dust is not merely grated down from the rocks at the sides of 
 the fissure, but is at first of such indescribable fineness as to remain 
 long suspended in the air. The heated rock beneath the surface is in 
 fact so charged with water that, when it comes towards the surface, 
 the water expands into steam, blowing out the films like soap bubbles, 
 so that they cool and contract ; and becoming broken, fall into the 
 finest powder. 
 
 Eruptions of Ashes. An excellent example of a dust eruption took 
 place in the volcano of Coseguina in 1835. This cone is situate on 
 the Bay of Fonseca, in Nicaragua, and is about 500 feet above the sea. 
 Slight noises were heard, and smoke was seen on the i gth of January, 
 and on the 2oth a cloud was thrown up, which, seen from San Antonio, 
 48 miles south, looked like an immense plume of feathers ; at first 
 white, then grey, yellow, and finally crimson, and expanding rapidly 
 in every direction ; columns of fire shot up, and there were severe 
 earthquakes. On the 22d the sun shone brightly, but in the direction 
 of the cloud there was intense darkness. A fine white ash began to 
 fall, and in half an hour the day, at San Antonio, became darker 
 than the darkest night, so that people could touch without seeing 
 each other ; the cattle came in from the surrounding country, and the 
 
ASH DEPOSITS ON THE OCEAN FLOOR. 183 
 
 fowls went to roost. At twelve o'clock on the following day, objects 
 could be distinguished at a distance of twelve yards, but the light 
 was thus obscured for two days longer. All the time a fine 
 impalpable white dust fell, which covered the ground at San Antonio 
 to a depth of 2 J inches, in three layers ; the lowest dark, the next 
 greyish, and the upper whitish. The light continued partly obscured 
 for twelve days more the darkness, of course, being due to the 
 quantity of ash in the air. At Nacaome, 24 miles north, the ashes 
 which fell four or five inches deep, had a fetid, sulphurous smell. 
 On the 23d, the sky was light enough to show that a fresh eruption 
 had taken place ; and in three hours darkness returned as on the 
 2oth. When the atmosphere became clearer next day, the houses 
 were covered with ashes to a depth of eight inches. Twenty-four 
 miles south of the crater, ashes covered the ground to a depth of more 
 than 10 feet, destroying pine woods. The thickness of the ash 
 deposit varied somewhat with the wind, but so fine was the dust 
 that it was carried as far as Chiapa, 1200 miles to the north, while 
 at St. Ann's, in Jamaica, 1700 miles N.E., the sun was obscured on 
 the 24th and 25th of January, and showers of fine ashes fell over 
 the whole island, so that they must have travelled at the rate of 
 170 miles a day. 
 
 The surface of the Pacific, uoo miles S.E. of the volcano, was 
 covered with ashes, and a ship ran through 40 miles of floating 
 pumice. This eruption of Coseguina is especially interesting, because 
 Aconcagua and Corcovado were active at the same time. 
 
 The eruption of Vesuvius in 472 is said to have covered all 
 Europe with ashes which even fell in Constantinople ; and on various 
 occasions the ashes from the volcanoes of Iceland have covered the 
 North Sea, and fallen in Scandinavia. Perhaps the most remarkable 
 evidence, however, of the extreme fineness of the dust is furnished by 
 those great deposits of red clay which characterise the central regions 
 of the Atlantic and Pacific Oceans. 
 
 Red Clay in the Deep Sea. Mr. Murray reports that the deep- 
 sea clays and deposits at a greater depth than 2000 fathoms appear 
 to be always due to the decomposition of ashes and volcanic materials. 
 The red clays owe their colour to oxide of iron ; the chocolate-coloured 
 clays are tinged with oxide of manganese, a mineral that abounds in sea- 
 bed regions covered with augitic materials. Most of these clays con- 
 tain little carbonate of lime, and those of the North- West Pacific 
 abound in siliceous organisms. Amorphous matter rarely makes up 
 one-half of the clay, and the remainder consists of quartz, mica, pumice, 
 peroxide of manganese, and other minerals. Peroxide of manganese 
 is always present, and sometimes makes up one-half the deposit, but 
 quartz and mica are only characteristic of the North Atlantic. Occasion- 
 ally copper, cobalt, and nickel are found in the clays. Pumice and 
 scorias are universally distributed, some of the bottoms at 2900 fathoms 
 being largely made up of finely-divided pumice. Pumice was dredged 
 by the Challenger in at least 80 stations in masses from the size of a 
 pea to that of a cannon-ball, and is most abundant in the neighbour- 
 
1 84 VOLCANIC MUD. 
 
 hood of volcanoes, and in the deep-sea clays far from land. It is 
 more frequently found in the Pacific than in the Atlantic. The pumice 
 is sometimes coated with peroxide of manganese, and may be white, 
 grey, green, or black, as it is felspathic or augitic. It contains crystals 
 of sanidine, augite, hornblende, olivine, quartz, leucite, magnetite, and 
 titaniferous iron. Magnetic iron is found in all the masses. Although 
 a good deal of the pumice may be derived from volcanoes which girdle 
 the Pacific, yet no inconsiderable quantity is derived from land, being 
 washed down from the mountains by the rain, and floated to sea by 
 the rivers. Thus, in Iceland, a ferry is said to have been blocked 
 for days by floating pumice. Quantities of pumice float on the 
 Amazon, brought from the region of its head waters. The river 
 Chile, in Peru, has cut gorges 500 feet deep through pumice, and 
 carries the fragments to sea. 
 
 Mud Streams. Mud streams frequently descend from those vol- 
 canoes which throw out fine ashes. This may be due to different 
 causes. The vast quantity of steam thrown out becomes condensed 
 into rain, and this falling on the mountain, washes down the ashes in 
 torrents of hot mud. Such streams are well known to have descended 
 from Vesuvius in the great eruption of 79, and to have overwhelmed 
 Pompeii and Herculaneum. This substance, however, has now be- 
 come hardened into a compact tuff by the development of zeolites and 
 other minerals in its substance, in precisely the same manner as 
 Bunsen found that basalt, ground to a powder and left in water, con- 
 solidated, when the water evaporated, into a mass so hard as only to 
 bo broken with the hammer. Humboldt has recorded how the 
 volcanoes of Ecuador have discharged torrents of mud so as to fill up 
 valleys ; and it is well known that the cone of Cotopaxi has repeatedly 
 melted the great glaciers upon it, which descend to about 14,000 feet, 
 and the water thus liberated has carried down the ash. The eruption 
 of Imbambaru in 1691 poured out not only mud, but a considerable 
 quantity of fishes, which would indicate that the crater of the volcano 
 had become a lake, in which a species of Pimelodus had lived. In the 
 geological formations, as will be subsequently seen, examples of forests 
 buried in ashes, and vegetation overwhelmed by mud streams, are found 
 among the primary rocks of Arran, and the tertiary rocks of Mull. 
 
 Bombs. As the amount of steam becomes reduced, and the rock 
 is less permeated by it, the explosive force in the volcanic throat is 
 diminished ; and the rock-material, though still blown out into cellular 
 structure by the expansion of the steam, is ejected in much larger 
 fragments, which are termed lapilli and scorice. At length the supply 
 of explosive steam near to the surface becomes exhausted, and the 
 fluid rock ceases to be shattered by its expansion. But from time to 
 time fresh supplies of condensed vapour rise through the molten rock 
 as it ascends, and catch up masses of lava, which are often rotated as 
 they rise in the air, and become fashioned into the often rounded and 
 sometimes hollow masses termed " volcanic bombs." These, however, 
 are rarely thrown far, and usually occur near to the cone. But 
 grander paroxysmal outbursts of steam have lifted large masses of 
 
LAVA-FLOWS OF VESUVIUS AND ETNA. 185 
 
 rock to a height of 25,000 feet, and some, thrown up from Heckla, 
 have been seen by ships 180 miles out at sea. Humboldt speaks of 
 masses weighing 200 tons ejected from Cotopaxi, while the plain 
 around the volcano is strewn for many miles with great masses of 
 lava which have been thus accumulated. 
 
 Lava Streams. All this time the lava has been slowly rising in 
 the volcanic throat ; for in place of the pressure of superincumbent 
 rock which formerly confined it, there is now no pressure above but 
 the earth's atmosphere. The reduced pressure must operate on the 
 liquid rock much in the manner of a pump ; while below, there is 
 the lifting power of the steam, which Bischoff states to be alone 
 sufficient to expel the lava ; and above all there is lateral contraction 
 thrusting the rocks together so as to force the fluid to the surface. 
 We need not now inquire into the height to which lava streams are 
 stated to have been thrown ; it is enough to recognise the fact that 
 the molten rock rises in increasing volume. If the volcano is low, 
 lava fills the crater till the stream overflows its rim, or bursts down 
 its weakest side, and escapes like a torrent. If the volcano is high, 
 fissures may appear on the flanks, and give exit to the molten rock, 
 which then usually disappears from the crater. 
 
 The character of the lava stream depends upon the fluidity of the 
 lava, the amount thrust out, and many circumstances of the eruption. 
 \Vhen the streams are short they are frequently permeated with 
 vapour, which expands the rock into a cindery mass ; and as these 
 vesicles burst on its surface, the rock acquires a rough or reticulated 
 and stringy aspect. Such vapour cavities are always elongated in the 
 direction in which the lava flows, and by their expansion help to 
 arrest its movement. They usually disappear when the stream is 
 large ; and then the current, instead of dividing and subdividing as it 
 descends, is apt to move irresistibly over all obstacles and spread 
 itself in a wide sheet. In the last three centuries many lava streams 
 from Vesuvius have* reached the sea, frequently descending upon 
 the town of Torre del Greco. In 1631 twelve or thirteen branches 
 reached the coast in broad masses, and still cap the cliffs. Some of 
 these streams were five miles long, and their extreme distance apart, 
 on the coast, was seven and a half miles. Among other well-known 
 streams in this district was the lava flow of 1794, one branch of 
 which passed through Torre del Greco in a sheet 1130 feet broud 
 and 15 feet thick, and extended 360 feet into the sea. The cele- 
 brated lava flow from Etna in 1669, issuing on the flank of the 
 mountain, ran for fifteen miles, passing over part of Catania, and 
 entered the sea in a mass 1800 feet broad and 40 feet thick. In 
 Hawaii, some lava-flows are 25 miles long; in Iceland, streams from 
 Skaptar Jokul have run for 60 miles ; in Greenland, they are longer 
 still, though even those are far surpassed by the great lava sheets 
 of the west of i^orth America. 
 
 Temperature. The temperature of the lava is very variable. In 
 the Vesuvian lavas the heat is above the melting point of silver, but 
 does not always melt copper. A stream from Etna in 1766 in a 
 
1 86 THE COOLED SURFACE OF LAVA. 
 
 quarter of an hour melted down a hill of volcanic matter 50 feet high 
 and carried it away. On the other hand, a stream from Kilauea in 
 1839, catching the branches of trees, scarcely scorched the bark, and 
 the lava hung from them in masses like stalactites ; while islets of 
 forest trees were swept along on the surface, without killing the 
 bamboos, and only partially injuring the foliage. 
 
 Kate of Flow of Lava. The rate at which the lava flows is neces- 
 sarily variable, but it may be remarked that the stream from Etna in 
 1669 ran 13 miles in the first 20 days, while it occupied 23 days 
 in covering the last two miles. The fluid lavas of the Sandwich 
 Islands have a far greater velocity, Dana stating the average progress 
 of the stream of 1839 at 400 feet an hour. 
 
 Aspect of Lava Fields. The island Hawaii exhibits in a striking 
 manner some of the varieties which fields of lava assume on cooling. 
 Large tracts consist of smooth solid lavas, with undulating ridges 
 and hillocks some 10 to 20 or 60 feet high, and with the surface 
 marked with folds and lines, such as may be seen on glassy slags. 
 When the elevations are broken through, they are seen to be hollow, 
 and due to the ascent of volumes of vapour, which have formed 
 subterranean caverns. Other regions are rough, and termed " clinker 
 fields." They are covered with angular blocks and rough slabs 
 of every possible size, lying in the utmost confusion. These fields 
 stretch for miles, and are characterised by a grey and black desolate 
 condition. The transition from one surface of lava to the other 
 is frequently abrupt. The clinker fields are supposed by Professor 
 Dana to result from a lava stream floating on its surface, the materials 
 of a stream which had cooled, in much the same way as ice is carried 
 by a river in spring ; and perhaps the fact that the clinker fields rise 
 20 or 30 feet above the smooth lavas favours this view. The fissures 
 in lava streams are often seen to have been filled after the crust 
 of the lava had cooled, so that miniature dykes are formed, and 
 raised a little above the surface. This aspect characterises the whole 
 of the southern and south-eastern part of the island, and is termed 
 by Dana "the wearying grandeur of desolation." The European 
 lavas are rarely smooth. They more frequently have an irregular 
 billowy appearance, because the hardened external film is long 
 carried on with the stream, and rolled over and over till the con- 
 solidated surface becomes extremely rugged. Professor Phillips 
 observed that the hardened crust of the Vesuvian lavas sometimes 
 forms tunnels, from which the lava may run out so as to leave an 
 arched roof, which, however, soon falls in as the lava cools. 
 
 It seems probable that one cause of the fluidity of greatly heated 
 lava must be its freedom from dissolved water, since steam, in [ex- 
 panding, can only tend to arrest its movement. 
 
 Surface of a Volcano. The form of the mountain thus built up 
 depends a good deal upon the nature of the ejected materials. Those 
 cones which are formed chiefly of ashes are often regular and conical ; 
 while those formed chiefly of lavas are frequently so rugged as not 
 to suggest at first sight a volcanic origin. The regularity, too, may 
 
LARGE CRATERS AND CRATER-LAKES. 187 
 
 be a good deal influenced by the wind, which often blows the ash so 
 that it accumulates more on one side of the mountain than the other. 
 The steeper inclination is always towards the summit, where it may 
 amount to 20 or 35, the slope gradually diminishing down the 
 flanks, till the level becomes horizontal. Volcanoes are often rendered 
 irregular by the truncation of the cone, and by the development of 
 parasitic cones upon their flanks. According to Sartorius Yon 
 Waltershausen, there are upwards of 700 minor cones on the flanks 
 of Etna ; and Dana mentions several thousand in the island of 
 Hawaii. These minor cones (according to Mr. Scrope) vary in size 
 from that of a hay-stack to 1000 feet in height, and two or three miles 
 in circumference. 
 
 Cones and Craters. All the time that the eruption is in progress, 
 the volcano undergoes changes of form, partly from the accumulation 
 of ejected materials on its flanks, partly from the building up of new 
 lateral cones upon it. But more important changes are developed at 
 the top of the mountain ; for> as the super-heated water rises towards 
 the surface, and flashes into steam in the throat, its explosive force 
 blows out the loose materials of which the cone was composed ; and 
 thus the mountain becomes truncated, and its conical upward termi- 
 nation is often replaced by a funnel-shaped pit, which does not always 
 become entirely obliterated by subsequent eruption. Thus, Monte 
 Somma, on the flank of Vesuvius, is a remnant of the ancient crater, 
 whose size marks the violence of its earlier eruptions. High up on 
 Etna there is a somewhat flattened platform, which probably marks 
 the limits of an ancient truncation of the mountain by the formation 
 of a crater which has since been filled up ; and upon which the upper 
 cone now rises. A still grander example of truncation is furnished 
 by Mauna Loa, which has a horizontal width of 20 miles at 18 feet 
 below its summit. These great craters sometimes remain permanently 
 as pits, and, according to Professor Milne, there are about 20,000 
 people dwelling in the crater of Asosan, in the island of Kiushiu in 
 Japan. This crater is 15 miles in diameter. Many extinct vol- 
 canoes seem to have exhausted their energy in a final effort of this 
 kind, which has blown much of the volcano into the air. Per- 
 haps the most remarkable examples of large craters are seen in the 
 crater lakes to the north of Rome. The lake Bracciano is nearly 
 circular, 6J miles in diameter, and its surface rises 540 feet above 
 the sea. The crater of Monte Albano is 6 miles in internal diameter, 
 almost entirely composed of volcanic dust, and has a central moun- 
 tain in its midst. The lake of Bolsena is over 10 miles long by 9 
 miles broad, approximating to the oval outline common among 
 craters. In its midst rise two islands, which are composed of vol- 
 canic tuffs, and show the characteristic quaquaversal dips which are 
 seen in cinder cones ; so that, large as these excavations are, and 
 vanished as are the ancient volcanoes, we observe no more than 
 extreme stages of truncation of the mountains by explosive forces, 
 accompanied by faint indications of nature's efforts to restore the 
 structures destroyed. 
 
188 CHANGING POSITION OF VOLCANIC VENTS. 
 
 It is an almost universal experience, after the crater of a volcano 
 has been thus formed, and the lava raised and poured out, that con- 
 siderable quantities of heated water rise through the molten mass in 
 the volcanic throat, and burst in great bubbles so as once more to 
 throw up cinders, which fall around the throat and begin to build up 
 a new cone within the crater. The views of Vesuvius published by 
 Sir William Hamilton are particularly instructive in this connection. 
 In 1756 this mountain possessed no fewer than three cones and 
 craters, rising successively one within another, the outermost being 
 girdled by Monte Somma. Then the innermost cone became obliter- 
 ated, and finally, in 1767, but one cone existed within the crater, 
 which in due time became filled up, so as to form a convex platform ; 
 until a new eruption, in 1822, blew out the entire centre of the 
 mountain, and then once more small craters began to appear as the 
 crater, became filled up again. More than once, two or more minor 
 cones have existed side by side within the crater of Vesuvius, as 
 though a fissure had formed across its floor, and permitted the 
 escape of the explosive forces along a line. 
 
 Volcanoes without Cones. It is not always, however, that cinder 
 cones are produced. In the summit crater of Mauna Loa, which is 
 termed Mokua-Weo-Weo, there were at the bottom of the pit that 
 serves as crater two cinder cones, one of them rising 200 feet in 
 height when examined by Professor Dana. Eut the Sandwich Island 
 volcanoes are remarkable for the absence of cones, in place of which 
 there are deep pits on the side of the mountain, and the so-called 
 cones are patches of lava which have been ejected on the flanks. 
 Kilauea, which is sixteen miles from the summit of the mountain, 
 has no cone, and the crater is a pit 7^ miles in circumference. 
 
 Cones arranged in Lines. Any one who carefully examines a 
 volcano, or still better, a volcanic district, will observe that the posi- 
 tion of the eruptive vent undergoes some change. Thus, in the island 
 of Vulcano, there is the ancient crater which forms the S.E. of the 
 island, with three other craters successively superimposed, before we 
 reach the small mass on the north termed Vulcanello, which also has 
 a succession of craters in a line. But it is more instructive to observe 
 the manner in which cones succeed each other on a grand scale in 
 such a chain as the Andes, or the Kurile Islands ; for we may reason- 
 ably infer that the cause, whatever it was, whicli determined the 
 shifting of the point of eruption along such a line as that of Vulcano, 
 is closely analogous to the cause which determines the intermittent 
 outbursts of eruptions along successive portions of a volcanic chain. 
 
 We have only to observe what has happened in the history of Etna 
 to learn the secret of the linear extension of volcanic vents. After 
 the central cone has become sufficiently massive and consolidated to 
 oppose a resistance which the explosive forces below cannot easily 
 overcome, they occasionally find an outlet by producing rents on the 
 mountain-side. Rarely these rents radiate, but more frequently they 
 are limited to one side of the mountain, and sometimes occur in 
 parallel bands. Among numerous illustrations given by Ferrara, we 
 
>-^" r\ O TiT-Tr* *^O 
 
 VOLCANIC FISSURES. 
 
 may remark that in 1536 a fissure of this kind was _ 
 
 which 12 cones were erupted one below another. In 1669 tne moun- 
 tain was split from the summit down two-thirds of its extent, and 
 from this fissure flowed the lava stream which destroyed Catania, 
 while upon it was built up the great double cone known as Monte 
 Rossi. On other occasions the lava has boiled out from fissures and 
 formed streams, all in the same line, at a number of points succes- 
 sively below each other. Some of these fissures have been traced, and 
 seen to be filled to a certain height with incandescent lava. One of 
 the latest examples was formed in 1874, and described by Professor 
 Orazio Silvestri, 1 who states that after the central crater had poured 
 out formidable columns of black smoke, sand, and scoriaa, a fissure 
 appeared on the north side of the central crater, which extended for 
 five kilometres, running east by north, and at a height of 2450 metres 
 was 50 to 60 metres wide where widest. Here a new elliptical crater 
 formed, and at its base ten small eruptive throats succeed each other 
 in a distance of 50 to 60 metres ; then others follow, so that there 
 are 22 minor cones in linear extension in a distance of half a kilo- 
 metre, and lower down there are 13 more cones, many of them only 
 a few yards in diameter. The superficial temperature of the lava 
 streams erupted was 70 cent. ; at a depth of half a metre it was 90 
 cent. This new mountain and its system of 35 subordinate cones and 
 lava streams, were all thrown up in a few hours. 
 
 Fissure Eruptions. These fissure eruptions vary greatly in import- 
 ance. We have already seen that fissures on Etna may pour out 
 lavas without passing them through a volcanic cone, but it may be 
 useful to remember that this is the usual condition in Hawaii, and 
 that the eruption is then on a grander scale. A stream issuing from 
 a fissure in 1839, at a height of 1244 feet, flowed for twelve miles and 
 ran down to the sea. For three weeks this fiery river continued to 
 pour on, converting night into day. The reflected glare of the lava 
 was visible 100 miles at sea, and fine print could be read at midnight 
 at a distance of forty miles. When this lava entered the ocean, it 
 was shivered into millions of particles, which were thrown up in 
 clouds that darkened the sky, and fell like a storm of hail on the 
 surrounding country. With such materials, it will be readily under- 
 stood that lava-flows occur which are independent of ordinary volcanic 
 activity. Indeed, Yon Eichthofen regards massive fissure eruptions as 
 the more important and fundamental phenomena, conceiving that just 
 as a minor cone may be parasitic upon a main cone, so the ordinary 
 volcano is parasitic upon the subterranean part of a massive eruption. 2 
 But as a rule the massive eruptions form mountain-ranges and show 
 no signs of craters, though volcanoes occur on their lower slopes, or 
 form a parallel series of outbursts after the rock in the main fissure 
 has solidified. In such cases the rock-material is the same in the 
 massive outburst and in the volcano. 
 
 1 "Notizie sulla eruzione dell' Etna, del 29 Agosto, 1874. Catania, 1874." 
 
 2 "Natural System of Volcanic Kocks," Mem. California Academy of Sciences, 
 vol. i. part 2, p. 66. 
 
190 INFLUENCE OF THE SEA ON ERUPTIONS. 
 
 The kind of rock emitted varies with the district. In the Andes 
 the bulk of the volcanic rocks consists of a dark or blackish rock 
 termed Andesite, which is also seen on the southern slopes of the 
 Carpathians. It forms the Hargitta range, the Vihorlat-Gietin range, 
 and the Eperjes-Kaschau range. It is emitted from Chimborazo, 
 Cotopaxi, Antisana, Tungurahua, Popocatepetl, Colima, Teneriffe, 
 which are said never to have changed the mineral character of their 
 lavas. 
 
 Other fissure eruptions on the Carpathians in Hungary and Tran- 
 sylvania consist of the dark volcanic rock termed Propylite. 
 
 Trachyte is still being ejected by most of the volcanoes of Central 
 America. It forms the base of Etna and the older rocks of the 
 Campi Phlegraei. It occurs in the Lower Rhine and Central France ; 
 and circles round the east of the Washoe Mountains. 
 
 Rhyolite, in Hungary, skirts the lower part of the Andesite 
 ranges, and forms the Tokay Mountains. Yast sheets occur on the 
 east of the Sierra Nevada. 
 
 Professor Joseph Le Conte states that the lava- floods of the 
 Sierras commence in Middle California as immense but separate 
 lava streams. In Northern California they become an almost con- 
 tinuous flood, which in Oregon is 2000 feet thick, and universally 
 spread. It streams away to the north through Washington Territory, 
 and on into British Columbia for an unknown distance. This lava 
 inundation has a length of 700 to 800 miles; the stream is 80 to 100 
 miles broad, and where cut through by the Columbia River is 2000 
 to 3000 feet thick. It is one of the most remarkable results of fissure 
 eruptions that is known. 
 
 Nearness of Volcanoes to the Sea. If we refer to a map record- 
 ing the distribution of active volcanoes, such as that given by 
 Professor Karl Fuchs, 1 it will be observed that nearly all the volcanic 
 regions of the earth are remarkable for their linear extension, and 
 in this we may see evidence of their arrangement along fissures 
 of great magnitude. And secondly, it may be noticed that with few 
 exceptions they are situate either in oceanic islands or comparatively 
 near to the sea. And when this distribution is compared with that 
 of the great regions of extinct volcanoes, which are for the most part 
 in the interior of continents, and far removed from the sea, we have 
 strong presumptive evidence that volcanoes need for their activity 
 more water than can usually be furnished from a sub-aerial source ; 
 for it would seem that as land is uplifted, and the volcano removed 
 from the sea, its eruptive power usually disappears. But we are 
 further compelled, by the theory of fissure eruptions, to conclude 
 that the situation near the sea is not due to the action of water alone, 
 but is a consequence of the ways in which the rocks, which undergo 
 compression so as to rise in island chains and mountain ranges, are 
 ever developing new fractures, parallel to the direction of their 
 upheaval; and while this cause may develop new heat, and give 
 access for new supplies of water to the region which is heated, it 
 1 " Vulkane und Erdbeben," 1875. 
 
GASES GIVEN OFF DURING ERUPTIONS. 191 
 
 renders the position of the volcanoes readily intelligible. Other 
 agencies, such as the accumulation of sediments, have sometimes 
 been supposed to develop additional pressure, and assist in augment- 
 ing the volcanic fires along coasts ; but on the view proposed, 
 \ve seem compelled to adopt the conclusion so long since enunciated 
 by Krug von Nidda, and regard volcanoes in their normal condition 
 as intermittent springs which throw out melted matters. 
 
 Relation of Volcanoes to Springs. There is, in fact, a perfect 
 sequence to be traced from the volcano which pours out molten rock 
 to that in which the water supply has become so great, relatively to 
 the rock-matter, that the materials ejected cease to be molten ; and 
 as the water preponderates they may become more and more invisible, 
 till the condition of a hot spring is reached. Professor Prestwich 
 has drawn attention to the fact that an artesian well at Naples, after 
 passing through 735 feet of volcanic beds, and 787 feet of more 
 or less water-bearing strata, furnished a spring which rose 8 feet 
 above the surface, or 81 feet above the sea-level. He urges that the 
 pressure of the water in the rocks which rise above the sea-level 
 keeps the sea-water out frojn the land. But when the water which 
 is stored in the mountain and neighbouring rocks "becomes expelled 
 and exhausted under the conditions of an eruption, then the pressure 
 is removed on the landward side, and an inflow of salt water from 
 the sea necessarily takes place, and modifies the explosive form of 
 the outburst. 
 
 Decline of Volcanic Activity. After the solid materials cease to 
 be ejected, and before the eruptive throat of a volcano is hermetically 
 sealed, the existence of various gases may be detected, and the de- 
 position of salts observed. Some of the gases appear to be given off 
 all through an eruption, others chiefly at its close. Among the most 
 frequent acids are sulphuric and hydrochloric. The gases comprise 
 nitrogen, hydrogen, and carbonic dioxide. When we seek for the 
 causes of the gaseous eruptions, we shall find some gases to be derived 
 from water, and others from the rocks beneath the surface. It is 
 well known that a considerable amount of atmospheric air is dissolved 
 in water, and that in sea- water there is some amount of carbonic acid 
 gas; it is also known that at a moderate temperature these gases 
 become expelled from water, and at a higher temperature the water 
 itself is decomposed into oxygen and hydrogen. The hydrogen 
 is chiefly found as sulphuretted hydrogen, and in combination with 
 chlorine, in the form of hydrochloric acid; the nitrogen is more 
 common in the form of sal ammoniac than in the free state. The 
 decomposition of the sulphuretted hydrogen has produced frequent 
 deposits of sulphur, which sometimes cap the mountain. The 
 frequent presence of salt, of hydrochloric acid, and the large per- 
 centage of soda in Vesuvian lavas, leave little doubt that sodium 
 chloride in some way obtains access to the heated regions, and in 
 some cases is decomposed. The most abundant gas is carbonic 
 dioxide, and this would appear to be always due to the action 
 of heated matter upon limestones beneath the surface, so that the 
 
192 SULPHUR SPRINGS AND MUD VOLCANOES. 
 
 gas is produced much as in a lime-kiln ; and the fact that limestone 
 fragments are ejected from Vesuvius leaves no doubt that the material 
 exists where it can be calcined. 
 
 As the mountain cools and contracts, small cracks appear about 
 its summit and its flanks. These are termed " fumeroles," and give 
 vent to steam and various vapours, which deposit brilliantly- coloured 
 crystals of salts, that are mostly soluble and are dissolved by rain. 
 
 The decline in eruptive power, however, is gradual, and at a lower 
 level on the flanks of mountains new phenomena often appear, and 
 testify to the changed condition of the interior regions. 
 
 Solfataras. This is especially seen in the formation of solfataras, 
 which are essentially hot springs wherein the dissolved acids decom- 
 pose the rock through which the water flows, so that a good deal of 
 mud is brought to the surface ; and as the sulphuretted hydrogen in 
 the water is decomposed, sulphur is deposited in the clay in nodular 
 masses. Such sources of sulphur-supply occur at the solfatara near 
 Naples, and in the deposits near Girgenti in Sicily, and in Iceland. 
 Professor Ansted described some remarkable deposits and solfataras 
 at Kalamaki, near the Isthmus of Corinth, where the sulphur-bearing 
 region is about a mile long and half a mile wide. Not only are the 
 marls loaded with sulphur, but it is deposited in a crystalline form 
 from hot vapour, in gorges and caverns. 1 
 
 Mud Volcanoes and Mud Springs. Another phase of declining 
 volcanic activity is exhibited in the formation of mud cones, which 
 are common not only in the volcanic regions of Mexico and Peru, but 
 in Iceland, and many localities in the South of Europe. Professor 
 Ansted mentions a mud eruption which took place in a flat plain 
 below Paterno, to the south-west of Etna, which threw out water in 
 strong jets, without visible vapour, though large bubbles of carbonic 
 acid gas arose through the muddy water. The temperature of the 
 water at the surface was 110 F. The fluid ejected was a thin mud, 
 which on exposure became a tenacious paste. A good deal of petroleum 
 floated on the surface of the water, and the neighbouring volcanic 
 rocks contain cavities full of naphtha, and have a strong bituminous 
 odour. Mud volcanoes are much more numerous on the eastern side 
 of the Crimea, reaching for about 50 miles from the straits of Kertch. 
 The cones about Yenikale pour out mud slowly from the top. The 
 fluid has a temperature of from about 55 to 65 F. It is tenacious, 
 often as thick as treacle ; and the streams, which are blackish when 
 dry, sometimes run for about 120 feet. The largest hill, near Kertch, 
 is about 250 feet high; and the main cone consists of clay, in which 
 are embedded small angular pieces of limestone and fragments of 
 earthy oxide of iron. 2 Other examples occur in North Italy, in 
 Parma and Modena. But among the most interesting are those dis- 
 covered by Captain Stifle 3 on the Mekran coast, which stretches 
 
 1 Ansted, Q. J. G. S., vol. xxix. p. 360 ; and Th. Fuchs, Verb. K. K. Geol. 
 Reichsanst : 1876, p. 54. 
 
 2 Royal Institution, Friday, May 1 1, 1866. 
 
 3 Quart. Jour. Geol. Soc., vol. xxx. p. 50. 
 
VOLCANIC NAPHTHA SPRINGS. 193 
 
 from Scinde to the mouth of the Persian Gulf, and are most numerous 
 in its eastern portion. Their situation is remarkable from the circum- 
 stance that, there are no traces of volcanic action on the coast. They 
 extend over 200 miles from Guadur to Kas-Kucheri, and vary in 
 height above the plain from 20 feet to 400 feet ; they are truncated 
 at the top, and have circular craters, in some cases 100 feet wide. 
 Here the mud is cold, somewhat thicker than treacle, and consolidates 
 into a compact substance. From time to time there is an ebullition 
 of gas. The mud ejected is chiefly clay, with some carbonate of lime 
 and a little quartz sand. Thus we remark a decline in temperature, 
 until there is no external indication of volcanic action, except the form 
 of the cone and its eruptive character. But there are other modes of 
 volcanic decline in which mechanically-suspended substances are never 
 brought to the surface, and the erupted matters are bituminous pro- 
 ducts or mineral and metallic matters in solution. 
 
 Petroleum Springs. Asphalt is constantly met with in connection 
 with mud volcanoes. It may be sometimes absent (as when Hum- 
 boldt visited the cones of Turbaco, near Carthagena in JSTew Granada, 
 in 1801), and yet found plentifully fifty years later. In other dis- 
 tricts the discharge of asphalt or petroleum is permanent. Pallas 
 and other travellers have described the presence of bituminous 
 substances in the materials thrown out from the mud volcano of 
 Taman, in the Western Caucasus; but probably the best known 
 locality is Baku, 1 on the south side of the Caucasus, in the Caspian 
 Sea. Here the ground is so saturated with petroleum that it is 
 obtained by sinking wells ; and Eichwald has remarked that the 
 neighbouring cones should rather be called naphtha volcanoes than 
 mud volcanoes, since the outburst always ends with a large emission 
 of naphtha. Occasionally the naphtha, which floats on salt water, 
 takes fire during an eruption, and the flames rise to some height. In 
 the springs near Balachana the naphtha is derived from Upper Tertiary 
 sandstones ; but here, as in so many other localities, they exist in the 
 neighbourhood of volcanic rocks. In the Dead Sea, the asphalt has 
 been described by Lartet, and its appearance is associated with ther- 
 mal springs in a region of old volcanic action. 
 
 In the island of Trinidad we see another phase of emission of 
 asphalt in the celebrated Pitch Lake, which covers an area of 99 acres, 
 and in the pitch banks which sometimes exist off the coast in that 
 island ; for here eruptive phenomena are entirely absent, except in a 
 few mud volcanoes scattered over the country. 2 
 
 The origin of the bituminous matter is not easily accounted for. 
 Professor Ansted mentions a portion of a tree in the condition of 
 lignite obtained from one of the mud volcanoes of Northern Italy ; 
 and the constant association of these springs with districts of tertiary 
 
 1 See Abich, "Mdmoires de 1'Academie, Imp. St. Pdtersbourg," viii. Se"rie, tome 
 vi., No. 5. 1863. Trautschold, Ueber die Naphtaquellen von Baku, ' ' Zeitschrif t 
 der Deutschen G^olog. Gesellschaft," xxvi. Bd. p. 257. 1874. 
 
 2 See Wall, Geol. Survey, Trinidad, where a belief is expressed that the mineral 
 pitch is formed on the surface, out of existing vegetation. 
 
 VOL. I. N 
 
194 INTERMITTENT ERUPTIVE SPRINGS. 
 
 coals evidently suggests that they are a consequence of the action of 
 subterranean heat upon vegetable matter which happens to be con- 
 tained in the strata. And this view is strongly supported by the 
 fact, that during the explorations of the Challenger certain hot 
 springs at Furnas in the Azores were visited by Professor Moseley, 
 in which the algse became converted by the heat of the water into 
 a green, creamy, or black elastic inflammable substance. 1 And 
 just as the mud volcanoes bring to the surface fragments of various 
 rocks, so in the naphtha and similar materials we have a product 
 which might naturally be associated with such strata. Indeed, the 
 petroleum springs of the coal region of North America sufficiently 
 prove that, given the vegetable matter and even such a moderate 
 temperature as might result from the crumpling which the carboni- 
 ferous rocks have undergone, there is no difficulty in obtaining the 
 petroleum by a process of slow distillation. The special interest, 
 however, of the tiaphtha springs is twofold, and lies partly in their 
 normal existence in regions where volcanic action is nearly extinct, 
 and in the extent to which they have in former times contributed to 
 form bituminous limestones and deposits like the asphalt of the Yal 
 de Travers and many localities in Switzerland and France. 
 
 Eruptive Hot Springs. Geysers offer another phase of volcanic 
 action in which the eruptive power remains, but instead of the 
 water being dissolved in the rock, or mechanically mixed with it, 
 only such mineral matters remain as can be dissolved in the water. 
 These phenomena mark the near extinction of volcanic energy, and 
 not only originate near to the surface, but appear to owe their 
 existence entirely to surface waters. It is remarkable that these 
 phenomena, though met with in regions so widely separated as the 
 northern island of New Zealand, the Yellowstone Valley in the 
 United States, and the south - west of Iceland, exist in areas 
 occupied by rhyolitic rocks. 
 
 Geysers of Iceland. In Iceland they are chiefly found in the 
 Keykiadal, about thirty miles south-west of Heckla, and are situate 
 in an oblong strip of land where the marsh terminates and the 
 mountains begin to rise. The water is furnished by cold streams 
 derived from melting snows, and from the neighbouring river. The 
 geyser basins are conical siliceous domes, built up of the material 
 deposited by the waters as they cool. One of these has a height 
 of 7 feet, and is 75 feet in circumference, while the eruptive 
 throat in the centre of the basin is a circular opening 14 inches 
 in diameter. Frequently these cones stand upon coloured clays, 
 and they are so intimately connected with mud eruptions that 
 in some places mounds occur, formed entirely of clay, from which 
 water is thrown into the air to a height of a few feet. The 
 well-known Great Geyser has a well-defined cone under the hill 
 on the north-east side of Geyser Island ; but the Strokr has no cone, 
 
 1 " On Freshwater Algse at the Boiling Springs at Furnas," &c., H. N. 
 Moseley, Jour. Linn. Soc., vol. xiv., No. 77, pp. 322, 325, 333; also W. T. 
 Thistleton Dyer, p. 326. 
 
GEYSERS OF THE FIREHOLE RIVER. 195 
 
 its mouth being on a level with the surface of the ground. The 
 Great Geyser only ejects water once in twenty or thirty hours, throw- 
 ing up a column 60 feet high, accompanied with clouds of vapour. 
 The Strokr may be caused to erupt at any time by artificially block- 
 ing its throat, and its outbursts have lasted for half an hour at a time. 
 But the phenomena change from time to time. In the middle of 
 the eighteenth century there were three or four eruptions in a day, 
 and some reached a height of 300 feet. Their activity is influ- 
 enced too by earthquakes ; and it is remarkable that no mention of 
 geysers is found in the old Icelandic writings. When the geyser 
 basin is full, the water is clear and in a state of ebullition, the 
 temperature being about the boiling point, but Bunsen found that in 
 the descending tube the temperature increased to 266 F. It would 
 hence seem that the water draining in from the neighbouring hills 
 has its temperature augmented by contact with heated rock, or water 
 coming from heated rock, until a portion of the mass is so far raised 
 in temperature that the water there, having boiled off the gases held 
 in solution, overcomes the pressure of the column above, and bursts 
 into vapour, so as to throw up the column above it in a fountain. 
 The fact that Bunsen was able by experiment to reproduce periodic 
 eruptions from a tube by heating its middle portion, goes far to 
 demonstrate the accuracy of his interpretation, though the details of 
 nature's mechanism beneath the surface are necessarily unknown. 
 
 Geysers of the Yellowstone. All over the great lava district of 
 the far west of North America geysers are numerous, though many 
 are extinct. They are specially instructive, since they are often sur- 
 rounded by mud volcanoes, and associated with calcareous springs, 
 which have deposited limestone terraces, so that all the constituents 
 of water-formed rocks are here differentiated, and poured out from 
 neighbouring vents, though the materials are in some cases mixed. 
 On the Yellowstone Lake the geyser called " Fishpot" is so close to 
 the water that, without moving, the fisherman has caught trout in 
 the lake, and cooked them in the boiling geyser. The Grotto arid 
 Giant Geysers of the Yellowstone throw up columns of water from 
 one to two hundred feet high, and the eruptions last from one to two 
 hours. A mud geyser in this district, at Crater Hill, has a small 
 basin, 60 feet in diameter, formed chiefly of layers of clay and silica, 
 situate in a larger basin with a higher rim, which measures 200 feet 
 by 150 feet; and on one side of the outer basin is a ravine with 
 holes in the banks, which are lined with sulphur. The geyser basins 
 on the Firehole river are upwards of 7000 feet above the sea; they 
 are rarely conical, more frequently globular, and often have overhang- 
 ing ledges. The Grand Geyser throws up a column 6 feet in diameter 
 and 200 feet high. 1 The temperature of the water is usually below 
 the boiling point, and the water itself is green. Many mineral sub- 
 stances have been detected in these geyser ^yater3, and among others 
 small crystals of gold. 
 
 1 See Professor F. V. Hayden, U.S. Geol. Survey of Montana, 1871, and 
 Montana, Idaho, Wyoming, and Utah, 1872. 
 
196 GEYSERS OF AUCKLAND. 
 
 Geysers of New Zealand. The geysers of New Zealand are in 
 some respects much less interesting. They extend over a line of 
 fracture in the Northern Island, which lies between the crater of 
 Tongariro and the crater of White Island in the Bay of Plenty. 
 Here geysers rise over a distance of about 17 miles, associated with 
 hot springs, which come up through rhyolitic rocks, formed of horn- 
 blende and sanidine felspar. Most of the waters are alkaline, owing 
 to the quantity of soda which they contain, and there can be no 
 doubt that the silica in them has been dissolved from out of the 
 volcanic rocks. The temperature of these waters varies from 80 to 
 200. Many of the smaller geyser springs throw up water to a height 
 of 30 or 40 feet. Some of the basins fill in ten minutes, and are in 
 eruption every two hours. The best-known geysers are on the lake 
 Rotomahana. One forms a hill above the lake, with a basin 100 
 feet in diameter and 1 5 feet deep, terminating in a pipe which is 8 
 feet wide. Its waters are of an intense sapphire blue, and as they 
 descend, form successive milk-white or pinkish terraces of silica, in 
 which the water gradually loses its intensity, and passes through a 
 turquoise colour in the middle terraces, to a faint blue tinge at the 
 base, where the overflow passes into the warm lake. When the silica 
 is deposited, according to Von Hochstetter, it is at first as soft as 
 gelatine, and gradually hardens into a mass which has a sandy texture 
 and chalky aspect, but is frequently infiltrated so as to resemble 
 chalcedony or opal. Fumeroles abound all over the geyser region, 
 and there are occasional minute mud volcanoes 3 to 4 feet high. So 
 near to the surface are the heated springs that they can often be 
 tapped at will by pushing a walking-stick into the ground. 1 Extinct 
 geysers occur in the Azores ; and the sinter deposits there have pre- 
 served leaves of trees perfectly. 
 
 The Boracic Acid Jets of Tuscany. About 15 miles S.W. of 
 Volterra many jets of vapour, charged with boracic acid, rise from a 
 narrow valley in the secondary limestone. . Mr. Hamilton 2 describes' 
 large fissures which have in this way been filled up by deposits of 
 calcareous sinter. By passing the vapour through water the boracic 
 acid is collected, and the water is then evaporated. Similar discharges 
 of vapour occur in the neighbourhood at Sarrezano, Castelnuove, and 
 Monte Rotondo. The vapour escapes from hundreds of vents often 
 with the noise of a steam boiler blowing off its steam. 
 
 Hot Springs. Hot springs are by no means limited to volcanic 
 regions, but they are most numerous where igneous rocks have been 
 intruded. 3 Therefore we regard them as indicating that surface 
 waters have penetrated to a depth where the rocks are still heated, and 
 thus warmed, have risen again to the surface by different channels. 
 Among the best-known examples in Europe are the sulphurous springs 
 of Aix-la-Chapelle (171 F.), the springs of Ems (131), Wiesbaden 
 
 1 Hochstetter, " New Zealand : its Physical Geography, Geology, and Natural 
 History." 
 
 Hamilton, P. Geol. Soc., 1844, p. 477. 
 
 3 See Daubeny's "Volcanoes," 2(1 ed., p. 544. 1848. 
 
ORIGIN OF HOT SPRINGS. 197 
 
 (147), Toeplitz (117), Carlsbad (164), Baden-Baden (153), Gastein 
 (117), Aix-les-Bains (116), Leukerbad (124), with many more in 
 Central France, the Pyrenees, Hungary, Italy, and other countries. 
 For the most part these springs are at moderate elevations above the 
 sea, Wiesbaden being 323 feet and Aix-la-Chapelle about 500 feet; 
 but at Gastein the spring rises at 3100 feet, and at Leukerbad at 4400 
 feet ; while in South America hot springs occur in Chile at a height of 
 upwards of 12,000 feet. Bischoff l has ingeniously calculated that by 
 supposing water to collect on the Balm Horn above the Baths of Leuk at 
 an elevation of 10,292 feet, where it is assumed to be liquid, it would, 
 by flowing down the clefts in the interior of the mountain, be raised 
 to a temperature of 125 F. on reaching Leukerbad, supposing the tem- 
 perature to rise 2^ F. for every 145 feet descended. Therefore, the 
 temperature of hot springs is never above that which may be derived 
 from the earth ; but their existence in regions of existing and former 
 igneous action may well be compared with the positions of geysers. 
 Some, like the hot springs of Bertrich, slowly decline in temperature ; 
 and earthquake disturbance has raised the temperature of others. 
 The substances dissolved in hot springs vary with the nature of the 
 rocks through which the water flows, and with the temperature. 
 
 The evolution of gas is often considerable. It is commonly 
 carbonic acid, as in nearly all the springs of volcanic districts, and 
 those which rise through faults, or else it is sulphuretted hydrogen. 
 Often there is a large evolution of nitrogen, as in the waters of Bath, 
 Buxton, and many continental springs. Frequently the waters are 
 so charged with carbonic acid as to form natural soda waters, and 
 sometimes carbonic acid escapes from fissures without any indication 
 of springs, as in the Mofettes, near Laach, and Gerolstein, and many 
 localities in Italy. The salts dissolved in the waters of springs are 
 usually carbonates and sulphates, but chlorides, phosphates, and 
 fluorides are also found. The waters of Carlsbad (which contain 
 about 463 grains of solid matter to the gallon) yield as the 
 principal substance sulphate of potash, but there are small quantities 
 of arsenic, iodine, bromium, antimony, gold, copper, chromium, 
 manganese, zinc, cobalt, nickel, titanium, barytes, strontian, lithium, 
 fluorine, selenium, phosphoric acid, and boracic acid, besides several 
 organic acids and traces of resinous substances. 
 
 The hot springs of Cornwall at Wheal-Ciifford, near Kedruth, 
 yield a large quantity of lithium, and are rich in chlorides of sodium 
 and calcium. Professor Daubeny 3 has given a statement of the salts 
 found in the principal thermal springs in a tabular view, from which 
 it will be seen how variable are the salts contained, and how different 
 are the geological formations through which the waters rise. But, 
 in every instance, the conclusion is forced upon us that these salts 
 are to a large extent such as would be yielded by the decomposition 
 
 1 Bischoff, " Physical, Chemical, and Geological Researches on the Internal 
 Heat of the Globe.'' 1841. 
 
 2 Lyell Address, Brit. Association, Bath, 1864, p. 65. 
 
 3 ' "Volcanoes," id ed. 
 
198 MINERAL DEPOSITS FROM HOT SPRINGS. 
 
 of the various felspathic and micaceous minerals, which, enter into 
 the composition of igneous and metamorphic rocks. 
 
 We thus observe that springs differ from volcanoes in bringing to 
 the surface soluble substances, one of which usually predominates in 
 a particular spring ; but if we regard hot springs, geysers, and mud 
 volcanoes as a group of phenomena closely connected, we shall find 
 in their waters all the materials which go to form the minerals that 
 build up igneous rocks, or which are differentiated in lavas. 
 
 Relation of Hot Springs to Mineral Veins. It is impossible in 
 examining the analyses of the waters of springs not to be struck with 
 the frequency with which they contain small percentages of copper, 
 tin, zinc, iron, and other metals ; and since it is well known that the 
 salts which they commonly bring to the surface are deposited as the 
 temperature decreases, so it has been inferred that the heated waters 
 have dissolved the metallic substances which were contained in large 
 masses of rock, and making their way from great depths into channels 
 which communicate with the surface, have lined the walls of these 
 fissures with crystalline deposits of metals, which have become sepa- 
 rated from each other in consequence of the differences of temperature 
 at which the different crystals are deposited. These are presumed to 
 have formed on the walls of the fissure in successive layers, because 
 the rock would always be colder than the water it contained ; and 
 thus it is conceived that the minute traces of metallic substances 
 which sometimes come to the surface, are but indications of larger 
 deposits of the same mineral, which the water of the spring parts 
 with at greater depths beneath the earth's surface. And it deserves 
 to be remembered that in regions where mineral veins are found to 
 be divided by fissures of a later date, which also contain the ores of 
 metals, the newer veins yield different metals and minerals to the 
 older ones, as though the plane of denudation of the present surface 
 of the earth had cut these deposits transversely at different distances 
 from the source, or in positions where the temperatures of deposition 
 were different. The deposits from hot springs at the surface are 
 chiefly alkaline salts, and salts of magnesia and lime ; but in mining 
 districts, where mineral veins occur, we have in almost all cases 
 evidence, not only of igneous action, or metamorphism and disruption 
 of strata, but also proofs of enormous denudation. So that it is 
 reasonable to conclude, especially with the experience of important 
 metalliferous deposits occurring in gravels, that the upper portions of 
 many mineral veins have been removed by denudation ; and it is 
 thus that we come upon the ores of metals in veins, which have 
 neither been erupted nor volatilised from below, but simply segre- 
 gated under the influence of water and volcanic heat, from the rocks 
 in which they were previously diffused. 
 
 Denudation of Volcanic Regions. Any area in which volcanic 
 phenomena are manifested with the greatest intensity, always shows 
 indications that the region has undergone extensive denudation. "We 
 need but to examine such sections as have been drawn across the 
 
DENUDATION OF VOLCANIC REGIONS. 199 
 
 Andes by Charles Darwin 1 and David Forbes 2 to recognise in the 
 vast inversion and incessant crumpling of the rocks, demonstration 
 that before the era of existing volcanic activity, enormous, almost 
 inconceivable, denudation must have taken place. This denudation, 
 which has bared the granitic rocks and exposed the rich metalliferous 
 veins at the surface, has reduced by no infinitesimal amount the 
 weight of superincumbent rock which had to be rent to form the 
 lines of volcanic fracture, and has lessened the height to which 
 volcanic forces heaved the products of their energy. 
 
 The same phenomena may be observed in all volcanic regions, so 
 that the manifestations of volcanic action which we have been con- 
 sidering may be held to depend not only upon the internal heat of 
 the earth, but also in some degree upon the agencies which act upon 
 its surface. 
 
 In the previous chapter we examined the ways in which heat acts 
 upon and changes the surface rocks ; we have now seen the nature 
 of the changes which are produced on the materials thus altered by 
 agencies at the earth's surface, and it only remains to inquire how 
 far an examination of the products of volcanic action justifies the 
 interpretation which has been here given. 
 
 1 "Geological Observations on South America." 
 
 2 Q. J. G. S., vol. xvii. p, 7, 
 
( 200 ) 
 
 CHAPTER XIV. 
 THE NATURE AND ORIGIN OF IGNEOUS ROCKS. 
 
 WHEN we contemplate the products of igneous energy, as seen in 
 lavas, dykes, and the materials of mountain masses, we are impressed 
 by their diversity. From Vesuvius alone Von Buch distinguished 
 eighteen distinct principal kinds of lava, besides many varieties ; and 
 the number of minerals found on this volcano forms a considerable 
 fraction of those of the whole world. It has always been difficult to 
 account for the variety of igneous rocks ; and hence many hypotheses, 
 from time to time, have been suggested in elucidation of observed 
 facts. There are three principal views, all supported by a large 
 amount of evidence, which are more or less worthy of attention, since 
 they have been adopted by able observers. 
 
 Hypothesis that the Earth's Crust is formed of Layers of 
 Different Igneous Rocks. First, there is the hypothesis that all 
 igneous rocks are derived from the more or less liquid interior of the 
 earth. In its crude form, as stated by the earlier writers, this view 
 had but little to recommend it, beyond its obvious simplicity. But 
 when the observed facts of basaltic lavas being in many cases poured 
 out after the trachytic lavas, as determined by Bunsen and earlier 
 writers, became generalised into the hypothesis of the existence beneath 
 the surface of two shell-like layers of rock material, the doctrine ac- 
 quired importance ; for the outer layer was assumed to be formed of 
 highly silicated or acidic rocks, of lower specific gravity, and more 
 perfectly oxidised, than the deeper-seated basic layer, which was 
 inferred to have been extruded after the acidic layer had been poured 
 out and cooled. 
 
 Subsequently, when Von Richthofen divided volcanic rocks into 
 five groups, it became necessary to admit the idea of five of these 
 magmas, forming successive shells or onion coat-like layers of the 
 original fluid earth, which had been successively erupted. And when 
 we bear in mind that this classification of lavas into propylite, ande- 
 site, trachyte, rhyolite, and basalt, represents the order in which they 
 are superimposed upon each other in Hungary, Transylvania, North 
 Germany, and the great volcanic region of North America an order 
 which experienced geologists have found never to vary there is an 
 a priori case in favour of thus accounting for the diversity of igneous 
 rocks. But we need to bear in mind that this sequence, even if it 
 should hold good for larger areas than those in which it has been 
 established, is, at best, an order which characterises certain eruptions of 
 the tertiary period ; and the moment we go further back in time, the 
 
EVOLUTION OF IGNEOUS ROCKS. 201 
 
 illusion vanishes, as Yon Cotta clearly saw ; for instead of finding 
 other zones, or magmas of the earth's substance tapped by ancient out- 
 bursts of the secondary and primary periods, we still meet with rhy- 
 olites, basalts, trachytes, and andesites ; but in an order of succession 
 in time which bears no relation to their occurrence in Richthofen's 
 tertiary system. We therefore are compelled to abandon the hypo- 
 thesis that volcanic rocks are derived from the original products of an 
 igneous fusion of the earth ; and no law has ever been suggested 
 which would on this hypothesis, show that the order of their succes- 
 sive appearance in the Primary period could be harmonized with their 
 order in the Tertiary period. It is an attempt not to disentangle and 
 explain the problem, but to escape from it by assuming results sup- 
 posed to follow from original igneous fusion of the earth. 
 
 Chemical Hypothesis of Davy and Daubeny. Another view 
 which cannot be altogether ignored is the chemical theory, originated 
 by Sir Humphrey Davy, modified and supported by Daubeny, and 
 combated by Bischoff. This conception, also starting from an original 
 igneous fusion, rested on the hypothetical idea, that the interior of 
 the earth consisted of the elements in an unoxidised condition ; and 
 as water and the atmosphere obtained access to these substances from 
 the earth's surface above, so chemical changes were assumed to be 
 developed, which gave rise to volcanic energy. We need not now 
 delay to examine this view in detail. The potency of chemical action 
 to produce important changes in rock materials, and possibly in the 
 relative level of land, may be regarded as established ; but when \ve 
 remember the demonstrated continuity of volcanic outbursts with 
 plutonic phenomena, we seek in vain for any evidence which would 
 allow us to believe that the internal heat of the earth, crystallisation 
 of igneous rocks, and eruption of lavas, are exclusively or even prima- 
 rily chemical processes. 
 
 Hypothesis of Igneous Evolution. Finally, there is the hypo- 
 thesis which regards all igneous rocks as metamorphosed from ancient 
 sedimentary strata, in the manner indicated in a previous chapter. 
 This may be termed the hypothesis of igneous evolution, which sup- 
 plements the hypothesis of organic evolution. For since the most 
 ancient fossiliferous rocks contain highly-organised groups of animals, 
 and even genera which still survive, we are compelled to believe, if 
 the hypothesis of organic evolution is true, that strata immensely 
 more ancient than those preserved must have existed, and represented 
 long eras of past time, during which the earlier steps of organic evo- 
 lution took place. And since no such rocks have been found, a belief 
 in organic evolution enforces the conviction that such strata were 
 metamorphosed, worn into sediments once more, and metamorphosed 
 again, with many repetitions of the process, effacing from the earth 
 all traces of its ancient life in times antecedent to the Primary era. 
 The evidence on which this conclusion rests is essentially palseonto- 
 logical; but it will be sufficient to restrict ourselves now to the 
 observed facts of igneous outbursts, and see how far the origin of 
 igneous rocks can be explained on this hypothesis. 
 
202 DR. STERRY HUNTS SUGGESTIONS. 
 
 Views of Sir John Herschel. It is to Sir John Herschel that we 
 must attribute the enunciation of the origin of volcanic and igneous 
 phenomena in metamorphism of sedimentary deposits. In 1836 he 
 maintained that the temperature of the earth's crust must be raised 
 in consequence of the accumulation of sediments, and insisted that 
 the heat thus developed in the strata would result in the develop- 
 ment in them of a crystalline condition, and that, under the influence 
 of included water, they might become heated to the melting point, so 
 that gases would be given off, and the phenomena of earthquakes and 
 volcanic eruptions result. He further appealed to the transfer of sedi- 
 ments by denudation, as the primary cause which has initiated 
 internal changes which result in elevation and depression of the 
 earth's crust. As late as 1861, we find this distinguished philosopher 
 urging, in his " Physical Geography," that the accumulation of sedi- 
 ments, and their denudation, is capable of producing any amount of 
 pressure on the one hand, and relief from pressure on the other, that 
 the geologist can possibly require, without calling in the aid of 
 unknown causes. 1 These views, however, in so far as they concern 
 the recognition of pressure and water as agents in volcanic action, 
 were anticipated by Poulett Scrope in 1825 ; 2 and in so far as concerns 
 the nature of the products of metamorphism of sedimentary rocks, 
 identical views were published by Keferstein in 1834. But these 
 hypothetical ideas produced little or no impression, and were pro- 
 pounded anew by Sterry Hunt. 3 
 
 Views of Sterry Hunt. Hunt's views are similar to our own, 
 which were suggested by other evidence. They are well stated in the 
 following paragraph : 
 
 " Two things become apparent from a study of the chemical nature 
 of eruptive rocks ; first, that their composition presents such varia- 
 tions as are irreconcilable with the simple origin generally assigned to 
 them; and, second, that it is similar to that of sedimentary rocks 
 whose history and origin it is, in most cases, not difficult to trace. I 
 have elsewhere pointed out how the natural operation of mechanical 
 and chemical agencies tends to produce among sediments a separation 
 into two classes, corresponding to the two great divisions above 
 noticed. From the mode of their accumulation, however, great varia- 
 tions must exist in the composition of the sediments, corresponding 
 to many of the varieties presented by eruptive rocks. The careful 
 study of stratified rocks of aqueous origin discloses, in addition to these, 
 the existence of deposits of basic silicates of peculiar types. Some 
 of these are in great part magnesian, others consist of compounds like 
 anorthite and labradorite, highly aluminous basic silicates, in which 
 lime and soda enter, to the almost complete exclusion of magnesia and 
 other bases ; while in the masses of pinite or agalmatolite rock we 
 have a similar aluminous silicate, in which lime and magnesia are want- 
 ing, and potash is the predominant alkali. In such sediments as 
 
 1 L. c., p. 117. 
 
 2 See "Considerations on Volcanoes," by G. Poulett Scrope. 1825. 
 
 3 "Chemical and Geological Essays," 1^75, pp. 8, 15, 44, 66. 
 
ORIGIN OF GRANITES. 203 
 
 those just enumerated we find the representatives of eruptive rocks 
 like peridotite, phonolite, leucitophyre, and similar rocks, which are 
 so many exceptions in the basic group of Bunsen. As, however, they 
 are represented in the sediments of the earth's crust, their appearance 
 as exotic rocks, consequent upon a softening and extravasation of the 
 more easily liquefiable strata of deeply buried formations, is readily 
 and simply explained." 1 No other interpretation gives so simple and 
 logical an explanation of the variety of lavas which volcanoes emit. 
 
 Texture of Volcanic Rocks. In any endeavour to comprehend 
 their variety, we are always thrown back from considerations of 
 texture and mineral structure to the fundamental facts of chemical 
 composition. The texture of lavas may be nearly paralleled by that 
 of iron-furnace slags ; and we have seen at the Lowther Iron Works 
 at Workington, slags which have run their full length exhibiting a 
 compact earthy fracture and deep blue colour, while other portions 
 of the flow which have descended like the waters of a cataract, have 
 consolidated into stalactitic sheets and films of nearly transparent 
 glass ; while in one portion of the stream, where the slag had overrun 
 a minute spring, the whole mass was elevated into a cone of yellowish 
 white cavernous pumice, with a well-defined cup at the summit. 
 And occasionally, under special conditions, these slags are found per- 
 fectly crystallised, with a development of Humboldtilite and other 
 minerals. This circumstance, no less than the differences seen in 
 extinct volcanoes between the texture and the mineral composition of 
 lava streams, and the parent masses from which they were poured 
 out, seem to render any purely mineralogical classification impossible. 
 And it has been experimentally proved that slight changes in the 
 chemical composition of the mass necessarily result in the develop- 
 ment of different minerals. Wo therefore proceed to examine the 
 origin of granite as a type of the history of igneous rocks ; because if 
 the hypothesis of igneous evolution can be applied to granites and 
 rhyolites, we make lio doubt that its application to other igneous 
 rocks must follow. 
 
 Origin of Granites. It is convenient first to turn our attention 
 to the granites, and the lavas which most nearly correspond to them 
 in composition because they are perhaps the best known. Not only 
 do granites vary greatly in the relative proportions of their mineral 
 elements, but they also exhibit considerable variation in their con- 
 stituent minerals. For although we may use the general formula of 
 quartz, felspar, and mica to describe the rock, yet the felspar or mica 
 may be almost any member, or members, of these families of minerals, 
 and they may be supplemented or partly replaced by minerals which 
 are no essential component of granite, and are local in their develop- 
 ment. And when the chemical composition of granite is examined, 
 the variation is almost as remarkable ; for although we may regard 
 the normal composition as including silica, alumina, peroxide and 
 protoxide of iron, lime, magnesia, soda and potash, and water, yet 
 
 1 See also Clarence King, U. S. Geol. Exploration, Fortieth Parallel, vol. i. 
 p. 112, on the " Genesis of Granite and Crystalline Schists." 
 
204 CHEMICAL DIVERSITY OF GRANITES. 
 
 sometimes in addition to these there are perceptible quantities of 
 oxide of manganese, phosphoric acid, lithia, and fluorine, while not 
 infrequently the protoxide of iron, or even all the iron, may be absent, 
 as may be the magnesia and the water. Even in British granites the 
 percentage of every constituent is very variable ; thus the silica ranges 
 ^rom as low as 55*20 in the granite of Ardara to as high as 80^24 in 
 the granite of Croghan Kinshela ; so that, judged by this test, the 
 Ardara rock might be termed basic, while the Croghan Kinshela rock 
 is typically acidic. 
 
 The alumina varies from 11-14 P er cent, at White Gill, Skiddaw, 
 to 20 per cent, in the granite of Glen in Donegal. The peroxide of 
 iron ranges from '23 at Botallack, to 7*3 in some of the granites of 
 Leinster ; while the protoxide of iron which is so frequently absent, 
 amounts sometimes to upwards of 2 per cent. The lime varies from 
 J per cent, in some of the Cornish granites, to upwards of 5 per cent, 
 in some of those from Donegal. The magnesia, which may be a 
 mere trace, amounts to 3^ per cent, in the granite of Ardara. Soda 
 may be but -J per cent, in Cornish granites, and 5^ per cent, in some 
 of the Leinster rocks. Potash is less than \ per cent, in one of the 
 Leinster granites, and more than 8J per cent, in the granite of Chy- 
 woon Morvah in Cornwall. The manganese never quite amounts to 
 i per cent., and the water is never more than 2 per cent. If we 
 further included the elvans in our survey, we should find in some 
 respects yet greater variations, since the percentage of silica may fall 
 as low as 47, and the water and magnesia rise above 6 per cent. each. 
 
 Turning from the examination of these rocks to discover deposits 
 out of which they might have been formed by metamorphism, we have 
 not to seek far. We may take such clays as are associated with the 
 coal strata, or used in the potteries, and find their composition to 
 include the same chemical elements as granite, though from the 
 analyses available, we may not be able to exactly parallel the granite 
 of any particular district. These clays consist of silica, alumina, 
 peroxide, and occasionally protoxide of iron, lime, magnesia, potash, 
 and sometimes soda, and water ; while occasionally there are traces of 
 phosphoric acid, organic matter, and various elements locally distri- 
 buted. If we further compare the percentages, we shall find the 
 silica to vary between 44 per cent, and 77 per cent., while the 
 alumina varies between 14 per cent, and 34 per cent. The lime may 
 be less than \ per cent., or as much as 3 per cent. ; the magnesia 
 rarely exceeds i per cent. The potash ranges as high as 3 J per cent., 
 and the soda, which is not often detected, is in some cases | per cent., 
 while the water may vary from i per cent, to 25 per cent. These 
 facts may be seen sufficiently set out in the accompanying tables, 
 which give the chemical compositions of certain clays. I'irst we 
 compare the granite of Creetown with a slate from Prague analysed 
 by Zirkel, and detect but little more difference than might occur in 
 different samples of the same rock. The Cornish granite of Redruth 
 may be compared with the American triassic sandstone of Cotton- 
 wood. If this granite is compared with the clay of Hillscheid or 
 
RELA TION OF SOME GRANITES, SANDSTONES <&- CLA YS. 205 
 
 Comparison of the Composition of Granites and Clays. 
 
 
 Cree- 
 towu 
 Granite. 
 
 Prague 
 Slate. 
 
 Redmth 
 Granite. 
 
 Cotton- 
 wood 
 Sand- 
 stone. 
 
 Bendorf 
 Clay. 
 
 Hill- 
 
 scheid 
 Clay. 
 
 Loss. 
 
 Silica 
 
 67*04 
 
 67XO 
 
 74. '60 
 
 74 '74 
 
 7C*44 
 
 77 'OT 
 
 78-61 
 
 Alumina .... 
 Peroxide of iron 
 Protoxide of iron 
 
 17*20 
 
 3-I5 
 MI 
 
 I5-89 
 5-85 
 
 l6'2I 
 
 1-16 
 
 14-14 
 79 
 
 17-09 
 1-13 
 
 14-06 
 I'35 
 
 j I5-26 
 
 Lime .... 
 
 2*Q2 
 
 2*24. 
 
 28 
 
 1-61 
 
 48 
 
 7C 
 
 
 Magnesia .... 
 Soda 
 
 I '20 
 V2"? 
 
 3-67 
 2'I I 
 
 48 
 
 1-18 
 
 39 
 
 Q2 
 
 31 
 
 '47 
 
 -9i 
 
 Potash 
 
 7'QO 
 
 I '2T. 
 
 3 '44 
 
 t?"2Q 
 
 C2 
 
 1-26 
 
 3'33 
 
 Manganese oxide . 
 Lithia 
 
 
 
 *5* 
 
 'IO 
 
 
 
 
 
 Water 
 
 
 
 l'2T> 
 
 1-88 
 
 4'7I 
 
 TI7 
 
 1-89 
 
 
 
 
 
 
 
 
 
 Bendorf, it will be seen that the chief differences between them are 
 that the granite contains some soda not found in the clay, and three 
 times as much potash, while the clay contains much more water than 
 the granite. These differences are small compared with those seen in 
 different so-called granites, though they would determine distinct 
 felspar formation. The percentage of lime and magnesia in clays 
 must necessarily vary with the organisms clays contain, so that the 
 only difficulty which presents itself exists in the small percentage of 
 soda and the large percentage of water. Some alum-shales in Sweden 
 contain 3 to 8 per cent, of potash, and some are free from water. 
 From the fact that crystals of sodium chloride are met with in purbeck, 
 triassic, and carboniferous clays in this country, the deficiency of soda 
 is not perhaps insuperable, even without appealing to probable sources 
 of salt supply, in the contact of sea-water with heated regions beneath 
 the earth's surface, or growth and decay of marine plants. And the 
 presence of combined water in clay, though greater than the quantity 
 which is found in granite, is perhaps not more than we may assume 
 Avould be easily expelled or altered by chemical combination, under 
 the conditions of igneous fusion, since it often disappears in slates. 
 If, then, it is no rare circumstance for certain sandstones and clays 
 to have an average chemical composition which is nearly identical 
 with that of granite, we may fairly believe that such strata, 1 under 
 requisite conditions of heat, pressure, and cooling, such as would be 
 presented in the central axis of a mountain mass, would become 
 changed by metamorphic action into granite rock. 2 
 
 1 See pp. 206 and 210. 
 
 - It may not here be out of place to draw attention to the celebrated instance 
 at Carlingford, in which Dr. Haughton affirms that veins of granite penetrating 
 into carboniferous limestone become converted into syenite, by assimilating a 
 certain portion of the limestone. Mr. R. H. Scott confirms Dr. Haughton's de- 
 termination, so that the facts may be presumed to be clear ; yet we cannot help 
 drawing attention to the circumstance that the Carlingford granites all contain 
 7 to 8 per cent, of alkali, and Dr. Haughton's analysis gives no trace of alkali in 
 his syenitic rock. Such a result would not have been anticipated from adding 
 limestone t > granite substance under such conditions. 
 
206 
 
 ANALYSES OF CLAYS. 
 
 
 
 Bovey Traccy.* 
 
 O O *o O l *"> 
 
 i^oo -i ^ : : " 
 
 
 . 
 
 Bovey Tracey .* 
 
 vo p o yo p 
 
 r^ ON HI HI : : HI 
 
 VO N 
 
 SH CIA? 
 
 - 
 
 Stourbridge. 
 
 O N N rf- O OO OO OQ OOO 
 
 io N HI bobbo N t* 
 
 H 
 
 h 
 
 
 ia Clays, 
 
 Newcastle Coal 
 Measures. 
 
 vo t^ p VO . r-^ HI TJ- yo 
 
 voi^ cs b b N b b 
 
 vo N HI 
 
 
 13 
 o 
 
 
 ^ HI to T^" . N CO . COVO 
 
 5j 
 
 
 
 Tj- CO 
 
 3 
 
 
 Cornwall China Clay. 
 
 N rj- 1^ vo Tj- 00 r^ 
 CO t^ N CO . rf . . O vO 
 
 vbosb b : b : :i;2 
 
 
 
 
 ^o" vS ^ . sr o" . ^. 
 
 
 
 
 ^- ON O O O N CO 
 
 
 
 
 ON * <U ** O ON O 
 
 co a\ o i^, . o . - rj- o 
 
 
 
 
 ^t--. J N H, b b 
 
 
 | 
 
 
 Ot^ -rhO O^ COO 
 VOCO MVO. O u ."^O 
 
 
 
 
 
 Tj- CO HI 
 
 r CLAVS. 
 
 
 Clay atove Chalk. 
 Houdan (Seine-et-0i.se). 
 
 vo o ON co vo o oo ^}- 
 
 '^ co HI 'HI : b : 2 N vb 
 
 J- VO -*3 HI HI 
 
 FOREIG> 
 
 
 Eberuhahu. 
 
 99 r 5 *" r^ P ^ f^ 
 
 -)--)- l_ _ Q Q VQ 
 
 VO N 
 
 
 
 
 
 OO O OO HI N vo ON 
 M O t^vo . vo CO - co 
 
 B 
 
 5 
 
 
 OO O HI O ' O M ' vo 
 vO N 
 
 
 S 
 
 
 
 
 0000 10VO J^. - \o 
 
 
 I 
 
 
 ^ vo <S vo 
 
 
 1 
 
 
 <* ON coOO H, N 
 rf o HI rt- . co vo . r^ 
 
 
 
 
 vo t->. HI o o O ^r 
 
 t^ HI 
 
 
 
 Hillscbeid. 
 
 c^o vo 10 t^ vo r->i 
 
 O O co o-> . rf- M . HI 
 
 j^V MO : b ~ ' io 
 
 
 
 
 
 i 
 
 
 
 "go ' ^ ^ 
 
CONDITIONS OF ORIGIN OF IGNEOUS ROCKS. 207 
 
 Moreover, the variations in chemical composition which result in 
 the elaboration of the different felspars and micas, in granites of cer- 
 tain localities, are precisely analogous to the differences in chemical 
 composition of clays, which result from the mechanical conditions of 
 washing, and transport at the time of deposition. These clays have 
 been experimentally analysed by washing, so as to be separated into 
 several parts, each with a chemical composition of its own. And 
 since the first washing removes all the larger particles of sand, it is 
 obvious that the percentage of silica in a clay is merely an expression 
 of the nearness of a deposit to shore, or indicates slope of sea-bottom, 
 or the presence or absence in the ocean of silicious organisms when 
 the deposit was forming. 
 
 Evolution of Igneous Rocks. Stratified rocks present gradations 
 of chemical composition, which commencing with analogues of the most 
 extremely acidic portion of the igneous rocks, gradate into others which 
 represent the extremes of the basic series. And if these sediments 
 were successively melted up and cooled, we should expect the sequence 
 to begin with quartz rock in which but few accidental minerals are 
 scattered, and pass down through an intermediate series into crystal- 
 line limestones, in which earthy minerals are rare. The great central 
 part of the group has the chemical composition which is required to 
 produce all the known types of igneous rocks, if it were open to the 
 influx of sea-water. And the local conditions of more or less perfect 
 separation of the mechanical substances, and products of the growth 
 and decay of various groups of vegetable and animal organisms, may 
 explain the fact that almost every locality has its local varieties of 
 the rock families, to which its igneous rocks belong. In nature 
 a perfect geographical gradation of altered sediments can never 
 be expected, because it could only occur along an existing coast- 
 line, which was at right angles to some ancient shore-line, which 
 furnished sediments in horizontal sequence ; and then we should 
 require these sediments to have been melted up along their extent, 
 without mixture with other deposits. The vast thickness of ancient 
 clays, as exemplified in the older primary strata, may account in 
 part for the absence of liquefied silicious rocks ; but the greater fusi- 
 bility and capacity of the felspathic sediments for aqueo-igneous 
 changes, would influence the extrusion of rocks of this character, to 
 the exclusion of the extreme terms of the horizontal series of aqueous 
 rocks deposited. Geological sections show, however, that strata of 
 different mineral character are superimposed on each other. If, then, 
 such sediments are inferred to become so folded, compressed, and 
 heated by internal contraction of the earth's crust as to be melted up 
 in succession, according to the order in which they rest upon each 
 other, and to become partially mixed in the process of eruption, there 
 ceases to be any difficulty on chemical grounds in accounting for 
 either the normal trachytic or normal pyroxenic groups of Bunsen, 
 or for any of the intermediate or extreme types of eruptive rocks 
 succeeding each other from the same eruptive throat. And when the 
 chemical compositions of the several groups of volcanic rocks are 
 
208 CLARENCE KING'S CLASSIFICATION OF LAVAS. 
 
 compared, there are no such, differences between them as would suggest 
 difficulties, on that account, in recognising their homologues among 
 stratified deposits. This hypothesis shows how the microscopic 
 crystalline texture may help to unfold the evolution of lava, and yet 
 how subordinate from a physical point of view are the refinements of 
 such analysis, since they manifest what may be termed mechanical 
 accidents which have governed the development of certain minerals in 
 natural association in igneous rocks. 
 
 Clarence King's Views on American Volcanic Rocks. Mr. 
 Clarence King conceives the volcanic rocks to belong to an altogether 
 different type from the plutonic rocks, and to have originated in 
 different ways. He bases his distinction upon the occurrence of 
 glass inclusions in the crystals of lavas, and fluid inclusions in the 
 crystals of plutonic rocks. To us this difference is chiefly indicative 
 of the different conditions of pressure under which the rocks respec- 
 tively cooled, which allowed the water of volcanic rocks to escape. 
 It is, moreover, by no means a universal law, and the fact that one 
 whole class of volcanic rocks, the propylites, is characterised by fluid 
 inclusions in the quartz, prevents us from attaching fundamental 
 importance to this condition. And in consequence of this circum- 
 stance, Mr. Clarence King, who finds no difficulty in accepting the 
 metamorphic origin of granites, does not see his way to accept the 
 metamorphic origin of lavas. Further, he finds each of the types of 
 lava, in the far west of the United States, to present such mineral 
 differences that each may, as a rule, be said to present three modifi- 
 cations, characterised respectively by quartz, hornblende, and augite, 
 as in the following scheme: 
 
 Clarence King's Classification of Tertiary Volcanic Rocks of America. 
 
 !Quartz-Propylite. 
 Hornblende-Propylite (rarely micaceous). 
 Augite-Propylite. 
 SQuartz-Andesite (or Dacite). 
 Hornblende- A ndesite (rarely micaceous). 
 Augite- An desite. 
 I Quartz-Trachyte. 
 
 Trachyte . . j Mica-Trachyte (rarely hornblendic) . 
 ( Augite-Trachyte. 
 
 iQuartz-Rhyolite (or nevadite). 
 Mica-Rhyolite (rarely hornblendic). 
 Basalt. 
 
 The propylite and andesite are regarded as of pre-miocene age, the 
 trachyte, and so-called neolite, are classed as post-miocene. Each of 
 these groups is supposed to have been erupted in succession from 
 subterranean lakes in which fusion was produced as a consequence 
 of denudation, which removed the vertical pressure of superincumbent 
 rock, and so enabled the heated mass to become liquid. It is presumed 
 that with successive ages of geological time denudation increased, and 
 fusion extended deeper and deeper. We have failed to discover any 
 evidence of such successive denudations, though the denudation of 
 the great American table- land was beyond doubt enormous ; and the 
 
CLARENCE KING'S VOLCANIC HYPOTHESIS. 209 
 
 circumstance that these lavas are superimposed on each other in great 
 thickness over broad areas, is some evidence that no denudation 
 adequate to liquefy rock in so grand a scale could have taken place. 
 
 It was suggested by Scrope that separation of crystalline sub- 
 stances in the magmas might take place, so that specific gravity would 
 exercise an influence in arranging igneous materials in order of their 
 densities ; and Clarence King also takes it to be certain that the 
 crystals in lavas were mostly formed before eruption. Hence, each 
 of these supposed fiery lakes is believed to have been a cauldron in 
 which the light quartzose rock floated towards the surface and was 
 erupted first ; and the heavy augitic rock sunk towards the bottom, 
 and was erupted last, so as to account for the threefold type of each 
 rock given in the table. These views seem to rest on the facts ob- 
 served by Charles Darwin in the Galapagos Islands, that crystals 
 were abundant in the lower part of a lava stream and wanting above, 
 and this circumstance has been observed again by King in the lavas 
 of Kilauea. But although the fractured crystals and fluxion structure 
 of rhyolites prove that the crystals formed before the mass become 
 absolutely solid, the Galapagos and Kilauea observations are more 
 likely to show that the lavas cooled more slowly where in contact 
 with the mountain than when exposed to the air ; and, therefore, that 
 the crystals were produced under conditions of slower cooling in har- 
 mony with all other observations. And if we were once to admit the 
 formation of crystals in the heated rock and their arrangement accord- 
 ing to specific gravity as supposed, it would be difficult to see why 
 the process should not have gone a step further, by separating the 
 minerals themselves, and obliterating the magmas or varieties of the 
 volcanic rocks. The sequence manifested by the American rocks of 
 the Fortieth Parallel certainly needs explanation ; but Clarence King's 
 hypothesis, though preferable to Richthofen's, which would derive the 
 lavas from successively lower zones of the earth's interior, leaves much 
 to be desired. 
 
 If we compare the varieties of granite with the varieties of any of 
 Clarence King's volcanic rocks, we might easily construct a parallel 
 series, except that the augitic member would be wanting ; but the 
 absence of such a rock may not be thought unaccountable when we 
 remember that according to Mitscherlich, Berthier, and G. Rose, 
 the actinolite and grammatite varieties of hornblende, when fused 
 in a porcelain furnace, yield crystals which have the form of augite. 1 
 And, therefore, the granites, hornblendic granites and syenites, might 
 be held to present the same gradations from acidic to basic types, 
 which are seen in the American groups of volcanic rocks, The other 
 igneous rocks may be similarly arranged. 
 
 Relation between Plutonic and Volcanic Rocks. If lavas are 
 poured out on the surface, we may fairly conclude that parent masses 
 of the same rocks consolidate beneath the surface under different con- 
 ditions of temperature, pressure, and liquefaction. And if corre- 
 spondence in chemical composition can be established between certain 
 
 1 " Phillip's Mineralogy," by Brooke and Miller p. 302. 
 VOL. I. O 
 
210 
 
 OF SANDSTONES. 
 
 Neocomian Sandstone, 
 
 -H i^>. t*N. i_o C3 co vo 
 
 00 -i ~> co . . . -3- . Tf ON 
 
 00 Ti- iovo 
 rj-OO OO >O 
 
 Hunstanton. 
 
 C4 
 
 ON 10 ON O ' ' O 'MO 
 ^t" IN 
 
 O O covo 
 
 
 
 Bunter Sandstone. 
 
 10 TJ- 10 OOC^O OOOO 
 
 MONco:c\)o)o .voo 
 
 t-. rj- vo O 
 
 1 
 
 i^io . 0\iC . o" .'SjvO 
 
 1-1 i_i O vo 
 
 ON 1-1 Tj-oo 
 
 | 
 
 c^^ " ~ ' 
 
 o o o 
 
 Millstone Grit Sandstone. 
 
 h 
 
 O 
 
 O ON vo O t^-* v O 3) t^H rooO 
 ^" ON . CO cooo O ^ <S coo 
 
 T}- ro O ON 
 r^ co O c^ 
 
 1 
 
 iCSJ SJo . . !? ^:^ 
 
 0*w ^R 
 
 K 
 
 joooo -oo|oo 
 
 w o 
 
 Cambrian Grit, Barmouth. 
 
 OOor^ NUJt^co rooo 
 voc^Oro .O^ONcooi 
 
 v^m^ S 
 
 
 OONN ^^QO^- 
 
 
 
 
 
 
 
 
 
 
 
 
 
 'i'i!l|||| '.g 
 lllllllllll 
 
 J *wl 
 
 Brown's Hole, Eocene. 
 
 ^- 10 O .OO^J- P^ OO 
 
 
 a 
 
 Battle Mountain Carboni- 
 
 ^2" co ^ rj M ^ 
 
 
 a, * erous - 
 
 10 >o co ' MOO'-' 
 
 
 Near Cottonwood Canon. 
 
 xot^O 'OOfOOO 
 
 
 
 voc-jvo .t-^t^voUr^ 
 
 
 
 ONt^co 'OO-tf-^rO 
 
 
 Between Cottonwood and 
 <j Unionville Canons, Tdassic. 
 
 r* . M r^ 'P r r 1 ^^ 
 V V b '<-> b ^o b -> 
 t** ti 
 
 
 
 
 
 
 
 
 
 "il| '-I ' 
 
 ililillil 
 
 in << FR PR H! ^ PH cc 
 
 
RELATION OF RHYOLITES TO SANDSTONES. 211 
 
 lavas and plutonic rocks, we are justified in believing that they are 
 connected in origin. This connection is at present best made out in 
 the case of the granites and felsites or altered rhyolites. For, first, 
 there are the experiments of Sir James Hall and others, proving that 
 when granite is melted and cooled slowly it forms a substance like 
 felsite, and when it cools more rapidly the product is a glass like 
 obsidian. And, secondly, there are the descriptions by Professor 
 Judd of the tertiary felsitic rocks of the inner Hebrides, which can 
 be shown to have originated from extinct volcanoes of which the 
 central cores are granites ; and the same fact is demonstrated in 
 the primary period, by the occurrence of felsites in Scotland under 
 circumstances which show that they have been derived from granite 
 masses. Therefore the step is sufficiently probable which would 
 lead us to derive felsites and all the imperfectly crystallised or glassy 
 forms of granitic rocks from the metamorphism or melting up of 
 certain clays, and their extrusion or ejection on the earth's surface ; 
 but some may have been derived from sandstones. 
 
 Igneous Rocks related to Sandstones. There are no igneous 
 rocks which correspond entirely in chemical composition with the 
 few sandstones which have been examined. Those analysed by Mr. 
 John Arthur Phillips all contain a smaller percentage of alumina 
 than is usual in rhyolites ; but rhyolites vary sufficiently among them- 
 selves in this respect, as may be seen from the analyses collected by 
 Justus Roth, 1 to justify a comparison. The tertiary rhyolite, described 
 by Mr. Hardman, from Tardree, near Antrim, has, however, but 5-10 
 per cent, of alumina to 76*96 per cent, of silica. The analyses of 
 American sandstones correspond with rhyolites very closely ; the only 
 difference being a slight variation in the proportions of potash, and 
 
 
 
 Rhyolite of 
 Mont 
 Dore. 
 
 Cottonwood 
 Triassic 
 
 Sandstone. 
 
 Silica . 
 Alumina 
 Peroxide of iron 
 Manganese 
 Lime 
 Magnesia 
 Soda 2 . 
 Potash 
 
 74-80 
 
 I4 ! 47 
 1-03 
 trace 
 0'43 
 
 6-63 
 1-69 
 
 7474 
 14-14 
 079 
 
 i V 6i 
 0-39 
 0*92 
 
 V2Q 
 
 Water 
 
 CO 2 trace 
 0^96 
 
 i' ; 88 
 
 
 
 
 soda. "We therefore suggest that rhyolites are often sandstones which 
 have been liquefied and erupted. The way in which the quartz 
 
 1 " Beitriige zur Petrographie der plutonischen Gesteine." 4to. 1873. 
 
 2 In the Rhyolite of Otting, the proportions of the alkalies are reversed, being 
 soda 1-55, potash 5-25. 
 
212 
 
 CLASSIFICATION OF IGNEOUS ROCKS. 
 
 occurs in grains, in this and certain other volcanic rocks, is probably 
 not entirely unconnected with the occurrence of quartz in separate 
 grains in the rock which was metamorphosed ; while, in some cases, 
 we attribute the formation of glass inclusions to the action of heat 
 upon the fluid inclusions in quartz grains, by which the water and 
 alkaline chloride contents have fused the silica with which they were 
 in contact, so as to convert the fluid cavity into a glass " inclusion." 
 If rhyolites are typically the lavas of granites, and do not form a 
 distinct and more highly silicious group, it is because of the diver- 
 sity of rocks included under the name granite ; and just as certain 
 sandstones graduate into certain clays, so certain rhyolites repre- 
 sent some highly acidic granites. 
 
 Zirkel's Classification of Igneous Rocks. 1 Zirkel has always 
 recognised the tertiary lavas as representative in time of the older 
 lavas, and as connected with plutonic rocks. His classification of 
 these rocks, according to the predominant or typical mineral of the 
 felspar family, expresses chemical as w r ell as mineralogical differences 
 between the chief groups, and the affinities which he believes to 
 exist between the older and newer rocks. 
 
 With Quartz or excess of 
 Silica. 
 
 Granite. 
 
 Granite Porphyry. 
 
 Felsite Porphyry. 
 
 Rhyolite. 
 
 Obsidian. 
 
 Perlite. 
 
 Pumice. 
 
 Pitchstone. 
 
 With Hornblende. 
 
 Quartz-Diorite. 
 
 Diorite. 
 
 Porphyrite. 
 
 Hornblende-Porphyrite. 
 
 Quartz-Propylite. 
 
 Propylite. 
 
 Dacite. 
 
 Hornblende - Andesite. 
 
 /. Orthoclase Rocks. 
 
 Without Quartz, with Plagio- 
 clase. 
 
 Syenite. 
 Augite-Syenite. 
 Quartzless Orthoclase 
 Porphyry. 
 
 Trachyte. 
 
 Augite-Trachyte. 
 
 //. Playioclase Rocks. 
 
 With Augite. 
 Diabase. 
 
 Augite-Porphyry. 
 Melaphyre. 
 
 Augite-Andesite. 
 
 Felspar-Basalt. 
 
 Tachylyte. 
 
 Without Quartz, with Nephe- 
 liue or Leucite. 
 
 Foyaite. 
 
 Miascite. 
 
 Orth oclase -Porphyry. 
 
 Phonolite. 
 
 Leucite rock. 
 
 Sanidine rock. 
 
 With Mica. 
 Mica-Diorite. 
 
 With Diallage. 
 Gabbro. 
 
 With Hypersthene. 
 Hypersthenite. 
 
 With Olivine. 
 Serpentine (Forellenstein). 
 
 777. Nepheline Rocks. 
 Nephelinite. 
 Nepheline Basalt. 
 
 IV. Leucite Rocks. 
 Sanidine Leucite Rocks. 
 Leucite Basalt. 
 
 The Basic Series of Volcanic Rocks may be similarly paralleled 
 by analyses of shales, clays, slates, and schists, see p. 225 and p. 285 ; 
 
 1 U. S. Geol. Explor. Fortieth Parallel, vol. vi. p. 6, 1876; compare also 
 Zirkel, "Lerhbuch der Petrographie," vol. i. p. 450, 1866. 
 
THR 
 
 EXPERIMENTAL FORMATION OF IGNEOUS R0(. 
 
 3. . 
 
 and as closely as rhyolites and granites, though the number of analyses 
 of sedimentary and foliated rocks hitherto made is too few to demon- 
 strate correspondence with all the varieties of igneous rocks which 
 might be selected. This deficiency may be remedied hereafter. But 
 if the views enunciated are sufficiently supported by facts, further 
 details will become available, and repeat the general principles which 
 we have attempted to exemplify. 
 
 It remains to be shown that rocks and minerals can be produced 
 artificially. 
 
 Experimental Formation of Volcanic Rocks. Messieurs Fouque" 
 and Levy by a series of ingenious experiments have succeeded in 
 forming volcanic rocks artificially, not indeed by transforming sedi- 
 mentary deposits, but by heating together constituent minerals of the 
 rocks. Thus andesite and andesite porphyry are obtained by com- 
 bining four parts of oligoclase with one part of augite, and heating 
 them together for three days in a platinum crucible half embedded 
 in the furnace. In this process oxide of iron may be produced at the 
 expense of the augite. When a little lime is added microliths of 
 labradorite are formed, and augite forms more abundantly round these 
 than round the oligoclase. In a similar way augite-andesite has been 
 obtained by mixing ten parts of oligoclase and one part of amphibole, 
 when the amphibole is changed into pyroxene. Basalts have been 
 made artificially by a double heating of a black glass, having a composi- 
 tion which corresponds to 6 of olivine, 2 of augite, and 6 of labradorite. 
 This is raised to a white heat, maintained for forty-eight hours, above 
 the melting point of pyroxene and labradorite, when peridote is formed 
 in a brown glass. Subsequently the mass is exposed for forty-eight 
 hours to a red heat, when microliths of labradorite, augite, iron oxide, 
 and picotite are formed. 
 
 By combining nepheline and augite in the proportion of 3 to i "3, 
 nephelinite results after two days' heating ; but when the augite is in 
 the proportion of i to 10, it gives rise to octohedrons of spinelle and 
 dodecahedrons of melanite garnet. In the same way 9 parts of leucite 
 are combined with i part of augite and heated for three days, when 
 crystals of leucite were found to be surrounded by augite and oxide 
 of iron. In this production of leucitite double fusion is necessary, in 
 consequence of the different fusibility of leucite and pyroxene. The 
 formation of Iherzolite is more difficult and less complete. And from 
 negative results, Fouque and Le"vy infer that the rocks containing 
 quartz, orthoclase, black mica, and amphibole, have not been formed by 
 the agency of heat alone. For the methods of artificial formation of 
 the minerals which enter into the composition of igneous rocks, we 
 must refer to " Synthese des Mine"raux et des Roches " by these 
 authors. 1 
 
 1 The three preceding chapters are the matter of Royal Institution lectures, 
 delivered March 15, 25, and April i, 1882. 
 
CHAPTER XV. 
 
 THE GRANITIC OR PLUTONIC GROUP OF ROCKS. 
 
 FROM a geological point of view it is convenient to associate together 
 all the deep-seated rocks which are completely crystallised, for they 
 occur under similar conditions. The term granitic applied to these 
 rocks, which have a granular texture like granite, merely indicates 
 similar conditions of solidification for the rocks which are associated 
 under that name. From the point of view of evolution it would be 
 more natural to associate the volcanic rocks with their corresponding 
 plutonic representatives. The student in a granitic district will not, 
 however, usually be able to trace the granite into rhyolite, the syenite 
 into trachyte, or diorite into andesite, even if there be some indica- 
 tions of a passage from gabbro into dolerite. Hence, though impor- 
 tant questions of theory are involved in establishing the unity of the 
 plutonic and volcanic series, no practical inconvenience will be found 
 in grouping together the massive igneous rocks, which form axes of 
 upheaval, and have been exposed at the surface by denudation. 
 
 Mineral Composition of Granite. The term granite in its modem 
 sense was first used by Werner. 1 As already indicated, this rock is 
 typically a crystalline mixture of orthoclase (and oligoclase), quartz, and 
 mica, varying in texture with the size of the crystals, which are some- 
 times as fine as mustard- seed, and sometimes as large as the closed 
 fist. When the crystals are large the texture is more irregular. 
 When an un crystalline matrix separates the crystals from each other, 
 the rock is passing into felsite, a term still conveniently used for 
 altered rhyolitic rocks. 
 
 The orthoclase constituent, which usually occurs in twin crystals, 
 cleaves with a pearly fracture and gives granite its characteristic 
 colour, being pink, red, or brownish red, reddish brown, white, 
 yellowish grey, green, or reddish grey, and is even blue in Connecticut 
 and the Pyrenees. 
 
 The oligoclase is less transparent, contains more soda than ortho- 
 clase, is more fusible, and has a grey or greenish tinge. In some 
 granites, these felspars are in about equal quantity. Frequently in 
 Scotch granites the oligoclase surrounds the orthoclase crystals as a 
 rind. Professor Zirkel points out that at Schreibershau, in the 
 
 1 See Zirkel's " Petrographie, " 1866, a general history of plutonic rocks, in- 
 dispensable to the student, from which we have quoted numerous facts. 
 
THE MINERALS IN GRANITE. 215 
 
 Riesengebirge, the orthoclase is flesh-red and the oligoclase snow- 
 white, as in some Scotch granites ; and at Viborg, in Finland, the 
 orthoclase is flesh-red and the oligoclase green. Though in such 
 cases the oligoclase is formed subsequently, yet in the granite of 
 Beyrode in the Auvergne, the two minerals alternate in the same 
 crystal, or the oligoclase may have been first formed. 
 
 Albite also has been recognised in granite, especially in the Mourne 
 Mountains, and the chemical composition appears to indicate the exis- 
 tence of labradorite and other kinds of felspar in some localities, espe- 
 cially in Ireland, and at Strontian. 
 
 Quartz usually occurs in more or less angular grains, but not often 
 with the crystalline faces perfectly developed. It varies in colour 
 like the felspar, being blue in Monte-Rosa and sometimes blue in 
 the Mourne Mountains, and red in the Jagernthal in the Vosges, 
 though commonly colourless. At Gablonz in Bohemia the quartz 
 crystals are larger than the orthoclase. 
 
 The mica generally occurs in thin plates which are often hexagonal. 
 Crystals are rare. It varies in colour, being silvery white, brown, or 
 black. The white potash mica is more diffused than the black mag- 
 nesian mica, which is brown or green in polarised light. Both kinds 
 often occur together. At Penig, in Saxony, the mica is olive-green. 
 Certain large-grained granites contain lithia mica. Haughton recog- 
 nises the silver-grey mica margarodite in the granite of the south-east 
 of Ireland, and other varieties of the mineral in other localities. As 
 is well known, these minerals are usually mixed together without any 
 trace of a schistose arrangement. 
 
 Mica is the most variable, element in granite. It is more or less 
 replaced in the Alps and some parts of the Schwarzwald by talc. In 
 the granites of the Pyrenees graphite is associated with mica. Some- 
 times hornblende is associated with magnesia mica, forming a syenitic 
 granite. Apatite is generally present. Zircon is occasionally found. 
 In porphyritic granite large crystals are only formed by the orthoclase : 
 the well-known Carlsbad twins are among the finest examples of these, 
 sometimes reaching in the Pyrenees a length of six inches. Occasion- 
 ally the crystals are broken and reunited. 
 
 Granite is usually compact, but sometimes porous, and in South 
 America allows water to pass through it so freely as to be used for 
 filters. And it may contain cavities which have the walls covered 
 with crystals of the chief constituent minerals as well as various acces- 
 sory minerals, as may be seen at Lugano, Baveno, Mourne, and in 
 many localities for European granites. 1 
 
 Chemical Variation in Composition. The silica in granite varies 
 from 62 to 82 per cent. ; the alumina from 7 to 19 per cent. ; the iron 
 oxides from less than a quarter per cent, to 6 per cent. ; lime from 
 13 per cent, to 5-5 per cent. Magnesia may amount to 2 per cent., 
 or show little more than a trace; potash varies from 2 to 7 per cent., 
 
 1 For a list of the accessory minerals found in the granites of different locali- 
 ties the student should consult Zirkel's " Le irbuch der Petrographie," vol. i 
 p. 481. 
 
216 VARIETIES OF GRANITIC ROCKS. 
 
 and soda varies from a trace to 6 -3 per cent. Water may be absent, 
 and never amounts to more than 2 per cent. 
 
 Chemical Classification of Granites. Haughton proposed in 
 I856 1 to divide granites into a potash group and a soda group, 
 because the percentage of these alkalies not only determines the 
 kind of felspar in the rock, but usually has a relation to the percent- 
 age of silica. Thus in the granite of Croghan Kinshela, in Wexford, 
 the soda amounts to 5-58 as against '4 of potash, while the silica is 
 So per cent. At Baveno the soda amounts to 6-12 and the potash to 
 3 '5 5 per cent., while the silica is 74*82. When the percentage of 
 potash increases, the amount of silica frequently diminishes, though 
 there is no necessary relation between these substances. 
 
 Mineral Varieties of Granite. The percentage of quartz in 
 granites varies, according to Haughton, from 20*0 to 35*0, though 
 Delesse states it as high as 60 'o in a granite of the Vosges. 
 
 The mica varies from 4*0 to 27*0 in Irish granites, while at Tholy, 
 in the Vosges, it is stated to be only one per cent. 
 
 The felspar varies from 40*0 to 69*0 in Irish examples, though the 
 highly quartzose granite of the Vosges, already referred to, has but 
 35-0 per cent. 
 
 So great are the variations in mineral composition of granites that 
 it becomes necessary to recognise the value of adventitious minerals 
 in distinguishing local modifications of the rock. Besides the type, 
 the following varieties may be defined : 
 
 Granitite, formed of red orthoclase, much oligoclase, little quartz, 
 and little dark-green magnesian mica. It is especially distinguished 
 by the absence of potash mica, the predominance of oligoclase, and the 
 reduced importance of the quartz. It contains magnetic iron and 
 titanic iron. Many Irish granites are intermediate between the typical 
 granites and granitite. In British geology the rock has no certain 
 representative, and the term is chiefly used in the Kiesengebirge, 
 Harz, &c. Augite-bearing granitite occurs in dykes in the metamor- 
 phic rocks of Laveline in the Vosges, at Titisee in the Schwarzwald, 
 &c. It is less common than hornblendic granitite. The augite is 
 formed in green prismatic crystals or crystalloids. 
 
 Protogine, or talc granite of the Alps, has the same composition as 
 granite, but contains in addition a pale green talc-like mineral. Its 
 quartz is easily broken. The oligoclase has a greenish tinge, while the 
 orthoclase is grey. The mica is usually in six-sided plates. The talc 
 is only freely developed when the rock becomes schistose. It is well 
 seen in Mont Blanc and the Western Alps. Between Schneeberg and 
 Eibenstock, in the Erzgebirge, it contains no mica, and the felspar is 
 flesh-red. 
 
 Syenitic Granite or hornblendic granite is intermediate between 
 granite and syenite. It contains less quartz than granite, and horn- 
 blende to a large extent takes the place of the mica, which is always 
 dark. It forms the summits and central parts of the Vosges. The 
 accidental minerals are sphene, zircon, chlorite, iron pyrites. In 
 1 Quart. Jour. Geol. Soc , vol. xii. p. 177. 
 
STRUCTURE OF GRANITE. 217 
 
 Jersey the syenitic granite is red. The red rock at Syene, in Egypt, 
 is a syenitic granite. Characteristic syenitic granite is seen at Stron- 
 tian, in Argyleshire. 
 
 Gneissose Granite is granite which has a schistose character. 
 
 Graphic Granite is also schistose, but consists of orthoclase and 
 quartz, so arranged in parallel layers that a transverse fracture 
 exhibits the quartz in forms, suggesting letters of an Oriental lan- 
 guage. It occurs near Ilmenau, and by Limoges, &c. 
 
 Pegmatite is a kind of giant granite, in which the crystals of 
 orthoclase are sometimes a foot long, and the white mica occurs in 
 large flakes. It is only known in other granite, and generally contains 
 tourmaline, garnet, topaz, &c. It is seen near Penig, in Saxony. 
 Sometimes the greater part of the rock is formed in a milk-white 
 quartz. It occurs in Ireland, according to Zirkel, in Carlingford Bay, 
 and is frequently cavernous, with the walls of the cavities covered 
 with crystals. 
 
 Haplite consists almost entirely of orthoclase and quartz, with very 
 little mica. It is seen at Gottleube in Saxony. 
 
 Tourmaline Granite is granite in which the mica is partly replaced 
 by schorl. It is seen in Cornwall, and is also known as Luxulianite. 
 The felspar is flesh-coloured, and there is very little quartz. 
 
 Beresite is a variety of granite, rich in iron-pyrites, and poor in 
 mica. It occurs in dykes at Beresowsk, in the Ural mountains. 
 
 Many other local varieties occur. Thus, in the Fichtelgebirge, at 
 Vordorf, the granite is rich in Epidote, and in Cornwall granite con- 
 tains tin. 
 
 Joints in Granite. Granite is generally characterised by joints 
 or division planes. In Cornwall they usually run from N.jST.W. to 
 S.S.E., or at right angles to the direction in which the granitic masses 
 extend through the country. Near the Land's End, especially off the 
 coast and on the tops of the moors, the granite has a rude columnar 
 structure. In the Harz Mountains there are three principal divi- 
 sion planes, one in the direction in which the rock extends, another 
 at right angles to this direction, and a third which is horizontal and 
 less developed. In the province of Constantine, in Algeria, granite 
 is found in regular columns, with five or six sides, which, at a distance, 
 have the aspect of basalt. 
 
 In the Riesengebirge and Fichtelgebirge granite often has a 
 spheroidal structure, with the spheres ranging in size from 2 inches 
 to 2 feet. The structure is concentric, and the kernel is sometimes 
 formed of crystals of orthoclase. ISTear Oporto concretions in granite 
 are 50 feet in diameter. 
 
 Decomposition of Granite. The decomposition of granite is well 
 seen in Cornwall, especially near St. Austell, and at Cornwood, in 
 Devonshire. It is even more marked at St. Yrieix, south of Limoges. 
 Decomposition results from the solvent action of carbonic acid, dis- 
 solved in water, acting upon the soda, potash, magnesia, lime, iron, 
 or other soluble constituent of the rock, resulting in the production 
 of a friable rock called arkose. 
 
2i8 INCLUSIONS AND CONCRETIONS IN GRANITE. 
 
 Inclusions in Granite generally have an irregular polyhedral form. 
 They usually belong to the schistose rocks. The larger fragments 
 are sharply angular, but many are rounded, like rolled stones and 
 boulders, some of which, according to Zirkel, have a length and 
 breadth of many thousand feet ; one of the largest occurs between 
 Carlsbad and Eibenstock. Sometimes granite veins pass through 
 these inclusions ; one such, first instanced by Dr. Forbes, is seen in 
 Whitesand Bay at Land's End. 1 Sometimes the smaller included 
 fragments, which often lie parallel to each other, are so abundant as 
 almost to give the granite the aspect of a breccia. In the Fichtelge- 
 birge, near Reizenstein, the granite is so blended with clay slate as to 
 form a fine brecciated mass, and as a rule the included fragments 
 belong to the neighbouring rocks ; and varying with the locality, 
 comprise gneiss, mica schist, clay slate, &c., which have undergone a 
 further metamorphism. 
 
 Limestone has been found in granite in the Pyrenees, and can be 
 readily identified with the deposit from which it was derived. These 
 fragments have a crystalline structure, which is better developed on 
 the surface than in the interior ; and while the latter retains its dark 
 colour, the surface for the depth of an inch is a snow-white marble. 
 
 Zirkel remarks that these inclusions must be carefully distinguished 
 from the small concretions in granite, which usually have a finer 
 texture, and abound in mica. 
 
 Concretions in Granite. Mr. John Arthur Phillips observes that' 
 a concretionary patch generally resembles an enclosed pebble, and 
 usually has the outline clearly defined. Such masses, well seen in 
 granite buildings, often enclose crystals of felspar similar to that in 
 the surrounding granite. 
 
 In the granite of Lamorna, fine-grained black inclusions are very 
 abundant ; they are sometimes traversed by grains of the surrounding 
 granite, and may also contain large crystals of quartz. The felspar 
 is partially orthoclase with a considerable proportion of plagioclase. 
 The orthoclase often encloses grains of quartz and patches of triclinia 
 felspar. The quartz contains the usual fluid cavities ; the mica is 
 chiefly dark brown, and often penetrated by well-formed crystals of 
 magnetite. This granite contains light-brown tourmaline and a few 
 small crystals of apatite. 
 
 The pebble-like masses when examined under the microscope are 
 found to be a granular mixture of quartz with felspar and an abun- 
 dance of dark mica ; but the plates of mica are mostly parallel. 
 Imperfect garnets are sometimes found in these concretionary patches. 
 
 The grey granite of Penryn is almost free from patches of this 
 kind, and they have not been observed in the granites of St. Austell. 
 At Gready, in the parish of Luxulian, the granite contains spheroidal 
 bodies which resemble water- worn pebbles of fine-grained greenstone. 
 These concretions show the black and silvery-white micas, often 
 inter-laminated, but differ chiefly from the granite in a finer grain, and 
 greater abundance of black mica. On analysis the chief difference 
 1 "Treatise on Primary Geology," Henry S. Boase, 1834. 
 
GRANITE VEINS. 219 
 
 is that the concretion contains less silica, more iron, more lime, more 
 soda, and less potash. There are no inclusions or concretions in the 
 Cheese-ring granite near Liskeard. In several localities, as at Foggen 
 Tor, near the Prince's Town prison on Dartmoor, concretionary patches 
 occur in which the outlines are not sharply defined. 
 
 Similarly the granite of Westmoreland, at Shap, frequently ex- 
 hibits rounded patches of a dark colour, in which Mr. Phillips has 
 detected crystals of felspar, partly contained in the concretion and 
 partly in the granite, 1 as may be seen in the St. Pancras Railway 
 Station. These concretions vary in size from a pea to a water-melon, 
 and, like all the others, owe their dark colour to the abundance of 
 mica. They have a ground mass of quartz, felspar, and mica, and con- 
 tain accidental minerals such as magnetite, titanite, apatite, and horn- 
 blende. Occasionally, however, the patches, owing to the absence of 
 black mica, are lighter in colour than the surrounding rock. 
 
 In the granite of Aberdeen, the pebble-like concretions are almost 
 unknown. This granite consists of orthoclase, a large proportion of 
 oligoclase, quartz, white and black mica, minute garnets, and occasional 
 crystals of apatite and sphene. The quartz is frequently traversed by 
 hair-like crystals of rutile. Fragments of schistose rocks sometimes 
 occur. Mr. Phillips describes one lenticular mass weighing over a 
 hundredweight found in the Dyce quarry, north-west of Aberdeen. It 
 was covered with a layer of mica so as readily to separate from the 
 rock. On being broken the mass consisted of granite half as fine 
 again in texture as the surrounding rock, and only differed in contain- 
 ing more oligoclase. Near Peterhead, ovoid concretions from the size 
 of a nut to that of an apple are found. They are nearly always dark, 
 fine in texture, and sharply defined. On analysis they contain less 
 silica and less potash, but more iron, lime, and alumina than the 
 surrounding rock. The granite of Ballachulish contains both concre- 
 tions and fragments .of foliated rocks. Many of the Irish granites 
 abound in dark-coloured concretions. 2 
 
 Granite Veins. The size of granite veins is very variable. They 
 may be fine and interlace like a network. Such, are well known in 
 Glen Tilt. At Wicca Pool they often enclose pieces of mica slate. 
 They are numerous in Cornwall. On the Continent they are well seen 
 on the south side of the Brocken in the Harz. Sometimes in such 
 veins the mica disappears, and further on occasionally the felspar also 
 disappears, so as to leave at last nothing but veins of quartz at the 
 termination. These changes are well seen in Arran, in the Valley of 
 the Drummond, and at St. Marie in the Pyrenees. Granite veins 
 three miles long have been described as penetrating the granite near 
 Rossau in Saxony. The texture is often coarse in the middle of a 
 large vein, but it almost invariably becomes fine-grained or felsitic 
 towards the border, showing that the granite was consolidated before 
 the vein penetrated into it. Occasionally, however, the vein may 
 have a coarse texture, while the granite which it penetrates is fine. 
 
 1 Quart. Jour. Geol. Soc., vol. xxxviii. p. 217. 
 8 J. A. Phillips, Q. J. G. S., vol. xxxvi. p. i. 
 
220 LOCALITIES FOR GRANITE. 
 
 Granite is sometimes supposed to be formed at successive periods 
 in a mountain chain, and in the ThuringerWald, Credner defines three 
 granites having such relations to each other. 
 
 Contact Metamorphism. In the island of Skye, where the granite 
 comes in contact with limestone, the latter rock is changed into a 
 crystalline marble. Many minerals are developed in the limestone 
 near the contact, but the most common are garnet, vesuvian, epidote, 
 spinelle, hornblende, augite, and mica. Clay slate is modified in 
 Cornwall, between Constantino and Penryn, into mica schist and 
 gneissose rock, where it comes into contact with granite, with the 
 development of tourmaline, chiastolite, and other minerals. Similar 
 phenomena are seen in the Pyrenees, Saxony, Brittany, and most of 
 the granitic localities. Hornstone is sometimes produced at the con- 
 tact by a process of silicification of very fine-grained sandy rocks, 
 though ordinary sandstones become altered into quartzites. 
 
 Modes of Occurrence of Granite. In the south of Russia there is 
 an elliptical granitic area of nearly 4000 square miles. It reaches from 
 Owracz in Volhynia in a S.E. direction to the neighbourhood of Tagan- 
 rog, and in a westerly direction stretches nearly to Brody in Galicia. 
 In Saxony, between Gorlitz and the Georgenthal in Bohemia, there 
 is an area of forty square miles of granite. Between the Tagus and 
 Guadiana there is a granite plateau. In those cases the granite is 
 assumed to be horizontal. And many instances are quoted by Zirkel 
 in which granite is an overlying rock, and follows the undulations of 
 slates and schists on which it rests. Among these are the well-known 
 instance on the banks of the Irtish in Siberia, originally described by 
 Von Humboldt ; Marhallac's examples in the islands of Milhau in the 
 the Cote du Nord in France ; and others in the Harz and the Erzge- 
 birge. But granite more frequently occurs in consecutive masses like 
 chains of islands which have been exposed by denudation, and are 
 elevated with the great folds of the earth's crust. 
 
 The principal granitic districts in Europe comprise : 
 
 FRANCE. Eastern part of the Vosges, much of the high land of the 
 Auvergne, the district between JS T antes and Parthenay, the Pyrenees, 
 and Brittany. 
 
 GERMANY and AUSTRIA. West of the Schwarzwald, in the Oden- 
 wald, south of the Thuringerwald, in the Harz, much of the Eich- 
 telgebirge, several areas in the Erzgeberge, Oberlausitz, in Bohemia, 
 the Biesengebirge, the Sudetic Alps, the highest peaks of the Tatra 
 in the Carpathians, the Bohmerwald. 
 
 SWITZERLAND and ITALY. Mount Blanc, St. Gothard, &c., Velte- 
 line Alps, Trientine Alps, &c., Corsica and Elba. 
 
 SPAIN. N.W. province of Galicia, the Sommo-Sierra, the Gua- 
 darrama Mountains, the Sierra Morena. 
 
 SCANDINAVIA. A large part of the peninsula. 
 
 RUSSIA. East side of the Ural, and a large area in the south. 
 
 Varieties of Granite in the United States. The granites of 
 North America are classed by Clarence King into eruptive and meta- 
 morphic, and he groups the eruptive series into four divisions. The 
 
GEOLOGICAL ANTIQUITY OF GRANITE. 221 
 
 first type or muscovite granite comprises granites formed of quartz, 
 orthoclase, minute and unimportant crystals of plagioclase, and mus- 
 covite, with a small percentage of microscopic apatite. This granite 
 lies to the west of Reese river, long. 117 W. It is seen in Nevada, 
 in the Kavenswood hills, in the Shoshone range, in the Pah-tson 
 mountains, in the Truckee range. 
 
 The second type or biotite granite consists of quartz, orthoclase, 
 little or no plagioclase, and biotite, with microscopic apatite. It is 
 seen in the Ombe range, west of Salt Lake, in Nannie's Peak in the 
 Seetoga range, at Mount Tenaho in the Cortez range, on the Wah- 
 weah mountains, in the Montezuma range and in the Truckee range, 
 where it is associated with the muscovite granite. 
 
 The third type or hornblende granite consists of quartz, orthoclase, 
 little or no plagioclase, biotite and hornblende, with microscopic 
 apatite. Its distribution corresponds very much with that of the 
 biotite granite, with which it is often in close proximity. It is well 
 seen in Granite Canon in the Cortes range, and at Granite point in 
 the Augusta mountains. 
 
 The fourth type or plagioclase granite consists of quartz, plagio- 
 clase, orthoclase, and a large percentage of biotite, hornblende, tita- 
 nite, and apatite. The plagioclase often equals and sometimes exceeds 
 the quantity of orthoclase. Such a granite approximates towards the 
 diorites. 1 
 
 Geological Age of Granite. The age of granite is always newer 
 than the rock which it penetrates, and older than a stratum deposited 
 upon it. It is rare to be able to fix both of these limits of age. 
 ]>ut the more ancient or Silurian granites are found in the Harz, 
 Thuringerwald, Saxon Erzgebirge, Yosges, Christiania in Norway. 
 The granite of Cornwall and Devon is of post-carboniferous date, as 
 also that of Arran. The protogine granite of the Alps is newer than 
 the Lias. And at Predazzo in the Tyrol there are granites of 
 secondary age, and other instances have been quoted near Champo- 
 leon in France, of granite overlying and altering secondary lime- 
 stones. In the Pyrenees both Liassic and Cretaceous limestones are 
 altered by contact with granite. In the Banat the granite is of 
 tertiary age, and similar examples are quoted by Darwin in Chile, 
 and Sawkins in Jamaica. 
 
 Syenite. 
 
 Syenite is a kind of granite which is typically free from quartz. 
 Orthoclase is the predominant mineral, and is associated with horn- 
 blende, biotite, and augite in proportions which vary with the locality. 
 When muscovite appears it is always as a secondary product, due to 
 decomposition of the orthoclase. The minerals in syenite are arranged 
 in the same way as in granite. 
 
 The orthoclase in large-grained syenites is often rich in colour- 
 less microliths and small plates of specular iron. Sometimes fluid 
 1 Clarence King, TJ. S. Geol. Explor., 4Oth Parallel, vol. i 
 
222 DISTRIBUTION OF SYENITE. 
 
 inclusions contain small cubic crystals. The orthoclase twins gene- 
 rally follow the Carlsbad law ; they are sometimes white, but commonly 
 red. In many Scandinavian syenites and in the Thuringerwald, the 
 mineral has a blue lustre. Plagioclase, when present, resembles that 
 of granite. In augite-syenite, plagioclase is better preserved than 
 orthoclase. 
 
 Hornblende is usually green ; when it is brown, as at Laurvig ill 
 Norway, the brown colour is deep as in trachytes and andesites. It 
 commonly occurs in lamellar or columnar crystals, and encloses mag- 
 netite, apatite, brown mica, and titanite. 
 
 The biotite may be green or brown, or both colours occur in the 
 same film. It is associated with the hornblende. The crystals or 
 irregular grains of augite are usually green, but when the rock is very 
 rich in plagioclase, the colour is yellowish brown. Quartz occurs 
 occasionally, and is probably an accessory mineral due to alteration of 
 the orthoclase. 
 
 The predominant mineral composition indicates the division of 
 syenite into three types, which may be termed hornblende-syenite, 
 mica-syenite, and augite-syenite. 
 
 Geographical Distribution of European Syenites. Among the 
 European localities for syenite are Plauen, near Dresden, many places 
 on the southern slope of the Thuringerwald, in the Odenwald, Meis- 
 sen, in Saxony ; in Moravia it extends 30 miles from south of Kienitz, 
 through Brunn, to north of Boskowitz ; in the mountains of Lower 
 Silesia, a large mass of syenite extends from Glatz to Ullersdorf. A 
 rock of syenitic character, classed by Zirkel as a syenite-granite-por- 
 phyry, stretches from north to south in the east of the Banat from 
 Kudernatch to Moldawa. A somewhat similar rock occurs in the 
 Bihargebirge in south-east Hungary, penetrating Neocomian rocks. 
 In the Vosges, massive syenite appears between Windstein and Ballow, 
 north of Geromaguy. In the Tyrol it forms the centre of the eruptive 
 mass at Predazzo, and the great mountain mass of Monzoni, charac- 
 terised by red orthoclase, white oligoclase, with films of hornblende 
 and brown mica. At Monzoni, the Triassic limestone is converted for 
 a thickness of 100 feet into crystalline limestone, with the develop- 
 ment of many accessory minerals. In the South of Norway, syenite 
 is seen around Christiania, penetrating slates and limestones, and in 
 Finland it occurs near Viborg. 
 
 North- American Syenite. There is only one exposure of syenite 
 in the region of the 4oth parallel survey. It forms the Cluro Hills 
 in the Cortez Eange, Nevada, and consists of flesh-red orthoclase and 
 greenish hornblende. Under the microscope, indications of plagioclase 
 are detected, and there are microscopic grains of quartz, with fluid 
 inclusions. 
 
 Hornblende Syenite is generally large-grained, and formed of 
 orthoclase and amphibole, with titanite as an accessory. The type is 
 not so common as is usually supposed, but good examples are seen at 
 Plauen and Leuben in Saxony, and Biella in Piedmont. When the 
 rock is decomposed, calcite and epidote often appear as decomposition 
 
MINERAL VARIETIES OF SYENITE. 223 
 
 products. And when syenite forms dykes, it contains no titanite and 
 less plagioclase than massive syenite. As in trachytes, of which it 
 is a deep-seated representative, its orthoclase is rich in soda. The 
 varieties of hornblende syenite diverge chiefly towards hornblendic 
 granite and granitite in the Vosges, towards quartz diorite in the 
 Odenwald and Ascheffenburg ; while at Laurvig the rock is inter- 
 mediate between syenite and gabbro, containing olivine, and apparently 
 diallage and hypersthene. 
 
 Mica Syenite. Mica syenite is often termed minette. It usually 
 occurs in dykes. It consists of a fine-grained ground mass of ortho- 
 clase, with biotite. The felspar is rarely fresh, it is white or red, and 
 has assumed a microcrystalline texture. Plagioclase is nearly always 
 absent. Though the mica is usually brown, it is sometimes green, 
 or the colours are combined in the same film ; the crystals are six- 
 sided. Apatite, magnetite, and pyrites are met with. Calcite and 
 quartz occur as decomposition products. The best types of mica-syenite 
 occur in Calabria, in the Vosges, and Odenwald. At Framont this 
 rock contains green hornblende. At Wackenbach it contains blue 
 glaucophane. In Lower Alsace the minettes contain augite, which is 
 sometimes altered into chlorite or delessite. The minettes of the 
 Southern Schwarzwald are rich in augite. Titanite is usually absent 
 from mica-syenites. 
 
 Augite Syenite. This rock consists of orthoclase, plagioclase, 
 and augite, with some titanite, hornblende, biotite, pyrites, magnetite, 
 and apatite. The two felspars vary in relative quantity. The plagio- 
 clase often has a glassy character. The augite is generally green, but 
 occasionally brown, and like the felspar includes the rarer minerals. 
 The green augite often approximates to Uralite, and is sometimes 
 changed to a fibrous chloritic substance. Biotite is generally mixed 
 with augite and hornblende. Titanite only occurs when the rock is 
 massive. Augite syenites occur in the Vosges in dykes. A Uralite 
 syenite occurs in the Ural at Turgojak. 
 
 Foyaite, Elaeolite Syenite. Where the continuation of the Sierra 
 Morena enters Portugal it forms the Sierra de Monchique, the north- 
 west of Algarve. This range consists of Devonian slates and sandstones, 
 through which rise the dome-shaped masses of crystalline rocks known 
 as the Foya, 2968 feet high, and the Picota, 2410 feet high. These 
 crystalline rocks cover an area of about 84 square miles. From Foya 
 this rock is named Foyaite ; it is a nepheline syenite, in which the 
 naked eye easily distinguishes orthoclase, the elaeolite variety of nephe- 
 line, and hornblende. Orthoclase is predominant ; elaeolite shows hex- 
 agonal outlines ; hornblende occurs in long, slender, greenish-black 
 prisms. The accessory minerals are brownish-yellow titanite, dark lamel- 
 lar biotite, magnetite, and occasionally pyrite. Under the microscope, 
 nosean and sodalite occur as accessories with triclinic felspar, muscovite, 
 haematite, and apatite. The orthoclase is nearly always crystallised 
 with layers of oligoclase. The crystals of elaeolite are usually ill- 
 defined. The hornblende and augite are in almost equal quantity, and 
 intergrown with each other. Both are green. The nosean and sodalite 
 
224 THE SYENITE GROUP OF ROCKS. 
 
 are similarly combined, and both are sometimes embedded in the nephe- 
 line. The biotite is brown. When muscovite occurs it is always 
 associated with the felspar and nepheline, and is never combined with 
 the biotite. Titanite is characteristic, and always has a delicate 
 yellow colour. This rock presents a singular chemical correspondence 
 with phonolite, of which it may be regarded as the plutonic represen- 
 tative. Its composition is as follows : 
 
 Analysis of Foyaite. 1 
 
 Silica 56-23 
 
 Sesquioxide of iron . '17 
 
 Protoxide of iron . . 6' 21 
 
 Alumina 2 3' 1 S 
 
 Lime 2*39 
 
 Magnesia .... -40 
 
 Soda 3-84 
 
 Potassa 5-33 
 
 Titanic acid ... '27 
 
 Phosphoric acid . . '13 
 
 Sulphuric acid ... -09 
 
 Chlorine ... . '07 
 
 Water ro6 
 
 Zircon-Syenite. This rock is a granular compound of orthoclase, 
 elseolite, and zircon, with a little hornblende. It is excellently seen 
 in the south of Norway, between the entrance to the Christiania 
 Fjord and Langesand Fjord, stretching north to Skunsfjeld, south of 
 Konigsberg. Another mass occurs north of Christiania, in the island 
 of Seiland in West Finnmark, and at Cape Comfort and Kittiksut in 
 Greenland. 
 
 The orthoclase is large and shows a silvery play of colour, which 
 may be blue, green, yellow, or red. There is some difference of 
 opinion as to the identification of the felspar. The elseolite has a 
 greasy aspect ; the hornblende is black and rich in soda ; quartz is 
 rare, and zircon is found in long columnar crystals. The rock in 
 Maridal contains 66-39 P er cent, of silica and 1379 per cent, of 
 alumina. Fifty accessory minerals have been described from the 
 Norwegian zircon-syenite, and occur chiefly on the margins of the rock 
 as contact minerals, though many are scattered through it. 
 
 Miascite. Miascite is a large-grained granite-like compound of 
 orthoclase, elseolite, and biotite, differing from zircon-syenite in the 
 substitution of black mica for zircon. The felspar is white or grey ; 
 the elaeolite has the aspect of quartz ; and the black mica, in thin 
 plates, is dark green. Sodalite is frequently met with. Hence this 
 rock approaches to some of the nepheline and nosean-bearing phono- 
 lites. Octahedral zircon is sometimes present in miascite as an 
 accessory mineral. Ilmenite occurs in crystals some inches in 
 diameter. 
 
 This rock is chiefly known from Miask in the Ilmengebirge to 
 the east of the Ural, whence it extends north between granite on the 
 east and gneiss on the west. 
 
 A variety which is rich in soda, and contains hornblende and 
 mica, is termed ditroite, from Ditro in Transylvania. 
 
 1 Dr. C. P. Sheibner, Q. J. G. S., vol. xxxv. p. 42. 
 
RELATION OF SYENITE TO SOME SLATES. 
 Analyses comparing Syenite witli Cambrian Slates. 
 
 225 
 
 SYENITE. 
 
 Stiege in the 
 Harz. 
 
 Monte Margola 
 near Pradazzo. 
 
 Weidenthal, 
 near Melibocus. 
 
 Vetakollen, 
 Norway. 
 
 Silica 
 
 56-36 
 
 58'05 
 
 60-97 
 
 62*52 
 
 Alumina . 
 
 20-05 
 
 I7-7I 
 
 16-44 
 
 I4'I3 
 
 Protoxide of iron 
 
 7-96 
 
 8-29 
 
 10-58 
 
 7-38 
 
 Manganese oxide 
 
 
 
 0'08 
 
 
 Lime 
 
 7-22 
 
 5'8l 
 
 5'*4 
 
 3-36 
 
 Magnesia . 
 
 4'12 
 
 2-07 
 
 1-80 
 
 i'5o 
 
 Soda 
 
 170 
 
 3 '24 
 
 0-80 
 
 3^5 
 
 Potash . 
 
 274 
 
 2-98 
 
 3*4i 
 
 6-25 
 
 "Water and loss 
 
 O'62 
 
 i '34 
 
 1-03 
 
 I'2O 
 
 CAMBIUAN SLATES.* 
 
 Welsh Slate. 
 
 Chiastolite Slate, 
 How Gill. 
 
 SkMdaw Slate, 
 Bed Pike. 
 
 Silica 
 
 60-50 
 
 6575 
 
 54'48 
 
 Alumina .... 
 
 1970 
 
 14-18 
 
 20-72 
 
 Peroxide of iron . 
 
 
 trace 
 
 0-98 
 
 Protoxide of iron 
 
 7^3 
 
 7'3o 
 
 8-18 
 
 
 1*12 
 
 1*17 
 
 1-62 
 
 Magnesia .... 
 
 2-20 
 
 2-34 
 
 1-94 
 
 Soda 
 
 2 -2O 
 
 I-q8 
 ~" 
 
 6'2I 
 
 Potash .... 
 
 3'l8 
 
 3-26 
 
 3'20 
 
 Sulphuric acid . 
 Loss on ignition and water . 
 
 3*30 
 
 0-29 
 373 
 
 2-06 
 
 Diorite. 
 
 Dioiites are essentially combinations of plagioclase and horn 
 blende, but in some localities orthoclase, apatite, and magnetite or 
 titanic iron appear as subordinate constituents ; while titanite, mag- 
 nesia-mica, and pyrites are sometimes present as accessories. The 
 plagioclase was formed first, and occurs in grains or small rods ; but 
 when the rock is porphyritic, these crystals are tabular. In the 
 diorites of Guernsey, the Yosges, and many other localities, the twins 
 cross at a right angle, or an angle a degree or two less. Fluid inclu- 
 sions are rare. The mineral is sometimes so clear as to resemble quartz, 
 but is generally opaque ; and when the plagioclase is decomposed, cal- 
 cite is often developed. The orthoclase in diorite is like the orthoclase 
 of granite. It is chiefly found in quartz-diorite. Hornblende occurs 
 in films or short columns. In the deeper-seated North-German rocks 
 the crystals are broad and large. On the western slope of the Vosges 
 the needle-diorite occurs, in which hornblende forms small slender 
 needles among the rod like crystals of felspar. Its colour is com- 
 monly green, but sometimes brown, as in the quartz diorites of the 
 Odenwald and Vosges. It incloses magnetite, apatite, and sometimes 
 plagioclase and titanite. Where mica is present its crystals penetrate 
 
 I 
 
 1 Clifton Ward. Q. J. G. S., vol. xxxii. pp. 5, 22. 
 
 VOL. I. 
 
226 HISTORY OF DIORITE. 
 
 the hornblende, but biotite is often a product of decomposition of the 
 green hornblende. The commonest result of decomposition is a fibrous 
 structure in the hornblende, which is often more or less converted 
 into epidote, though chlorite is the commonest decomposition product. 
 At the Lac d'Aydat it is altered into a substance like serpentine. 
 
 When quartz is well developed it always resembles the quartz of 
 granite, and often contains fluid inclusions and microliths ; but it is 
 sometimes a secondary product. Augite is only found in diorites 
 with fibrous hornblende ; it occurs in clear reddish-brown grains, 
 which are readily converted into chlorite. Garnets occur in diorite 
 near Freiberg and at Aschaffenburg. 
 
 Typically the rock often has a true granitic structure ; but some- 
 times, as in the Yosges, Southern Schwarzwald, and parts of Cornwall, 
 it assumes a slaty texture, as though it were a metamorphosed slate. 
 The orbicular diorite of Corsica has a concentric concretionary struc- 
 ture, and is rich inanorthite ; an approach to this structure is recorded 
 from Pondieres, in the Auvergne, though in the former locality the rock 
 contains augite, and in the latter hornblende. The best-known quartz 
 diorites occur at Quenast in Belgium, and Catanzaro in Calabria. 
 In the latter locality it consists of oligoclase, hornblende, augite, mica, 
 quartz, and chlorite, and has a porphyritic structure. Tonalite may be 
 classed as a quartz-mica diorite. Ophite differs in no respect from 
 ordinary diorite, but a Spanish example contains a true amorphous 
 base, as well as gas and fluid inclusions in the crystals. Eosenbusch 
 regards porphyrites as porphyritic rocks, which would otherwise be 
 classed as mica-diorites. Teall refers the Scotch porphyrites to the 
 andesites. 
 
 North-American Diorites. Among the more typical diorites l of 
 the Virginia range is Mount Davidson, 7827 feet high, which is 
 surrounded by the volcanic rock termed propylite. In this diorite 
 the plagioclase is often fresh, and has fluid inclusions. There is no 
 orthoclase, but a good deal of quartz. The hornblende is dark-green, 
 fibrous, and more or less altered, the interstices in it being filled with 
 epidote and calcite. Sometimes quartz occurs in considerable quantity. 
 There are small exhibitions of diorite in Washoe, in basalt canon, 
 others in the Peavine Mountain in Nevada, and in Truckee canon, 
 where the rock resembles that of Ilmenau in the Thuringerwald. 
 Some of these diorites are poor in hornblende and biotite, and rich in 
 quartz and apatite. At the Hot-Spring Hills in the Pah-Ute range, 
 the diorite consists almost entirely of plagioclase, with veins and 
 spots of hornblende and magnetite. Many localities in Nevada pre- 
 sent slight differences in composition, but there is rarely anything 
 remarkable in its mineral composition, though at the foot of the 
 Augusta Mountains tourmaline appears to be a constituent, and the 
 granular rock of Winnconneca Peak contains dihexaedral quartz, en- 
 closing cubes of salt in the fluid inclusions, like those seen in the 
 Belgian diorite from Quenast, used for paving Paris. 
 
 An exceptional variety of dioritic rock occurs as a dyke in granite 
 1 Zirkel : Micros. Petrog. 
 
HISTORY OF GABBRO. 227 
 
 to the N.E. of the Havallah range, where quartz forms a sort of 
 colourless ground mass in which the hornblende is distributed. 
 Larger crystals of quartz and hornblende occur, and there is some 
 brown mica. 
 
 Gabbro. 
 
 Gabbro consists essentially of plagioclase and diallage. Olivine is 
 rarely an essential constituent. Magnetic iron and titanic iron occur, 
 apatite is abundant ; and many gabbros contain hornblende, rhombic 
 pyroxene, brown biotite and quartz as accessories. Gabbros in 
 Guernsey contain hornblende as a chief constituent, but it is probably 
 a decomposition product. Some gabbros approximate to the diabase 
 variety of dolerite. Other gabbros have the plagioclase converted into 
 saussurite, and the diallage more or less replaced by smaragdite. 
 
 The plagioclase is either labradorite or anorthite, the latter species 
 being present when olivine occurs. Orthoclase has only been 
 recorded in the gabbros of Monzoni and Yal di Susa. 
 
 The diallage often tills up the interspaces between the plagioclase ; 
 it aggregates round the olivine when olivine is present. At Volpers- 
 dorf, diallage and rhombic pyroxene are intimately associated. The 
 pyroxene may be hypersthene, enstatite, or bronzite. This inter- 
 growth of pyroxene parallels the intergrowth of felspars. Parallel 
 interlamiriation of crystals of diallage or hornblende with plagioclase 
 occurs. The hornblende is sometimes brown, sometimes green, and 
 sometimes results from the decomposition of diallage. The diallage 
 may be nearly colourless, greenish, or brownish. Biotite is the con- 
 stituent next in importance after diallage ; it is abundant in the rock 
 at Waldheim in Saxony and Todtmoos in the Southern Schwarzwald. 
 When enstatite and bronzite occur, they decompose and form bastite. 
 
 In olivine gabbros, sometimes olivine, sometimes plagioclase, pre- 
 dominates. When the quantity of felspar is small, the development 
 of serpentine is greatest. Occasionally diallage is absent. The 
 gabbro of Penig is rich in hypersthene, and has little olivine, and so 
 resembles that of Loch Scavaig and the Cuchullin Hills. At Haus- 
 dorf the gabbro is free from olivine. A rock of this kind at Harzburg 
 is rich in biotite, and contains augite and quartz. 
 
 The silica in gabbro usually varies between 43 and 50 per cent., 
 alumina 13 to 20 pef cent., with an average of 10 per cent, of iron, 
 10 per cent, of lime, and 10 per cent, of magnesia soda and potash. 
 
 Geographical Distribution of Gabbro. In Bohemia, near Eons- 
 berg, the gabbro contains crystals of diallage several inches in 
 diameter, which are surrounded with crystals of hornblende. In the 
 Vosges the gabbro is intrusive, and is sometimes converted into 
 serpentine. In Italy it is seen between Genoa and Savona, and is 
 associated with serpentine in the coasts south of Livorno. In Corn- 
 wall it is associated with the serpentine of the Lizard. 
 
 Murchison describes hypersthenite in the Stanner Rocks, near 
 Kingston, by Old Radnor, where it is intrusive in Wenlcck limestone. 
 
( 228 ) 
 
 CHAPTER XVI. 
 HISTORY OF BRITISH PLUTONIC ROCKS. 
 
 Age of Igneous Rocks. When in any country a certain class of 
 rocks, as, for instance, the slate rocks, has been convulsed and thrown 
 into new positions before the deposition of another series upon them, 
 as, for instance, the carboniferous rocks, and we find a mass of 
 granite occupying the axis or nucleus of the dislocation, it is certain 
 that such granite is older than the carboniferous system, because it 
 was uplifted with the older slates. If, in addition, this granite sends 
 veins through the slate rocks so as to prove that it was uplifted in a 
 melted state, we must infer that it is (considered as a solid) of more 
 recent origin than those slates ; and, in fact, that the antiquity of its 
 latest fusion is exactly measured by the date of the convulsion. 
 
 If there be no veins thrown off from the mass of granite, and 
 no other satisfactory proof of its having been uplifted in a melted 
 state, the age of the igneous rock is indefinable, except by saying 
 that it is older than a given stratified rock. Such a case occurs in 
 the Ord of Caithness. It appears, then, that in any case of convul- 
 sion, the era of the elevation of the igneous rock is determined by the 
 convulsion, but whether the rock was actually generated at that time 
 from a melted state requires other evidence. Now this consolida- 
 tion from a melted state is what fixes the age of an igneous rock. 
 Granite may perhaps have remained in the deep parts of the earth 
 at a melting temperature through many geological periods, but its age 
 as a rock is counted from the period when its fusion ceased. 
 
 In Derbyshire the carboniferous limestone is interlaminated for 
 great lengths with a basaltic rock (toadstone), which has evidently 
 been poured out at certain intervals by an ancient submarine volcano 
 while the limestone was in formation. The age of such a rock is 
 fixed by the age of the limestone. 
 
 The basalt of dykes which pass through certain strata is, of 
 course, not more ancient than the newest strata divided ; if at 
 any point the dyke should be covered by newer strata which are 
 undisturbed by the dislocation accompanying it, wo may generally 
 admit that the basalt is older than these strata. Such a case, 
 perhaps, occurs in those dikes of the Durham coal-field, which 
 do not penetrate the magnesian limestone; but it is not always 
 
GRANITE IN THE WEST OF ENGLAND. 229 
 
 to be affirmed, because the dikes are there often unaccompanied by 
 dislocation. 
 
 These instances are sufficient to show the truth of two proposi 
 tions of general application to this subject. 
 
 When igneous rocks accompany convulsions, we can always 
 fix the minimum of their geological antiquity ; when they throw 
 off veins or intrude in the shape of dykes, or interpolated beds, 
 among stratified rocks, we are able to assign the maximum of their 
 antiquity. 
 
 Guided by these views, and restricting our illustrations as much 
 as possible to the British Isles, we proceed to describe some details 
 of the characteristic phenomena of plutonic rocks, and to fix the eras 
 of their production. 
 
 In referring to the works of authors who have written on granite, 
 we have sometimes followed them in using the terms eruptive granite 
 and metamorphic granite, because these terms conveniently express 
 differences in the relations of granite to the associated rocks ; and not 
 as implying that the origin of granite may be different at different 
 points in the same chain ; for we have already expressed an opinion 
 that most eruptive granites are only^ metamorphic granites which 
 have been injected into fissures or otherwise displaced by faulting 
 from the positions in which granitic structure was first developed. 
 
 South-West of England. 
 
 Granite of Cornwall. The granite of Cornwall, as described by 
 Mr. John Arthur Phillips, F.R.S., is usually coarse-grained, and in 
 addition to quartz, felspar, and mica, almost invariably contains schorl. 
 Sometimes the mica is partly replaced by a talc-like mineral, and the 
 granite thus becomes a kind of protogine. This rock is cited by Dr. 
 Boase 1 as occurring a few miles north of Penzance, but its existence is 
 denied by Dr. Haughton.* The mica frequently becomes almost re- 
 placed by schorl, and in many places the felspar disappears also. The 
 felspar is orthoclase, sometimes mixed with albite. The black or dark 
 brown mica is either muscovite or lepidomelane, and the pearly white 
 or pink mica is lepidolite. The quartz is usually granular, but some- 
 times in distinct crystals. It is transparent or white, occasionally 
 bluish or grey, and abounds in microscopic cavities, which may be 
 either partly full or empty. 
 
 Mr. Phillips has found that at a temperature of 185 centigrade 
 some of the bubbles in the quartz greatly diminish in size, and infers 
 that they disappear at very varying temperatures ; and since many of 
 the cavities appear to be full, it may be concluded that the depth at 
 which the rock consolidated, as inferred by the method of Dr. Sorby, 
 is open to considerable doubt. 2 
 
 Schorl Granite of the West of England. There are six principal 
 masses of granite in Devon and Cornwall, including the Scilly Isles, 
 
 1 "Treatise on Primary Geology," 1834, p. 20. 
 
 2 Q. J. G. S., vol. xxxi. p. 330, J. A. Phillips. 
 
230 GRANITE MASSES IN DEVON AND CORNWALL. 
 
 besides smaller patches at St. Michael's Mount, Tregonning and Godol- 
 phin Hills, Cam Brea, Cam Marlts, near Redruth ; Cligga Point, 
 near St. Agnes ; Castle au Dinas and Belovely Beacon, near St. Columb 
 Major and Roche ; Kit Hill and Hingston Down, near Callington ; 
 and at Lundy Island. 1 
 
 The range of the granite is 24 JST.E. Schorl is chiefly developed 
 in it on the confines of the various masses. 
 
 Dartmoor. The granite of Dartmoor is coarse-grained, with the 
 mica sometimes white, sometimes black. It is schorlaceous where it 
 joins the slates. The schorl often occurs in radiating nests, especially 
 to the west of Buckfastleigh, but ordinary granite often passes into 
 schorl rock. After the mica disappears, the felspar vanishes, and 
 the rock at last consists of quartz and schorl. Schorl rock is seen at 
 Holne Lee. 
 
 Bodmin Moor. The granite of the Brown Willy district is similar, 
 consists of quartz, felspar, and two micas, is often porphyritic, but 
 not particularly schorlaceous, though schorl enters into its composition 
 near St. Cleer. 
 
 St. Austell. The granite district of St. Austell or Hensborough 
 is much more variable, and much richer in schorl than those of either 
 Brown Willy or Dartmoor, and is more decomposed. On the west of 
 this district the mica becomes scarce and often absent, and is replaced 
 by a talcose mineral and schorl. Schorlaceous veins are abundant in 
 the St. Austell granite in the western and central portions. A thick 
 vein of dark mica traverses the granite of St. Dennis Down. 
 
 Cam Menelez. The Cam Menelez or Falmouth granite is occa- 
 sionally porphyritic from the presence of felspar crystals, but the 
 grain is regular at Constantine and Mabe. It is poor in schorl. 
 
 Land's End. At the Land's End the granite abounds in schorl, 
 and often passes into schorl rock, as on the west of Penzance, at 
 Leity, near Lelant, and on the east of Trecobben Hill. At Trevalgon, 
 near St. Ives, the schorl rock contains large crystals of felspar. The 
 schorl veins in the granite at Wicca Cove, near Zennor, -are often 
 several inches thick. This granite contains much pinite. 
 
 Scilly Isles. The granite of the Scilly Islands is a somewhat 
 coarse compound of quartz, felspar, and the two micas. Schorl is 
 rare. It is traversed by veins in which the grain is finer. 
 
 De la Beche remarks that schorlaceous veins occur in the granite 
 of Kit Hill, and that the mass of Belovely Beacon graduates into 
 schorl rock on the north. The granite of St. Agnes Beacon is on the 
 north a mottled mass of quartz and felspar, containing large crystals 
 of flesh-coloured felspar and semi- crystallised quartz ; on the south the 
 rock becomes more quartzose, and is fine-grained and traversed by 
 small veins and strings of black oxide of tin. 
 
 On the coast it is composed of quartz, felspar, and a talcose 
 mineral, with occasional specks of mica, and is often marked by lines 
 of felspar crystals. At Godolphin Hill the rock is a base of felspar 
 
 1 " Report on Cornwall, Devon, and West Somerset, De la Beche," 1839, p. 156. 
 
GRANITES AND ELVANS. 
 
 231 
 
 with white crystals of felspar, and nodules of clear quartz and black 
 mica embedded thickly. Schorl veins are there common. 
 
 All these granites decompose so as to develop a tabular structure, 
 which is horizontal, and gives the rock a stratified appearance. 
 
 In Kingston Down a hard granite alternates with a decomposed 
 granite so as to give a stratified appearance which coincides with the 
 dip of the adjoining slates. 
 
 Alternations of schorl rock and granite give a similar stratified-, 
 appearance near St. Austell. 
 
 At Pardenick Point, and other places near the Land's End, the 
 granite resembles a collection of huge basaltic columns. 
 
 The Dartmoor granite is of post-carboniferous age, but it pierced 
 through a district which had previously experienced considerable 
 igneous action. Large veins, like dykes, are seen in the Serpentine 
 at Kynance, and in other places. The veins are generally a com- 
 pound of felspar and quartz. 
 
 
 CORNISH GRANITES. 1 
 
 CORNISH ELVANS. 2 
 
 
 
 
 A 
 
 
 
 >. 
 
 ' 
 
 
 
 zz 
 
 
 $ 
 
 
 
 -- 
 
 i 
 
 e3 
 
 ^ 
 
 i3 
 
 
 ^ J 
 
 
 o 
 
 ^ 
 
 
 
 00 o 
 
 w 
 
 
 
 
 o p 
 
 3 
 
 g 
 
 G Z 
 ctf 
 
 fJ0 
 
 II 
 
 1 
 
 S-s 
 
 
 fil 
 
 3 
 
 1 
 
 ff 
 
 
 1 
 
 1 
 
 ll 
 
 
 a 
 
 3 
 
 I 
 
 O 
 
 EH 
 
 1 
 
 1 
 
 EH 
 
 Silica 
 
 74-69 
 
 74*54 
 
 70-65 
 
 69-64 
 
 72-82 
 
 72*51 
 
 71-46 
 
 47*35 
 
 Alumina . 
 
 l6'2I 
 
 14-86 
 
 16-16 
 
 J 7'35 
 
 I5T2 
 
 
 i5'38 
 
 2O'6O 
 
 Ferrous Oxide . 
 
 1-16 
 
 ' 2 3 
 
 0*52 
 
 1-94 
 
 trace 
 
 '3-87 
 
 2-27 
 
 I -60 
 
 Ferric Oxide . 
 
 
 2 '53 
 
 !'53 
 
 1-04 
 
 **7S 
 
 trace 
 
 0*30 
 
 3-10 
 
 Lime. 
 
 0*28 
 
 
 o'55 
 
 I '40 
 
 0-52 
 
 o'6o 
 
 0-47 
 
 4*72 
 
 Magnesia . . 
 
 0-48, 
 
 ... 
 
 
 O"2I 
 
 i -06 
 
 1-52 
 
 0'22 
 
 6'12 
 
 Soda . . . 
 
 1-18 
 
 3 '49 
 
 '54 
 
 3 '51 
 
 0-51 
 
 o'43 
 
 2-79 
 
 358 
 
 Potash . 
 Manganese Oxide 
 
 3'64 
 0-58 
 
 373 
 trace 
 
 8-66 
 trace 
 
 4-08 
 
 trace 
 
 6-25 
 trace 
 
 6-65 
 0*62 
 
 5*51 
 
 trace 
 
 6-29 
 trace 
 
 Lithia . -j 
 
 O'lO 
 
 trace 
 
 
 3 hosphoric 
 Acid. 
 
 
 Fluorine. 
 
 
 Fluorine. 
 
 t 
 
 
 
 
 trace 
 
 " 
 
 trace 
 
 
 trace 
 
 Hvgrometric 
 Water 
 
 o*34 
 
 0-87 
 
 o*33 
 
 0-13 
 
 0*26 
 
 o'n 
 
 o'43 
 
 o'34 
 
 Combined . 
 
 0-89 
 
 trace 
 
 0-89 
 
 '59 
 
 2-03 
 
 0-49 
 
 1-27 
 
 6 V ii 
 
 Elvans. The term Elvans is applied to long lines of granitic and 
 felspar porphyry rocks which cut the slates and granites, and there- 
 fore being newer are conveniently distinguished from the granite 
 veins which they occasionally resemble. 
 
 The elvans have almost the same chemical and mineralogical com- 
 position as the granites, though their ingredients are differently aggre- 
 
 1 J. A. Phillips, Q. J. G. S., vol. xxxi. p. 330. 
 3 Ibid., vol. xxx vi. p. 8. 
 
 Ibid. 
 
232 CORNISH EL VANS. 
 
 gated. The quartz, instead of forming a kind of crystalline residue as 
 in granite, is usually enclosed with the crystallised felspar in a f el- 
 spathic or quartzose-felspathic base. Mica, schorl, chlorite, and pinite 
 are sometimes accessory minerals ; the quartz is often in double hexa- 
 gonal pyramids with the angles removed and rounded. The felspar 
 is usually in large well-defined crystals, though sometimes so minute 
 as only to be seen with the lens. When the elvan occurs in granite, 
 its texture is generally finer than when it cuts through slate. It 
 rarely penetrates between the cleavage planes of slate. In the prin- 
 cipal mining districts the elvans are parallel to most of the productive 
 tin and copper lodes, running a few degrees north of east, though in 
 other parts of the country the elvans run nearly north and south, and 
 almost coincide in direction with the cross courses which often yield 
 ores of lead and iron. Under the microscope the quartz is seen to 
 contain hair-like crystals of schorl. 1 
 
 Distribution of Elvans. The elvan dykes of Cornwall vary from 
 a breadth of a few feet up to 300 or 400 feet ; they are parallel to 
 each other, and can be traced for several miles. One of the longest 
 runs from Wheal Darlington, near Marazion, for twelve miles by 
 Wheal Fortune, Corbus, Treganhorn, Cayle, Roseworthy, and Cam- 
 borne to Pool, sending off a branch near Cayle about five miles long, 
 which passes by Carnbrae Green, Cassawson, and Tregear into the 
 Carnbrea granite on the west of Camborne Beacon. Another runs 
 along Penzarrce Pier. Only one has been observed in the Scilly Isles, 
 on the northern part of St. Mary's, running 25 N.W. It has a grey 
 felspatho-quartzose base, containing crystals of light-coloured felspar 
 and quartz. At the Land's End elvans sometimes contain schorl ; in 
 the central part of the dyke they are often granitic in texture. The 
 elvan which runs through St. Hilary abounds in crystals of pinite. 
 Elvans are numerous between Redruth and Gwennap, and seem to cut 
 into the granite. The elvan on the west of Killaganoon is so decom- 
 posed as to be worked for crucible clays, but they are usually hard. The 
 colour of the rock sometimes changes in a short distance from a rose 
 tint to greenish, and the texture from porphyritic to arenaceous. The 
 Cubert elvan in the centre is a compound of quartz-felspar and mica, 
 though the mica is rare. The upper part of the elvan of "Watergate 
 Bay appears to have a concretionary structure. North of the Hens- 
 borough granite the elvans are very variable in colour. A group of 
 elvans near St. Austell runs 20 N.E. They sometimes contain nests 
 of sulphide of copper, and occasionally small nests of plumbago. The 
 Pentnau elvan contains fragments of slate rocks, especially in the 
 branch from it which runs along the shore to the Black's Head. 
 Elvans also occur round Brown Willy. The elvan called Roborough 
 stone, on Morwell Downs, abounds in crystals of quartz formed of 
 hexagonal pyramids base to base. Elvans are much divided by joints 
 like the granite. Elvans everywhere have the appearance of having 
 been produced towards the close of the eruption or upheaval of the 
 granite masses near which they occur. 
 
 1 Q. J. G. S., J. A. Phillips, vol. xxxi. p 334. 
 
GRANITE AND SYENITE OF LEICESTERSHIRE. 233 
 
 Charnwood Forest. 
 
 The igneous rocks of Charnwood Forest in Leicestershire l are 
 chiefly syenite, granite, and diorite. 
 
 Syenite. The syenites occur in isolated masses in the neighbour- 
 hood of Groby, extending north-west from that place, where they are 
 surrounded by trias. At Cliff Hill the syenite forms a conspicuous 
 rugged ridge, and another mass is seen at Stanton Field; another 
 occurs at Hammer Cliff. All these exposures are termed by Professor 
 Bonney and Mr. Hill the southern syenites. At Groby the rock is 
 rather coarsely crystalline, and contains dark-green hornblende, with 
 pink and greenish felspar, with small masses of yellowish-green 
 epidote, and occasional grains of pyrites. When the rock is more 
 finely crystalline it is generally of a red colour. The rock is also 
 quarried at Markfield on Cliff Hill. It is generally traversed by bold 
 joints, which usually have a brown ferruginous coating. Under the 
 microscope the rock consists of orthoclase and plagioclase felspar, 
 often a good deal decomposed, and sometimes stained green, with 
 inter-crystallised quartz. The hornblende is often somewhat decom- 
 posed so as to appear fibrous, and it is frequently replaced by the 
 mineral epidote, and viridite. The quartz rarely occurs in grains 
 such as are seen in granite. Apatite is found in long clear six-sided 
 prisms. The more compact syenite consists chiefly of inter-crystal- 
 lised felspar and quartz, the felspar being sometimes replaced by a 
 zeolite. The quantity of hornblende is small. 
 
 The rocks termed the northern syenites by Messrs. Hill and Bonney 
 are seen at Bawdon Castle, and to the north-west. A patch also 
 occurs at Great Buck Hill. At first sight the aspect of the rock is 
 different from that of the southern district, being less coarsely crystal- 
 line and more altered, passing occasionally into a dull green compact 
 rock. The circumstance that these rocks are rather more basic than 
 the others is associated with the view that they were probably less 
 deep-seated in their origin. Under the microscope the K>ck appears to 
 contain less quartz. The hornblende is almost entirely replaced by 
 products of its decomposition, such as viridite, chlorite, and epidote. 
 The iron is chiefly in the form of ilmenite. In so far as these rocks 
 differ from the typical syenites they make an approach towards the 
 diorites. 
 
 Mount S orel Granite. On the left bank of the Valley of the Soar 
 is a mass of rock, forming the height called Buddon Hill and terminat- 
 ing at the village of Mount Sorel. Various outlying neighbouring 
 patches occur on the north-east and south-west, which are all probably 
 connected beneath the trias, so that the granite hills rise from the 
 surrounding strata as from a sea. The colour is usually pinkish, but in 
 the Mount Sorel pit it is sometimes grey. The rock is occasionally 
 slightly porphyritic, and consists of quartz, felspar, black mica, and 
 
 1 Rev. E. Hill, M.A., and Professor Bonney, F.R.S., Quarterly Journal 
 Geological Society, vol. xxxiv. p. 211. 
 
234 REPUTED GRANITE OF ANGLESEA. 
 
 dark-green hornblende, occasionally with pyrites and epidote. Occa- 
 sionally felsite veins occur in the rock, which is divided up by three 
 or four systems of joints. Under the microscope the quartz and 
 felspar are occasionally inter-crystallised, as in the syenite. 
 
 The felspar is chiefly orthoclase, but is mixed with oligoclase. It 
 is inferred by Professor Bonney and Mr. Hill that all these rocks of 
 Charnwood Forest were intruded at about the close of the Silurian 
 period, and are of the same age as the granites and syenites of the 
 Lake country. 
 
 Diorite. To the north of Brazil Wood is a knoll of dark diorite 
 which consists of white plagioclase felspar and hornblende. 
 
 Granite of Anglesea. The reputed granite of Anglesea furnishes 
 a notable instance of the way in which the nomenclature of rocks 
 sometimes changes, with a new interpretation, for most recent writers 
 deny that the island contains any granite at all. 
 
 According to Sir Andrew Ramsay the granite runs across the 
 island in a broad irregular belt nearly 12 miles long. Its greatest 
 width is less than 3 miles. Where best developed it is composed of 
 quartz, felspar, and black and silvery mica, but it is usually coarse, 
 with the felspar not well crystallised, and the mica often absent. It 
 is regarded by Dr. Hicks as a Dimetian rock. All along the south side 
 towards Caernarvon Bay, it is bordered by a belt of hard felspatho- 
 silicious-looking rock, sometimes faintly laminated or more clearly 
 foliated. This band Dr. Hicks names Halleflinta, and identifies as 
 Pebidian. Near Bodwrog portions of foliated rocks are interlaced 
 with the granite, and further west there is a mass of altered rocks 
 entirely surrounded by granite. The foliated semi-crystalline rock 
 often almost melts into the granite. Elsewhere it passes insensibly 
 into foliated hornblendic gneiss. On the west the boundary between 
 the granite and gneiss rocks is equally obscure. In some places 
 innumerable granite veins ramify into the gneiss, and so alter it that 
 the two rocks are inseparable. Round Handrygarn much of the 
 granite is hornblendic. Professor Eamsay remarks that it was im- 
 possible not to be impressed with the idea that the granite and its 
 veins are merely the result of a more thorough metamorphism than 
 was attained in the production of the associated gneiss. And he 
 observes that the stratified rocks near to its margin dip indifferently 
 towards it and from it, as if part of the strata had been used up in 
 the making of the granite itself. In the west of the island, near 
 Llan-Trefwll, there is a small patch of granite, round which the Cam- 
 brian rocks are much altered, and several small hornblendic patches, 
 which were formerly regarded as greenstones. West of Treath Dwlas 
 a larger granitic mass stretches two miles inland. 1 
 
 1 Ramsay, North Wales. These rocks have since been described by Dr. 
 Hicks, Quarterly Journal Geological Societ}% vol. xxxv. p. 295, as Precambrian. 
 Professor Bonney terms the rock Granitoid gneiss, and gneiss. Dr. Callaway 
 (Q. J. G. S., vol. xxxvii. p. 210) names much of the rock Granitoidite. 
 
THE LAKE DISTRICT GRANITES. 235 
 
 Granitic Rocks of the Lake District. 
 
 The igneous rocks of the Lake District were well described by the 
 late Mr. Clifton Ward. 1 
 
 The Skiddaw Granite occurs in small inlying masses, the largest 
 being only a mile long and half a mile wide. It is seen in the valley 
 of the Caldew, and in the course of Sinen Gill, and is everywhere 
 surrounded by Skiddaw slates, which are greatly metamorphosed by the 
 contact. There is no evidence that these granite masses were the cores 
 of old volcanoes, since the volcanic rocks which occur in the surround- 
 ing strata are more basic and chiefly belong to the basaltic family. 
 
 The felspar is partly orthoclase and partly triclinic, the latter 
 showing the usual coloured bands when seen under polarised light in 
 microscopic sections. 
 
 The quartz contains many fluid cavities with moving bubbles 
 and hair-like crystals, as well as others which are stout and long. 
 The mica is mostly dark-brown. There are a few grains of magnetite. 
 Mr. Ward infers that the Skiddaw granite has never been covered by 
 a greater thickness of strata than 38,000 feet. 
 
 The liquid cavities in the quartz are considered to indicate a 
 pressure equal to 52,000 feet of superincumbent rock, it being supposed 
 that this pressure was exerted in the upheaval of the overlying rocks, 
 and that the Skiddaw granite consolidated at a temperature of about 
 680 Fah. 
 
 The Eskdale Granite is only met with surrounded by the volcanic 
 series of Borrowdale. From the thickness of these strata it is con- 
 cluded that not more than 22,000 feet of rock could ever have covered 
 the granite. The granite metamorphoses the rocks with which it is 
 in contact. The pressure which is indicated by the condition of the 
 crystals is stated by Mr. Ward at 42,000 feet. Dykes of quartz-felsite 
 or elvanite from this rock indicate a pressure of 53,000 feet. 
 
 The Wastdale Granite is another mass, coloured reddish by its 
 felspar, which is partly orthoclase and partly triclinic, and contains 
 dark-brown mica. An analysis shows it to contain, as compared with 
 the Skiddaw granite, rather less silica and more alumina, less potash 
 and more soda and more iron. 
 
 The Wastdale granite is assumed to be in connection with the 
 Eskdale granite. At Wastdale Head numerous veins run into the 
 overlying volcanic series. 
 
 The Snap Granite, so well known on Wastdale Crag and Wastdale 
 Pike, is characterised by large flesh-coloured or reddish-brown crystals 
 of felspar. Like the other masses, it is incapable of being connected 
 with any volcanic outburst. The geological evidence indicates that 
 it could not have consolidated under a greater pressure than about 
 14,000 feet of superincumbent strata, but the calculated pressure 
 indicated by the microscopic structure is 46,000 feet, at a temperature 
 of 502 Fah. This granite is inferred to have cooled very slowly. 
 The metamorphism in the surrounding rocks is less intense than in 
 
 1 Quarterly Journal Geological Society, vol. xxi. p. 568 ; vol. xxxii. p. i ; Me- 
 moirs of the Geological Survey : Northern Part of the English Lake District, 1876. 
 
236 RHYOLITES IN RELATION TO THE GRANITES. 
 
 the Eskdale district. Patches of fine-grained micaceous granite some- 
 times occur in the middle of the Shap mass. 
 
 The Ennerdale and Buttermere Syenite and Syenitic Granite. 
 The Ennerdale rock is for the most part red and of coarse grain. 
 Usually hornblende is absent or in small quantity, but it is sometimes 
 so abundant as to give with the mica a dark and grey tinge to the 
 rock ; and then its texture is fine-grained. It is well seen over the 
 summit of Red Pike. The quartz appears to occupy the interstices 
 between the other minerals, and is not crystallised. Magnetite is 
 sometimes plentiful. Where the syenite meets the overlying volcanic 
 ash it becomes a hornblendic felsite. It has a distinct aspect from 
 the Eskdale granite, though it adjoins that rock at the foot of Wast- 
 water. It extends north and south for about nine miles, and forms in 
 part a boundary between the Skiddaw slates and the volcanic series. 
 The liquid cavities in the quartz indicate a pressure at consolidation 
 equal to 35,000 feet of rock. This mass may have furnished the 
 ashes in the uppermost part of the volcanic series. 
 
 St. John's Quartz Felsite. This rhyolitic rock varies in colour 
 from red to white, and consists of a felspathic base containing crystals 
 of felspar and quartz, with a little magnesia, mica, and some altered 
 hornblende. The felspar is chiefly orthoclase. Grains of magnetite 
 occur. The quartz contains fluid cavities which indicate a pressure 
 equal to 40,000 feet of rock. The masses in which it occurs are 
 rarely more than a mile long. The crystals of quartz in this rock are 
 generally double pyramids, the intermediate prism not being developed. 
 This formation of complete crystals is difficult to reconcile with the 
 theoretical estimate of pressure. 
 
 The Armboth and Helvellyn Dyke. This rock also consists of 
 quartz felsite or rhyolite, and is traced towards the St. John's rock. 
 The dyke, which is 20 to 30 feet wide, is a red felspathic base, with 
 crystals of pink felspar and quartz, with green mica and a steatitic 
 mineral. Most of the quartz is crystallised, but some is interstitial. 
 The percentage of silica is less than in any of the other rocks. The 
 pressure indicated by this rock corresponds to 46,000 feet, it being in 
 accordance with other observations that the pressure is greater in a 
 dyke than in the mass from which it proceeds. 
 
 Quartz Felsite of Fairy Crag. This is a similar area to that 
 of St. John's Vale, and occurs among the Skiddaw slates. It in- 
 dicates pressure of 49,000 feet. These rhyolitic rocks may be re- 
 garded as granitic materials which have not perfectly crystallised. 
 Mr. Ward has noticed that as the rocks severally approach towards 
 the granite masses, especially that of Eskdale, they undergo a regular 
 modification by metamorphism. First, at a distance of several miles, 
 the rock, which may have been a bed of volcanic ash, shows under the 
 microscope hazy and indistinct outlines of the fragments, and fre- 
 quently a kind of streaky-flowing structure round the larger particles, 
 though remains of original lines of bedding are observable. Nearer 
 to the granite the altered rocks put on a purple tint and develop 
 specks of mica, which are sometimes gathered into nests as the granite 
 
LAKE ROCKS OF BASIC TYPE. 
 
 is approached, while finally many small crystals of felspar, am 
 sionally quartz, become developed in the purple micaceous base. 
 
 There are a few felstone dykes in the Lake District. One ex- 
 tends from White Pike and crosses part of Matterdale Common. It 
 is thought to be connected with the St. John's quartz felsite. Narrow 
 dykes of banded felstone occur in the south-west of Buttermere. 
 
 Basic Rocks of the Lake District. 
 
 Minette. 1 Professor Bonney describes minettes between Winder- 
 mere and Sedbergh, running east and west in dykes a foot or two 
 wide, typically seen at Cross Haw Beck. The rock is called minette- 
 felsite at Backside Beck, Helm Gill, south of Haygarth, &c. ; mica- 
 diorite at Stile-End Farm ; and kersantite at Holbeck Gill. The dykes 
 are probably of Old Red Sandstone age. The Sale Fell rock, according 
 to Mr. Ward, is a pink felspathic base, consisting of orthoclase and 
 triclinic felspar, generally crystalline. It contains dark green mica, 
 probably biotite, and the quartz, which is interstitial, contains many 
 cavities. 
 
 Diorites of the Lakes. Many small intrusive bosses and dykes of 
 diorite burst through the Skiddaw slates and volcanic series of Borrow- 
 dale. Several occur on the hill-side north of the railway, between 
 Cockermouth and the Bassenthwaite lake. They are generally fine- 
 grained, with white felspar and dark hornblende and specks of iron 
 pyrites. Quartz occurs, with liquid cavities and moving bubbles. 
 Another quartz diorite occurs on the summit and side of Hind Scarth. 
 It contains a good deal of quartz, with many needle-like crystals. 
 Other masses are found at Burtness Comb. The hornblende is a 
 good deal altered, and glass cavities as well as liquid cavities occur 
 in the quartz. A mass of diorite with picrite is seen at Little Knott 
 on the east of Bassenthwaite lake. It is almost entirely formed of 
 hornblende, the quantity of felspar being very small. 
 
 Dolorites or Diorites. Mr. Ward mentions four principal ex- 
 hibitions of dolorite, first at Wythop Fells, a fine-grained dark- 
 green rock, with chlorite replacing much of the plagioclastic felspar. 
 Unlike other diorites, this contains many spaces filled with quartz, 
 which contains many cavities with bubbles. A small mass occurs at 
 Castlehead, Keswick. It is a mixture of pale felspar with augite 
 and a soft dark-green mineral. Most of the augite is replaced by 
 pseudomorphs. Another mass is found at Swirral Edge, Helvellyn. 
 It also has the augite greatly altered and converted into a soft green 
 substance. Across the upper part of Longstrath Valley, Borrowdale, 
 run several dykes, which contain felspar crystals embedded in a fel- 
 spathic base, and mixed with a soft green chloritic mineral. 
 
 We hesitate to group any of these rocks as dolorites, and are 
 inclined from the quartz they contain to regard them as perhaps 
 related to quartz diorites. They may perhaps be connected with the 
 Ennerdale syenite. 
 
 1 Bonney and Houghton, Q. J. G. S., vol. xxxv. p. 165. 
 
2 3 S 
 
 DISTRIBUTION OF SCOTCH GRANITE. 
 
 
 LAKE ROCKS. 1 
 
 SCOTCH GRANITES. 
 
 Eskdale.2 
 
 Granite of White 
 Gill, Skiddaw. 
 
 Sye' itic Granite of 
 Buitormere. 
 
 Quartz FeLsite of 
 St. Johns. 
 
 Peterhead.3 
 
 Rossof MulU 
 
 Bell's Grove, 
 Strontian.5 
 
 Creetown 6 
 
 Granite S. of 
 Great How. 
 
 Silica 
 Alumina . 
 Ferrous Oxide 
 Ferric Oxide 
 Lime. 
 Magnesia . 
 Soda . 
 Potash 
 Manganese Oxide 
 Phosphoric Acid 
 Hygrometric ) 
 Water . I ) 
 Combined . . J 
 Loss . 
 
 73-57 
 1375 
 2*IO 
 
 o'6i 
 i -06 
 o'39 
 4'S 1 
 3'S* 
 
 O'OI 
 
 75-22 
 11-14 
 1-77 
 trace 
 1-62 
 i -08 
 3 '99 
 4-5i 
 
 0-14 
 
 71-44 
 I5-34 
 
 I '10 
 
 1*23 
 i -06 
 0-72 
 3'95 
 4 '43 
 
 o'n 
 
 67-18 
 16-65 
 
 a**5 
 
 o'55 
 2 '35 
 I- 54 
 4-03 
 2*91 
 
 0-17 
 
 73 '70 
 14-44 
 
 i '49 
 o*43 
 i -08 
 trace 
 4-21 
 
 4'43 
 trace 
 trace 
 
 0'2I 
 
 0*40 
 
 74-48 
 
 l6'20 
 
 O'2O 
 0'13 
 0-27 
 378 
 4^6 
 
 o'6o 
 
 62-09 
 17*60 
 074 
 478 
 4'95 
 3-I7 
 4-08 
 
 3'25 
 0-40 
 
 67-04 
 17-20 
 0*41 
 
 3 -I 5 
 2-92 
 
 T'2O 
 
 3 <2 5 
 3-90 
 
 0-66 
 
 0*50 
 
 0-58 
 
 i'54 
 
 
 
 
 
 Granite in Scotland. 
 
 The granite masses of Scotland are chiefly developed along the 
 course of the Grampian chain, about Peterhead, west of Aberdeen, 
 on both sides of the Dee extending almost to Braemar; one large 
 mass includes Ben Mac Dhui and Cairngorm, another includes Loch- 
 nagar. A smaller mass occurs in Glen Tilt, another mass around 
 Loch Luydan, east of Glencoe, and about Loch Etive, a considerable 
 mass includes Ben Cruachan. The base of Ben Nevis is of granite, 
 and there is a remarkable mass of granite at Strontian around the 
 head of Loch Sunart. Granite occurs in the Ross of Mull, and in 
 the central part of that island. Many smaller masses are found in 
 the north of Scotland, as at Strathie Point, and between Strath 
 Ullie and Strath Halladale ; and in the south of Scotland in Arran, 
 south of Loch Doon, in the upper course of the Dee, and in the well- 
 known mass of Criffel in Kirkcudbrightshire. 
 
 Granite of Banff. The granite of Banff came into existence after 
 the formation of the old metamorphic rocks. It sends veins into 
 them in the lower Craigellachie district. Mr. Jamieson, 7 however, 
 regards the granite as a metamorphic rock formed partly out of the 
 argillaceous and arenaceous beds, and the greenstone of the Portsoy 
 
 1 Ward, Q. J. G. S., vol. xxxii. p. i. 4 Haughton. J. R G. S. I., vol. i. p. 30. 
 
 2 Ibid., vol. xxxi. p. 597. 5 Scott, vol. i. p. 265. 
 
 3 Phillips, vol. xxxvi. p. 13. 6 Haughton, R. Ir. Acad., xxiii.; p 607. 
 
 7 Q. J. G. S., vol. xxvii. p. 105. J. E. Jamieson, on the Granite of Banff. 
 
GRANITES OF THE WEST OF SCOTLAND. 239 
 
 district. Granitic structure alternates with gneiss structure in the 
 southern part of Ben Aigan. In the hill called Little Conval, near 
 Dufftown, the S.E. base is a large-grained rock with red felspar and 
 little mica, but higher up it becomes finer-grained, and at the top is a 
 mixture of small-grained red felspar and quartz. But the felspar is 
 redder than is usual in gneiss, and the quartz is less in amount. 
 
 Grampians. Of the Grampian chain, Professor Judd remarks 
 that where the lower portions of the masses are exposed by extensive 
 denudation, the rock presents the character of a typical granite, such 
 as is well seen in the Ross of Mull ; but where it rises into lofty 
 peaks it becomes more and more hornblendic, and then graduates into 
 felsite, which rock is commonly more or less porphyritic. And it is 
 worthy of notice that a similar series of changes in the character of 
 the rock is often to be observed when the granite is traced from its 
 central portion towards the outer margin. At the junction of the 
 granite and stratified rocks numerous veins, which often include 
 angular fragments of the strata, penetrate the water- formed deposits, 
 and large masses of stratified rocks, altered on their surface, and pene- 
 trated by granite veins, are found enveloped in the igneous mass, so 
 that the granite is entangled with the strata. Granite and felstone 
 dykes are well seen in the Passes of Glencoe and Brander, and are 
 always numerous near the central igneous masses. 
 
 The central mass of Ben Nevis consists of hornblendic granite, 
 passing by insensible gradations into ordinary granite on the one 
 hand and syenite granite on the other. The period at which these 
 granitic protrusions took place was posterior to the deposition of the 
 Cambrian rocks, which were already metamorphosed when the granite 
 penetrated them ; and since this granite never penetrates either the 
 secondary or tertiary strata, it is inferred to belong to the newer 
 palaeozoic period. Professor Haughton describes the granite of the 
 Koss of Mull as coarse-grained, with abundance of quartz, pink 
 orthoclase, and a little black mica. It contains 74 per cent, of silica 
 and 1 6 per cent, of alumina. 
 
 The Granite of Loch Etive is described by Mr. R. H. Scott, F.R.S., 
 as consisting of a fine-grained rock in which the felspar is mainly 
 anorthic. It sometimes contains sphene, and includes many frag- 
 ments of slaty rocks, which are often angular and but little altered ; 
 it is traversed by dykes of red quartz felstone, and has all the char- 
 acters of an elvan. The neighbouring granite is grey, and consists of 
 anorthic felspar, quartz, and black mica. Granite extends on Loch 
 Leven for two and a half miles, and has an elvan character where 
 quarried at Ardsliiel and close to the pier at Ballachulish. It is 
 succeeded in the usual way by gneiss, mica slate, and roofing slate. 
 The gneiss of Loch Linnhe often assumes a granitoid character. Up 
 Glen Tarbert, to within four miles of Strontian village, it is absolutely 
 bare of vegetation, and yet shows no sign of chemical disintegration. 
 
 The Granite of Strontian extends eight miles east and west ; it is 
 dark and coarse-grained, with red orthoclase, white felspar, quartz, a 
 large proportion of black mica and hornblende, with crystals of sphene, 
 
=40 GRANITES OF THE SOUTH OF SCOTLAND. 
 
 and perhaps zircon. Lenticular masses or nests of black mica occur, 
 in which are crystals of white felspar, quartz, and sphene. On the 
 south shore of Loch Sunart the Strontian granite is more micaceous 
 and hornblendic, and contains large masses of hornblende rock. 
 
 Galloway Granite. Mr. Irvine remarks of the mass of granite 
 called Cairns More of Fleet, which is one of the three large granitic 
 bosses of Galloway, that its colour is grey, shading in places to pink, 
 and its texture coarse, so that on the top of the hill crystals of quartz 
 and felspar are two inches long. The granite usually consists of 
 quartz, orthoclase, plagioclase, and black and white mica ; but some- 
 times the mica may be almost replaced by hornblende, and frequently 
 the quantity of quartz is small. The black mica, called Lepidomelane, 
 is the most common, but in Cranmer, Craiglowrie, and Craigherron 
 Hills a white mica occurs, probably Margarodite. Quartz frequently 
 occurs in hexagonal prisms. No mass of fine-grained granite occurs 
 in the area, but dykes and veins of elvanite are found all through the 
 heart of the granite, and in the surrounding metamorphosed area. 
 The rock consists of white orthoclase felspar, with a little mica, iron 
 pyrites, and quartz. Occasionally, at the circumference of the granite, 
 the rock passes into a compact felstone, with some crystals of horn- 
 blende. Many patches of altered stratified rocks are caught up in the 
 granite ; the largest, in Blair Buie's Hill, is 600 feet long by 150 feet 
 broad. 1 
 
 The Granite of Loch Ken 2 consists of pink and white orthoclase 
 and plagioclase felspar, quartz, mica, some hornblende, and a little 
 iron pyrites. But the quartz may disappear entirely, and hornblende 
 often replaces the mica. Crystals of sphene abound near Loch Aber- 
 loch. Near the margin the rock is foliated, and then hornblende is 
 most abundant. Veins abound on Bennan Hill west of Loch Ken. 
 Near the margin of the granite, felstone dykes are frequent. 
 
 Arran. 
 
 General Features. The Island of Arran has been very often 
 described by eminent geologists. Jameson, MacCulloch, Necker, 
 Murchison and Sedgwick, Oeynhausen, Von Dechen, and Ramsay 
 have all written ably on the inexhaustible subject of this little world 
 of geological phenomena. The leading features of Arran are its moun- 
 tainous and truly Alpine scenery in the northern extremity, and the 
 elevated plateaux of its southern portion. These latter are generally 
 partly of syenite, partly porphyry, partly basalt, with many basaltic 
 dykes and dykes of pitchstone passing through the red sandstone 
 strata. 
 
 Granite occupies the central district of the northern half of the 
 Island of Arran, and is especially seen in the mountains. It is sur- 
 rounded by clayslate, and schistose rocks. Sir Andrew Ramsay 
 
 1 Mem. Geol. Surv. Scot., Sheet 4, 1878, p. 18. 
 
 2 Ibid., John Home in Explanation Sheet 9, 1877. 
 
GRANITE IN ARRAN. 241 
 
 remarks l that the granite of Goat Fell and the principal mountains is 
 a large-grained variety, in which felspar predominates and mica is 
 comparatively rare. Some of the felspar is light-brown, other crystals 
 are pure white, and a third is glassy, so as to resemble quartz. The 
 quartz too is very variable, being white, pale yellow, grey, light or 
 dark brown, and sometimes almost black. This mineral frequently 
 occurs in hexagonal prisms in cavities in the rock. There are large 
 areas in the interior between Ben Ghnuis and Ben Mhorroinn 
 which consist of a fine-grained granite, and in some of the veins the 
 texture is so fine that the rock might be mistaken in hand-specimens 
 for a sandstone. 
 
 In veins the quartz and mica sometimes disappear, leaving the 
 rock in the form of a compact felspar. The mica is in very small 
 black scales. Frequently the granite rises in perpendicular cliffs 
 
 
 Fig. 54. From the top of Goat Fell (Arran). 
 
 which are well seen on Goat Fell, the west side of Ben Ghnuis, and 
 on the peak of Caisteal Abhael. The junction of the granite and 
 slate is seen in Glen Kosa and Glen Sannox, the slate being pene- 
 trated by veins. A mass of fine-grained granite forming part of the 
 hills which surround Glen. Dubh at the upper part of Glen Cloy, 
 contains a large proportion of reddish felspar. It is partly surrounded 
 by syenite and porphyry, and the syenite has metamorphosed the 
 sandstone into quartz rock. 
 
 Alterations of Stratified Rocks in Arran. No new minerals are 
 produced in the slate where the granite touches it, nor in the red 
 sandstones where they are hardened by the basaltic dykes. This 
 hardening is very various in degree, and the causes of these differences 
 are not very evident even upon the examination of many cases. The 
 
 1 " Geology of the Isle of Arran from Original Survey," 1841. 
 VOL. I. Q 
 
242 GRANITES OF THE NORTH-WEST OF IRELAND. 
 
 hardening effect is sometimes communicated to a distance of two or 
 three feet into the neighbouring rock, but generally not to more than 
 a few inches. The hardened parts sometimes stand up in narrow 
 crests. Where dykes cross, it has been found that one of the planes 
 of intersection of the basalt dykes has been marked by the occurrence 
 of a very narrow band of black pitchstone. The base of the pitchstone 
 pillars of the interposed bed in Corygills is decomposed to a kind 
 cf kaolin where it touches the sandstone below. 
 
 It is impossible to say what was the geological epoch of the later 
 eruptions of Arran, further than that they were posterior to the red 
 sandstone. They may be as modern as the basaltic eruptions of the 
 north of Ireland. 
 
 Fig. 55. Granitic Ridges, Glen Sannox (Arrau). 
 
 Granites of Ireland. 
 
 Donegal. The granite axis of Donegal extends from Malin Head for 
 60 miles S. "W. to near Ardara. It is marked by the two great valleys 
 of Glenveagh and Gweebarra, which are nearly in the centre of the 
 granite band which traverses Donegal from Glen in the KE. to 
 Doocharry Bridge in the S.W. The granite is at first nine miles 
 wide, and afterwards spreads out to a breadth of 18 miles. It is 
 separated by the sea and by quartz rock from the granite of Dunaff 
 Head and Malin Head. There is an isolated patch to the S.E. of the 
 granite axis, divided into two portions by the Barnes More or Great 
 Gap. Still further to the S.E. granitic veins are numerous in the 
 gneissose rocks and metamorphic slates, where Donegal borders Fer- 
 managh, at Beleck and Castle Caldwell. This granite has a stratified 
 structure, the beds are nearly vertical, and run parallel to the great 
 valleys mentioned. The joints are nearly at right angles to the 
 
GRANITES OF NORTH-EAST OF IRELAND. 
 
 -43 
 
 planes of the cleavage structure. This granite, according to Professor 
 Haughton, is interstratih'ed with the quartz rock, mica-slate, and 
 limestone with which it is associated. This interstratification is seen 
 on both northern and southern outcrops. At Glentchen the granite 
 contains beds of quartzose mica slate, beds of gneiss, and beds of 
 sphene rock, the latter consisting of quartz, orthoclase, and sphene. 
 Nearer the southern boundary thin beds of limestone run vertically in 
 the granite for several miles. On the northern border the granite 
 passes into stratified rocks by insensible gradations, passing into 
 felspathic gneiss with black mica, hornblende slate, and micaceous 
 quartz rock. At Castle Caldwell some of the granite veins consist of 
 quartz, pink orthoclase, white mica, black mica, and schorl, all in 
 large crystals, while other of the veins consist of quartz, pink ortho- 
 clase, yellowish-green oligoclase, black mica, sulphuret of molybdenum, 
 and copper pyrites. The percentage of silica in these granites varies 
 from 55 to 75 per cent. 
 
 
 
 
 DONEGAL GRANITES.! 
 
 ! 
 
 
 
 
 
 
 
 
 i 
 
 
 1 
 
 
 
 
 
 ! 
 
 J 
 
 C5 
 
 Unismenagh. 
 
 Glenveagh. 
 
 Poison Glen. 
 
 Arranmore. 
 
 
 
 5 
 
 Tory Island. 
 
 Glenvengh. 
 
 Ardmalin. 
 
 Poison Glen. 
 
 Doocharry Brit 
 
 Anagarry. 
 
 Barnesmore. 
 
 Dunlewy. 
 
 Silica 
 Alumina . 
 
 55 '20 
 19-28 
 
 58-44 
 
 20*00 
 
 65-80 
 
 12 '80 
 
 68-00 
 16-80 
 
 68-20 
 i5'96 
 
 68-80 
 16-40 
 
 68-96 
 17-40 
 
 69-2069-36 
 16-40 i6'oo 
 
 70*00 
 16-36 
 
 70-64 72-24 
 15-6414-92 
 
 73 '4 
 15-20 
 
 73-6075-24 
 13-80 13-36 
 
 Iron Per- 
 oxide . 
 
 6-08 
 
 6- 44 
 
 6-64 
 
 3-68 
 
 3'69 
 
 2-60 
 
 2-52 
 
 2*09 
 
 3 '3 
 
 2'8o 
 
 2-64 
 
 1-63 
 
 .. 
 
 2*00 
 
 0-60 
 
 Iron Pro- 
 toxide . 
 
 0*46 
 
 2-05 
 
 o'i8 
 
 o"6$ 
 
 I '00 
 
 0*65 
 
 .. 
 
 I '00 
 
 0-30 
 
 o'o8 
 
 
 0-23 
 
 .. 
 
 .. 
 
 .. 
 
 Lime 
 
 5-08 
 
 4-72 
 
 2-92 
 
 4'05 
 
 2-92 
 
 i '75 
 
 2-80 
 
 I '03 
 
 2-29 
 
 I'I2 
 
 2 '47 
 
 1-68 
 
 i -60 
 
 0-79 
 
 2*25 
 
 Magnesia . 
 
 3'66 
 
 i '57 
 
 i '75 
 
 o'95 
 
 0-78 
 
 0-85 
 
 0-41 
 
 0-85 
 
 o'54 
 
 0' 7 I 
 
 0-15 0-36 
 
 0*07 
 
 o'so 
 
 0*14 
 
 Soda . 
 
 4-63 
 
 3-8i 
 
 4-16 
 
 4'32 
 
 3'75 
 
 3 '75 
 
 3 '03 
 
 4 '20 
 
 4'i7 
 
 4'i3 
 
 3-8i 
 
 3'5i 
 
 2-88 
 
 4-29 
 
 4-86 
 
 Potash 
 
 3-i7 
 
 2 82 
 
 4-40 
 
 2 '04 
 
 4-14 
 
 5 - 3i 
 
 5 '25 
 
 5'25 
 
 4 '47 
 
 4-66 
 
 4'53 
 
 S'io 
 
 7-32 
 
 5'22 
 
 3*27 
 
 Manganese 
 Protoxide 
 
 Water 
 
 0-96 
 0-64 
 
 
 
 
 
 
 
 
 
 
 
 0-32 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 North-East of Ireland. The granites of the north-east of Ire- 
 land are chiefly found on the borders of counties Down, Louth, and 
 Armagh, though a small outburst occurs near Cushundun in County 
 Antrim. It forms three natural areas : First, the granite district of 
 Mourne, which is a circular mass nearly nine miles in diameter north 
 of Carlingford Bay ; second, the district of Carlingford, five miles in 
 
 Haughton, Q. J. G. S., vol. xviii. p. 408. 
 
244 
 
 GRANITES OF CARLINGFORD AND NEWRY. 
 
 diameter, which is also circular, and lies to the south of Carlingford 
 Bay ; and third, the district of Newry, which extends from Slieve 
 Crook, on the north-east, for twenty-eight miles south-west, to Fork 
 Hill and Jonesborough, with an average breadth of six miles. In the 
 Mourne mountains the granite is fine-grained, and abounds in cavities 
 filled with crystals of the minerals which form the granite. This 
 rock consists of quartz, orthoclase, albite, and green mica. The 
 quartz is of a smoky-brown colour, and occurs in hexagonal crystals 
 in the cavities of the rock. The orthoclase is opaque white. The 
 albite occurs in the interstices of the orthoclase and quartz, the 
 mica is dark green and nearly opaque, and contains an unusually 
 large percentage of iron. Among the accidental minerals are 
 beryl, chrysoberyl, octahedral fluor spar, topaz, and peridote. 
 This granite somewhat resembles the elvans of Cornwall. The 
 mineral composition is stated at quartz 28, orthoclase 44*2, albite 
 27-8. 
 
 
 NORTH-EAST OF IRELAND GRANITES.* 
 
 
 Carlingford. 
 
 Newry. 
 
 
 "j 
 I 
 
 -p 
 
 ! 
 
 "C 
 
 > "3 
 
 1 
 
 t> 
 
 i Station. 
 
 |l 
 goi 
 
 >> 
 
 
 i 
 
 s 
 
 9 
 ^ 
 
 1 
 
 4 
 
 
 t* 
 
 
 
 o >. 
 *& 
 
 It 
 
 
 o 
 
 *& 
 
 
 
 N 
 
 ,.d 
 
 a ^ 
 
 , c3 
 
 
 i 
 
 a 
 
 w 
 
 
 
 w 
 
 if 
 
 fl 
 
 
 RJ 
 
 w 
 
 o 
 
 . S 
 
 yt 
 
 
 
 O 
 
 Oo 
 
 s 
 
 Silica 
 
 70-48 
 14-24 
 372 
 
 71-41 
 12-64 
 4-76 
 
 47*52 
 28-56 
 
 71-24 
 
 64-60 
 14-64 
 6-04 
 
 62-08 
 15-92 
 772 
 
 66-08 
 6-76 
 
 0-18 
 
 74-20 
 
 10-84 
 1-88 
 
 Alumina .... 
 Iron Peroxide . . 
 Iron Protoxide . . 
 
 Lime 
 
 1-48 
 0-40 
 
 i -80 
 0-63 
 
 15*44 
 
 i- 4 8 
 0*64 
 
 2-80 
 
 5*52 
 
 2-16 
 
 1-20 
 I-32 
 
 2 84 
 
 trace 
 
 Magnesia. . . . 
 
 Soda 
 
 3*66 
 4-26 
 
 3*03 
 
 5*47 
 
 
 3-13 
 4-09 
 
 4-02 
 
 3*34 
 2-19 
 
 375 
 
 273 
 
 477 
 3-12 
 
 Potash .... 
 
 Loss 
 
 i*59 
 
 
 
 1-50 
 
 1-13 
 
 0-89 
 
 2-19 
 
 0-83 
 
 
 In the Carlingford district there are two varieties of granite which 
 pass into each other ; and the summit of the Carlingford mountains 
 is formed of syenite, which passes into the granite in one locality, 
 and also passes into hornblende rock. In this district there are 
 numerous greenstones. According to Dr. Haughton, one variety of 
 granite which has the grains of medium size consists of quartz, 
 felspar, and green mica. The second variety, which is very fine- 
 grained, consists of quartz, white felspar, and hornblende. Although 
 different mineralogically, these granites are very similar chemically ; 
 
 1 Haughton, Q. J. G. S., vol. xii. pp. 196-199. 
 
GRANITES OF THE SOUTH-EAST OF IRELAND. 245 
 
 Tooth are potash granites, the first consisting of quartz 2070, felspar 
 6 6 '3 7, mica 1276, and the second consisting of quartz 17*16, felspar 
 67*18, hornblende 15*40. The syenites of Carlingford consist of 
 hornblende and anorthite. The formation of anorthite is attributed 
 to the addition of carboniferous limestone of the second variety of 
 granite when fused ; and in the locality of Grange Irish, the granitic 
 dykes which pierce the carboniferous limestone are found to be 
 changed into coarse-grained syenite. 
 
 In the Newry district, there is potash granite to the south of 
 the town and soda granite to the north. The potash granite is 
 similar to the first-noticed granite of Carlingford. The soda granites 
 
 characterised by the presence of jet black mica and reddish trans- 
 ucent felspar. 
 
 Leinster. 1 The granites of the south-east of Ireland occur in 
 the counties of Dublin, Carlow, Kilkenny, Wicklow, and Wexford. 
 Professor Samuel Haughton describes the main chain of granite 
 hills as extending from Booterstown, county Dublin, to Poulmounty 
 in the south of Carlow, within five miles of New Ross. This chain 
 has an unbroken length of sixty-eight miles, and is from eight miles 
 to fifteen miles broad. Secondly, to the east of the main chain, and 
 parallel to it in Wicklow and Wexford, are about twenty isolated 
 districts, where granite breaks through the Cambrian slates. This 
 chain extends forty-three miles from Ballinaclash, county Wicklow, 
 to Camaross Hill, county Wexford. 
 
 The granite of the main chain varies but little in appearance, 
 and consists of quartz, orthoclase, silvery grey mica, and black mica. 
 The quartz is uniformly grey and transparent. The orthoclase is 
 invariably white and opaque, and occasionally forms large porphyritic 
 crystals. Dr. Haughton thinks that albite probably occurs, though 
 he has been unable to identify it. The grey mica is in plates varying 
 from ^\jth of an inch to three inches diameter ; and occurs in flat right 
 rhombic prisms, or in hexagonal tables ; it belongs to the species 
 called Margarodite. The black mica is only in small quantity ; the 
 only accidental minerals are schorl, beryl, apatite, garnet, fluor spar, 
 spodumeme. In chemical composition this granite varies but little, 
 the silica only ranging between 70 and 74. The mineralogical com- 
 position is mica 13*37, felspar 61*18, quartz 24*98. 
 
 Rev. H. Lloyd (1833) notices that to the south of Dublin, opposite 
 the village of Black Rock, the granite is composed of rounded 
 masses which he thinks to be water-worn and cemented in a granite 
 matrix. 2 The microscopic structure of the granite of Ballyknockan, 
 county Wicklow, has been described by Professor Hull. 3 
 
 1 Haughton, Quart. Jour. Geol. Soc., vol. xii. 1856, p. 171. 
 
 2 Geol. Soc. Dublin, vol. i. part i. p. 83. 
 
 3 Journal R. Geological Society of Ireland, vol. iv. p. 6 ; and in Geol Mag. 
 
2 4 6 
 
 VARIOUS GRANITES OF LEINSTER. 
 
 
 MAIN CHAIN OF LEINSTER.I 
 
 ISOLATED GRANITES 
 OF LEIN3TER.2 
 
 
 
 
 1 
 
 ' 
 
 
 
 
 
 
 
 
 
 1 
 
 1 
 
 |1 
 
 
 jL 
 
 |j 
 
 a 
 O* 
 
 
 jA 
 
 Sd 
 
 "bo 
 
 
 
 4 
 
 b 
 
 
 d 
 
 s 
 g 
 
 [j 
 
 a 
 
 H 
 
 
 1 
 
 ^3 
 
 
 
 1 
 
 1 
 
 ll 
 
 
 
 !s 
 
 1 
 
 | 
 
 
 
 
 1 
 
 I 
 
 JH 
 
 
 II 
 
 J3 o 
 
 M 
 
 & 
 
 1 
 
 H 
 1 
 
 -** 
 
 ^>s 
 
 1 
 
 1 
 
 in 
 
 1 
 
 8 
 
 1 
 
 | 
 
 I 
 
 |l 
 
 Silica 
 
 70*28 70*32 70*38 
 
 70*82 
 
 73*00 73*20 73*24 73*28 74*24'7i'8o 
 
 66*60, 68*56. 70*32 
 
 80*24 
 
 63*80 
 
 Alumina . 113*64 16*14 
 
 12*64,14*08 
 
 13*64 15*48 15*45 12*64 13-64 11*72 
 
 13 -26 14*44.11 -24 
 
 12*24 
 
 17-60 
 
 
 
 
 
 I 
 
 
 
 
 
 
 
 
 
 
 Iron Per- 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 oxide . 
 
 2 '60 3*20 
 
 3*16 
 
 3*47 
 
 2*44 
 
 1*72 
 
 1 '60 2 '00 
 
 1*40 
 
 3*88 
 
 7*321 5*04 
 
 4*80 
 
 0*72 
 
 3*4 
 
 Iron Prot- 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 O'CQ 
 
 Lirne 
 
 2*04 
 
 34 
 
 2*84 
 
 2*65 
 
 , 84 
 
 
 Q> 
 
 1*72 
 
 I* 4 8 
 
 , 
 
 .. 
 3*361 3*85 
 
 3*01 
 
 o-8 9 
 
 2-70 
 
 Magnesia . 
 
 .. 
 
 .. 
 
 o'53 
 
 0*31 
 
 0*1 1 
 
 
 .. 
 
 .. 
 
 .. trace 
 
 1*22 0*43, 0*73 
 
 trace 
 
 1*00 
 
 Soda . 
 
 2*82 
 
 3 '39 
 
 
 2*31 
 
 3 'S3 
 
 3*i8 
 
 3*18 
 
 2*97 
 
 2-72! 3*06 
 
 3*6o 3 '36' 3*39 
 
 5-58 
 
 S'io 
 
 
 
 
 
 
 
 | 
 
 
 
 
 
 I 
 
 
 
 Potash . 
 
 5'79 
 
 4*65 
 
 5 '9 
 
 4-64 
 
 4*21 
 
 4-80! 4*59 4*70 
 
 3 '95 
 
 4'77 
 
 2*31 
 
 2*781 2*27 
 
 0*40 
 
 2*61 
 
 'Loss . 
 
 .. 
 
 0*96) 1*16 
 
 i '39 
 
 !*26 
 
 .. 
 
 1*20 1*04 
 
 1*20 
 
 o'95 
 
 2*34 
 
 1*00 
 
 1*62 
 
 .. 
 
 0*76 
 
 
 
 
 
 
 
 I 
 
 
 
 
 
 Carbo 
 
 nate 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ;of Li 
 
 me. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 i 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 1*62 
 
 
 
 The Isolated Granites of Leinster. The isolated granites differ 
 in mineral composition much more than those of the main chain. 
 They are referred to four geographical groups. The first or western 
 group extends in a broken manner from near Rathdrum, for about 
 10 miles to Aughrim, and is well seen in Cushbawn Hill. In the 
 north it contains red felspar and black mica ; but in most cases it is 
 a fine-grained granite, with grey quartz, white felspar, and minute 
 particles of grey and dark-green mica, so small in quantity that 
 the rock may be said to consist of quartz 17*4 and felspar 82*6. 
 On analysis it contains 70 per cent, of silica; 1*34 of carbonate 
 of lime is supposed to be infiltrated from the overlying limestone 
 gravel. 
 
 The second group of these granites consists of Croghan Kinshela 
 and Conna Hill, and has a length of about six miles. On its north 
 are the famous gold mines of Wicklow. This granite is chiefly com- 
 posed of quartz 38, albite 62, but contains a variable quantity of 
 chlorite, sometimes nearly wanting, sometimes plentiful. The quartz 
 in this rock occurs in small rounded granules. It is a soda-granite, 
 and contains 80 per cent, of silica. The brilliant white colour is due 
 
 1 Haughton, Q. J. G. S., vol. xii. pp. 177, 186. 
 
 58 Ibid., pp. 182, 183, 184; R. Irish Acad., vol. xxiii. 
 
FOLIATED GRANITE OF GALWAY. 247 
 
 to tlie albite, but it is sometimes stained by chlorite, which appears 
 to take the place of mica. 
 
 The third group of isolated granites commences S.W. of the 
 village of Oulart in County Wexford, and extends at intervals for 15 
 miles to Camorus Hill. These granites consist of grey quartz, white 
 felspar, which occasionally become yellow or pink, and black mica, 
 which is probably mixed with hornblende. Professor Haughton 
 considers it to consist of quartz 6*44, felspar 89*69, mica 3*60. 
 
 The fourth group of granites is at Carnsore, and consists princi- 
 pally of grey quartz and reddish felspar, frequently associated with 
 green mica and a variety of hornblende, which is irregularly distri- 
 buted. This is a potash granite, and neglecting the mica, consists of 
 quartz 21*50, felspar 78*50. 
 
 Hence Professor Haughton concludes that the Leinster granites 
 belong to two types ; first, the potash granites of the main chain and 
 of Carnsore ; and, secondly, the soda granites of the isolated series. 
 The potash granites are subsequent in date to the Cambrian rocks 
 and older than the carboniferous limestone. The soda granites are 
 also newer than the Cambrian rocks, but there is no evidence of their 
 exact age. In I858 1 Dr. Haughton regarded the isolated granites as 
 formed by irruption of granite of the main chain adulterated or mixed 
 with the materials of the rocks through which it burst. 
 
 Professor Haughton states that the eruptive granites can be dis- 
 tinguished from the metamorphic granite 2 by their felspars never 
 including any lime felspar, and by containing albite in addition to 
 orthoclase. Among such granites he would class those of Cornwall 
 and Devonshire, Leinster, Mourne, and Peterhead. The granites 
 which he believes to be metamorphic contain, in addition to the 
 orthoclase, oligoclase or labradorite, but albite is never found. As 
 examples of such granites may be quoted those of Donegal, Galway, 
 West and Central Scotland, and Aberdeen. 
 
 Galway. A large tract of granite stretches west from the tow r n 
 of Galway to Bertraghboy Bay and Dogs' Bay, which has been 
 described by Mr. Kinahan. 3 Professor Hull, F.RS., 4 has examined 
 its microscopic characters. It is a foliated rock, probably of metamor- 
 phic origin at Firbogh, in Galway Bay, and consists of quartz, dull 
 waxy felspar probably oligoclase, and dark-green mica in nearly equal 
 proportions, with porphyritic crystals of flesh-coloured orthoclase. 
 The quartz is never crystallised, and envelops all the other minerals. 
 It contains numerous fluid cavities. Orthoclase occurs in small crystals 
 as well as in larger masses. It is penetrated in two directions by irre- 
 gularly parallel lines, like chains, of microscopic beads. Scattered 
 flakes of brown mica are imbedded both in the silica and felspar. 
 Colourless mica occurs in twisted flakes. There is probably a little 
 magnetite. 
 
 1 Trans. Royal Irish Acad., vol. xxiii. p. 611. 
 
 2 Proc. Royal Society, vols. xvii. xviii. 
 
 3 Explanation of Sheet 105, Geol. Survey, Ireland. 
 
 4 Geological Magazine, May 1873. 
 
248 
 
 GRANITE VEINS IN SCOTLAND. 
 
 Mayo. A considerable mass of granite extends from N.E. to S.W. 
 in County Mayo. Professor Hull describes 1 that of Aillemore as 
 forming two mountains named Coroock Brack and Knockaskeheen. 
 It is of a grey colour, usually fine-grained, consists of quartz, ortho- 
 clase, and probably oligoclase, and dark-green mica. Occasionally it 
 has the aspect of graphic granite. It is traversed by joints which run 
 IS", by 10 W. It is surrounded by schists, and is older than the May 
 Hill sandstone. 
 
 The orthoclase under the microscope is often cloudy, but some- 
 times shows cross-banded structure, as in the Firbogh granite. The 
 quartz is similar to that of Galway. The mica often includes grains 
 of magnetite. 
 
 This rock is said by Mr. Symes to be a true eruptive granite, 
 without a trace of foliation, and sends veins into the surrounding 
 rocks. 
 
 Granite Veins. 
 
 Granite Veins. It is difficult to find a satisfactory example of 
 any extensive tract of granite without the occurrence of granite veins 
 ramifying through the neighbouring rocks. They occur in Cornwall, 
 Cumberland, and Arran, in Ben Cruachan, at Strontian, in Glen Tilt, 
 and generally throughout the Highlands. The same is true for the 
 continent of Europe ; and perhaps we nowhere find a better example 
 of the elevation of granite in a solid form, than that described by 
 Murchison at the Ord of Caithness. This granite, on its northern 
 flank, supports the old red conglomerate, whilst to the south it 
 occupies a cliff on and near the shore, the verge of which affords 
 a remarkable breccia, compounded from all the beds of the oolitic 
 series that occur on this coast. This breccia of sandstone, shale, 
 and limestone, is tilted off from the granite wherever that rock 
 protrudes upon the shore, whilst the strata are regularly developed 
 
 where the granite recedes into the 
 interior. No veins or portions of 
 the granite are to be met with 
 in or above the oolitic breccia, 
 which, by its disturbed position, 
 appears to fix the maximum of 
 antiquity of the elevation of the 
 granite as not older than the coral- 
 line oolite. 
 
 Tornidneon. The granite veins 
 of Tornidneon in Arran pass from 
 a body of very coarse - grained 
 granite through nearly vertical 
 laminae of dark quartzose clay 
 K & slate \ the line of junction dividing 
 
 the whole side of a hill. One 
 
 he veins encloses fragments of slate, and divides itself into 
 1 Hull, Journal of the Geological Society of Ireland, vol. iv. p. 4. 
 
 'w^ 
 
 of 
 
GRANITE VEINS IN CORNWALL. 
 
 249 
 
 branches which cross the laminae of slate, cutting off both the quark 
 zose and argillaceous layers. The granite becomes much finer- 
 grained along the veins, and 
 nearly in proportion to their -SSili^E 
 smallness ; so that in the narrow- 
 est veins it is nearly compact. 
 Strings of fine-grained granite 
 divide the coarser sort 
 
 Glen Tilt. In Glen Tilt, 
 MacCulloch has described nume- 
 rous and valuable facts of this ; 
 nature. At the bridge beyond 
 Forest Lodge, granite, hornblende 
 slate, and crystalline limestone 
 are very curiously associated. 
 Veins of red granite here divide 
 the other rocks, and enclose frag- 
 ments of them. The singular 
 interlacements of the rocks are 
 here shown by the sketch taken 
 1826. 
 
 Fig. 57- 
 
 Professor John Phillips in 
 
 1. Crystalline limestone laminated by hornblende and red felspar in curved lines 
 or detached masses, round which the laminse of limestone bend, crossed by 
 granite and red felspar veins. 
 
 2. White quartz rock and red felspar crystallised. 
 
 3. Felspathic rock, red, with layers of black hornblende. 
 
 4. Limestone laminated with felspar. 
 
 5. The same with less felspar. 
 
 6. Hornblende and felspar in layers. 
 
 7. Laminated limestone. 
 
 (a.) Red felspar vein a little quartz. 
 
 8. 9. Hornblende, with layers, masses, and veins of white quartz and red fe'spar, 
 
 which substances often occur together, making binary granite of very large 
 grain. 
 
 10. Limestone, with red granite veins. 
 
 1 1. Limestone, red granite veins, and white calcareous spar veins, which divide the 
 
 granite veins. 
 
 12. Red granite, composed of red compact or crystallised felspar, white quart,/, 
 
 and black or gray mica, and encloses hornblende masses which are divided 
 by veins of granite ramifying from the general masses of thafc rock. 
 
 Cornwall The extremity of Cornwall has long been famous 
 for the great variety of curious phenomena connected with the granite 
 veins which there divide the argillaceous slate, hornblende slate, 
 and greenstone rocks, all included by the miners under the title of 
 killas. So many writers of eminence, both English and foreign, have 
 described and reasoned upon these occurrences, that it is difficult to 
 select from the immense variety. The following is Majendie's account 
 of the veins at Mousehole, three miles south-west of Penzance : 
 " At this period the clay slate ceases, and the granite commences, 
 forming a promontory which runs out in a southern direction from 
 the central ridge. The slate is of a grey colour ; it is in strata nearly 
 
250 
 
 VEINS AND VEIN-STONES. 
 
 horizontal, but having a slight dip to the east ; it increases in 
 hardness near the junction. The granite, which is generally coarse 
 and porphyritic from the large embedded crystals of felspar, becomes 
 here of a finer grain, with black mica and light flesh-red felspar. 
 On the north it laps over the schist. At this spot numerous granite 
 veins, varying in width from about a foot to less than an inch, pass 
 through the slate ; the two principal veins proceed nearly east from 
 the hill above, for more than fifty yards, until they are lost in the 
 sea. One of these, not far from its first appearance, is divided and 
 heaved several feet by a cross vein consisting of quartz intermingled 
 svith slate ; fragments of slate appear also in the granite veins. The 
 most remarkable vein, after proceeding vertically for some distance, 
 suddenly forms an angle, and continues in a direction nearly hori- 
 zontal, having slate above and below." 1 
 
 The Devonian rock, locally called killas at this place, has much 
 the aspect of greenstone, and it appears generally true that the clay- 
 . / slate is much altered in character round 
 all the granites of Cornwall and Devon. 2 
 The veins of granite are generally most 
 fine-grained towards the walls. Von 
 Oeynhausen and Von Dechen mention 
 three principal veins at Mousehole, one 
 3j to 10 feet wide; quartz veins cross 
 the direction of the granite veins, and 
 sometimes divide them, and apparently 
 alter their character. Schorl occurs ir- 
 regularly in the granite, and in some of 
 the quartz veins. The intricate character 
 of the veined masses of Mousehole will 
 be best understood by consulting the dia- 
 gram (fig. 58), copied from the sketch of 
 the distinguished Prussian geologist above 
 named. 
 
 s 8 - At Cape Cornwall a granite vein heaves 
 
 a quartz vein in a direction contrary to the general law. In the 
 Lizard district granite veins divide serpentine. 
 
 Veinstones. Under this name Dr. Sterry Hunt, F.R.S., designates 
 a class of veins which are neither injected from below nor infiltrated 
 from above, but derived from the rock which immediately surrounds a 
 closed fissure in which water accumulated. This origin of felspathic 
 and granitic veins by segregation was first indicated by Daubree, and 
 subsequently taught by Dana. In schists in the Appalachian region 
 of Canada are veins with flesh-red orthoclase mixed with chlorite 
 and white quartz. Some veins are made up of orthoclase, quartz, and 
 mica ; they have the character of coarse granite, and include a number 
 of rare minerals in small quantity. Granitic veinstones abound in 
 the Montalban series in the United States. Sometimes the felspar 
 
 1 Cornwall Geol. Soc. Trans., vol. i. 
 
 2 J. A. Phillips " On so-called Greenstones of Cornwall,'' Q. J. G. S., vol. xxxii. 
 
PHONOLITE OF THE WOLF ROCK. 251 
 
 crystals are enclosed in vitreous quartz, sometimes quartz lines each 
 side of the vein, or occupies the middle, or alternates with bands of 
 orthoclase, or orthoclase mixed with quartz, as in graphic granite. 
 Garnet and tourmaline are common as accidental minerals. Such 
 veins are distinguished by their shortness and irregularity. These 
 concretionary veins are frequent in the Laurentian rocks of Canada, 
 which are alternations of gneisses and limestones ; but they pass on 
 the one hand into metalliferous veins, and on the other into veins 
 with calcite, apatite, and various calcareous and magnesian silicates, 
 accompanied by orthoclase and quartz. Banded structure and drusy 
 cavities are characteristic evidences of deposition from water, though 
 the walls may be coated with hornblende, phlogopite, or other minerals 
 usually accounted igneous. 1 
 
 Syenite of Ailsa Craig. 2 Ailsa Craig is an island ten miles west 
 of Girvan, 1113 feet high, 1500 yards long, and 1250 wide. It is a 
 fine-grained syenite, showing close parallel vertical joints on the south 
 and west sides. Dykes of dolerite 'run into it in a N. W. direction. 
 
 Phonolite. 
 
 The Wolf Kock lies nine miles S. W. of the Land's End, and at high 
 water is covered by the sea to a depth of two feet. The rock has a 
 yellowish grey base containing clear crystals of glassy felspar. Under 
 polarised light, crystals of felspar and nepheline are seen to be em- 
 bedded in a fine-grained matrix of nepheline, felspar, and hornblende. 
 In thick sections the felspar and nepheline are well-coloured * but in 
 thin sections colours are only shown by the hornblende, the hexagonal 
 sections being black and the rectangular sections white. The felspar 
 also has the aspect of a mosaic of dark and light stones ; it frequently 
 encloses crystals of nepheline and hornblende, and ' gives all ' the 
 characters of orthoclase ; it contains many glass cavities and minute 
 crystals. Hornblende is found in small green prisms sometimes 
 crowded about grains of magnetite. 3 
 
 The greater part of the rock consists of nepheline, in crystals which 
 vary from T Jo^ * ToW^li of an inch across. This rock, probably of 
 Primary age, is classed by Mr. Allport as a porphyritic phonolite. 
 The following is the analysis by Mr. J. A. Phillips, F.K.S. : 
 
 Silica. . . . 
 Alumina. . . 
 Peroxide of iron 
 
 56-46 
 . 22-29 
 
 2'7O 
 
 Magnesia . . . 
 Phosphoric acid . 
 Potash .... 
 
 trace 
 trace 
 
 2'8l 
 
 Protoxide of iron 
 
 'Q7 
 
 Soda 
 
 11*11 
 
 Manganese . 
 
 trace 
 
 Water .... 
 
 2 XX 
 
 Lime 
 
 I '4.7 
 
 
 
 
 
 
 
 1 T. Sterry Hunt, Chemical and Geological Essays, 1875, P- J ^3- 
 
 2 Mem. Geol. Surv. Scot. Explanation Sheet 7, 1869. Arch. Geikie. 
 
 3 Allport, Geol. Mag., vol. viii. p. 247, 1871. 
 
( 2 5 2 ) 
 
 CHAPTER XVII. 
 THE HISTORY OF VOLCANIC KOCKS. 
 
 Method of Study. Igneous rocks can only be studied with ad- 
 vantage in their natural occurrences on the surface of the country ; 
 and the student will always spend his time most profitably who gains 
 practical knowledge of the behaviour and variation of rocks in the 
 field by detailed study of some classical district. 
 
 A useful preparation for such an investigation is a preliminary know- 
 ledge of the common minerals which combine to compose these rocks. 
 And this can hardly be attained better than by collecting the specimens 
 on the old lava streams of a volcanic district, such as the Auvergne, 
 Siebengebirge, or Eifel ; but in cases where such practical work is not 
 immediately convenient, specimens of rock-forming minerals (see p. 
 22) must be obtained from some trustworthy dealer. When their 
 crystalline forms and other physical properties have been sufficiently 
 examined, duplicate specimens of each should be sliced and mounted 
 for study under the microscope with polarised light, so as to form a 
 standard series, by which similar minerals may be recognised, when 
 they occur in combination in igneous rocks. Having the commoner 
 types of minerals thus prepared, and furnished with a microscope, to 
 which a polariscope is fitted, the student is qualified to begin practical 
 work. He must prepare, or have slices prepared from typical volcanic 
 rocks, and will soon be able to identify the constituent minerals in 
 them by comparison with his types, so that the rock slices will be- 
 come a series of standards. He will then be in a position to add to 
 the store of knowledge by determining the nature of the rocks found 
 in districts which may come under examination. 
 
 The Characters of Minerals in Polarised Light. The common 
 rock-forming minerals can often be identified under the -ifiicroscope 
 by their crystalline forms and method of arrangement ; but under 
 polarised light identification is more often certain. The following 
 notes on the optical characters of minerals may be useful (but see 
 Rosenbusch : " Mikrosk. Physiog. der petrog. wichtigen Minera- 
 iien." 1873): 
 
 Quartz usually shows one colour in the centre, with two or three 
 other colours around it. The colours are very bright and clear in 
 thick slices, while in thin slices they are white or grey blue. 
 
 Orthoclase sometimes shows cross hatching. Occurring in twins, 
 
OPTICAL CHARACTERS OF MINERALS. 253 
 
 it usually is blue on one side of the twin plane and yellow on the 
 other, but the colour varies with thickness of the slice, &c. 
 
 Oligoclase exhibits many alternations of blue and yellow in thin 
 bands. This condition of repeated twinning is common in triclinic 
 felspar. 
 
 Labradorite only assumes a darker tinge when the polariser is 
 rotated, and shows banded structure due to twinning. 
 
 Anorthite is distinguished from labradorite by showing brighter 
 colours than the other felspars. 
 
 Nepheline. In thin slices its colours are very pale, and vary, 
 between crossed nicols, from dark milky-blue to brownish-yellow. 
 
 Leucite shows no colour, though when the slices are sufficiently 
 thick the mineral has a bluish-white tint, paler than nepheline. 
 
 Black Mica gives green, yellow, and brown tints when the sec- 
 tion is hexagonal ; when cut across the lamellae, and the plates are 
 very thin, there is a carmine tint. 
 
 White Mica, in which the divergence of the optic axes is greater, 
 shows yellow and red colours, but the colours are clear. 
 
 Amphibole. In this the colours may be either pale green or blue, 
 or yellowish, or dark brown between crossed nicols. They are brighter 
 than those of pyroxene. The Actinolite variety gives emerald green ; 
 glaucophane, clear blue; hypersthene is intermediate between 
 pyroxene and amphibole ; epidote is citron yellow to brown. 
 
 Pyroxene. The colours of augite are not so clear as those of 
 epidote, nor so bright as those of olivine. They are often yellow and 
 red, sometimes green-brown and rose, varying with the rock. 
 
 Olivine in very thin slices is colourless, but in thicker sections its 
 colours are brilliant red and green. Apatite varies from pure white 
 to bluish white or yellowish. 
 
 Titanite between crossed nicols gives deep-yellow and brown 
 colours, but less bright than those of augite and hornblende. 
 
 Tourmaline is brown or dark green, rarely blue or rose colour. 
 It may be compared with hornblende, biotite, and epidote. 
 
 Frequently it is necessary to measure the angles of crystals to 
 identify some minerals with certainty ; this can be done by means of 
 graduated circles and a rotating stage, such as are found in Rosen- 
 bush's petrological microscope. We have also used crossed spider 
 lines in the eye-piece, one of which is capable of being rotated, as an 
 instrument for measuring crystal angles under the microscope, with 
 the advantage that the object is not moved. 
 
 Texture of Igneous Rocks. The structures of igneous rocks 
 which are most easily recognised, admit of being classed according to 
 the conditions of solidification into three types : first, crystalline ; 
 secondly, semi-crystalline ; and thirdly, uncrystalline. These divisions 
 are adopted by Professor Zirkel in his "Microscopical Petrography." 
 
 (i.) Crystalline Rocks. Granite is the best type of a crystalline 
 rock, in which the texture is macroscopic, or such that the crystals 
 may be distinguished by the naked eye ; but the same type of crystal- 
 line structure may exist when the texture is cryptocrystalline, or such 
 
254 STRUCTURE AND TEXTURE OF LAVAS. 
 
 that the crystals can only be recognised under the microscope. No 
 perfect boundary can be drawn between perfectly crystallised rocks 
 and those in which a minute quantity remains of the original material 
 which has not been differentiated into crystals. 
 
 (2.) Semi-crystalline Rocks. The semi-crystalline rocks are a 
 large class, in which the greater part of the material is an amorphous 
 substance, through which are scattered crystals that may be either 
 microscopical or macroscopicaL The crystals may be few, or so 
 numerous as to form nearly the whole of the rock. These semi- 
 crystalline rocks present several varieties, according to the condition 
 of this uncrystallised material. 
 
 First, it may be purely glassy, consisting of glasses which yield 
 no colours in polarised light. 
 
 Secondly, the glass may be partially devitrified, by the formation 
 of grains and needles. The needles are usually black and hair-like, 
 the grains angular or rounded. They have been termed crystallites and 
 globulites, but neither polarise, and they are both therefore regarded 
 as glass richer in iron. The dark needles or trichites are aggregated 
 into branched or net-like masses. The globulites are common in 
 dolerites and other basic rocks, but rare in rhyolites. 
 
 Third, devitrification may proceed so far that the glass is entirely 
 replaced by such small bodies as those described. 
 
 Fourth, the microfelsitic mass sometimes presents an amorphous 
 substance, free from glass, without transparency, and incapable of 
 being resolved into separate particles. This condition is more charac- 
 teristic of the quartz porphyries and rhyolites than of basic rocks. It 
 will readily be understood that it is often difficult to distinguish the 
 original nature of a rock which has undergone some of these phases of 
 devitrification ; and chiefly on this account, the volcanic rocks of the 
 Primary period have only recently been shown to be essentially the 
 same as those of the Tertiary period, but somewhat decomposed. 
 
 (3.) Uncrystalline Rocks. The uncrystalline type consists of a 
 volcanic rock which was originally amorphous, sometimes glassy, like 
 obsidian or tachylyte, and often in the microfelsitic state. Between 
 these rocks, therefore, and the semi-crystalline rocks there is a com- 
 plete transition. 
 
 Ground Mass and Base. Zirkel uses the term ground mass to 
 indicate the part of a rock between visible crystals which shows no 
 structure to the unaided eye ; but when the rock is examined under 
 the microscope, and a similar homogeneous appearance is seen between 
 crystals, that undifferentiated paste is named the base ; and it is this 
 base which presents the varied conditions of texture which we have 
 enumerated in semi-crystalline rocks. 
 
 Fluxion Structure. Fluidal structure is a term applied to more 
 or less glassy rocks, in which streams of microliths or needle-like 
 crystals undulate and bend in their arrangement about a larger crystal. 
 Such fluxion structure is often seen in the least crystalline basalts, 
 trachytes, and phonolites. These conditions are best observed under 
 the microscope between crossed nicols, and under a low power. The 
 
MICROSCOPIC TEXTURE OF LAVAS. 255 
 
 appearance seen resembles the curvature and waved lines on some 
 kinds of marbled paper. Those rocks in which the fluidal structure 
 is best developed, are rich in broken crystals. 
 
 Microliths. Among constituents found in the base of some lavas 
 are microlitks, which may be described as imperfectly formed needle- 
 shaped crystals. They may belong to many minerals, such as felspar, 
 hornblende, augite, mica, or apatite ; and if the microlith can be 
 identified, it is referred to its mineral species. Garnets have no 
 tendency to form microliths, and specular iron occurs in six-sided 
 plates. When the microlith is colourless it is a belonite, when it is 
 black and opaque it is a trichite. 
 
 Opacite, Ferrite, and Viridite. Three other terms are used in 
 describing the base of rocks to designate substances which cannot be 
 certainly identified. First, opacite exists as black opaque grains, 
 and is found among the products of decomposition of minerals. It 
 may consist of oxides of iron, titanium, manganese, graphite, and 
 various earthy silicates. Secondly, ferrite is a rust-coloured material 
 of amorphous form, which cannot be identified, but is probably sesqui- 
 oxide of iron. Third, viridite is a fibrous or scaly-green transparent 
 substance, consisting of silicates of iron and magnesia. It results 
 from the decomposition of olivine, augite, and hornblende. The fibres 
 are referred to such a mineral as delessite, the scales to such a mineral 
 as chlorite. 
 
 Propylite. 
 
 In treating of the several kinds of volcanic rocks, 1 we have followed 
 the grouping of Bichthofen, chiefly because these rocks have been so 
 fully described in elaboration of his researches. And on this account 
 it has seemed desirable to give, in addition to a history of the 
 European volcanic rocks, a statement of the conditions under which 
 such rocks are found and their mineral variations in the western 
 volcanic region of the United States. 
 
 Propylite. Eichthofen states that around Bisytritz, in Northern 
 Transylvania, propylite forms cones. It also occurs at Nagybanya 
 and Kapnik in Hungary. Massive eruptions of it are found on the 
 southern slopes of the Carpathians ; and it appears in every case to 
 have been ejected through fissures, since no traces of volcanoes of 
 propylite are known in Europe. The rock is always porphyritic, and 
 consists of a microcrystalline paste, of a dark-green or greenish-brown 
 colour, or red and grey. In the ground mass are scattered crystals of 
 white or light-green oligoclase, and dark-green fibrous hornblende. 
 The paste is formed of the same ingredients, with titaniferous iron ; 
 so that the colour of the rock is always green, and it closely resembles 
 oligoclase trachyte, and consists of the same ingredients as hornblende- 
 andesite. Some varieties contain rounded grains of quartz, and other 
 varieties hold crystals of augite. 
 
 1 Rosenbusch's "Mikros. Physiographic der massige Gesteine " is a valuable in- 
 troduction to a knowledge of volcanic rocks, from which we have quoted many facts. 
 
2 5 6 
 
 HISTORY OF PROPYLITE. 
 
 Relation of Propylite to Silver. Propylite is frequently as- 
 sociated with veins of silver, as in the Carpathian mountains, and 
 more strikingly in North America. It forms one of the walls of the 
 famous Comstock lode. It is connected with silver veins in the 
 Aurora district, with some of those in Silver Mountain, with 
 the Moss lode of Arizona, and with the silver veins of some parts 
 of Mexico. 
 
 ' AMERICAN PROPYLITES.I 
 
 AMERICAN QUARTZ- 
 PROPVLITES. 
 
 AMERICAN 
 
 DlOKITES. 
 
 
 
 34 
 
 
 
 3 
 
 
 
 jq 
 
 
 
 g | 
 
 1 
 
 A 
 
 s 
 
 a 
 
 A 
 
 o 
 
 >C 
 
 d 
 
 1 
 
 
 
 11 
 
 
 
 > tn 
 
 a 
 1 
 
 6 
 
 p 
 
 li 
 
 
 
 sj 
 
 1 
 
 fi| 
 
 a 
 
 o? i^ 
 
 a 
 
 o 
 
 3 
 
 
 
 
 a 2 
 
 
 
 a 
 
 o| 
 
 13 o 
 
 
 
 
 1 
 
 If 
 
 
 
 
 
 
 
 
 o 
 
 0> 'aS 
 
 
 8 
 
 -*^ " 
 
 o 
 
 a 
 
 ^ 
 
 O 4> 
 
 
 s 
 
 
 * 
 
 i 
 
 53 
 
 1 
 
 5 
 
 fe 
 
 ~ 
 m 
 
 H^ 
 
 Silica . 
 
 58-66 
 
 60-33 
 
 64-62 
 
 66-34 
 
 68-44 
 
 74 '4 1 
 
 5671 
 
 60 '20 
 
 Alumina 
 
 17-90 
 
 1974 
 
 11-70 
 
 14-80 
 
 14-86 
 
 2-84 
 
 18-36 
 
 jg-ce 
 
 Peroxide of iron 
 
 
 0-70 
 
 
 4-07 
 
 
 
 
 
 Protoxide of iron 
 
 4-11 
 
 2-50 
 
 8 : 39 
 
 
 3-80 
 
 I3'30 
 
 6-45 
 
 4'37 
 
 Manganese . 
 
 
 trace 
 
 
 
 
 
 
 
 Lime . 
 
 5 ; 87 
 
 372 
 
 8-96 
 
 2'9Q 
 
 i- 9 o 
 
 0*40 
 
 6-ii 
 
 4-41 
 
 Magnesia 
 Soda . 
 
 2-03 
 2-07 
 
 4'oi 
 4'36 
 
 1-18 
 
 0-92 
 5'l6 
 
 3-22 
 
 i '-28 
 
 3-92 
 
 2 *2O 
 
 3 '20 
 
 Potash . 
 
 3-19 
 
 1-62 
 
 i -95 
 
 
 5 -o8 
 
 6'02 
 
 2-38 
 
 3-87 
 
 
 
 
 PO 5 
 
 C0 2 
 
 C0 2 
 
 
 
 Lithium 
 
 
 
 
 
 trace 
 
 1-03 
 
 0-94 
 
 ... 
 
 
 trace 
 
 Loss on Ignition . 
 
 6'53 
 
 3-13 
 
 I'02 
 
 2-13 
 
 2-26 
 
 179 
 
 1-94 
 
 2-97 
 
 Propylite of the Virginia Mountains. Propylite is a widely 
 distributed rock in the United States, and is met with in the form of 
 tuffs as well as in solid rock. The most important outburst de- 
 scribed by Clarence King occurs in the higher part of the Virginia 
 range, and extends from Pyramid Lake on the south to the Sierra 
 Nevada. Before the propylite was erupted the Virginia mountains 
 consisted of slates, limestones, schists and quartzites, disturbed by 
 intrusive granite. Great masses of diorite had already burst through 
 these, and formed the highest peaks, such as Mount Davidson. Then 
 the propylite was thrown out from fissures, which run in the direction 
 of the range, and extend from the summit of the range to its base 
 on both sides. On the south and east the propylite flood ran to 
 Carson Plain, and on the west to Steamboat Valley ; and only the 
 highest portions of the diorite peaks were lifted above the products 
 of this outburst. The eruption was intermittent, and the material 
 was ejected in a viscous condition. The first outbursts were of 
 olive-green propylite, crystalline and porphyritic. The second, on 
 the north and south of Mount Davidson in Washoe, is a propylitic 
 breccia enclosed in an ordinary propylitic matrix. The third out- 
 
 1 U. S. Geol. Surv., Fortieth Parallel, vol. i. p. 560 ; Table VIII. 
 
MINERAL VARIETIES OF PROPYLITE. 257 
 
 burst formed narrow dykes, which often stand up in bold remnants 
 thirty or forty feet above the surface. The texture sometimes becomes 
 fine, compact, and fissile, like hornblendic slate, where it is in con- 
 tact with diorite. Propylite is more easily decomposed than any other 
 volcanic rock, forming white, yellow, and red clays. North of 
 Tuscarora it is overlain by rhyolite. In the Toyabe range, near 
 Boone Creek, hornblende propylite is overlaid by rhyolite and basalt. 
 At Kaspar's Pass, north of Hot Spring station, at the S.W. of 
 Montezuma range, the propylite is also covered by rhyolite and 
 basalt. At Berkshire Canon, propylite lies to the east of a lofty 
 mass of melaphyre, and is invaded by quartz-propylite and andesite, 
 and overflowed by trachyte, which in turn is covered by rhyolite, 
 succeeded by basalt. 
 
 Minerals in Propylite. The green hornblende of propylite is 
 frequently changed into epidote, a change which is never seen in 
 the brown hornblende of andesites. Frequently the felspar crystals 
 are filled with hornblende material, which is changed into bright 
 yellow epidote in the Washoe country. Apatite occurs in short 
 thick rounded prisms, having a hexagonal section. In Sheep Corral 
 Canon the propylite is grey, and in the Truckee and Montezuma 
 ranges the low hills of propylite have a yellowish green-grey colour. 
 In the Fish Creek mountains of Nevada, the hornblende is composed 
 of aggregates of green microliths ; and augite occurs which is re- 
 markable for its pale-yellow colour; brown mica and apatite are 
 found. In Storm Canon the rock is pale yellow and reddish grey. 
 The hornblende in andesite never exhibits the parallel staff structure 
 seen in propylite. At Tuscarora, in the Cortez range, green and dark- 
 brown hornblende occur together. 
 
 Quartz Propylite. In that part of the Cortez range which lies 
 south of the Humboldt river is a mass of propylite, with quartz 
 propylite resting upon it and forming the summit of Cortez peak. 
 At Papoose Peak the quartz propylites again come to the surface, 
 extend for about eight miles, and then pass beneath overlying dacite. 
 The general colour of the surface is soft grey, pinkish and salmon 
 colour, varied with green and olive hornblende. The ground mass 
 consists of clear dark plagioclase, more or less fibrous hornblende, 
 microscopic quartz, with fluid cavities sometimes including cubes of 
 salt. The hornblende has the prismatic staff-like form characteristic of 
 the propylitic rocks. There are the usual titanites. The larger 
 felspars are all dull and slightly kaolinised. At Waggon Canon the 
 rock contains a few laminae of brown mica. Biotite seems to be 
 characteristic of the latest injections. Quartz propylite has the aspect 
 of having been erupted in an almost solid condition, showing no 
 tendency to spread out into thin sheets. According to Zirkel, one of 
 the finest exhibitions of this rock is in Berkshire Gallon, Virginia 
 range ; the ground mass is grey, rich in macroscopical crystals of 
 dark-green hornblende, and dust of hornblende in laminae, crowds all 
 the larger felspars. In some localities apatite is present, and occa- 
 sionally there is a little sanidine. The rock at Cortez peak, seen 
 VOL. I. B 
 
258 COMPOSITION OF ANDESITES. 
 
 under the microscope, presents the aspect of being perforated with 
 innumerable pinholes from the abundance of minute quartz grains. 
 
 Quartz propylite forms the hills of Golconda. It is a dark-grey 
 yellow rock, with clear quartz grains about the size of peppercorns. 
 The larger felspars are more or less decomposed. The quartz contains 
 fluid inclusions with moving bubbles, and rather resembles that of 
 diorites in the matter of inclusions. No quartz is visible in the 
 ground mass. Propylites vary greatly in their percentage of silica ; 
 and in chemical composition may be instructively compared with 
 diorites, syenites, and certain slates. 
 
 Andesites. 
 
 Andesites are rocks which consist typically of crystals of oligoclase 
 and columnar hornblende, combined with more or less of a glassy 
 ground mass, small particles of magnetic iron,, and a few flakes of 
 mica. Augite, olivine, magnetite, and hauyine are occasionally present. 
 Andesites vary in colour from grey to dark green, and when horn- 
 blende abounds may be dark brown or black. They vary chiefly in 
 possessing or wanting an amorphous base, and, when a base exists, in 
 the relative proportion of crystals which it includes. The commonest 
 type of andesite is porphyritic, with a microcrystalline ground mass 
 which has large crystals developed in it. They are chiefly of felspar, 
 but include hornblende, mica, and quartz. Chemical analysis some- 
 times yields as much silica as occurs in a dacite. Andesites some- 
 times exhibit a fluid structure, characterised by a parallel arrangement 
 of small, slender crystals, or by the extension of such crystals in curves 
 round larger particles. 
 
 Hornblende-andesite is commonly regarded as the volcanic equi- 
 
 
 QUARTZOSE HORNBLENDE- 
 
 ANDKSITE. 
 
 QUAKTZLESS HORN- 
 
 BLENDE-ANEESITE. 
 
 AUOITE ANDESITE. 
 
 
 
 
 
 ^ 
 
 
 
 
 
 i 
 
 10 
 
 
 
 
 
 ".- 
 
 $ 
 
 to 
 
 
 
 
 
 M 
 
 
 ! 
 
 1 
 
 .j 
 
 03 
 
 E 
 
 CM 
 
 |l 
 
 olkenbu 
 
 s 
 
 1 
 
 i 
 
 1 
 
 -3 
 
 5 
 
 o 
 
 I 
 
 eda Lavs 
 
 
 $ 
 
 5 
 
 < 
 
 
 
 [J 
 
 03 
 
 ^ 
 
 
 Pd 
 
 W 
 
 Silica . . 
 
 76-66 
 
 69-47 
 
 65-46 
 
 6ri3 
 
 62-38 
 
 5 9 -22 
 
 57-60 
 
 60-06 
 
 57-88 
 
 5476 
 
 Alumina . 
 
 12-05 
 
 14-98 
 
 I5'36 
 
 16-44 
 
 16-88 
 
 I3'59 
 
 20-53 
 
 i6'59 
 
 19-09 
 
 13-61 
 
 Peroxide of 
 
 
 
 
 
 
 
 
 
 
 
 iron 
 
 f2***3Q 
 
 2 "Q T 
 
 
 
 7 '33 
 
 5 '55 
 
 
 
 
 
 Protoxide of 
 
 
 
 
 
 
 
 
 
 
 
 iron . . 
 
 i -08 
 
 1*04 
 
 6-65 
 
 9 '23 
 
 ... 
 
 4 '03 
 
 8- 7 6 
 
 1 1 '37 
 
 8-92 
 
 15-60 
 
 Manganese 
 Lime 
 Magnesia . 
 
 1-25 
 trace 
 
 4 : 68 
 0-98 
 
 
 
 
 
 
 
 
 2^ 
 
 6*25 
 
 376 
 
 3 "49 
 0-82 
 
 f-66 
 
 6-66 
 1-70 
 
 5-S6 
 2*40 
 
 3-65 
 trace 
 
 6-44 
 I- 35 
 
 Soda . . 
 Potash . . 
 
 3'55 
 2 '94 
 
 4-46 
 
 4-09 
 
 i '33 
 
 j-2'99 
 
 4-42 
 2-94 
 
 5'3* 
 4-64 
 
 3-04 
 1*46 
 
 3 'bo 
 i '45 
 
 j-9-64 
 
 I '21 
 
 
 
 
 
 
 Water 
 
 
 
 
 
 
 Loss. . . 
 
 1-12 
 
 o'35 
 
 o*34 
 
 0-44 
 
 0-87 
 
 
 
 
 
 
 
 
 
 
MINERAL VARIETIES OF ANDESITE. 239 
 
 valent of diorite ; and though mica is more characteristic of diorite 
 than of andesite, most of the varieties of diorite may be paralleled by 
 varieties of this rock. When andesites are decomposed their percent- 
 age of silica is much reduced. Koch divides the Hungarian andesites 
 into seven varieties, of which two are classed by Eosenbusch as augite- 
 andesite, and five as hornblende-andesite. 
 
 The latter comprise (i.) Labrador-biotite-garnet andesite: (2.) 
 Labrador - biotite - garnet - augite andesite ; (3. ) Labrador - amphibole 
 andesite; (4.) Labrador-amphibole-augite andesite; and (5.) Labra- 
 dor-amphibole-biotite andesite. The minerals thus named occur as 
 crystals in a ground mass, which may be clear as glass, or opaque 
 with crystallites, or rich in orthoclase, as in (i) and (2). 
 
 Concretionary Inclusions in Andesite. The hornblende andesites 
 of the Siebengebirge are remarkable for the concretions and small 
 angular inclusions which they contain. The concretions are well de- 
 veloped at Great and Little Eosenau, and in the Stenzelberg, where they 
 are known to the quarrymen as black granite. They vary from an inch 
 or two in size to a diameter of many inches, and consist of conspicuous 
 columns of hornblende imbedded in a ground mass of andesite. The 
 size of the hornblende crystals and the amount of ground mass be- 
 tween them both vary. When the proportion of hornblende is small, 
 the external shape of the concretion may be undefined, but when the 
 hornblende crystals are in contact, and include the matrix between 
 them, then the external form of the concretion is that of hornblende, 
 as well defined as though the matrix were limited to the centre. 
 Large porphyritic crystals of compact hornblende occur. The crystals 
 appear to have grown gradually at the expense of the surrounding 
 rock, the large forms being built up by the gradual increase in size 
 and blending of many small crystals. In the augite-andesite of the 
 Lowenburg we find similar concretions, only the section is octagonal, 
 and therefore presumably determined by the augite. The texture 
 of these concretions is often the finest possible. The separation 
 between the concretion and the surrounding rock is clean and easy, as 
 though a slight film of kaolin parted them. These concretions are 
 not unlike the mica concretions in granite already described, which 
 may have had a like origin. At Little Kosenau silica sometimes 
 separates so as to form quartz veins in the andesite; the ground 
 mass is cryptocrystalline. It contains, with crystals of plagioclase 
 and oligoclase, much hornblende ; but sometimes biotite and augite 
 replace hornblende. The percentage of silica varies from 46 to 51. 
 
 Mineral Condition of Andesite. The plagioclase is usually 
 microtine in the glassy varieties of the rock ; but in some dacites statf- 
 like crystals of triclinic felspars are frequently aggregated into com- 
 pound crystals ; and it may be that the plagioclase is partly labrado- 
 rite and partly oligoclase, with some other felspars. The crystals con- 
 tain inclusions of the base, inclusions of glass and steam cavities, 
 microliths, and occasional fluid cavities. In Hungary and Transyl- 
 vania fluid cavities abound in dacite, and are absent in andesite. The 
 crystals sometimes are made of fragments united together, some- 
 
260 ANDESITES IN EUROPE. 
 
 times they are more or less decomposed, especially in the andesites of 
 the Stenzelberg in the Siebengebirge. Sanidine is often present. 
 Magnesia-mica occurs in dark-brown or red hexagonal plates, frequently 
 surrounded and penetrated by magnetite. It is usually fresh, always 
 occurs in large plates, and is absent from the ground mass. Horn- 
 blende is usually found in prismatic crystals, either brown or green, 
 commonly green when the rock contains quartz. When hornblende 
 is brown, it is usually surrounded by magnetite ; it then contains ovoid 
 inclusions of glass. The mineral is sometimes decomposed in the 
 greenstone-like dacites. Like mica, it rarely occurs as a part of the 
 ground mass. Augite occurs in small grains and columns as a con- 
 stituent of the ground mass, and also in crystals, but is less abundant 
 than hornblende and mica. It is not decomposed. Quartz occurs in 
 grains and crystals in variable proportion in many andesites. It differs 
 from the quartz of rhyolite, and resembles that of quartz-porphyry in 
 rarely containing glass inclusions ; and it abounds in fluid inclusions, 
 with cubic crystals, except in the Department of Var. 
 
 Tridymite is rare, but is seen in the hornblende-andesite of Dub- 
 nik. Apatite occurs in long colourless needles, and in short crystals 
 tinged blue or brown. It is common in the Stenzelberg in the 
 Siebengebirge, and is always present in the Sengelberg, near Salz in 
 the Westerwald. Titanite is a common accessory mineral in the 
 Siebengebirge, the Department of Var, and Tres Montanas in Canary. 
 Hauyine is characteristic of the andesites of the Canaries. Garnet is 
 found in biotite-andesites near Buda-Pest, and some other Hungarian 
 localities. As products of decomposition, andesites contain opals and 
 chalcedony in Servia and Hungary. 1 
 
 Localities for Andesites. Among the more important European 
 localities for andesites are Shemnitz, Kremnitz, the St. Andra-Visegrad 
 Mountains, near Buda-Pest in Hungary ; the Transylvanian Erzge- 
 birge, the south of Servia, the Smrkouzgebirge in Styria ; at Stary 
 Swietlan, near Banau in Moravia, where the rock abounds in siderite, 
 calcite, and various carbonates. 
 
 In the Auvergne hornblende-andesites are seen in the lava from 
 the Puy de Moutchie, Eigolet-Haut, and Plateau de Durbize, where 
 they make a transition towards trachytes. A rock free from augite 
 occurs in the valley of the Dordogne near Mont Dore. These ande- 
 eites sometimes contain a little crystallised quartz, as at the foot of 
 the Briingelsberg in the Rhondorf Valley, in the Great Breiberg, and 
 at Kelberg in the Eif el ; at the Puy de Chaumont and Liorant in 
 Cantal, and many localities in Hungary, Moravia, and other regions. 
 
 In Italy andesites occur in the Eganean Hills at Monte di Ferro 
 di gran Pietra, Monte della Croce and Teolo. 
 
 In the Andes of Ecuador at Pululagua, the andesite is almost free 
 from augite. A similar rock occurs at Toluca in Mexico, but it con- 
 tains quartz, olivine, hornblende, and biotite. 
 
 Hornblende-andesite is met with in the Caucasus near Kasbek. 
 
 1 Rosenbvusch : Min. Physiog. 
 
ANDESITES IN AMERICA. 
 
 261 
 
 American Andesites. In tlie United States andesites are more 
 widely distributed than propylites. They extend in scattered ex- 
 posures from the Cedar Mountains, in the great Salt Lake desert, to 
 California, and generally appear as massive eruptions. The relics of 
 an enormous crater at Lassen's Peak indicate an immense andesite 
 volcano ; and andesite volcanoes extend at intervals along the axis of 
 the Sierra Nevada and Cascade ranges. As in Europe, the rock is 
 divided into three groups, termed hornblende-andesite, dacite, and 
 au<nte-andesite. 
 
 
 AMERICAN ANDESITES. 1 
 
 AMRKICAN QUARTZ- 
 ANDKSITKS(DACITES). 
 
 
 o 
 
 a 
 
 
 m 
 
 Jf 
 
 
 
 1 
 
 gj 
 
 d 
 
 o 
 
 a 
 
 5 
 
 i 
 
 
 <"! 
 
 |J 
 
 O 
 
 | 
 
 J 
 
 
 PEJ 
 
 
 
 
 O 
 
 n 
 
 o Q 
 
 o 
 
 
 0. . 
 
 "5 
 -Ji 
 
 1 
 
 
 
 5J 
 
 j 
 
 
 I 
 
 |l 
 
 a 
 
 1 
 
 ji 
 
 1 
 
 
 Jj 
 
 
 
 
 Jj 
 
 55 
 
 
 Silica . 
 
 58-33 
 
 6l'I2 
 
 62-71 
 
 67-63 
 
 69-33 
 
 70-25 
 
 Alumina 
 
 18-17 
 
 ii *6i 
 
 I2'IO 
 
 1 8 -08 
 
 17-9 
 
 14-90 
 
 Peroxide of iron 
 
 
 11-64 
 
 H79 
 
 ! 2-17 
 
 
 2-57 
 
 Protoxide of iron 
 
 6-03 
 
 ... 
 
 
 S 217 
 
 4-1 
 
 176 
 
 Manganese 
 
 
 ... 
 
 
 trace 
 
 ... 
 
 trace 
 
 Lime 
 
 6*19 
 
 4*33 
 
 8-34 
 
 3-16 
 
 1-6 
 
 2-39 
 
 Magnesia 
 
 2-40 
 
 0*61 
 
 1-31 
 
 1-14 
 
 i '3 
 
 0-83 
 
 Soda 
 
 3-20 
 
 3'85 
 
 073 
 
 2-87 
 
 2-0 3-24 
 
 Potash . 
 
 3-02 
 
 3'5 2 
 
 
 386 3-6 
 
 3-22 
 
 Loss on ignition 
 
 0-76 
 
 4'35 
 
 
 1-49 2-1 
 
 
 The hornblende-andesite described by Clarence King, frequently 
 exhibits on weathering a lavender tint, but, freshly broken, is dark 
 brownish, with a compact, homogeneous, half-glassy matrix, including 
 small white crystals of plagioclase, occasional brown micas, the 
 lamellar form of hornblende distinctive of andesite, and a few rounded 
 grains of quartz. This rock is seen on the Divide between Gosiute 
 Valley and Deep Creek. Andesites are found in the Cortes range break- 
 ing through the propylite of the Tuscarora region where the rock is 
 dark, containing black hornblende and bright plagioclase in a greenish- 
 grey ground mass. At Carlin Peaks in the Cortes range, the dark- 
 grey andesite rises in a mass 1200 feet above the surrounding rhyolites. 
 At the head of Clan Alpine Canon the summits of the Augusta range 
 are formed of andesite, which shows a rudely columnar structure. 
 The long hornblende prisms have a rude parallel arrangement, and 
 under the microscope the crystals of hornblende are often seen to be 
 fractured, with the pieces displaced. Sometimes there is a percentage 
 of sanidine in the rock, which gives a roughness to its weathered 
 surface. At the upper end of the canon of Truckee Kiver the andesites 
 
 
 1 U. S. Geol. Surv., Fortieth Parallel, vol. i. p. 576 ; Table IX. 
 
262 DIFFERENCE BETWEEN PROPYLITE AND ANDESITE. 
 
 are fully 12,000 feet thick, and often have the aspect of trachyte, a 
 condition which varies with the quantity of orthoclase. 
 
 In the Washoe district three zones of fissures occur through which 
 hornblende-andesite was poured out so as sometimes to cut the diorite 
 and propylite, and form table-like masses. The rock poured out from 
 the dykes in Crown Point ravine is TOO feet thick, but the most 
 important spread covers the country near Devil's Gate in Gold Canon. 
 Nearly all the felspar is fresh plagioclase, with rare sanidine in Carls- 
 bad twins. The hornblende is decomposed into a light-green substance, 
 and the rock contains no augite, biotite, quartz, tridymite, or olivine. 
 The ground mass is a dark-grey aggregate of ledge-shaped felspars, 
 felspar microliths, and grains of magnetite, with a yellowish half- 
 glassy base, as in the andesites of the Siebengebirge. Many varieties 
 of andesite occur on the south side of the entrance to Truckee Canon. 
 The ground mass varies in colour from grey to reddish. The felspars 
 often contain glassy or half-glassy grains. When augite, which is 
 greenish, occurs, it is rich in glass inclusions, which are always absent 
 from the brown hornblende, as in the Siebengebirge in Hungary ; but 
 at times no large hornblende crystals are developed. A typical andesite 
 occurs in Berkshire Canon, where the rock contains no augite. Half- 
 glassy andesite is found on the west shore of Pyramid Lake in Nevada. 
 The ground mass is made up of colourless felspar, brownish microliths, 
 magnetite, and a brown glass. Characteristic andesites occur in the 
 Augusta Mountains in Nevada, and one from Augusta Canon resembles 
 the andesite of the Wolkenburg, having hornblende crystals a milli- 
 metre long, usually fractured, sometimes into thirty or forty pieces. 
 Above Tuscarora, andesite has a brownish-grey felsitic ground mass, 
 with a macroscopical plagioclase and green hornblende. Multitudes 
 of black trichites give a wavy fluidal structure to the rock. Some of 
 the andesites in the Washoe Mountains are remarkable for the Iamina3 
 of brown mica, so that they might be termed mica-andesites, especially 
 as the quantity of hornblende is very small. 
 
 Distinction between Propylite and Andesite. The chief con- 
 stituents of propylite and andesite are identical. They are, however, 
 easily distinguished by observation in the field. Zirkel observes that 
 the propylite ground mass is greenish grey, while the andesite ground 
 mass is pure grey or brown ; propylite is rich in minute particles of 
 hornblende, while andesite only contains large crystals of hornblende ; 
 propylite felspars are filled with hornblende dust, while the andesite 
 felspars are free from it. The hornblende in propylite is green, but 
 in andesite it is almost invariably brown, and has a black border never 
 seen in propylite. Propylite hornblende is built up of thin needles, 
 and therefore does not cleave. This is never seen in andesite. 
 Microscopical epidote is common in propylite, but is almost unknown 
 in andesite. A glassy propylitic ground mass is unknown, w r hile 
 andesites sometimes have a half- glassy ground mass. 
 
STRUCTURE AND DISTRIBUTION OF DA CITES. 263 
 
 Dacite. 
 
 Dacites are quartz-hornblende andesites. They were associated 
 with quartz-trachytes till distinguished by Stache. Their colour is 
 always of a blackish grey-green or dark-brown. The texture is either 
 compact or granular, and typically the rock consists of quartz, oligo- 
 clase, and hornblende, probably with a second felspar, and occasionally 
 mica, with which may be associated a little augite and olivine. Dacites 
 differ from andesite in the character of the ground mass, which when 
 present in dacite has a rhyolitic structure with a tendency to form 
 spherulites, while the andesite ground mass is an aggregate of 
 microliths. 
 
 Hungarian Dacites. In Hungary and Transylvania the dacites 
 have no glassy base, and have been divided by Doelter according to 
 their resemblance to granite, trachyte, and porphyry. Some, as at 
 Rodna and the Vlegyasza Mountains, have no true ground mass, but 
 a fine-grained matrix, with isolated larger crystals, and present the 
 conditions of granite. Near Roclna, biotite is sometimes so abundant 
 as to form a biotite dacite, and the trachyte dacite is generally rich 
 in biotite and poor in sanidine, with scattered particles of quartz, 
 which, however, does not occur here in the ground mass. In the 
 neighbourhood of Nagyag the porphyritic dacites take on the aspect 
 of quartz-porphyries. The dacites around Kapnik often contain small 
 masses with a glassy base, and frequently include pyrite. Timazite 
 is a rock sometimes to be classed with the dacites, and sometimes 
 with the andesites. At Les Crottes, in the Department of Var, the 
 dacite has a granitic texture, while at St. Raphael the rock is micro- 
 felsitic. Both these dacites are rich in sanidine, the crystals some- 
 times being 20 mm. long, so that the rock approximates towards 
 rhyolite. 
 
 Dacites also occur at Neu Prevali in Carinthia and Monte Alto in 
 the Euganean Hills. Among the specimens which have been analysed 
 are examples from Besobdal, Ararat, Kasbek, Ebendaher, and between 
 Iveshet and Kobi. A dacite from the volcano of Mojanda in Ecuador 
 has a microcrystalline ground mass which includes rounded grains of 
 quartz, andesine, hornblende, and biotite. 
 
 American Dacites. In the eastern half of the Cortez range 
 dacite forms a nearly continuous field for a distance of fourteen or 
 fifteen miles, but behaves like granite, forming massive eruptions 
 15,000 feet thick, and shows no tendency to extend itself laterally 
 from the region of fissure or to form sheets. Its prevailing character 
 varies, and the colour varies from purple to chocolate and brown, 
 though growing pale to the north of Waggon Canon, where the tinte 
 are olives and greys. At the south it is covered with rhyolite, and 
 on the east is overlain for many miles by basalt. The behaviour of 
 this rock in the field is more like propylite than andesite, and in hand 
 specimens it is not easily distinguished from quartz-propylite. Dacite 
 wants the resinous lustre and glassy fracture of the andesites. The 
 
254 MINERAL STRUCTURE OF TRACHYTE. 
 
 crystals of felspar are often decomposed, and the rock is sometimes 
 brecciated. North of Waggon Canon the quartz crystals in the rock 
 are very dark, and rarely visible to the eye. But at Shoshone Peak 
 the quartz grains are frequently from one-eighth to one-quarter inch 
 in diameter, and here the ground mass is sometimes so coarse that 
 the crystals may be distinguished by the eye. Dacites are also well 
 seen in the Virginia range, and there the large quartz grains contain 
 fluid inclusions, and frequently stand out prominently on the smooth 
 weathered surface. The dacite breccia, which breaks through the 
 purple dacite of this range, contains a good deal of dark-brown mag- 
 nesian mica, as well as more free quartz than the earlier rock. Be- 
 longing to the close of the eruption in this region is an apple-green 
 dacite, which is richest of all in quartz, and the grains are frequently 
 double six-sided pyramids. 
 
 In the hills about American City, Washoe, the ground mass of 
 the dacite is brownish or greenish grey, and contains striated felspar 
 and quartz grains as large as peas. Sometimes the structure of the 
 ground mass is rhyolitic, but the rock is distinguished from rhyolite 
 by its plagioclase and hornblende. The ground mass may be minutely 
 spherulitic. Fluid inclusions occur in the felspar crystals, but are 
 absent from the quartz, and sometimes the felspar is speckled with 
 calcite. The hornblende crystals contain yellowish-green viridite, 
 rhombohedral calcite, brownish-red oxide of iron, and yellowish-green 
 epidote, which almost replace the hornblende. The circumstance that, 
 on analysis, the potash is usually equal to the soda, suggests the 
 presence of sanidine in the ground mass. 
 
 Trachyte. 
 
 Trachytes are felsite-like rocks, which abound in sanidine. They 
 are regarded as the volcanic equivalents of syenite, and as corre- 
 sponding to the old quartzless porphyries. The trachytes of some 
 parts of the Elkhead range, and other districts in North America, 
 have quartz as a normal constituent ; and thus, on the one hand, 
 trachyte closely approximates towards rhyolite just as the trachytic 
 rocks in "Washoe approximate towards andesite, owing to the amount 
 of their plagioclase. 
 
 While sanidine is always the principal constituent of trachyte, it 
 is associated with plagioclase, and hornblende, biotite, or augite ; 
 or all of these minerals may be among its constituents, but they 
 always occur in relatively small quantities. Magnetite, titanite, and 
 apatite are usually associated with them, but are unimportant as con- 
 stituents of the rock. 
 
 Sanidine commonly occurs in crystals, which show a tabular or 
 ledge-like section. The regular outer border may be absent, and 
 the entire rock occasionally may be granular. Sanidine crystals are 
 usually Carlsbad twins, but sometimes simple prisms. Baveno twins 
 are mentioned by Rosenbusch from the dark trachyte of Toreggia in 
 the Euganian Hills, and from the Puy de Dome. Sanidine is often in 
 
CHEMICAL COMPOSITION OF TRACHYTE. 
 
 265 
 
 fragments conspicuous for their irregular corners and angles ; and not 
 ^infrequently complete sanidine crystals are seen, when polarised, to 
 be a cemented agglomerate of angular sanidine, so that two pro- 
 cesses of crystallisation appear to have taken place in the rock in 
 succession, one dating perhaps before the eruption, and the other 
 after it. At Capo Negro in Ischia, the reddish lavas have the 
 sanidine in parallel threads, but the crystalline arrangement is some- 
 times in concentric strips, and a crystal often shows zones of growth 
 in the interior. 
 
 The plagioclase probably includes several different felspars. 
 Tschermak has proposed to distinguish plagioclase of glassy texture 
 in andesites as microtine. The sanidine is frequently invested by 
 plagioclase ; and this condition is well seen in the trachyte of Battag- 
 lia, in the Euganean Hills. 
 
 
 
 
 AMERICAN 1 
 
 ViACHYTES. 
 
 L 
 
 
 
 
 || 
 
 
 | 
 
 3 M - 
 
 
 
 
 1 
 
 K^ 
 
 s 
 
 II 
 
 bo 
 
 1 
 
 
 ^ -*' 
 
 1 
 
 a" 
 
 00, 
 
 55 
 
 i 
 
 
 
 
 O ^ 
 
 s * ^* 
 
 M G 
 
 C ^1 
 
 p< 
 
 r^ 
 
 
 o> > 
 
 S > > 
 
 fl 
 
 O r^ 
 
 
 
 O 
 
 
 1* 
 
 j|3S 
 
 W 
 
 |s 
 
 1 
 
 o 
 
 
 H 
 
 I 
 
 
 
 
 
 
 
 s 
 
 Silica . 
 
 50-36 
 
 53-25 
 
 56-45 
 
 61-95 
 
 67-81 
 
 70-30 
 
 Alumina 
 Peroxide of iron 
 
 I7-OO 
 
 6-12 
 
 14-42 
 trace 
 
 19-85 
 4 "95 
 
 15-80 
 trace 
 
 15-83 
 
 trace 
 
 13-65 
 
 Protoxide of iron 
 
 3-84 
 
 6-00 
 
 0-97 
 
 576 
 
 3'4i 
 
 5-41 
 
 Manganese 
 
 0-30 
 
 trace 
 
 O'll 
 
 
 
 trace 
 
 Lime 
 
 8-85 
 
 6'oi 
 
 7-70 
 
 4*24 
 
 3-66 
 
 1-92 
 
 Magnesia 
 
 3-02 
 
 5-06 
 
 2-66 
 
 263 
 
 1-36 
 
 0-40 
 
 Soda 
 
 
 3-I3 
 
 3-16 
 
 4-50 
 
 5-10 
 
 3'45 
 
 Potash . 
 
 I '95 
 
 4-58 
 
 3-84 
 
 
 0-67 
 
 4'5 
 
 Lithium . 
 
 
 
 trace 
 
 trace 
 
 trace 
 
 
 
 
 
 
 
 C0 2 
 
 
 
 
 
 
 
 '49 
 
 
 Loss on ignition 
 
 5'35 
 
 7-63 
 
 0-38 
 
 i'34 
 
 173 
 
 0-56 
 
 
 Hornblende sometimes occurs in well-developed crystals, some- 
 times in ill-defined grains. It is distinguished with difficulty from 
 augite ; the cleavage being the only satisfactory means of differentia- 
 tion, and in some small columnar crystals this is absent. Hornblende 
 in trachyte is almost invariably dark brown, but is green at Mocsar 
 and Tepla in Hungary ; and is often green when found in micro- 
 liths. It is sometimes enveloped in magnetite, and may contain 
 microliths of apatite, and glass and gas inclusions. In the ground 
 mass, hornblende occurs in bundles of small needles, like those seen 
 in phonolite. Hornblende occurs alone in the trachytes of the 
 Azores, at Kieshiibel near Schemnitz, and at Kuhlsbrunnen in the 
 
 1 U. S. Geol. Surv., Fortieth Parallel, vol. i. p. 604; Table X. 
 
266 GEOGRAPHICAL DISTRIBUTION OF TRACHYTE. 
 
 Siebengebirge ; it is associated with augite in the trachytes of Krem- 
 nitz, of Alsberg in the Rhon, and at Dernbach near Montabaur, and 
 Castelnuova in the Euganean Hills ; it is associated with biotite in 
 some trachytes of the Siebengebirge, and in other trachytes horn- 
 blende is absent. 
 
 Augite may occur in microscopic or macroscopic crystals, which may 
 be well defined and regular, or irregular, or in columns or grains. 
 Twin crystals are frequent, as are fractured crystals. The inclusions 
 comprise glass, magnetite, and microliths of apatite. According to 
 the position of the cleavage so is the colour of the crystal. It is 
 often green when the axis is parallel to the short diagonal of the 
 nicol, and reddish brown when the axis is perpendicular to it. 
 Augite is not so liable to decomposition as hornblende or 
 biotite. 
 
 Biotite is not known in the ground mass of trachyte. It occurs 
 in thin hexagonal plates, which are often surrounded with magne- 
 tite ; the colour is deep brown, but exceptionally may be blood-red, 
 as in the Yallee de la Cour in the Auvergne, at Capo Negro in Ischia, 
 and at Mocsar in Hungary. Biotite is also found in Italian trachytes, 
 at Yenda di Teolo, and Monte Zacon in the Euganean Hills. In the 
 Auvergne it is found in the trachytes of Mont Dore, Val de 1'Enfer, 
 Plateau de Durbieze, Ravin de la Craie, and in these French localities 
 is associated with augite. 
 
 Apatite usually occurs in long needles with hexagonal section ; 
 but it is also found in short thick columns, of a grey tint, which 
 may incline to brown, blue, or violet. It abounds in some of the 
 bombs of trachyte at the Laacher See, and at Alsberg in the Rhon. It 
 is less abundant in the Drachenfels ; and it is found in the trachyte 
 of Dernbach near Montabaur, and near Kelberg in the Eifel. 
 
 Magnetite is scattered in the ground mass in grains and in 
 crystals. It is often adherent to the crystals of augite or hornblende, 
 but never adheres to felspar. 
 
 The accessory minerals are titanite, olivine, sodalite, hauyine, and 
 nosea, with quartz and tridymite; though in American trachytes, 
 quartz and olivine both occasionally become important constituents. 
 Leucite occurs sparingly in the trachyte of Arso-Stromes. 
 
 North American Trackytes. Clarence King distinguishes four 
 important areas of trachyte, separated from each other by intervals 
 of four degrees of longitude in the region explored by the Fortieth 
 Parallel Survey. These masses occur in the Rocky Mountains, in 
 the Wahsatch range and Salt Lake region, in the Pinon and Cortez 
 ranges, and in the Virginia and Lake ranges near Pyramid Lake. 
 Several of these masses of trachyte have been forced out through 
 great fissures, which can sometimes be traced to lines of fault. 
 
 From the hills at the foot of the "Washoe range a flood of 
 trachyte extends 40 miles to Pyramid Lake, and in this district caps 
 all the prominent hills. Zirkel remarks that the whole of this mass 
 has been ejected through a narrow dyke less than TOO feet wide, 
 which pierces propylite in the pass north of Gould and Curry mill. 
 

 AMERICAN TRACHYTES. 267 
 
 Much of this overflow consists of breccias, in which the fragments 
 range from the size of a pebble to a diameter of 20 feet. The 
 breccias are capped with sanidine trachyte, varying from 100 
 to 1000 feet in thickness. The older trachytes are rich in plagio- 
 clase, though sanidine slightly preponderates ; and they contain more 
 brown hornblende than the newer rocks, and thus approximate to 
 andesites. Such trachytes are seen in Mount Rose and Sugarloaf. 
 The newer trachytes are poor in plagioclase, richer in sanidine, 
 contain much less hornblende, and are usually rich in laminae of 
 biotite. The felspar frequently contains kernels of yellowish-brown 
 glass, and the hornblende is intermediate in colour between green 
 and brown. Aggregations of tridymite occur like those in the 
 trachytes of the Siebenbiirgen in Hungary, the Euganean Hills in 
 North Italy, Mont Dore, and the Puy de D6me in Central France. 
 In the north and middle parks, cretaceous rocks are dislocated, and 
 have blocks and fragments wedged in the flood of lava. Some of 
 the hills and cones of this district are termed by Zirkel granite 
 porphyries, and by Clarence King trachytoid porphyry. They have a 
 fine-grained and nearly homogeneous ground mass, which consists of 
 orthoclase quartz and a little hornblende, with occasionally plagio- 
 clase, apatite, titanite, magnetite, and pyrite. The hornblende is 
 green, and the quartz contains fluid inclusions, but no glass ; while in 
 Steve's ridge the hornblende is brown, and the quartz contains glass. 
 In the Henry mountains, Gilbert found both green and brown horn- 
 blende together in trachyte, where the quartz contained fluid in- 
 clusions. 
 
 Zirkel remarks of the quartz trachyte of Steve's ridge in the 
 Elkhead Mountains, that the sanidine crystals are more than an inch 
 long, and the rock closely resembles the trachyte of the Drachenfels ; 
 but the felspar, though behaving like sanidine, has the crystal faces 
 which characterise the old compact and dull orthoclase of porphyritic 
 granite and felsite 'porphyry. The rock also contains grains of 
 quartz of the size of peas, which are broken by many cracks ; but 
 there is no microscopical quartz. In other localities, as at Camel 
 Peak in the Elkhead Mountains, olivine occurs with the quartz, and 
 the rock has augite for the predominant mineral. But at the Little 
 Snake River in Colorado the cracked quartz occurs in grains as large 
 as hazel nuts, with large glassy sanidine crystals, and large plates of 
 mica ; but shows much augite in the ground mass. At City Creek in 
 the Wahsatch range, microscopic cavities in the rock are encrusted 
 in tridymite. Tridymite occurs at Silver Creek, near Kimball's 
 station. 
 
 The rocks to t-he west of the Elkhead Mountains, and which extend 
 from Hantz Peak to Camel Peak and south from Steve's Ridge in a 
 broad field 35 miles long, are all sanidine trachytes. Some have a 
 rough porous crystalline ground mass in which cracked grains of quartz 
 occur like those of rhyolite, but there is no quartz in the ground 
 mass. 
 
 Besides these minerals the trachyte there contains hornblende, a 
 
268 MINERAL STRUCTURE OF PHONOL1TE. 
 
 little mica, a comparatively large amount of augite, and some olivine. 
 The rock forms rounded dome-shaped hills and sharp cones. 
 
 On Skellig's Ridge a dyke of trachyte from 20 to 50 feet thick 
 rises out of the sandstone to a height of 150 feet, and extends with 
 vertical walls and horizontal columnar structure for five or six miles, 
 with only one break. 
 
 The walls are pitted wherever the grains of quartz have weathered 
 from its surface, and they here occur in double six-sided pyramids. 
 This rock combines the constituents of basalt and rhyolite. 
 
 Many American trachytes are remarkable for the quantity of their 
 augite. Thus in the hills between Sheep Corral Canon and Wads- 
 worth, the trachytes contain abundance of sanidine, associated with 
 pale-green augite, brown hornblende, and some plagioclase in a ground 
 mass of colourless crystals and microliths. At Truckee Ferry the 
 augite prisms are grouped in oval, imperfectly radial accumulations, 
 and another example of augite trachytes is seen between Green River 
 and Bitter Creek. 
 
 Near the Wahsatch Range so little of the ground mass is left that 
 the trachyte has frequently a granitoid aspect. 
 
 Near Salt Lake City tridymite and quartz occur together in the 
 purple trachyte ; and the hornblende crystals contain fluid inclusions 
 with moving bubbles as well as simple gas cavities. The augite 
 crystals are here free from the magnetite so characteristic of basalts. 
 In the upper valley of Susan Creek the trachyte contains granular 
 aggregations of rose-coloured garnets and hexagonal grains of hauyine. 
 Trachytes vary in colour, but are commonly tints of green, grey, and 
 purple, and make transitions on the one hand to the tints of the 
 rhyolites, and through the black varieties approximate to the augite 
 andesites and basalts. 
 
 Trachyte eruptions are usually free from lines of bedding, by 
 which the material may be traced up to the vent from which it 
 flowed. In the North- American region it may have either rhyolite 
 or basalt resting upon it. 
 
 Phonolite. 
 
 Phonolite is defined as a quartzless rock, consisting of a com- 
 bination of sanidine, with nepheline and leucite. Hence it is essen- 
 tially a nepheline or leucitic trachyte, and has the same relation 
 to trachyte that nepheline-syenite bears to syenite. Phonolite is 
 probably a volcanic representative of nepheline-syenite. There is no 
 known combination of orthoclase and leucite in the older series of 
 rocks. Though these minerals predominate in phonolite, pyroxene 
 and amphibole are essential though subordinate constituents, and 
 hauyine is often abundant. Titanite, apatite, and magnetic iron are 
 diffused in the ground mass. Plagioclase is absent in most true 
 phonolites, and, when present, is as rare as in the mica-syenite, 
 called minette. Sanidine occurs in long rod-shaped crystals, which 
 may form the greater part of the rock, or occur sparingly between the 
 

 NATURAL HISTORY OF RHYOLITE. 269 
 
 nepheline and leucite. Nepheline is often as important as sanidine 
 in the ground mass. It may give six-sided or quadrate sections. 
 This mineral is fresh and clear as water, and not easily determined. 
 Like leucite, which also enters the ground mass, it may occur in 
 microscopic crystals. Plagioclase occurs in the phonolites of the 
 Auvergne, which are poor in nepheline, and is rare in the phonolites 
 of the Rhb'n and the Kaiserstuhl. Nosean and hauyine may be absent 
 from phonolites which contain no nepheline ; but usually both these 
 minerals are more or less important. Hauyine may be macroscopic as 
 well as microscopic : its colour is variable, black, brown, blue, yellow, 
 green, or colourless. It is rare for hornblende to occur, unless 
 associated with augite, and frequently the augite predominates. The 
 larger crystals of hornblende may be brown or green. Hornblende- 
 bearing phonolites occur at Teplitz and Aussig, and at Spansdorf 
 and Grosspriesen, and on the road from Mont Dore to Murat, &c., 
 hornblende occurs with much green augite. Both those minerals 
 may occur as porphyritic crystals or as part of the ground mass. 
 More rarely augite is present without any hornblende. The leucitic 
 rocks are richest in augite. Many phonolites in Cantal and Heidel- 
 berg contain magnesia-mica. The apatite is blue and brown, and 
 may form thick prisms. Titanite is either greenish yellow, clear 
 yellow, or orange-red. Tridymite is common in the phonolite at 
 Aussig. Olivine occurs at the Roche Sanadoire, and some other 
 minerals occur exceptionally in other localities. The base is often 
 globulitic, as at Hohenwiel. It may be amorphous or micro-felsitic 
 in some places, or occasionally granitic. A phonolite-like obsidian is 
 found at Teneriffe. Occasionally when the rock has a trachytic character 
 it is porous, and often has the cavities filled with zeolites. Spotted 
 phonolites are found in Teneriffe. The phonolites include seven 
 mineral varieties, which may be grouped as nepheline-phonolite, 
 nosean or hauyine phonolite, and sanidine-phonolite. 
 
 Rhyolite. 
 
 The name rhyolite was first used by Yon Eichthofen in 1860, to 
 define Hungarian and Transylvanian lavas, which consist of crystals 
 of quartz and sanidine, scattered through a felsitic ground mass. A 
 similar rock is well seen in the Lipari Islands, from which it was 
 named Liparite, by Justus Roth, in 1861. It has been found in the 
 Euganean Hills, in Rhenish Prussia, the Auvergne, Iceland, the 
 Rocky Mountain region of North America, the Northern Island of 
 New Zealand, and several of the islands of the Greek Archipelago. 
 
 Rhyolites are the volcanic equivalents of granites, and are identi- 
 cal with quartz porphyry, quartz felsite, and felsitic pitchstone, associ- 
 ated with the Primary strata. 
 
 No volcanic rock presents greater varieties of texture and micro- 
 scopic structure than rhyolite. Some rhyolites are entirely crystal- 
 line ; others have crystals in greater or less quantity scattered through 
 an amorphous base ; while a third type is absolutely free from 
 
270 
 
 CLASSIFICATION OF RHYOLITES. 
 
 crystals. The first of these is the rarest, and is usually known as 
 granitic rhyolite or nevadite. The second type is perhaps the most 
 typical, being common wherever rhy elites occur. It corresponds to 
 the old quartz porphyry and felsite. The third type comprises the 
 hyaline rhyolites, which include perlite, obsidian, and pumice, and 
 includes the old felsitic pitchstones. No region has yielded so 
 many varieties of rhyolite as that examined by the Fortieth Parallel 
 Survey in North America, where Zirkel finds every type of rhyolitic 
 structure hitherto recognised in other parts of the world, and groups 
 the typical rhyolites into fifteen varieties. 
 
 
 EUROPEAN FELSITE PORPHYRY.! & 
 
 
 i 
 
 
 . 
 
 
 
 
 3 
 
 
 w 
 
 o" 
 
 23 
 
 O 
 
 
 
 
 
 jrf 
 
 i 
 
 f 
 
 "5 
 
 
 
 ^*" 
 
 
 f 
 
 i 
 
 I 
 
 1 
 
 
 
 |.5 
 
 
 1 
 
 
 
 
 
 i 
 
 I 
 
 I 
 
 2 
 
 
 
 
 i 
 
 1 
 
 F 1 
 
 "ej 
 
 S 
 
 "o ft 
 
 
 5 
 
 > 
 
 H 
 
 
 
 
 ft 
 
 H 
 
 Silica 
 
 68-54 
 
 7i'S5 
 
 7578 
 
 7975 
 
 84-IO 
 
 87-20 
 
 76-44 
 
 Alumina . 
 
 9 '49 
 
 16-60 
 
 I2'l6 
 
 12*00 
 
 IO-50 
 
 6'oo 
 
 12-64 
 
 Peroxide of iron 
 
 8-60 
 
 
 177 
 
 ... 
 
 ... 
 
 
 O'29 
 
 Protoxide of iron 
 
 3-23 
 
 6-40 
 
 0-51 
 
 175 
 
 I -10 
 
 3-66 
 
 0-51 
 
 Manganese 
 
 trace 
 
 ... 
 
 
 
 trace 
 
 trace 
 
 
 Lime 
 
 '54 
 
 i -60 
 
 079 
 
 2'10 
 
 0*04 
 
 0*60 
 
 ... 
 
 Magnesia . 
 
 0-42 
 
 0-85 
 
 
 O'24 
 
 0-03 
 
 0-08 
 
 0-27 
 
 Soda 
 
 3'H 
 
 0-20 
 
 1-16 
 
 0'20 
 
 
 trace 
 
 3 '41 
 
 Potash . 
 
 5' 11 
 
 trace 
 
 6-28 
 
 0-15 
 
 j I'lO 
 
 trace 
 
 4-29 
 
 Titanic acid 
 
 1-36 
 
 
 
 
 
 
 
 Phosphoric acid 
 
 o 
 
 trace 
 
 
 
 
 
 
 o 19 
 
 Water . 
 
 030 
 
 2-44 
 
 i'39 
 
 3-6o 
 
 I 'Q"? 
 
 2-30 
 
 1-46 
 
 Granitic Rhyolite. The granitic rhyolite, which is always rare, 
 is distinguished by being entirely crystalline. The ground mass 
 of this Nevadite consists of well-defined microscopic grains of quartz 
 and felspar, through which larger crystals are scattered. 
 
 Zirkel's Classification of Khy elites. We briefly summarise 
 Zirkel's classification of the typical rhyolites. The commonest type 
 presents a microfelsitic structure, sometimes becoming more or less 
 granular, and usually characterised by imperfectly formed sphserolites. 
 It generally contains ferrite and opacite. 
 
 A second variety consists chiefly of microfelsite, with some 
 polarising particles and single dark axiolites ; or the microfelsite may 
 be traversed by fibrous strings arranged axially, with a distinct middle 
 division ; or it may consist of a network of such strings, enclosing 
 radially fibrous and concentric spherolites in its meshes ; or the meshes 
 may include more or less distinct aggregations of a crystalline granular 
 
 1 Justus Roth : " Beitrage Petrograph der Pluton." Gesteine, 1873, taf. xiii.-xiv. 
 
CONDITION OF THE QUARTZ IN RHYOLITES. 271 
 
 character, or the meshes may disappear altogether, leaving simple 
 aggregations of sphserolites, or the meshes may expand into confused 
 aggregations of bunches of parallel fibres ; or a felt-like structure 
 may be formed of short confused fibres, or the sphserolites may be mixed 
 with aggregations of cumulites. Other rhyolites consist of aggre- 
 gations of colourless polarising particles and colourless glass, but these 
 are rare. Alternating bands of microfelsite and light-coloured glass 
 are also rare ; but it is commoner to find a half-glassy mass made up 
 of little thin microliths almost passing into obsidian, but containing 
 crystals of quartz, sanidine, and biotite. The homogeneous glass of 
 other rhyolites is sometimes contorted and undulated with dark- 
 brown grains ; or spherolites and axiolites may replace the glass, and 
 hajft fluxion structure marked by similar bands of grains ; or the 
 lignt-coloured homogeneous glass may be traversed by perlitic cracks, 
 and have a fluxion-structure marked by narrow zones of microfelsite 
 on both sides. 
 
 Characteristics of Rhyolites. Rhyolites are best characterised 
 by fluid structure and fibrous aggregates. The wavy fluidal struc- 
 ture is due to several causes. First, coloured bands are formed by 
 parallel layers of needles and grains of ferrite and opacite. Secondly, 
 the different layers of the rock may vary in texture, as when crystal- 
 line granular layers alternate with spherolite layers ; or when micro- 
 felsitic layers alternate with less crystalline layers ; or perfectly 
 granular layers with imperfectly granular layers. The corrugations of 
 fluidal structure are also sometimes marked by the alternations of 
 layers of colourless glass with brownish-yellow globulitic glass. 
 
 The fibrous concretions of rhyolites comprise four types : first, 
 those in which the fibres radiate from a centre as in spherolites ; 
 second, those in which the fibres are arranged longitudinally about 
 an axis forming axiolites; third, those in which the fibres are 
 parallel to each other forming bunches or bundles ; and fourthly, those 
 in which the fibres are confusedly mixed. These fibrous aggregates 
 are wanting in trachytes. 
 
 Khyolites are. even better characterised by their mineral composi- 
 tion. The crystalline quartz component is a consequence of the high 
 percentage of silica in the rock ; being often in excess of what could 
 be used up in forming felspar, some has crystallised separately. 
 
 Quartz in Rhyolites. The quartz of rhyolites may be in frag- 
 mentary grains, or in double six-sided pyramids, divided by a six-- 
 sided prism, the latter condition being developed in proportion to the 
 crystalline texture of the rock. The glass inclusions sometimes have 
 this crystalline form, and then contain dark bubbles ; but the moving 
 bubble and the fixed bubble are never found in the same quartz. 
 The quartz is poor in mineral inclusions, but contains ovoid gas 
 cavities, and ovoid inclusions of colourless glass, in which are opaque 
 needles. Microliths occur, and fluid inclusions have been described 
 in rhyolites from Lipari, Samothrace, the Auvergne, and Kis Sebes, 
 but they are evidently rare ; for among all the rhyolites of North 
 America, only two were found with fluid inclusions in the quartz, 
 
272 
 
 MINERAL CONSTITUENTS OF RHYOLITE. 
 
 and on both, Zirkel expresses doubts as to the grains being originally 
 generated in rhyolite. Tridymite occurs in the ground mass in 
 uiinute crystals ; and it also occurs in well-marked aggregates of 
 hexagonal plates, but is only abundant when the proportion of quartz 
 is small ; and it is absent from the glassy varieties of the rock. 
 
 Felspar in Rhyolite. The predominant felspar is sanidine, clear 
 as water, and sometimes in twin crystals of the Carlsbad type. 
 Occasionally lamellae of plagioclase occur in the sanidine. The felspar 
 is richer in gas cavities than the quartz. The glass inclusions may be 
 either clear or coloured. They are ovoid or many-sided, or ledge- 
 like forms, with angular indentations. Their microliths are isolated, 
 as are the plates of biotite. The felspar in the granitic rhyolite of 
 the Kotorua Lake in New Zealand is opaque. 
 
 Plagioclase occurs in small, clear, long, isolated crystals. Their 
 principal cleavage-plane agrees with that of albite and oligoclase. 
 The amount of plagioclase, however, is less than might be expected 
 from the percentage of soda in the analyses, so that soda is probably 
 diffused in the ground mass. 
 
 
 
 AMERICAN ] 
 
 iHYOLITES.l 
 
 
 
 Lassen's Peak, 
 California. 
 
 Harlequin 
 Canon. 
 
 Pine Nut 
 Canon. 
 
 Mopung Hills. 
 
 Silica 
 
 68-84 
 
 70^5 
 
 75 '7 
 
 77-00 
 
 Alumina . 
 
 1773 
 
 
 II -40 
 
 1 1 '54 
 
 Peroxide of iron 
 
 
 1-24 
 
 0-53 
 
 0-69 
 
 Protoxide of iron 
 
 3'ii 
 
 1*20 
 
 1-28 
 
 0-42 
 
 Manganese 
 
 
 O'll 
 
 trace 
 
 
 Lime 
 
 3-58 
 
 I'M 
 
 0-61 
 
 0'43 
 
 Magnesia 
 Soda 
 
 0-90 
 
 0-27 
 379 
 
 O'll 
 
 trace 
 2 -45 
 
 Potash 
 
 3'59 
 
 5-60 
 
 8'33 
 
 672 
 
 Lithium 
 
 trace 
 
 trace 
 
 trace 
 
 trace 
 
 
 
 
 C0 2 
 
 
 
 
 
 trace 
 
 
 Loss on ignition 
 
 1-50 
 
 1-38 
 
 1-74 
 
 077 
 
 Mica, Hornblende, and other Constituents in Rhyolite. The 
 magnesian mica is of a brown tint, rarely green. Its films are often 
 bent by fluxion structure. It frequently occurs in hexagonal plates, 
 which are usually visible, but may be microscopic. 
 
 Hornblende is sometimes associated with the mica, sometimes 
 separate. It always occurs in brown plates or needles. 
 
 Augite is commonly seen in small grains or microscopic crystals, 
 but in some of the American rhyolites the ground mass is an almost 
 wholly crystalline aggregate of comparatively large grains of felspathic 
 quartz and augite. 
 
 Contrary to an otherwise general rule, augite in these rocks is 
 
 1 U. S. GeoL Surv., Fortieth ParaUel, vol. i. p. 652 ; Table XI. 
 
TERTIARY RHYOLITES OF NORTH AMERICA. 273 
 
 associated with quartz. Like the hornblende, it is usually associated 
 with magnetite. Apatite and magnetite are diffused in many rhyolites, 
 but both are sometimes absent. There are few, if any, accessory 
 minerals in rhyolite, and when found they are always integral parts 
 of the ground mass. 
 
 North American Rhy elites. Rhyolite is the predominant super- 
 ficial volcanic rock of the Fortieth Parallel, where its oldest eruptions 
 appear to date from the beginning of the Pliocene period. It accom- 
 panies the trachyte in the Rocky Mountains, but there are no important 
 exposures between the Rocky Mountains and the western side of the 
 great Salt Lake Desert. Between the ii4th meridian and the borders 
 of California, rhyolites are widely spread. Many of the exposures 
 occur at the angles of great flexures of the rocks. Against the base 
 of Mount Richthofen rhyolites are poured out in immense streams. 
 Here their colour is dark, and the ground mass is a fine-grained 
 mixture of broken crystals of sanidine and dark quartz, in which are 
 contained large clear grains of quartz, shining black hornblende, and 
 large fractured sanidine ; but besides the sanidine, in some localities 
 orthoclase crystals are found, but then the rock has a more felsitic 
 character, and the ground mass is usually light. The rhyolite of 
 Desert Buttes is sometimes warm grey, sometimes salmon colour, and 
 contains spherolites and lithophyses, and is reticulated by veins of 
 translucent chalcedony. At times the rhyolites are glasses, variously 
 banded and tinted, containing sanidine and quartz. 
 
 In the region of the Washoe Mountains, to the south-west of Salt 
 Lake Desert, rhyolites are grandly spread, presenting excellent ex- 
 amples of viscous flow, and of variations in rock structure, sometimes 
 being green and compact, like hornstone, at other times brick-red and 
 porphyritic, with sanidine and prisms of hornblende. 
 
 North of Spring Canon the rhyolites include pumice, and bril- 
 liantly tinted glassy and half-glassy rocks, the quartz in which has 
 inclusions with moving bubbles. At Clover Peak the rock is as black 
 as basalt, and free quartz has a brilliant olive-green tint. The tuffs 
 of this region are sometimes creamy or light-grey in colour, sometimes 
 pale-green or olive-tinted. The rhyolites of the Sierra Nevada are 
 among the most important in the world. They form the Augusta, 
 Fish Creek, Shoshone, Toyabe, Cortes, Seetoya, and part of the 
 Pinon ranges, and the Mallard Hills. This belt was explored for a 
 length of 200 miles and a breadth of 40 to 80 miles. 
 
 North of the Humboldt River at Osino Canon, the rock is rich in 
 crystalline constituents, and contains sanidine, biotite, and quartz. 
 Biotite is also met with in the ground mass of rhyolite in other locali- 
 ties, but is usually rare. In Pinon Pass there is more black mica 
 than in any other exposure. In the Shoshone Mesa the rock presents 
 several varieties besides the ordinary type, with sanidine, cracked 
 quartz, a little plagioclase, and occasional mica. One of these is 
 pearl grey, rich in tridymite, and poor in crystalline secretions. Others 
 are dark pearlites, with more or less augite, and characterised by 
 spherolitic concretions. In several localities, as at Cortes Peak, 
 VOL. I. S 
 
274 A UGITE-A NDESITE. 
 
 breccias are seen, and in Owhee Bluffs pink and red angular frag- 
 ments are embedded in the thin lava streams like pieces of inlaid 
 wood. At New Pass, in the Desatoya Mountains, the rhyolites are 
 not less than 1000 feet thick. The breccias in this pass are white 
 and green below, and pink and red above. The sanidine, found in a 
 porphyritic rhyolite on the west, has a play of colour like labradorite, 
 and sanidine with this property is also found in the Pahute and 
 Augusta Mountains. It is noticed that the soda in this sanidine is 
 almost equal to the potash. At Antimony Canon the rhyolites are 
 6000 feet thick, and throughout the region of the Augusta and Fish 
 Creek Mountains the whole country is covered with rhyolite to the 
 thickness of from 2000 to 7000 feet, and this exposure is nearly 100 
 miles long, by from 12 to 20 miles wide ; but the character of the rock 
 seems to have changed a little with each successive outpouring. On 
 the eastern base of Pahute range, the rhyolite is a minutely microf elsitic 
 rock, approaching the texture of porcelain, and has few minerals that 
 can be recognised by the naked eye. In Eayless Canon, in the 
 southern end of the Montezuma range, is a ridge of rhyolite a mile 
 long, and 300 to 400 feet high, made up of well-developed prismatic 
 columns, varying from an inch to two feet in diameter. In almost 
 every district the colour is very variable, and frequently passes through 
 such tints as white, pale green, pale lilac, bright indian red, deep 
 purple, and olive brown. Its laminated structure is often marked by 
 bands of colour as fine as the leaves of a book, though it is as fre- 
 quently structureless ; and in mineral composition it presents as wide a 
 range as any known rock. Tertiary rhyolite is never covered by any 
 lava except basalt. 
 
 Augite Andesite. 
 
 Augite andesite is essentially a combination of augite with plagio- 
 clase, and is free from olivine. This absence of olivine is held by 
 Eosenbusch to similarly characterise diabase, and to distinguish that 
 rock from basalt and melaphyre. These rocks further differ from 
 basalts in frequently containing hornblende and biotite, and the quan- 
 tity of augite is less than in basalts. The ground mass may be crystal- 
 lised or glassy, and occasionally shows fluidal structure. The char- 
 acteristic crystals in it are oligoclase and augite. 
 
 The percentage of silica ranges from 55 to 60. Occasionally there 
 is a little quartz, which rarely occurs in basalt as an original element. 
 The silica more frequently is used up in the formation of sanidine, 
 which sometimes may make up as much as one-half of the felspar, 
 though the quantity is usually subordinate. Magnetite and apatite 
 are always found, and in a few localities tridymite occurs. The chief 
 products of decomposition in augite andesites, are chlorite, iron oxide, 
 calcite, opal, and chalcedony. 
 
 The plagioclase is found in rod-shaped crystals, but the larger 
 forms become twins. The felspar is often more or less replaced, and 
 is converted into opal in the augite andesites of Hungary. The 
 
 
GEOGRAPHICAL DISTRIBUTION OF 
 
 sanidine is only to be recognised by its crystalline form and optical 
 properties. Bastite is a product of the decomposition of enstatite. 
 Augite, like the plagioclase, contains inclusions of glass, but they 
 rarely show devitrification as in the felspar ; they are similarly asso- 
 ciated with steam pores, microliths, and occasional fluid inclusions. 
 Olivine and plates of mica are very rarely detected in the ground 
 mass. 
 
 Quartz Augite Andesite. Zirkel divides augite andesites into 
 two groups according to the presence or absence of quartz. Most of 
 the andesite from the American Andes belongs to the quartz-bear- 
 ing variety, and contains from 57 to 67 per cent, of silica. It is 
 recorded, among other localities, as from Chimborazo, at a height of 
 17,916 feet, Gua^apichincha, Cotopaxi, Antisana, Riobamba, Tun- 
 guragua. At Palissade Canon in the Cortez range, in the region of 
 the Fortieth Parallel Survey, an exceptional rock of this kind occurs 
 with 62 per cent, of silica, which has a granular crystalline texture, 
 is free from glass, abounds in well- crystallised quartz, has no horn- 
 blende or olivine, and but little sanidine, and consists chiefly of 
 plagioclase, with some augite and brown mica. 
 
 Distribution of Augite Andesite in Europe. The variety of 
 augite andesite which is free from quartz is found chiefly in lava 
 streams; it is seen in Iceland, at Hals, in the Hecla lava of 1845, 
 though the ashes of that eruption differ in chemical composition. It 
 occurs at Portillo in Teneritfe, at Serra Varalau, in the Yal del Bove. 
 It forms much of the volcanic summit of Radicofani in Tuscany, where 
 it contains olivine. The augite andesite of Reykjavik is of a greyish 
 or reddish colour, and consists of oligoclase, augite, and olivine, with a 
 felspar in loosely-connected thin plates. The rock which caps the 
 Lowenburg in the Siebengebirge appears to contain nepheline, and is 
 grouped by Zirkel as an augite andesite. The Transylvanian augite 
 andesites have a crystalline ground mass at Nagy Banya, but generally 
 the ground mass varies from a macrocrystalline to a microlithic state, 
 with the remains of a more or less glassy base ; and many varieties of 
 texture occur in these rocks at Tokaj, Schemnitz, and other localities 
 in Hungary. A black variety of the rock with a resinous lustre 
 occurs at Bagonya in Hungary. In the Auvergne, augite andesites 
 composed the lava-flows which ran down from the Petit Puy de Dome, 
 Parion, and from the Puy de la Nugere towards Volvic, where they 
 become crystalline. Augite andesites are well developed in Santorin 
 in the lavas of 1865. 
 
 The South American augite andesites from Tunguragua, Cotopaxi, 
 and Antisana, though containing 63 to 67 per cent, of silica, contain 
 
 quartz, but are rich in brown glass. 
 
 North American Augite Andesites. To the south-west of Salt 
 ke, where an angular bend occurs, near the southern extremity of 
 the Cedar Mountains, is an outburst of andesite which occupies the 
 entire angle. In external appearance it is quite like basalt ; it occurs 
 in thin sheets, and often shows rude columnar joints. Where broken 
 it shows a large amount of pale grey glass, with crystals of plagioclase, 
 
 : 
 
276 NORTH AMERICAN AUGITE-ANDES1TE. 
 
 augite, and a little hornblende, with some brown biotite. The augite 
 always predominates over the hornblende. Occasionally, as at the 
 mouth of Spring Canon, this andesite contains a little sanidine. In 
 Melrose Mountain biotite predominates, so that the rock might be 
 termed a mica andesite ; but Clarence King regards it as evidently 
 comparable to the rock of Spring Canon, with which it may have 
 been originally continuous. In Egyptian Canon this rock exhibits a 
 columnar structure ; but Clarence King describes the columns as 
 cylindroids rather than prisms. Sometimes considerable quantities 
 of apatite are seen under the microscope. It is difficult to draw a 
 distinction between this rock when free from hornblende, and basalts 
 which are free from olivine. It is also seen on the eastern side of the 
 Cortez range, north of Jacobsville, on the Reese river, and in many 
 other localities. The crystals vary much in size, and frequently con- 
 tain both in the augite and plagioclase yellow or brown glass inclusions. 
 The rock is commonly covered by rhyolite, but occasionally, as in the 
 Reese river, by basalt. The augite andesite of Augusta Canon is 
 overlaid by trachyte, and it is on this account that the American 
 geologists have grouped it rather with the andesites than with the 
 basalts. 
 
 In the Fortieth Parallel District it occurs to the west of Basalt 
 Creek in Washoe, where it is a resinous brownish black rock with 
 ledge-shaped felspar crystals, and shows under the microscope a 
 brownish-yellow glass ground mass. It abounds in yellowish green 
 augite with colourless felspar microliths, and black grains of magnetite, 
 with larger crystals of augite, sanidine, and plagioclase. Like nearly 
 all the known rocks of the same class, it contains 58 per cent, of silica. 
 Other augite andesites occur in the hill to the west of Steamboat 
 Valley, Nevada. In a ravine north of the Truckee Road, by the 
 Truckee river, the augite is green, and the rock contains half-decom- 
 posed olivine with abundance of sanidine. Besides these exceptional 
 augite andesites, typical types of the rock occur in the Foot Hills south 
 of Wadsworth, near the Truckee river. The felspars are remarkable 
 for the quantity of their inclusions, which are chiefly kernels of ground 
 mass. Pale-yellow sections of augite occur with light-brown horn- 
 blende, surrounded by a narrow black border. Similar rocks are 
 found in Augusta Canon, Augusta Mountains, and similarly free from 
 olivine. Sometimes there is very little sanidine ; but at Susan Creek 
 Canon, Nevada, the augite is so full of glass inclusions, that upwards 
 of seven millions would occur in a cubic millimeter. Augite andesite 
 is found in the Foot Hills of the Cortez range, in Independence 
 Valley, Nevada, arid in Waggon Canon. In the rocks to the west 
 of White Rock, Cedar Mountain, the augite andesite contains some 
 brown biotite. In the eastern portion of the Fortieth Parallel Terri- 
 tory, these rocks have a pale grey glass base, while in the western 
 territory the glass base is brownish. 
 
 Augite andesite occurs in the Palau Islands, at Kyneton in Vic- 
 toria, Australia, and in Java. 
 
 
NATURE AND TYPES OF BASALT. 
 
 277 
 
 Basalt. 
 
 Basalt, anamesite, and dolerite, are typical basaltic rocks, which 
 vary in texture from the compact condition of black biscuit china 
 seen in basalt, to a finely granular state in dolerite. These lavas 
 have a dark colour on the newly-fractured surface, varying through 
 shades of greyish brown, blue, and greenish black ; but where the 
 external surface is weathered, the rock is commonly a pale drab, 
 though the tint varies with chemical and mineral composition and 
 texture. Basaltic rocks have a high specific gravity and basic com- 
 position. Their silica rarely sinks below 40 per cent. ; a lower 
 percentage of silica is usually associated with large percentages of 
 iron, and sometimes of lime. The silica rarely exceeds 56 per cent. 
 The alumina has no necessary relation to the silica, though the average 
 amount ranges between n per cent, and 28 per cent. The lime, 
 magnesia, potash, and soda all vary in amount, and on this variation 
 depends the mineral composition of the rock. Basalt abounds in 
 labradorite and augite, generally contains magnetite and olivine, and 
 sometimes may have a little quartz and sanidine. 
 
 
 
 
 
 BASALT. 
 
 
 
 
 
 2 
 
 t 1 
 
 . 
 
 i 
 
 
 Sao i 
 
 ,' 
 
 
 a 
 
 
 J 
 
 i 
 
 u 
 
 Sal 
 
 I! 
 
 
 bo 
 
 g 
 
 
 
 _G 
 
 t-<5^ 
 
 l ^t'-^ 
 
 3 
 
 
 || 
 
 ll 
 
 1 
 
 3 
 
 1 
 
 II 
 
 "> N 3 
 
 || 
 
 
 r 
 
 IT 
 
 6 
 
 O 
 
 ft 
 
 "o 
 
 (3 
 
 1 
 
 Silica 
 
 36-68 
 
 42-64 
 
 44-85 
 
 46-32 
 
 49-27 
 
 5i-4 
 
 56-05 
 
 Alumina . 
 
 14-34 
 
 17-11 
 
 17-56 
 
 11-86 
 
 18-54 
 
 14-0 
 
 
 Peroxide of iron 
 
 22-30 
 
 5-29 
 
 
 4'5 
 
 6-98 
 
 
 10-30 
 
 Protoxide of iron 
 
 
 4-80 
 
 13-75 
 
 7-26 
 
 5-62 
 
 8'i 
 
 
 Manganese 
 
 ... 
 
 '45 
 
 
 0*14 
 
 
 
 trace 
 
 Lime 
 
 15-59 
 
 I 4'5^ 
 
 12-83 
 
 10.43 
 
 10-38 
 
 12-0 
 
 6-66 
 
 M?gnesia 
 
 9 -i8 
 
 7-34 
 
 9-74 
 
 11-82 
 
 3-76 
 
 7-1 
 
 I-52 
 
 Soda 
 
 077 
 
 1-38 
 
 0-90 
 
 2-10 
 
 2'22 
 
 3-6 
 
 0-98 
 
 Potash 
 
 3-93 
 
 3'43 
 
 0-24 
 
 4-09 
 
 3'45 
 
 3-8 
 
 3' 2 9 
 
 Water 
 
 
 
 o'6o 
 
 I'75 
 
 
 
 3-50 
 
 Zirkel's Classification of Basalt. Zirkel distinguished several 
 varieties of basalt according to the texture of the rock, which is 
 sometimes even-grained, and free from ground mass and porphyritic 
 crystals ; or it may be very fine-grained with porphyritic crystals, and 
 only a trace of glass ; or there may be a homogeneous glassy or half- 
 glassy ground mass half-filled with crystals ; or there may be large 
 crystals in an ill-defined amorphous mass. 
 
 But besides these varieties of texture due to conditions of cooling, 
 almost every locality affords varieties distinguished by mineral com- 
 position ; and sometimes this variation is so marked that the felspar 
 
2/8 MINERALS COMPOSING FELSPAR BASALTS. 
 
 basalt may have its felspar more or less absolutely replaced by neplie- 
 line, or by leucite, so as to be conveniently distinguished as nepheline 
 basalt or leucite basalt. The former is not necessarily free from fel- 
 spar, and may contain a little leucite ; the latter may include nepheline 
 with leucite, but rarely felspar. 
 
 The felspar basalts are more characteristic of Western Europe ; 
 the leucite and nepheline basalts are typical of the Erzgebirge, Eifel, 
 and Italy. 
 
 Felspar Basalts. Most basalts are felspar basalts ; the plagio- 
 clase cannot be determined with certainty, though there is rarely any 
 doubt that it is labradorite. It' occurs in long prismatic crystals, 
 which are the predominate constituent of the rock, and cross each 
 other in various directions. They show with polarised light the 
 coloured striae indicative of laminated twin structure, and occasionally 
 two sides of the crystal show blue and yellow colours. 
 
 If the lamination is absent, the felspar is identified as orthoclase, 
 as in the dolerite of Eowley in Staffordshire, where clear structureless 
 glassy felspar also occurs. 1 
 
 Felspars often include augite, magnetite, and apatite. The felspar 
 is sometimes partly decomposed, when the interior of the crystal is 
 turbid, or a little chlorite occurs in it ; and pseudomorphs of felspar 
 in chlorite are found at Mugdock Tunnel, near Glasgow ; Craigie Hill, 
 near Edinburgh ; Deep More in Staffordshire, and Matlock in Derby- 
 shire. 
 
 Pseudomorphs are easily recognised under the microscope, because 
 the optic axes vary in every part of the crystal, so as to produce a play 
 of colours in all positions of the prisms. 
 
 Augite is usually found in well-formed crystals, which have a 
 brownish tinge, and show bright colours in polarised light. It gives 
 an eight-sided section, while hornblende occurs in six-sided crystals. 
 In polarised light, augite, as a rule, shows no change of colour, though 
 in the rock at the Necropolis Hill, Glasgow, it is slightly dichroic, 
 and shows a purple tinge. Hornblende is always dichroic, and as the 
 polariser is rotated the colour changes from yellow to brown. Twin 
 crystals of augite are common, and laminated crystals are found at 
 Bowling, near Dumbarton. Minute grains of magnetite occur in 
 augite crystals ; and in the Campsie Hills, near Glasgow, the mineral 
 contains cavities in lines parallel to the sides. When augite decom- 
 poses it yields a grey fibrous substance, or a turbid granular substance, 
 and is often more or less perfectly replaced by chlorite. When horn- 
 blende is present it may be brown or black, and is distinguished from 
 the augite by its shining fracture, as at Mayen in the Eifel, many 
 places in the Westerwald, at Schima and Kostenblatt in Bohemia, 
 and in Heilenberg and Gickelsberg in Saxony. 
 
 Olivine is one of the more conspicuous minerals in basalt, often 
 
 forming glassy oil-green grains like drops. At Dalsmynni much of 
 
 the ground mass of the basalt is formed of grains of olivine ; at Unkel 
 
 on the Khine, and occasionally in the Auvergne, the olivine crystals 
 
 1 See Allport : Carbon. Doler., Q. J. G. S., vol. xxx. 
 
 
MINERALS IN EUROPEAN BASALTS. 279 
 
 vary in size from a diameter of half an inch to four inches. Near 
 Wiesbaden similar masses of olivine reach a diameter of two feet. In 
 Bohemia the olivine is usually well crystallised. It is generally present 
 in the dolerites of Scotland, but in the North of Ireland and in Iceland 
 there is rarely any trace of olivine, though it is sometimes abundant 
 in definite bands. In polarised light it shows brilliant red and green 
 colours ; it never contains any mineral except magnetite, but often 
 includes glass cavities. In England olivine is usually more or less 
 altered ; and in the midland counties it is always changed into a green 
 substance like serpentine. The alteration begins at the surface of the 
 crystal and extends inward along fine cracks. Serpentinous pseudo- 
 morphs are seen in the doierite of the Glee Hills, of Little Wenlock, 
 Matlock, and Eowley. Olivine is partly converted into hematite in 
 the basalt of Duncarnock in Lanarkshire, and at Bowling, near Dum- 
 barton. It is replaced by calcite in the South Staffordshire Coalfield, 
 and is sometimes replaced by zeolites. 
 
 Bronzite is found in the basalt at Oberwinter on the Rhine, and 
 in the Lohrberg in the Siebengebirge. Iron pyrites and blende are 
 both found in the basalt of Unkel. 
 
 In Antrim and some other localities native iron is found in small 
 particles ; and at Ovifak in Greenland, nickel-bearing iron is plentiful 
 in basalt, and sometimes occurs in very large masses. Titaniferous 
 iron is often found in the Rhine basalts in large visible grains. 
 
 Magnetite is invariably present in British basalts, and, being in 
 octahedrons, shows as an opaque black square, but it is sometimes so 
 clustered that the form cannot be recognised. Specular iron is recog- 
 nised by its blood-red colour, and occurs in thin hexagonal plates. 
 
 Brown mica is present, in some Scotch dolerites, in irregular 
 polygonal plates or strips. In sections parallel to the cleavage plane, 
 biotite is not dichroic, and is always dark between crossed nicols. 
 Sections at right angles to the cleavage plane vary from pale to very 
 dark-brown when the polariser is rotated. Mica is seen in the basalt 
 at Yeitskopf, at Kriifter Ofen, and the Laacher See, and has been 
 noticed near Teplitz and Bilin in Bohemia. 
 
 Apatite is always present in British dolerites in slender hexagonal 
 needles ; it is very common in the felspar and augite, and never 
 encloses any mineral except magnetite. 
 
 Quartz and sanidine have been detected in many localities in 
 North America; and in Europe are found in the basalts of the 
 Siebengebirge. 
 
 The percentage of water varies from 7 \ per cent, in basalts which 
 are rich in zeolites, to nothing in those which are free from zeolites. 
 When the basalt contains nepheline, decomposition of that mineral 
 yields natrolite; but most zeolites in basalt are due to the decom- 
 position of labradorite. 
 
 Agates and chalcedony often fill steam cavities in vesicular basalt. 
 
 The glassy matrix, when present, is seen to be a structureless 
 substance, filling the interstices between crystals. Its structure is 
 sometimes felsitic or even cryptocrystalline. When felsitic it always 
 
 
280 GEOLOGICAL HISTORY OF DOLERITIC ROCKS. 
 
 shows double refraction. It often has a granular aspect, but its 
 texture is due to consolidation during incipient crystallisation. Fel- 
 sitic texture is well seen at Inch Knock, near Coatbridge in Lanark- 
 shire, and in Kaimes Hill and Dalmahoy Hill, near Edinburgh. 
 
 The glassy base is often more or less altered. At first a fine 
 grey dust appears in it, and with further change the glass may be 
 replaced by specular iron, chlorite, calcite, and quartz. Grains of 
 quartz, clear and crystalline, of secondary origin, are common in 
 Scotch dolerites. 
 
 Representatives of Dolerite in Time. Mr. Allport, in 1874, 
 urged that the names of greenstone, melaphyre, and diabase should 
 be discarded, since they are essentially synonyms for basaltic rocks. 
 Melaphyre is only a dolerite of carboniferous age ; and diabase is a 
 dolerite which is so far decomposed that the mineral chlorite is 
 diffused in it. And as the oldest rocks are most altered, it results 
 that most diabases are of Cambrian, Silurian, and Devonian age ; x 
 they are basalts modified by the action of infiltrating water. 
 Hence, accepting these views, we give no detailed account of the 
 rocks so named. 
 
 Modes of Occurrence of Basalt. Basalt forms lava streams 
 which may be traced to their connection with the parent cone in 
 many of the Tertiary volcanoes of the Inner Hebrides, the Auvergne 
 and the Eifel. Interstratified sheets of basalt are characteristic of 
 the British Carboniferous, Devonian, and older Primary rocks. In 
 the form of dykes, however, basalt has a much wider geological 
 horizon, being intrusive in almost every kind of plutonic, meta- 
 morphic, and sedimentary rock, and in all periods of time. 
 
 Contemporary basalt is absent from the secondary rocks in Eng- 
 land, but occurs in Germany, developing prismatic structure in the 
 triassic sandstones which it covers ; while in the Jurassic rocks near 
 Dettingen, basalts are accompanied by massive tuffs and agglomerates. 
 Basalt induces a columnar structure in the Quadersandstein, near 
 Kribitz and Zittau. 
 
 Difference between Basalt and Augite Andesite. The differ- 
 ence between basalt and augite andesite consists essentially in the pre- 
 sence of olivine, for it is only when olivine is present that the rock 
 can be classed as basalt. The percentage of oligoclase may undergo any 
 amount of variation ; andesine and anorthite may both be associated 
 with the labradorite ; and when sanidine occurs it is commonly in 
 twin crystals. 
 
 Tachylyte. The glassy form of basalt called tachylyte is very 
 rare in this country, and has only been described at the edges of 
 basalt dykes, where the rock has a lustrous pitchy aspect, with ex- 
 tremely minute columnar structure and a dark-brown base, full of 
 cumulites which are regarded as the embryos of magnetic crystals, 
 though well-formed crystals of basalt minerals are scattered through 
 the dark-brown glass. Professor Judd has met with this rock at 
 Screpidale in the east of Raasay, at Beal in Skye, at Some north-west 
 1 Allport : Carbon. Dolerites, Q. J. G. S., vol. xxx. 
 

 GEOGRAPHICAL HISTORY OF DOLERITES. 281 
 
 of Mull, and Gribun west of Mull, and east of the Treslmish Islands, 
 and opposite Lamlash in the Holy Island, east of Arran. 
 
 In some continental localities trachylyte is spherulitic, as at 
 Bobenhausen, and it is perlitic at Marostica near Eassano. 1 
 
 Geographical Distribution of Basalt. The large areas occupied 
 by basalt can only be adequately appreciated with the aid of a geo- 
 logical map. They occupy a great space in the southern Eifel ; there 
 is a larger district in the Westerwald, and immense spreads in the 
 Vogelsberg, the Rhon, Mittelgebirge, and other parts of northern 
 Bohemia. Smaller exhibitions are seen in the Oelberg, in Petersberg, 
 and the Nonnenstromberg in the Siebengebirge, at Unkel, at Scheids- 
 berg, near Remagen ; the Landskron, near Neuenahr ; Steinheim, near 
 Hanau ; Wenneberg in Ries, Kemnath in the Fichtelgebirge, Groditz- 
 berg and Striegau in Silesia, Suhl in the Thiiringerwald ; and many 
 places in Saxony, Bohemia, Moravia, Styria, Hungary, and Transyl- 
 vania. In North Italy basalts occur at Fonte del Capo, near Avesa ; 
 Vestena Nuova, south of Monte Bolca ; Radicofani, in Tuscany ; and 
 among the lavas of Etna. In the Auvergne, basalts are seen at Mont 
 Rognon, the Plateau of Cozent, the Plateau of Prudelles, the Puy de 
 Charade, Puy de Come, Puy de Coliere, and the lavas of Gravenoire. 
 There are a few localities for basalt in the extreme south of Sweden, 
 such as Sosdala and Hoor. Basalts are well known in Greenland, 
 at Ovifak in Iceland, the Faroe Isles, and Inner Hebrides ; at Paran- 
 agua in Venezuela, in the Galapagos Islands, in the Sandwich 
 Islands, north of Melbourne in Victoria, St. Helena, the Isle of 
 Reunion, at Funchal in Madeira, at Palma, and at Cruz in 
 Teneriffe. 
 
 Types of Basalt in North. America. The basalts of the Fortieth 
 Parallel belong to two types. In the Elkhead region, and the 
 Kawsoh Mountains in Western Nevada, these rocks are nepheline- 
 basalt ; but in all other localities there is no trace of nepheline, 
 and the rocks are felspar basalts. The latter rock usually consists 
 of plagioclase, augite, and olivine ; but on the upper Snake River it 
 is composed of quartz, plagioclase, augite, and magnetite, without any 
 olivine, so that it makes a transition to the neighbouring augite-bear- 
 ing quartz trachyte ; but the ground mass is free from quartz. South 
 of the Yampa River, hauyine occurs as an inclusion in colourless 
 crystals of plagioclase. 
 
 The nepheline basalt in the Elk Mountains is light grey and very 
 porous. In the fine ground mass the eye distinguishes augite and 
 olivine ; while the microscope shows biotite, magnetite, nepheline, 
 plagioclase. and gothite. A dyke called the Rampart, only six feet 
 wide, rises to a height of 30 to 60 feet, and extends for four or five 
 miles. It is formed of basaltic columns arranged horizontally. It is 
 
 1 Hyalomelane was defined as distinguished from tachylyte, by not forming a 
 gelatinous-silica with acids. But Professor Judd and Mr. Grenville Cole give 
 strong reasons for rejecting the term. The rock occurs in Germany, at Ostheim 
 in the Witterau, and at Sahaburg in the Reinhardswald. A similar rock oc- 
 curs in the Ruby Valley range, in the Fortieth Parallel Region, U. S. A, 
 
282 BASALTIC ROCKS OF THE ROCKY MOUNTAINS. 
 
 free from triclinic felspar, is rich in biotite, and shows augite, nephe- 
 line, and sanidine. Sometimes triclinic felspar is present with 
 nepheline, as at Hantz Peak and Fortification Peak. 
 
 
 
 AMERICAN BASALTS. 
 
 i 
 
 
 Summit of Elk- 
 
 Stony Point 
 
 Ombe Range, 
 
 
 head. 
 
 Range. 
 
 Nevada. 
 
 Silica 
 
 48-60 
 
 48-40 
 
 <^4'8o 
 
 Alumina 
 
 1578 
 
 I7'95 
 
 JT^ 
 
 I7-58 
 
 Peroxide of iron . 
 
 3-22 
 
 2-28 
 
 0-97 
 
 Protoxide of iron 
 
 7'2I 
 
 8-85 
 
 8-84 
 
 Manganese 
 
 ... 
 
 trace 
 
 trace 
 
 Lime . 
 
 8'34 
 
 1005 
 
 8-22 
 
 Magnesia 
 
 10-13 
 
 6-99 
 
 4'47 
 
 Soda 
 
 377 
 
 2-86 
 
 3*14- 
 
 Potash 
 
 1-65 
 
 1-03 
 
 1-16 
 
 
 
 TiO- 
 
 
 
 
 0-24 
 
 
 
 trace 
 
 trace 
 
 ... 
 
 
 PO 5 
 
 CO 2 
 
 
 
 O'H 
 
 0-84 
 
 
 Loss on ignition . , . 
 
 1-30 
 
 0-34 
 
 0-94 
 
 
 
 
 ! 
 
 Modes of Occurrence of American Basalt. There is frequently 
 evidence that the basalt was extremely liquid, and flowed for long 
 distances, spreading out in thin sheets, which are superimposed on 
 each other. Felspar basalt extends over hundreds of miles in Nor- 
 thern California, Oregon, and Idaho, and the surface of the country 
 appears to be made up of continuous sheets. 
 
 In the Fish Creek Mountains basalt is seen to have come to the 
 surface through true craters, in the rhyolitic cones. This rock con- 
 tains large crystals of sanidine an inch long, and its base cannot be 
 resolved into its constituent minerals. 
 
 In most cases the basalt appears to have been poured out from 
 fissure eruptions, as in the Pahute range. The sheets, frequently 1000 
 feet thick, are exposed on table-lands ; while in some canons, like 
 those of Clarke's Station, the thickness amounts to 2000 or even 3000 
 feet. 
 
 The degree to which the rock is crystallised and the minerals 
 developed in it vary much with locality. In the Kugby chain, basalt 
 breaks with a curved fracture, and rings like bottle-glass, and is a 
 dark-brown glass, which is representative of obsidian, though the 
 glassy conditions are less developed than in the island of Hawaii. 
 After crystals of felspar, augite, and olivine have formed, the amount 
 of glass which remains is very variable ; and the basalts of the hills 
 to the north of Sou Springs have the whole of the ground mass 
 crystallised, and are remarkable for absence of glassy material. On 
 
 1 U. S. Geol. Surv. Fortieth Parallel, p. 676 ; Table XII. 
 

 HISTORY OF LEUCITE BASALT. 283 
 
 Shoshone Mesa the basalt contains quartz, and occasionally augite is 
 deficient or almost wanting. The olivine crystals are often so abun- 
 dant as to give a cleavage to the rock. Occasionally they form nearly 
 the whole ground mass, and the crystals often contain an abundance 
 of picotite. 
 
 On the south of Black Eock Mountain, in Mud Lake Desert, is a 
 coarse dolerite in which the plagioclase crystals are an inch long, and 
 the augite crystals a fourth of an inch in diameter. The ground 
 mass of this rock is slightly vitreous. 
 
 Aspect of Basaltic Rocks. In colour basalt may be black, choco- 
 late, dark grey, or greenish, but the latter colour is always due to abun- 
 dance of olivine. It frequently exhibits horizontal columnar structure 
 indicative of having cooled in dykes. It sometimes occupies eroded 
 valleys in the older rhyolite. Where decomposed, the basalt is charged 
 with seladonite and chalcedony. It shows all stages of texture, from 
 compact crystalline, through porous, to highly vesicular and scoriaceous 
 rocks as light as a sponge. Basaltic tuffs are met with, and sometimes 
 form an earthy and sometimes a true pelagonite. Even those which 
 are compact vary in texture ; some basalts are evenly granular and 
 poor in glass, like the common British types. Others have a ground 
 mass formed of a very fine crystalline aggregate of microliths of felspar 
 and augite, with larger crystals of felspar and olivine, and occasionally 
 of augite also. A third type is a homogeneous yellowish-brown glass 
 half filled with crystals, while sometimes the basalt is a mixture of 
 small and large crystals, with wedge-shaped masses of globulitic glass 
 between the crystals, and is then regarded as closely related to augite 
 andesite. Many of the American basalts closely approximate to those 
 of Europe. The basalt near American Elat Creek in Washoe, like 
 that of Scheinnitz in Hungary, has no glassy base, and consists of an 
 aggregate of fine-grained, pale-coloured augite and black magnetite, 
 with macroscopical and large microscopical crystals of plagioclase and 
 sanidine, often with olivine. But the minerals undergo considerable 
 change with the locality. East of Spanish Spring Station, in the 
 Virginia range, the augite is green. Near Wadsworth the plagioclase 
 includes augite and olivine. The olivine itself often contains octa- 
 gonal crystals of picotite, a variety of chrome spinel, also found in 
 the basalts of Germany, Bohemia, Hungary, and Italy. The nephe- 
 line basalts contain tridymite and sanidine. On the Upper Little 
 Snake river grains of quartz occur surrounded by augite. 
 
 Leucite Basalt. 
 
 Leucite basalts consist of augite and leucite. Mica occurs in 
 microscopic films in the fine-grained ground mass which contains 
 porphyritic crystals of augite, and usually olivine. The rock is very 
 rarely glassy, though a glassy condition is seen in the magma of the 
 Vesuvian lavas of 1822 and 1858. The basalt of Schackau in the 
 Khon Mountains is particularly rich in leucite. On both slopes of 
 the Erzgebirge the basalt consists chiefly of leucite, augite, and 
 
284 
 
 DISTRIBUTION OF LEUCITE BASALTS. 
 
 nepheline, though in some cases the quantity of nepheline is less than 
 in others. In a few localities olivine is added, and sometimes Hum- 
 boldtilite. This rock is well seen at Pohlberg, near Annaberg. In 
 the eastern Mittelgebirge, leucite-basalt occasionally contains a little 
 felspar, probably sanidine, and abounds in trichites. Near Aussig 
 the felspar is much more developed, and at Rothweil, in the Kaiser- 
 stuhl in Baden, the quantity of leu cite is diminished, so that it is not 
 always seen in hand specimens. At Stoffelskuppe in the Thiiringer- 
 wald, the leucite-basalt is free from felspar, and at Westberg, near 
 Hofgeismar, it is rich in large crystals of nepheline. In the Eifel, 
 the basaltic lavas contain leucite in many places, especially near Wehr 
 on the Laacher See, Veitskopf, the Forstberg, Burresheim, near St. 
 Johann. Other localities are Uedersdorf, Wehrbusch near Daun, and 
 Birresborn and Gerolstein. 
 
 
 
 LKUCITE 
 
 BASALT. 
 
 
 
 II 
 
 || 
 
 
 
 A 
 
 
 C- ^ 
 
 - 3 
 
 
 
 a 
 
 
 ^t 
 
 i*j 
 
 o 
 S 
 
 
 
 o oj -3 
 
 - 
 
 f ^ 
 
 i 
 
 * 
 
 
 a 
 
 6 
 
 
 
 5 
 
 Silica . 
 
 45-93 
 
 4803 
 
 52-08 
 
 47-48 
 
 Alumina 
 
 16-72 
 
 20-78 
 
 17-30 
 
 21-26 
 
 Peroxide of iron 
 
 ... 
 
 4-72 
 
 
 12 '39 
 
 Protoxide of iron 
 
 10-68 
 
 3'27 
 
 6-52 
 
 
 Manganese . 
 
 
 trace 
 
 ... 
 
 ... 
 
 Lirne 
 
 10-57 
 
 10-18 
 
 12-23 
 
 8;54 
 
 Magnesia 
 
 5-67 
 
 i'i6 
 
 1-25 
 
 
 Soda 
 Potash . 
 
 6-83 
 1-68 
 
 7-12 
 3-65 
 
 j 9-63 
 
 2-39 
 3 '42 
 
 Water . 
 
 
 
 
 
 Loss 
 
 0-59 
 
 0-17 
 
 0-59 
 
 035 
 
 The famous porous basalt of Niedermendig, worked for mill- 
 stones and paving-stone, contains leucite and nepheline, with augite, 
 hauyine, and olivine, with triclinic felspar in some specimens. Chemi- 
 cally, these rocks may be compared with Malvern schists and shales. 
 
 Leucite Basalts of North America. In North America this rock 
 is found in the Leucite Hills of Wyoming. The American leucite 
 rocks are yellowish grey and finely porous, and contain brownish mica 
 in long stripes peculiar to the rock. Leucite crystals are microscopic, 
 but abundant beyond anything known in European rocks. The 
 crystals are too small to show the twin structure indicated by alter- 
 nating dark and polarising bands. The rock contains pale-green prisms 
 and needles, which are referable to augite. There is a small quantity 
 of magnetite, and a few comparatively thick crystals of apatite. The 
 colour of the European leucite rocks is darker, because the augite 
 
HISTORY OF NEPHELINE BASALTS. 
 
 285 
 
 crystals are larger and more abundant, and there is more magnetite. 
 There is an entire absence of felspar in the American rock. 1 
 
 
 SCHISTS AND SHALES, MALVERN.S 
 
 
 Schist. 
 
 Schist. 
 
 Shale. 
 
 Shale. 
 
 Shale. 
 
 Silica . 
 
 43-61 
 
 45-82 
 
 49*37 
 
 53-97 
 
 64-37 
 
 Alumina 
 
 1 9 "34 
 
 16-39 
 
 21-47 
 
 23-24 
 
 18-62 
 
 Oxide of iron 
 
 17-02 
 
 l6'2O 
 
 I3-39 
 
 9-5I 
 
 2*07 
 
 Manganese 
 
 0-15 
 
 0'45 
 
 030 
 
 
 ... 
 
 Lime 
 
 2-31 
 
 1-46 
 
 075 
 
 I- 5 8 
 
 17 
 
 Magnesi.-i 
 
 7'10 
 
 7-8 r 
 
 100 
 
 366 
 
 077 
 
 Akalies and loss 
 
 4-96 
 
 5-68 
 
 7'06 
 
 8-04 
 
 8-86 
 
 Loss on ignition 
 
 5*51 
 
 6-19 
 
 6-66 
 
 ... 
 
 3-84 
 
 
 
 
 
 
 
 Nepheline-Basalt. 
 
 Nepheline-basalts consist typically of nepheline, augite, magnetic 
 iron, and olivine. Some varieties contain triclinic-felspar and a 
 little leucite, the latter being more often found. Biotite and Hum- 
 boldtilite are sometimes met with. Usually the structure is granular, 
 with a certain amount of glassy base; the porphyritic condition is 
 comparatively rare. Good types of the rock occur at Pflasterkaute in 
 the Thiiringerwald, and Kohlback, near Bayreuth, in the Western 
 Erzgebirge, near Ador Between Joachimsthal and Flatten a rock 
 occurs with more augite than nepheline. Nepheline-basalts are found 
 in the Mittelgebirge, and at Kaltennordheim in the Khb'n, where the 
 nepheline is recognised by its six-sided section, and is associated with 
 triclinic felspar. In the Swabian Alps the nepheline-basalt is altered, 
 chiefly by the decomposition of the nepheline. The nepheline-dolerite 
 of Katzenbuckel, near Eberbach, in the Odenwald, contains large 
 crystals of nepheline, which are sometimes altered. They lie in a 
 ground mass, rich in green augite microliths, small crystals of nephe- 
 line and nosean, magnetic iron, and a little glass. A rock of this 
 character is found at Oberbergen in the Kaiserstuhl. It contains 
 green or brown augite, nosean, nepheline often penetrated by micro- 
 liths of green augite, with sanidine and garnet penetrated by apatite. 
 Some lavas of the Eifel are characterised by a predominance of 
 nepheline, which is associated with Humboldtilite. Such a rock is 
 seen at Hannebacher Ley, north of the Laacher See : in addition to 
 these minerals, it contains well-crystallised augite, magnetic iron, and 
 a very little leucite. At the Scharteberge, near Kirchweiler, the rock 
 is a compound of nepheline, Humboldtilite, and augite, with hauyine, 
 olivine, and magnetic iron. A similar rock forms the Mosenberg, and 
 occurs near Bertrich by the Moselle. 
 
 1 For further details see Zirkel, Basaltgesteino ; Zirkel, Microg. Petrog. ; 
 Rosenbusch, Micro. Physiog. ; Clarence King, Report Fortieth Parallel, vol. i. 
 3 Timins, Q. J. G. S. vol. xxiii., p. 357. 
 
286 THE HISTORY OF OLIVINE ROCKS. 
 
 Influence of the Magnesia in Basalts on the Production of Dolomite. 
 
 Leopold Von Buch, 1 by a survey of the southern flank of the Alps, 
 was led to believe (i) that the elevation of the eastern range of the 
 Alps, since the tertiary epoch, was contemporaneous with and de- 
 pendent on the eruption of the basaltic rock termed melaphyre; (2) 
 that the dolomites of the Alps were produced from ordinary limestone 
 at the same time and with the same dependence. The line of dolo- 
 mites and melaphyres extends (interruptedly) from Bleiberg to Lake 
 Lugano ; but the presence of dolomitic limestone in other situations 
 than where melaphyre shows itself, must render inconclusive the 
 inferences drawn from their association in the Alps. At Lugano it is 
 rather near the augitic rock than in contact with it, that the limestone is 
 dolomitised. Between the dolomite and melaphyre of the peninsula 
 of Lugano, mica schist and porphyry intervene ; and on Monte 
 Argentera, the limestone which lies upon the melaphyre is not dolo- 
 mitised. De Beaumont observes that it is even rare to find the 
 dolomites near Lugano in actual contact with melaphyre. 
 
 Yon Buch assumes, it is to gaseous eruptions accompanying the 
 basaltic eruption, that we must ascribe the alteration. But we might 
 with more reason attribute the change to the influence of waters 
 charged with magnesia, liberated during denundation from the decay 
 of augite in long periods of time. 
 
 Peridotite. 
 
 Rosenbusch includes pikrite, Lherzolite, olivine-rock, eulysite, and 
 Dunite under the name peridotite. 
 
 Pikrite is usually a combination of olivine and augite, and is often 
 more or less altered into serpentine ; and according to the amount of 
 change depends the development of magnetite at the expense of pico- 
 tite, which is embedded in the olivine. The olivine is sometimes 
 embedded in the augite. The rock approximates to those varieties of 
 olivine-dolerite in which there are few crystals of plagioclase. The 
 augite frequently decomposes into a fibrous chloritic mineral. In the 
 Fichtelgebirge, where pikrites occur in many localities, titaniferous 
 iron is common, but in the pikrites of the Rhenish Uebergangsgebirge 
 it is replaced by magnetite. Hornblende and biotite occur as accessory 
 minerals. 
 
 Olivine-Diallage Pikrite. Another rock of this group consists of 
 olivine and diallage, and may be compared to an olivine-gabbro from 
 which the felspar has disappeared. It may include magnetic iron, 
 titanic iron, and chromic iron, and as accessories, hornblende and 
 magnesia-mica. This rock is usually more or less converted into 
 serpentine. It is well seen between Bensheim and Darmstadt. 
 
 At Schriesheim, near Heidelberg in the Odenwald, the pikrite is 
 a mixture of olivine, hornblende, and some plagioclase, with a little 
 
 1 Ann. des Sci. Nat., torn, xviii. pi. vii. 
 
LHERZOLITE. 287 
 
 biotite. Serpentites derived from this rock are found in Elba, and at 
 Monte Ferrato in Tuscany. In Elba the rock is very rich in olivine, 
 while in the Tuscan localities it is subordinate to the diallage. 
 
 Eulysite is a similar rock, and may be described as an olivine- 
 iliallage with accessory garnet. The Bohemian garnets are derived 
 from this rock. It is sometimes converted into serpentine. 
 
 The garnet-olivine rock of Mohsdorf in Saxony is not to be 
 separated as a distinct rock, though the garnet is sometimes wanting, 
 and then the rock is a compound of olivine and diallage. 
 
 Olivine-Enstatite consists of olivine, rhombic pyroxene, enstatite, 
 bronzite or hypersthene, and always contains magnetite or chromite. 
 It is seen in Russdorf in Saxony. A hornblendic variety occurs at 
 Crube Varallo in the Monte Eosa district, and a variety rich in garnet 
 is found near Heiersdorf in Saxony, where the rock also contains 
 zircon. Several examples of bronzite serpentines are iue to the 
 decomposition of olivine-enstatite. 
 
 Lherzolite, according to Professor Bonney, is a crystalline aggre- 
 gate of olivine, enstatite, and diopside with some picotite. Its tex- 
 ture is granular, and the grain may be either fine or coarse. It occurs 
 around the small tarn of Lherz, in the Eastern Pyrenees, in the depart- 
 ment of Arriege. It is a tough rock of olive-green colour, with olivine 
 for the predominant mineral ; it contains emerald-green spots of 
 diopside, dull waxy green spots of serpentine, resinous-looking grains 
 of enstatite, and minute black grains of picotite. The exterior weathers 
 to a bright yellowish or a rusty brown. There are seven exposures 
 in the district, all intrusive in the Lias. Under the microscope olivine 
 is seen in roundish grains, showing between crossed nicols, colours vary- 
 ing from greenish yellow to yellowish green, and from a bright pink 
 to a purple pink, The enstatite is in irregular or long grains ; it is 
 colourless in ordinary light, but in polarised light varies from pale 
 yellow or grey to some, tint of blue. The diopside rarely occurs in 
 distinct crystals. In polarised light the colours are less clear than 
 those of olivine, and usually vary from rich yellowish brown to puce. 
 Picotite may occur in groups of grains or films. Its colour is olive 
 green to umber-brown, and is dark between crossed nicols. The 
 serpentine, which appears to result from the decomposition of the 
 olivine, shows between crossed nicols a pale golden tint. 1 
 
 The Lherzolite at Konradsreuth in the Fichtelgebirge, according 
 to Gunibel, 2 is accompanied by an altered olivine slate now converted 
 into talc slate and actinolite schist. It is associated with eklogite 
 and diorite, and with them is contained in syenitic gneiss. The 
 Lherzolite here, too, is converted into serpentine, and it is probable 
 that many serpentines will prove to have been originally a rock of 
 this kind, especially the serpentines of the Vosges, the Lizard, and 
 the department of Yar. 
 
 Dunite, originally described from New Zealand, consists of olivine 
 and chromic iron. In the south of Spain, at Serrania de Honda, the 
 
 1 Bonney "On the Lherzolite of the Arriege," Geol. Mag., Feb. 1877. 
 
 2 Gumbel, Fichtelgebirge, 1879. 
 
288 NATURE AND ORIGIN OF SERPENTINES. 
 
 Dunite is decomposed into serpentine. A similar rock appears at 
 Kranbat in Upper Styria, and at several places in the Vosges, near 
 St. Etienne, &c. There is rarely any enstatite or diallage, but both 
 are found, and are sometimes decomposed into chlorite or amphibole. 
 
 Serpentine. 
 
 Serpentine is now known to result from reconstruction of the 
 mineral constituents of many kinds of igneous rocks, especially of 
 those rich in olivine and enstatite. The whole group of peridotites or 
 olivine rocks may be converted into serpentines, though a few serpen- 
 tines have been described to which no such origin can be attributed. 
 Examples of divine-serpentines have been enumerated under the 
 peridotites ; but at Todtmoos in the South Schwarzwald, a serpentine 
 is seen which indicates that the rock from which it was formed con- 
 sisted of a mixture of diallage and enstatite, and contained as acces- 
 sories biotite and hornblende. In the Bluttenthal in the Vosges, the 
 serpentines are said to be decomposed hornblende slates. But even 
 in these cases it is difficult to affirm positively that the rock 
 was always- free from olivine ; though nothing is needed for the 
 production of serpentine but an abundant infiltration of silicate of 
 magnesia in chemical combination with water. The ground mass is 
 colloidal, and contains crystals and fibres of the minerals which it 
 replaces. It is typically green. When serpentine occurs as an 
 accessory, it may replace olivine, hypersthene, augite, or amphibole ; 
 but black mica, amphibole, and augite are more frequently converted 
 into chlorite. MM. Fouque and Levy regard the viridite of volcanic 
 rocks as a phase of serpentine. Any rock which includes these 
 minerals may be more or less converted into serpentine. When 
 serpentine results from the decomposition of enstatite, bastite is fre- 
 quently formed. 1 Serpentine often occurs in schists, when it may be 
 due to the presence of glauconite in the original sedimentary rock 
 which was metamorphosed in other cases it is certainly intrusive, but 
 even then it may have had a similar metamorphic origin. 
 
 Serpentine in Relation to Diallage Rock. The abundance of 
 serpentine in the Pyrenees, Apennines, and other parts of the south 
 of Europe, has long been known. Diallage rocks, which are equally 
 abundant, often occur in connection with the serpentine, and there is 
 now no doubt as to the fact that these two rocks are very intimately 
 related. Few conclusions of this nature appear better authenticated 
 by observation than the gradation of diallage rock into serpentine, in 
 the Alps, the Apennines, Corsica, and Cornwall. 
 
 In the Northern Apennines Brongniart remarked the following 
 general order of succession downwards : i. Serpentine. 2. Diallage 
 rock, in the upper part assuming the aspect of serpentine (at Eochetta, 
 north of Borghetto, near Spezia), consisting partly of red crystallised 
 limestone. 3. Jasper rock in thin laminae. Below these are lime- 
 stones and marly schists, common in the Apennines. In Monte 
 1 Fouque* and Levy, Roches Eruptives Fransaises, 1879. 
 

 SERPENTINES OF ITALY. 289 
 
 Kamezzo, north-west of Genoa, the serpentine rests on limestone and 
 talc schist, the limestone is in thin tortuous beds, and is as it were 
 dissolved with the shining slate and steatite-schist. The direction of 
 the serpentinous masses in the Northern Apennines, to which the 
 elevation of that part of the range is ascribed, is east- south- east, 
 which is the same as that of the Pyrenees, and of some serpentine 
 rocks about Como. 
 
 Serpentines of the Riviera. West of Genoa several examples 
 of dull-green serpentine are seen on the shore on both sides of Pegli. 
 Several neighbouring headlands consist of gabbro, and both rocks, 
 according to Professor Bonney, closely resemble those of Cornwall. 
 Fragments of serpentine are brought down to the shore above Pegli 
 by the Yarenna Torrent, some full of crystals of bronzite, like the 
 serpentine of Cadgwith in Cornwall. Serpentine forms the coast 
 from Framura for several miles as far as Bonasola. It shows a sub- 
 spheroidal structure, and is sometimes a rusty red, sometimes greenish. 
 At Levanto, where the rock is quarried, one variety is purplish or 
 brownish-black, veined here and there with dull green, and with 
 crystals of bronzite. In other places it is lighter and greener. A 
 gabbro, consisting of sassurite and diallage, here as elsewhere, is 
 associated with the serpentine, and has an intrusive aspect. 
 
 Serpentines of Tuscany. The celebrated Verde di Prato occurs 
 at the village of Figline, about three miles from Prato, in the Lower 
 Arno. This serpentine is of a purplish or greenish brown, and forms 
 a large mass on the upper part of the hill. Through the ground mass 
 are scattered small green crystals of enstatite and films of white 
 steatite. The serpentine is intrusive, according to Professor Bonney, 
 and he states that gabbro penetrates into the serpentine. As in BO 
 many other localities, microscopic evidence leads to the conclusion 
 that these serpentines are altered olivine rock. 1 
 
 Serpentines South-east of Leghorn. Mr. Hamilton described 
 the serpentine in the western part of Tuscany, S.E. of Leghorn. 
 It extends from the copper mines of La Cava, and along the hills 
 running from Monte Catini to Castellina. At Monte Rufoli it covers 
 an area of many miles. At Monte Catini it is soft and soapy, of a 
 grey-green colour. The copper ore lies between the serpentine and 
 the red gabbro. The serpentine masses strike from N.W. to S.E., 
 which is parallel to the axis of the Apennines. Veins of steatite, 
 known as Pietra di Sarto, frequently occur in the serpentine, so as to 
 have a commercial value. At Monte Eufoli the serpentine is covered 
 with forests of ilex. Here occur the famous chalcedony quarries, 
 which resemble quartz veins, but only extend a few feet below the 
 surface. The serpentine around them is soft and decomposed. Ser- 
 pentine is well exposed at Sibbiano, and at the village of L'Impruneta, 
 where it often contains red veins. It is everywhere associated with 
 the red gabbro. 
 
 1 Bonney, Geol. Mag., Aug. 1879, p. 362. 
 
 VOL. I. 
 
( 2 9 o ) 
 
 CHAPTER XVIII. 
 
 THE HISTORY OF VOLCANIC ACTIVITY IN BRITAIN. 
 
 THE British region was greatly disturbed by volcanic pnenomena 
 during the whole of the Primary period, and during the earlier por- 
 tion of the Tertiary period volcanoes were active in Western Scot- 
 land ; but except at its commencement in the Triassic rocks of 
 the south-west of England, the whole of the Secondary epoch was 
 free from volcanic outbursts. During a large part of the time re- 
 corded by the Primary strata, the British region was a volcanic archi- 
 pelago throwing out vast quantities of volcanic ashes and lavas, 
 which not infrequently form much of the thickness of some of the 
 older Primary rocks. 
 
 Evidence for the Former Existence of Volcanoes. The ex- 
 istence of these volcanoes is affirmed chiefly on the evidence of the 
 volcanic ashes and agglomerates j for although intrusive lavas may 
 be injected in sheets into strata and between them, long subsequent 
 to their deposition, no source for volcanic ashes is conceivable except 
 the throat of an active volcano. The very sites where such ancient 
 centres of volcanic activity stood are usually unknown, and often 
 only inferred, with more or less uncertainty, from dykes and bosses of 
 volcanic rock. But although long since levelled, like Graham Island, 
 and having the throats or necks covered up by later deposits, denu- 
 dation, which has exposed the old strata forming the structure of the 
 country, yields a history of volcanic action, which we propose to 
 trace. 
 
 In the following pages we begin with the most ancient eruptions, 
 and follow the outbursts of volcanic action during successive periods of 
 geological time. If there be any general differences between modern 
 and ancient volcanic effects, it is in part attributable to the circum- 
 stance that the modern phenomena best known to us are such 
 as happen on land subaerial eruptions while the evidence of 
 the ancient volcanoes is chiefly gathered from subaqueous lava 
 streams, and submarine beds of ashes. Much of the modern andesite, 
 trachyte, basalt, and other melted rock has been exposed by 
 eruption, and indurated at the surface ; much of the ancient 
 porphyrite, felsite, and greenstone was solidified under the pressure 
 of seas, and decomposed and indurated by prolonged infiltration. 
 
THE MOST ANCIENT BRITISH VOLCANOES. 
 
 Pre-Cambrian Volcanoes. 
 
 Pre-Cambrian Volcano of St. David's. The oldest group of 
 rocks in this country, named pre-Cambrian, is largely volcanic. The 
 oldest portion, constituting the Dimetian formation of Dr. Hicks, 
 is a variable rock, regarded as intrusive syenite by Professor Kamsay, 
 and as intrusive granite by Dr. Archibald Geikie, in its typical 
 exhibition at St. David's ; and this part of the series is generally 
 admitted to be everywhere free from volcanic rocks ; but, so soon as 
 we pass above these ancient gneisses, with their interstratified schists 
 and limestones and granitoid rocks, and come to the series named by 
 Dr. Hicks Arvonian and Pebidian, we find proofs of remarkable 
 volcanic activity. It is possible that the so-called "Arvonian" 
 formation is only the lower part of the " Pebidian." It consists 
 chiefly of felsites of the type known as " halleflintas," with quartz 
 felsites and breccias. On the hypothesis of the so-called Dimetian 
 being intrusive, this felsitic condition of the rock immediately above 
 it is a matter of some interest, because it corresponds to the state in 
 which the underlying rock would appear in its outer portion in 
 consequence of rapid cooling, and a similar rock is found on the 
 margins of many important masses of granite. According to the views 
 of Dr. Hicks, these felsitic rocks are ancient rhyolitic lavas. The 
 overlying Pebidian series, however, consist to a large extent of 
 volcanic ash, interstratified with micaceous and chloritic schists. The 
 lowest beds include, in the agglomerates, spherulitic felstone lava ; 
 and higher up are many alternations of felstone and volcanic tuff. 
 Hence the whole series is precisely such as might have been fur- 
 nished if the granitoid rocks of St. David's had been the base of 
 an old volcano, and the materials ejected from it had sometimes 
 decomposed into muds and sometimes formed ordinary volcanic 
 rocks. 
 
 In North Wales the three subdivisions of pre-Cambrian rocks 
 have been established by Dr. Hicks, Professor Hughes, and other 
 observers, and show similar characteristics. 
 
 Pre-Cambrian Volcano of the Wrekin. Mr. Allport, who has 
 contributed greatly to establish the identity of volcanic rocks of the 
 Primary and Tertiary periods, notices a narrow ridge of igneous rock 
 which extends two miles and three-quarters through Ercal Hill and 
 the Wrekin. A second ridge lies to the west. Both these rocks 
 are highly acidic, and are associated with beds of volcanic ash 
 greatly indurated, which are well seen on the south of Lawrence 
 Hill. 
 
 The lavas are reddish-brown felsite, or altered pitchstone. They 
 frequently exhibit on their weathered faces numerous parallel lines, 
 which sometimes show complicated folding. These lines indicate the 
 presence of streams of microliths, such as are seen in Hungarian 
 perlites of Tertiary age. Sometimes the spherulitic rock of the 
 Wrekin consists of bright-red spherulites, set in a grey or yellowish- 
 
2 9 2 VOLCANIC PHENOMENA OF THE WREKIN. 
 
 Sreen matrix ; and under the microscope each central red spot and its 
 border exhibit a radiated fibrous structure. Frequently the spheru- 
 litic structure is replaced by perlite structure, and when the rock is 
 examined by the lens, it then shows small concave or convex sur- 
 faces. The original glassy base of these rocks has undergone the 
 change of devitrification. Quartz veins are not uncommon in the 
 rock, and nodules of chalcedony are abundant. The old lava contains 
 72 per cent, of silica, 14^- per cent, of alumina, 6 per cent, of potash, 
 and 2 per cent, of soda. 1 
 
 The volcanic agglomerates consist of ash of varying size, fre- 
 quently somewhat vesicular, having the cavities filled with chlorite 
 and crystalline quartz. Epidote is frequent in the coarser ash. 
 
 The Ercal is a hill overlooking "Wellington, quarried for a bright 
 red felspathic rock of a granitoid character, which suddenly changes 
 to compact felstone, which is apparently the Wrekin rhyolite intrusive 
 in the red rock. The north-east end of Lawrence Hill consists of 
 felstones similar to those of Ercal, with felspathic tuff, which contains 
 fragments of pitchstone and felstone. 
 
 The tuff which forms the Wrekin is continued and appears on 
 the northern slope of Primrose Hill, and there intrusive diorite is 
 found, which is similar to the diorite of the Malvern district. 
 
 The Lea rock to the N.W. is a great mass of rhyolite, remarkable 
 for its spherulitic and perlitic structure. 
 
 In the pre-Cambrian rocks of Lillieshall Hill there are two asli 
 bands, each about 10 feet thick, soft and ferruginous, and excavated 
 by weathering. According to Dr. Galloway, the minimum thickness 
 of the alternations of hornstone, ashy slates, shales, and felspathic 
 agglomerates there seen is 1500 feet. 
 
 Other examples of the same alternations of pre-Cambrian felstones 
 and tuffs occur at Rock ward en, where the felstones are spherulitic, 
 at Charlton Hill, at both ends of Lawley Hill, the centre of which 
 is an intrusive mass of greenstone. At Caer Caradoe there are also 
 pre-Cambrian rocks with felstones and ashy shales, and the volcanic 
 series is traced onward by Cardington Hill and Ragleth Hill. 
 Hence the chain of hills which forms the axis of South Shropshire is 
 of pre-Cambrian age, and largely formed of volcanic materials of a 
 rhyolitic type, though sometimes broken through by dolerite and 
 gabbro. 2 
 
 Pre-Cambrian Volcano of Caernarvonshire. In north-west Caer- 
 narvonshire, quartz-felsite or rhyolite is well exhibited in the vicinity of 
 Bangor, Caernarvon, and Llyn Padarn. These rocks were regarded by 
 Professor Ramsay as metamorphic, and as stretching for thirteen 
 miles in the Cambrian strata. Professor Bonney 3 believes that the 
 
 1 Allport : " Ancient Devitrified Pitchstones and Perlite," Q. J. G. S., vol. 
 xxxiii. p. 449. 
 
 2 Galloway : " Pre-Cambrian Rocks of Shropshire," Q. J. G. S., vol. xxxv. 
 p. 643. 
 
 3 Bonney: " Quartz Felsite in North-West Caenarvonshire," Q. J. G. S.,May 
 1879. 
 
THE VOLCANIC HISTORY OF NORTH WALES. 293 
 
 rocks at Bangor, and part of those at Llyn Padarn, are ancient lava- 
 flows, which in strictness should be termed rhyolites. This felsite, 
 under the microscope, shows a ground mass containing crystalline grains 
 of quartz, with orthoclase and plagioclase. The rock constantly ex- 
 hibits the streaky structure characteristic of acidic lavas, while to the 
 north-east of Llyn Padarn the felsite is associated with agglomerate. 
 These masses are referred by Professor Bonney to the Pebidian group 
 of Dr. Hicks. 
 
 On the eastern side of the Malvern Hills, near the Herefordshire 
 Beacon, there is a small area of compact felsitic rocks of uncertain 
 age, which Professor Bonney regards as probably Pebidian. 1 
 
 Pre- Cambrian Volcano of Charnwood Forest. In Charnwood 
 Forest Professor Bonney and the Rev. E. Hill have described rocks 
 probably of pre-Cambrian age, which consist of slates alternating 
 with thick masses of rhyolitic agglomerate, less glassy than those of 
 the Wrekin, and alternating with beds which appear to be composed 
 of volcaiiic materials, sorted by water and somewhat triturated. 
 
 Cambrian Volcanoes. 
 
 Cambrian Volcanoes of North. Wales. The great importance 
 of volcanic rocks in the history of the Cambrian Formations of North 
 Wales may be best appreciated by examining a geological map, which 
 represents on a sufficient scale the country from the south of Bar- 
 mouth through Cader Idris, the Arans, and the Areing Mountains 
 into Caernarvonshire. On the Geological Survey map broad strips of 
 reddish colours indicate the areas now occupied by beds of volcanic 
 ashes, felspathic lavas, and quartz porphyries, which were poured 
 out from sub-aerial and sub-marine volcanoes during the whole of the 
 Cambrian ages ; and in the Malvern district the Holly bush sand- 
 stone contains dolerites 1 which are interbedded, and at least as old 
 as the Lingula Flags of North Wales. The lava occurs as lenticular 
 masses, which now stand up in rounded bosses, because they have 
 decomposed less rapidly than the muddy sediments in which they 
 were covered up. 
 
 Volcanic Rocks in the Lingula Flags. The Lingula Flags of 
 North Wales in their higher beds so resemble volcanic ashes that 
 between Capel Arthog and Penmaen, west of Dolgelly, there appears 
 to be no doubt about their having originated in volcanic action. 
 But, otherwise, the igneous rocks of the Lingula beds are for the 
 most part dykes and intrusive masses of dolerite, though some 
 masses of felsite occur. Many of these dykes run in the line of 
 strike, but they are never interstratified, and there is no evidence of 
 their geological age. 
 
 Volcanoes of the Tremadoc Slates. Volcanoes were probably 
 active during the succeeding Trernadoc period, for near the top of 
 that series the slates contain pisolitic iron ore, such as, in other 
 
 1 Holl : Q. J. G. S., vol. xxi. p. 72. 
 
294 
 
 AREN1G VOLCANOES IN NORTH WALES. 
 
 localities, is associated with volcanic rocks, and is known to result 
 from their decomposition. 
 
 Arenig Volcanoes. It was during the succeeding Arenig period 
 that the volcanoes in the Welsh district attained their greatest acti- 
 vity. There are no remaining traces of the throats up which the 
 igneous matter was ejected, for these outbursts seem analogous to recent 
 submarine eruptions among the Canaries, which rose from the floor of 
 a moderately deep sea, and in a few years were so completely worn away 
 that no definite indications of their position can now be ascertained. 
 
 In Cader Idris the Arenig rocks include a jointed porphyry, some- 
 what hornblendic, and therefore possibly andesitic, which is 1700 
 feet thick, and extends northward continuously to beyond Aran Mowd- 
 dwy. Upon these lava streams are 300 feet of blue slates, formed of 
 muds which may have been to a large extent derived from the decom- 
 position of volcanic materials ; and higher up are i oo feet of por- 
 phyritic felspathic ashes, and finally 500 feet of a greenstone, which 
 is full of air cavities, and therefore was poured out under moderate 
 pressure of water or air. Many intrusive sheets of greenstone also occur. 
 Lower down the mountain, the newer beds also alternate with fel- 
 spathic ashes and lavas, which are rudely columnar, and are about 
 1500 feet thick; but Sir Andrew Kamsay is doubtful whether the 
 lavas are really contemporaneous, and out of the 3600 feet of igneous 
 materials interstratified with the slates, believes that only 400 feet of 
 ash-beds can with certainty be regarded as derived from craters which 
 were active during the Arenig period. 
 
 The Arans, however, have the same general structure as Cader 
 Idris, and there is here less room for doubt as to the contemporaneous 
 character of the interstratified igneous rocks, because the lavas have 
 baked the slates over which they flowed, giving them the texture of 
 porcelain, while the slates which rest upon the lavas are unaltered. 
 The volcanic ashes are frequently vesicular, and sometimes water 
 worn, and thin away rapidly on the north side of the Aran chain, as 
 though that were furthest away from the volcanic crater. 
 
 Evidence of this thinning out of the ashes and lava is best exhi- 
 bited in a table, constructed from Sir Andrew Ramsay's data: 
 
 
 Cader 
 Idris. 
 
 Ai-an 
 Mowddwy. 
 
 Moelddu. 
 
 Arenig. 
 
 Moelwyn. 
 
 Upper ashes and agglo- j 
 merates . . . . ) 
 
 Feet. 
 100 
 
 traces 
 
 ... 
 
 800 
 
 ... 
 
 Felspathic lavas . . . 
 
 2850 
 
 2400 
 
 1150 
 
 250 
 
 400 
 
 Lower ashes and agglo- ) 
 merates . . . . \ 
 
 2700 
 
 3400 
 
 1 100 
 
 ... 
 
 2700 
 
 from which we learn that though the contemporary ashes and lavas 
 extend from Cader Idris to the east of Festiniog, and though the ash 
 
VOLCANIC ROCKS OF THE LAKE DISTRICT. 295 
 
 extends from six miles south of Barmouth to six miles west of Bala 
 Lake, all the beds die away and disappear at a little beyond Tremadoc 
 on the north, and towards Llanegryn on the south. The lower ashes 
 have their greatest thickness in Aran Mowddwy, where the lava also 
 is thickest. The lower ash disappears in the Arenig. The lavas have 
 thinned to an insignificant amount in Arenig, while the upper ash 
 there attains its greatest thickness. This deposit formed a lenticular 
 mass 23 miles long, thinning away on the south near Penmaen, and 
 on the north at Cwmorthim. The volcanic centres of this region are 
 placed by Kamsay to the eastward of a line drawn from Tremadoc to 
 Llangryn, and he suspects that the felspathic masses of Tyddyn-Rhiw 
 and Gelli-llwyd-fawr near Dolgelly, and of Y-Foel-ddu near Aran 
 Mowddwy, and part of the Arenig, are probably remains of the necks 
 of volcanoes, because the lower ashes and felstones attain their 
 greatest development near Dolgelly and Aran Mowddwy, and the 
 upper ashes attain their greatest thickness near Arenig. 
 
 The greenstones do not appear to have at any time reached the 
 surface of the country, and therefore it is difficult, if not impossible, 
 to fix their age ; even the great mass of Rhobell Fawr, which is seven 
 miles long and three miles broad, appears to have been an intrusive 
 overflow between planes of bedding, like the masses termed laccolites. 
 
 In the lake country contemporary volcanoes poured out great 
 thicknesses of felspathic lava and ashes, which are interstratified in 
 the formation known as Skiddaw slate. 
 
 Llandeilo Volcanoes. In the succeeding period, known as the 
 Llandeilo Flags or Lower Bala Rocks, volcanic action still continued 
 with great vigour. In the group of green slates and porphyries of the 
 Lake district, the volcanic rocks make as prominent a feature as in 
 strata of the same age in Wales ; but according to Professor Bonney 
 these are rather to be referred to the andesites, since they correspond 
 to them in chemical composition. 
 
 The Volcanic Rocks of the Borrowdale Series. The greenslates 
 and porphyries of Sedgwick, termed the Borrowdale series by Pro- 
 fessor Nicholson, cover a large area of the central part of the Lake 
 district, extending 25 miles E.N.E. along the strike, and 13 miles 
 S.S.E. in the direction of the dip. These rocks are estimated at 5000 
 to 6000 feet thick, and a large part of the thickness consists of asli 
 beds and lava streams. In Borrowdale, the Skiddaw slates are over- 
 lain by dark -green compact lava, which is sometimes rudely columnar. 
 This is succeeded by felspathic ash and agglomerate, with many 
 minor beds of lava, the highest bands being amygdaloidal ashes, in 
 which the cavities are commonly filled with quartz. These rocks are 
 well seen rising through Cat Bells, Barrowside, Maiden Moor, and 
 Narrow Moor. A similar sequence is seen in the valley of Gates 
 Garth Beck, which flows into the head of Buttermere. In many 
 places the ashes and breccias are cleaved, and form fine-grained slates, 
 sometimes green, sometimes purple, as may be seen in the quarries at 
 .Dale Head and Honister Crag. Sometimes a subordinate bed of fine 
 purple felstone occurs with large greenish crystals of felspar. Similar 
 
296 VOLCANIC ROCKS ABOUT SNOIVDON. 
 
 beds are traced in the vale of St. John, in Matterdale, in Eycott Hill 
 between Ulleswater and Haweswater, above Shap, and in other 
 places. 1 
 
 Volcanic Rocks of Bala Age. The nodular felsites in the Bala 
 rocks of North Wales are well exhibited in the valley of the Conway 
 and Bett \vs-y-Coed, and have been studied by Professor Bonney near 
 the Conway Falls Inn. On both sides of the house the rock is a 
 compact felsite. It is succeeded by a bed with wavy lamina and 
 films of a green mineral, soon passing into more or less fissile and 
 coarsely spheroidal rock, with nodules as large as a pigeon's egg, 
 which, when weathered, give the rock the aspect of a conglomerate. 
 Other sections are seen by the wicket-gate leading to Conway Falls 
 from the road to Pandy Mill. The typical felsite is a compact bluish- 
 grey rock, which shows corrugated structure under the microscope, 
 due to the arrangement of microliths ; and nearly all the specimens 
 exhibit fluxion structure. The rock, which has a schistose character, 
 contains vesicles, which have become infiltrated with crystalline 
 < quartz, and with limonite, so that the rock is identified as a vesicular 
 rhyolitic lava. The nodular spherulitic rock shows no trace of a 
 radial structure, but closely resembles the ordinary felsites of the 
 neighbourhood. In the upper part of Conway Mountain, the yel- 
 lowish felsite contains spheroids, sometimes two inches in diameter, and 
 frequently hollow in the centre. This felsite is also seen in the 
 Diganwy Hills, where the ovoid masses have a cherty aspect, but 
 under the microscope they show fluxion structure like the matrix of 
 the rock. The ordinary cream-coloured felsite of the Conway moun- 
 tain has almost the aspect of a bedded mudstone. Professor Bonney 
 accounts for the formation of the nodules by contraction of the vesi- 
 cular rock, determined by the presence of a cavity, with roughly con- 
 centric cracking of the mass in cooling. 2 
 
 Felstone of the Glyders. In a lava-flow from the Bala beds 
 forming the Glyders, on the north side of the Pass of Llanberis, Mr. 
 Frank Kutley has described perlitic and spherulitic structure. The 
 rock now termed felstone was originally a vitreous lava of the kind 
 named rhyolite, and the felstone character is entirely due to devitrifi- 
 cation. 3 
 
 On the south side of the Capel Curig road, near Bedd-Gelert, is a 
 greenish-grey rock, which shows the usual characters of a de vitrified 
 rhyolitic lava, with spherules one-eighth of an inch in diameter, ar- 
 ranged in bands. The spherules are an aggregate of minute colourless 
 granules, probably garnets, and pale-green scales, probably of chlorite. 
 The rock was originally a pitchstone or obsidian. 
 
 Another rock of a greenish-grey colour, which presents a banded 
 
 1 Nicholson : "Greenslates and Porphyries of the Lake District," Q. J. G. S., 
 vol. xxvii. p. 599. 
 
 2 Bonney : "Eelsites in the Bala Group of North Wales," Q. J. G. S., vol. 
 xxxviii. p. 280. 
 
 a Rutley : "On Perlitic and Spherulitic Structures in the Lavas of the Glyder 
 Fawr," Q. J. G. S., vol. xxxv. p. 508, 
 
VOLCANIC PEAKS OF THE GRAMPIANS. 297 
 
 surface on weathering, is found in the Bala beds, about a mile N.W. 
 of the summit of Snowdon, and closely corresponds with obsidians 
 from volcanic districts in its fluxion structure. 
 
 A dark-grey felstone, with a fissile structure, is seen between 
 Pont-y-Gromlech and Gorphwysfa, so that it might be termed a felsite 
 schist, or an indurated volcanic ash, unless examined under the micro- 
 scope ; but its microscopic texture corresponds with many obsidians 
 and rhyolites, and is probably to be regarded as a devitrified obsi- 
 dian, because the base is entirely devoid of crystalline structure. 
 
 Skomer Island, off the coast of Pembrokeshire, contains volcanic 
 rocks which are associated with strata belonging to the Llandeilo or 
 to the Bala series. They are banded and spherulitic, and what were 
 once obsidians are now felstones. They closely resemble those of the 
 Yellowstone district of the United States. With the felstones occur 
 other rocks, classed by Mr. Rutley as basalt, and as quartz oligoclase 
 trachyte. 1 
 
 The Silurian period, comprising the Wenlock and Ludlow rocks, 
 appears to have been one in which volcanic action was inter- 
 mitted. 
 
 Devonian and Old Red Sandstone Volcanoes. 
 
 Old Eed Sandstone and Devonian Volcanoes. With the Old 
 Red Sandstone and Devonian period, eruptions began again with 
 great vigour, but their locality is removed from Wales and the 
 Lake district. Interbedded volcanic rocks abound in North Devon, 
 West Somerset, and South Devon ; but northward there is no trace 
 of an Old Red Sandstone volcano till we reach the south and centre 
 of Scotland. 
 
 Volcanic Peaks of the Grampians. Professor Judd has drawn 
 attention to a series of granite masses which burst through the 
 Cambrian strata ; and, forming the axis of the Grampian chain, 
 extend from the Ross of Mull in the S.W. in a N.E. direction to 
 Peterhead. The largest of these masses are Cairngorm, Ben Nevis, 
 and Ben Cruachan. Professor Judd remarks, that when the granite 
 boss is greatly denuded, as in the Ross of Mull, the rock exposed 
 is a typical granite ; but where it rises into lofty peaks, it becomes 
 more and more hornblendic, and graduates externally into a felsite, 
 which is more or less porphyritic. Everywhere, when in contact 
 with stratified rocks, the granite sends veins into them, which de- 
 monstrates that it was sufficiently heated to be rendered fluid by the 
 removal of the rock pressure upon it ; and the existence of the 
 fissures now filled with granite is an evidence of the influence which 
 such alteration of pressure exercised. Professor Judd's section of 
 Ben Nevis may be regarded as demonstrating that lava and ashes 
 were poured out from old volcanoes, of which the granite bosses of 
 the Grampians are the cores. The Ben Nevis rocks consist at the 
 
 1 Rutley : "Devitrified Rocks from Bedd-Gelert and Snowdon," Q. J. G. S.. 
 xxx vii. p. 403. 
 
293 ANDESITE LAVAS OF THE CHEVIOTS. 
 
 top of felstone lavas, alternating with volcanic agglomerates ; below 
 these is a bed of felsite resting on another felsite, which graduates 
 downward into a fine-grained granite. Under this is the coarse 
 porphyritic granite, which burst through the Cambrian rocks. On the 
 one hand, it graduates into syenitic granite, and on the other into 
 granite, consisting of orthoclase, oligoclase, quartz, and hornblende. 
 
 Thus the granite graduates externally into sucn rocks as granite is 
 known to become when it cools rapidly, and it is overlain bv such lava 
 and ashes as would have been poured out from a volcano having a 
 granitic base. Therefore, when it is found that great lava-sheets in 
 the Scottish Lowlands are associated with the Lower, Middle, and 
 Upper Old Red Sandstone in the district of Lome, that lava streams 
 also occur upon the northern flank of the Grampians, and that the hill 
 ranges of southern Scotland are composed of volcanic rocks, it seems 
 highly probable that the Grampian range was elevated as a chain of 
 volcanic islands, 1 not unlike some of those which now occur in the 
 Indian Ocean or the Pacific. 
 
 The activity of these volcanic centres during the Old Red Sand- 
 stone and succeeding Carboniferous periods, must have been largely 
 determined by the action of the compressing forces which elevated 
 those areas out of the ocean ; and it is worth remarking that the 
 Old Red Sandstone is a shallow-water deposit, even if it was not of 
 lacustrine origin. 
 
 Cheviot Andesita Lavas. The Cheviot district consists chiefly 
 of quartzless porphyritic rock, such as is usually termed porphyrite. 
 Under the microscope it is a compact felsitic ground mass, usually 
 purple or red, with crystals of triclinic felspar. Volcanic ash and 
 breccia are found on both the English and Scotch sides of the 
 eruption. And a black resinous rock, which has been called pitch- 
 stone-porphyrite, occurs near Cherrytrees in Roxburghshire, in a cliff 
 near Yetholm, in the Coquet between Windy Haugh and Blindburn, 
 and in the Usway between Battleshields Haugh and Fairhaugh. 
 These rocks belong to the same group as ordinary porphyrites, and 
 are regarded by Mr. Teall 2 as ancient altered andesites, belonging to 
 the Lower Old Red Sandstone period, during which the Pentland, 
 Ochil, and Sidlaw Hills were formed. Porphyrites of later date 
 occur about Kelso, which belong to the Tuedian beds at the base of 
 the Carboniferous formation. These rocks can scarcely be distin- 
 guished from the andesites of Santorin, and Tokaj in Hungary. The 
 felspars in the ground mass form a felted aggregate of microliths. 
 The augite in most if not in all the andesites is subordinate to a 
 rhombic pyroxene which is regarded as hypersthene. An andesite 
 lava from near the summit of Ararat, contains a twinned monoclinic 
 and a dichroic pyroxene ; and this condition is found in the andesites 
 of Southern Servia, and the districts near Schemnitz, Kremnitz, and 
 Eperies. The Cheviot andesite gives on analysis 
 
 1 Judd, Q. J. G. S., vol. xxx. pp. 295, 289, &c. 
 
 2 J. J. H. Teall : Geol. Mag. March, April, May, 1883. 
 
 
OLD RED SANDSTONE VOLCANOES. 299 
 
 Silica . . . -63-0 
 Alumina . . .14*9 
 Iron Oxide . . 47 
 Lime .... 4*8 
 
 Magnesia . . . 2'8 
 
 Soda .... 4'O 
 
 Potash . . . . l'9 
 
 Loss .... 4 - o 
 
 Thickness of Volcanic Rocks of the Lower Old Red Sandstone. 
 
 In the Lower Old Red Sandstone country, between the base of the 
 highland mountains and the southern uplands, there were two lines 
 of contemporary volcanic vents from which vast lava streams and 
 accumulations of ashes were emitted. 
 
 Notwithstanding denudation, more than 5000 feet of volcanic 
 rocks are measured at the northern end of the Pentland Hills, with- 
 out reaching the top ; and in the Ochill Hills more than 6000 feet 
 of similar rocks are seen without reaching the bottom. The Sidlaw 
 and Ochill Hills are composed of felstones and porphyrites, inter- 
 bedded tuffs and agglomerate, which extend for 60 or 70 miles, and 
 rise 2000 feet above the sea. 1 
 
 Volcanoes of the Middle Old Red Sandstone. The Middle Old 
 Red Sandstone of Ayrshire abounds in interbedded rocks of volcanic 
 origin. They are often very slaggy and amygdaloidal, and are seen 
 in successive layers on the coast at Turnberry Point. The porphyrites 
 are generally separated from each other by thin beds of sandstone. 
 The Kirkoswald, Maybole, and Brown Carrick districts show the vol- 
 canic rocks of the Middle Old Red Sandstone, forming conspicuous 
 hills of pink porphyrites and dark compact dolerites ; and towards 
 the N.W. they sometimes pass into a kind of coarse sandy tuff. Vol- 
 canic rocks form the range of cliffs at Culzean, where the porphyrites 
 are dark green or purple. The porphyrite series, ends on the shore 
 near the heads of Ayr. In the Straiton and Dalmellington district 
 the Middle Old Red Sandstone reappears, with similar massive por- 
 phyrites to those of the Brown Carrick Hills. 
 
 In this district the Lower Old Red Sandstone also contains dark- 
 purple, fine-grained, amygdaloidal porphyritic rocks. 
 
 Further east, the middle of the Lower Old Red Sandstone is almost 
 entirely made up of purple and greenish slaggy and amygdaloidal 
 tuffs, with occasional bombs of porphyrite. 
 
 The upper volcanic series is well seen in the district south of Irvine, 
 overlying sandstones with Ceplialaspis Lyelli ; and similar rocks occur 
 on the same horizon in Lanarkshire. 2 
 
 Volcanoes of the Upper Old Red Sandstone. In the Upper Old 
 Red Sandstone of the Pentland Hills great consecutive sheets of fel- 
 stone, with occasional bombs and volcanic ash, are interbedded. The 
 larger beds, such as the felstone of Kips Hill, extend to the S.W. for 
 six or eight miles. The lowest bed, forming Warklaw Hill, is a com- 
 pact blue rock, which in its higher part becomes porphyritic and 
 amygdaloidal, and ultimately vesicular. It is succeeded by pale rose- 
 
 1 " Carboniferous Volcanic Rocks of the Firth of Forth," Trans. Roy. Soc. 
 Edin., voL xxix. p. 441, and Brit. Assoc., Dundee, 1867, sec. p. 49. 
 
 - Mem. Geol. Survey, Scotland ; Explanation of Sheets 13, 14, 15, 22, 23. 
 
3oo DEVONIAN DOLE RITES. 
 
 coloured felstones, and various other felstones follow, interstratified 
 with ash and agglomerate. 1 
 
 The prevailing porphyrite extends into Peeblesshire, Lanark, and 
 Ayr. It forms the greater part of the hills between the Lyne Water 
 and the Clyde, but the true bedded character is less marked there than 
 in the Pentland Hills. The porphyrites thicken to the S. W. towards 
 Symington, where several necks of felstone probably mark volcanic 
 vents. 2 
 
 Devonian Volcanoes of Cornwall. Prior to the upheaval of 
 the great masses of granite of Devon and Cornwall, and during the 
 deposition of the Devonian rocks which those upheavals disturbed, 
 enormous quantities of doleritic lavas were poured out from vol- 
 canoes in the Cornish area. The recognition of the igneous rocks is 
 not always easy, because the old slates are frequently so metamor- 
 phosed as to put on the characters of doleritic rocks ; and trap rocks 
 and ash beds so graduate into slates that the change is almost im- 
 perceptible Their history has been unravelled almost entirely by Mr. 
 John Arthur Phillips, F.R.S. The largest group of these rocks is 
 situate in the neighbourhood of Penzance, where they consist of a 
 series of fissile greenish slates, containing compact crystalline beds 
 without trace of lamination. Several beds are well seen around the 
 shores of Mount's Bay. 
 
 Those slates which have undergone the least metamorphism 
 consist of crystalline felspar or diallage, magnetite, titanic iron, and 
 occasional specks of pyrites, prisms of apatite, and flakes of brown 
 mica. The more altered rocks consist of a colourless transparent 
 base, through which hornblende and viridite are diffused, with 
 pseudomorphs of augite and decomposed felspar. On the eastern 
 side of the Valley of Tollarn the large crystals of felspar are well 
 preserved. The rock at Battery Point, and the Chapel Rock, 
 are hornblendic lavas ; but Mr. Phillips regards the hornblende 
 as being, sometimes at least, a product of metamorphism. These 
 rocks contain from 43 to 47 per cent, of silica, 18 to 21 per 
 cent, of alumina, 9 to n per cent, of ferrous oxide, 6 to 12 
 per cent, of lime, 4 to 7 per cent, of magnesia, and i to 3 or 
 4 per cent, of potash and soda. The composition of the killas, or 
 Devonian clay-slate, is often almost identical, and though the 
 percentage of silica is sometimes higher, it may also be lower. 
 Other altered dolerites occur in the Gurnard's Head; and the 
 headlands at the extreme limit of Porthglaze Cove are so changed 
 that apatite is the only unaltered mineral remaining. In the St. Ives 
 Bay district the rocks are very similar to those in Mount's Bay, and 
 it is just as difficult to distinguish whether the dark mineral in the 
 rock was augite or diallage. There is usually present some granular 
 quartz and a little viridite. Near Camborne an igneous band occurs, 
 stretching further east to South Koskear, which is known as blue 
 elvan, and consists of garnets and axinite. Two miles west of 
 
 1 Geikie : Mem. Gcol. Surv. Scot. Edin., 1861. 
 
 2 Mem. Geol. Surv. Scot. ; Explanation of Sheet 24. Arch. Geikie. 
 
DOLERITES OF CORNWALL. 301 
 
 Camborne a bluish-green dolerite, with a slaty structure, stands out 
 some 30 or 40 feet above the surface. It is hornblendic, and closely 
 resembles the hornblendic slates of Penzance ; and many other rocks 
 which have been classed as greenstones are also hornblendic, like the 
 beds at Newlyn East. At St. Stephens there is another blue elvan, 
 which has many of its felspar crj^stals replaced by schorl and cassi- 
 terite, but like all the other rocks has the chemical composition of 
 dolerite. I31ue elvans are only met with in the neighbourhood of 
 granite. 1 
 
 A mile west of St. Austell there is a dolerite which extends in a 
 south-easterly direction to the sea near Duporth. It is about 90 feet 
 thick, and only preserves crystalline structure in the central part. 
 Occasionally orthoclase is present with the plagioclase ; the augite is 
 frequently replaced by hornblendic pseudomorphs. In the cliff 
 section at Duporth, the rock has the aspect of an aggregation of 
 boulders cemented by a mineral like asbestos. This is due to de- 
 composition along lines of fissure. Various other altered dolerites 
 occur at Tregorrick and Hallane. 
 
 Another greenstone region stretches from Trevose Head on the 
 west to beyond Camelford on the east. Near the coast the rocks con- 
 sist of foliated ash beds, vesicular lavas, and augite lavas. When the 
 rock is vesicular it is often termed dunstone. At Pentire Point there 
 is a dark-green lava with abundant microliths of hornblende. The 
 dolerite at St. Tidy is composed of plagioclase, viridite, green horn- 
 blende, with minute garnets, a little apatite, and occasional grains of 
 quartz. 
 
 The ancient lavas of Northern Cornwall have often a greenish- 
 grey colour and an amygdaloidal structure. At times amorphous, they 
 are frequently divided up into blocks by joints, and at other times 
 occur in foliated sheets, foliation evidently resulting from the move- 
 ment of the rock in a fluid state. The amygdaloidal lava of Pentire 
 Point contains 43 per cent, of silica. The cavities are generally filled 
 with crystalline calcite and viridite, or quartz and chlorite. Near 
 South Petherweir the dolerite is almost entirely unaltered. Near 
 Liskeard, dolerites are well developed ; and in South-East Cornwall 
 vesicular lavas again become plentiful. The transformation of the 
 augite into hornblende often begins with an external hornblendic 
 fringe, and at last the crystal is replaced by a mass of hornblende 
 microliths, but sometimes the augite becomes converted into uralite. 
 Mr. Phillips suggests that some of the slaty hornblendic rocks which 
 have the composition of dolerites may originally have been flows of 
 volcanic mud; such rocks are limited to "Western Cornwall. The 
 interstratified condition of the vesicular lavas admits of no question, 
 but the eruptive dolerites also are probably of the same age as the 
 strata in which they occur, because they do not traverse the granite, 
 but are disturbed by it. 2 
 
 1 J. A. Phillips : Q. J. G. S., vol. xxxii. p. 155. 
 
 2 J. A. Phillips : " On the so-called Greenstones of Central and Eastern 
 Cornwall," Q. J. G. S., vol. xxxiv. p. 471. 
 
302 VOLCANO OF BRENT TOR. 
 
 Volcano of Brent Tor. Mr. Allport suggested that Brent Tor, 
 which is four miles west of Tavistock, presents many of the features 
 of a volcano. The rocks consist of purple-bedded ash, with vesicular 
 and amygdaloidal dolerites, described by Mr. Rutley 1 as having 
 a fissile texture, and abounding in small crystals. They sweep 
 round Brent Tor in a semicircle, dipping from it at a low 
 angle ; and it is suggested that the old volcano has been faulted 
 through the cone, so that the ashy beds, by being thrown down, are 
 much better preserved on one side than on the other. The altered 
 dolerite about Tavistock is probably only a prolongation of the lava- 
 flows from Brent Tor. 
 
 On the east of Dartmoor there are several dolerites of similar 
 character. Those seen near Hennock, to the N.E. of Bovey Tracy, 
 have the augite but slightly altered. Near Torquay eruptive dolerites 
 are exposed in Babbicombe Bay and in Austis Cove. These dolerites 
 are converted into a serpentinous rock at Clicker Tor, S.E. of 
 Liskeard, but more frequently show the alteration due to the develop- 
 ment of hornblende. 2 
 
 Volcanic Rocks of the Mendip Hills. At Down Head Com- 
 mon, near Shepton Mallet, is a large exposure of intrusive rock, which 
 was regarded by Mr. Charles Moore as a dyke. It occurs in the Old 
 Red Sandstone. At Down Head the rock appears to be a felstone of 
 dark-grey colour, with minute crystals of hornblende and much 
 magnatite. At Stoke Lane in the Mendips, the lava has the cha- 
 racters of a pitchstone porphyry. It is a brownish-grey rock, with 
 minute vitreous crystals, which are colourless or greenish. Under the 
 microscope it is seen to consist of orthoclase and plagioclase, with a 
 little magnatite and a green mineral termed viridite. Mr. Rutley 
 has also described basalts and dolerites from the Uphill Cutting, 
 Great Western Railway ; Wrigton Warren, near Bristol ; Wood Spring 
 Hill, Charfield Green, and Damory. Some of these lavas are 
 amygdaloidal, and some are greatly decomposed. 3 
 
 Felsite of Bittadon. This is an intrusive mass in the grey 
 unfossiliferous slates which form the upper part of the middle 
 Devonian rocks, and like the slates is affected by cleavage, so that 
 Professor Bonney dates its intrusion before the close of the Carboni- 
 ferous period. It was originally, he says, probably a sanidine 
 trachyte, with hardly enough quartz to be a rhyolite, and may have 
 been not unlike some of the Drachenfels trachyte ; but the original 
 minerals have undergone much alteration. The mass of the rock is 
 greenish-grey, thickly studded with small reddish-white crystals, 
 which are mostly orthoclase. A few small grains of quartz are 
 visible. Under the microscope it is crowded with indistinct micro- 
 liths, brown and green granules, and occasional black specks and 
 
 1 F. Rutley : "On Schistose Volcanic Rocks on the West of Dartmoor," Q. J. 
 G. S., vol. xxxvi. p. 285. 
 
 2 Allport : " Metamorphic Rocks Surrounding the Land's End," Q. J. G. S., 
 vol. xxxii. p. 418. 
 
 3 Q. .T. G. S., vol. xxiii. p. 452 ; and Mem. Geol. Survey: East Somerset 
 and Bristol Coalfields, H. B. Woodward, pp. 14-208. 
 
 
VOLCANIC ROCKS OF THE COAL FIELDS. 303 
 
 yellowish-green streaks. There is a little plagioclase, but the base 
 appears to be chiefly formed of sanidine. Though clearly intrusive, 
 the rock resembles some of the altered volcanic ash in the volcanic 
 series of Borrowdale. 1 
 
 Carboniferous Volcanoes. 
 
 Carboniferous Volcanoes of South Staffordshire. In the South 
 Staffordshire Coalfield there are several masses of basalt, some of 
 which appear to be interstratified with the coal measures. 2 The most 
 important of these outbursts is the columnar basalt, which spreads 
 over an area two miles long by one mile broad in the Rowley Hills, near 
 Dudley. There occur sometimes, just under the basalt, considerable 
 beds of volcanic ash and agglomerate, which seem to show that thu 
 lava was ejected in a true volcanic eruption. The coal, on which 
 it rests, is altered so as to become earthy, and has nearly lost its 
 inflammability. Where this change has taken place, veins sometimes 
 penetrate the coal, which consist of what Professor Jukes termed 
 " white rock," chiefly distinguished from basalt by yielding on ana- 
 lysis a large percentage of carbonic acid and water and a small per- 
 centage of silica, differences which Professor Jukes attributed to the 
 assimilation of a portion of the coal by the basalt vein. There is no 
 trace of the original throat through which the basalt was ejected. 
 Some of the basalts of the Dudley Coalfield appear to be on a different 
 horizon from that of the Rowley Hills, which is 600 feet above the 
 thick coal. 
 
 Among the minor masses is one at Barrow Hill, 10 miles west of 
 Dudley. A smaller columnar mass occurs at Pouk Hill, near Walsall, 
 and is below the thick coal. A fourth mass is seen south of Nether- 
 ton. Professor Jukes also distinguishes, under the name of green- 
 stone, sheet-like masses of basaltic rocks which occur in the lower 
 coal-measures between the Rowley Hills up to Wolverhampton, Bil- 
 ston, and Bentley. On the whole, these eruptions may be referred to 
 the close of the Carboniferous period, because the rocks have been 
 faulted with the coal-measures, so that the more important sheets 
 appear to be contemporaneous. 
 
 Mineral Character of the Carboniferous Dolerites. Dolerites in 
 the South Staffordshire Coalfield are quite typical. In the Rowley 
 Rag the texture is finer than at Pouk Hill. The plagioclase occurs 
 in the usual long prisms, well striated, mixed with pale-brown augite, 
 with green pseudomorphs of olivine, a mineral which at Pouk Hill is 
 usually unaltered. There is always some apatite and a little amor- 
 phous glass. 
 
 At Deep More, N.W. of Walsall, the dolerite is usually much 
 altered, so that the felspar is replaced by chlorite, and chlorite is dis- 
 tributed throughout the rock. Where it meets the coal or shales, it 
 
 1 Geological Magazine, 1878, p. 207. 
 
 2 Jukes' Mem. Geol. Surv. South Staffordshire Coalfield, 1859. 
 
304 CARBONIFEROUS VOLCANIC ROCKS. 
 
 becomes nearly white, and then its constituents are completely decom- 
 posed, though the felspar is usually unaltered. The Titerston Glee 
 Hills are capped by columnar dolerite, in which the olivine is nearly 
 unaltered. A similar rock is found at Knowle Hill near Kinlet, but 
 where in contact with the sandstone on which it rests, both the augite 
 and felspar are converted into a yellow granular substance. At Whit- 
 wick colliery, where unaltered New Red Sandstone rests on dolerite, 
 most of the augite has a purple tinge. The dolerite ridge near Shat- 
 terford has the minerals altered only above the sides of fissures, and 
 in places assumes a porphyritic texture, when it abounds in grains of 
 magnetite. At Swinnertoii Park, eight miles N.E. of Stafford, the rock 
 abounds in augite and olivine, with comparatively little plagioclase. 
 
 Diorites of Warwickshire. In the Warwickshire Coalfield diorites 
 occur in the district about two miles south of Nuneaton. The several 
 binds and masses are limited to the lower part of the coal measures, 
 which are here unproductive, and to the millstone grit. They run 
 in the planes of bedding, but are clearly intrusive, since the rock is 
 altered above and below them. They are well seen in the railway 
 cutting near Chilvers Coton. The eruption was previous to the 
 deposition of the Trias. These diorites vary a good deal in composi- 
 tion. That seen near Marston Jabet has the external appearance of 
 basalt, and contains small crystals of hornblende of a clear-brown 
 colour, lying in a matrix of triclinic felspar, with numerous grains of 
 magnetite, and a few hexagonal needles of apatite. But sometimes 
 the ground mass is a good deal altered, being converted into a sub- 
 stance like serpentine, and the felspar becomes turbid. The diorite 
 of Purley Park, near Atherstone, consists of a mass of plagioclase 
 crystals, with a few crystals of orthoclase. Crystals of brown horn- 
 blende are abundant, and crystals of yellowish augite frequent. There 
 are many pseudomorphs after olivine, usually replaced by calcite and 
 viridite. This is the only augite-diorite recorded in this country, but 
 the quantity of augite varies much in different specimens. 1 
 
 Basalt in Arran. Much of the southern half of Arran consists 
 of sheets and masses of basalt of Carboniferous age, with intrusive 
 sheets in places. The dolerite of the Clauchland Hills extends east- 
 ward to Dunfion, and reaches the coast at Clauchland Point. A 
 similar sheet caps Ross Hill, near Lamlash. Sheets of dolerite form 
 a succession of terraces in the cliffs between Deppin and Benan Head 
 on the S.E. coast, and these three or four sheets are traced inland in 
 the beds of streamlets. The lowest sheet, seen at Kildonan Castle, has 
 the characters of an augite -andesite. On Auchenhew and Levencorroch 
 Hills the dolerite is columnar. 2 
 
 Trees in Volcanic Ash in Arran. On the north-east coast of 
 Arran, near the base of the carboniferous series, eleven distinct beds of 
 volcanic ash occur in a distance of 400 feet, alternating with layers 
 of shale and coal, which are inclined at an angle of 37. Mr. 
 E. A. Wunsch records the occurrence of twelve or fourteen stumps of 
 
 1 Allport: "Diorites of Warwickshire Coalfield," Q. J. G. S., vol. xxxv. p. 637. 
 
 2 Allport : " Carboniferous Dolerites," Q. J. G. S., vol. xxx. 
 
HISTORY OF ARTHUR'S SEAT. 
 
 305 
 
 trees in the volcanic ashes on two or three distinct horizons. The 
 height of the trunks is limited by the thickness of the ash, which is 
 three feet. 1 
 
 Quartz-Felsite of Corriegills. Professor Bonney has drawn atten- 
 tion to the great quartz-felsite dyke in Arran, on the Corriegills shore, 
 south of Erodick, as having the base traversed by parallel joints which 
 divide the rock into plates like tiles. Higher up there is rude vertical 
 prismatic jointing, and higher still a recurrence of the platy structure. 
 Other examples of predominant fissile structure are seen in the pitch- 
 stone veins at Corriegills and Dunfion, while the latter rock at Tormore 
 is sometimes also rudely columnar. 
 
 The great pitchstone at Corriegills shows under the microscope 
 quantities of microlithic dust, with larger belonites, either singly or 
 in groups, aggregated in patterns like algae. Sometimes the pitchstone 
 shows a rough perlitic structure. 
 
 Fig. 59. Drumadoon (Arran). 
 
 On the shore north of Drumadoon the felsite is divided by a dyke 
 of basalt. On the one side of the dyke the felsite is compact and 
 flaggy, but on the other side it is porphyritic. 2 
 
 Arthur's Seat. One of the most interesting remains of an extinct 
 volcano of the latter part of the Primary period is seen in Arthur's Seat. 
 There the strata consist of sandstones and shales of estuarine origin, 
 \vhich belong to the lower part of the calciferous sandstone, and 
 alternate with stratified tuffs and sheets of dolerite and felspathic 
 lava. These rocks dip N.E. about 20, and form part of a great 
 anticlinal fold. Through these carboniferous rocks rise masses of 
 lava, dykes, and piles of volcanic agglomerates. Professor Geikie 
 formerly referred this latter outburst to the Permian period, but Pro- 
 fessor Judd has adopted the view that the interstratified lavas, and 
 
 K l E. A. Wiinsch : Trans. Geol. Soc., Glasgow, 1865-66, p. 97. 
 - Bonney : "Pitchstones and Felsites in Arran," Geol. Mag., 1877. 
 VOL. I. U 
 
3 o6 THE LAVAS ROUND ARTHUR'S SEAT. 
 
 those which formed the core of the old volcano, are portions of one 
 continued eruption ; so that, according to the latter writer, the history 
 of Arthur's Seat, after the formation of the fundamental rocks in the 
 Lower Carboniferious period, included the eruption of ashes, basalts, 
 and augite-andesites, and the injection of the great masses now known 
 as St. Leonard's Crags, Salisbury Crags, and Samson's Eibs. Fossils 
 are found in the stratified tuffs of St. Anthony's chapel, showing that 
 the eruption was at first submarine. Coarse agglomerates lie around 
 the central basaltic throat of Arthur's Seat, just as they would do 
 around the eruptive throat of a volcano, though Professor Judd sug- 
 gests that the position of the eruptive outlet may have been changed 
 from time to time. 1 
 
 Mineral Character of the Lavas near Edinburgh. Arthur's 
 Seat exhibits examples of contemporary interbedded dolerites in the 
 masses known as Long Eow, the Haggis Knowe, and St. Anthony's 
 Chapel. The rock sometimes shows a fluidal structure, the long axes 
 of the felspar prisms being more or less parallel to each other. The 
 intrusive dolerites of Arthur's Seat form the three ridges known as 
 Mount Heriot, Salisbury Crags, and the Dassies. In the Salisbury 
 Crag bed, the felspar and augite crystals are sometimes visible to the 
 eye, and are associated with grains of calcite and analcime, with 
 prehnite and pectolite. It always contains crystals of orthoclase as 
 well as plagioclase. Calcite appears to have replaced part of the 
 glassy matrix. The Dassies consists of a rock which is partly decom- 
 posed, and is green from the chlorite developed in it. 
 
 At Dalmahoy, eight miles S.W. of Edinburgh, the black dolerite 
 is semi-crystalline. At Ratho the rock is coarse-grained, with the 
 augite sometimes altered into a fibrous, brownish -green substance. 
 Similar rocks are seen at Springbeth, near Queensferry on the Forth. 
 At Corstorphine Hill the rock is described by Mr. Allport as a true 
 gabbro ; it is a granular compound of plagioclase, diallage, and a little 
 magnetite, with serpentine occupying the interspaces between the 
 constituents. 2 
 
 Salisbury Crag. In Salisbury Crag is a very fine section of basalt 
 in places 80 feet thick, enclosed between stratified sandstone, con- 
 glomerate, shale, and ironstone nodules, and it is easily seen that both 
 the igneous and sedimentary rocks were altered at their formation. 
 Masses of sandstone and conglomerate, of various forms and magni- 
 tudes, are insulated in a confused manner within the basalt, and por- 
 tions of basalt interposed among the sandstones. No dyke appears ; 
 but small veins of calcareous spar, occasionally metalliferous, cross the 
 line of junction. The accompanying drawings and references will 
 sufficiently explain the most interesting phenomena observed, and 
 give a general view of the face of the cliff as it appeared to Pro- 
 fessor Phillips in 1826. The letters of reference, a, b, c, mark points 
 of which details are given below. On a nearer examination, the 
 point a shows basalt gradually changing to a red colour and finer 
 
 1 Judd : Q. J. G. S., vol. xxxi. p. 131. 
 
 2 Allport : Carboniferous Dolerites, Q J. G. S., vol. xxx. 
 
 
SALISBURY CRAG. 3 c 7 
 
 grain near its upper surface, on which rest beds of sandstone, iron- 
 stone, and shale, as under : 
 
 1. The upper part of a dolerite mass, fine grained, and of reddish colour. Veins 
 
 of calcareous spar, with micaceous iron ore, divide the upper part of this mass, 
 and pass through Nos. 2 and 3 above. 
 
 2, 3. Mass of siliceous sandstone, mixed with softer green portions. 
 
 4. The same sort of hardened sandstone, with less of the softer parts (here and 
 
 there a purple tinge). 
 
 5. Argillaceous, compact, hard shale of a purplish or green colour, and subcon- 
 
 choidal fracture. 
 
 6. Red argillaceous ironstone in green shale. 
 
 7. Sandstone beds, reddish and indurated. 
 
 Fig. 60. 
 
 At the point b (fig. 60) a nearly similar series of alternating stone 
 and shale rests on very similar dolerite. A portion of sandstone is 
 engaged in the trap, and other signs of violent intrusion occur. 
 
 Fig. 61. 
 
 Fig. 62. 
 
 At the point c (fig. 60) hard red sandstone flags, without ironstone, 
 rest on reddened greenstone. 
 
 A large quarry at the south end of Salisbury Crag affords an ex- 
 
 Fig. 63. 
 
 cellent section of sandstone beds below the dolerites. Figs. 61 and 62 
 are taken from this quarry. 
 
 In fig. 63 the dolerite, reddening below, rests on jasperised sand- 
 stone, which is much broken and confused in places. Below this is 
 
303 CARBONIFEROUS VOLCANOES OF THE LOTHIANS. 
 
 green shale, covering red and white sandstone with conglomerate. 
 Fig. 64 shows portions of sandstone enclosed in the dolerite, which 
 grows redder towards the contact with the strata below. The aspect 
 of a portion of sandstone fairly enclosed in dolerite is seen in 
 fig. 64. 
 
 The Basin of the Forth. Professor Archibald Geikie distin- 
 guishes six districts in the basin of the Firth of Forth which were 
 characterised by volcanic activity during the Carboniferous period. 
 These are i, Edinburgh; 2, Haddingtonshire ; 3, Linlithgowshire ; 
 4, Stirlingshire ; 5, West Fife ; 6, East Fife. 
 
 Volcanoes of the Edinburgh District. In the Edinburgh dis- 
 trict the eruption began about the close of the Old Red Sandstone 
 period, when volcanic activity was general over the southern half of 
 Scotland. It formed the hills known as Arthur's Seat, Calton Hill, 
 and Craiglockhart Hill. In this district the earlier lavas are dole- 
 rites, and the later lavas are termed by Professor Geikie porphyrites. 
 Arthur's Seat is regarded as a prolongation of the old volcanic ridge 
 of the Pentland Hills, and is hardly two miles from the great vent on 
 the Braid Hills. The maximum thickness of the volcanic rocks at 
 Edinburgh is about 500 feet. 
 
 Volcanoes of the Haddington District. The East Lothian or 
 Haddington district covers an area of about 65 square miles, and 
 includes the Garlton Hills, and most of the coast from Dirleton to 
 D unbar. The volcanic masses here reach a thickness of about 1500 
 feet, and consist largely of tuffs, interstratified with various sediment- 
 ary deposits. The oldest lavas are dark-red augitic rocks ; and the 
 later lavas are dull red, pink, grey, brown, yellow, and white porphy- 
 rites. On the coast, both east and west of North Berwick, many old 
 volcanic throats are seen, sometimes consisting of agglomerate, some- 
 times of basalt. North Berwick Law, the Bass Rock, the headland 
 of St. Baldred's Cradle, and Traprain Law, are cited by Professor 
 Geikie as examples of these necks ; and he observes that volcanic 
 action was prolonged in East Lothian after it had died out in the 
 Edinburgh district. 
 
 Volcanoes of the Linlithgow District. In the West Lothian or 
 Linlithgow district there were many volcanic cones. The Binns Hill 
 of Linlithgow is one of these, consisting of fine green tuff, 350 feet 
 thick ; and S.W. from Binns the volcanic cones are grouped close 
 together, and threw out both ashes and dolerite. The thickness of 
 the volcanic rocks of the south of Linlithgow is about 2000 feet. 
 The intrusive sheets are of a later date, and Professor Geikie suggests 
 that some of them may be overflows from Tertiary dykes. 
 
 Volcanoes of Stirling. The Stirlingshire district embraces the 
 Eastern prolongation of the Campsie Fells, which consists chiefly of 
 porphyrites and tuff's at the base of the carboniferous system. These 
 rocks are 1000 feet thick at Kilsyth, and thin away to the east, 
 so as to disappear about a mile north of Stirling, which is 13 miles 
 from Kilsyth. Subsequently thick sheets of dolerite extended from 
 Kilsyth round the base of Campsie Fells to beyond Stirling. They 
 

 AUGITE-FELSPAR ROCKS OF SCOTLAND. 309 
 
 are intruded into the carboniferous limestone, but are probably of 
 carboniferous age. 
 
 Volcanoes of Fife. The volcanic rocks of Fife are separated by 
 the Dysart and Leven Coalfields. In West Fife one group of old 
 volcanic vents was situate in the district now occupied by the Saline 
 and Cleish Hills, and is represented by some cones of fine green tuff. 
 Another group lies six or eight miles to the east, near Burntislam 
 The tuffs and lavas occupy nearly the whole interval between 
 the Burdie House limestone, and the base of the carboniferous 
 limestone. The interstratification of the rocks is well seen on the 
 coast between Burntisland and Kinghorn, especially between Petty- 
 cur and Seafield Tower. These volcanic outbursts are contemporary 
 with those of West Lothian, and the basalts reach a thickness of 
 upwards of 1500 feet. 
 
 The East Fife district contains an extraordinary number of 
 volcanic vents, several of which are well seen on the coast They 
 extend in a band six miles wide from Leven to St. Andrews, where 
 about fifty volcanic vents are now filled with tuff and agglomerate or 
 masses of basalt ; but these rocks are almost unconnected with inter- 
 bedded volcanic rocks. The outbursts probably belong to the close 
 of the Carboniferous period. 
 
 Professor Geikie remarks that the Campsie Fells and Kilpatrick 
 Hills are only the north-eastern extremity of the great volcanic 
 plateau of Dumbartonshire, Renfrewshire, and Ayrshire, and that the 
 Garlton Hills of Haddington are to be connected with the outbursts 
 along the southern flank of the Silurian uplands from Duns in 
 Berwickshire ; by Kelso, Ruberslaw, Langholm, Birinswark, and the 
 Annan, to the mouth of the Nith at the foot of Criffel. 
 
 The Clyde Basin. In the Clyde Basin Coalfield interbedded dole- 
 rite and ash form the hills of Kilsyth, Campsie, and Kilpatrick on 
 the north, and the Renfrewshire hills on the south, where they occur 
 at the base of the carboniferous limestone. The intrusive sheet of the 
 Necropolis Hill, Glasgow, is a micaceous dolerite, sometimes dark 
 grey, sometimes reddish brown. 1 
 
 Varieties of Doleritic Rocks in Scotland. Dr. Archibald Geikie 
 describes the augite-felspar rocks in three varieties, which he terms 
 diabase, dolerite, and basalt. The diabase is more coarsely crystalline, 
 varies in colour with the tint of the felspar and with the development 
 of decomposition, so that although some are pink, most are green. 
 They are never amygdaloidaL Orthoclase usually occurs to the 
 almost total exclusion of plagioclase, and when plagioclase does occur 
 it is probably never labradorite. Augite is the most conspicuous 
 mineral under the microscope, and is but little altered. The other 
 minerals are titaniferous iron and apatite. No olivine crystals are 
 seen, though serpentine occurs as a decomposition product. Quartz 
 is occasionally present, but usually as a product of decomposition. 
 Brown biotite and small prisms of hornblende are found when the 
 rock is greatly altered. 
 
 1 Allport : " Carbon Dolerites," Q. J. G. S., vol. xxx. 
 
3 io BRITISH PIKRITES. 
 
 These dolerites in Scotland present no differences froni those of Ter- 
 tiary age. The rock has usually a dark-grey speckled character, and 
 seldom contains orthoclase or any original quartz. Sometimes the 
 ground mass is glassy, with dark trichites and microliths. The felspar 
 is probably labradorite, often contains minute particles of glass, and 
 may be studded with apatite. The augite is usually fractured, as in 
 the diabases ; the apparent fracturing being due to included triclinic 
 felspar. Oiivine is rarely recognisable. Dr. Geikie has adduced 
 evidence to show that the felspar crystals were already formed when 
 the rock was in a fluid state. 
 
 The basalts, as usual, are the finer-grained dolerites, and are well 
 seen in the rocks of Craiglockhart Hill and Long Row, near Edin- 
 burgh. The augite crystals are nearly unbroken ; olivine is almost 
 always visible. Octahedrons of magnetite occur, but apatite needles 
 are rare. At Mid-Tartraven in Linlithgowshire the olivine crystals 
 abound in magnetite. 
 
 Pikrite. Another group of rocks characterised as serpentine- 
 olivine is represented by pikrite. In Scotland it is only known from 
 Blackburn, near Bathgate, and the island of Inchcolm. It formed 
 a true lava-sheet at Blackburn, but appears to be intrusive at Inch- 
 colia. It is blackish-green, and contains olivine and augite, with brown 
 biotite, and has the crystals often united together with serpentine. 
 
 Pikrite is a rare rock in Britain. It occurs in situ at Little 
 Knott, east of Bassenthwaite. 1 Boulders of a similar rock have 
 been described by Professor Bonney from near Pen-y-Carnisiog 
 in Anglesey. This, like the pikrites of Fifeshire, described by 
 Professor Archibald Geikie, 2 is characterised by a ground mass 
 formed of small tufts of needle or blade-shaped crystals of horn- 
 blende, which from its optical properties resembles actinolite, and 
 is not regarded as an original constituent of the rock. Secondly, 
 there are both small and large green-coloured and strongly dichroic 
 hornblende crystals. Augite is found in colourless grains and 
 crystals, which are usually embedded in a chloritic mineral. 
 Opacite and rounded grains resembling magnetite occur with many 
 pseudomorphic constituents, which Professor Bonney believes to indi- 
 cate the former presence of olivine. The larger hornblende crystals 
 are of a brown colour, and are believed to have been originally augite. 
 In the Fifeshire rocks olivine still remains as the dominant mineral 
 well preserved. 
 
 Porphyrites. Many of the rocks which were formerly mapped as 
 felstones and porphyrites, Professor Geikie groups as felspar mag- 
 netites, and distinguishes from felstones under the name porphyrites. 
 They agree with the Old Red Sandstone lavas, and are among the 
 thickest and most widespread of the Carboniferous period, extending 
 through Berwick, Roxburgh, and Dumfries, and through the Garlton 
 Hills of Haddington, are seen at Calton Hill and Arthur's Seat, and 
 range through Dumbartonshire, Renfrewshire, and the north of Ayr- 
 
 1 Bonney on Hornblende Pikrite, Q. J. G. S., vol. xxxvii. p. 137. 
 3 Trans. Roy. Soc. Edin., vol. xxix. 
 
 

 PERMIAN VOLCANOES OF SCOTLAND. 311 
 
 shire. The rock has a dull, coarse-grained porphyry base, in which are 
 scattered triclinic felspar and sometimes orthoclase. The base varies 
 from dark chocolate or purple to pale yellow or white, and may be 
 greenish or bluish. It is frequently amygdaloidaL Porphyrites are 
 relatively less heavy than basalts. The microscope shows the ground 
 mass to be a clear colourless, felspar, so that about nine-tenths of a 
 typical porphyrite is felspathic. Next in abundance are octahedra 
 of magnetite. Augite is not always present. 
 
 Felstones. Finally, there are the orthoclase felspar rocks, termed 
 felstones. They consist of a finely granular felsitic ground mass, with 
 grains of quartz and crystals of orthoclase. Felstone is well seen on 
 the shore at Largo in Fife, where it forms a volcanic neck. It is 
 also seen as a yellow quartz felsite among the Campsie Fells stretching 
 into Ayrshire ; this rock was originally a rhyolite. 1 
 
 Permian Volcanoes. 
 
 Permian Volcanoes of Dumfries. In the northern half of the 
 Thornhill basin in Dumfriesshire, the lower part of the Permian 
 series consists of a succession of interstratined beds of porphyrite, 
 which are lava-flows associated with beds of tuff. From Nether 
 Dalbean the rock forms swelling slopes with occasional hillocks, which 
 extend southward. A large mass of it covers the carboniferous rocks 
 at Norton Castle, and it is seen to the south-east of Townfoot ; on 
 the west it occurs in the bed of the Nith. The porphyrite is identical 
 with that of the Permian volcanic rocks of Ayrshire, varying from a 
 line-grained compact rock to a dull earthly scoriaceous rock. It con- 
 sists of plagioclase felspar with much hematite, which often replaces 
 augite or other minerals. Most of the rock is much decomposed ; 
 it is best seen at the bend of the Nitli between Drumlanrigg and 
 Carron Eidge. Another grand section occurs half-way between Gate- 
 law Bridge and Kettleton Bridge on the left bank of Campsie Water. 
 These rocks rest upon the Carboniferous series, and are covered by 
 the usual brick-red Permian sandstones. Carron Water is the centre 
 of the volcanic Permian district of Thornhill, and here some of the 
 blocks of agglomerate weigh half a ton or more. In Garroch Water 
 a thin bed of ashy breccia divides these Permian rocks from the Car- 
 boniferous series. 2 
 
 Permian Volcanoes of Ayrshire. In Ayrshire many necks of 
 volcanic agglomerate mark the site of ancient volcanoes. The coal- 
 workings have shown that they descend vertically, and destroy the 
 coal-seams for some distance around them. Five such necks are seen 
 to the south-east of Symington, three to the east of Irvine, three 
 more near Stevenstone, besides many others. Several of these necks 
 occur along a line of fault. 3 
 
 1 Geikie : "Carboniferous Volcanic Rocks of the Firth of Forth," Trans. Roy. 
 Soc. Edin., vol. xxix. p. 437. See also Mem. Geol. Surv. East Lothian, 1866 ; 
 Geol. Edinburgh, 1861 ; and East Berwickshire, 1863. 
 
 2 Arch. Geikie : Mem. Geol. Surv. Scotland, Explanation Sheet 9, 1877. 
 
 3 Mem. Geol. Surv. Scotland, Explanation Sheet 22, Arch. Geikie. 
 
312 SERPENTINE OF CORNWALL. 
 
 Serpentine. 
 
 Serpentine of the Lizard. The serpentine district of the Lizard 
 is a wild moorland plateau furrowed by gullies and small coves with 
 cliffs, which often rise vertically for from one to two hundred 
 feet. 
 
 In this area, besides serpentine, the rocks shown are gabbro and 
 hornblende schist, with some granite and dolerite. The first junc- 
 tion of the serpentine on the west coast is seen near Polpoer. On 
 the north of a little chine the brecciated rock is all serpentine, on the 
 south it is hornblende schist. The serpentine is intrusive ; near by 
 other masses of hornblende schist are included in the serpentine, which 
 is sometimes dull-red, mottled with a dull-green mineral and occa- 
 sional flakes of bronzite. The different varieties which it exhibits 
 are all the result of decomposition. The rock is cut through by 
 veins of granite in many places, and at the contact the serpentine is 
 altered. At Cadgwith the rock is black ; it contains, according to 
 Mr. Hudleston, 36 per cent, of magnesia, 38 per cent, of silica, 12 
 per cent, of water, 8 per cent, of iron, and 2 per cent, of lime in the 
 matrix freed from crystals. 
 
 On the east coast the exposures are even more complicated than 
 on the west, the hornblende schist being frequently included in the 
 serpentine, while the latter rock is cut by gabbro veins and by 
 granite. The gabbro is of two ages ; the older variety has a dull-red 
 ground mass, with greyish-white felspar and small crystals of diallage, 
 and may easily be mistaken for serpentine. The newer gabbro is 
 coarser in texture and more decomposed. Professor Bonney believes 
 the intrusion of the serpentine took place after the metamorphism of 
 the hornblende schist, and that the metamorphism of the serpentine 
 was complete before the intrusion of the gabbros and hornblendic 
 dolerites which are found on the east coast. 1 
 
 The Lizard serpentine contains olivine, enstatite, olive green with 
 a metallic lustre, diallage, hornblende, augite, chrysolite, picotite, be- 
 sides magnetite, occasionally a little felspar, and products of decom- 
 position, such as steatite. It closely resembles Iherzolite, and is 
 regarded as an altered peridotite intrusive in hornblende schist. 2 
 
 Among the localities described are Coverack Cove, Mullion Cove, 
 Gue Graze, Lower Pradanack Quarry, Hill Quarry, Helston Road, 
 Goomhilly Downs, Kynance Cove, George Cove, Cam Sparnack, 
 near Cadgwith, and Balk. 
 
 Serpentine of Anglesea. Serpentine occurs at several localities 
 in Anglesea and in Holyhead. It is found in a bluish or greenish 
 schist, which is greatly crumpled near Ty Newydd, and is itself cut 
 through by gabbro, which sometimes has a serpentinous aspect. 
 Sometimes the serpentine is reddish, brecciated, and veined with 
 calcite. A larger mass of serpentine occurs near Rhoseolyn, at the 
 
 1 Bonney : Q. J. G. S., vol. xxxiii. p. 884. 
 
 2 Q. J. G. S., voL xxxix. p. i. 
 
SERPENTINES OF SCOTLAND. 313 
 
 south end of Holyhead Island. Here dark-green serpentine is 
 associated with gabbro. The serpentine is shattered and slickensided, 
 and is in contact with ophi-calcite, which is a breccia of dark serpen- 
 tine cemented by calcite infiltrated from the overlying Carboniferous 
 limestone. Under the microscope the serpentines give evidence of 
 being altered olivine rocks like Dunite, with various other associated 
 minerals now decomposed and replaced by ill-preserved pseudomorphs, 
 so that it is classed by Professor Eonney as an altered peridotite. 
 The rock at Porthdinlleyn in Caernarvonshire, formerly regarded as 
 serpentine, is classed by Professor Bonney as a diabase tuff on the 
 evidence of its microscopic structure. 1 
 
 Serpentine of Ayrshire. On the coast of Ayrshire serpentines 
 are exhibited at Lendalfoot and at other points between Balcreuchan 
 Port and Pinbane Hill. They are also seen at Balhamie Hill, near 
 Colmonell. In the latter exposure the rock has a rhomboidal joint- 
 ing, the joints coated with greenish or whitish steatite. It is full of 
 crystals of glittering bronzite or some similar mineral. The olivine is 
 completely converted into serpentine, and the rock is regarded by 
 Professor Bonney as an altered olivine enstatlte which contains 38 
 per cent, of silica, 35 per cent, of magnesia, and 4 per cent, of 
 alumina. The associated dolerite rocks are particularly interesting, 
 some of them having the aspect of being metamorphosed sediments, 
 while others have an igneous aspect. Some writers have regarded 
 them as diorites, but they contain plagioclase and augite. There are 
 also gabbros of two ages, one similar to the gabbro of the Lizard, very 
 rich in diallage, so as to be almost a mass of diallage crystals. The 
 serpentine is older than the diallage rock and gabbro, and is intruded 
 into rocks which were regarded by Murchison as of Bala age. 2 
 
 Serpentine dykes of Carboniferous age occur in Forfarshire. 
 
 These serpentines, like those of Portsoy in Banffshire, 3 are modi- 
 fied volcanic rocks, though really to be classed as metamorphic rocks. 
 Other serpentines appear to be derived from sedimentary rocks, meta- 
 morphosed in the usual way. (See Heddle, T.E.S., Edin., vol. xxviii.) 
 
 Secondary Volcanic Hocks. 
 
 Sir Henry De la Beche, nearly fifty years ago, gave an excellent 
 account of the felsitic rocks associated with the lower part of the New 
 Red Sandstone in Devonshire. He states that the section at Wash- 
 field near Tiverton gave the best evidence of a volcanic eruption, the 
 lava being covered by detritus containing angular volcanic fragments 
 which sometimes weigh a ton. These fragments contained quartz 
 and large crystals of glassy felspar ; 4 and nearly thirty years later Mr. 
 "W. Yicary, F.G.S., described all the volcanic rocks near Exeter in 
 more detail. 5 There are no more interesting volcanic rocks in Britain. 
 
 1 Bonney : "Serpentine of Anglesey," Q. J. G. S., vol. xxxvii. p. 40. 
 
 2 Bonney : "On Serpentine of Ayrshire Coast," Q. J. G. S., November 1878. 
 
 3 See also post, p. 388. 
 
 4 Proc. Geol. Soc., vol. ii., 1835, p. 196. 
 
 8 " Report and Transactions of the Devonshire Association for the Advance 
 of Science, Literature, and Art," part iv., 1865. 
 
3T4 TRIASSIC VOLCANIC ROCKS OF DEVON. 
 
 Killerton Park has the aspect of a great volcanic centre formed of 
 compact dark micaceous lava, vesicular in the higher portions, becom- 
 ing red, scoriaceous, and ashy in the neighbouring quarries at Bud- 
 lake and Silverton. The compact rock near Tiverton at Holmead, 
 almost like a mica schist for its abundance of mica, similarly corre- 
 sponds to the scoriaceous and ashy beds which extend from Washfield 
 to Loxbeare. The compact lava which forms Knowle Hill may repre- 
 sent the centre from which the porous lavas and ashes of Pocombe and 
 Northernhay flowed. In the Crediton district, Postbury furnishes the 
 compact crystalline rock which thins as it extends to Yeotown, while 
 highly vesicular rocks are met with at and about Spence Combe, often 
 jointed with "mountain cork." As in all similar rocks, the vesicles, 
 now amygdaloidal, are greatly elongated by flowing movement of the 
 rock, so as to often give an aspect of stratification. Other large vol- 
 canic masses occur near Haldan and at North Tawton. Their colour 
 is dark red or purple black. They vary in mineral composition ; are 
 remarkable for a clear felspar, which occurs in porphyritic masses 
 in the rock at Knowle and many other places ; and this is associated 
 with plagioclase, with a frequent abundance of brown mica, some 
 orthoclase, potash mica, some brown hornblende, and much hematite. 
 Hence some of these rocks seem to be related to the minettes. Some 
 Exeter rocks are referred to by Professor Bonney as basalts. 
 
 Tertiary Volcanic, Roclcs. 
 
 The British volcanic rocks of Tertiary age cover two distinct 
 areas ; first, the large district in the N.E. of Ireland extending 
 round Lough Neagh, which comprises nearly the whole of Antrim 
 and the adjacent part of Londonderry; and, secondly, the chain 
 of the Inner Hebrides, including Mull, Rum, Eigg, Canna, Muck, 
 and three-fourths of Skye. This line of old volcanic activity 
 extends N. to the Shiant Islands, and appears again in the Earoe 
 Isles, before terminating in the older volcanic districts of Ice- 
 land. Hence, the tertiary volcanoes of Britain are the southern 
 end of a band nearly 800 miles long, which is still active at its 
 northern extremity. The rocks in Ireland and in Scotland include 
 acidic series as well as basic series. The basic series of Scotland 
 is demonstrably the younger ; but in Ireland the rocks which have 
 been identified as trachyte and rhyolite appear to be older than the 
 basalts. 
 
 Acidic Rocks of the North of Ireland. The acidic rocks of 
 the North of Ireland have hardly received the attention which their 
 importance demands. They occur in many places, especially near 
 Tardree, in the neighbourhood of Templepatrick, near Broughshane ; 
 and at Ballyknock, south-west of Hillsborough. Some of these 
 rhyolites are regularly bedded, and intrusive in the dolerites, and 
 therefore younger than the dolerites. The analysis by Mr. Hard- 
 man does not differ from the composition of typical rhyolites of 
 Germany. 
 
 
IRISH TERTIARY RHYOLITE. 
 
 315 
 
 
 Rhyolite, Tardree, 
 near Antrim. 
 
 Silica 
 
 76.96 
 
 Alumina . 
 
 5.10 
 
 Peroxide of iron 
 
 2-34 
 
 Lime 
 
 7-06 
 
 Magnesia 
 
 0.30 
 
 Soda 
 
 1.82 
 
 Potash . 
 
 4.26 
 
 Water . 
 
 2.10 
 
 Basalts of North of Ireland. The coast between Belfast Lough 
 and Lough Foyle is one boundary of a large tract reaching westward 
 to Lough Neagh, and including the river Ban, which is almost wholly 
 occupied on the surface by basaltic rocks, rising at intervals to 
 eminences of 1320, 1820, 1864 feet above the sea. Under this im- 
 mense overlying mass of basalt are found several members of the 
 secondary series of strata not known elsewhere in Ireland, i. Chalk, 
 agreeing with the lower beds of the English series. 2. Mullattoe, 
 an Irish name for the Hibernian Greensand of geologists. 3. Lias 
 limestone (without any other rock of the oolitic system). 4. Beds 
 of red marl, gypsum, and salt, resting on variegated sandstone. 5. At 
 the north-eastern and south-eastern extremity, coal-measures, con- 
 sisting of red sandstones and shales with inferior coal, appear below 
 all the other strata. The mulattoe and lias are often wanting in the 
 section. The superincumbent basalt is estimated to have an average 
 thickness of 545 feet (in Benyavenagh it is 900 feet, in Knochlead 
 980 feet), and its superficial extent 800 square miles. 1 
 
 The immense mass of doleritic rocks in this district exhibits, 
 besides basalt, which' is the most abundant material, several other 
 varieties of rock. Near the Causeway, the cliffs consist of alter- 
 nating basalt and red ochre, in the following order downwards : 
 
 1. Basalt rudely columnar, 60 feet. 
 
 2. Red ochre or bole, 9 feet. 
 
 3. Basalt irregularly prismatic, 60 feet. 
 
 4. Columnar basalt, 7 feet. 
 
 5. Intermediate between bole and basalt, 8 feet. 
 
 6. Coarsely columnar basalt, 10 feet. 
 
 7. Columnar basalt, the upper range of pillars at Bengore Head, 54 feet. 
 
 8. Irregularly prismatic basalt, 54 feet. In this bed the wacke and wood coal 
 
 of Port Noffer are situated. 
 
 9. Columnar basalt, forming the Causeway by its intersection with the plane 
 
 of the sea, 44 feet. 
 
 10. Bole or red ochre, 22 feet. 
 
 11, 12, 13. Tabular basalt, divided by thin seams of bole, 80 feet. 
 14, 15, 1 6. Tabular basalt, occasionally containing zeolites, 80 feet. 
 
 1 See also Lee's Note-Book of a Geologist. 
 
316 ROCKS METAMORPHOSED BY BASALT. 
 
 Contact Metamorphism by Basalt. The stratified rocks in 
 contact with the trap have undergone remarkable changes in several 
 localities. 
 
 At Portrush, the lava, a rudely prismatic dolerite, overlies and 
 perhaps alternates with a flinty slate, which contains numerous 
 impressions of Ammonites, belonging to the Lias shales. This trans- 
 formation of Lias shale reminds us of the more extensive phenomena 
 of the same kind in Savoy. Most of the alterations of stratified rocks 
 on this coast are produced by basaltic dykes, which divide both the 
 overlying masses of trap and the subjacent strata. At the foot of the 
 hill called Lurgethan, basaltic dykes traverse the red sandstone con- 
 glomerate, which is altered near the contact so as to resemble compact 
 hornstone. 
 
 The coal-measures, underlying the basalt of Fairhead, are crossed 
 by dykes which have changed the ordinary shale into flinty slate, 
 
 Fig. 65. The Giant's Causeway. 
 
 hardened and pyritised the sandstone for 1 5 yards, and converted the 
 coal to cinder. The chalk is traversed by many dykes, and is con- 
 verted into a real marble for 10 feet or more from the contact with 
 the basalt. 
 
 The effects in approaching the contact are first a yellowish tinge of 
 colour, then a bluish-grey colour and compact texture, then a fine- 
 grained arenaceous aspect, next a saccharoid granulation, and finally, 
 close to the dyke, the chalk is altered to a dark -brown crystalline 
 limestone, with flaky crystals as large as those in limestone. The 
 flints in the altered chalk assume a grey-yellowish colour ; the altered 
 chalk is highly phosphorescent when headed. Examples are seen 
 near Belfast, at Glenarm, in Eathlin, and other places. Near the top 
 of the chalk which crowns the cliffs of Murloch Bay is an interposed 
 bed of wacke 5 or 6 feet thick. 1 
 
 Volcanic Mud Streams of the Hebrides. Before the volcanic 
 
 1 Conybeare, Trans. Geol. Soc., vol. iii. 
 

 VOLCANOES OF THE INNER HEBRIDES. 317 
 
 eruptions occurred in this land of the Inner Hebrides the whole 
 country appears to have been elevated out of the sea. The present 
 Duke of Argyll was the first to demonstrate the condition of the old 
 land by describing the section at Ardtun Head on the northern shore 
 of the Ross of Mull. There, near the base of the cliff, preserved in a 
 mud stream, formed by ruin washing down the fine volcanic ashes, 
 and afterwards sealed down by thick beds of basalt, are the leaves of 
 species of Platanus, and the large conifer Sequoia langsdorfii, which 
 occurs in the London Clay, and in the older tertiary deposits of Green- 
 land, Spitzbergen, Iceland, and Central Europe. 
 
 In Ireland a more abundant flora has been described by Mr. W. 
 H. Baily, from volcanic-mud beds beneath the basalt at Ballipallidy. 
 The volcanic activity of this region probably extended over a pro- 
 longed period of time, for several leaf-beds in the island of Mull are 
 separated from each other by ashes, indicating that the eruptions were 
 intermittent, as they are in Etna and Vesuvius. 
 
 The old volcanoes of the Inner Hebrides are now known from 
 nothing but skeletons of the cones. The loose ashes which formed the 
 upper parts of mountains have been worn away, or blown away, some- 
 times to the thickness of 5000 feet, and nothing remains where the 
 volcanoes stood but cores of granite or gabbro which formed the 
 matter erupted into the base or the throat of a volcano from which 
 the surrounding basalts or felstone lavas were ejected. 
 
 The Mull Volcano. One of the most interesting of the skeleton 
 volcanoes forms the S.E. of the island of Mull. Previous to the out- 
 burst the Mull country was formed of Upper Cambrian Kocks like 
 those seen on the opposite coast of Morvern. Through these rocks, 
 granite of Primary age rises to form the south-west of the island. 
 And resting on the Cambrian Mica-schist, secondary rocks are seen 
 beneath the tertiary lavas at many places round the coast. 
 
 Professor Judd 1 fixes the building of the old volcano as dating from 
 the Lower Tertiary period. The central mass consists of granite, but 
 all round the granite is the less crystalline rock called felsite, whicli 
 granite becomes when it cools rapidly. The altered rhyolites or felsites 
 are covered by beds of felsitic ashes and agglomerates ejected into the 
 air and subsequently consolidated by the action of rain. After a 
 period of activity of unknown duration the volcanic fires of Ben More 
 died away, and the volcano was probably denuded, so that most of 
 the ashes were washed away and the granites and felsites were 
 exposed on its surface. After this interval the volcanic fires broke 
 out again, but brought to the surface another kind of rock which 
 consolidated to form a central core of gabbro 2 or hypersthenite. 
 This gabbro burst through the granite and has sent sheets of fluid 
 rock into the granite in every direction, forming dykes. Ashes 
 were once more thrown out, and some of those basic agglomerates still 
 
 1 Q. J. G. S., vol. xxx. 
 
 2 The name gabbro is a convenient term which may be used in a generic 
 sense for this rock. There is no diallage in it, but it does not contain true 
 hypersthene. 
 
3i8 TERTIARY VOLCANO OF MULL. 
 
 remain resting upon the older acidic agglomerates, and penetrated by 
 dykes of gabbro. When gabbro is poured out as lava it becomes 
 basalt and dolerite ; and floods of basalt extend round Ben More in 
 unbroken sheets. They cover the whole of the North of Mull, and 
 lap along the west coast of Morvern. Indications of their extent are 
 seen in the now isolated patches which form Ulva, Staffa, the Treshnish 
 Islands, and the rock covering the Ross of Mull, but the lava extended 
 much beyond its present limits. Ben Yattan shows that it flowed far 
 into Morvern, while southward it may have extended into the Firth 
 of Lome. The circumference of the base of this volcano could not 
 have been less than 40 miles, and judging from the proportions of 
 Etna, its height may have been 14,000 feet. Like Etna the old Mull 
 volcano appears to have been covered with small cones towards the 
 close of the second period of its activity. One of these minor cones, 
 or puys, called Ben Sarsta, rises to a height of between 800 or 900 
 feet. It is situate behind Tobermory, and consists of a central core of 
 gabbro, surrounded by basalt or dolerite which is altered by contact 
 with the central intrusive mass. This gabbro may well have been 
 the site of the old crater, and in its centre is a small lake. The 
 country round is so covered with bog that neither the lavas nor ashes 
 poured out from this little cone can be distinguished, even if they 
 escaped the denudation under which the great volcano suffered. The 
 central mountain mass of Mull as it is now left has a diameter of 12 
 miles, and consists of peaks which rise to a height of 2000 to 3000 
 feet, the highest being Ben More. 
 
 Staffa. The cave of Staffa is 
 excavated in vertical prisms of 
 basalt, between rows of which 
 the eye rests on the distant 
 view of lona. Over the cave 
 the basalt is in smaller prisms, 
 lying obliquely. 
 
 Ardnamurchan. Another 
 great volcano existed in the penin- 
 sula of Ardnamurchan. The evi- 
 dences of its activity are not so 
 clear as those seen in Mull. The 
 
 Fig. 66.-Fingars Cave (Staffa). whole of ^ g w Qf ^ penin _ 
 
 sula consists of varieties of gabbro which form wild and barren moun- 
 tains. On the east the gabbro graduates into dolerite. These rocks 
 are similar to those of Mull, and similarly rest upon and break through 
 a series of acidic lavas. The peak of Meal-nan-Con and the neigh- 
 bouring heights to the S.W. are formed by intrusive felsite, which, 
 like that of Mull, passes into granite. It is penetrated on the east 
 and south by sheets and veins of newer amygdaloidal felstone, which 
 are interstratified with beds of ashes and scoriae, full of fragments of 
 the primary and secondary rocks, through which the volcano of 
 Ardnamurchan pierced. 
 
 Here, as in Mull, subordinate volcanic cones of later date appear 
 
THE RUM VOLCANO AND ITS LAVA STREAMS. 319 
 
 to have been formed on the flanks of the great volcano. The highest 
 mountain in Ardnamurchan is one of these. It rises to a height of 
 1759 feet, and is known as Ben Shiant. It consists of a succession 
 of grassy slopes formed of the softer and less compact rocks, which 
 rest upon columnar lava which terminates its slope in a spur. Ben 
 Shiant rises at the junction of the felspathic and basaltic lavas ; its 
 rocks contain glassy felspar and sometimes include porphyritic 
 pitchstones and rocks like compact felstones. They are referred to 
 the augite andesites by Professor Judd. 
 
 The Island of Rum is a third volcano. Around it lie the smaller 
 islands of Canna, Eigg, and Muck, which are portions of lava sheets 
 more or less interstratified with sedimentary deposits. They accumu- 
 lated gradually, and helped to mark the distance from which the lavas 
 from Rum extended. The foundation of the island of Rum is a mass 
 of Cambrian sandstone with overlying highly metamorphosed rocks, 
 and far away from the lavas the old strata present their normal 
 characters and are free from intrusive dykes. A number of peaks 
 which consist of gabbro rise to a height of about 2500 feet, piercing 
 through the stratified formations. To the east of the gabbro heights 
 is the older mountain of Oreval, which does not rise so high and is a 
 core of granite. Dykes and veins of gabbro almost innumerable 
 penetrate the felsites and granites of Rum ; and, therefore, show that 
 the basic lavas were thrown out subsequently. The ashes ejected 
 from the Rum volcano have nearly all been swept away by processes 
 of denudation, though some patches of felsitic ash still remain 
 preserved by coverings of felstone. 
 
 The Island of Eigg has been described in detail by Professor 
 Archibald Geikie, F.R.S. It is about five miles long, from one and a 
 half to three and a half miles broad, and attains a height of 1300 
 feet. It is an isolated part of a great basaltic plateau, and is so 
 tilted that while it is a thousand feet high at the north, it slopes 
 gently to the south. The volcanic rocks rest unconformably upon 
 estuarine and fresh- water shales and limestones of Jurassic age, which 
 have some marine beds at the top similar to the rocks seen in 
 Raasay. The volcanic rocks covering those beds are a succession of 
 dolerites and tuffs, which vary in thickness from a few feet to fifty 
 or sixty feet, and have an aggregate thickness of eleven hundred feet. 
 They vary in character, being sometimes amorphous and amygdaloidal, 
 and sometimes characterised by columnar structure, which may be 
 radiating. 
 
 Where the rock is amygdaloidal it is largely infiltrated with 
 quartz and chalcedony, calc spar and stilbite. There is a comparative 
 absence of tuffs and agglomerate in the Tertiary period, as compared 
 with their abundance in the Primary era ; but in Eigg many 
 examples occur of breccias interstratified with the dolerite, though 
 they are never thick. As is usual in volcanic districts, intrusive 
 bosses, sheets, dykes, and veins intersect both the underlying oolitic 
 strata and the volcanic rocks. One of these Professor Geikie 
 describes as a quartz felsite, forming a cliff two hundred feet high at 
 
320 VOLCANIC CORES AND LAVAS OF SKYE. 
 
 the north end of the island; it is sometimes columnar, and of a 
 pale-grey colour resembling the fine-grained quartz porphyries of Skye 
 and Raasay. One or two other masses of felstones occur in the 
 island. The Scur of Eigg is an elevated ridge two and a quarter miles 
 long, and rising three hundred to four hundred feet above the basalt 
 plateau. It consists of pitchstone and felstone, the former dark- 
 coloured and columnar. The porphyry is grey and interbedded in 
 the pitchstone. Under the microscope the base of this rock is glassy 
 or granular with crystals of orthoclase which are sometimes a quarter 
 of an inch long. The eruption of the pitchstone is considered to 
 have been long subsequent to the eruption of the basalt, and the Scur 
 of Eigg was formerly a river valley in which gravels and drift-wood 
 were buried under the products of successive volcanic eruptions. 
 Subsequent denudation has reduced Eigg to much the same condition 
 as the fragments of the older basaltic plateau of the Auvergne. 1 
 
 Skye. A volcano which was probable larger than any of the 
 others was situate in the south of Skye. The intrusive igneous rocks 
 burst through strata of Cambrian, Liassic, and Oolitic age, and often 
 transmute the lias into white granular and compact limestone at the 
 junction. The great central granite mass of the Red Mountains, 
 remarkable for their smooth contours, reaches a height of 2670 feet in 
 the pyramidal peak of Ben Glamaig. The granite gradually changes 
 at the circumference into felsites, and is pierced by contemporary 
 veins of the same rock. As in the associated volcanoes, this granite 
 is the core of the older or acidic cone. The later eruption formed the 
 wild hills of gabbro, jagged in outline, which are known as the 
 Cuchullin Hills and Ben Blabheinn, both more than 3000 feet high. 
 On passing outward from the central cores of gabbro, with its large 
 metallic bronzy crystals like hypersthene embedded in the grey labra- 
 dorite, the rock passes insensibly, first into dolorite, and then into the 
 liner-grained basalt. Patches remain of the old agglomerates and 
 deposits of volcanic ash, mostly mingled with felstone lavas. They 
 are well seen in the hill on the south of Ben Dearg ; and in the 
 island of Scalpa they lie on Primary and Secondary strata. The 
 basalt was remarkably fluid, and extended from the Cuchullin Hills 
 in every direction. Much of it may be now covered by the sea, but 
 large fragments stretch to the north-west and north, and form the penin- 
 sulas of Skye known as Trotternish, Vaternish, Duirinish, and Mingi- 
 nish. In the island of Kaasay an outlier of basalt forms the hill of 
 Dun Can. This volcano of Skye may perhaps have had its principal 
 crater in Loch Coruisk, which is a deep and desolated amphitheatre in 
 the gabbro. 
 
 Professor Judd has suggested that the remote island of St. Kilda 
 is an extinct volcano now represented by cores. M'Culloch describes 
 the eastern part as of granite, and the west as of gabbro, with the 
 hills surrounded by basalt. 2 
 
 1 Arch. Geikie, Q. J. G. S., vol. xxvii. p. 285. 
 8 Judd, Q. J. G. S., vol. xxx. p. 255. 
 
( 321 ) 
 
 
 CHAPTEK XIX. 
 
 CONCOMITANTS AND RESULTS OF VOLCANIC ENERGY. 
 
 BESIDES the igneous phenomena already described, there are some 
 other subjects, more or less associated with the geological history of 
 the earth and the reaction of the interior on the surface, which may 
 be termed concomitants of volcanic action. Among such are the 
 upheaval and depression of land ; breaks in the succession of strata, 
 which result from changes in relative level of land and water ; the 
 undulatory movements of earthquakes ; faults ; and the intrusion of 
 igneous matter into rocks in the form of dykes. It may be that 
 such phenomena are not always directly connected with volcanic erup- 
 tions, but no broad distinction can be drawn between those internal 
 movements in the earth which crumple strata, upheave mountains, 
 raise and depress continents, on the one hand, and the formation of 
 fractures through the rocks, by which vent is given on the surface to 
 the heated materials beneath, since both are consequences of contrac- 
 tions of the earth's crust. Hence we supplement the geological part 
 of volcanic history with the following considerations. 
 
 Earthquakes. 
 
 The study of earthquakes is the science of Seismology, which en- 
 deavours to trace the history and explain the origin of vibrations of 
 the earth's crust. 
 
 Dr. Daubeny's Views. Dr. Daubeny regarded the primary shock 
 of an earthquake as the result of local volcanic excitement, evidence 
 of the accumulation and elastic pressure of imprisoned gases ; and the 
 propagation of the motion was attributed to wave-like vibration in 
 the mass of the rocks. 
 
 Fissures in the rocks are occasioned by, and are the evidence of, 
 earlier convulsive movements. If we seek the cause of them, we 
 certainly find the greater part of the necessary evidence in the in- 
 numerable fractures and flexures of the earth's crust, of every geolo- 
 gical date, by which extraordinary disturbances of the strata have 
 happened ; for these very frequently occur in large areas, where no 
 other evidence of contemporaneous volcanic excitement can be dis- 
 covered. It follows obviously that many movements of the earth's 
 crust have been excited without the immediately preceding or coin- 
 cident local agency of volcanoes, as though all the differential effects 
 of volcanoes could be integrated into one energetic reaction of the 
 interior against the cooled and consolidated exterior crust. 1 
 
 1 Humboldt's definition of volcanic agency in " Kosraos " contains this view. 
 VOL. I. X 
 
3^2 NATURE OF EARTHQUAKES. 
 
 Darwin's Views. According to the comprehensive idea of Dar- 
 win, earthquakes and volcanic eruptions originate in some local frac- 
 ture and displacement of the bed of the neighbouring ocean ; the 
 volcanic effect spreads as far as the subterranean sea of molten rock 
 extends, but is excited to violence at one or more points, the most 
 favourably circumstanced at the time ; the convulsive movement is 
 propagated through the solid and liquid contents of the crust of the 
 earth, as far as the nature of these materials and the force of the blow 
 permit. It is important to remark that a blow is struck the earth- 
 quake is the evidence and this can only mean some rending of the 
 solid crust of the earth. Thus a convulsion seems the necessary pre- 
 cursor of the earthquake ; and to allow of the movement traversing 
 whole continents, we must suppose the blow to be given at a consi- 
 derable depth, expressed in miles, below the surface. 1 
 
 Michell 2 entertained the idea that the sheet of rocks constitut- 
 ing what we know of the crust of the earth was flexible enough to 
 bend to the agitation of an interior liquid ocean of melted rock. 
 The earthquake in this view is a wave in the rocks representing a 
 tide in the subjacent fluid. 
 
 Rogers 3 supposed waves of this kind, moving parallel to certain 
 axes, to be effective in permanently raising the ground ; and attributes 
 to such an agency in early geological periods the formation of the 
 anticlinal and synclinal hollows parallel to the Appalachian chain. 
 
 Mallet's Views. Mallet, 4 however, both by reasoning and ex- 
 periment, has shown that the earthquake movement is a wave of elas- 
 tic compression, whose rate of movement varies according to the elas- 
 ticity of the medium and the continuity of rock masses. There is 
 one rate of movement for the sea, another for the land ; one rate for 
 solid granite, another for broken granite, another for loose sand. 
 This movement near the surface is far less rapid than the elasticity 
 of the media might lead us to expect ; a circumstance dependent on 
 the many fissures of the rock, at each of which discontinuity and loss 
 of motion are occasioned. 
 
 Velocity of Earthquake Waves. The actual velocities of some 
 earthquake movements have been approximately ascertained as under 
 in a line directly across the wave : 
 
 Conception, earthquake, 1835 . . 30 . . Rogers, Rep. to Brit. Assoc. 1843. 
 Guadaloupe, .... 1843 . . 27 . . Do. do. 
 
 The Conception earthquake had its crest directed ET.N.E. from the 
 western border of Alabama to Cincinnati, a distance of 500 miles. 
 On this line it was felt simultaneously. The motion was to the 
 E.S.E., and was felt at successive times simultaneously on lines 
 directed to the JST.N.E. If a measure of velocity were taken on the 
 
 1 Stukeley assigned the improbable depth of 200 miles for the forces of an 
 earthquake in Asia Minor, A.D. 17, which embraced a circle of 300 miles in 
 diameter. 
 
 2 Phil. Trans., 1760. 3 Brit. Assoc. Reports, 1843. 
 4 Mem. Royal Irish Acad., 1846 ; Brit. Assoc. Reports, 1850, et seq. ann. 
 
 
I 
 
 EARTHQUAKE MOTION. 333 
 
 N.N.E. line, it would appear infinite; on the E.S.E. line, 30 miles a 
 minute; on intermediate courses, intermediate velocities. The apparent 
 velocity of the wave at the surface will be greatest near the point 
 vertically situated over the disturbance. 1 
 
 Mr. Mallet, 2 by careful experiments on the sands at Kingstown, 
 and in the granite rocks at Dalkey, obtained velocities of wave transit 
 as under : 
 
 Feet per Second. Miles per Hour. 
 
 Loose sand, Killiney . . . 825 .... 9^ 
 
 Solid granite, Dalkey . . . 1,306 . . . . 15 
 Fissured granite, do. ... 1,665 J 9 
 
 The velocity at the surface may be in these cases somewhat less 
 than that of sound in air, but is far less than was expected by Mr. 
 Mallet from his knowledge of the elasticity of stony media. To judge 
 by experiments on their elasticity, we might expect the sound wave 
 to travel 
 
 Feet in a Second. | Feet in a Second. 
 
 In air . . . 1,140 j In primary limestone . 6,696 
 
 ,, water . . . 4,700 i ,, carboniferous limestone 7,075 
 
 ,, lias .... 3,640 | ,, hard slate . . . 12,757 
 ,, coal sandstone . . 5,248 I ,, granite perhaps still higher rate. 
 oolite. . . . 5,723 ! 
 
 Professor Milne finds that in Japan different earthquakes travel 
 with different velocities, and that the velocity of a wave decreases as 
 it travels. In 1881, earthquakes between Tokio and Yokohama moved 
 at rates per second of 4500 feet, 3900 feet, 1900 feet, 1400 feet, 2454 
 feet, and 2200 feet. 
 
 Depths and Origin of Earthquakes. According to Mallet, the 
 depth of the point where the blow or concussion which is the origin 
 of earthquake movements take place may be as far down below the sur- 
 face as the versed sine of the arc cut off by the extreme points of the 
 space subject to tremor. In some cases this passes very deeply into 
 the earth. 
 
 Origin of Earthquakes. Taking into account all the phenomena 
 of earthquakes, Mallet admits for consideration the following modes 
 of origin of the impulse : 3 
 
 (i.) The operation of steam extricated by cooling from the spheroidal state. 
 
 (2.) Evolution of steam through fissures, and its irregular condensation under 
 pressure of sea- water. 
 
 (3.) Recoil from volcanic explosions. 
 
 (4. ) Great fractures and dislocations in the earth's crust, suddenly produced 
 by pressure or contraction in any direction. 
 
 Earthquake Motion. The actual movement of the ground, says 
 Professor Milne, is sometimes 8 millimetres, but often under i milli- 
 metre ; there are seldom more than two or three vibrations a second. 
 The motion of the ground towards the origin of the disturbance is 
 usually much greater than the motion away from it. 
 
 Professor Ewing of Tokio finds that in almost every instance the 
 
 1 Hopkins, Brit. Assoc. Reports, 1847. 2 Mallet, Rep. to Brit. Assoc. 1851-52. 
 3 Reports of Brit. Assoc., 1850. 
 
324 PERMANENT EFFECTS OF EARTHQUAKES. 
 
 motion of the ground begins very gradually, and it does not reach its 
 maximum for some seconds. An earthquake consists of many suc- 
 cessive movements, among which no single large movement stands 
 out prominently from the rest. The disturbance ends more gradually 
 than it begins. The area affected, duration and direction of move- 
 ment are very irregular and variable during the same earthquake. 
 Frequently the beginning of visible motion consists in a tremor of 
 short-period waves, about five to the second, followed by the principal 
 movements of one or two waves to the second. 1 
 
 Distribution of Earthquakes. British earthquakes are always 
 slight : one-third of all that are known are recorded from the county 
 of Perth, and most of the others are also Scotch. 2 In Europe earthquakes 
 are common in all the regions of active volcanoes ; and they have 
 especially disturbed Calabria, the country about the mouth of the Tagus, 
 Agram, and several of the Greek islands, like Chios. Many districts 
 which experience earthquakes are free from volcanoes, though the 
 most violent earthquakes occur in volcanic regions. The destructive 
 earthquakes of Asia Minor and Syria are connected with regions where 
 volcanic action has become extinct ; but in South America earthquakes 
 are frequently connected with outbursts of volcanoes along the line of 
 the Andes. The ground disturbed by an earthquake as frequently 
 sinks as rises ; and there can be no doubt that the fractures produced 
 by undulation of the rocks, which must develop the character of 
 master-joints, are favourable to dislocation. The movements rarely 
 last more than a minute, and frequently only for a fraction of a 
 minute, though one at Tokio lasted four and a half minutes. The 
 areas over which the disturbance extends are extremely variable, the 
 most extraordinary record being that of the Lisbon earthquake of the 
 ist of November 1755, which affected the North of Africa and 
 Western Europe, and appears to have crossed the Atlantic and 
 travelled to the valley of the Mississippi. 
 
 Effects of Earthquakes. Besides the production of cracks in 
 buildings in the lines of wave motion, tangents to which pass through 
 the centre of disturbance, we may cite as among the best-known per- 
 manent effects of earthquakes the formation of the Ran of Katch in 
 1819; the uplifting of the shore of Cook Straits ten feet in 1855 ; 
 and the well-known elevation of the Chili coast recorded by Darwin 
 in the "Voyage of the Beagle." 
 
 Causes of Earthquakes. Earthquake action has sometimes been 
 connected with variation in atmospheric pressure, and with the attrac- 
 tion of the sun and moon. Thus earthquakes are more numerous in 
 mountain regions, like the Alps, than in lower regions. The circum- 
 stance that they are most numerous in winter would apparently indi- 
 cate that the radiation of earth's solar heat in winter causes contraction 
 of the rocks, resulting in dislocations which produce perceptible vibra- 
 
 1 Memoirs of Science Department Tokio Daigaku. No. 9, Earthquake Mea- 
 surement, by J. E. Ewing, 1883. 
 
 3 On the 22d April, 1884, an earthquake of some severity was experienced 
 about Colchester, and felt along a line running N.W. by way of Leicester. 
 
 

 CAUSES WHICH ORIGINATE EARTHQUAKES. 325 
 
 tions of the earth's crust ; and a similar conclusion is indicated by the 
 well-known fact that the Swiss earthquakes mostly happen at night. 
 But the great earthquake vibrations, which have been measured to 
 extend to depths which vary from a mile or two down to more than 
 thirty miles, are clearly connected with the great internal earth-move- 
 ments which are consequences of the contraction of the earth's crust 
 from cooling. And while volcanoes are intelligibly accounted for as 
 distributed in lines of predominate anticlinal folding, we are unable to 
 account for many earthquake phenomena unless they are produced by 
 increased compression in regions of predominant synclinal folding. 
 For if the base of a synclinal fold is fractured owing to augmented 
 contraction of the rocks, and the fissure is formed at a depth of miles 
 beneath the surface, then the distinction of earthquakes from vol- 
 canoes becomes intelligible, and their frequent development in plains 
 and at sea, rather than in mountains, is such as might theoretically 
 have been anticipated from their frequent independence of volcanic 
 outbursts, and the dimensions of the areas affected. 
 
 Changes of Level in Land. 
 
 On the Elevation and Depression of Land. Whatever may 
 have been the earth's earliest cosmical relations, it appears first in 
 geological history as a spheroid of revolution, whose parts have taken 
 their relative place under the joint influence of gravitation to the centre 
 and rotation on an axis. The density increases toward the centre, the 
 surfaces of equal density are elliptical to the same axis as the external 
 oblately spheroidal surface. 
 
 This spheroid cools by radiation ; contraction of the whole miss 
 follows, so that the crust is pressed into accommodation with the 
 interior. Thus inequalities of the surface would be occasioned ; and 
 from the beginning a continual system of reciprocal depressions and 
 elevations would be established. The consequence would be, that the 
 surface of the spheroid would be wrinkled by folds of elevation and de- 
 pression, growing more and more deep, and with the progress of time 
 more and more complicated. In remarkable harmony with this view 
 is the well-known fact of the frequency of anticlinal and synclinal and 
 more complicated flexures of the palaeozoic strata in all parts of the world, 
 flexures which were often completed before the close of that period. 
 
 In later periods of the earth's contraction, local inequalities of 
 consolidation partly dependent on the earlier flexures, partly pro- 
 duced by the inequality of molecular aggregation, as by the separation 
 of different orders of silicates, calcium sulphate, or magnesian car- 
 bonates may have overcome the phenomenon of reciprocal depression 
 and elevation, and limited the areas in which elevation or depression 
 might take place and in which one might follow the other so that 
 the same tract might be alternately raised and depressed. 
 
 In nearly all cases depression must be supposed to be real and 
 gradual, that is to say, part of the earth's surface affected by it must 
 be gradually carried nearer to the centre than it was before ; the 
 
326 UPHEAVAL IN THE NORTH OF EUROPE. 
 
 elevation may have been in many cases only relative and gradual, but 
 in others real and unequal ; that is, the area may have been removed 
 farther from the centre than it was before, by a force of lateral pres- 
 sure subject to inequality and cessation. In some cases of real de- 
 pression, and in many more of real elevation over a limited area, 
 the solid crust must be supposed to have been intensely folded. 
 
 General upward pressure rarely if ever happens ; the crust would 
 be extended, and beyond a certain strain it would break, when the 
 broken parts would slide on one another so as to occupy a larger area, 
 and the result would be faults. Such conditions are apparently ob- 
 served in the region of the Rocky Mountains. 
 
 In a case of real depression of a given tract, followed after a long 
 interval by elevation, the effects would vary according to the area 
 moved and the vertical range of the motion. If the area were so 
 extensive as to include a larger arc on the earth's surface, the crust 
 would subside into a smaller area, and be wrinkled, or otherwise 
 affected by compression and augmented heat. On the re-elevation of 
 such an area, faults would probably be produced ; this seems to have 
 been the case in many of our coal-fields, whose flexures are traversed 
 by later faults. If the subsiding area were small or narrow, and the 
 downward movement great, the rocks would sink into a larger area, 
 and faults might be expected. On the re-elevation of such a tract 
 much local disturbance and complicated internal movements among 
 the masses of the rocks would probably follow, and this may have 
 happened in the Belgian and Somersetshire coal-fields. 
 
 The influences of these conditions have not yet passed away. 
 Scandinavia is rising and sinking, not in either case on account of 
 volcanic excitement, strictly so called, but by reason of internal changes 
 consequent on slow refrigeration. 
 
 Upheaval of Land. In Smith Sound raised terraces occur at 
 from 32 to i ro feet above high- water mark. Other evidences of rise 
 consist in the existence of ruined houses high above the water, as at 
 Hunde Island Dr. Kane assumed the upheaval to take place north 
 of latitude 77, south of which the land is depressed. Over many 
 places in the Arctic lands, as on the summit of the Coxcomb range in 
 Earing Island, shells of living species occur many hundred feet above 
 the sea-level, which would indicate recent upheaval. 
 
 On the coast of Norway, Professor Keilhau infers that the whole 
 country from Cape Lindernas to North Cape has been raised in com- 
 paratively recent times, the upheaval amounting on the S.E. coast to 
 more than 600 feet. Sir Charles Lyell in 1834 described the raised 
 sea-beaches at Uddevalla, which are 100 feet above the sea, and have 
 barnacles adhering to the gneiss. In 1730 a beach-mark was made 
 in the island of Loeffgrund, near Gefle, in the presence of Linnaeus, 
 which in 1839 had been elevated three feet; but farther north, by 
 the mouth of the river Tornea, at the head of the Gulf of Bothnia, 
 the land is rising at the rate of five feet in a century. The waters 
 over which the French expedition measured an arc of the meridian 
 are now replaced by meadows. 
 
DEPRESSION OF GREENLAND. 327 
 
 Similar evidences of elevation in recent times are found in the Medi- 
 terranean. Not only is the shore of Tunis becoming too shallow for the 
 approach of ships, but the coasts of Sicily, Sardinia, and parts of Tus- 
 cany tell the same story in the elevation of shell-beds, which sometimes, 
 as in Sardinia, contain pottery at a height of 200 feet above the sea. 
 The shores of the Adriatic, however, are undergoing depression. 
 
 Mr. Kinahan, in his " Geology of Ireland," has described numerous 
 raised sea-beaches and sea-margins, and others are well known on the 
 Devonshire coast and in Cornwall, where Mr. Ussher has referred to 
 them at Mount Edgecombe near Plymouth, Looe Island, St. Austell's 
 Bay, Falmouth, south of the river Helford, Coverack Cove, &c., &C. 1 
 
 Depression of Land. Depression is inseparable from elevation, 
 just as every synclinal fold is a portion of an anticlinal fold. Hence, 
 beyond the geographical limit of upheaval a coast is found to be sub- 
 siding, and the regions where this condition is seen are necessarily 
 adjacent to those which are being raised. On the Greenland coast, 
 in Igalliko Fiord, in 1779 a small rocky island was entirely sub- 
 merged at spring-tides, yet the walls of an old Norse house remained 
 visible. Fifty years later only the ruins rose above the water. In 
 many places farther south, in lat. 62, 63, 64, and 65, the ruins of 
 dwellings are seen which are overflowed by the tide. The Moravian 
 Mission settlements moved inland the posts on which the large 
 boats are kept. In Disco Bay, Dr. Kobert Brown records that a 
 blubber boiling-house was built about eight miles from the shore on 
 an islet, which sunk gradually till the water entered the floor of 
 the house at high tide, when it had to be abandoned. 
 
 In the Mediterranean, the standing pillars of the temple of Jupiter 
 Serapis, in the Bay of Baia3, are an example of the extreme steadi- 
 ness with which earth-movements both of elevation and depression 
 take place. 
 
 But the most striking instances of change of level at the present 
 day are recorded by the phenomena of barrier reefs and atolls. 
 
 On our own shores depression is proved by submerged forests, as 
 at Looe, Fowey, Falmouth, Mounts Bay, Padstow in Cornwall, Por- 
 lock in West Somerset, and many places in Norfolk, Sussex, &c. 
 
 The entire history of the strata is a record of a few grand oscilla- 
 tions of level, from which it has resulted that the same area has been 
 alternately covered by fresh-water deposits accumulated upon land, 
 and marine deposits which were superimposed when the region sunk 
 beneath the sea. 
 
 Is Steam a Cause of Upheaval or a Consequence 9 it is con- 
 ceivable that water might be conducted, in consequence of some 
 accident of the earth's crust, to the required contact with the heated 
 rock at some moderate depth below the surface, and thus high-pres- 
 sure steam be generated and accumulated until disturbance comes. 
 A gaseous force of some kind may be supposed ; thus the melted rock 
 is pressed up to the very summits of Tenerifle and the Andes ; and 
 masses of stone hurled out of Vesuvius have fallen at a distance of 
 1 Geol. Mag., 1879 
 
328 RELATION OF STEAM TO UPHEAVAL. 
 
 five or six miles at Pompeii. It is necessary to assume the stone to 
 have been thrown about 7000 feet above the summit of Vesuvius, or 
 more than two miles above the sea, and the force to do this work 
 may be supposed to have been seated one, two, or three miles below. 
 American volcanoes have thrown bombs to far greater distances. 
 
 That the steam or gas power, thus estimated for intensity, is also 
 of enormous volume and magnitude, appears from the continuity of 
 some eruptions, the amazing mass of rock which has been ejected, 
 the clouds of ashes, the rapidly formed hills, the land upheaved, and 
 the islands raised. Thus in forty-eight hours the volcanic forces 
 seated about the Lucrine lake raised by showers of ashes a hill called 
 Monte Nuovo, 440 feet high and 841 feet deep in the middle (1538). 
 Skaptar Jokul, or a great Icelandic volcano thus indicated, is reputed 
 to have thrown out three streams of lava, eight miles apart, which 
 covered an area of 1200 square miles; though it has been affirmed 
 that Icelandic records show that the mountain so named has never 
 been in eruption. 
 
 The pressure of steam equal to raise felspathic lava five miles 
 high may be called in round numbers 2000 atmospheres, a pressure 
 attempted at the request of the British Association by Mr. Hopkins 
 and Sir W. Fairbairn. These experimentalists accumulated pressures 
 equal to that of the highest mountains nay, equal to thirty-three 
 miles of water. Such a pressure, unrelieved by volcanic vents, might 
 lift large tracts of solid land. 
 
 Probably the western coast of South America, raised in 1822 for 
 i ooo miles in length, and in places for three or four feet, was uplifted 
 less by the explosive power of steam than by the crumpling of the 
 earth's crust, which takes place owing to lateral east-to-west contrac- 
 tion and vertical expansion of the rocks beneath the Andes, con- 
 sequent upon the formation of a vacuity by the discharge of volcanic 
 products ; and elevation in most cases must result in pressure, which 
 increases the heat beneath mountain chains, and thus generates steam. 
 
 Hopkins' Theory of Elevation. Mr. William Hopkins, F.K.S., 
 gave a mathematical form to the experiences of miners and geologists, 
 which had recognised the existence in a limited area of two sets of 
 dislocations placed at right angles to one another, which often yield 
 metallic matters of different kinds. These fractures may depend on 
 one system of movement under that district. Suppose an expansive 
 force gradually augmenting under the whole of a limited tract, and 
 capable of bearing up the whole mass of the strata there. Let these 
 strata be capable of extension, so that they should swell up into an 
 arch, but let their extensibility be limited, so that at last the arch 
 must break. It will depend mainly on the outline or figure of the 
 ground raised what shall be the direction of the fractures. If the 
 area be indefinitely very long as compared to the breadth, and the 
 sides be parallel, there will, in the first place, be one or more frac- 
 tures parallel to the length of the figure across the lines of greatest 
 tension and, secondly, other fractures depending on them at right 
 angles to them. Thus, in the mining districts of Alston Moor, the 
 
 he 
 
FRACTURES FORMED BY UPHEAVAL. 
 
 OF 
 
 -KLVJ 
 
 f 
 
 329 
 
 north-and-south fractures, parallel to the great Pennine fault, and the 
 east-and-west fractures, at right angles to these, compose a system in 
 accordance with the mechanical theory. 
 
 Again, if the force under a given district be determined by any 
 peculiarity of the rocks to a conical elevation, there will be radiating 
 primary fissures and secondary concentric ones. Such a case, per- 
 haps, occurs in the volcanic elevation of Mont Dore. An elliptical 
 elevation would have characters intermediate between the two, and 
 the same district may show traces of one of these superadded to 
 the narrow rectangular elevation first noticed. 1 Such a case occurs 
 in the Weald of Sussex. By cautiously employing this ingenious 
 mode of interpretation, we shall be able to determine, in any given 
 region where the fractures of the strata are well traced, the whole 
 area of the ground subject to any movement at a given time. 
 
 It is unnecessary to quote examples in which Mr. Hopkins' views 
 find a useful application. We will only state a single case of the 
 parallelism of trap dykes, which has been furnished by Archdeacon 
 Verschoyle, in the north-west part of Mayo and Sligo. 2 He describes 
 no less than eleven basaltic and amygdaloidal dykes, which, in a 
 space of n^ miles in breadth, traverse the northern part of the dis- 
 trict in a nearly east-and-west direction, and cut through all forma- 
 tions from gneiss to the Carboniferous limestone. One of these dykes 
 he traced between sixty and seventy miles, and believed it might be 
 followed much farther to the eastward. Two of the dykes are crossed 
 by others having a north-and-south direction. 
 
 Hopkins on the Elevation of the Weald. In the large ellip- 
 tically elevated area of the Weald, 150 miles long from east to west, 
 and 40 miles broad from north to south, between the chalk escarp- 
 ments, Mr. Hopkins recognises, besides the general broad anticlinal 
 slopes which determine the main features of the district, several lines 
 of flexure and fracture, and anticlinal axes ; and he also defines some 
 of those transverse lines of movement, depending on the main axis 
 and boundaries of the district, which are directly deducible from his 
 theory. He combines with the elliptical elevation of the Weald the 
 more elongated system of parallel movements of the Isles of Wight 
 and Purbeck. The remarkable breaks in the bounding chalk ranges 
 which give passage to the rivers flowing from the Wealden northward 
 and southward were supposed to correspond in situation with cross 
 fractures, indicated by theory, and occasionally proved by observa- 
 tion. One considerable decisive and simultaneous movement is ap- 
 pealed to for the dislocations of the elevated mass, and for the pro- 
 duction of its main physical features ; but there is still a necessity of 
 admitting a slow and gradual continental elevation to account for the 
 denudation of the district. 3 And we shall do well, before accepting 
 the origin of these river valleys in faults, to consider the history of 
 river valleys in the South of Ireland advanced by the late Pro- 
 fessor Jukes, F.R.S., 4 and the application of similar views to the 
 
 1 Camb. Phil. Trans., 1837. 2 Proc. Geol. Soc., 1833. 
 
 3 Geol. Trans., vol. vii. " 4 J. B. Jukes, Q. J. G/S., vol. xviii. p. 378. 
 
330 EROSION SIMULATING FRACTURE. 
 
 Wealden district enunciated by Mr. Whitaker, 1 from which it ap- 
 pears that the river valleys are older than the denudation of the Weald. 
 
 5 4 3 a 345 
 
 Fig. 67. General section of the Wealden, showing the probable 
 
 extent of denudation. 
 
 I Hastings sand. 2 Weald clay. 3 Lower greensand. 4 Gault. 5 Upper 
 greensand and Chalk. 
 
 John Phillips' Hypothesis of the Elevation of Land. Newton 
 supposed the spheroid to be homogeneous ; it has been found that 
 this supposition is by no means fitted to fulfil the observed conditions 
 of the problem of the earth's figure. And the irregularities of attrac- 
 tion indicated by the pendulum experiments, and of curvature demon- 
 trated by direct meridional measurements, seem to show that the con- 
 centric masses of the spheroid are not of uniform density. 
 
 This being allowed, there would seem no objection to supposing 
 that the densities along any one radius of the spheroid are variable, 
 by reason of internal movements among the unequally dense parts of 
 the concentric masses, and this would exactly answer the conditions 
 of the geological problem. For the length of any radius of the 
 heterogeneous spheroid would necessarily vary with the densities ; 
 and considering the small proportion of the height of the land above 
 the mean radius of the latitude, it is clear that small internal changes 
 in a length of 4000 miles would easily account for variations on that 
 line to the extent of 1000 feet or yards. This hypothesis would give a 
 gradual and prolonged elevation in some parts and corresponding de- 
 pressions in others ; it would not affect in a sensible degree the astro- 
 nomical elements of the planet, but would change more or less com- 
 pletely its hydrographical boundaries. 
 
 This view is an example of speculation which has no sound logical 
 basis, since we know nothing of internal movements in the earth due to 
 variable density. But the hypothesis has some historical interest as a 
 precursor of the modern doctrine of crumpling as the cause of elevation. 
 
 Disturbances of the Strata. 
 
 Proofs of Dislocation. When strata, originally level, or nearly so, 
 have been raised to high angles of inclination ; when beds, originally 
 continuous, are found to be broken asunder, and their separated por- 
 tions placed in new relations of position, one portion being raised or 
 depressed, or both deranged ; when layers, originally flat, are found 
 to be bent into extraordinary curvatures ; the conclusion is inevitable, 
 that contortions, if not convulsions, have happened in the places 
 where such phenomena occur. 
 
 1 Whitaker, Geol. Mag., vol. iv., 1867. 
 
 
 

 DEPOSITION SIMULATING UPHEAVAL. 331 
 
 Evidences that Strata were not Deposited in Inclined Layers. 
 
 General experience assures us of the fact, that agitated water deposits 
 sediment slowly in the form of strata whose upper surfaces continually 
 tend to become horizontal. This is seen in inundations from a river, 
 in shallow and ruffled lakes, and within the low-water margin of the 
 sea. The form of the bottom influences the horizontally of the 
 upper surfaces of the deposits in such a way that where the bottom 
 is like a pit, the stratified masses above are hollow on the faces ; but 
 these effects of the original inequality are rapidly obliterated by suc- 
 cessive coats of sediment, all becoming more and more nearly hori- 
 zontal. In perfectly tranquil water, through which any fine sediment 
 is equally diffused, the depth to which this will cover any part of the 
 bed depends on the depth of the supernatant water, and on the angle 
 of rest in water of that kind of sediment. The angle of rest in air for 
 earthy substances is about 45. If a river bring sediment into agitated 
 water, deposited strata tend to become horizontal, but with a constant 
 dependence upon the point where the river enters, such that, the 
 quantity of sediment being there always accumulating, a general 
 conical slope therefrom in all directions will modify the horizon- 
 tality of the strata. Or if a river bring sediment into calm water, 
 or into water suddenly deepening, so that all its lower parts may 
 be considered as calm, the conical slopes from the point where 
 the river enters will be much more abrupt than in the former case, in 
 a certain proportion to the calmness and depth of the water. This is 
 the case in the deep lakes which receive sediment from the torrents 
 of the Alps. 
 
 On considering these cases with reference to stratified rocks, it is 
 evident that instances coming within the class of conical deposits 
 radiating round a point can only be of very limited occurrence, not 
 likely to affect a general argument, and are, in fact, almost unknown. 
 The lacustrine deposit of the Weald of Sussex has been regarded by 
 Mr. Topley as an area of seeming upheaval, in which the thinning 
 out of the beds away from the district is a true explanation of their 
 dip. 1 It is very doubtful how far this can be recognised in the thin- 
 ning out of any marine formation, and certainly it does not apply 
 conspicuously to any class of marine deposits now in progress : at 
 the same time we must admit that, in all cases, the action of 
 the sea grows less and less sensible far from shore where the 
 water deepens, and the sediment brought by rivers and floods must 
 be formed in attenuated masses, thickest towards the shores. This 
 effect will be in exact proportion to the falling velocity of the 
 particles in water, so that pebble beaches lie in steeper slopes and 
 cover shorter breadths than sands, while fine clays will spread farther 
 into deeper water. But all these slopes in water are very gradual, 
 so that even against the rocky eastern coasts of England, the deep 
 waters have been filled up by sediments, which now assume a gently 
 declining surface under the water, and often only a moderate slopo 
 above it. 
 
 1 Topley, Q. J. G. S., vol. xxx. p. 186. 
 
3j2 GEOLOGICAL EPOCH OF AN UPHEAVAL. 
 
 Pebble Beds indicate Upheaval. When we find traces of a sudden 
 and complete change in the succession of aqueous deposits, so that the 
 quiet deposition of clay or limestone is interrupted by a tumultuous 
 aggregation of pebbles, we know that there has been some agitation 
 of the water. This may have happened either from a periodical 
 or accidental change in the drainage of the neighbouring land, or from 
 some extensive change in the relations of land and sea. The latter 
 interpretation may be adopted if these indications of agitation are 
 very extensive, arid if there be proof of local conglomerates following 
 upon local convulsions. 
 
 Another indication of some distant convulsion affecting the 
 relations of land and sea seems to be afforded from the occurrence 
 of a bed of marine shells among fresh- water estuary deposits without 
 any local unconformity of stratification ; though the alternation of 
 fresh-water and marine conditions in the Lym Fiord in Jutland 
 shows that prevailing winds changing their direction or force may 
 account for some such phenomena. 
 
 Method of Fixing the Age of an Upheaval. In geological in- 
 quiries concerning time, the answer is always expressed in terms of the 
 relative antiquity of stratified rocks ; and a convulsion is fixed in geolo- 
 gical time when it can be shown to have happened after the deposition 
 of one stratum, and before the deposition of another. If the strata 
 which thus limit the period of the convulsion be consecutive terms 
 of the series of deposits, the most precise result is obtained ; but if 
 these limiting strata be not consecutive, the age of the disturbance 
 is known only within a given range. 
 
 An example of accurate determination of the geological era of a 
 convulsion is afforded in the North of England, where the newest of 
 the coal strata are found to be dislocated under the oldest red sand- 
 stones of the Permian system. 
 
 Instances of less precise determinations are common enough : for 
 example, in the Mendip hills the dislocated mountain limestone is 
 covered by undisturbed oolite ; and, as far as this observation goes, 
 the convulsion may have happened during any part of the long period 
 occupied in producing the coal, red marl, and lias strata. In this 
 case, however, by tracing the line of the dislocation to other localities, 
 other strata are found to be so related to the limestone as to fix the 
 geological date of its disturbance within narrower limits. 
 
 If the dislocated strata be not actually seen covered by others 
 which are undisturbed, another set of data must be employed. It 
 may happen that around the disturbed rocks some newer stratum 
 spreads in such a manner as to give sufficient reason to conclude that 
 it was deposited since the period of the convulsion. This is, in most 
 parts, all that can be observed with respect to the red marl around 
 the igneous rocks of Charnwood Forest, and there would be satisfactory 
 evidence that the slaty rocks of that district were upraised before 
 the period of the New Red Sandstone ; and, in fact, we have found 
 instances where the red marl does really cover with level beds the 
 
 broken edges of slate. 
 
 
GREAT FAULTS. 333 
 
 If no horizontal or undisturbed strata be visible in any part of tho 
 dislocated tract, either in superposition or in juxtaposition, the limit 
 of least antiquity vanishes, and we are in danger of imagining too 
 modern a date for the convulsion ; if the newer members of the dis- 
 located group of strata be concealed, there is danger of ascribing too 
 high an antiquity to the convulsion. 
 
 Eolation of Igneous Rocks to Convulsions. The almost universal 
 coincidence of convulsive dislocation of the strata with eruptions of 
 plutonic rocks seems enough to prove their common dependence 
 upon one pervading cause of internal movement. In the same man- 
 ner as the modern earthquake precedes the eruption of lava, so the 
 ancient convulsion or fault preceded the injection of plutonic rocks. 
 Also precisely as in the present day the earthquake shakes countries 
 far removed from volcanic centres, so in more ancient periods many 
 tracts were dislocated, but the fissures thus formed were not filled 
 with melted rocks, at least near the surface. As far as at first appears, 
 the common dependence of the two orders of effect upon one cause is 
 merely to the amount that the mechanical transference of melted rocks 
 has been effected by the same internal pressure which dislocated the 
 strata ; whatever occasioned the pressure was also the cause of the 
 fluidity of the rocks when the pressure was reduced or removed. 
 
 Various mechanical modes besides contraction may be conceived 
 by which such pressure may have been occasioned, and various con- 
 ditions assumed for the production of melted rocks, and these may 
 be wholly distinct from one another; but the exhibition of these 
 rocks along the lines of convulsion can only be ascribed to the same 
 mechanical cause which produced the convulsion. 
 
 Fractures and Dislocations of Strata. 
 
 Faults. Those dislocations known as "faults" which break the 
 continuity of the beds along certain planes or fissures, and elevate or 
 depress one side, often plainly declare themselves to be the result of 
 single movements. Inspection of the phenomena in this country 
 will leave no room for doubt that the dislocated strata were put 
 into their present relations, not by a repetition of small and gra- 
 dual movements, but by one movement, which may have been slow 
 or rapid. 
 
 The extent of dislocation to which the name of fault accurately 
 applies is extremely various, the difference of level thus occasioned 
 being sometimes a few inches, in many cases 100 feet, in others 
 more than icoo yards. This marks out in very clear characters the 
 degree of force exerted in each case. Those dislocations which 
 make the greatest difference of level range through the greatest 
 lengths of country. The ninety-fathom dyke so named from the 
 observed extent of its dislocation ranges from the eastern sea across 
 the whole breadth of Northumberland ; and certain dislocations in 
 Yorkshire have ranges of ten, twenty, and thirty miles in one nearly 
 straight line. 
 
334 RELATION OF FAULTS TO UPHEAVAL. 
 
 Great Dislocations. As far as we know, the greater portion of 
 the convulsive movements of the earth's crust were accomplished by 
 means of " faults." One of the most magnificent examples of dis- 
 location in Europe is that grand break nearly along the line of the 
 western border of Durham and Yorkshire, from near Brampton by 
 Brough and Kirkby Stephen to near Kirk by Lonsdale, the effect of 
 which is to throw down to the west the strata of the Carboni- 
 ferous system more than 1000 yards through a length of seventy 
 miles. An axis of slate rocks rises along the line of fracture, 
 which is also partially marked by dykes of dolerite. On the 
 west the beds dip at high angles to the west ; on the east they 
 decline gently to the east. No proper plane of fault is traceable in 
 this case of enormous disruption, owing to the circumstances of the 
 country. This line of disturbance is cut off to the north by the 
 ninety-fathom dyke, and to the south by the Craven fault ; and 
 there is every probability that it is actually continued along the 
 lines of these faults to a direction right-angled, or nearly so, to its 
 own course. 
 
 Relation of Faults to Axes of Elevation. Amongst these faults 
 it is possible, perhaps, to distinguish two periods of disturbance, the 
 older one marked by a direction nearly east and west, which is that 
 of most of the metalliferous veins, the other by a direction from 
 north to south, which is that of several whin dykes and some few 
 lead veins. These different directions may have taken their rise from 
 the two directions of the axes of convulsion which bound the district, 
 and may mark successive periods of folding, and elevation of the 
 ancient sea-bed still evident in axes of compression. 
 
 The limits of time by which the faults are defined are in many 
 instances nearly coincident with the limits of uppermost Coal-measures 
 and New Bed Sandstone. 
 
 Some cases of disturbance are of a complicated nature. Such 
 are the extraordinary retroflexures of the calcareous strata adjoining 
 the Alps, the retroverted dips in the coal-fields of Somersetshire and 
 Belgium, and the flanks of the Malvern hills. In some of these cases, 
 as on the western side of the Malverns, and the western side of the 
 Appalachian chain, we find the curvature often repeated in many 
 synclinals and anticlinals; we see the slopes on each anticlinal steeper 
 on the western side ; and we remark that the anticlinals, taken suc- 
 cessively from east to west, grow less and less steep, and more and 
 more broad, as we proceed farther and farther from the mountain 
 chain. 
 
 Dykes. 
 
 Dykes are concomitants of volcanic action, which owe their exist- 
 ence to the formation of fissures which extend from below upward, or 
 in the planes of stratification. The fissure forms a vacuum, which 
 draws the igneous rock up into its present position ; and sometimes 
 the fissure, by reducing the pressure, renders the igneous rock fluid. 
 How far dykes may be produced out of the material of the strata 
 
DISTRIBUTION OF DYKES. 335 
 
 which they penetrate by reduced pressure in a plane of intrusion is 
 still an open question. Dykes are formed during changes of level 
 of land, and are exposed at the surface by denudation. 
 
 Ben Nevis. The abundance and variety of felspar porphyry in 
 great masses on the summit of Ben Nevis and in the valley of Glen- 
 coe are familiar to every traveller in the Highlands. The porphyry of 
 Ben Nevis was shown by Von Oeynhausen and Von Dechen to have 
 been erupted through the granitic basis of that mountain. The por- 
 phyries along the vertical precipices of Glencoe send veins through 
 the subjacent granites, in number proportioned to the proximity of 
 the situation to the great mass of porphyry. This rock varies through 
 every stage, from claystone to felspar porphyry, the transition being 
 sometimes gradual and sometimes sudden. Agglomerate composed 
 of fragments of claystones and porphyries like those on Ben Nevis, 
 and some in Cumberland, are often seen in Glencoe. 
 
 Ben Cruachan. In the mountain of Cruachan, which overlooks 
 Loch Awe, the hornblendic granite and schist rocks are traversed by 
 a great variety of large felstone and porphyry dikes, and some changes 
 of appearance happen to the mica schist. MacCulloch l describes the 
 porphyry dykes as perpendicular, 
 varying from 3 to 50 feet in breadth, 
 traversing alike the schist and the 
 granite veins, but not intermingling 
 with either. Dykes of porphyry, of 
 different kinds and colours, may run 
 near or in contact with each other ; 
 but in all cases these and other 
 dykes of basalt or porphyry are very 
 distinct at the edges, though firmly 
 
 united to the rock which encloses them. Fig. 68 shows veins of 
 granite traversing the schist of Cruachan, themselves crossed by dykes 
 of two kinds of porphyry. 2 
 
 Cumbrian Mountains. In the Cumbrian mountains felstone por- 
 phyries occur in many situations, and with a great diversity of 
 character. Some have a basis of translucent grey or green felspar, 
 and included crystals of glassy felspar and quartz ; others are com- 
 posed of a red, opaque, granular felspar basis, and red felspar and 
 quartz crystals, as in rhyolites ; the basis of others is compact felspar 
 or hornstone, and some have a dark andesitic base, with small white 
 opaque felspar crystals. Most of them, like the amygdaloids and 
 dolerites of the same region, occur in overlying masses, as well as 
 dykes. They seem to have a geographical dependence of a peculiar 
 kind on the foci of granitic eruption. They are not abundant in or 
 very near to the granite of Wastdale, Skiddaw, or Shap, but occur at 
 small distances from each of those masses. The Valley of St. John 
 shows pale red felspar porphyry overlying slate, well crystallised red 
 porphyry in Armboth Fell, and various kinds of felspathic rocks under 
 Helvellyn. Dykes of variable greenish porphyry divide the slates of 
 1 Geol. Trans., iv. 2 Ibid., iv., pi. vi. 
 
336 DYKES IN OLDER PRIMARY ROCKS. 
 
 High Pike, and a solitary red dyke ranges east and west of the granite 
 of Shap Fells. No porphyry occurs very far from the granites. 
 
 North Wales. In North Wales felspathic porphyries and dolerites 
 are so connected by alternate bedding with the slates as to have been 
 subjected to the same elevations and undulations of dip ; and thus not 
 only prove their high antiquity, but also suggest views as to the fre- 
 quent recurrence of igneous action at the same points of the ancient 
 bed of the sea during the Cambrian period. Dykes are even more 
 abundant than interstratified igneous rock. For details reference may 
 be made to Sir A. Ramsay's " North Wales." 
 
 Cornwall. We may believe that all the complicated rocks, wholly 
 or partially crystallised, composed of felspar, quartz, and mica, which 
 are included between the slaty rocks of Cornwall, and which traverse 
 them, are either the result of submarine eruptions during the forma 
 tion of the Devonian slate ; of the subsequent action of the heated 
 granitic masses upon the killas ; or are subsequent eruptions of melted 
 rock into fissures caused by convulsion, or result from some gradual 
 conversion and transfer of mineral ingredients. It is difficult to reason 
 on dyke phenomena so remarkable as those of Cornwall without refer- 
 ence to other districts. The student may consult the writings of J. 
 A. Phillips, F.R.S., on Cornwall, in the "Journal of the Geological 
 Society," already quoted, for details of their distribution. 
 
 Fig. 69. Caldron Suout, Teesdade. A waterfall iu subprismatic basalt of the 
 " whiu sill." 
 
 Basalt of Teesdale. The basaltic formation of Upper Teesdale 
 in Yorkshire was described by Professor Sedgwick, and its continua- 
 tion through Northumberland was traced by Mr. Button. The great 
 mass of basalt called the " whin sill " forms a layer of irregular thick- 
 ness, enclosed among strata of the Carboniferous limestone series, 
 generally on the same horizon, so that in the valley of the Tync 
 
INTRUSIVE BASALT OF TEESDALE. 
 
 337 
 
 its place in the section is constant, and it occupies generally the 
 same situation in Teesdale, though in Weardale another layer of 
 basalt occurs. We cannot doubt that its thickness at different places 
 was effected by their proximity to the eruptive channel. In the short 
 space of six miles, from Caldron Snout to Hilton Beck, its thickness 
 is diminished by 200 or 300 feet to 24 feet, and farther south it 
 disappears totally. But to the northward the range is interruptedly 
 continued to the sea-coast of Dunstanborough. 
 
 No dykes pass from this mass in Teesdale into the rocks above 
 or below ; so that a first view of the case suggests the idea that it 
 was poured out as a mass of submarine lava upon the yet incomplete 
 deposit of the Carboniferous limestone. Professor Sedgwick, how- 
 ever, 1 maintained that it was injected from below amongst these 
 
 Fiy. 70. High Force, Teesdale. A waterfall in columnar basalt of the " whin sill" 
 over limestone. 
 
 strata, and that it penetrated between the planes of the strata by 
 violently separating them. 
 
 The strata in contact are altered by the basalt in several ways, 
 as may be seen about the High Force. The subjacent shales have a 
 prismatic structure so as to be mistaken for basalt, are generally grey 
 or whitened, and rendered brittle by condensation, but not much 
 hardened. The sandstones are in several places highly hardened, 
 rendered brittle and full of fissures, and much whitened. The lime- 
 stones below the shale are remarkable for having their top bed 
 full of iron pyrites ; those above the basalt but not in contact with 
 it are frequently changed from a full blue, hard, rather crinoidal 
 limestone, first into a pale blue, crystallised, soft marble ; and finally 
 
 1 Camb. Phil. Trans. 
 VOL. I. Y 
 
338 CLEVELAND DYKE. 
 
 into a loose, granular, saccharoid rock, in which, nevertheless, some 
 traces of organic remains, such as crinoidal column, remain. But the 
 most remarkable effect is the generation of garnets in the contiguous 
 shale under the basalt of Cronkley Scar ; a case analogous to that 
 described by Professor Henslow l in connection with the dykes of 
 Plas Newydd. 
 
 The igneous rocks themselves are chiefly a fine-grained dark 
 basalt, changing to a coarse-grained dolerite. Contemporaneous 
 veins of very beautiful hypersthenic rock pass through the basalt 
 in several points, and it is traversed by a few productive lead 
 veins. 
 
 The connection of several extensive basaltic dykes with this 
 great "whin sill" has been rather assumed than proved. 
 
 These dykes pass in directions to the east-north-east, east-south- 
 east, and nearly east, and they take straight lines through all sorts 
 of rocks. Their respective breadths, and the quality of the rock in 
 each, are nearly uniform, though in these particulars they differ from 
 one another. 
 
 Cockfield and Armathwaite Dyke. The Cleveland or Cock- 
 field dyke, in particular, ranges for seventy miles through the coal 
 series, where it chars the coal, hardens the sandstones, and whitens 
 the shales ; the lias shales and sandstones of the oolite series 
 are affected like the coal system below. Generally it is a nearly ver- 
 tical dyke, but at Cockfield Fell is subject to oblique expansions 
 of a singular kind. The dyke which passes east-north-east is remark- 
 able for having a small vein of lead ore running by the south-east 
 side of it, and for converting the shales through which it passes 
 to the state of a soft, whitish shale, called "pencil-bed," like those 
 in connection with the whin sill. It does not cut through the 
 magnesian limestone. 
 
 This dyke, well described by Sedgwick, appears first about six miles 
 south of Whitby, near Maybecks on Swerton High Moor. It extends 
 W.JST.W. by Egton Bridge, Amthorpe, Ryehill, and Ay ton, to Mun- 
 thorpe in the Vale of Cleveland. Its thickness increases along this 
 line from 18 feet at Maybecks to 80 feet at Great Ay ton, though the 
 top of the dyke is only 20 feet thick at the latter place. In this 
 distance of 20 miles the dyke is intrusive in Oolite and Lias, and 
 forms a conspicuous feature on the moors east of Selhone. West of 
 Nunthorpe it extends by Stainton and Preston, where it makes an 
 abrupt bend. It reappears after an interval at the village of Bolam, 
 and extends to Cockfield Fell. Near Bolam it is 200 to 300 yards 
 wide; near Cockfield it terminates abruptly beneath the stratified 
 rocks, and in this district it is quite vertical. The altered shales and 
 sandstones in contact extend to a distance of 20 to 30 yards. Though 
 absent from the surface for some distance, it is proved to extend over 
 Woodland Fell to about one mile east of Middleton. 
 
 The dyke of the Eden valley, which extends from Eenwick to 
 Armathwaite, is a portion of the same intrusive mass, and nine 
 
 1 GeoJ. Trans. 
 
 .ex- 
 

 DYKES IN CARBONIFEROUS ROCKS. 339 
 
 posures connect it with Cockfield. This outburst is of post-jurassic 
 age ; Mr. Teall suggests that it may perhaps be Miocene. In chemical 
 composition it closely resembles the augite-andesites of the Continent. l 
 
 The Hetts Dyke. According to Professor Sedgwick this dyke 
 ranges from the escarpment of magnesian limestone at Quarrington 
 Hill, east of Durham, in a W.S.W. direction by Tarsdale, Hetts, Whit- 
 worth, through the collieries of Bitchburn and Hargill Hill, and up 
 the Bedburn Beck valley to Egglestone Moor. The thickness increases 
 westward from 6J feet to 15 feet. At Hetts the dyke leads to the 
 north at a high angle. It bakes and indurates the rocks with which 
 it is in contact. It differs from the Cleveland dyke in wanting por- 
 phyritic crystals of felspar. Two miles to the north of the Hetts 
 dyke is another dyke of the same composition ; and there is a hori- 
 zontal sheet between the two at sixty fathoms below the surface. 
 Sedgwick regarded this dyke as palaeozoic. 
 
 The High-Green Dyke. The High-Green dyke, which crosses 
 the Parret, runs west to east, and is 50 feet thick along the stream. 
 Its central part is coarsely crystalline. On the north wall it is 
 cellular. 
 
 The Acklington Dyke. This dyke stretches from the coast at 
 Bondicar near Acklington, where it is 30 feet thick, through the Car- 
 boniferous rocks and Cheviot porphyrites, and runs for many miles 
 across the South of Scotland. It is well seen near Newton and 
 Chennel, about eighteen miles west of Acklington. 
 
 The Hebburn Dyke, according to Professor Lebour, emerges from 
 beneath the magnesian limestone near Cleadon, passing W.N.W. by 
 Hedworth and Hebburn to the Tyne. The Coley Hill dyke, west of 
 Newcastle, is sometimes supposed to continue the line. This dyke 
 is 44 to 50 feet thick. It cuts no formation newer than the Coal- 
 measures. 
 
 The Tynemouth Dyke is seen at Tynemouth on the shore 
 in contact with the Coal-measures. It is 10 feet wide, and divided 
 into two parts by a quartz vein 6 inches thick. Mr. Teall regards 
 the Coley Hill dyke as a part of the Tynemouth dyke. 
 
 Bnmton Dyke. Professor Lebour traces this dyke 2 from West 
 Allendale over the South Tyne to west of Haydon Bridge ; over the 
 North Tyne, near Wall, to St. Oswald's chapel, near Brunton ; and 
 it is last seen in the Bingfield Burn ; so that its main direction is 
 from N.E to S.W. 
 
 The Seaton and Hartley Dykes. Several dykes run parallel to 
 each other from N.W. to S.E., and are exposed on the shore between 
 Seaton and Hartley. The dyke near Seaton, according to Mr. Teall, 
 is 10 feet thick at the bottom of the quarry, 5 feet thick at the top. 
 Another of the dykes is occupied by a rubbly mass in the centre. 
 
 The Morpeth Dyke is only seen crossing the Wasbeck, near 
 Morpeth. 
 
 These are a few examples of dykes discussed by Mr. Teall, but 
 they indicate some of the general features of such phenomena. 
 
 1 Teall, Q. J. G. S., vol. xl. 2 "Geology of Northumberland." 
 
340 SUPPOSED BREAKS IN SUCCESSION. 
 
 Breaks in Succession of the British Strata. 
 
 There are two kinds of breaks in succession : first, the physical 
 interruption in stratification, which is marked by unconformity ; 
 secondly, the palseontological break, which consists in a change in 
 fossils without any necessary variation in the rock sequence. The 
 latter condition can only claim to be a break in succession on the 
 hypothesis that the change in life has been brought about by upheaval 
 in some adjacent sea, which has caused the life to migrate away from 
 the upheaved region, and thus has disturbed the succession of life, in 
 an adjacent area which we may be examining, by causing an immigra- 
 tion which has changed its fauna. 
 
 Breaks in the Primary Eocks. The nature of the break which 
 divides the Harlech and Bangor groups of Lower Cambrian rocks from 
 the unfossiliferous Pre-Cambrian is essentially a palasontological break 
 due to the absence of fossils from the lower rocks, though the older 
 beds are more highly volcanic. The fauna of these Lower Cambrian 
 rocks includes 18 genera and 32 species, of which one-third pass 
 into the Menevian. The Menevian beds again are defined in the 
 St. David's area by a palaeontological break. The fauna comprises 
 24 genera and 52 species, of which 19 pass into the Lower Lingula 
 flags. The Lower Lingula flags is a palseontological group with 17 
 genera and 36 species, chiefly Crustacea, of which 2 pass into the 
 Middle Lingula flags and 8 into the Upper Lingula flags. The Upper 
 Lingula flags contain 16 genera and 41 species, of which 10 pass into 
 the Lower Tremadoc and 7 into the Upper Tremadoc. Still there is 
 no visible unconformity in the series, and therefore to the physical 
 geologist they are essentially one group, which suggests many changes 
 in the relations of land and water in adjacent areas, but shows that 
 no new axis of upheaval was developed in the British region. 
 
 The Arenig rocks rest upon the Tremadoc. The deposit contains 
 96 species. Forty Graptolites here make their appearance for the 
 first time ; and of the 3 1 Crustacea only 6 live on from the Tremadoc, 
 and but 3 pass up to the Llandeilo. Of the 149 species in the bed 
 all but 38 are peculiar to it. 
 
 The Llandeilo beds contain 80 genera and 175 species; and 38 
 genera and 73 species pass up from them to the Bala beds. The 
 fauna of the Bala group admits of subdivision into three. The 
 Middle Bala contains 610 species, of which only 102 pass up to the 
 Lower Llandovery. The Lower Llandovery beds, though easily dis- 
 tinguished from the underlying beds by their sandy character, show 
 no unconformity in Central Wales, but near Llandovery, Sir Andrew 
 Ramsay and Mr. Aveline recognise a slight unconformity at Noeth- 
 Grug. These beds contain 204 species, of which 104 pass up. 
 
 Between the Lower and Upper Llandovery there is every- 
 where a perfect unconformity ; and Sir A. Ramsay remarks that the 
 newer beds rest on the denuded edges of the Lower Llandovery, 
 sometimes on the Caradoc sandstone of the Bala series, and at Builth 
 
 
UNCONFORMITY INFERRED FROM FOSSILS. 341 
 
 and the Longmynd, on contorted and denuded Llandeilo and Cambrian 
 rocks. The Upper Llandovery contains 91 genera and 261 species, of 
 which 59 genera and 126 species pass up to the Wenlock. There is 
 no clear and satisfactory unconformity of a visible kind here, though 
 the Upper beds of the Llandovery series are sandstones and conglome- 
 rates indicative of shallower water conditions than those of the typical 
 succeeding Wenlock beds. The Wenlock contains 536 species of 
 fossils, of which 126 pass into the Lower Ludlow. The Ludlow 
 fauna includes 137 genera and 392 species, and the beds have 129 
 species in common. 
 
 There is a complete break in life between the Silurian and over- 
 lying Devonian series, but there is no sign of unconformity or a break 
 of any kind between the Ludlow rocks and the Old Red Sandstone. 
 Yet only 20 species pass up into the Devonian, which is about one- 
 thirteenth of the species in the Upper Ludlow rocks, and less than 
 one-third of the fauna of the passage-beds. The Devonian rocks con- 
 tain 195 genera and 544 species, of which 32 genera and 51 species 
 pass up to the Carboniferous ; but Sir Andrew Ramsay observes that 
 round the Forest of Dean and the South Wales coal-field there is no 
 sign of unconformity between the Old Red Sandstone and the Car- 
 boniferous series. Yet, although conformable to the strata above and 
 below, the Old Red Sandstone includes distinct unconformities in 
 Scotland. 
 
 The Carboniferous series is generally conformable from top to 
 bottom, but the beds exhibit many oscillations of level. In the 
 Forest of Dean, the Millstone Grit rests unconformably on the moun- 
 tain limestone, but there is nowhere a gap that would correspond to 
 the change in life. The fauna comprises no less than 515 genera and 
 2409 species, of which only 51 species are derived from the Devonian, 
 and only 8 species pass up into the Permian. 
 
 The Permian rocks show a marked unconformity resting on all 
 the Primary strata. ,The fauna is poor, and, so far as Brachiopoda are 
 concerned, is largely composed of species which live on from the 
 Carboniferous. Thus there appears to be a more marked physical 
 distinction between these strata than suggests itself on an actual 
 comparison of Permian with Carboniferous fossils ; and we find that 
 the physical break is not coupled with a pala3ontological break. 
 
 Breaks in the Secondary Strata. We now come to the great 
 stratigraphical break which marks the commencement of the New 
 Red Sandstone ; and nowhere is there an actual passage downwards 
 into the Permian. The Triassic rocks are very imperfectly represented 
 in this country, and the series includes some unconformities, since 
 near Ormskirk the New Red Marl lies unconformably on the New 
 Red Sandstone. Only one Trias species appears to range into the 
 Lias. There is no appearance of unconformity between the New Red 
 Marl and the Rhsetic beds, although the latter are a marine series. 
 
 The Lias is so closely connected with the Rhsetic beds, that the 
 separation between them has only been made during the last twenty 
 years, and there is no visible unconformity in the Lias ; but the per- 
 
342 PHYSICAL AND PAL&ONTOLOGICAL BREAKS. 
 
 centages of species which pass between the divisions of the Lias are 
 remarkable. Thus about one-third of the species pass from the top 
 zone of the Lower Lias into the Middle Lias ; and from the top of 
 the Middle Lias only about 5 per cent, pass into the Upper Lias ; 
 while from the Upper Lias 27 per cent, pass into the Upper Lias 
 sands, and 2 1 per cent, into the Inferior Oolite. One hundred species 
 are recorded from the Rhsetic beds, 281 genera and 1830 species 
 occur in the Lias, of which 1080 are in the Lower Lias, 562 in the 
 Middle Lias, and 418 in the Upper Lias. Forty-five species survive 
 to the Inferior Oolite, and 1 1 to the Great Oolite ; about half in both 
 cases being bivalves. 
 
 There is no complete physical break between the Inferior Oolite 
 and the Lias, though the mineral character of the bed alters, and 
 between Yorkshire and Gloucestershire this might be considered to 
 amount to an unconformity. The Inferior Oolite contains 1000 
 species, of which 65 extend into the Fuller's Earth, and 175 into 
 the Great Oolite. The Fuller's Earth thins out entirely to the N.E. 
 of Cheltenham. It is only known to contain no species, of which 
 65 are common to the Inferior Oolite, and the same number range up 
 to the Great Oolite. 
 
 The Great Oolite contains 820 species, of which 84 range to the 
 Forest Marble and 120 to the Cornbrash. The Forest Marble con- 
 tains 136 species, of which 48 range up to the Cornbrash. Between 
 Yorkshire and Dorsetshire all these lower oolites may be regarded 
 as unconformable to each other, though no actual unconformity is 
 seen. 
 
 The Cornbrash may be regarded as forming a break with the un- 
 derlying strata, since it is the only deposit of the Lower Oolites which 
 ranges through England from Dorsetshire to Yorkshire. It contains 
 244 species, of which 56 range up into the Kellaway Rock, and 48 
 range down to the Forest Marble. The Kellaway Rock has 168 
 species, of which 60 pass into the Oxford Clay. The Oxford Clay 
 contains 73 genera and 194 species, of which 48 pass up into the 
 Coralline Oolite and 25 into the Kimmeridge clay. Still there is no vis- 
 ible physical break in the succession in the British area. The Corallian 
 fauna, which abounds in lamellibranchs, gasteropods, echinoderms, and 
 ammonites, comprises 116 genera and 334 species, of which only 14 
 are corals. The Kimmeridge Clay only receives 33 species from the 
 Coralline Oolite, and of the 263 species found in the bed, only 22 
 survive into the Portland Oolite. The Portland Oolite, greatly 
 limited in its area, has a fauna of 128 species, of which none survive 
 to newer rocks. 
 
 The Purbeck beds pass so insensibly into the underlying Portland, 
 that the difference is only to be detected by the fossils ; but since the 
 Portland beds are marine, and the Purbeck largely fresh water, an un- 
 conformity must exist. Similarly an unconformity must be inferred 
 for the overlying Wealden beds, the distribution of which is different 
 from the Purbeck beds ; but there is no trace of an unconformity 
 between the Wealden and the Lower Greensand till we pass west and 
 
UNCONFORMITY AND CHANGE IN LIFE. 343 
 
 find the Greensand of Dorset and Devonshire resting unconformably 
 on the oolites and older rocks. 
 
 About 1 8 per cent, of the Lower Greensand species pass up into 
 the Gault, but 40 per cent, of the Gault species pass into the Upper 
 Greensand. The Upper Greensand has 20 per cent, passing into the 
 Chalk Marl. The Chalk Marl has 59 per cent, passing into the 
 Lower Chalk, and the Lower Chalk has 3 1 per cent, passing into the 
 Upper Chalk. It is doubtful if any species really survive from the 
 Chalk into the Tertiaries. 
 
 Breaks in the Tertiary Strata. In this country the physical 
 break between the Secondary and Tertiary strata is of the largest 
 kind, and presumably consisted in an upheaval of the sea-bed, so that 
 what had previously been an area covered with organic deposits 
 free from sediments was succeeded, after a certain amount of denuda- 
 tion, by sediments derived from crystalline rocks. But though the 
 Tertiary rocks repeat in their subdivisions differences in life as re- 
 markable as those which have been mentioned already in the succes- 
 sive beds of the Primary and Secondary series, and further contain 
 conglomerates, fresh-water deposits, and several zones rich in land 
 vegetation, indicating oscillations of level, there are no visible physical 
 breaks till after the close of the Hempstead beds, with which the 
 Lower Tertiaries terminate. Whether any newer deposits were 
 ever accumulated in the British area it is now impossible to determine, 
 but the land may well have been upheaved and dry during the 
 Middle Tertiary or Miocene times. It is certain that an immense 
 physical break is indicated by the interval between the Lower Ter- 
 tiaries and the Upper Tertiary Crags, which exhibit an absolute un- 
 conformity with the strata on which they rest. The Bed Crag rests 
 unconformably on the Coralline Crag, and the Boulder Clay rests un- 
 conformably on the Red Crag. 
 
 Thus the number of physical breaks in the British area is inade- 
 quate to account for the breaks in the succession of life. There is no 
 evidence of denudation which would warrant an explanation of the 
 breaks in life by assuming missing deposits ; and we are hence com- 
 pelled to attribute the seeming breaks, in the main, to disturbances 
 in the relations of land and water in adjacent seas which affected 
 the distribution of life in the region which is now Britain, and at the 
 same time varied the mineral character of the sediments in which the 
 fossils became preserved. 
 
 Table of Disturbance in the British Area. 
 
 The following table prepared by Prof. John Phillips shows the 
 geological periods of many remarkable convulsions in Great Britain, 
 and the places where some of the most considerable effects are 
 manifested : 
 
344 
 
 CHIEF BRITISH CONVULSIONS. 
 
 Geological Period of the 
 - Convulsions. 
 
 During the deposi- 
 tion of the Harlech 
 and Bangor grits . 
 
 . After the Silurian) 
 strata and before' 
 the Carboniferous f 
 system . . .) 
 
 During the Carboni- ) 
 ferous period . \ 
 IL Before the adjacent ( 
 rocks of the Per- 1 
 niian system taken j 
 generally . . ( 
 
 During the Permian ) 
 period . . . ) 
 
 III. After the earlier I 
 Permian period ? . \ 
 
 After the later Per- ) 
 mian period . . \ 
 
 During the Oolitic I 
 period ? . , \ 
 
 IV. After the Oolitic 
 period . 
 
 During the Chalk 
 
 period ? 
 After the Chalk 
 
 period . 
 
 V. After the Eocene 
 deposits 
 
 Effects Noted. 
 
 Production of Conglo- \ 
 merates . . . j 
 
 Porphyry and dolerite J 
 and Trappean Con- > 
 glomerates . . ) 
 
 Disturbed position of I 
 Primary rocks ; vol- < 
 canic outbursts . I 
 
 Production of Old Red ) 
 Conglomerates . C 
 
 Marine bed among es- ) 
 tuary deposits . ^ 
 
 Numerous dislocations,^ 
 fissures of dykes and f 
 veins, anticlinal axes, f 
 &c. . . .) 
 
 Production of Conglo- ) 
 merates . , . 
 
 Veins of lead,&c., Great 1 
 or 9O-fathom dyke . \ 
 
 Production of New ) 
 Red Conglomerates \ 
 
 Unconformity. Kello- \ 
 way rocks in contact I 
 with the Lower Oo- \ 
 lite group, excluding j 
 the upper portion . / 
 
 Unconformity of strata ) 
 between Oolitic and > 
 Chalk systems . \ 
 
 Estuary deposits. Peb- 
 ble beds of Lower 
 Greensand 
 
 Pebble beds, wasted 
 surface of chalk 
 
 Vertical strata . 
 
 Marine deposits be- 
 tween lacustrine beds \ 
 The crag . 
 
 Localities of some of the 
 Phenomena. 
 
 Derwent Water Cum- 
 berland, North Wales. 
 
 Grasmere in Westmore- 
 land, Radnorshire, Me- 
 rioneth, &c. 
 
 The Grampians, Lam- 
 mermuirs, Cumbrian 
 Mountains, North 
 Wales. 
 
 The Highland Border, 
 Cumbria, &c. 
 
 Yorkshire. 
 
 In all Coal Districts of 
 this era, both in Europe 
 and America. Charn- 
 wood, Crossfell fault. 
 
 North of England, north 
 of Germany. 
 
 Yorkshire, Pontefract, 
 Mendip hills, Tyne- 
 mouth castle, border of 
 Cumbrian group (Kirk- 
 by Stephen). 
 
 North of England. 
 
 Cave, Yorkshire. 
 
 Yorkshire wolds, Dorset- 
 shire cliffs. 
 
 Weald of Kent and Sus- 
 sex, Isle of Wight, &c. 
 
 Hertfordshire, Vale of 
 
 Thames. 
 Isle of Wight, Isle of 
 
 Purbeck. 
 
 Isle of Wight. 
 Essex and Norfolk. 
 
 The Roman numerals are applied in the above list to all periods 
 where considerable movements are traced in direct effects of disloca- 
 tion and conformity. 
 
 The next table presents the results of a more extended survey of 
 direct convulsive effects on the continent and islands of Europe, as 
 they appeared to E. de Beaumont on the first proposal of his ingeni- 
 ous views of subterranean movement : 
 
GREAT EUROPEAN CONVULSIONS. 
 
 345 
 
 Table of Disturbance in the European Area. 
 
 No. 
 
 Geological Period of the 
 Convulsions. 
 
 I. (i and 2, E. deB.)) 
 Before the Old [ 
 Bed Sandstone . ) 
 II. (3, 4, 3, K de B.) 
 a. Before the Ro- 
 thetodteliegende . 
 
 6. Before the Zech- 
 stein . 
 
 c. Before the New ) 
 Red Sandstone . \ 
 
 III. (6, E. deB.) Be- 
 fore the Lias 
 
 IV. (7, E. de B.) Be- j 
 fore the Lower > 
 Greensand . . ) 
 
 V. (8, E.deB.) Be- j 
 fore the uppermost > 
 Chalk beds . . ) 
 
 VI. (9, E.deB.) Be- 
 fore all the Ter- 
 tiary rocks . 
 VII. (10, E. de B.) 
 Before the Nagel- 
 flue . 
 
 VIII. (11, E. de B.) 
 Before some Gla- 
 cial beds 
 
 IX. (12, E. de B.) 
 During the forma- 
 tion of other Pre- 
 Glacial beds 
 
 Effects Noted. 
 
 Anticlinal axes and 
 
 great faults of the 
 
 Slate system . 
 Immense disruptions 
 
 and faults of the Coal 
 
 system . 
 Immense dislocations 
 
 and faults of Coal 
 
 strata 
 Immense dislocations 
 
 and faults 
 Mountain ridges of 
 
 Zechstein, &c. 
 Abrupt and distorted 
 
 strata of Oolitic sys- 
 tem 
 Abrupt elevations of 
 
 Greensand and 
 
 Lower Chalk . 
 
 Elevations of Chalk / 
 and Greensand . I 
 
 Detached ridges . . -| 
 
 Newest Tertiaries up- 
 lifted 
 
 Some Diluvial beds 
 covulsed . 
 
 Localities of some of the 
 Phenomena. 
 
 The Hunsdriick 
 Taunus. 
 
 and 
 
 Calvados, south-west bor- 
 der of the Vosges. 
 
 Westphalia, Belgium. 
 
 Vosges, and Black 
 
 Forest. 
 Thuringerwald and Boh- 
 
 merwald. 
 Mont Pilat, Cevennes 
 
 (perhaps the Erzge- 
 
 birge). 
 
 Mont Viso, Devolny. 
 
 Pyrenees, Northern Ap- 
 ennines, the Morea. 
 
 Corsica, Sardinia, Au- 
 vergne. 
 
 The range of the Wes- 
 tern Alps, Diablerets, 
 Mont Blanc. 
 
 The range of the Eastern 
 Alps from the Valais 
 to Austria. 
 
 Fig. 71. E. de Beaumont's System of Elevations. 
 
 1. Snowdon. 
 
 2. Ballons, Bocage. 
 
 3. Crossfell. 
 
 4. Pays Bas. 
 
 5. Rhine. 
 
 6. S.W. of Brittany. 
 
 7. M. Pilas, &c. 
 
 8. M. Viso. 
 
 9. Pyrenaeo-Apennine. 
 
 10. Corsica. 
 
 11. W.Alps. 
 
 12. Alps. 
 
346 
 
 TEACHINGS OF ELIE DE BEAUMONT. 
 
 Elie de Beaumont's Generalisations. The following is De 
 Beaumont's view of his first five systems, including applications in 
 Great Britain for comparison with the details of Professor Phillips' 
 groups I. and II. : 
 
 Geological Period of the 
 Convulsions. 
 
 1. During the de-\ 
 position of the I 
 lower Palaeozoic V 
 strata, anterior to I 
 upper Silurians .) 
 
 2. Posterior to the 
 upper Silurians, 
 anterior to Old 
 Red Sandstone . 
 
 }$. After the CoaH 
 I strata and before [ 
 Rothetodtelie-f 
 gende . . ) 
 1 4 After the Coal i 
 II. / strata, and before > 
 the Zechstein . ) 
 
 5. After the Coal] 
 strata, and before ( 
 the Biinter Sand- { 
 stein . . .1 
 
 Effects Noted. 
 
 Elevation of mountain 
 chains 
 
 Great faults, and anti- 
 clinals 
 
 Immense disruptions { 
 and faults of the coal. C 
 
 Ditto . . ( 
 
 Great disruptions 
 
 Localities of some of the 
 Phenomena. 
 
 Snowdon, Anglesea. 
 
 Grampians, Lammer- 
 muirs, Cumbrians. 
 
 From Derbyshire to 
 Northumberland along 
 the Western border of 
 Yorkshire, Malvern. 
 
 Westphalia, Belgium, 
 Mendip, South Wales. 
 
 Vosges and Black Forest, 
 from Basle to Mayence, 
 Faults in magnesian 
 limestone of northern 
 counties. 
 
 Elie de Beaumont's Hypothesis. It has long been known in 
 mining countries that the faults take parallel directions, and some- 
 times two or more systems of dislocations, crossing in certain angles, 
 were found to be of different antiquity. That dislocations were in 
 some respects to be compared to the effects of earthquakes was also 
 well understood, but no one before De Beaumont appears to have 
 carried his notions of the coincidence between the lines of convulsion 
 and the direction of the great physical features of the globe so far as 
 to venture on the construction of a general system. This excellent 
 geologist believed that there is a constant dependence between the 
 direction of the dislocation and the geological epoch of its occurrence, 
 such that all the dislocations of the same age are parallel to one and 
 the same great circle of the sphere ; and that, in most instances, dis- 
 locations of different ages are parallel to different great circles, which 
 intersect one another at assignable angles owing to the shrinkage 
 and parallel crumpling of the earth's crust. This general hypothesis 
 is not to be tested by single or small dislocations. It must be 
 examined on a great scale, by means of very exact and numerous 
 data. The facts known are not clear and numerous enough to de- 
 monstrate this hypothesis ; and on the other hand there are not facts 
 to warrant the unconditional rejection of it. It is certainly founded 
 on. important data, and in several instances agrees well with observa- 
 tion. The principal difficulty of applying satisfactory tests to its 
 application, arises from the uncertainty of the exact date of many of 
 the most characteristic convulsions. We cannot positively tell 
 
SEQUENCE OF EARTH-MOVEMENT IN BRITAIN. 347 
 
 whether the dislocations of the Grampians and Lammermuirs, which 
 take parallel courses, were geologically synchronous or not, because 
 the beds dislocated are not the same. Even in the case of the great 
 faults which followed upon the Carboniferous system, the limits of the 
 geological epochs of their occurrence are often too vague for the ap- 
 plication of such a theory. Eothetodteliegende and magnesian lime- 
 stone cover the coal of the North of England unconformably, and thus 
 define the date of the convulsions. But in the South of England 
 these rocks are of rarer and less regular occurrence, and often entirely 
 wanting, and then the New Red Sandstone above the coal gives only 
 a vague approximation to geological time. 
 
 Three great Groups of Earth-Movement in Britain. The sub- 
 joined diagram (fig. 72) is intended to show the directions of three 
 great movements of strata in Britain 
 which appear to be grouped in trace- 
 able systems. 
 
 The earliest is that N.E. and .2 
 
 S.W. system which includes Snow- \ 
 
 donia and a large tract about it. v* v? 
 
 By this the Cambrian strata, as \\ \ 
 
 understood by Sedgwick (including ^'s'/ 
 the Lower Silurian of Murchison), ^tr \ 
 
 have been much disturbed in North ^ , \ 
 
 Wales, so that unconformity ap- 
 pears between them and the lower 
 part of the Upper Silurians. 
 
 Another great system of move- 
 ment is typified in the North of 
 England by the great faults and 
 anticlinals of the Pennine chain, 
 varying from N.N.W. to S.S.E. 
 
 Nearly parallel to .this are the 
 
 dales of the Nith and the Annan, 22- 
 
 the Dee and the Clwydd. These Fig. 72. 
 
 dislocations precede the Whole Threc Sy8tems O f Subterranean Movement in 
 System. Britain. 
 
 A third series of parallel or * Clyde. 8. Dee. 15. s. Wales. 
 
 ,, , . , 2- Eden. 9. Clwydd 16. N. Devon. 
 
 nearly parallel movements affects 3 . nibble. 10. Menai. I7 . Mendip. 
 the south of Ireland, South Wales, <; Sf " $ J Icre . 
 
 and the South Of England. In 6. Nith. 13. Bala. 20. Surrey. 
 
 South Wales, the Mendip Hills, 7 " Ken " 22 I4 'i s Kf Wight!" S ' 
 and North Devon, it disturbs all 
 
 the strata earlier than Permian ; and in the Isles of Purbeck and 
 Wight, and the Weald, it disturbs all the Eocene strata. This 
 appears a case of nearly the same direction, and nearly the same kind 
 of movement (anticlinals and synclinals), affecting a given district in 
 different geological times. The earlier movement was continued both 
 eastward and westward, so as to embrace a length of fully 700 miles 
 of the earth's surface from Bantry Bay to Elberfeld. The later 
 
348 FOCUS OF ELEVATION. 
 
 movement affected a large breadth of country from the Isle of Wight 
 to beyond the Thames valley, and is parallel to the upheaval of the 
 Alps, and the axis of elevation of central Europe of which it is a 
 consequence. 
 
 Elevation the Consequence of Convulsion. That the effect of 
 convulsions has been, generally, to raise the convulsed area, will 
 appear evident by considering what is the focus of the disturbance 
 and the direction of its energy. The mountain chains and groups are 
 most certainly the foci of the disturbing forces ; for as we pass to- 
 wards them, from all sides the number and intensity of the dislocations 
 continually increase, and the inclination and contortion of the strata 
 grow more and more violent. The direction of the disturbing force 
 is clearly seen to be horizontal, while its effects are vertical or nearly 
 so, and thrust the folded masses outwards from the central regions of 
 the earth. It is like an "expansive force, which employed its principal 
 efforts along certain lines and about certain centres, breaking and 
 bending the strata in the highest degree, but also lifting them up 011 
 all sides around. Although the Mediterranean lies between the Atlas 
 chain and the Alps, the elevation of mountain chains and groups was 
 generally unaccompanied by any neighbouring violent depressions, 
 because the upheaval is only a part of a predominant upward bending 
 of the earthy crust. The inclination of the strata from mountain 
 chains for the most part gradually subsides to a gentle slope, and 
 finally vanishes in nearly horizontal planes. In the mountain chain 
 itself various and suddenly reversed dips may be met with corre- 
 sponding to the violence of the disruption, but by careful study the 
 general tendency of the convulsion may be clearly deciphered. 
 
 The same data will not, however, by any means give us right to 
 conclude that the mountains so brought into existence were raised 
 above the sea, because, though we may know the absolute height of 
 the vertical movement, this will avail us nothing in our ignorance of 
 the original depth of the water. We must see whether mountains 
 bear on any part of their surface traces of those later marine deposits 
 which spread around their bases ; if they do, we may be sure they 
 were not elevated above the sea till after the date of these strata ; and 
 the Alps, for instance, bear upon their crests portions of oolitic, 
 cretaceous, and tertiary strata, and are thus proved to be of more 
 recent elevation than the geological age of the strata uplifted. If the 
 newer marine strata around their bases have been deposited horizontally 
 against the slopes of the mountains, we are entitled to believe that 
 the mountains had been previously reared above the sea. This con- 
 clusion, however, it must be always borne in mind, does not inform 
 us correctly concerning the height they were reared above the sea, 
 but leaves us to infer that they have since partaken of another move- 
 ment by which the newer strata have been placed at their present 
 elevation. 
 
COLLATERAL EFFECTS OF UPHEAVAL. 349 
 
 Collateral Effects of Upheaval and Depression. 
 
 Relation of Lines of Upheaval to Magnetic Intensity. M. 
 
 Necker, in a communication to the Societe dHistoire NaturelU de 
 Geneve, traced a very unexpected coincidence over large portions of 
 the northern hemisphere, of the direction of the strata, and the curves 
 of equal magnetic intensity, as drawn by General Sabine. One of 
 these curves, that of 297 seconds, traverses Scotland in a direction 
 north-east and south-west, which is exactly that of the strata. It 
 keeps the same direction by Christiania in Norway, where the strata 
 trends north-east and south-west, and passes through Sweden where 
 the same direction of strata predominates. On arriving at the Gulf 
 of Bothnia the magnetic curve turns north-west and south-east, which, 
 according to Strangways, is the direction of the southern border of the 
 Swedish and Russian granite. 
 
 The curve of 308 seconds enters Europe by Lisbon, and passes 
 south-west and north-east through the Spanish peninsula, which is 
 nearly the line of most of the long sierras between the great rivers ; 
 it passes by the Cevennes, and goes parallel to the Alps in their north- 
 east course to the Tyrol, but there turns south-east, as do also the lines 
 of stratification through Carniola, Istria, Croatia, Dalmatia, and the 
 Morea. Parallel to these are the Carpathian mountains. The same 
 correspondence between the magnetic curve and the lines of folding 
 of strata is traced through the Crimea and along the Caucasus. 
 
 In North America the magnetic curve and the stratification range 
 north-east and south-west along the whole eastern coast ; in the Rocky 
 Mountains both extend from north north-west to south south-east ; in 
 Mexico the magnetic curve takes the parallel of the Cordillera of 
 Anahuac north-west and south-east, and ranges along the south coast 
 of New Spain. Farther to the south the curves resume their course 
 north-east and south-west, which, according to Humboldt, is the direc- 
 tion of the strata in* Venezuela, and between the Orinoco and the 
 Amazons. The chain of the Himalaya, which in Nepaul bears north- 
 west and south-east, and turns north-east at the north-east extremity 
 of Bengal, is parallel to the curve of 297 seconds which was first 
 noticed. Whether the thermal conductivity of strata governs their 
 magnetic intensity, or whether alternation of mineral character governs 
 inductive electrical action of strata on each other, or whether the 
 compressions and tensions of contortion have modified magnetic char- 
 acters of strata, are problems not undeserving the attention of the 
 physical geologist, but too special for examination here. 
 
 Possible Changes in Ocean Level from Depression of Land. 
 The variability of the ocean level in consequence of displacements of 
 the solid land may be stated under three hypothesis : 
 
 First, We may suppose no vacuum to exist below the crust of 
 the earth, nor any receptacle into which the solid land could sink, but 
 that a sinking in one place should be compensated by a rising in 
 another, so that the cubic dimensions of the globe remain unchanged. 
 
350 STABILITY OF OCEAN LEVEL. 
 
 Moreover, to put an extreme case, it may be a condition that the land 
 shall sink so that water shall cover the whole surface. In this case 
 the level of the ocean would rise, that is, the mean radius of its 
 curved surface would be lengthened, by a quantity depending on the 
 mass of the solid land submerged, and on the relative area of land and 
 water. This relation of area is more than 3 water to i land. The 
 cubic content of the solid land may be thus estimated. In England, 
 Wales, and Scotland, the average height of those conspicuous moun- 
 tain masses which appear to give shape to the whole country is about 
 30,000 feet; and if we consider this as the apex of a cone whose base 
 is the whole area, we shall have the mean height of the land above 
 
 3000 
 the sea = feet. The mountain masses, however, do not really 
 
 affect, by their special elevation, more than a fraction of the area of 
 the British Isles ; the far greater part of the land depends on heights 
 not exceeding 1000 feet. If the mountain tracts be called half of the 
 area, and the hilly and more depressed parts half, we shall find the 
 
 mean height of the whole mass of land I - ) = 666 feet. 
 
 But on account of the valleys which divide the principal masses, we 
 may reduce this to say 500 feet. 
 
 This principle applied to the continents of Asia and America would 
 give in round numbers about 2000 feet mean altitude of land; and as 
 the area of the expanded ocean would be four times as great as the 
 land is now, the total mean elevation of the water, by the submersion 
 of the whole mass of land, would be about 500 feet ; a quantity too 
 small to be of use in explaining any but the lesser order of geological 
 phenomena, and which may be considered as the extreme limit of 
 oceanic rise under these conditions. 
 
 Secondly, We may suppose the existence of cavities into which 
 the solid land might sink, so that there may be no elevation in another 
 place corresponding to the given depression. To put this also to 
 extreme, we may imagine the very improbable case that a mass of 
 solid materials, equal in bulk to all the solid land above the water, 
 should sink into a cavity, and that the surface of the submerged land 
 should be level. The level of the ocean would be nearly unaltered, 
 except in a small degree, by reason of its shallow expansion over the 
 area of the land. We might go on to suppose even the enormously 
 improbable case of cavities existing so large as to admit twice the 
 whole solid mass of the continents, and that these should sink with 
 an equal bulk of materials into these cavities. Even in this case the 
 ocean level would only be lowered 500 feet. 
 
 TJiirdly, If we suppose contemporaneous or successive elevations 
 and depressions, however extensive, the ocean level would oscillate 
 about a constant line. 
 
 It is evident, therefore, that by no stretch of conjecture, that is 
 not absolutely absurd, can we torture the known laws of terrestrial 
 arrangements into agreement with the hypothesis of any but small 
 changes of level of the ocean ; a conclusion which enables us to 
 
STRUCTURE OF MOUNTAINS. 351 
 
 argue upon that level as a general standard to which we may refer all 
 the effects of internal movements, in whatever period, and by what- 
 ever forces produced. It fixes no limits to the effects of the temporary 
 violence induced in the ocean by such movements, because these effects 
 would be in proportion to the impulse with which they originate. 
 
 Mountain Ranges. 
 
 Study of Mountains. Long chains and insulated groups of moun- 
 tains form, so to speak, the skeleton of the earth, and are the funda- 
 mental features of its topography ; their insulated groups characterise 
 kingdoms, their long connected chains divide the races of mankind, 
 and define the geographical limits of the distribution of land animals. 
 The principal ranges of mountains everywhere contain in their axes 
 similar rocks, which are often the lowest, and among the oldest, 
 with which we are acquainted. By lateral contraction they have been 
 lifted to their present heights, so as to break through and rise by denu- 
 dation from beneath the strata which were superimposed upon them 
 in succession. 
 
 These mountain-forming materials comprise gneiss, mica schist, 
 slate, and many associated rocks, resting upon and often pierced by 
 granite and similar crytallised compounds. 
 
 Though we speak of long-continued chains and belts of mountains, 
 it is certain that to be crossed in groups is the real character of moun- 
 tain association, and that the chains and belts are nothing but ap- 
 proximated groups. A geological map is in this respect a most valu- 
 able instructor; from it we see that, instead of the plains being 
 commonly insulated among the mountains, the newer strata spread 
 wide, and round the bases of the mountains, as the ocean encircles 
 islands and continents. We may observe that the most insulated 
 and many of the loftiest eminences on the surface of the earth are 
 volcanic summits. The most connected ranges of uniformly high 
 ground are formed by limestones. 
 
 Elie de Beaumont supposed that all ranges of mountains which 
 were uplifted at the same period are parallel to the same great circle 
 on the sphere. Parallel ranges are an effect that lateral pressure would 
 produce. 
 
 If a great circle be conceived to pass round the earth through 
 Xatches and the mouth of the Persian Gulf, and the directions of 
 mountain chains be compared with it, it will appear that the Pyre- 
 nees, part of the Apennines, the Dalmatian and Croatian ranges, and 
 part of the Carpathians, are parallel to it. In accordance with the 
 researches of some geologists, M. de Beaumont supposed that all these 
 mountain chains were thrown up at the same geological epoch. 
 
 Another circle may be traced on the sphere parallel to the Alps, 
 from the Valais to Styria, and to this system we may refer the Atlas, 
 the Caucasus, the Balkan, the Himalaya, &c. ; and, according to the 
 hypothesis of M. de Beaumont, these must have been all raised since 
 
352 ORIGIN OF MOUNTAIN CHAINS. 
 
 the deposit of the tertiary strata. This is the effect that lateral con- 
 traction should produce. 
 
 Lateral Displacement as the Sole Cause of the Formation of 
 Mountain Chains. Professor Heim states that a contraction of only 
 Y^yth of the earth's circumference would be sufficient to fold all the 
 rocks in the mountain masses which would be crossed by a meridian 
 traversing the Alps. Even the central crystalline mass of the Alps 
 has undergone enormous lateral compression, and is now reduced by 
 crumpling to half the width to which it once extended. These 
 phenomena of compression have been especially studied in the moun- 
 tain mass of the Toedi, which is an enormous block of Jurassic lime- 
 stone in the Orisons, separated by prodigious denudation from the sur- 
 rounding masses. In this neighbourhood many of the folds are highly 
 complicated. One great contortion, bent over towards the north, 
 piles upon the Nummulitic rocks, the Jurassic rocks, the Verrucano, 
 which is partly of Carboniferous age, the porphyry of "Windgselle, and 
 gneiss. In the direction of the Windgselle mountains, this fold breaks 
 up into a number of minor folds ; and at the southern border of the 
 central mass, the chain of the Piz Tumbrif is formed from a fold 
 which has itself been folded, by which the middle zone forms greatly 
 compressed synclinals. 
 
 Mountains of the European Basin. 
 
 The Ural Chain forms the western limit of the European basin. 
 It is a water-parting, which must have come into existence as a long 
 island, whose eastern slope the older geographers separated from the 
 western slope, as though they were distinct parts of the world. This 
 chain is one of the older features in geological history, though certainly 
 newer than the western contour of the continent. Broken as that 
 contour is by the inlets of the North Sea and the Bay of Biscay and 
 the isolation of Britain, it needs but the help of a geological map of 
 Europe, and a map of the hundred-fathom soundings, to recognise 
 that the Scandinavian chain, now ending with the peninsula, strikes 
 away south-west to Shetland, the western Highlands of Scotland, and 
 north of Ireland, and is prolonged further south beneath the sea, so as 
 to have outlined the eastern side of the great Atlantic Valley, or to 
 have formed a ridge parallel to the Dolphin ridge, before the Atlantic 
 was denned by land. 
 
 Old ridges, generally rising to the west of Britain, but sometimes 
 elevated also to the east, come again and again under notice of the 
 British physical geologist, as parent masses from which the sedimen- 
 tary materials of primary and secondary rocks were derived, and vary- 
 ing elevation of which governed the succession of British strata. 
 
 But the European basin in its present form is essentially a product 
 of forces which began to operate with the Tertiary period. This eleva- 
 tion runs eastward through Europe and Asia, and links the south 
 of Europe with the north of Africa, in a way of which the Mediter- 
 ranean, at first sight, gives no indication. Grand contractions of the 
 
PARALLEL MOUNTAIN CHAINS. 353 
 
 earth's crust, travelling from north to south, have crumpled up the 
 rock masses into parallel ridges running east to west, the slopes of 
 which form the great Eurasian and North African continent ; for the 
 north of Africa, like Europe, has its chief extension from east to west. 
 
 In endeavouring to understand the conformation of Europe, we 
 may begin by noticing the east to west extension of the Cantabrian 
 mountains and Pyrenees. Parallel to these, crossing the tableland of 
 Castile, are the Guadarama mountains, mountains of Toledo, the 
 Sierra Morena, the Sierra Nevada, divided from each other by valleys 
 more or less deep ; while farther south are the chains of the Little 
 and Great Atlas, parted from the Sierra Nevada by a deeper valley 
 which admits the ocean. Turning next to the east of Europe, we 
 find in Asia Minor another tableland, comparable to Spain, bordered 
 to the north by the Caucasus and traversed by parallel ranges, which 
 run east and west. 
 
 We may then perhaps conceive how it has come about that the 
 intermediate Mediterranean region acquired its peculiar contours, 
 owing to downward or synclinal folding, which has not sunk moun- 
 tains out of sight like those east and west, but has prevented them 
 from being formed, by using up the materials of the earth's crust in 
 the production of folds of a different order. Whoever will experiment 
 on the contraction or crumpling of materials on spherical surfaces, 
 will see that, with a predominant anticlinal elevation, an arrangement 
 of primary folds at right angles to the compression is produced, but 
 whenever a moderate synclinal depression is formed on the flank of 
 the main axis of elevation, then lateral chains or spurs are formed, 
 more or less at right angles to the main ridge, but under inverse con- 
 ditions to those in which William Hopkins demonstrated the two 
 orders of fractures dependent upon strain in an anticlinal elevation. 
 
 Between the Caucasus and the Pyrenees lie the whole system of 
 mountain ranges -of the south of Europe, which consists of main chains 
 and spurs. The chain of the Cevennes runs north as though it were 
 a spur dependent u-pon the Pyrenees ; but the mountain axis of the 
 Pyrenees, interrupted for a time by depression in the Gulf of Lions, 
 becomes prolonged into the Mont des Maures in France, and is 
 deflected northward parallel to the Cevennes, forming the Graian 
 and Cottian Alps, before it resumes its main direction east, in the 
 Pennine Alps, the Lepontine Alps, the Eha3tic and Noric Alps, which 
 strike away eastward to the Carpathians. Dependent upon this 
 great range are parallel subordinate ranges ; and spurs are given off to 
 the south and north. It seems to be a characteristic of a spur that 
 an intervening area of depression separates it more or less from the 
 main chain upon which it depends ; for we find not only the chain of 
 Corsica and Sardinia running south from the Gulf of Genoa as a 
 spur from the Atlas or the Alps, but the Apennines extending S.E. 
 from the Alps as the axis of a peninsula which is a secondary con- 
 sequence of the great Alpine elevation. Similarly the Julian and 
 Dinaric Alps, like the northern spurs of the Balkans and the Sieben- 
 biirgeii of eastern Transylvania, run south-east. If the southern 
 
 VOL. i. z 
 
354 ORIGIN OF SPURS FROM MOUNTAIN CHAINS. 
 
 mountain spurs form peninsulas which point to the south, it is only 
 because the depression of the basin between Asia Minor and Spain, 
 is coupled with great uplifting of the complicated folded mass of the 
 Alps to the north, just as the north to south chains of Asia are spurs 
 formed in consequence of the elevation of the mountain axis of Asia. 
 North of the Alps other chains at right angles to the main chain, as 
 though they were spurs, slope towards the North Sea, though the 
 plain of North Germany is now raised too high for the water to divide 
 them into peninsulas. 
 
 The Vosges and the Schwarzwald both run northward, and the 
 Bohmerwald may perhaps be regarded as another range placed as 
 though it were a northern spur of the Alps ; just as the Sudetic Alps 
 and Riesengebirge are parallel chains lying farther north and depen- 
 dent on the Carpathians, but more denuded, and of older origin. The 
 irregularities of direction of the chains and their deflections are 
 governed, it would seem, by the directions of more ancient mountains 
 which interfere with the flexures of newer date. And it would even 
 seem that mountain chains of ancient date parallel to each other may 
 come to play the part of spurs to a newer mountain range. 
 
 It might be reasonably urged that, in the same way that lateral 
 spurs, running north and south, are given off by the Alps, so the 
 mountain systems of Europe and Asia are to be regarded as a grand 
 series of chains which are similarly dependent upon the more ancient 
 disturbances which originated the Ural chain and Caspian Sea. The 
 mountain system of Central Asia, however, presents this remarkable 
 difference from that of Europe, that whereas the western region is 
 broken by the predominant synclinal depression of the Mediterranean 
 valley, the eastern portion has the corresponding area entirely up- 
 heaved, so as to constitute the great tableland between Turkestan and 
 China, limited by the Himalayas and the Altai Mountains. The 
 distribution of mountains in Asia shows that the peninsulas which 
 run south are the effects of the formation of lateral chains, crumpled 
 up at right angles to the grand upheaval of the mountain axis to the 
 north, and in consequence of that upheaval. 
 
 This predominant direction of so much of the land of the Old 
 World, in an east to west line, is no less remarkable than the cor- 
 responding direction of the remainder of the land of the world from 
 north to south ; but these different directions of land masses are to 
 be regarded as mutually dependent, in the same way as a mountain 
 chain and its spurs. If we crumple the surface of one side of a globe 
 into ridges running east and west, the material of the crust is so used 
 up by the contractions that no corresponding series of ridges having 
 the same direction could be developed on the opposite side of the 
 globe, for the reason that we have already indicated in explaining the 
 origin of lateral spurs. But since the ridges running east and west do 
 not use up on a large scale material of the earth's crust in an east and 
 west direction, it follows that the predominant contractions which are 
 formed on the opposite side of the globe must develop mountain 
 ridges running from north to south. Thus the Andes and Rocky 
 
GEOLOGICAL SUCCESSION OF CONTINENTS. 355 
 
 Mountains, with the Alleghanies and Cordilleras of Brazil, may be 
 taken as skeleton contours, which are correlative with and dependent 
 upon the formation of the axis of the Eurasian continent ; while 
 the north to south mountains of the east and west sides of Southern 
 Africa, of Madagascar and Australia, show that a wide extension of 
 land from west to east about the northern parts of the world cannot 
 be upheaved without the development of correlative elevation in an 
 opposite direction, so as to use up a corresponding portion of the 
 earth's crust on the opposite side of the earth in the direction of 
 tension, which initiates contraction at right angles to itself. Hence, 
 the east to west direction of the Great Antilles is dependent upon 
 the north to south direction of the Andes. The deflection of Cen- 
 tral America north-west is governed by the direction of the Appal- 
 lachian chain north-east. The Suliman mountains and Ghats have a 
 like dependence upon the Himalayas. The existence of the oceans we 
 take to follow the same law as inland seas or other basins, and to be 
 dependent on the contractions which have upheaved the continents. 
 
 We have entered into this statement concerning mountains 
 because the views enunciated seem to show an indication of law in 
 the distribution of land and water at the present day. Hence it may 
 be inferred, perhaps, that such a law has never been absent from the 
 earth ; and that in its natural development we find an interpretation 
 -of the great geological mystery of upheavals and depressions, which 
 caused the same region of the earth in past ages to have been 
 occupied successively by ocean and by land. It harmonises the 
 evolution of mountains by the radiation of the earth's heat, under 
 the principle that pressure of contraction must give rise to tension at 
 right angles to the contraction ; and that all chains are hence approxi- 
 mately at right angles to each other, and therefore succeed each other 
 in this order, both in space and in time. 
 
 A mountain chain once formed can never be obliterated or 
 ignored by a newer chain crossing the same district ; and the rocks of 
 the folded region, no matter how denuded or depressed, will always 
 exhibit the conditions of their origin. In such a sense mountains 
 and continents may be said to be permanent ; but just as the forma- 
 tion of the great east to west ridges of the mountain axis of the Old 
 World drew the land up above the ocean to the north and south, in 
 Tertiary times, so we may anticipate that the line of contraction 
 which is outlined in the Dolphin and Challenger ridges would 
 by further elevation gradually draw the lands on both sides of the 
 Atlantic beneath the sea, and vary the geography of the world much 
 as it has changed in bygone times. In successive ages the great con- 
 tractions of the earth's crust are repeated, and upheaval and depression 
 take place in later ages along old lines of contraction ; but contraction 
 in one direction may be succeeded in the next period of geological 
 time by a contraction which is approximately at right angles, result- 
 ing in grand unconformity of the newer strata, because the origin of 
 rock material is different. 
 
( 356 ) 
 
 CHAPTER XX. 
 
 METAMORPHISM. 
 
 SUBTERRANEAN heat has transformed to a certain extent strata of all 
 ages which were exposed to its action ; and thus the Lias shale of 
 Savoy, for example, approximates to the character of clay slate. In 
 such cases, we may indicate the alteration by Ly ell's term metamorphic, 
 and designate by it all those parts of aqueous strata which have been 
 transformed in structure or appearance by subterranean heat, or heat 
 developed by pressure applied since their deposition. All strata may 
 become metamorphic. 
 
 Effects of Internal Heat. We have seen the effects produced by 
 plutonic rocks upon strata which they penetrate ; these effects depend 
 on the degree of heat communicated, and the substances operated on. 
 As examples of these effects we may name structural metamorphism, 
 molecular metamorphism, and chemical metamorphism : 
 
 1. The consolidation of stratified rocks is exemplified in the in- 
 duration and contraction of shale, and in the development of new 
 faces or joints in it, which sometimes meet one another rhomboidally, 
 sometimes follow the columnar relations of the adjoining basalt, and 
 sometimes imitate slaty cleavage. 
 
 2. The partial fusion or cementation of some part of the substance 
 of a rock, so as to agglutinate its grains and solidify and harden the 
 whole mass. Thus sandstone is converted to a granular quartz rock. 
 
 3. The complete vitrification or recomposition of the rock, thus 
 converting shale into Lydian stone, and fine sandstone into a kind of 
 jaspar, or even into schists or igneous rocks. 
 
 4. The complete rearrangement of the particles into granular or 
 crystalline forms, as in the instance of chalk in Ireland, and limestones 
 in Yorkshire, the Isle of Skye, and Carrara. 
 
 5. The production of minerals not before existing in a distinct 
 state in the substances affected. The development of pyrites, asbes- 
 tus, anthracite, plumbago, garnet, &c., along the contact of igneous 
 and aqueous rocks is a very characteristic and general effect, which 
 appears to result from the actual transfer of the metallic and other 
 matter through the solid substance of the rock. If Yon Buch's 
 notion of the impregnation of strata with magnesia in the vicinity of 
 augitic rocks should be substantiated, it must be considered as a 
 remarkable example of such transfer of mineral material. 
 
METAMORPHISM BY PRESSURE. 357 
 
 6. The sublimation of some portion of the neighbouring substances. 
 Thus the charring of coal, the expulsion of sulphur and bitumen from 
 shale, are directly connected with the heating power of the igneous 
 rock. It is probable that some peculiar conditions were required for 
 such effects of contact metamorphism in submarine depths, where most 
 of these operations were performed. 
 
 Metamorphism of Rocks. 
 
 Structural Metamorphism. In daily experience, we see some 
 degree of consolidation effected in calcareous deposits by the concre- 
 tionary or crystalline coherence of the particles. But we scarcely 
 perceive any induration of clay or agglutination of sandstone without 
 infiltration of salts, enormous pressure, or the application of heat. By 
 the subsidence of the strata to some thousands of feet or yards, which 
 has unquestionably happened in very many cases, these favourable 
 influences were brought into action. The oldest strata were upon the 
 whole sunk to the greatest depth, and in consequence have experienced 
 the greatest amount of pressure and heat, and these are on the whole 
 the most consolidated; the clays have become slate and the sands 
 quartzite. 
 
 Effects of Pressure. The lowest strata of the coal basin of South 
 Wales, which were deposited nearly at the sea-level, were necessarily 
 sunk during the latest palaeozoic periods about 12,000 feet below the 
 surface. In these a partial slaty cleavage appears. The Old Red 
 Sandstone strata, several thousand feet thick, which were still deeper 
 and more heated, are more marked by cleavage ; and the Silurian and 
 Cambrian, still deeper by thousands of feet, are even more distin- 
 guished by that structure. It is chiefly in what were the deeper 
 parts of the basin that this effect occurs ; for on the north side of 
 the South Wales coal-field the Silurian strata and the Old Red 
 Sandstone are often free from cleavage, and cleavage is there only 
 partially exhibited in the Cambrian. 
 
 Farther to the north, as in the district of the Malvern Hills, 
 Woolhope, Abberley, Dudley, the Silurian rocks and all above them 
 are free from cleavage, or exhibit it only in a very slight degree, and 
 along some small and limited spaces. Yet these districts are marked 
 by great and violent flexures, and even reversals of the strata, so that 
 pressure seems sometimes to have failed entirely in producing cleavage. 
 This is the more curious, because, in the same country, parallel to the 
 once heated basalt dyke of Brockhill, the Old Red Sandstone shales 
 have developed a rude cleavage. In these districts there is no reason 
 to admit more than 5000 to 8000 feet of depression. 
 
 The Cambrian rocks of Charnwood Forest and the base of Ingle- 
 borough are full of cleavage, crossing great curvatures of the strata. 
 Those curvatures preceded the formation of the Old Red Sandstone. 
 There is no cleavage in any of the upper or middle palaeozoic strata, 
 which, in the utmost depression which we can trace, may have been 
 sunk some 5000 or 6000 feet deep in Yorkshire, and some 8000 feet 
 
358 MOLECULAR METAMORPHISM. 
 
 deep in Lancashire ; but the Cambrian and Silurian rocks in which 
 cleavage occurs must have been depressed twice as deep. 
 
 From considerations of this kind we are led to admit that depth 
 in the earth that is, the heat, and pressure, and molecular action 
 favoured by depth, and of which depth is a measure is one of the 
 main agencies favourable to the generation of slaty cleavage in the 
 strata. Pressure is clearly necessary. For the direction of the planes 
 of cleavage is parallel to the great axis of movement in the district 
 which determines the strike of the rocks ; and Professor John Phillips's 
 researches, enlarged by the investigations of Mr. Sharpe, left no doubt 
 that the compression of the rocks is in the direction at right angles to 
 the cleavage planes. Mr. Sorby succeeded in producing cleavage struc- 
 ture by artificial pressure in clay originally quite destitute of it. Some 
 further illustrations were added by Dr. Tyndall, who used pressure to 
 develop cleavage in wax. Hence we arrive at the following general 
 view. A large area of country subsides parallel to a certain axis of 
 movement, is thrown into parallel folds, and is transferred to a hotter 
 and narrower space hotter as compared to the surface, narrower as 
 the chord is shorter than the arc. Lateral pressure operates on all the 
 strata ; heat more particularly on the argillaceous parts ; plates of 
 mica become scattered through these strata, and are by the pressure 
 made to assume positions which are not all parallel, but tend to 
 parallelism, and thus effectually cause fissility in the stone. 1 
 
 Cleat. Though there is no slaty cleavage in the coal strata of the 
 northern counties, or indeed in Wales, there is a structure called 
 " cleat" in the substance of coal which is of the same order, quite as 
 regular and extensive, and due to as general a cause. This consists 
 in a series of parallel fissures, often very fine and numerous, which 
 cut across the strata of coal in planes nearly vertical to the strata, and 
 in directions seldom deviating much in the large area of a coal-field. 
 In the northern coal-fields this direction is N.N.W. and S.S.E., or 
 nearly so. It scarcely occurs except in the coal, is not affected by 
 faults, is not parallel to axes of movement, and varies in character 
 from bed to bed. This structure is of a very peculiar type in the 
 anthracite of Wales. 
 
 Cleavage on Mont Blanc. In a survey of the structure of Mont 
 Blanc, 2 Mr. Sharpe traced no fewer than nine parallel axes of slaty 
 cleavage, crossing the gneissic, calcareous, and argillaceous strata, 
 which dip in various directions, a phenomenon analogous to 
 that observed by the same geologist in the country of the High- 
 lands. 3 
 
 Molecular Metamorphism. Under this head we class the con- 
 version of earthy carbonate of lime into crystallised marble, which has 
 been effected naturally, by the proximity of igneous rocks, in many 
 places, as in Teesdale by the basalt or whin sill, in Raghlin by 
 basaltic dykes, and artificially by Sir James Hall in a heated gun- 
 barrel. On a large scale, saccharoid limestone is a great example, 
 proving the pervading influence of high heat through a mass of deep- 
 
 1 Sorby, in Q. J. G. S., &c. 2 Geol. Proceedings, 1855. 3 Phil. Trans., 1855. 
 
STAGES IN METAMORPHISM. 359 
 
 seated rocks, by which the carbonic acid was retained in them and 
 the particles of the rock entirely rearranged. Similarly the change of 
 loose sand or argillaceous sandstone to solid sandstone or quartzite, 
 or jasper, or rhyolite, or granite, is, in the first place, a case of cementa- 
 tion of the grains through heat, followed by gradual solution of the 
 rock in the water which it contained, and more or less perfect crystal- 
 lisation, by which in many cases twin crystals of highly complex 
 structure have been formed by the enlargement and blending of 
 crystals which were at first microscopic. In the consolidation of clay 
 slate, the particles are not merely pressed together, but more or less 
 confluent at the edges, from crystallisation, 1 and in basalt and basaltic 
 dykes we see a more perfect development of molecular metamorphism. 
 
 Stages in Chemical Metamorphism. The most extreme change 
 induced by heat, and the chemical actions which heat and water 
 acting under pressure set up, is an alteration in the nature of a 
 rock. Such a case, in its simplest form, may be typified by the gene- 
 ration of garnet in the vicinity of dykes and large igneous masses, 
 or in the artificial combinations of the furnace. Near the dyke of 
 Plasnewydd, in Anglesea, Professor Henslow collected in the altered 
 and jasperised shales grey garnets ; in the rock of the mountain called 
 the Gable, near the granites of Wast Water, are multitudes of beauti- 
 ful red garnets. We are thus led by easy analogy to view the in- 
 numerable garnets in the mica slate of the Highlands as generated in 
 those foliated rocks by chemical combinations which originated under 
 the same influence of heat, as that by which the limestone which lies 
 in them has become crystalline, and by which the schists have acquired 
 their granitoid aspect. 
 
 Ratio of the Metamorphism of Strata. On considering the 
 series of strata in relation to the degree of their metamorphism, it 
 is impossible not to perceive that in most countries metamorphism 
 increases continually with the age of the rock. It is impossible at 
 present always to point out exactly the amount of changes which 
 have been produced on the primary strata by the general and con- 
 tinued communication of heat from below; gneiss, for example, is 
 in some cases almost identical with granite, in other cases approxi- 
 mates to sandstone. Yet when we consider the bedded and 
 laminated character of this rock, and observe that its constituent 
 minerals, even when united into a dense rock, are not crystallised 
 with regular external forms, successively modifying one another in a 
 certain order, we understand that the rock has been solidified by 
 a species of crystalline growth at the edges of the constituent sub- 
 stances, which, carried to extreme, under the requisite pressures, 
 would have reconverted the whole into granite. 
 
 Similar remarks apply to mica schist, w^hich, on the one hand, 
 varies to gneiss, and on the other to clay slate ; and it is observable 
 that the fusible mineral garnet, which is generated at moderate heats 
 in rocks in contact with basalt, is very generally intermixed with the 
 laminae of gneiss and mica slate. 
 
 1 Sorby, Address Geol. Soc., 1880, p. 47. 
 
360 CONDITIONS OF FOLIATION. 
 
 The limestone of Teesdale is a hard rock, but where it touches the 
 basalt of that country it has become crystalline. The shales are also 
 altered (fig. 70, p. 337). The upper portions of the slate system in 
 Shropshire and Radnorshire, where that system is immensely thick, 
 show similar changes. 
 
 Decreasing Effects of Pressure and Heat in Newer Strata. The 
 effects of general pressure and heat continually decrease among the 
 superior strata of the Saliferous, Oolitic, and Cretaceous systems, and 
 seem almost wholly lost in the tertiary strata of this country. It is 
 chiefly to this graduated effect of heat that we may ascribe the dis- 
 tinctness of the rocks in different parts of the series. Thus, to take 
 the calcareous rocks, we have a series gradually changing in proportion 
 to their antiquity, from crystalline limestone, through highly condensed 
 carboniferous limestone, to compact lias, concretionary oolite, marly 
 chalk, and tertiary lacustrine marls. Among sedimentary deposits 
 there is a series from gneiss through the hard sandstones associated 
 with the carboniferous limestone to the sands of the oolites, chalk, 
 and tertiaries ; and another series from cleavable slate, through jointed 
 greywacke slate, hard coal shale, compact red marl, lias and lower 
 secondary clays, gault, and clays of the tertiary period. There is 
 properly no sand, clay, or marl among the older strata. Indurated 
 shale, hard gritstone, and compact limestone are of rare occurrence 
 among the younger rocks. 
 
 Effects of Heat in forming Granitoid Strata. Mr. Sharpe re- 
 garded the foliation of gneiss generally as due to metamorphic action 
 of the same kind as that which produced cleavage in slate, only more 
 prolonged and more intense, and this view is now generally accepted. 
 
 These foliated rocks, which have the aspect of being derived from de- 
 composed granitic rocks, with subordinate and associated strata equally 
 devoid of organic remains, constitute, according to the testimony of 
 observers, the lowest geological group, and in most countries were origi- 
 nally water-formed deposits. From the effects of metamorphic agents 
 upon these rocks, their natural analogy to granite is sometimes so much 
 heightened as to cause some uncertainty in distinguishing between 
 them, and enforce a conviction that the distinction between them is 
 one of degree, and not of kind. The rocks of this whole series might 
 without impropriety be termed granitoid strata. 
 
 Foliation. Foliation is certainly in most cases independent of 
 stratification, and Darwin observes that even when its direction cor- 
 responds to the strike of strata its dip is different. David Forbes 
 records at Crianlorich in Perthshire, beds of blue limestone resting on 
 mica schist, but with the limestone foliated by the development of 
 plates of mica so as to resemble gneiss ; and while the foliation in 
 the limestone appeared to be identical with the planes of bedding, in 
 the bed above the limestone the foliation is twisted and irregular. 
 At Jaegerborg near Christiansand foliated limestone abounds, and con- 
 tains patches of gneiss, besides being capped with gneiss, which is 
 itself capped with granite. 1 
 
 1 Q. J. G. S., vol. xi. p. 167. 
 
FELSPARS IN METAMORPHIC ROCKS. 361 
 
 Mineral Constituents of Metamorphic Rocks. 
 
 Any theory of relation between metamorphic rocks and plutonic 
 rocks must be based upon similarity in the composition of constituent 
 minerals in the rocks which are compared. Interesting researches 
 made by Professor Heddle show absolute correspondence between 
 the metamorphic and plutonic forms of minerals 1 in the same district. 
 And if any theoretical value is to be attached to the chemical com- 
 position of different rocks, the tables of composition of the chief 
 minerals which we reproduce will go far to show within what limits 
 a rock may vary in chemical composition owing to the proportions 
 in which its mineral elements are combined. 
 
 Felspars. Orthoclase is found in gneiss at Glen Urquhart, Dee- 
 side, and central Sutherland associated with hyaline quartz, carrying 
 Lepidomelane or Haughtonite, and occasionally apatite. In chlorite 
 schist it is associated with chlorite and rutile. It is occasionally 
 found in porphyry, and exceptionally in syenite, as on Morven and 
 Froster Hills, and near New Leslie in Aberdeenshire associated with 
 hornblende, menaccanite, and sphene. More rarely it is found in 
 crystalline limestone. Orthoclase enters into a greater number of 
 rocks than any other felspar. 
 
 Albite has only been found in Scotland in the red granite of 
 Peterhead, which contains hornblende and epidote ; and in another 
 hornblendic and serpentinous rock from Fetheland Point to Hillswick 
 Ness in Shetland. It forms the moonstone of Stromay. 
 
 Anorthite or lime-felspar -is recorded in the diorite of Fetlar, and 
 some parts of the gabbro of Ayrshire, but is not a felspar found in 
 limestones. 
 
 Latrobite, the lime-potash felspar, is found in the crystalline lime- 
 stone of Glen Gairn. 
 
 Labradorite is nowhere found in Scotch metamorphic rocks, being 
 limited to gabbro, diallage rock, and dolerite. 
 
 Oligoclase combined with hornblende forms the hornblendic gneiss 
 of Cape Wrath, and makes the bulk of the grey granite of Aberdeen, 
 where it is associated with a little orthoclase, a little quartz, a little 
 muscovite, and much Haughtonite, forming the compound termed 
 granitite. 
 
 Andesine is in Scotland a characteristic felspar of gneiss, just as 
 oligoclase is characteristic of granite. Andesine is especially found 
 in limestone. 
 
 These felspars are practically identified with the microscope, in 
 thin sections under polarised light, by the angle between the directions 
 of "Extinction" of adjacent laminae. When the angle exceeds 62 
 30' the felspar is Anorthite. If the angle is between 37 and 62 
 30' the felspar is probably Labradorite. If the angle is less than 37 
 it may be Oligoclase or Albite. 2 
 
 1 M. F. Heddle, Mineralogy of Scotland. Trans. R. S. Edin., vols. xxviii. 
 and xxix. ; and Journ. of the Min. Soc., vols. ii., iii., iv. 
 - Fouque and Levy, Roches Eruptives 
 
-,62 
 
 METAMORPHIC ORTHOCLASE AND ALBITE. 
 
 COMPOSITION OF ORTHOCLASE IN SCOTCH ROCKS. 
 
 Locality. 
 
 Rock. 
 
 Colour. 
 
 4 
 3 
 
 S3 
 
 Alumina. 
 
 || 
 
 I 
 
 Magnesia. 
 Lime. 
 
 Potash. 
 
 C8 
 
 Water. 
 
 Ben Capval 
 
 Granite dyke in 
 
 Blue 
 
 64-86 
 
 18-47 
 
 67 
 
 
 71! .. 
 
 12-98 
 
 I-8 9 
 
 5 
 
 Stromay, 
 
 gneiss 
 Dyke .... 
 
 Grey 
 
 65'35 
 
 17-68 
 
 92 
 
 .. 
 
 25 
 
 68 
 
 13*13 
 
 
 18 
 
 Harris 
 
 
 
 
 
 
 
 
 
 
 
 
 Glen Fer- 
 
 Granite vein in 
 
 Pink 
 
 6 3'99 
 
 17*06 
 
 2*47 
 
 .. 
 
 07 
 
 *52 
 
 14-85 
 
 "53 
 
 '65 
 
 nate 
 
 mica slate 
 
 
 
 
 
 
 
 
 
 
 
 Cowhythe, 
 Portsoy 
 
 Clattering 
 
 Granite veins in 
 talc slate 
 Dyke of red por- 
 
 Flesh | 
 Fawn 
 
 64-74 
 66-0 
 64*03 
 
 18*3 
 18-3 
 19-17 
 
 1*99 
 2-03 
 "3 
 
 22 
 
 04 
 
 "94 
 
 '97 
 
 i* 
 
 9-87 
 
 10 '02 
 11*84 
 
 3*34 
 3^9 
 
 "17 
 16 
 '57 
 
 Briggs . . 
 Rubislaw . 
 
 phyry 
 Veinstone in 
 
 Flesh 
 
 
 18-36 
 
 32 
 
 
 09 
 
 36 
 
 I3-05 
 
 2-58 
 
 09 
 
 
 granite 
 
 
 
 
 
 
 
 
 
 
 
 Lairg. . . 
 
 Veinstone syeni- 
 
 Buff 
 
 62-62 
 
 19-63 
 
 06 
 
 
 64 
 
 6 
 
 13-72 
 
 2-92 
 
 "13 
 
 
 tic granite 
 
 
 
 
 
 
 
 
 
 
 
 Tongue . . 
 
 Amazon stone 
 
 Green 
 
 64-2 
 
 18-39 
 
 *45 
 
 'IS 
 
 07 
 
 72 
 
 I275 
 
 2'95 
 
 '5 1 
 
 
 vein in syeni- 
 
 
 
 
 
 
 
 
 
 
 
 
 tic granito 
 
 
 
 
 
 
 
 
 
 
 
 Froster Hill 
 
 Syenite. . . . 
 
 White 
 
 63 '31 
 
 18-17 
 
 87 
 
 
 
 1*07 
 
 I 3' 2 7 
 
 2-06 
 
 81 
 
 Blirydrine . 
 Banchory . 
 Balvraid . 
 
 Micaceous gneiss White 
 ? Granite | White 
 Crystalline lime- j Blue 
 
 63-59 
 63*11 
 63-04 
 
 18-98 
 
 
 " 
 
 08 
 '57 
 
 '21 
 
 68 
 88 
 '97 
 
 12-53 
 13*06 
 14 '63 
 
 2-76 
 2*34 
 
 I*O2 
 
 "4 2 
 
 1* 
 
 
 stone 
 
 
 
 
 i 
 
 
 
 
 
 Struay, ) 
 
 Granite band in jf Pink 
 
 65- 
 
 17-03 
 
 i *43 
 
 6 9 i .. 
 
 '73 
 
 13-82 
 
 I* 
 
 "So 
 
 Ross . . f 
 
 gneiss ( Blue 
 
 64*19 
 
 17-39 
 
 
 46 j . . 
 
 69 13-31 
 
 I'96 
 
 56 
 
 Canish, 
 
 Porphyry in j Brick 
 
 
 17-36 
 
 \1 7 
 
 38 
 
 
 
 I-6 9 
 
 1*12 
 
 Sutherland 
 
 quartzite 
 
 
 
 
 
 
 
 
 
 
 
 Sanidine, 
 
 
 
 
 
 
 1 
 
 
 
 
 Corriegills . 
 
 Pitchstone por- 
 
 Colour- 
 
 66-85 
 
 17-24 
 
 '42 
 
 
 '06 I '22 
 
 9*2 
 
 4^2 
 
 86 
 
 
 phyry 
 
 less 
 
 
 
 
 
 i 
 
 
 
 Kinkell . . 
 
 Tuff Yellow 
 
 63-07 
 
 18-68 
 
 2*47 
 
 06 
 
 .. 2 '2 
 
 6-62 5-5 
 
 i-39 
 
 COMPOSITION OF ALBITE IN SCOTCH ROCKS. 
 
 ALBITK, OR SODA FELSPAR. 
 
 
 
 
 
 3 
 
 M 
 
 
 
 
 
 
 1 
 
 
 
 cS 
 
 
 
 
 'I 
 
 
 
 
 
 Locality. Rock. 
 
 Colour. 
 
 i 
 
 
 
 3 
 
 1 
 
 g 
 
 V 
 
 s 
 
 J 
 
 d 
 
 
 t 
 
 
 
 
 <3 
 
 h 
 
 s 
 
 
 I 
 
 1 
 
 r 
 
 Stromay, Harris 
 
 Granite dyke 
 in hornblen- 
 
 Grey. 
 
 66-97 
 
 19-46 
 
 6 
 
 *2I 
 
 2-04 
 
 X-23 
 
 9'54 
 
 
 
 i die gneiss 
 
 
 
 
 
 
 
 
 
 
 Colafirth, Shet- ! Serpentine 
 land . . j and talc rock 
 
 White 
 
 66*8 
 
 I7-83 
 
 1-13 
 
 14 
 
 i '5 
 
 92 
 
 11-52 
 
 4 8 
 
 Colafirth . 
 Hillswick . 
 
 ' 'Quartz vein" 
 In hornblen- 
 dic rocks 
 
 White 
 Pink . 
 
 66-84 
 66-71 
 
 16-73 
 19-81 
 
 2-42 
 '9 
 
 '37 
 9 
 
 '94 
 1*38 
 
 '73 
 1-26 
 
 10*76 
 9'23 
 
 89 
 
 "54 
 
 deavlandite, Ben 
 Bhreck, Tongue 
 
 Veinstone in 
 syenitic gra- 
 
 White 
 
 67-79 
 
 18-76 
 
 I-43 1 
 
 
 "52 
 
 76 
 
 10-49 
 
 16 
 
 
 nite 
 
 
 
 
 
 
 
 
 
 
 Fe 2 O 3 , also -08 Mn. 
 

 OLIGOCLASE, ANDESINE, AND LABRADORITE. 363 
 
 OLIGOCLASE : SODA LIME FELSPAR. 
 
 | 
 
 & 
 
 Alumina. 
 
 fii 
 
 s * 
 
 feO 
 
 ||| 
 
 d 
 S 
 J 
 
 Potash. 
 
 c! 
 
 | 
 
 02 
 
 Water. 
 
 . Locality. 
 
 Rock. i Colour. 
 
 I 
 
 Rispond, 
 
 Granite vein in j White i 61-85 J 21*7 
 
 3 '37 I '2 : '9 
 
 4'i3 
 
 1-63 
 
 6'95 
 
 '37 
 
 Sutherland 
 
 hornblendic 
 
 
 
 
 
 
 
 Coyle, Aber- 
 
 gneiss 
 Actinolite slate . i Cream i 63-54 
 
 21-45 
 
 1-86 
 
 '23 
 
 3-88 
 
 1-07 
 
 7-64 
 
 '44 
 
 deen 
 
 I j 
 
 1 
 
 
 
 
 
 
 Barra Hill, 
 
 Black serpen- Milk . j 64^67 
 
 22'l8 1 t'44 
 
 01 
 
 1-89 
 
 i "54 
 
 7-62 
 
 'i5 
 
 Old Mel- 
 
 tine 
 
 
 J 
 
 
 
 
 
 
 drum 
 
 
 
 
 
 ! 
 
 
 
 
 
 Dvce, Aber- 
 
 Veinstone in White 
 
 64-85 1 23-2 
 
 
 ., i '2 i - 9 6 
 
 3'77 
 
 8-12 
 
 01 
 
 deen 
 
 grey granite 
 
 
 
 ! 
 
 
 
 
 Sclatty,near 
 
 Granite vein . . White 
 
 59'53 
 
 2 1 'OS 
 
 i'8i 1 .. -88i 3-63 
 
 4"73 
 
 7 '23 
 
 1-88 
 
 Buxburn 1 
 
 
 
 ! i 
 
 
 
 
 Rubislaw . 
 
 Veinstone in White 
 
 62-53 
 
 23-52 
 
 1-28! .. -36! 4-97 
 
 1-32 
 
 6'ig 
 
 6 
 
 
 granite 
 
 
 
 i i 
 
 
 
 
 
 Craigie 
 
 Granite . . . White 
 
 61*58 
 
 22* 
 
 1-28 
 
 I 
 
 i 32 
 
 4-19 
 
 i '52 
 
 8-27 
 
 '54 
 
 Buckler, 
 
 i 
 
 
 
 
 ! 
 
 
 
 
 
 Aberdeen 
 
 i 
 1 
 
 
 
 
 i 
 
 
 
 
 
 Lairg. . . 
 
 Vein in syenitic j Colour- 
 
 62-81 
 
 22*92 
 
 16 
 
 .. i-o8 
 
 4'25 
 
 84 
 
 8-53 
 
 29 
 
 
 erranite less 
 
 I 
 
 
 [ 
 
 
 
 
 
 Canisp . . i Porphyry . . . j Cream 
 
 64-44 ' 20-47 
 
 88 
 
 38 .. | 1-3 1-13 9-96 
 
 1-46 
 
 COMPOSITION OP SCOTCH ANDESINE. 
 
 ANDESINE : LIME AND SODA FELSPAR. 
 
 
 
 % 
 
 OJ 
 
 
 
 
 
 
 
 
 ! * 
 
 s 
 
 rt 
 
 
 
 
 
 
 
 
 d 
 
 = O 
 "g 1 .2 
 
 I 
 
 1 
 
 QJ 
 
 A* 
 2 
 
 
 C 
 
 Locality. 
 
 Rock. 
 
 Colour. 
 
 o 
 
 
 S 
 
 
 a? 
 
 a 
 
 i 
 
 1 
 
 *| 
 
 
 
 
 CO 
 
 3 
 
 (2, 
 
 s 
 
 s 
 
 a 
 
 H-l 
 
 1 
 
 ^ 
 
 Glen Urquhart . 
 
 Crystalline 
 
 White 
 
 58-38 
 
 22-5 
 
 2-12 ! -15 
 
 tr. 
 
 5 '34 
 
 3'2 
 
 5'2i 
 
 3-4! 
 
 
 limestone 
 
 
 
 
 
 
 
 
 
 
 
 Glen Gairn, \ 
 Aberdeen . . f 
 Glen Gairn . . f 
 Crathie . . . ) 
 Portsoy. . . . 
 
 Crystalline ( 
 limestone -I 
 in gneiss ) 
 
 ? diabase . . 
 
 Blue. 
 
 White 
 White 
 White 
 
 57'i8 
 
 56-96 
 
 56-3 
 58-36 
 
 24-04 
 23*81 
 23'34 
 
 1*12 
 
 '94 
 '97 
 '24 
 
 tr. 
 
 12 
 
 09 
 5 
 
 6'ii 
 
 7-98 
 9 '35 
 8-24 
 
 2-83 
 
 2-56 
 1-49 
 
 6*85 
 4-72 
 7-84 
 
 1*6 
 
 1-62 
 1-82 
 '53 
 
 COMPOSITION OF SCOTCH LABEADORITE. 
 
 LABRADORITE : LIME SODA FELSPAR. 
 
 
 d 
 
 
 i 
 
 4 1 
 
 ^ 
 
 
 
 
 
 
 d 
 
 | 
 
 S-3 
 
 5P 
 
 a 
 
 3 
 
 a 
 
 & 
 
 Locality. 
 
 Rock. 
 
 Colour. 
 
 3 
 
 CO 
 
 
 lo 
 
 1 
 
 ! S 
 S j 3 
 
 1 
 
 cS 
 
 c3 
 ^ 
 
 Portsoy (massive) Gneiss 
 
 White . 
 
 53"3 
 
 29-85 
 
 'n 
 
 
 61:11-44 
 
 6 4 
 
 4-21 
 
 '4 2 
 
 Loch Scavaig 
 
 Gabbro 
 
 Grey . . 
 
 50-81 
 
 2Q- 4 8 
 
 '25 
 
 
 12^12-69 
 
 "55 
 
 3'9 2 
 
 2- 4 8 
 
 Glen Bucket 
 
 Diorite 
 
 White . 
 
 5*59 
 
 
 3'5 
 
 
 '59 1I ' I 7 
 
 2'l8 
 
 2-56 
 
 -1-42 
 
 Balta . . . 
 
 Gabbro 
 
 White . 
 
 53*14 
 
 29*9Q 
 
 '25 
 
 
 "21,12-3 
 
 '47i 3'86 
 
 '21 
 
 Portsoy (crystals) 
 
 Gabbro 
 
 Grey . . 
 
 52*41 
 
 28'q6 
 
 '15 
 
 '9 1 
 
 5410-85 
 
 1-61! 3-48 
 
 "93 
 
 Kildrummy 
 
 Mica-Gneiss 
 
 Cream . 
 
 5i'3i 
 
 26-76 
 
 1-82 
 
 76 
 
 
 
 6*43 
 
 68 
 
 Kinneff . . 
 Balvraid, Glenelg 
 
 Porphyrite 
 Serpentinous 
 limestone 
 
 Colourless 
 Wax 
 
 53'i9 
 47 '44 
 
 26-43 
 28*02 
 
 2-85 
 
 '34 
 
 tr. 
 
 92 9-68 
 41.11-03 
 
 
 
 4 '59 
 4-61 
 
 '731 
 
 5 
 
364 
 
 MICAS IN METAMORPHIC ROCKS. 
 
 ANORTHITE, OR LIME FELSPAR. 
 
 
 
 d 
 
 <o 
 
 "3 
 
 
 
 
 
 
 
 
 
 
 'fi 
 
 o 
 
 
 
 4 
 
 
 
 
 
 Locality. 
 
 Rock. 
 
 Colour. 
 
 | 
 
 1 
 
 o 
 T 
 
 i 
 
 cs 
 
 a 
 
 bo 
 
 03 
 
 6 
 I 
 
 "I 
 
 cj 
 
 ij 
 
 3 
 
 
 
 
 02 
 
 < 
 
 PM 
 
 i 
 
 
 ~ 
 
 M 
 
 PH 
 
 DQ 
 
 ^ 
 
 Fetlar, Shetland 
 
 Anorthite 
 
 Cream . 
 
 46-92 
 
 30"77 
 
 
 
 tr. 
 
 oq 
 
 16-34! i "5 
 
 3-07 
 
 i '54 
 
 
 diorite 
 
 
 
 
 
 
 
 
 
 
 
 
 Lendallfoot, Ayr 
 
 Gubbro 
 
 Greyish 
 
 44-22 
 
 3 1 '44 
 
 i '95 
 
 .. 
 
 
 
 I 
 
 14-18 
 
 1-48 
 
 1-63 
 
 3-69 
 
 Delnabo, Glen 
 
 Crystaline 
 
 Green . 
 
 46-42 
 
 21-86 
 
 
 5 '92 
 
 69 
 
 2-92 
 
 18-38 
 
 1-26 
 
 I-6 9 
 
 i -08 
 
 Gairn 
 
 limestone 
 
 
 
 
 
 
 
 
 
 
 
 
 LATROBITE, OR POTASH LIME FELSPAR. 
 
 
 
 d 
 
 3 
 
 
 
 
 
 
 
 
 
 1 
 
 O 
 
 O 
 
 1 
 
 
 
 
 
 
 
 
 Locality. 
 
 Rock. 
 
 Colour. 
 
 I 
 
 
 
 3 
 
 1 
 
 f. 
 
 fcD 
 
 1 
 
 O 
 I 
 
 a 
 
 rt 
 
 5 
 
 
 
 
 02 
 
 33 
 
 1 
 
 
 
 S 
 
 
 a 
 
 1 
 
 02 
 
 
 
 
 
 | 
 
 
 
 
 
 
 
 
 _ 
 
 ~\ 
 
 Delnabo, Glen 
 
 Crystaline 
 
 f Rose . 
 
 45-231-04 
 
 3'43 
 
 tr. 
 
 68 
 
 1*2 
 
 5-21 
 
 7-12 
 
 "49 
 
 " 
 
 Guirn 
 
 limestone 
 
 (Rose . 
 
 46-87 
 
 29-31 
 
 2-31 
 
 "ii 
 
 x- 15 
 
 1-38 
 
 6-46 
 
 2 
 
 85 
 
 4'J 
 
 Micas in Metamorphic Rocks. Muscovite, according to Professor 
 Heddle, is chiefly found in granite veins, which he terms " exfiltra- 
 tive veins," but which we regard as vein-stones. Such veins, remark- 
 able for their tortuous course and branches, were by Jameson and the 
 older writers termed contemporaneous veins. Muscovite is specially 
 cited from the great vein of Capval, in Harris, where an abundance 
 of ferrous oxide gives it a green colour, though the finest specimens 
 are known from Bigsetter Voe, in Mainland, Shetland, and Glen 
 Skiag and Loch Glass, in Ross-shire. This mica is comparatively 
 rare in granite in Scotland. Margarodite is another potash mica, but 
 with more soda ; it is characteristic of gneissose rocks, specially in 
 Shetland, but it occurs in granular limestone in Aberdeenshire, Banff, 
 and the central parts of Scotland. 
 
 Of the black micas, biotite i characteristic of crystalline lime- 
 stones, no other mica occurring in them in Scotland except marga- 
 rodite. Among the well-known localities for it are Glen Urquhart, 
 Glen Laggan in Inverness, Shinness in Sutherland, and Glen Beg in 
 Glenelg. Lepidomelane is another black mica, found in gneiss on 
 the north shore of Loch Shin, in Sutherland. Its colour is yellowish- 
 brown to chocolate-brown. This is the ordinary black mica of the 
 granites of Ireland. A blacker mica is Haughtonite, remarkable for 
 ' its large percentage of iron. It is found in veins in hornblendic gneiss, 
 in the graphic granite which occurs in the gneiss, at Rispond, Suther- 
 land, and in the micaceous gneiss of many localities in Koss and 
 Sutherland. This mica is frequently associated with oligoclase, 
 especially in veins, but it is an essential constituent of grey granite, 
 
MICAS IN GNEISS AND LIMESTONE. 
 
 365 
 
 which Heddle states to be formed of oligoclase, quartz, and Haugh- 
 tonite, with some orthoclase. It is considered likely that lepidome- 
 lane is the dark mica of some gneisses, such as the bronzy gneiss of 
 Tiree, while Haughtonite occurs in others. Haughtonite is readily 
 distinguished from biotite under the blow-pipe, since biotite whitens, 
 and Haughtonite increases in blackness. 
 
 WHITE MICAS. 
 
 
 Margarodite. 
 
 
 
 . 
 
 4 
 
 
 
 
 
 
 
 - 
 
 
 
 
 
 
 g 
 
 o 
 
 
 
 
 
 
 
 
 
 
 c 
 
 
 
 
 a 
 
 
 3 
 
 J 
 
 
 O 
 
 a 
 
 
 Locality. 
 
 Rock. 
 
 Colour. 
 
 i a 
 
 3 
 
 g 
 
 2 
 
 1 
 
 o 
 
 a 
 
 1 
 
 1 
 
 4 
 
 o 
 
 
 
 
 
 02 3 
 
 
 
 1 
 
 3 
 
 
 s 
 
 * 
 
 m 
 
 S 
 
 
 Lambhoga . 
 
 Kaolin in 
 
 Faint 
 
 50-77,31-71 
 
 1-32 
 
 .. 
 
 23 
 
 95 
 
 79 
 
 5*" 
 
 'S? 
 
 
 7 '97 
 
 Vanleep, 
 
 gneiss. 
 Withkyanite 
 
 yellow . 
 White . 
 
 45'4329'65 
 
 8*33 
 
 
 "02 
 
 79 
 
 
 6*94 
 
 2-27 
 
 
 5 '29 
 
 Shetland 
 
 in quartz 
 
 
 i 
 
 
 
 
 
 
 
 
 
 
 
 vein in mica 
 
 
 
 
 
 
 
 
 
 
 
 
 
 gneiss 
 
 
 1 
 
 
 
 
 
 
 
 
 
 
 Vanleep, 
 
 In mica-slate 
 
 White . 
 
 45*4230*30 
 
 6-87 
 
 
 82 
 
 6 
 
 2-6 
 
 6-09 
 
 2 '01 
 
 i -06 
 
 5 'oi 
 
 Shetland 
 
 
 
 i 
 
 
 
 
 
 
 
 
 
 
 Botriphine, 
 
 With kyanite 
 
 White . 
 
 4S'i 29*9 
 
 7*87 
 
 
 '03 
 
 62 
 
 72 
 
 7*84 
 
 2*56 
 
 tr. 
 
 5-51 
 
 Banffshire 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Glenbucket 
 
 Crystalline 
 
 White . 
 
 46-1831-83 
 
 4* 1 
 
 
 
 1-66 
 
 1*23 
 
 8'8i 
 
 I'3I 
 
 .. 
 
 S'7 1 
 
 
 limestone 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Muscovite. 
 
 
 
 
 
 
 
 
 
 
 
 
 Ben Capval 
 
 Granite vein 
 
 Green 
 
 43-0832-85 
 
 *73 
 
 2- 7 6J 
 
 H 
 
 1-07 
 
 3, 
 
 9 "08 
 
 8 4 
 
 
 
 9-12 
 
 BLACK MICAS. 
 
 Biotite. 
 
 
 
 1 
 
 1 
 
 , 
 
 
 
 
 
 
 
 Locality. 
 
 Rock. 
 
 Colour.' 
 
 | 
 
 4 
 
 i 
 
 3 
 
 
 
 I 
 
 2 
 
 1 
 
 4 
 
 a 
 
 3 
 
 1 
 
 i 
 
 o 
 1 
 
 1 
 
 
 
 02 
 
 3 
 
 i 
 
 ~ 
 
 3 
 
 S 
 
 CM 
 
 00 
 
 ^ 
 
 ^ 
 
 Glen Urqu- 
 hart 
 
 Crystalline 
 limestone 
 
 Pinchbeck 
 brown 
 
 38-69 
 
 17-66 
 
 '25:12-95 
 
 
 i '16^7*54 
 
 8*92 
 
 13 
 
 V 
 
 2*14 
 
 
 in gneiss 
 
 
 
 
 
 
 
 
 
 
 
 
 Lnggan . . 
 
 Shinness . . 
 
 Limestone 
 Limestone 
 
 Bronzy 
 Brown 
 black 
 
 39*5 
 39"77 
 
 15-04 
 16*68 
 
 *24 
 
 65 
 
 10-23 
 6-73 
 
 '75 
 62 
 
 2-' 2 
 
 18-46 
 20*92 
 
 -T 
 
 62 
 
 48 
 
 '73 
 
 3*21 
 
 5*4 
 
 Glen Beg . 
 
 Limestone 
 
 Chocolate 
 
 
 
 39 
 
 10" 
 
 33 
 
 i'.S9 
 
 IQ* 
 
 8-22 
 
 26 
 
 32 
 
 3*34 
 
 
 
 brown 
 
 
 
 
 
 
 
 
 
 
 
 
 Hillswick . 
 
 Hornblendic 
 
 Bronzy 
 
 39'8 
 
 14-19 
 
 2*59 
 
 n-s8 
 
 24 
 
 i 
 
 18-32 
 
 8-43 
 
 2"I1 
 
 *S6 
 
 2-52 
 
 
 gneiss 
 
 brown 
 
 
 
 
 
 
 
 
 
 
 
 
 Milltown, 
 
 Edenite rock 
 
 Very pale 
 
 40 '3 1 
 
 12-58 
 
 1*81 
 
 3'3S 
 
 38 
 
 7*58 
 
 21* 
 
 6-56 
 
 *95 
 
 
 5*74 
 
 Urquhart 
 
 
 green to 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 rich 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 brown 
 
 
 
 
 
 
 
 
 
 
 
 
 Lepidomelane. 
 
 Achadhaphriz 
 Tongue . . 
 
 Gneiss 
 Vein in 
 
 Brown 
 Deep 
 
 40-38 
 
 ,2-XZ 
 
 T4'53 3*03 
 
 3'.7 
 
 1-03 
 
 ,|,,3 
 
 1-8 
 
 
 
 3'57 
 
 
 granite 
 
 brown 
 
 40*08 
 
 I2-4I-I3-47J 2-67 
 
 62 
 
 1-08^4-661 7*57 
 
 2*15 
 
 
 5-29 
 
 
 
 
 
 I 1 
 
 
 
 I 
 
 
 ! 
 
366 
 
 CHLORITIC MINERALS. 
 
 HAUGHTONITE. 
 
 
 CS 
 
 
 
 <o 
 
 
 | 
 
 
 
 
 
 1 
 
 | 
 
 s 
 
 o 
 
 09 
 
 ! 
 
 4 
 
 I 
 
 ^3 
 
 
 
 LJ 
 
 
 
 
 Locality. 
 
 Rock. 
 
 Colour. 
 
 
 
 
 S'P. 
 
 oJ'S 
 
 s 
 
 | 
 
 S 
 
 1 
 
 a 
 
 "? 
 
 
 
 
 02 
 
 ^ 
 
 feO 
 
 fc<O 
 
 s 
 
 3 
 
 Si 
 
 PL, 
 
 02 
 
 ^ 
 
 Roneval, 
 
 Granite vein 
 
 Black 
 
 3? . l6 
 
 15' 
 
 7-69 
 
 i7'35 
 
 1*04 
 
 i '3 
 
 8-88 
 
 8-18 
 
 1-6 
 
 2*12 
 
 Harris 
 
 
 brown 
 
 
 
 
 
 
 
 
 
 
 
 Locb-na-Muilne, 
 Lewis 
 
 Granite vein 
 in gneiss 
 
 Brown to 
 black 
 
 36-46 
 
 17*25 
 
 4-18 
 
 15-33 
 
 54 
 
 69 
 
 12-23 
 
 9-2 
 
 66 
 
 3 '39 
 
 Rispond, 
 Sutherland 
 
 Gneiss 
 
 Deep 
 black 
 
 36 '54 
 
 22*28 
 
 2 '43 
 
 i6'oi 
 
 78 
 
 1*25 
 
 IO* 
 
 8-26 
 
 '79 
 
 1-96 
 
 Kinnaird Head 
 
 Vein in 
 
 Black : 
 
 35-67 
 
 17 '95 
 
 7-19 
 
 1 8 "06 
 
 2* 
 
 i '4 
 
 i'5 
 
 9-27 
 
 3'8i 
 
 3 '2 
 
 
 gneiss 
 
 films 
 
 
 
 
 
 
 
 
 
 
 
 Lairg .... 
 
 Vein in 
 
 Dark 
 
 35-56 
 
 16*69 
 
 1-88 
 
 18-04 
 
 6 9 
 
 2-72 
 
 8-47 
 
 9.9 
 
 "ii 
 
 S'7 1 
 
 
 syenitic 
 
 green 
 
 
 
 
 
 
 
 
 
 
 
 
 granite 
 
 
 
 
 
 
 
 
 
 
 
 
 Portsoy . . . 
 
 Diorite 
 
 Bronzy 
 
 34-08 
 
 17-34! 3-61 
 
 18*70 
 
 38 
 
 3 '23 
 
 I0< 54 
 
 6-78 
 
 1-194-05 
 
 
 brown 
 
 
 
 
 
 
 
 
 
 
 Ben Stack . . Granite 
 
 Brown 
 
 35 '69 
 
 20*09 
 
 2-23 
 
 14-01 
 
 I* 
 
 1-89 
 
 J 4'77 
 
 7-38 
 
 '53 2-46 
 
 
 black 
 
 
 
 
 
 
 
 
 
 I 
 
 Chlorites are essentially metamorphic minerals, and according to 
 Heddle, the true clilorites never occur in volcanic rocks, while the 
 saponites are confined to volcanic rocks. Chlorite has been found 
 in granite at Rubislaw and at Girdleness. The chlorites comprise 
 glauconite, talc chlorite, penninite, ripidolite, chlorite, and chloritoid. 
 They often have a cry ptocry stall ine structure, so that they are only 
 to be identified by chemical or external characters. Penninite is 
 found in serpentine in hornblendic gneiss in Scalpa, Harris, in the 
 mica schist of Glen Lochy, in Perthshire, and in Shetland. Ripi- 
 dolite is found in limestone, in mica slate, in chlorite slate, and in 
 hornblendic gneiss, especially at Cape Wrath. Chlorite is found in 
 serpentinous beds, in micaceous gneiss which is free from felspar. 
 It occurs in the mica slate which ranges from north-east to south-west 
 through the lower Highlands of Scotland, in granular limestone parallel 
 to the limestone of Glen Tilt, and in chlorite slate to the west of 
 Portsoy, in Banffshire. Chloritoid is a Shetland mineral, as is talc 
 chlorite ; both are found near Hillswick. 
 
 The saponites comprise Delessite, chlorophseite, Hullite, saponite, 
 and seladonite. They may be regarded as decomposition products, 
 and in that sense as metamorphic, but there are no British meta- 
 morphic rocks, properly so called, in which they form an important 
 constituent. Delessite abounds about Dumbuck ; it has a dull lustre 
 and dark colour. A variety in the tufa at Elie Ness contains 5 per 
 cent, of soda and 3 per cent, of potash ; like saponite, it always con- 
 tains about 20 per cent, of manganese. Chlorophaeite, when fresh, 
 resembles a green jelly, when weathered it resembles drops of asphalt ; 
 it contains less alumina, the iron is chiefly ferric oxide, instead of 
 ferrous oxide ; the magnesia is reduced to one-half, and the water is 
 half as much again. Saponite is soft, greasy, green, and translucent ; 
 it is richer in silica and poor in alumina ; the iron is small in quantity 
 and the water large. The saponites are regarded as products of the 
 decomposition of augite and olivine. 
 
AUGITE AND HORNBLENDE. 
 
 367 
 
 CHLORITE. 
 
 
 
 1 
 i 
 
 a 
 
 
 
 1 
 
 
 , 
 
 
 | 
 
 Locality. 
 
 Rock. 
 
 Colour, j 
 
 .a 
 
 !| 
 
 N 
 
 1 
 
 d 
 
 1 
 
 4 
 
 4 
 
 g 
 
 
 
 iJL 
 
 5 
 
 o 
 
 o 
 
 
 
 a 
 
 % 
 
 1 & 
 
 ^ 
 
 Fethaland, 
 
 Mica 
 
 Bright 
 
 2 4'3 
 
 20-86 
 
 3-57 
 
 16-72 
 
 '55 
 
 i '5 
 
 22'2 
 
 .. 
 
 
 
 11-55 
 
 Shetland 
 
 gneiss 
 
 green 
 
 
 
 
 
 
 
 
 
 
 
 Ben Derag, 
 
 Mica 
 
 Grass 
 
 24-72 
 
 21-57 
 
 62 
 
 26-16 
 
 '47 
 
 '45 
 
 12*86 
 
 i '73 
 
 05 
 
 10-89 
 
 Perthshire 
 
 slate 
 
 green 
 
 
 
 
 
 
 
 
 
 
 
 Craig-aii 
 
 Mica 
 
 Dark 
 
 24-29 
 
 21*18 
 
 *IO 
 
 18-74 
 
 8 
 
 1-66 
 
 2 1 "03 
 
 1*29 
 
 56 
 
 10 '08 
 
 Lochan 
 
 slate 
 
 green 
 
 
 
 
 
 
 
 
 
 
 
 Lude . . . 
 
 Lime- 
 
 Olive 
 
 23-92 
 
 22-98 
 
 I'll 
 
 19-54 
 
 28 
 
 2'45 
 
 I7-26 
 
 
 
 11-78 
 
 
 stone 
 
 green 
 
 
 
 
 
 
 
 
 
 
 
 Lude . . . 
 
 Lime- 
 
 Bright 
 
 24-66 
 
 23-19 
 
 64 
 
 20-58 
 
 29 
 
 4 
 
 17-79 
 
 .. 
 
 
 12*12 
 
 
 stone 
 
 green 
 
 
 
 
 
 
 
 
 
 
 
 Portsoy, 
 
 Chlorite 
 
 Bright 
 
 26-71 
 
 20*42 
 
 3 '47 
 
 1 3 '99 
 
 '73 
 
 
 2 3'9 
 
 
 
 11*17 
 
 Banffshire 
 
 slate 
 
 green 
 
 
 
 
 
 
 
 
 
 
 
 Loch Lag- 
 
 Lime- 
 
 Grass 
 
 26*25 
 
 19*22 
 
 1-67 
 
 16-44 
 
 I'02 
 
 124-35 
 
 
 .. 
 
 11-67 
 
 gan, Inver- 
 
 stone 
 
 green 
 
 
 
 
 
 
 
 
 
 
 
 ness-shire 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Girdleness, 
 
 Granite 
 
 Dark 
 
 24-77 
 
 20*16 
 
 1-38 
 
 27 '39 
 
 *6i 
 
 '9 i3'34 
 
 .. 
 
 .. 
 
 I2'05 
 
 Kincardine 
 
 
 green | 
 
 
 
 
 
 
 
 
 
 Augite and Hornblende in Metamorphic Rocks. 
 
 The varieties of augite and their alteration products are frequently 
 met with, though they are scarcely so important as the minerals of 
 the hornblende group. Crystalline or granular limestone is rich in 
 the paler coloured varieties of both minerals. Malacolite, which is a 
 white or blue variety of augite, forms layers in the limestone of Loch 
 iS hi n, in Sutherland, crystals often being a foot long. The limestone 
 of Totaig, which is embedded in micaceous gneiss, is full of nodules 
 of it. It is found in the limestone of Glen Tilt, and near Serpentine 
 in the hills of Coyle, in Aberdeenshire. The sap-green variety of 
 sahlite is often associated with malacolite. Sahlite is well known 
 from the marble of .Tiree, which is characterised by numerous minute 
 crystals of the dark-green variety, about the size of shot. Another 
 of these augitic minerals is coccolite, of a deep rich green colour. 
 Diallage is recorded from the diallage rock of Unst, in Shetland, 
 where the crystalline masses of diallage are embedded in fine 
 granular labradorite. It is also recorded by Professor James Geikie 
 at Pinbain, near Lendalfoot, in Ayrshire. True augite does not 
 occur in the metamorphic rocks of Scotland, though it is found in 
 the volcanic rocks of Rum and Skye. 
 
 Augite and hornblende, in Scotland, are both connected witli the 
 formation of serpentine. Heddle states that the serpentines of Unst, 
 in Shetland, are derived from diallage ; that one of the beds to the 
 west of Portsoy is derived from gabbro, and the other from eupotide ; 
 while the bed to the east of Portsoy is derived from an augitic rock. 
 The same author states that a bed of serpentine in the Farrid Head, 
 Sutherland, is unquestionably derived from gneissic schists. 
 
 Among hornblendic minerals asbestos is plentiful in the granular 
 
368 
 
 VARIETIES OF PYROXENE. 
 
 limestone of Shinness, where tremolite also occurs, forming crystals 
 nearly a foot long. The magnesia lime amphibole, called edenite, 
 is found in the localities already alluded to Milltown, Glen Urqu- 
 hart, &c., which have yielded so many minerals in the granular 
 limestone. Actinolite, on the other hand, is characteristic of horn- 
 blendic gneiss, especially at Hillswick, in Shetland, and typical 
 hornblende has been found by Heddle on the north-west of Ben 
 Spinna in Durness, but hornblende is rather associated with the 
 diallage rocks of Shetland and the diorite of Portsoy. Like augite, 
 it undergoes the usual alterations by taking up water, peroxidation of 
 the iron, diminution of the lime, and partial removal of the silica, 
 all these changes being attributable to the action of waters passing 
 through the earth. 
 
 By means of the following tables the varieties of Amphibole and 
 Pyroxene may be contrasted ; but beyond the fact that Amphibole 
 usually contains rather more magnesia and less lime, there is no 
 striking difference in composition ; and it would not be possible to 
 estimate the chemical composition which would give rise to one 
 variety rather than another if a sedimentary rock were metamorphosed. 
 The evolution of these minerals seems to depend upon the other 
 mineral constituents developed in the process of crystalline change. 
 
 PVROXENE. 
 
 
 | d -o 
 
 j 
 
 1 i 1 
 
 
 
 IS 
 
 i 
 
 
 
 
 . 1 
 
 
 
 ,_; 
 
 
 
 
 
 t 
 
 
 
 05 
 
 
 
 
 
 
 
 
 g 
 
 Locality. 
 
 Rock. 
 
 Colour. 
 
 s' 
 
 3 
 
 1 
 
 g 
 
 1 
 
 0) 
 
 1 
 
 A 
 
 4 
 
 | 
 
 3 
 
 
 
 
 02 
 
 3 
 
 
 
 
 
 j 
 
 3 
 
 
 & 
 
 & 
 
 
 
 ( 
 
 Shinness 
 Totaig . 
 
 Limestone . 
 Limestone in 
 mica gneiss 
 
 White 
 Blue . 
 
 53-06 
 
 50-69 
 
 '03 
 
 1-77 
 93 
 
 '47 
 
 07 
 
 23-63 
 25-78 
 
 19-29 
 18-09 
 
 5 
 
 i '43 
 
 1 '54 
 2-62 
 
 |1 
 
 Glen Tilt 
 
 Limestone 
 altered by 
 
 White 
 
 53*24 
 
 
 
 2-71 
 
 " 
 
 *i3 
 
 22-77 
 
 18-86 
 
 
 2-18 
 
 1 
 
 Glen 
 Muick, 
 
 granite. 
 Serpentine . 
 
 Blue . 
 
 5i' 
 
 
 
 i '37 
 
 .'59 
 
 38 
 
 26-36 
 
 17-08 
 
 "63 I "12 
 
 26 
 
 \ 
 
 Coyle 
 
 
 
 
 
 
 
 
 
 
 
 
 *\ 
 
 Ben 
 Chourn 
 
 Granular 
 limestone 
 
 Leek- 
 green 
 
 54'48 
 
 
 
 
 3*13 
 
 "24 
 
 22-82 
 
 17-58 
 
 '44 
 
 '79 
 
 "4 2 
 
 I; 
 
 Tiree . i Marble . 
 
 Dark- 
 
 50-54 
 
 4-69 
 
 4-14 
 
 08 
 
 69 
 
 23*59 
 
 14-4 
 
 '3 1 
 
 "63 
 
 1-48 
 
 3\ 
 
 
 
 green 
 
 
 
 
 
 
 
 
 
 
 
 W 
 
 Eslie . 
 
 Limestone . 
 
 Grass- 
 
 49'5 
 
 i*96| - 
 
 1 1 '06 
 
 4 24-08 
 
 io'8i '57 
 
 8 
 
 fig 
 
 a 
 
 
 
 green 
 
 
 1 
 
 
 
 
 
 
 
 1' 
 
 Balta . j Metamorphic 
 { diallage rocks 
 
 Grass- 
 green 
 
 50-23 
 
 5-84 
 
 
 5 '22 
 
 
 11-23 
 
 2i-59 
 
 I "2 
 
 58 
 
 4-i7 
 
 Q 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Glen Beg 
 
 Limestone . 
 
 Dull- 
 
 54-22 
 
 ' J 7 
 
 
 6'7 
 
 '4 ! i9'57 
 
 16-97 
 
 5 
 
 '45 
 
 q6 
 
 
 
 ; green 
 
 
 I 
 
 | ( 
 
 
 
 Halival, 
 
 Gabbro . 
 
 Dun- 
 
 50-54 
 
 3 '35 
 
 I- 34 
 
 4-42 
 
 '23 
 
 21-42 
 
 17-05 -25 
 
 '53l '7 1 
 
 ^ 
 
 I Rum 
 
 green 
 
 
 
 
 
 
 
 
 
 I 
 
VARIETIES OF AMPHIBOLE AND GARNETS. 
 
 369 
 
 AMPHIBOLE. 
 
 
 
 
 4 
 
 1 
 P 
 
 
 
 
 
 
 
 
 I 
 
 | 
 
 umina. 
 
 rric Oxi 
 
 
 
 i 
 
 n 
 
 i 
 
 
 
 1 
 
 1' 
 
 | 
 
 c3 
 
 | 
 
 1? 
 o 
 a 
 
 -? 
 
 i 
 
 a 
 
 3 
 
 M 
 
 o 
 
 
 j 
 
 "3 
 
 e 
 
 
 
 1 
 
 3 
 
 1 
 
 (2 
 
 1 
 
 P 
 
 Horn- 
 
 Kyle 
 
 Horn- 
 
 Rich 
 
 5 1 "46 
 
 2 96 
 
 2 '45 
 
 9-66 
 
 1-07 
 
 20*07 
 
 10*46 
 
 68 
 
 1-30 
 
 68 
 
 bleude 
 
 of 
 
 blendic 
 
 green 
 
 
 
 
 
 
 
 
 
 
 
 
 Dur- 
 
 gneiss 
 
 
 
 
 
 
 
 
 
 
 
 
 
 iiess 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Horn- 
 
 IMta, 
 
 Diallage 
 
 Dark 
 
 4586 
 
 878 
 
 
 I4 <X 5 
 
 "13 
 
 9*81 
 
 14-4 
 
 82 
 
 t'43 
 
 2' 3 t 
 
 blende 
 
 Shet- 
 
 rock 
 
 green 
 
 
 
 
 
 
 
 
 
 
 
 
 land 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Edenite 
 
 Urqu- 
 
 Granular 
 
 Blue 
 
 5i*3i 
 
 2"2I 
 
 16 
 
 7.66 
 
 '49 
 
 11-17 
 
 20-87 
 
 2 "2 
 
 * 4 6 
 
 2*12 
 
 
 hart 
 
 lime- 
 
 black 
 
 
 
 
 
 
 
 
 
 
 
 
 
 stone 
 
 
 
 
 
 
 
 
 
 
 
 
 Actino- 
 
 Hills- 
 
 Horn- 
 
 Bright 
 
 55" 
 
 I'SI 
 
 '99 
 
 3 '45 
 
 '3 
 
 10*38 
 
 23-30 
 
 1*12 
 
 1-09 
 
 2-89 
 
 lite 
 
 wick 
 
 blendic 
 
 green 
 
 
 
 
 
 
 
 
 
 
 
 
 
 gneiss 
 
 
 
 
 
 
 
 
 
 
 
 
 Tremo- 
 
 Urqu- 
 
 Granular 
 
 Yellow 
 
 57 '3 
 
 6 '67 
 
 i '08 
 
 3-22 
 
 30 
 
 12*36 
 
 16-61 
 
 
 
 2'5 
 
 lite 
 
 hart 
 
 lime- 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 stone 
 
 
 
 
 
 
 
 
 
 
 
 
 Tremo- 
 
 Shin- 
 
 Lime- 
 
 Silvery 
 
 55-15 
 
 85 
 
 1-61 
 
 '7* 
 
 06 
 
 i3'3i 
 
 24'*3 
 
 '44 
 
 "21 
 
 2'5 
 
 lite 
 
 ness 
 
 stoue 
 
 white 
 
 
 
 
 
 
 
 
 
 
 
 Asbes- 
 
 Shin- 
 
 Lime- 
 
 Granu- 
 
 56-86 
 
 "23 
 
 48 
 
 2"I2 
 
 23 
 
 12-53 
 
 23-92 
 
 '43 
 
 '53 
 
 2-52 
 
 tos 
 
 ness 
 
 stone 
 
 lar 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 lime- 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 stone 
 
 
 
 
 
 
 
 
 
 
 
 Amian- 
 
 Balta 
 
 Diallage 
 
 Grey 
 
 56'i5 
 
 i '53 
 
 38 
 
 3'" 
 
 '77 
 
 11-71 
 
 22*46 
 
 18 
 
 6 9 
 
 2'5 
 
 thus j 1 rock 
 
 green 
 
 
 
 
 
 
 
 
 
 
 Garnets. Pyrope or magnesia iron and alumina garnet, though, 
 often found in serpentine, is in Scotland only recorded from basalt. 
 It occurs to the east of Elie, in Fife, and is known as the Elie ruby. 
 The common garnet, or iron alumina garnet, is characteristic of 
 micaceous gneiss and mica slate, especially at Killiecrankie, where 
 garnets are of the size of bullets, and embedded in a paste of mica. 
 The almandite is found in the micaceous gneiss of Sutherland ; and 
 the precious manganese garnet occurs in granitic belts in micaceous 
 gneiss near Strathpeffer in Ross-shire. 
 
 
 
 
 
 
 
 6 
 
 o 
 
 
 
 
 
 Mineral. 
 
 Locality. 
 
 Rock. 
 
 Colour. 
 
 
 | 
 
 3 
 ' 
 o 
 
 1 
 to 
 
 1 
 
 cS 
 
 
 c5 
 
 
 
 
 
 
 1 
 
 | 
 
 1 
 
 1 
 
 1 
 
 | 
 
 1 
 
 1 
 
 
 
 
 
 35 
 
 3 
 
 o 
 
 fe 
 
 & 
 
 A 
 
 3 
 
 * 
 
 
 
 Grossular . . | Creag Mohr . 
 
 Limestone 
 
 Pea-green 
 
 39-83 
 
 9'74 
 
 15*07 
 
 n 
 
 '3533'57 
 
 I -01 
 
 "05 
 
 Cinnamonstone; Glen Gairn . . 
 
 Limestone 
 
 Garnet 
 
 39-2721-98 
 
 i '49 
 
 3 '93 
 
 3331-88 
 
 6 
 
 18 
 
 Pyrope . . . 
 Common garnet 
 
 Elie .... 
 Burra Voe, Yell 
 
 Basalt 
 Gneiss 
 
 Port wine 
 Pinkish 
 
 40-92 
 37 '3 
 
 22-45 
 
 5-46 
 7'47 
 
 8-n 
 
 24-02 
 
 46 
 2*14 
 
 5-04 17-85 
 4'43 3'53 
 
 .. 
 
 
 
 
 red 
 
 
 
 
 
 
 
 
 
 Do. do. 
 
 Killiecrankie . 
 
 Mica slate 
 
 Brownish 
 
 37'59 
 
 13*66 
 
 
 32'S 1 
 
 4'47 
 
 4-12 
 
 3-46 
 
 32 
 
 
 
 
 red 
 
 
 
 
 
 
 
 
 
 Do. do. 
 
 MeallLuidh . 
 
 Mica slate 
 
 Red brown 
 
 37'7 
 
 14-8 
 
 4-56 
 
 32-97 
 
 2'37 
 
 5^9 
 
 1*81 
 
 ,, 
 
 Do. do. 
 
 Knock Hill . 
 
 Diorite 
 
 Port wine 
 
 37' 11 
 
 14*9 
 
 10*13 
 
 32*41 
 
 I "21 
 
 2-17 
 
 2*93 
 
 
 Almandite 
 Precious garnet 
 
 Clach an Eoin 
 Glen Skiag . . 
 
 Mica-gneiss 
 Granite in 
 
 Brown red 
 Red 
 
 39'93 
 35'99 
 
 19-81 
 
 13*69 13*29 
 8-6423-27 
 
 I* 
 15-24 
 
 9'i3 
 '4 
 
 3'3i 
 '47 
 
 '25 
 
 Do. do. 
 
 Loch Garve 
 
 gneiss 
 Granite 
 
 Red brown 
 
 3 6 " J 5 
 
 21-94 
 
 15-15 
 
 ' 
 
 1 5 '09 
 
 7-85 
 
 2 "07] I 62 
 
 '3 1 
 
 
 
 vein 
 
 
 
 
 
 
 
 
 
 Do. do. 1 Struay Bridge 
 
 Granite 
 
 Red 
 
 35-66 
 
 15-8 
 
 
 11*43 
 
 . 
 
 
 *o6 
 
 VOL. I. 
 
 2 A 
 
3/0 NATURE AND ORIGIN OF GNEISS. 
 
 Gneiss. 
 
 Gneiss: its Origin. That the materials of the mechanically aggre- 
 gated gneiss rocks are derived from the disintegration of more ancient 
 granite and other crystallised compounds, is an opinion which is 
 strongly suggested by examining the composition of gneiss. 
 
 The component minerals of gneiss and granite are the same 
 quartz felspar, and mica. They are mixed with the like accidents and 
 permutations, and occasional admixture of other minerals; and are 
 subject in both rocks to the same extreme variation of size. But these 
 rocks differ in the mode of arrangement among their constituent 
 minerals. 
 
 The ingredients of granite are so connected together by con- 
 temporaneous or nearly contemporaneous crystallisation, that one 
 mineral penetrates and is intimately united with another ; and we are 
 compelled to conclude that they were not accumulated in distinct crystals 
 ready formed, but that the minerals never had a separate existence as 
 solids until their different geometric forms were slowly developed by 
 crystallisation. 
 
 Gneiss almost always suggests, by some degree of imperfection 
 of the edges and angles of the quartz and felspar, and much more 
 decidedly by the laminar arrangement of the mica and consequent 
 minute foliation of the rock, that its materials, ready made and 
 crystallised, were brought together and arranged by water. Gneiss 
 in all its gradations, from a rock resembling granite to a fine-grained 
 fissile mass hardly distinguishable from clay slate, may be compared 
 with a series of sandstones, many of which are composed of granitic 
 detritus, and strongly allied to it in structure, but not having 
 undergone metamorphism, show unobliterated evidences that they 
 were aggregated by water. Gneiss, however, is not always the 
 crystallisation of materials in the layers in which they were aggregated, 
 but more frequently a chemical integration of minerals under the sol- 
 vent influence of water contained in the rock which was transformed ; 
 and this water first dissolved the constituents of the minerals, and 
 then often became included in the minerals themselves. 
 
 In a great majority of instances, gneiss immediately rests upon 
 granite ; and we may suppose that the two rocks are effects of the 
 actions of pressure and heat under different conditions. 
 
 Mineral Constituents of Gneiss. Gneiss is essentially a mass of 
 quartz and felspar, foliated with thin films of mica which are some- 
 times exposed by fracture. As in granite, the felspar is usually ortho- 
 clase, but oligoclase is sometimes associated with the orthoclase, though 
 oligoclase is more frequent in hornblende gneiss and protogine gneiss ; 
 there are varieties of gneiss in which orthoclase is the only fel- 
 spar. Occasionally albite is associated with orthoclase. Orthoclase 
 varies in colour in gneiss quite as much as in granite, and is sometimes 
 found in porphyritic crystals. The quartz occurs either in grains or 
 small lenticular plates made up of many crystals united together. The 
 
TEXTURE AND VARIETIES OF GNEISS. 371 
 
 mica may be either potash mica or magnesia mica, and occasionally 
 both micas are found in the same rock. Sometimes the mica sur- 
 rounds the crystals of felspar, giving that mineral a lenticular form. 
 Hornblende is an important constituent in many gneisses of the West 
 of Scotland, and chlorite and talc are found in some gneisses of Scot- 
 land, so that gneiss has often been divided into mica gneiss, hornblende 
 gneiss, and chlorite gneiss. 
 
 Texture of Gneiss. Gneiss varies in texture with the condition 
 of the mica. In the common type, mica is found in separate laminae, 
 dividing the felspar and quartz. But when the foliated texture is 
 indistinct, owing to the imperfect continuity of the mica films, the rock 
 is termed granitic gneiss, and makes a transition to granite. On the 
 other hand, the mica may be so abundant as to isolate the quartz and 
 felspar in lenticular masses ; and in section this condition gives a 
 delicate veined aspect to the rock. Sometimes the mica shows 
 parallelism, giving the foliye of the rock as regular an aspect as 
 exogenous growth in wood ; and this condition further developed 
 imparts a platy cleavage to the gneiss. 
 
 Though the disintegrated materials of granite sometimes com- 
 pose the substance of gneiss, fragments of granite are most rarely 
 discovered in it. 1 But while gneiss may sometimes be a product 
 of denudation of granite subsequently metamorphosed, more frequently 
 the gneiss is simultaneously formed ; and granite is only the same 
 rock material more irregularly crystallised. 
 
 Transition from Gneiss to Granite. Professor Heddle recognises 
 three varieties in this form of metamorphism. First, there is a gradual 
 increase of granitic texture over the gneissic texture. This condition 
 may be seen on the north-east of the hill of Scoltie, near Banchory, 
 where granitic veins of segregation or veinstones appear in the gneiss, 
 while a couple of miles north the gneiss is becoming granite, owing 
 to a steady increase in the number of the granitic bands. The veins 
 themselves become larger, are free from foliated texture, and the granite 
 was a fine grain. Further on towards the hill of Fare the granite bands 
 progressively augment, and the gneissic bands are seen dwindling 
 away. 
 
 The second form of granitic metamorphism is abrupt, and is well 
 seen in the quarries of Tillyfourie, where the gneiss is highly con- 
 torted, broadly banded, and passes suddenly into granite without any 
 trace of intermediate mineral condition. Yet there is no space where 
 the one can be said to be in contact with the other. The openings 
 between the folias of the gneiss cease to exist along a wavy course, the 
 only sign of change being that sometimes the dark mica of the granite 
 has its plates parallel to the line of the transition near to the un- 
 altered rock. 
 
 Thirdly, there is metamorphism by a gradual fading of granite into 
 
 1 MacCulloch says that in certain varieties of mica schist fragments of granite, 
 of quartz rock, and of limestone are embedded in it. John Phillips suggests 
 that he may not always have been careful to avoid admitting conglomerates among 
 mica schist and gneiss. 
 
372 MODES OF OCCURRENCE OF GNEISS. 
 
 gneiss. This is seen in quarries of the stony hill of Nigg ; the gneiss 
 is fine grained, plicated, and darkly striped ; its layers gradually pass 
 into granular dark grey granite. Fragments of gneiss are often found 
 in the Aberdeen granite, and Heddle regards the kidney-shaped inclu- 
 sions, rich in black mica, as portions of gneiss. 1 
 
 Bedding of Gneiss. Beds of gneiss are of various thicknesses, 
 and the laminae of which they consist are subject to such extra- 
 ordinary curvatures, that it is often difficult to trace them. 
 
 Where gneiss alternates with other rocks, such as hornblende slate, 
 quartz rock, limestone, or mica slate, the stratification is rendered 
 very evident, but otherwise the beds are less regular, and often dis- 
 continuous. 
 
 The contortions of the laminae of gneiss are most numerous and 
 surprising where, as frequently happens, veins of granite, quartz, 
 or felstone divide the gneiss. These veins cross the laminae at 
 various angles, and generally cause some peculiar twists along their 
 sides ; veins not infrequently insinuate themselves between the 
 laminae, and in this case, when thick and extensive, may be mis- 
 taken for alternating strata. Some cases of supposed alternation 
 between gneiss and granite may be thus explained, and in other cases 
 the rock called granite may be really a coarsely-granular gneiss. 
 
 Minerals in Gneiss. Gneiss being one of the most widely-distri- 
 buted foliated rocks, is a rich repository of minerals, both in the 
 Old World and America. Garnets frequently, zircon, beryl, disthene, 
 epidote, tourmaline, rutile, oxide of tin, oxide of iron, sulphide of 
 molybdena, more rarely, are disseminated in its laminae. The veins 
 of quartz, calcareous spar, carbonate of iron, and sulphate of baryta, 
 which divide it, contain the sulphides of lead, copper, and zinc, 
 native silver, tin, &c., in Sweden, Germany, and Brazil; and many 
 other minerals occur in the calcareous strata which alternate with or 
 are enveloped by layers of gneiss. We can only suppose that the 
 metallic substances diffused in the sea were extracted by organisms, 
 or precipitated with the sediments, and that the minute and diffused 
 particles became collected by solvent water under the influence of 
 metamorphism. 
 
 Rocks Associated with Gneiss. Gneiss alternates with granite 
 in the Riesengebirge and in Quito, and in some cases graduates into 
 the character of granite, as on the southern declivity of the Titlis and 
 Jungf rau (the age of this gneiss, however, may be more recent) ; more 
 frequently it exchanges beds with mica schist, hornblende schist, 
 and granular limestone and clay slate. These rocks are sometimes 
 in such small quantity as merely to mark lines of division in the 
 mass of gneiss, but at other times they swell out to great thickness. 
 
 The limestone beds in particular are remarkably local and irregular 
 in their occurrence, and instead of extending, like the more recent 
 calcareous strata, through large tracts of country, appear in the form 
 of large lenticular masses, enveloped on every side by the predominant 
 
 1 Heddle, Trans. Roy. Soc. Edin., vol. xxix. p. 6; see however supra, p. 218 
 and p. 259. 
 
GEOLOGICAL ANTIQUITY OF GNEISS. 573 
 
 rocks of gneiss. The term subordinate, on a great scale, is not inv 
 properly applied to these lenticular masses, though in local geology 
 their occasional great extent and comparative regularity may entitle 
 them to be classed under an independent title. 
 
 By the substitution of hornblende for mica, gneiss gradually 
 changes to hornblende schist ; the loss of its felspar approximates it 
 to mica schist, the diminution of its mica produces a resemblance to 
 quartz rock. A finely granular slate, with more evidence than usually 
 appears of watery friction among the particles, almost makes a transi- 
 tion from gneiss to sandstone, as at Dalnacardoch. A more minute 
 admixture of its ingredients, with a predominance of chlorite, gives it 
 the aspect of argillaceous slate. These gradations are observed most 
 frequently at the junctions and alternations of the several rocks. 
 
 Gneiss forms Intrusive Veins. Yon Gotta remarks that it was 
 formerly believed that all gneiss is of metamorphic origin, but it has 
 been established that many kinds of gneiss are eruptive. In the min- 
 ing districts of the Erzgebirge there is a red gneiss containing 75 per 
 cent, of silica, and a grey gneiss containing 65 per cent, of silica ; and 
 this red gneiss or gneissite is said sometimes to form dykes and veins 
 in the common grey .gneiss. 
 
 Grey Gneiss Associated with. Mineral Veins. The grey gneiss is 
 more frequently associated with metalliferous veins rich in silver, and 
 this relation has been attributed to the influence of the iron yielded 
 in greater quantity by the decomposition of the mica in the grey 
 gneiss. 
 
 Geological Age of Gneiss. Gneiss commonly occurs beneath 
 the sedimentary rocks. It has hence often been regarded as the 
 oldest rock, and there is a disposition among geologists to refer any 
 gneiss beneath primary rocks to the most ancient or Archaean period 
 of Dana. But since slates occur of all geological ages, and granite 
 originated in every period of geological time, we are compelled to 
 believe that schists must have similar differences in antiquity. 
 But gneiss of these? several periods has not yet been determined, 
 because it is almost impossible to fix the age of the base of the 
 gneiss, though the age of the stratified rock above usually offers no 
 difficulty. On the Continent there has been an attempt to separate 
 an older and a younger gneiss, the older rock being beneath the 
 strata, and the younger gneiss resting upon them. But such an ano- 
 malous succession suggests the probability of its being due to some 
 grand inversion of the country ; though, since metamorphism may be 
 of any geological age, there is nothing impossible in gneiss of any 
 age resting upon any strata. 
 
 Fundamental Gneiss. Among the Continental localities for 
 gneiss, Zirkel mentions the Saxon Erzgebirge. This rock near 
 Freiberg has been divided into an older grey gneiss free from inclu- 
 sions, and a younger gneiss, grey or red, which includes fragments 
 of the older gneiss and various slates. Much of Bohemia and 
 Moravia is formed of the younger gneiss. Similar rocks are seen 
 in the Sudetic mountains, the Eulengebirge, and Kiesengebirge, and 
 
374 GNEISS RESTING ON STRATA. 
 
 the west of the Schwarzwald and Bohmerwald. Gneiss is an im- 
 portant rock in Central France, and it forms much of Scandinavia 
 and Finland. With these rocks are grouped the gneiss which skirts 
 the coast of Brazil, and that which constitutes the Laurentian rocks 
 of Canada. 
 
 The gneissose rocks of the Outer Hebrides and "West Coast of Scot- 
 land were first distinguished by Sir E. I. Murchison as " fundamental 
 gneiss ; " and he subsequently endeavoured to identify the rock with 
 the Labrador series of Canada. This gneiss extends from Cape 
 Wrath to Loch Broom, Loch Maree, and Loch Torridon ; and here, 
 as in the long island of the Hebrides, it is a mass of granitoid gneiss, in 
 which hornblende and quartz are much more important than felspar 
 and mica. Its strike is from north-west to south-east. 
 
 Gneiss which Rests upon Stratified Rocks. North-east of Chem- 
 nitz and west of Freiberg three large masses of gneiss, many thousand 
 feet thick, extend in a line. They rest upon Silurian slaty rocks, and are 
 covered by carboniferous conglomerate. Gneiss extends about Munch- 
 berg, north-west of the Fichtelgebirge,and fills an elliptical basin of eight 
 square miles in the newer clay slate. The gneiss appears to rest 
 unconformably on the rocks beneath. This condition is attributed by 
 G umbel to inversion, but is sometimes supposed to indicate a younger 
 gneiss. In West Finnmark a thick gneiss rests upon clay slate, lime- 
 stone, and mica slate. In Central Norway crystalline slates rest upon 
 Silurian strata ; and David Forbes described sections above Christian- 
 sand in which bands of granite about ten feet thick alternating with 
 limestone are interstratified in gneiss. All these rocks are mixed in 
 extraordinary confusion on the Torresdale river, where foliated gneiss 
 passes into granitic gneiss, and both alternate irregularly with granite 
 and limestone. 
 
 In the Alps, about the Col de Geant, near Mont Blanc, by the 
 Grimsel and St. Gothard, for example, gneiss is found folded between 
 beds of limestone which cannot be older than the Lias. Hence the 
 gneiss must have originated after the Lias was formed. 
 
 Mica Schist. 
 
 Its Origin. Mica schist, like gneiss, appears sometimes to 
 have derived its ingredients from the destruction of granitic rocks, 
 though it contains but little felspar. We may conjecture that the 
 felspar was decomposed during the disintegration of granite, and 
 mostly carried away, leaving the quartz and the mica to be arranged 
 by the water in the alternate layers which render this rock so remark- 
 able. Such ideas are readily suggested by the structure of the Mill- 
 stone grit and Carboniferous sandstones of the North of England ; 
 though its association with granite favours the belief that it originated 
 under similar conditions. 
 
 The foliation of mica schist is subject to much unevenness in 
 
NATURE AND ORIGIN OF MICA SCHIST. 
 
 375 
 
 consequence of the irregular size and arrangement of the pieces of 
 quartz ; and the undulations thus occasioned on the micaceous surfaces 
 are often further mo- 
 dified by interspersed 
 garnets, the growth 
 of which appears to 
 have pushed aside the 
 other ingredients. Be- 
 sides this minute in- 
 equality, the laminae 
 of mica slate are liable 
 to the same contor- 
 tions and curvatures 
 those of gneiss ; 
 
 as 
 
 -Mija Schist at Loch Lomond. 
 
 Fig. 73- 
 
 the same difficulty often occurs in tracing its beds ; similar and very 
 numerous veins of quartz traverse and mingle with its layers ; and 
 when in contact with granite it is locally penetrated by similar granite 
 veins. Small cavities lined with crystals appear among the most con- 
 torted parts. 
 
 Mineral Constituents of Mica Schist. The kind of mica in 
 mica schist varies with the locality. In the St. Gothard the soda 
 mica Paragonite is found. In some localities the yellowish- white 
 potash mica is rich in water, and forms the species Damourite. The 
 colours of the mica vary, but dark magnesia mica is most common. 
 This mineral determines the colour of the schist, which is grey, or 
 greenish-grey, or yellow-grey, or may be brownish black. 
 
 The quartz occurs in grains, scattered between parallel layers of 
 mica scales. As the quantity of quartz increases, the grains become 
 large, flattened lenticular plates, among which films of mica are diffused. 
 Occasionally the quartz becomes so abundant as to be only separated 
 into layers by thin films of finely-divided mica, and such varieties 
 make a transition to quartzite. The varieties which are poorest in 
 quartz always have*sinall grains of quartz enveloped in the laminae of 
 mica. The varieties of texture are similar to those of gneiss ; but the 
 crumpled wavy structure is one of its most typical modifications. 
 
 Ripple-Drift in Mica Schist. Dr. Sorby has described the struc- 
 ture of mica schist, in which he recognises the " ripple-drift " seen in 
 sandstones. This drift structure is by no means rare in the mica 
 schist of the Highlands of Scotland. It may be well studied between 
 Aberdeen and Stonehaven, and between Arrochar and Tarbat. In 
 many specimens of typical mica schist the original grains of sand may 
 be recognised under the microscope, because the original quartz is full 
 of fluid cavities, and is surrounded by the clear quartz of the schist, 
 which may similarly enclose grains of felspar sand. 1 
 
 Flexures of Mica Schist. The accompanying sketches (figs. 73, 
 
 75) were taken from the mica schist near the anticlinal axis of 
 
 these beds which crosses the upper part of Loch Lomond. On a 
 
 f/rcat scale, the laminations of gneiss and mica schist are suffi- 
 
 1 Q. J. G. S., vol. six. p. 401. 
 
376 
 
 QUARTZ ROCK AND QUARTZITE. 
 
 Fig. 74- 
 
 ciently parallel to give the idea of disturbed surfaces of deposition ; 
 on a small scale, by close examination, innumerable centres of local 
 forces, producing minute, recurring, and anastomosing curvatures, appear. 
 These minute flexures are due, not so much to general or 
 external pressure of the whole mass, as to the mechani- 
 cal displacements effected in the mass by the generation 
 of new minerals, as garnet, or the aggregation of others, as 
 quartz, or felspar, or both. Thus the mica and chlorite 
 which generally meet the surfaces of lamination appear to 
 have been shouldered about without being fused, twisted 
 in their structural planes, and subject to that curious 
 minute folding which is often observed as one of the 
 effects of cleavage structure in delicate and pliable shells, 
 in slates, for which the term " creep " was introduced by Professor 
 John Phillips (fig. 74). 
 
 Accessory Minerals in Mica Schist. Various minerals are dis- 
 seminated through it, as garnet, which may be two inches in diameter 
 or almost invisible. It is common in the Alps, abundant in Bohemia, 
 rare in the Thuringerwald, and less rare in Scotland and Ireland. 
 Emerald, beryl, disthene, tourmaline, felspar, epidote, hornblende, 
 graphite, staurolite, chlorite, talc, fluor spar, baryta, are sometimes 
 abundant. The accessory minerals give names to many local varieties 
 of mica schist, such as garnet mica schist, chlorite mica schist, tour- 
 maline mica schist, hornblende mica 
 schist, &c. ; and sometimes the ac- 
 cessory becomes the typical or 
 dominant mineral. Molybdena, 
 rutile, oxide of tin, wolfram, 
 oxide of iron, grey cobalt, native 
 gold occur. Its metallic veins are 
 of the same nature as those in 
 gneiss. 
 
 Physical Relations of Mica 
 Schist. Mica schist alternates in 
 the same way with quartz Tock, 
 hornblende schist, and the older 
 slates, and encloses similar deposits of limestone and dolomite. It 
 seems, therefore, almost superfluous to say that the line of rigid dis- 
 tinction between mica schist and gneiss cannot be drawn in the field. 
 On a great scale the two rocks retain their typical characters over large 
 tracts of country, and their distribution is hence essentially the same. 
 
 Quartz Rod: 
 
 Quartz rock, in the greater number of instances, especially when 
 occurring in veins, seems more recent than mica schist and gneiss, though 
 by easy changes in composition it becomes nearly identical with them. 
 The internal evidence of texture seems to decide the question of the 
 origin of quartz rock, and to prove that, however altered by subsequent 
 
 Fig. 75- Mica Schist, q quartz, c chlorite. 
 
OCCURRENCE OF CRYSTALLINE LIMESTONE. 377 
 
 metamorphic action, it was originally a mechanical deposit. The 
 degree of compactness which it exhibits varies extremely, in some 
 cases approaching the loose granular character of sandstone, 1 in other 
 cases it has the density of the quartz of veins. In this latter case it 
 seems that the mass is composed of fragments so firmly united as to 
 suggest the idea of their having been soldered together by fusion or 
 cementation by infiltration, since their deposition from water. Perhaps, 
 also, in some cases, what has been considered as quartz rock may be 
 really an expanded quartz vein. 1 
 
 Minerals in Quartzite. In South America quartzite is the repo- 
 sitory of many rich ores and metals. Native gold is found in Brazil in 
 a stratified quartz rock, and micaceous iron ore appears to be intimately 
 related to quartz rock. The "flexible sandstone" of the same country 
 is a granular rock with drusy cavities containing topaz and amethyst. 
 
 Crystalline Limestone. 
 
 Its Origin. Crystalline limestone is in general stratified ; it fre- 
 quently alternates with gneiss and mica schist, and sometimes retains 
 argillaceous partings ; it was therefore a w r ater-f ormed deposit. Its state 
 of granular or saccharoid crystallisation is due to changes developed 
 since its deposition, and partly occasioned by the action of heat on 
 contained water : this change is more obvious in the deeper-seated 
 than in the newer calcareous deposits. 
 
 The beds of crystalline limestone, whether distinctly stratified or 
 not, are in general detached and limited, and so entirely enveloped in 
 gneiss and mica slate, as to form but subordinate members of those 
 widespread rocks. This fact is in harmony with the occurrence of 
 thin and imperfect limestone bands among the Devonian slates of 
 West Somerset and North Devon, and appears to indicate that in the 
 earlier periods of British geological time the precipitation of calcareous 
 matter was of a more local and limited nature than those depositions 
 which produced the widespread strata of Carboniferous Limestone, 
 Lias, Oolite, and Chalk. 
 
 However we may seek to explain it, the fact is undoubted, that 
 during the deposition of the rocks which were afterwards converted 
 into gneiss and mica slate, a large quantity of calcareous sediment 
 was deposited, not in one uniformly extended stratum, but at 
 scattered points and in unequal quantity. And this irregularity of 
 deposition continues to be observed in an inferior degree in the 
 limestones of Cambrian and Silurian age, which are often lenticular ; 
 but above this horizon the calcareous strata become more abundant, 
 regular, and extensive. 
 
 Minerals in Crystalline Limestone. Though crystalline lime- 
 
 1 Some of the quartz rocks of Scotland and Anglesea have a conglomeritic 
 character. In Garveloch, Von Dechen noticed rounded masses of granite, quartz, 
 and corneous limestone embedded in a basis of clay slate, passing to quartz or 
 mica schist. Mr. Sharpe cautions us that some of the quartz rocks of MacQulloch 
 are of later date (Phil. Trans. 1851). 
 
378 CHARACTERS OF CRYSTALLINE LIMESTONE. 
 
 stone is a simple rock, its aspect admits of many variations from 
 unequal admixture with other mineral substances. Of these, the most 
 frequent are mica, talc, and steatite, the latter of which often com- 
 municates a green or mottled colour to the whole rock. Crystals of 
 augite occur in the Island of Tiree, garnets and felspar crystals in the 
 Col de Bonhomme, and tremolite occurs in it in some places. Argil- 
 laceous slate lies upon its laminae. It sometimes assumes a brecciated 
 character, as if composed of limestone fragments, and more rarely it 
 contains fragments of gneiss and mica slate. 
 
 It is used for statuary carving and architectural decoration ; con- 
 tains a great variety of minerals ; and locally is traversed by veins of 
 quartz, felspar, and granite, and by veins of cobalt, galena, and iron. 
 
 Contains no Organic Remains. The limestone associated with 
 gneiss and mica slate is usually destitute of organic remains. Gneiss 
 and mica schist were therefore formerly classed as hypozoic, or beneath 
 the strata which contain evidences of palc&ozoic life. It has been said 
 that if we suppose the crystalline limestones which are devoid of 
 organic remains to have derived their peculiar texture from changes 
 subsequent to their deposition, developed under the influence of water, 
 subterranean pressure, and heat, it is probable that the absence of 
 organic remains may be a consequence of this change. 
 
 Summary. To conclude this discussion, we may collect in a small 
 compass the possible speculations of the origin of the gneiss and 
 mica schist. 
 
 Gneissic foliation is often the original, or the remains of the 
 original lamination, imparted to the mass by the successive accumu- 
 lation of its particles under water, and altered by subsequent 
 action of water, pressure, and heat. These circumstances are suffi- 
 cient in some cases to modify the internal texture, and almost to 
 obliterate the structure, and thus to reconvert the sandstone into 
 gneiss and the gneiss into granite \ in other cases the grains of 
 quartz, felspar, and mica are united together, and new minerals 
 of easier fusion like garnet are generated ; and in other cases the 
 sandstone acquires a superior degree of coherence, and becomes 
 quartzite. This view reconciles the diversity of the gneissic and 
 schistose beds with probable differences of mechanical origin of 
 their materials and degrees of applied heat ; and takes account of 
 the general truth that the foliation of the whole gneissose and 
 micaceous series is parallel to the great axes of earth-movement. 
 The excessive abundance of minute flexures, the occurrence of many 
 cavities, and the frequency of intrusive quartz veins, are observed 
 on and near to the summits of the arches of the laminae. 
 
( 379 ) 
 
 CHAPTER XXI. 
 
 DISTUIBUTION OF GNEISS AND MICA SCHIST. 
 
 In England, &c. The extent of country occupied by old gneiss 
 and mica schist with their associated rocks is enormous, and there 
 are few districts of large area where granite appears without being 
 surrounded by foliated rocks. The order of their succession and 
 their relative thickness are very uncertain. In some districts gneiss, 
 in others mica slate, in others quartz rock, is the only rock seen, and 
 that is immediately succeeded by clay slates. There are even cases 
 where the whole metamorphic system is wanting, and large areas of 
 granite are immediately invested by clay slates and limestones con- 
 taining organic remains. In England, for example, gneiss and mica 
 schist, crystalline limestone and quartz rocks are almost unknown ; 
 but in Ireland, and especially in Scotland, they are abundant, and 
 include many gradations of mineral composition, such as chlorite slate, 
 talc slate, hornblende slate, &c. The general order of succession 
 among the older primary strata in Scotland may be represented in 
 a diagram as in fig. 76, but it 
 must be remembered that all 
 the terms of the series are sel- 
 dom co-existent in the same 
 section. 
 
 In Cornwall and Wales the 
 granitic rocks are almost univer- 
 sally succeeded by modifications 
 of clay slate, Anglesea alone 
 exhibiting a quartzo - micaceous 
 group below all the Cambrian 
 slates. In Cumberland the gra- 
 nite of the river Caldew is in- 
 deed covered by rocks having the Fig ?6t 
 character of gneiss, mica schist, 
 
 dark hornblende slate (provincially called whintin), and chiastolite 
 slate, but their area and thickness are inconsiderable, and the latter 
 rock soon changes to clay slate. At a place called Martindale, at the 
 eastern foot of Caldbeck Fells, is a fine-grained variety of gneiss in 
 very thin laminae. Granite veins are rarely known to divide any of 
 the rocks of this region, except on a small scale between Skiddaw 
 
380 ORIGIN AND SUCCESSION OF LIZARD SCHISTS. 
 
 and Saddleback. Gneiss occurs, sometimes exchanging its mica for 
 hornblende, on the east flanks of the southern parts of the Malvern 
 Hills, where it is much intermixed with dolerite. It may be regarded 
 as older than the Cambrian rocks. 
 
 Schists of the Lizard, The serpentine of the Lizard district, 
 which has an intrusive aspect, seems to rise as a boss within a girdle 
 of schists. These schists, which form a single natural group, are 
 nevertheless divisible into three series, which Professor Bonney dis- 
 tinguishes as micaceous, hornblendic, and granulitic. The lowest 
 series consists of compact dull green schists and brown mica schists. 
 The middle group is formed of black hornblendic schists, which are 
 shining, and banded with felspar or epidote. The uppermost series 
 is formed by pinkish-grey rock, composed chiefly of quartz and felspar, 
 but banded with micaceous layers and seams of dark hornblende. 
 
 The lowest group is well shown on the southern coast of the Lizard, 
 especially about the point called the Quadrant ; and in the next cove 
 there is a faulted junction between the micaceous series and the 
 hornblendic series. The hornblendic group is well shown in Houzel 
 Bay ; and at Hot Point the rocks show lenticular bedding, with 
 remarkable instances of false bedding and indications of ripple-drift. 
 These hornblendic schists are frequently rich in epidote, and some- 
 times have a granitoid character. At the bay under the Balk quarry, 
 where the first intrusion of serpentine is seen, the commencement of 
 the granulitic series is found. These rocks consist of distinctly strati- 
 fied beds of massive quartz, alternating with schistose bands. The 
 quartzite is sometimes felspathic, and occasionally contains a little mica 
 or hornblende, but the darker schistose layers are rich in these 
 minerals. Lenticular bedding and current bedding are rare in this group. 
 The hornblende schists are well developed on various parts of the coast, as 
 in Polurrian Cove, and on Pradanack Point, where the schists become 
 very massive, and in hand specimens resemble a diorite. Professor 
 Bonney calculates the maximum thickness of the whole series at about 
 3300 feet, of which rather less than three-fourths would belong to the 
 hornblendic group, and about 300 feet to the granulitic group. These 
 schists have lost their original structure, and many have become highly 
 crystalline, but the current-bedding appears to be preserved, especially 
 at Hot Point, although the constituents of the rock have undergone 
 a crystalline change. It is suggested that the metamorphic agency 
 softened the whole rock mass, so that the materials crystallised much 
 as they would have done had the rock material been igneous ; and in 
 places the unaltered or undigested rock remains, surrounded by 
 crystalline minerals which form granitoid bands. Professor Bonney 
 thinks that such hornblendic schists may originally have been 
 basaltic tuffs. Such an origin would explain the close correspondence 
 between some of the altered rock and diorite, which furnishes evidence 
 of unusual interest concerning the transition between the water-formed 
 and fire-formed rocks. The progress of metamorphism may have been 
 extremely slow, for the change consisted in the replacement of the 
 materials of the rock under conditions of pressure and crumpling such 
 
METAMORPHISM IN SOUTH DEVON. 381 
 
 as earth-movements might produce. The age of these schists is un- 
 certain, but they present a close resemblance to the micaceous and 
 chloritic schists of Anglesea and Holyhead, and they have similarly 
 been grouped with the Archaean series. 1 
 
 Schists of South. Devon. The rocks of South Devon between 
 Start Point and Bolt Tail are highly metamorphosed. They consist 
 to a large extent of mica schist, though in many places this rock gives 
 place to chloritic schist, as at Prawle Point and in Bolt Tail itself, 
 and on the east side of Salcombe estuary. This region has always 
 appeared interesting because Sir Henry De la Beche was inclined to 
 regard it as indicating progressive metamorphic action, which increased 
 in intensity southward, while Sedgwick and Murchison inclined to 
 the belief that the southern schists show no transition to the northern 
 slates. Their views have been confirmed by Professor Bonney, who 
 finds that there is a passage from slates and phyllites to rocks with 
 incipient metamorphism into schists, which is too rapid to indicate 
 transition. This author observes concerning the mica schists that in 
 some cases the structure of the crystalline constituents indicates that 
 folding has been the first stage in the process of metamorphism, 
 and that the chemical or mineral change has been subsequent to the 
 mechanical one. In other cases the folding and crystalline changes 
 appear to have been produced together, while in a third case the rock 
 was crystalline before it was folded. Professor Bonney arrives at 
 the conclusion that the metamorphism was long anterior in date to 
 the great lateral compression which has corrugated the rocks. Dr. 
 Holl regarded the slates at Tor Cross as newer than the Plymouth 
 Limestone, but Professor Bonney thinks that the Cornish metamor- 
 phic rocks suggest that these may be a prolongation of the schists of 
 the Lizard, which may have included the gneiss of the Eddystone, and 
 that they too date back to the Archaean period. 2 
 
 Metamorphic Rocks of Anglesea. Anglesea exhibits schists in 
 grand development. North of the Holyhead Road they are green 
 an,d purple, and often micaceous, and lie in wavy lamination. North 
 of Church Bay they are a white or yellow felspathic schist; and 
 between Carmel's Point and Church Bay gneiss is well developed, 
 and the bedding has so entirely disappeared that the rocks have the 
 aspect of igneous masses. In this part of Anglesea the bedding and 
 foliation cross each other. Holyhead Island is formed of gnarled 
 schists, in which stratification, foliation, and cleavage are frequently 
 indistinguishable. Nowhere are the strata more violently contorted 
 than in the cliffs and islets round Holyhead. The eastern half of 
 Holyhead is described by Sir Andrew Ramsay as a kind of quartz rock. 
 Professor T. M'Hughes indicates four great masses of gnarled schists, 
 the Holyhead mass, the Amlwch mass, the Llangefui mass, and the 
 Menai mass. 3 
 
 Foliation in the Volcanic Series of St. Davids. Professor A. 
 Geikie remarks that a fine foliation, arising from the development of 
 
 1 Bonney, Q. J. G. S., vol. xxxix. p. i. 2 Q. J. G. S., vol. xl. p. i. 
 
 3 Q. J. G. S., vol. xxxvi. 
 
382 METAMORPHIC ROCKS OF SCOTLAND. 
 
 micaceous minerals along the planes of stratification, has been exten- 
 sively developed in the volcanic group of St. Davids (as we have 
 observed in ashes in the Eifel) and in the overlying sedimentary 
 rocks. It has affected many fine tuffs, the paste of some coarse 
 tuffs, and some shales. Where it occurs the rock is usually pale 
 apple green to pearl grey, sometimes pink, with a lustre like silk 
 and a soapy surface. Under the microscope the base of the schist 
 is a felted aggregate of minute scales of nearly colourless mica. 
 Chlorite and some other minerals occur with it. The schists which 
 are inters tratified with the sandstones and shales above the conglome- 
 rate present almost identical characters. 1 
 
 Gneiss in Scotland. Gneiss is abundant in Scotland, particularly 
 in the northern and western parts, and being exceedingly variable 
 in composition, is very often indistinguishable from mica schist, under 
 which head M. Bou6 preferred to class many of its varieties. 
 
 Gneiss constitutes almost the whole mass of lona, Tiree, Coll, 
 Rona, and the Long Island of the Hebrides, and enters largely into 
 the composition of the Shetland Isles, which are in some measure to 
 be viewed as a prolongation of the Hebridean group, just as the 
 Orkneys appear to be an extension of the eastern rocks of Caithness. 
 Housa, Burra, Whalsay, Out Skerries, and Yell, and the western parts 
 of Fetlar and Unst, and part of the mainland of Shetland, are gneiss. 
 The remainder of the mainland is principally mica slate, and the tw<? 
 rocks are partially separated from each other by an interrupted deposit 
 of limestone. The gneiss is often porphyritic, as in Unst ; at Hagra- 
 sattervoe 2 it appears to contain masses of granite, as well as to be 
 traversed by veins of syenite and talcose granite. Kaolin is derived 
 from it in the Mainland and in Fetlar. Gneiss exists in the Orkneys, 
 around the granite of Stromness. 3 
 
 In the Hebrides this rock changes often from the typical mixture 
 of quartz, felspar, and mica, by the substitution of talcose minerals 
 and hornblende for mica, by the omission of the quartz, and by the 
 inter-lamination of argillaceous schist. Some varieties are extremely 
 slaty, and suffer rapid decomposition; others approach nearer to 
 granite, and present rude and naked surfaces and precipitous faces, 
 with few brooks and little alluvium. The direction of the strike of 
 the strata in the Hebrides is north-west and south-east, but the 
 inclination of the beds is obscured by frequent contortions. These are 
 most frequent in the vicinity of the granitic veins which divide all the 
 gneiss rocks, except those which are associated with clay slate. The 
 drawing which MacCulloch gave of the contorted laminae of gneiss 
 and hornblende slate in connection with ramifying granite veins 
 near Cape Wrath seems to justify his views, for the laminae of gneiss 
 are often peculiarly bent or apparently dislocated along the line of 
 the veins, and sometimes masses of this rock are curiously enveloped 
 in their substance. 
 
 1 A. Geikie, Q. J. G. S., vol. xxxix. p. 310. Where also see Prof. Geikie's 
 account of the metamorphism produced by contact of granite with stratified rocks. 
 
 2 Hibbert, Edin. Phil. Jour., vol. ii. 3 Boue, " Gdologie de 1'Ecosse." 
 
SCHISTS OF THE HIGHLANDS. 383 
 
 Rocks in West of Scotland. The veins are not often filled 
 with granite of the ordinary kind, but with a compound rock, in 
 which felspar highly predominates, so as to form in several places 
 (Harris, South Uist, Eona, and Coll) a real graphic granite, which in 
 Coll contains garnets. Veins of quartz, occasionally metalliferous, 
 likewise traverse the gneiss of Coll and Tiree. Garnet, rose quartz, 
 zircon, hornblende, epidote, fluor spar, iron pyrites, and sulphide of 
 molybdena, occur in the gneiss. 
 
 In Eona, Coll, and Tiree, mica schist alternates universally with 
 the gneiss. Gneiss occurs in many places, as round the granitic 
 mountains of Braemar and Lachin y gair, at Kincardine, in Eoss- 
 shire, and other points in the extreme North of Scotland ; but the 
 most abundant and interesting mass adjoins the granite of Strontian. 
 It forms the beautiful and picturesque region around Loch Sunart, 
 which strongly resembles the Trossachs of Loch Katrine, being 
 equally rich in wood and remarkable for intricate confusion of rugged 
 surface. The curvature to which its laminae are here subject are very 
 numerous and extraordinary ; veins of quartz, felspar, and granite are 
 extremely common, garnets abound in it at certain points, and in the 
 metalliferous veins are carbonate of strontian, harmotome, and cal- 
 careous spar. On the eastern side it is bounded by porphyritic masses, 
 but in other directions appears to be overlaid by mica schist, to which 
 its composition approximates. 
 
 Mica Schist in the Highlands. But the principal part of the 
 Highlands is occupied by mica schist, whose layers range with more or 
 less regularity north-east and south-west, notwithstanding the inter- 
 ruption to their continuity by the unstratified rocks of the Braemar 
 mountains and the groups of Ben Cruachan and Ben Nevis. 
 
 The south-eastern limit of this vast crystalline area is the line of 
 the foot of the Grampians from the Firth of Clyde to Stonehaven. 
 Deposits of red sandstone, lias, and a coal-bearing part of the oolites 
 border the eastern coast from the river Spey to Duncansby Head, 
 and extend through the Orkneys. Eocks of igneous origin associated 
 with schistose rocks mostly occupy the base of the volcanic regions of 
 St. Kilda, Skye, Eum, Eigg, Mull, parts of Ardnamurchan and Mor- 
 ven. Within these limits, and with the exception of irregular masses 
 of igneous rocks and of gneiss, the whole of the vast space belongs to 
 the mica slate series, with its included quartz rocks, limestones, 
 serpentines, potstones, associated hornblende slates, and talcose slates, 
 and overlying clay slates. 
 
 Metamorphic Rocks on the Southern Slope of the Grampians. 
 Professor Nicol remarks that the clay slate extends along the 
 southern flank of the Grampians from Arran to Stonehaven. In 
 Arran it is parted from the mica slate by granite. In Bute the 
 clay slate rests on mica slate, and both dip south-east. This dip 
 extends along the Firth of Clyde, and at Gareloch the junction 
 of the mica slate and clay slate is well seen. The mica slate 
 runs up Loch Long, and is so twisted and contorted that the dip 
 is undeterminable. The rock is a lustrous slate, which towards the 
 
384 THE SOUTH SLOPE OF THE GRAMPIANS. 
 
 Gareloch becomes less crystalline and more sandy, and disappears 
 beneath the clay slate near Eoseneath. The lower clay slates are 
 micaceous, deep blue or purple ; the upper are light blue or green and 
 silvery. This is the normal order of the beds on the south border of 
 the Western Grampians. 
 
 Structure of Ben Lomond. Ben Lomond consists at the north 
 end of mica slate, often quartzose, greatly contorted, but with the dip 
 becoming more regular near Eowardennan on Loch Lomond. The 
 mica slate passes sometimes into grey talc slate when the beds become 
 intersected by a parallel vein of fine-grained syenite. Farther on, at 
 Sallachie, the clay slate rests conformably on the mica slate, and 
 according to Professor Nicol the bedding and cleavage coincide. 
 
 Slate Rocks of the Trossachs. To the north of Callander the mica 
 slate and clay slate are well exposed in the Pass of Leny and on the 
 banks of Loch Lubnaig. In the Pass of Leny the lowest beds of 
 clay slate show distinct grains of quartz of sedimentary origin. Then 
 succeeds a great mass of chlorite slate full of reticulate quartz veins 
 associated with foliated rocks consisting of quartz and felspar, and 
 these rocks pass into blue talc slates, which are succeeded by mica 
 slates dipping to the north at an angle of 73 degrees. Ben Ledi 
 is a great mountain on the west of Loch Lubnaig, which consists 
 almost entirely of mica slate greatly contorted. Near Strathearn and 
 Comrie the clay slate is seen in contact with the syenite of Ben 
 Chonzie. The slates show distinct foliation. The syenite has the 
 hornblende more or less replaced by mica, and has the felspar abundant. 
 It sends veins into the slate without producing much alteration. But 
 as the slate rocks approach the syenite they become harder and more 
 crystalline. Then lenticular masses of red felspar and quartz become 
 mixed with the blue slate till the whole rock is converted into a fine- 
 grained gneiss. 
 
 Mica Slate of Loch Earn. Mica slate predominates along the 
 north bank of Loch Earn, where it is greatly contorted, but with a 
 general dip to the north-east ; and Professor Nicol suggests that it here 
 forms an anticlinal axis, which probably continues the contorted beds 
 of Loch Long, Loch Lomond, and Loch Lubnaig. 
 
 Central Gneiss and Quartzite. The gneiss of the Black Mount 
 and Breadalbane Highlands, according to Professor Nicol, forms a 
 synclinal trough and rests on both sides on mica slate. Near Killin 
 at the head of Loch Tay the mica slate is mixed with hornblende 
 slate and limestone. Eound Tyndrum, mica slate is intermixed with 
 beds of quartzite, with a varying dip to the south-east or north- 
 west. At Urchay Bridge gneiss rests on mica slate on both sides of 
 the valley. Beyond Urchay the gneiss continues by Inveroran and 
 King's House to the top of Glencoe. This idea of the gneiss being 
 the newer formation was opposed to the views of Murchison and 
 Geikie. High up the pass of Glen Shee towards Braemar are certain 
 limestones overlain by gneiss, which passes up into quartzite and forms 
 the higher parts of these mountains. Prom Ben Turk Bridge, where 
 there is a grey micaceous limestone dipping south-east, the valley 
 
THE NORTH-WESTERN HIGHLANDS. 38^ 
 
 consists of gneiss with veins of granite. Professor Nicol remarks that 
 the whole series seems to be conformable, and as though the gneiss 
 were the more metamorphosed lower portion, and the quartzite the 
 less metarnorphic higher portion of one formation, which rests on the 
 granite of Ben Macdhui and the Cairngorm mountains, without evidence 
 of disturbance at the junction. 
 
 Gneiss of the North- West Highlands of Scotland. The North- 
 West Highlands of Scotland comprise a strip of about a hundred miles 
 of country, running from N.N.K to S.S.W. through the west of 
 Sutherland and Eoss. This region is formed almost entirely of gneiss 
 and similar metarnorphic rocks which were fully described by Sir K. 
 I. Murchison and Professor Nicol. The succession of rocks from the 
 west to the east is first fundamental gneiss, sometimes diversified by 
 patches of igneous rocks ; second, quartzite with limestone ; and 
 thirdly, the eastern gneiss. Whether this is a true succession has 
 seemed doubtful since the discovery of fossils in the limestone at 
 Durness. The succession has been accepted by experienced sur- 
 veyors Murchison, Eamsay, Geikie, and Harkness. It was contested 
 by Nicol, who regarded the eastern gneiss as a repetition of the western 
 gneiss consequent upon faulting, so that the occurrence of fossils of 
 the genera, Maclurea, Murchisonia, and Orthoceras in the Durness 
 limestone, and similar fossils in the limestones of Assynt and the 
 quartzite of Loch Erriboll, would present no anomalies. The sections 
 are such that considerable latitude in opinion may be allowed to all 
 observers ; for the contortions, the slips, the changes in mineral 
 character, the occurrence of igneous rocks which break the succession, 
 and such-like phenomena, impose caution. Dr. Hicks, who has exa- 
 mined the southern part of this country, accepts the view of the younger 
 gneiss being a separate formation, and regards the older stratified rocks 
 as resting unconformably on gneissic and pre-Cambrian rocks, which 
 have a different strike and with him the unconformity stands in place 
 of the fault, while the western gneiss is made to reappear to the east. 
 
 Hebridean Gneiss. Dr. Galloway has examined the sequence in 
 Loch Broom, Assynt, and Loch Erriboll, and regards the eastern 
 gneiss as newer than the western gneiss, but older than the Assynt 
 series. The lower rocks of this region, called fundamental gneiss or 
 western gneiss, are also termed the Hebridean. 
 
 The Hebridean gneiss, according to Heddle, is composed almost 
 entirely of pink orthoclase, dark-green hornblende, small-flaked black 
 mica, and quartz with a greasy lustre. The hornblende and mica are 
 especially aggregated into broadly-banded stripes. At the south-west 
 of Sutherland, at Loch Iiiver, and towards Loch Polly, the gneiss is 
 greatly convoluted ; its layers become smaller, and the banding is less 
 prevalent. Where the rock is exposed to the sea, near Cape Wrath, 
 the felspar bands weather out, leaving the hornblende standing 
 isolated. At Cape Wrath the gneiss dips south-east ; its prevalent 
 strike is north-west to south-east ; but in Sutherland its dip is south- 
 west. 
 
 Caledonian Gneiss. The eastern gneiss is termed the Cale- 
 VOL. i. 2 B 
 
3?6 FOSSIL-BEARING ROCKS OF ASSYNT. 
 
 (Ionian. It includes in the Erriboll area a lower division, the Arnaboll 
 series, well developed in Ben Arnaboll, which consists of grey grani- 
 toid felspathic gneiss, overlain by dark hornblendic gneiss and mica 
 gneiss, which present a striped aspect. The upper part of the Cale- 
 donian consists of the ordinary flaggy gneisses, exposed on Loch 
 Hope and Ben Hope. This Hope series preserves its character from 
 Loch Erriboll to Loch Broom, and eastward to Lairg in Sutherland 
 and Ben Wyvis in Koss. The upper gneiss is thin-bedded and 
 highly quartzose, sometimes passing into quartzose schist, and some- 
 times into felspathic gneiss. 
 
 The Assynt Series. A third group is the Assynt series, which 
 now lies between the western and eastern gneiss. The rocks are 
 comparatively unaltered. The quartzites contain annelid remains, and 
 possibly plants. The lowest of these Assynt rocks are subdivided 
 into the Tomdon sandstone below and Ben More grit above. This 
 grit, composed mainly of quartz and gneiss, has the fragments, some- 
 times several inches in diameter, embedded in a chloritic ground mass, 
 and passes up into green grits and other fine sediments, sometimes 
 sandstone, sometimes shale. The Torridon rocks appear to be about 
 300 feet thick, and though the bottom conglomerate covers miles of 
 country, it is n<Dt more than 100 feet thick. 1 
 
 Over the Torridon sandstone rocks are the quartzites and seamy 
 quartzite with felspar grains, covered by the Annelid-bearing quartzite, 
 or pipe rock, characterised by vertical burrows, which is seen between 
 Loch Broom and Loch More, and on Loch Erriboll. These quartzites 
 are about 300 feet thick. 
 
 Next succeed the brown flags, fine-grained sandy rocks, some- 
 times argillaceous, sometimes dolomitic, and these rocks graduate 
 upward into the grit, which contains Salterella Maccullochii. Finally, 
 the Assynt series includes the dolomite, which in its lower part is 
 dark grey or black, and in its upper part nearly white. The dolomite 
 is about 300 feet thick. The total thickness of the Assynt series is 
 upwards of 1000 feet. 
 
 All these rocks have been greatly contorted, so that the Assynt 
 series is doubled back upon itself all along Loch Erriboll in a com- 
 pressed synclinal fold, which brings the quartzite over the dolomite ; 
 and there is an overthrow of the eastern gneiss or Caledonian forma- 
 tion upon the Assynt series, and sometimes the Caledonian is brought 
 over the Assynt series by a reversed fault. On Loch Broom the 
 Assynt series dips to the north-east, while the Caledonian series dips 
 to the south-east. The Hebridean gneiss on Loch Broom is in con- 
 tact with and slightly overlies every member of the Assynt series, 
 and in its overthrow is sometimes accompanied by the Torridon sand- 
 stone. Dr. Calloway 2 regards the Caledonian gneiss as having been 
 deposited unconformably upon the Hebridean rocks ; but the relations 
 of the two gneisses have yet to be demonstrated. 
 
 1 See Drawings of Scenery in a paper by Heddle, in Journal Mineral Soc., 
 1880. 
 
 2 Q. J. G. S., vol. xxxix. p. 355. 
 
SCENERY OF METAMORPHIC ROCKS. 387 
 
 Scenery of Gneiss and Mica Schist. The mountains of this 
 system of rocks are formed into little groups separated by deep valleys 
 and long lakes, while their summits rise often more than 3000 feet 
 above the water. Their bases are usually and thickly covered with 
 birch underwood, and sometimes with forests of oak. Scenes of a 
 truly alpine character are very rare in Scotland, and perhaps nowhere 
 occur except in the Cuchullin mountains of Skye and the granite 
 peaks of Arran. The general outline of the mountains is pyramidal, 
 but this form, elegant at a distance, is broken on a near survey by 
 fantastic projections and bare cliffs and channels, which after storms 
 are changed into a multitude of waterfalls. The mountains of quartzite 
 and red sandstone rise abruptly from the great tableland of funda- 
 mental gneiss forming the lower hills on the West Coast of Scotland. 
 The greater elevations are smooth rounded cones, spire-like peaks, or 
 long serrated ridges, whose summits shine under the sun as though 
 capped with snow, and they send down streams of fragments to the 
 sea-lochs which wash their bases. The valleys which are destitute of 
 lakes are usually wild and barren and covered with scattered rocks. 
 Several of the most remarkable valleys in the Highlands follow the 
 strike of the strata ; as, for example, the extraordinary valley of lakes 
 united by the Caledonian Canal, whose highest point is but 90 
 feet above the sea. The valley of the Spey, Glen Tilt, Loch Tay, 
 Loch Long, Loch Fyne, Loch Awe, are other examples. The longi- 
 tudinal valleys are remarkably narrow, as if mere slits in the country, 
 while the numerous transverse valleys are in general more expanded. 
 
 One of the most interesting valleys in Scotland is Glen Roy, which 
 branches off from Glen Spean in Lochaber. Narrow, parallel, con- 
 tiguous terraces, perfectly level as seen from a distance, and continu- 
 ous along the whole length of the glen, mark the higher part of its 
 bordering slopes with singular and most surprising terrace deposits 
 of boulder clay, often laminated and crumpled like the contorted drift 
 of Norfolk, the effect of ancient local operations of ice and level water. 
 It-has sometimes been conjectured that these terraces are traces of the 
 ancient margin of the sea, left uninjured during a subsequent elevation 
 of the whole country, to the extent, perhaps, of 1500 feet. But the 
 limitation of the terraces and the material of which they consist 
 rather suggest that the valley was dammed with a barrier of ice 
 which varied in level and held the waters in Glen Koy as a lake enclosed 
 by glaciers. 
 
 As might be expected, the forms of the mountains, and especially 
 the shape of their summits, are often characteristic of the kind of rock 
 which constitutes them. Compare, for instance, the irregular head 
 and broken slopes of the Cobbler and other mountains of mica slate, 
 with the smoother sides and less angulated chloritic top of Ben 
 Lomond, and the conical summits of quartz rock on Benan, Sche- 
 hallion, and the Paps of Jura. 
 
 Neither are the features of the valleys and waterfalls independent 
 of the nature of the rocks which they traverse. The unequal hard- 
 ness of mica slate, in particular, is often evident in the rapid streams 
 
388 SCENERY OF QUARTZITE. 
 
 by singular hollows and pits in their course and deep cavities under 
 the cascades. A waterfall near Loch Earn Head exhibits this feature 
 remarkably. 
 
 Examples of gneiss-like mica slate are found in Glen Tilt, Dalna- 
 cardoch, and many other points of the Blair Athole country, near 
 Tyndrum, and sparingly around the granite mountains of Arran. In 
 some specimens, as in Glen Roy, it appears composed of little else 
 than mica folded and twisted round garnet crystals ; in other cases, as 
 by Ben Nevis, the garnets form almost distinct layers. In some cases 
 (Glen Roy) the white mica and quartz form very smooth and attenu- 
 ated laminae, like those of cleavage ; in others (Trossachs, Loch Earn) 
 the quartz is in thick irregular plates, which mark one of the grada- 
 tions to quartz rock. 
 
 Quartz Eock in Scotland. Quartzite is exhibited in a sea cliff 
 in the Whitten Head in Sutherland. Towards the sea it presents 
 caverns and stacklike pillars, but its general form is rounded. The 
 quartzite shows no trace of granular structure, and is free from veins. 
 Quartzite is seen opposite to Kyle Akin, in Skye, almost vertical, 
 highly tilted at Loch Kishhorn, and from Loch Broom to Loch 
 Urigill there is continuous quartzite underlying the limestone. Quartz 
 rocks and quartzose mica slates are seen in the North of Scotland, in 
 Moidart, along Loch Shiel and Loch Eil, and the eastern side of 
 Loch Linnhe. Above the granite of Glen Tilt quartz rocks abound 
 in Ben y gloe, and in several mountains round the granite of Braemar, 
 and may be well studied in the valley of the Bruar near Blair. They 
 reappear in Mount Alexander, and on the sides of Loch Rannoch 
 constitute the pyramidal summit of Schehallion, and on the borders 
 of the granitic desert of Rannoch Heath are traversed by granitic 
 and porphyritic veins. Quartz rock extends from Jura into Islay, 
 and is found in Shetland. 
 
 Talcose Slates in Scotland. Talcose and chloritic slates, holding 
 an intermediate mineralogical character between clay slates and mica 
 schists, occupy for the most part an intermediate geological position. 
 They may be well studied on the banks of Loch Lomond and 
 Loch Fyne, and several points on the south slope of the Grampians, 
 where they are often rich in quartz, and remarkable for minute 
 undulations and greater contortions. Chlorite slate is also found in 
 the Long Island, and in Eetlar and Unst. The very common associa- 
 tion of garnets with mica schist and gneiss is one of the effects of 
 heat applied to those rocks since their deposition. 
 
 Hornblende Slate in Scotland. Hornblende rocks, especially 
 hornblende slate, occur in combination with mica slate. Hornblende 
 slate is seen plentifully in Glen Tilt, and is much traversed by granite 
 veins; on both sides of the Pass of Killiecrankie ; south of Sche- 
 hallion ; north of Ben More ; in the upper part of Loch Lomond, and 
 under Ben Cruachan. 
 
 Serpentine in Scotland. Serpentine, a rock whose geological 
 relations are still imperfectly understood, occurs to a small extent in 
 Scotland at manyplaces, accompanied generally with chlorite or steatite, 
 
SOME SCOTCH SERPENTINES. 389 
 
 and diallage rock. It is said to be most frequent among the upper 
 beds of chlorite slate, though occurrences of serpentine in small quan- 
 tities accompany the limestones of lona, Glen Tilt, Harris, and 
 Tiree. On the south side of the Grampians it occurs at Cortachie, 
 on the North Esk, but through the North of Scotland its localities are 
 more scattered. (Near Drimnadrochit, near Inverness.) The beautiful 
 serpentine of Portsoy, said to be employed in some of the apartments 
 at Versailles, forms "three vertical bands," one of them enclosed 
 between hornblende rocks, another between hornblende rocks and 
 crystalline limestone, and the third between quartzose talc slate and 
 mica slate, which is covered by beds of limestone, hornblende slate, 
 and talc slate, and the junction of all these rocks is softened by a 
 mutual exchange of ingredients. In Scalpa, an irregular, highly- 
 inclined bed of serpentine, one hundred yards thick, traverses the 
 gneiss promontory of the lighthouse, and exhibits at its boundaries 
 against the gneiss abundance of hornblende crystals, layers of talc 
 slate, and a sublaminated structure. It contains steatite, asbestos, 
 &c. The granite veins here observed traverse both, the gneiss and 
 its included serpentine, and in the latter rock chlorite is added to the 
 ingredients of the vein. 
 
 Serpentine exists in Lewis, and occurs in Shetland in considerable 
 abundance and beauty, both in the Mainland, in Fetlar, and at Brassa 
 Sound in Unst, where it contains chromate of iron in sufficient abun- 
 dance to be of considerable value in commerce. 
 
 Potstone is found in Glenelg, opposite to Skye, and in the ser- 
 pentine of Scalpa. But the most remarkable rock of this kind is 
 found at St. Catherine's, near Inveraray, on the opposite side of Loch 
 Fyne. It is imperfectly slaty, and has been employed in the erection 
 of the Duke of Argyle's mansion ; other localities are the districts of 
 Strathearn and Breadalbane. 
 
 Crystalline Limestone in Scotland. The white crystalline 
 marbles of lona are found in rocks sometimes referred to mica slate, 
 but considered by MacCulloch to be gneiss. The variously coloured 
 marble of Tiree, with its embedded augite and hornblende, lies in 
 alternating gneiss and mica slate. That of Glen Tilt, characterised 
 by accompanying tremolite, lies in a quartzose mica slate associated 
 with hornblende slate. 
 
 Boue, following up the notices of MacCulloch, traces the line of 
 the Glen Tilt limestones to the east and to the west. In the western 
 direction they proceed from Gow's Bridge, crossing the hills at Lude, 
 and tending toward the south, pass through the Glen of Fincastle and 
 across the valley of the Tummel. It is conjectured that limestone 
 of the same range continues by Mount Alexander and the base of 
 Schehallion, from whence it proceeds through Glen Lyon to the side 
 of Loch Tay at the foot of Ben Lawers, reappears in Crien Larich, 
 at the entry of Strathfillaii to the west of East Tarbet, in Knapdale, 
 and the head of the valley of Croe. 
 
 Eastward from Glen Tilt this limestone is traced in the course of 
 the North Esk, and in the valley of the Dee, near Braemar, &c. 
 
390 CRYSTALLINE LIMESTONE IN SCOTLAND. 
 
 So extensive a range of limestone rocks in the direction of the 
 strata of mica slate may be regarded as being thoroughout a nearly 
 contemporaneous deposit. The limestones on Loch Laggan and Loch 
 Eil in Inverness, and at numerous other points- in Aberdeenshire, 
 are referred by Boue to the same era. 
 
 The marbles of Sutherland were formerly worked in the Inchna- 
 damph district, near Loch Awe; but they contain gritty particles 
 which interfere with the polishing. Yellow arid green serpentinous 
 marbles occur with others of a chocolate-brown and light-red colour 
 near Ledbeg, and a white marble runs from the south of the Ledbeg 
 river through Loch Urigill towards Elphin. The Loch Ailsh marble 
 forms two beds, a lower white saccharine marble being separated 
 from the upper by a layer of argillaceous chert. The upper bed is 
 less perfectly metamorphosed. This limestone contains about 5 per 
 cent, of carbonate of magnesia, and over 4 per cent, of silica. 
 
 Second Range of Crystalline Limestone. A second range of 
 limestones, lying chiefly in argillaceous and chloritic mica slate, is 
 considered to be of more recent origin. The points where it is seen are 
 near Blairgowrie, at the foot of Ben Vorlich, on the north side of 
 Loch Earn, Balquhidder, Inveraray, Knapdale, and Lorn ; and the 
 limestones of Balachulish, Cairndow, and Dalmally, as well as those 
 which run from Boharm to Banff, are classed with this series. 
 
 Perhaps the relations between all these points may not have been 
 correctly ascertained ; but there seem excellent reasons for admitting 
 that these calcareous rocks, like those which are more perfectly traced 
 among the newer strata, were produced at a few definite periods, 
 and not mere irregular formations having no relation to each other in 
 respect of time. (See Geikie's Geological Map of Scotland.) 
 
 Garnets in Statuary Limestone. Garnets are among the most 
 characteristic products of metamorphism. As a rule, granules of 
 quartz are scattered through the garnets, though in some granitic 
 veins, as to the east of Portsoy, the quartz is arranged in the garnet 
 crystals in a radiating manner. The colourless water garnet, a double 
 silicate of lime and alumina, is remarkable for being free from iron. 
 It is known from limestone quarries in the wood on Craig Mohr, 
 opposite to Balmoral. The lime and alumina iron garnet known as 
 grossular is a rare form found embedded in limestone in green crystals 
 of the colour and size of peas. The lime iron and alumina garnet, 
 termed cinnamon stone, is common in the limestone at Delnabo, 
 Glengairn. The formation of these and the other minerals found in 
 granular limestone is explained by pointing out that the limestone, in 
 developing the crystalline structure of calcite, has extruded from 
 itself the foreign substances it contained, much in the same way that 
 sea water, when it freezes, separates the brine in small globules. 
 Professor Heddle suggests that the high specific heat of limestone may 
 have to be considered as a source of heat liberated when the rock 
 passes from an amorphous to a crystalline condition, and cites many 
 examples of the association of calcareous beds with argillaceous mica- 
 schist, quartzite, and serpentine, especially in Banff, Aberdeen, and 
 

 METAMORPHISM IN IRELAND. 391 
 
 the north of Perth, to show that those parts of the Upper Limestone 
 beds which are associated with serpentine are richest in mineral;?, 
 where both beds are included in gneiss ; while the comparatively 
 implicated Lower Limestone in mica-schist is poor in minerals. 1 
 Limestones contain garnets as they approach granite. 
 
 The Isle of Man. The granite of the central axis of the Isle of 
 Man, seen at Barrule and North of Laxey, is succeeded by very little 
 gneiss and mica slate, much Cambrian schists, from which the mica- 
 schist and gneiss are metamorphosed, and quartz rock. The mica- 
 slate is traversed by veins of quartz and schorl. 2 
 
 North of Ireland. The older strata of the North of Ireland may 
 be considered as in part a prolongation of those of Scotland ; thus the 
 extensive spread of mica-slate in Londonderry and Donegal is a pro- 
 longation of the line of the chain of the Grampians, continued through 
 Jura and Islay ; and the clay slate ridges which border the Mourne 
 mountains run in the direction of the Mull of Galloway and the clay 
 slate chain of the South of Scotland, while between these two systems 
 of slates, occupying a basin-shaped depression, are red sandstone, car- 
 boniferous limestone, and other strata of newer origin, corresponding 
 to those which separate the analogous chains in Scotland. 
 
 The mica slate rocks are principally of the chloritic varieties with- 
 out garnets, but contain hornblende. Laminated crystalline limestone of 
 different colours, containing talc, quartz, hornblende, or pyrites, with 
 veins of quartz, chlorite, and calcareous spar, occur in the mica slate 
 in many parts of Antrim and Londonderry. Hornblende slate like- 
 wise forms distinct bands in the mica slate of this region, and felspar 
 porphyry is interposed. 3 
 
 South of Ireland. In the south-eastern part of Ireland mica slate 
 forms two ranges along the eastern and western boundaries of the 
 granite, and wherever it occurs is in direct contact with the granite. 
 On the eastern side of the granite it runs in a narrow course north- 
 east and south-west, dipping deeply south-east, and consists of alter- 
 nate layers of mica and quartz of extremely variable thickness. On 
 the eastern brow of Rochetown Hill mica slate runs into a natural 
 hollow of the granite, still retaining the north-east and south-west 
 direction of its strata. On Maulin Hill it is singularly and fantasti- 
 cally contorted on a small scale. There is a prolongation of the 
 body of mica slate at the head of Glenmacanass, gradually narrowed 
 in its western progress, and constituting a wedge-like mass inserted 
 into the body of the granite, and enclosing apparently a bed of granite 
 six to ten yards in width, besides irregular masses of granite incorpo- 
 rated with the slate. In the district, greenish, sectile, chloritic slate lies 
 embebbed in the mica slate, and is used for various purposes of 
 architecture and ornamental carving. 
 
 In Glenmalur occurs a remarkable instance of decided alternation 
 of granite and mica slate. In a space of 208 fathoms no less than 
 
 1 Heddle, Trans. Roy. Soc. Scot, vol. xxviii. p. 311. 
 
 2 Henslow, in Geol. Trans. ; Cmnming's " Isle of Man." 
 
 3 Berger in Geol. Trans, and Mem. Geol. Surv. Ireland. 
 
592 METAMORPHISM IN THE PYRENEES. 
 
 five distinct alternations of granitic beds, with as many layers of mica 
 slate, are clearly traced, and several of these beds are made up of 
 similar alternations of granite and mica slate or quartz and mica slate. 
 The great mass of granite is below, and the great mass of mica slate 
 above, constituting the hill called Lugduff. Granitite abounds in this 
 slate. 
 
 Similar alternations occur in other neighbouring places, making a 
 total thickness of one-third of a mile, and the whole system ranges 
 north-east and south-west, and dips south-east. On the north-east 
 they probably abut and terminate against the granite. The mica 
 slate on the summit of Lugnaquilla is likewise interstratified with 
 granite. Clay slate bounds it on the east, and at length coming into 
 contact with the granite, cuts off its farther progress to the south. 
 
 On the western side of the granite the mica slate is still less ex- 
 tensive. It is found to enclose beds and elliptical masses of granite 
 in Glenismaule ; and it is mentioned that a granite vein, four to eight 
 inches wide, ranging 25 north of west, cuts off the mass of alternating 
 strata, without occasioning any displacement. In the same valley are 
 two distinct beds of compact doleritic rock in the mica slate, one 
 four feet wide, the other two feet. Andalusite abounds in the mica 
 slate of this country ; and dolerites alternate with it. 
 
 The frequency of the phenomenon of alternation between mica 
 slate and granite is a feature in the geology of this part of Ireland 
 long since discussed by Mr. Weaver. 1 
 
 In Brittany. The tract of oH rocks in the north-western part of 
 France is one of the most extensive in Europe. The granite, gene- 
 rally the most elevated, is separated from the secondary strata by 
 gneiss and mica slate, and lower palaeozoic slates, into which they 
 pass almost indefinitely. In the departments of Calvados and La 
 Manche these two systems appear as zones around the granite, the 
 gneiss being within the clay slate. Quartz rocks of blue colour, and 
 pegmatites with tourmaline, are associated with them ; and they are 
 traversed by veins of quartz and granite. 2 
 
 In the Pyrenees. The granitic masses of the narrow chain of the 
 Pyrenees having been uplifted in much confusion, are very irregularly 
 bordered ; in several places they are overlaid by gneiss and mica 
 slate, but generally by the latter series. Charpentier regarded the 
 gneiss of the mountains which border the valley of Soulan as inti- 
 mately connected by gradation and alternation with the subjacent 
 granite, so as to be necessarily united therewith into one formation. 
 In many instances gneiss and granite are described as alternating in 
 very thin layers. In other cases, vast blocks of micaceous gneiss of 
 100 cubic fathoms bulk are buried at intervals in granite, always 
 preserving one constant relative position or direction of the lamina. 
 These are thought by Charpentier to be of contemporaneous origin 
 with the granite, which passes into them at the sides, and thus inter- 
 laminates the gneiss. 
 
 Mica slate, in the same manner, is intercalated with granite in a 
 
 1 Geol. Trans., vol. v. 2 De Cavnnont, Geol. du Cavaldos. 
 
METAMORPHIC ROCKS IN FRANCE. 
 
 great many places, and quartz and felspar bands occur in the granite. 
 In many places in the Pyrenees the " granite " contains beds of strati- 
 fied granular limestone such as in other districts lies in the gneiss, 
 with graphite, talc, fluor spar, mica, hornblende, &c. 
 
 Boue, Dufrenoy, and other writers, have proved beyond a doubt 
 the powerful action of heat along the Pyrenean chain, as shown not 
 only by the usual subcrystalline character of the slates, but also by 
 the metamorphism of chalk into the condition of crystalline limestone, 
 with the development of abundance of metallic and granitic veins at 
 the line of junction of the altered, stratified, and igneous rocks. The 
 age of the eruption of granite along this chain is determined by 
 observations of Dufrenoy to be, at least in part, posterior to the Chalk. 
 
 Igneous theory does not necessarily require that all these beds 
 of seeming granite should be altered gneiss, nor that the beds of por- 
 phyry should be considered as altered clay slate. Alternating igneous 
 and aqueous action is exemplified in modern operations of nature ; 
 but certainly in many cases, both in Cornwall and Cumbria, it ap- 
 pears the more correct view to suppose a gradual and partial rear- 
 rangement of the materials of the rock through the long-continued 
 action of heat. 
 
 In Central France. The great central plateau of old rocks in 
 France, from which the Loire, Vieime, Dordogne, &c., take their 
 source, is chiefly a granitic and porphyritic tract, surrounded by oolitic 
 and carboniferous rocks, but slates and gneiss rocks appear in the 
 valley of the Yienne, and occupy a large part of the southern boundary. 
 Near Limoges are alternating beds of granite and gneiss, and some 
 subordinate beds of pegmatite and hornblende rock : the gneiss passes 
 by one variation to granite, by another to mica slate. The ranges of 
 strata near Limoges are north-east and south-west, and they are 
 crossed by decomposing elvan courses to north-north-east. Tin veins 
 occur near Yaulry in gneiss as well as in granite. Towards the 
 borders of the district the gneiss becomes less granitic, more associated 
 with hornblende slate, and encloses deposits of micaceous limestone. 
 Serpentine lies in this gneiss in many places. The pegmatites and 
 kaolins of St. Yrieux, which have resulted from decomposition of 
 this rock, form numerous veins and strings in the gneiss and horn- 
 blende slates, whicli sometimes intercalate themselves between the 
 laminse. Quartz rock of bluish colour exists in the Black Mountain 
 and elsewhere. Oxide of iron abounds at many points in the gneiss ; 
 galena, phosphate of lead, carbonate of copper, antimony, and haematite 
 are the products of the veins. 
 
 The most remarkable alterations of secondary limestones take 
 place, according to Dufrenoy, along the line of junction with the 
 granitic and porphyritic masses. Thus the Lias and Oolite become 
 metamorphic, and are traversed by metalliferous veins, just as the 
 slates in Cornwall and Brittany are metalliferous, principally in the 
 same situation. 
 
 Other Localities in Europe. After these details of the circum- 
 stances attendant on gneiss and mica slate at so many interesting 
 
394 MET A MORPHISM IN NOR WA Y. 
 
 points, we can only add a few general observations on the range 
 and extent of these rocks in other countries. Gneiss and mica 
 slate in small quantity occur in the Vosges, and gneiss more abun- 
 dantly in the Black Forest. 
 
 The long irregular chain of the Alps contains a vast quantity of 
 gneiss and mica slate, variously extended around the talc-bearing 
 granite cores of Mont Blanc and St. Gothard, from the Mediterranean 
 almost to the Danube. 
 
 Gneiss and mica slate do not reappear around the granitic base of 
 the Carpathians. Their place is supplied in this chain by a vast 
 deposit of clay slate. 
 
 The mountains which encircle Bohemia are, on all the southern 
 half, granite. Gneiss and mica slate are superadded on the west, and 
 the former rock in particular abounds in the Erzgebirge. The Riesen- 
 gebirge granite is bordered on the north by gneiss, on the south and 
 east by mica slate, and these rocks are associated with granite in the 
 range which divides the drainage of the Oder and the Elbe. 
 
 These rocks are most extensively spread over the northern parts 
 of Europe, and extend from Copenhagen round the Gulf of Bothnia, 
 and along the Ural chain toward the Caspian Sea, and in the Caucasus. 
 
 Fossil-Bearing Schists of Norway. Dr. Hans H. Reusch has 
 shown that fossils occur in the schists of Norway to the south of 
 Bergen, in the neighbourhood of Osb'ren. The fossils occur on two 
 horizons. In the southern horizon, grey, blue, crystalline limestone 
 occurs in black clay slate, and bears fossils which are only indicated 
 by clear outlines. The limestone is concretionary, and some of the 
 concretions at Kuven contain the corals Halysites and Syringophyllum. 
 Both these genera also occur in the calcareous band on the Os river ; 
 and in that locality and at Bauerhof Valle a few Gasteropods are seen 
 in section. On the other horizon the fauna is richer, and is seen on 
 the road to the north-east of Ulven. The road traverses a grey slate, 
 apparently micaceous, which contains layers of calcareous concretions. 
 These beds yield in the different exposures Favosites and Graptolites. 
 The richest locality is in the Bauerhof Vagtdal, where the rock is a 
 grey Muscovite slate with inclusions of brown mica, and shows under 
 the microscope a good deal of quartz and rutile associated with the 
 mica. The fossils are Trilobites, cup-corals, chain-corals, and Brachio- 
 pods, and their mode of occurrence indicates that the foliation is in a 
 different plane from the bedding. The Trilobites comprise Calymene 
 and Phacops. The Gasteropods are referable to Subulites or Murchi- 
 sonia, and perhaps Pleurotomaria. The Brachiopod genera have not 
 been determined. The corals are Cyathophyllum, Halysites catenularius, 
 probably Syringophyllum organum, and some other types. The Grap- 
 tolites comprise Rastrites and Monograpsus, and there are indications 
 of Crinoids. This fauna indicates strata of Wenlock age. 
 
 In 1865 Sismonda described an Equisetum from an isolated block 
 of gneiss, probably derived from the Veltline. 
 
 Metamorpliic Rocks in America. In America, Humboldt describes 
 gneiss as less abundant along the high chains of the Andes than along 
 
GNEISS IN AMERICA. 395 
 
 the lower mountains of Caracas, in Orinoco, Brazil, New Spain. It 
 is occasionally auriferous, and contains micaceous crystalline limestone. 
 The most considerable masses of mica slate mentioned by this traveller 
 are those of the Cordillera of the shore of Venezuela. This rock in 
 the Andes is less rare on the north than on the south of the Equator. 
 Nowhere, perhaps, is the total suppression of mica slate more frequent 
 than in the Cordilleras of Mexico and South America. 
 
 The eastern primary range of North America passes through the 
 United States from the St. Lawrence to the Mississippi in a direction 
 nearly parallel to the coast, and at first 100 miles distant from it. 
 
 The prevailing and characteristic rock is a syenitic gneiss, in which 
 the divisional planes are obscure, and frequently evanescent, when 
 the rock is undistinguishable from the syenite which forms part of 
 this great metamorphic group. Gneiss retains in general its place 
 next the granite, which, however, is of small extent ; it is often suc- 
 ceeded by hornblendic, micaceous, and talcose schist, and granular, 
 sometimes dolomitic limestone, seldom pure enough for fine statuary. 
 It is traversed by granite veins at Haddam in Connecticut. 
 
 This is the principal metalliferous band in the Eastern UnitedStates, 
 yielding magnetic iron ore in veins and beds, near Lake Champlain, 
 in New York, New Jersey, Pennsylvania, and Maryland ; on the 
 southern side of Lake Superior, in the vicinity of Montreal, in Wis- 
 consin, in the Iron Mountain and Pilot Knob in Missouri, and in 
 Arkansas. Copper ore occurs in Lake Huron and on the northern 
 shore of Lake Superior. Lead ore lies in these rocks in northern 
 New York ; lead and copper in Pennsylvania ; zinc or red oxide, 
 mixed with franklinite, occurs in New Jersey ; phosphate of lime 
 has been found in New York and New Jersey ; kaolin marble, build- 
 ing stone, firestones, hones, steatite, plumbago, and many fine 
 crystallised minerals, as apatite, zircon, spinelle, sphene, augite 
 tourmaline, may be added to this list. 
 
 The range of the rocks is from the high country north of the St. 
 Lawrence, westward to the sources of the Mississippi, and southward 
 along the elevated parts of Maine, New Hampshire, New York, New 
 Jersey, Pennsylvania, Maryland, Virginia, and North Carolina, to 
 Alabama, with isolated belts in Missouri, Arkansas, and Texas. 
 
 Gneiss of North America. Gneiss varies in mineral composition, 
 and presents analogies towards different igneous rocks. At the north 
 end of the Lake range in Nevada, the felspar is almost entirely 
 triclinic, with a little orthoclase. There are also present biotite, quartz, 
 and green hornblende, so that it corresponds to quartz-mica-diorite, in 
 the same way that common mica gneiss corresponds to granite, and 
 hornblende gneiss corresponds to quartz syenite. The quantity of 
 prisms of apatite is enormous in this rock, as in all gneisses which 
 abound in hornblende. In Clover Canon many varieties of gneiss 
 occur, some representing the diorite gneiss, but with the titanite in 
 greyish yellow sharp sections ; other Clover Canon gneisses contain 
 inclusions of liquid carbonic acid. In Europe, liquid carbonic acid is 
 found in quartz in the granitic gneiss of the St. Gothard, and grey 
 
396 MICA-GNEISS AND HORNBLENDIC GNEISS. 
 
 gneiss from Freiburg in Saxony. In another part of the Humboldt 
 range, the texture of the gneiss approaches more nearly to granite ; 
 and in the Secret Pass the rock contains crystals which resemble 
 zircon, though no zirconia has been detected on analysis. 
 
 Hornblende gneisses generally predominate in the Ogden and 
 Farmington Canons in the Wahsatch range, Utah. One type is made 
 up of orthoclase, with a good deal of plagioclase, quartz, brown mica, 
 much hornblende, and apatite. The quantity of plagioclase is in some 
 cases much greater. At other times a mineral like zircon is present, 
 and in some of these the quantity of hornblende is small. Here too 
 some of the hornblende gneiss contains garnet. 
 
 In the same district mica gneiss is found, which is free from 
 hornblende, is rich in felspar and quartz, and poor in brown mica and 
 deep red garnet. 
 
 Zirkel contrasts the mica gneiss with the hornblende gneiss, showing 
 that in the former orthoclase preponderates ; there is little plagioclase ; 
 the quartz is full of fluid inclusions ; apatite and zircon are rare 
 or absent, and titanite is entirely absent. In hornblende gneiss 
 plagioclase sometimes preponderates. Fluid inclusions in quartz are 
 comparatively rare. Apatite and zircon are usually abundant, and 
 titanite is sometimes present. 1 
 
 Succession of North American Crystalline Rocks. The lowest 
 group of the Laurentian Rocks of North America is named the 
 Ottawa series. It consists of granitic gneisses, red and grey, often 
 highly contorted, formed of orthoclase, with quartz and hornblende. 
 Next in succession is the Grenville series, quartzose gneisses, and 
 quartzites, with great beds of Dolomite, and the Limestone which 
 yields Eozoon. It is said to be 17,000 feet thick. The Norian or 
 Labrador series is supposed to rest unconformably on the older 
 gneisses. These gneisses contain Labradorite or Anorthite, are 10,000 
 feet thick, and spread over Labrador and north of the St. Lawrence. 
 The Huronian series is a yet newer group of metamorphic rocks, con- 
 sisting, according to Dr. Bigsby, of chloritic, siliceous, and hornblende 
 slates ; but it also contains limestones, serpentines, and rocks of a 
 greenstone character. It has been compared to the Pebidian rocks of 
 Wales. The Montalban series is a succession of micaceous schists and 
 friable grey gneisses, passing into micaceous quartzites and hornblendic 
 gneiss. Newer still is the Taconian series, which forms the Taconic 
 hills. It consists of quartz rocks and schists, with crystalline magnesian 
 limestone, some serpentine, and great deposits of iron ores. Attempts 
 have been made to parallel these rocks in Europe, but the method of 
 research has yet to be developed which would justify the correlation. 2 
 
 1 Zirkel, Micros. Petrog. 
 
 2 See Hicks : " Succession of the Archaean Rocks of America, compared with 
 the Pre-Cambriau Rocks of Europe," Proc. Geol. Assoc., vol. viii. No. 5. 
 
( 397 ) 
 
 CHAPTER XXII. 
 
 MINERAL VEINS. 
 
 Origin of Mineral Veins. Facts observed in mining districts have 
 strongly enforced the belief that the water which descends through 
 fissures to regions beneath the earth's surface, becomes so heated as 
 to dissolve and hold in solution a multitude of metallic and mineral 
 substances. Different kinds of mineral matter may be deposited suc- 
 cessively from heated water as its temperature becomes lower ; and 
 hence the belief that, as waters on their way downward dissolve the 
 minute particles of metallic substances which are scattered in the 
 rocks with which they come in contact, so those waters on their way 
 back towards the surface, traversing the fissure of some fault which 
 gives them passage, become cooled, so as to throw down in a small 
 portion of the fissure, ores or other mineral matter which had been 
 invisible while diffused in the rocks. This belief is strongly sup- 
 ported by the fact that in passing down in a lode, it is no uncommon 
 thing for the metallic contents to change. 
 
 Mineral Veins now Forming. Many ores, like those of copper, 
 tin, zinc, and lead, have the metal combined with sulphur, and it is 
 well known that many hot springs, like those of Sicily and Lake 
 County in California, deposit sulphur at the surface; and in the 
 district of Lake County the sulphur contains a little mercury in the 
 form of cinnabar. The sulphur is here deposited upon volcanic rock, 
 and the sides of the fissures are frequently coated with chalcedony, in 
 which are both pyrites and cinnabar ; so that the sulphur deposit has 
 been worked as a quicksilver mine. In a sinter bed, precipitated from 
 a hot spring in the county of Colusa in California, Mr. Oxland dis- 
 covered deposits of silver ; and in this county Mr. Melville Attwood 
 discovered cinnabar on the surfaces of a fissure which had become 
 covered by a subsequent deposit of brilliant metallic gold. At Steam 
 Boat Springs in the State of Nevada, which are about seven miles 
 N.W. of the silver mines of the great Cornstock lode, heated waters 
 or steam are constantly given off. 1 The fissures appear to have been 
 subjected to repeated widenings, and have their walls lined, sometimes 
 to a thickness of several feet, with incrustations of silica containing 
 hydrated ferric-oxide, and occasionally crystals of iron pyrites. A 
 similar group of fissures occurs a mile to the west, but they now only 
 
 1 J. A. Phillips, Phil. Mag, 1868, p. 321 ; also Q. J. G. S.. vol. xxxv. p. 390. 
 
398 ORIGIN OF THE MATERIALS IN MINERAL VEINS. 
 
 give off steam and carbonic acid. The silicious deposits have here 
 accumulated in the principal fissure to a thickness of ninety yards in 
 each side of the opening. This silica is sometimes in the condition of 
 chalcedony, but most of it is crystalline. The quartz contains oxides 
 of iron and manganese, and small quantities of iron and copper pyrites. 
 In 1878 this older fissure was opened by a tunnel to a depth of fifty 
 feet below the surface, when the vein stuff yielded cinnabar, from 
 which 3 per cent, of mercury was obtained, while the deposits accumu- 
 lated at the surface, from the overflow of the water, only contain traces 
 of mercury. Similar accumulations of cinnabar occur near the hot 
 springs of Calistoga, at the foot of Mount St. Helena. In the great 
 Comstock lode the vein has the gangue consisting of silica and calcite, 
 and yields silver and gold. At a depth of 2660 feet the water issues 
 from the rock at a temperature of 157 F. These waters contain forty- 
 two grains of solid matter to the gallon, partly sulphates, partly car- 
 bonates, and a little chloride. Hence we may infer that the metallic 
 and non-metallic substances alike have been often deposited in the 
 fissures of ancient faults by the waters of hot springs, and that when 
 the temperature of the water is high, these metals may reach the sur- 
 face, but as a rule the temperature is not sufficient for the surface 
 deposition in quantity of anything but salts of lime and quartz, and 
 in such cases the metals only reach the surface in consequence of sub- 
 sequent denudation of the upper part of the vein stuff. It is well 
 known that nearly all metalliferous veins occur in regions characterised 
 by intrusive igneous rocks, volcanic phenomena, or faults. Such 
 influences, by furnishing the heat required to dissolve the metal, must 
 usually be held accountable for the formation of the veins. 
 
 Mr. J. Baddeley has pointed out that a great circle cutting the 
 equator in 10 E. long, and 170 W. long, and reaching to lat. 45 
 north and south, coincides with the principal gold-producing mines of 
 the world. 
 
 Source of Metals. Nothing can be learned concerning the origin 
 of metals. Hence, there is no greater d priori difficulty in deriving 
 metallic veins from invisible materials in the surface rocks, than 
 there would be in obtaining them from imaginary deep-seated masses 
 of metal. Deep-sea exploration, however, has discovered an unex- 
 pected abundance of manganese in concretionary nodules on certain 
 sea-beds ; and it has long been known that the precious metals exist 
 in the ocean in sufficient quantity to become deposited on the copper- 
 sheathing of ships, and affect its commercial value when removed. 
 Dr. Forschammer, by chemical examination of sea water and certain 
 marine organisms, made a valuable contribution to materials for a 
 theory of mineral veins of segregation. He found the following 
 chemical elements : Oxygen, Hydrogen, Chlorine, Bromine, Iodine, 
 Fluorine, Nitrogen. Sulphur forms salts of baryta, strontia, lime, 
 magnesia ; and Phosphorus remains as phosphate of lime when water 
 is evaporated. Carbon deoxidises the peroxide of iron either to 
 protoxide or sulphide, and thus colours deep-sea sands and clays. 
 Silicon is seen forming the skeleton in the calcined cup sponges of 
 
SEA-WATER AS A SOURCE OF MINERALS. 399 
 
 Singapore, except that the large pores are lined with oxide of iron. 
 Boron is found in the rock salt of Strassfurth in Germany, and this 
 is presumably of marine origin, while the plant Zostera marina 
 contains much Boracic acid. Silver, though difficult to detect in sea 
 water, is yet so abundant in the coral Pocillipora alcicornis, that one 
 cubic foot of coral yields about half a grain of silver. Copper is 
 frequently found in the lime salts of sea animals and plants, and in 
 the coral Pocillipora there is six times more copper than silver ; the 
 ash of Fucus vesiculosus contains copper. Lead is more abundant 
 in shells and ashes of sea plants than even copper. Pocillipora 
 alcicornis contains eight times as much lead as silver ; lead also 
 occurs in the Fucus vesiculosus. Zinc is found in the ashes of sea 
 plants, such as Fucus vesiculosus, and in Zostera marina it occurs to 
 the extent of i part in 3000. As the mineral Blende, it is common 
 in the Ammonites cordatus from the Oxford clay of St. Ives. 
 Cobalt occurs in Zostera marina, in the large cup sponge from 
 Singapore, and in fossil sponges from the chalk. Nickel, in minute 
 quantities, seems to occur with Cobalt. Iron in great quantity is 
 found in the ashes of sea-weeds and lime-salts of sea animals. 
 Manganese forms nearly 4 per cent, of the ash of Zostera marina. 
 Aluminium is more abundant than any metal except iron and per- 
 haps manganese. Magnesium usually forms i per cent, of shells, 
 though it forms 13 J per cent, of the shell of Serpula filigramma. 
 Calcium, as carbonate of lime, forms '003 of sea water, and is 
 abundant in all shells, &c. Strontium is found in the ashes of 
 Fucoids. Baryta is common in sea animals and abundant in the 
 ashes of sea plants. And there are the universally diffused substances 
 Sodium and Potassium. 
 
 These substances being thus diffused in nature and accumulated 
 by organisms, the strata become charged with them ; and hence, 
 when metamorphic changes raise the temperature and dissolve the 
 whole substance of the strata, with the production of multitudes of 
 fractures, joints, and faults, it is easy to conceive the steps of change 
 by which these minute particles become dissolved, extruded by crystal- 
 lisation of the rock, and deposited in veins. 
 
 DaubreVs Views on the Origin of Ores. Mr. Daubre'e urges l 
 that the ores which occur as sulphides are all to be attributed to the 
 action of sulphurous waters upon metallic substances. Thus Roman 
 coins buried in mud in the warm spring at Bourboune-les-Bains are 
 more or less converted into copper pyrites and other cupric sulphides. 
 Lead pipes have yielded a coating of galena, which M. Daubree attri- 
 butes to the decomposition of phosgenite under the reducing influence 
 of organic matter. Iron pyrites appear to be frequently formed under 
 the influence of decaying organic matter, especially plants, but it 
 assumes a more characteristic form in the waters of sulphurous springs. 
 In Iceland it is developed by the action of sulphuretted hydrogen on 
 the iron in basaltic rocks. 
 
 Difference between Dykes and Veins. Though in some instances 
 1 " Geologic Experimentale," 1879. 
 
400 CONDITIONS OF MINERAL MATTERS IN VEINS. 
 
 the distinction between rock dykes and mineral veins is imaginary, 
 they are in general clearly contrasted by the nature of the substances 
 which they contain. In dykes the crystallised minerals are of the 
 same kind as those great interior masses of consolidated rock from 
 which they often are evidently ramifications. In veins metallic sub- 
 stances occur which are mostly not known to exist in nature except in 
 these situations, and in others very similar or distinctly related to 
 them by position and minerals. To this general rule quartz is one of 
 the most striking exceptions ; yet even in this instance it is remark- 
 able that the quartz of veins is of a different aspect from that mingled 
 with the ingredients of granitic rocks. We must therefore take the 
 presence of metallic matter and certain non-metallic substances usually 
 connected therewith, and commonly called vein-stuff, as the leading 
 characteristic of the mineral veins, whose history we are now to 
 examine. 
 
 Substances in the Veins. The simple minerals which occur in 
 veins and analogous situations are far more numerous than those which 
 are found as component parts of rocks. Igneous rocks, and especially 
 those of modern volcanic origin, hold a very great variety of non- 
 metallic substances, some of which also occur in veins; but it is 
 almost exclusively in veins that we find the metals in their pure state, 
 or alloyed with one another, or mineralised by combination with 
 sulphur, oxygen, chlorine, &c., or converted into salts by union with 
 various acids. Every elementary substance yet discovered by chemists 
 exists in the earth ; and it is probable that none of these are entirely 
 absent from the solid contents of mineral veins. 
 
 Alloys. The metallic substances seldom occur pure. Sometimes 
 they are found in alloys, similar for the most part to those now pro- 
 ducible by the chemist, thus silver, antimony, cobalt, nickel, iron, are 
 alloyed with arsenic ; silver and nickel with antimony ; lead, gold, 
 silver, and bismuth with tellurium ; silver with mercury ; platinum 
 with gold, &c. The only known circumstance which stands as ante- 
 cedent to the production of such alloys is heat, produced by either 
 chemical action or pressure; and perhaps there is no single fact con- 
 nected with the theory of veins on which a belief in the influence of 
 heat in their production might be more securely based. 
 
 Oxides and Salts. The metallic oxides prevalent in veins are 
 produced under various relations to heat, moisture, and contact with 
 gaseous substances ; and have various degrees of permanence when 
 exposed to high temperatures, either separately or combined. Metallic 
 salts are not rare in veins, and in the same way vary in their origin 
 and degree of permanence. Metallic oxides and salts, in very many 
 instances, are derivative compounds from sulphides and other primary 
 combinations. This is frequently the case with oxide of iron, car- 
 bonate of copper, and probably carbonate, phosphate, and other salts 
 of lead. 
 
 Non-Metallic Minerals. Besides the metallic ores which impart 
 to many veins their most striking, if not most constant characters, 
 various earthy minerals lie in these repositories, and, as will afterwards 
 
STRUCTURE OF VEINS. 401 
 
 appear, under certain definite relations to the enclosing rocks as well 
 as to the included metals, and with a less distinct dependence on the 
 local situation or mining district. These earthy substances are usually 
 called the gangue, vein- stuff, or matrix of the ore. Generally they are 
 crystallised, as quartz, fluor spar, calcareous spar, phosphate of lime, 
 the sulphate and carbonate of baryta, strontian, &c. ; sometimes they 
 appear massive, as quartz and several other minerals, when the vein 
 has no cavities in it ; and sometimes the vein-stuff is entirely soft 
 argillaceous matter, of different aspect in different mining districts. 
 
 Rider. In some veins masses of the neighbouring rocks are en- 
 closed and penetrated to a great extent by little strings of the ore and 
 spar, so as occasionally to be worth the trouble of working. The vein 
 is said in this case to bear a rider. It, in fact, sometimes becomes 
 under these circumstances a double vein. More rarely pebbles and 
 other marks of water action are stated to occur in soft veins. 
 
 Mode of Aggregation of the Ingredients. A variety of appear- 
 ances deserve special notice as indicating some of the conditions under 
 which the vein was filled. In some cases, for instance, the whole 
 breadth of the vein is occupied by one kind of substance, as lead ore, 
 or quartz, or sulphate of baryta; in other instances, the metallic 
 matter is interspersed in small masses through a basis, such as quartz ; 
 but, generally, the different substances which fill the vein are ranged 
 in a definite order of succession from the sides of the vein toward the 
 middle, in which, commonly, the metallic matter occurs in an irregular 
 vertical table, called a rib of ore. These conditions are best observed 
 in the proper veins, but are also to be noticed in the nests and detached 
 masses of ore and vein-stuff which sometimes occur in the vicinity of 
 the veins. 
 
 General Idea of a Mineral Vein. The ordinary conception of a 
 mineral vein is well exemplified in some Derbyshire specimens, not 
 rare in collections, which, when cut across, show, in 
 the middle, masses or a continuous rib of galena, and b c a cb 
 on each side of this, to the extreme edges of the 
 mass (or narrow vein), layers of fluor spar and car- 
 bonate of baryta in frequent alternation, all the 
 materials being crystallised together without leaving 
 any cavities, yet preserving their own character of 
 structure. 
 
 Thus, in the diagram, a is the middle rib of 
 galena, b b, c c, the alternating bands of barytic spar and fluor spar ; 
 d d, the masses of rock which enclose the vein, are called the walls or 
 cheeks of the vein. 
 
 Supposed Successive Deposition of the Substances. The con- 
 templation of these specimens seldom fails to impress upon the 
 mind a conviction that the several bands of mineral substances 
 were deposited on the cheeks or walls of the vein in succession, the 
 middle being filled last of all ; and this theoretical notion has been 
 illustrated by comparing a mineral vein to a narrow gallery whose 
 walls were covered by many successive coats of plaster of different 
 
 VOL. I. 20 
 
402 NATURE OF RIBS OF ORE. 
 
 colour and composition. Werner adopted the notion of the unequal 
 antiquity of the vertical layers of the vein so implicitly as to speak 
 of the middle ribs as always younger. It is difficult to resist 
 this impression, especially \vhen, in addition to the circumstance of 
 the succession of the laminae, we observe that these laminae are so 
 crystallised as to turn their free terminations towards the centre of 
 the vein, and in that direction to imprint the next layer with their 
 own forms, just as crystals forming in a vessel shoot their point toward 
 the part still remaining liquid, and in that direction are covered 
 by the subsequently formed crystals. Very similar inferences are 
 suggested by certain agates, and more distinctly by geodes in basalt, 
 and the crystallised cavities in limestones, in the interior of shells, 
 &c. ; in which cases the hollow towards which the crystals pointed 
 still remains unfilled. 
 
 This first impression becomes somewhat modified when, instead of 
 confining ourselves to a cabinet specimen, we examine the whole 
 extent of a mine ; for here, in the first place, it is very often found 
 that the regular succession of minerals from the side to the centre is 
 a limited though repeated phenomenon ; that the rib of ore is of 
 short horizontal, and sometimes still shorter vertical extent, diminish- 
 ing to nothing, or diffused in small grains through the contiguous 
 spars ; that different metals are found in the same vein at different 
 depths, and at distant points along its course ; and that both the 
 quantity of metal and the presence of spars are dependent on the 
 hardness, and perhaps on some other physical properties of the rocks 
 which the vein divides and replaces. 
 
 Chemical Relations of Metals. The phenomena of crystallisation 
 before alluded to can hardly be thought to prove the successive intro- 
 duction of the mineral laminas into the vein ; though very probably 
 they do demonstrate the order of crystallisation of these substances. 
 
 In some cases we observe indications that one kind of mineral has 
 been formed round another as a nucleus ; as, for example, sulphide 
 of copper round iron pyrites in a part of Caldbeck Fells, Cumberland, 
 and more frequently in many places carbonate of copper and carbonate 
 of lead round the sulphides of those metals. As we have remarked, it 
 is very often the case that the metallic matter of the vein is collected 
 into the middle and forms there a distinct tabular mass, called a rib of 
 ore, more rarely it is disseminated into the gangue. Generally, only one 
 kind of metal abounds in the same part of the vein, but the same vein 
 may yield lead above and copper below, copper above and tin below, 
 or lead in one place and copper in another. The observation is fre- 
 quent that ore is collected into certain vertical portions of a vein 
 which are worked above and below a level, and between which little 
 but vein-stuffs is found in the horizontal drift. There is a vague 
 notion amongst miners that veins are most productive in the deep 
 workings, and it is at least probable that they are less rich very near 
 the surface. 
 
 Association of Minerals. Werner insisted on the fact, that cer- 
 tain associations of minerals can be traced in veins. He noticed the 
 
RAKE VEINS, PIPE VEINS, AND FLAT VEINS. 403 
 
 concurrence of lead glance, and blende, or calamine, and copper pyrites ; 
 of cobalt, copper, nickel, and native bismuth ; of tin, wolfram, tung- 
 sten, molybdena, and arsenical pyrites ; of topaz, fluor spar, apatite, 
 schorl, mica, chlorite, and lithomarge ; of brown ironstone, black iron- 
 stone, manganese, and heavy spar. He says where tin occurs, ores of 
 silver, lead, and cobalt, and vein-stuffs of heavy spar, calcareous spar, 
 and gypsum are rarely found. Cinnabar and other ores of mercury 
 scarcely ever occur with the ores of other metals, except iron ochre 
 and iron pyrites. 
 
 Rolled and Fragmental Masses. That fragmental masses of the 
 neighbouring rocks should be found in mineral veins, cannot be thought 
 surprising. It is a common occurrence in mining districts, both in 
 Primary and Secondary rocks. Thus gneiss at Joachimsthal, clay 
 slate in Cornwall, limestone in Cumberland, are included in the veins. 
 Werner mentions a vein in Danielstollen at Joachimsthal, fourteen 
 inches wide, which at one hundred and eighty fathoms depth was 
 almost entirely composed of rolled pieces of gneiss, some of them 
 nearly spherical. In the Stoll Kefier, near Kiegelsdorf, a vein of 
 cobalt was cut through by another vein of sand and rolled pieces. 
 These examples seem trustworthy, but we must always be careful to 
 discriminate between rolled pebbles and concretionary masses. 
 
 General Forms of Veins. Mineral veins are usually distinguished 
 by miners into several kinds, according to their form and direction, 
 because these circumstances are the most influential in the arrangement 
 of their works. Ralte veins, the most common and characteristic, may 
 be considered to fill long, narrow joint-fissures or faults, which pass 
 in a vertical or highly inclined direction downwards from the surface 
 through a great thickness of the subjacent rocks, whatever these may 
 be, and preserve nearly the same angle of inclination and the same 
 linear direction through their whole course. Pipe veins are also highly 
 inclined, and pass downward in the same manner, but they rather 
 resemble irregular channels than fissures, and are subject to great 
 swellings and contractions of their diameter. They sometimes pass 
 downward along the stratification ; in other cases penetrate through 
 the substance of the strata. The copper mines in the neighbourhood 
 of Ecton, in Staffordshire, are in pipe veins. The irregular cavity of 
 copper ore, which forms the celebrated Parys mine in Anglesea, and 
 the iron mines of Dannemora in Sweden, may be pipe veins, or 
 stock works. Flat veins or streaks, as far as we are acquainted with 
 them, seem hardly to deserve a special name, being only portions of 
 rake veins which have been changed in their inclination, and made 
 to pass for limited distances parallel to the beds. In the limestone 
 districts of the North of England, this happens principally in connec- 
 tion with certain limestone beds. The term Gash veins denotes such 
 as range for considerable lengths, like rake veins, but are wide at top, 
 and grow narrower downwards, till they entirely vanish. This is rare, 
 though many veins grow narrower downwards. 
 
 Strings. Perfect parallelism of the sides or walls of a rake vein, 
 which is the most regular of all, is a rare phenomenon. Most com- 
 
404 RIDERED ROCK. 
 
 monly, indeed, there is a definite boundary to the mineral masses 
 presented by the rocks on each side, but this is only on the great 
 scale ; and the operations of mining disclose to us innumerable cracks 
 and fissures in these boundary walls, which, when filled 
 by metallic or sparry matters, are called strings, and are 
 frequently worth the labour of following even to great 
 distances from the parent vein, if, indeed, we ought not 
 often to reverse this expression. The notion of miners 
 generally appears to be, that these strings are to be 
 viewed as feeders of the vein, and in proportion to their 
 frequency in many instances is the productiveness of the 
 vein. In the accompanying diagram, the vein is repre- 
 sented as receiving small branches or strings from the 
 neighbouring rock. A rock thus penetrated by strings is 
 sometimes said to be ridered. The rock masses which are 
 Fig. 78. often included in the vein are termed horses ; they are 
 usually well separated from the walls. In many rocks these ridered 
 parts are very greatly altered from their original state. 
 
 It sometimes happens that, in passing through rocks of varying 
 hardness, as limestone, shale, &c., the veins turn flat for a short 
 distance on the hardest and most connected beds (as, for example, 
 on the Tyne bottom limestone of Cumberland), and afterwards 
 continue their ordinary course. These flat parts usually send off 
 strings into the limestone, which may thus be ridered to a consider- 
 able distance. 
 
 Disseminated Veins, &c. Sometimes the mineral is dissemi- 
 nated through the parts of the rock adjoining a vein, or collected in 
 small nests and other closed cavities. This happens not only in the 
 Cornish mines, in killas and granite, but in those in the mountain 
 limestone tracts of the north of England, and even in magnesian 
 limestone. Generally speaking, we may be sure that this metallic 
 impregnation is so related to the veins, that it is an effect of the 
 same agent. Whatever filled the veins, also brought to small 
 distances from them some of their constituent minerals. Certain 
 metals and ores are more liable than others to this lateral diffusion. 
 Native silver, silver glance, red silver ore, native copper, tin ore, iron 
 pyrites, and red iron ochre, are specially noted as occurring in this 
 way ; though copper ore, pyrites, and lead glance seldom exhibit 
 this effect. Galena, however, blende, bitumen, calc spar, and quartz, 
 are found in closed cavities of shells, in mountain limestone, and in 
 other strata. 
 
 Tin Floors, Stockworks, &c. The dissemination of tin ores 
 through some of the rocks of Cornwall was noticed long since by 
 Hawkins, under the title of tin floors. 1 He observed that the whole 
 tenement of Botallack is said to be full of tin floors. At Zinnwald, 
 mineral beds or floors have long been the object of mining adventure. 
 There granite alternates with the tin floors, which consist of quartz 
 and mica, with tin ore, fluor spar, and wolfram. At Breitenbrunn a 
 1 Geol. Soc. of Cornwall Trans., vol. ii. 
 
STOCKWORKS. 405 
 
 floor of this kind has been very extensively worked in a gneiss 
 rock. 
 
 The stockwork of the German miners is to be considered as a 
 mass of rock impregnated with metallic matters, in numerous small 
 veins, which come together irregularly, so as to make particular parts 
 extremely rich. The working of such mineral repositories is directed 
 by quite other principles than those which serve for straight veins 
 of definite magnitude. The stockwork is generally opened like 
 a vast quarry, and the excavations are prosecuted irregularly in 
 the most favourable directions. Perhaps the copper mine of Parys 
 mountain in Anglesea, the iron mine of Dannemora in Sweden, 
 the tin ore mine of Geyer in Saxony, are examples of immense 
 stockworks. Werner considered the stockwork as peculiar to tin 
 ores. 
 
 Eelations of Veins to each other. The influence which veins 
 exert on each other may be in some measure ascertained by an 
 examination of the phenomena at the points where they come into con- 
 tact with or cross each other. At these points it is very often found 
 that the quantity of ore is suddenly increased to a large amount, and 
 for some distance, in either one or both of the veins. Many veins 
 are productive only near such points, or yield there peculiar ores and 
 minerals. This does not depend upon the enlargement of the vein 
 merely, but may indicate the influence of thermal or electric condi- 
 tions in the disposition of the materials of mineral veins. We have 
 heard miners say that in certain cases neighbouring veins are subject 
 to a kind of reciprocity, so that they are not both productive in 
 the same ground, but where one is rich the other is poor. 
 
 Age of Veins. When two veins cross, it almost invariably 
 happens that one of these cuts, or is continued, right through the 
 other, as a wall is sometimes continuous through another wall of brick 
 from top to bottom. Thus a vein of copper ore may cross and cut 
 through a vein of tin ore ; a vein of lead ore may cut through a vein 
 of copper ore, and all these be cut through by some other sparry vein 
 or porphyry dyke. It is supposed, by almost every writer on the 
 subject, that the relative antiquity of the veins which thus intersect 
 one another may be immediately determined ; and that in every case 
 the vein which is cut through is the older of the two. Werner took 
 this as the basis of his classification of veins, and most practical as 
 well as theoretical miners agree in his views ; but they are neverthe- 
 less controverted. 
 
 We cannot make a step in this argument, except upon the admis- 
 sion that the veins are posterior in date to the rock which encloses 
 them ; in other words, that the space in which the mineral masses of 
 a vein lie, once existed as a fissure in the rocks, and was subsequently 
 filled up by the accumulation of the sparry and metallic matters. 
 
 The Neighbourhood of a Vein. It is a general fact that the walls 
 of a vein partake in some degree of its characters, and that effects, 
 apparently depending on the vein, propagate themselves into the 
 neighbouring rocks. Thus the walls become more indurated, more 
 
4o6 "CONTEMPORANEOUS VEINS" NOT METALLIFEROUS. 
 
 crystalline, and for considerable distances are filled with the matters 
 of the vein ; and even the very substance of the rocks is often impreg- 
 nated with mineral combinations. In a country where the veins are 
 numerous, large masses of the rocks may in this way be ridered, as it 
 is termed in the North of England ; and if such a gradation of char- 
 acters could be relied on as a proof of contemporaneity of origin, this 
 may in a few cases lead to the conclusion that the veins and rocks are 
 coeval. 
 
 But the true conclusion on this point is that these effects are 
 locally related to veins; the ridering of the neighbouring rocks is 
 coeval with the production of the vein ; but since these rocks are 
 clearly defined from the veins, and fragments of them are enclosed 
 in the veins, and the mineralising influence which they have suffered 
 obviously depends on the influence of the veins, we cannot hesitate 
 to admit that these latter are of separate and subsequent origin. 
 
 Veins in Different Eocks. It is found that when veins divide 
 different sorts of rocks, their contents vary in an inconstant manner, 
 according to the nature of the rocks. The most usual notion on this 
 subject is, that the veins may be viewed as secretions from the rocks ; 
 and by some this is supposed to have happened after the production 
 of fissures ; by others, by a mere internal separation of the parts of the 
 mingled metallic and earthy mass. 
 
 This notion of the slow separation of the ingredients of rocks is in 
 accordance with the principles and facts of chemistry, and must be 
 often appealed to, if we would explain by true causes the phenomena 
 of mineral veins. 
 
 Contemporaneous Veins. There are combinations of minerals in 
 masses of various figure, which, upon very good grounds, are admitted 
 to be contemporaneous with the rocks in which they lie ; and if we 
 choose to call by the name of veins all such distinct combinations of 
 minerals, these certainly are contemporaneous veins. When in granite, 
 greenstone, &c., we find particular portions of those rocks either 
 linear, tabular, globular, or in any other figure, which have a different 
 proportion of ingredients from the other parts, and in consequence 
 become conspicuous and distinct, except at the edges, which graduate 
 without any sign of fissure into the ordinary mass of the rock ; 
 these may certainly be pronounced contemporaneous veins, and they 
 have been produced by a process of secretion or segregation during the 
 crystallisation of the rock. 
 
 In some instances veins of calcareous spar or other minerals lie 
 wholly included in limestone masses, and these are properly called 
 veins of segregation ; but they are not contemporaneous veins, for they 
 have clearly been fissures filled at some period since the consolidation 
 of the rock; and the proof is, that shells, corals, &c., are split, and 
 sometimes displaced by these sparry veins, which undoubtedly occupy 
 cracks left by the shrinking of the rock in the process of consolidation. 
 
 Allowing every just latitude to the doctrine of contemporaneous 
 veins, we must admit that most veins are newer than the rocks which 
 enclose them and have yielded their minerals. 
 
INTERSECTION OF VEINS. 407 
 
 Intersection of Veins. The most simple case is when two straight 
 veins cross without any change of direction, or any lateral displace- 
 ment; and the order of effects appears to be the production of a 
 fissure, and the filling of this by a vein which was afterwards broken 
 through by another fissure, and this, in its turn, received another 
 mineral vein. It seems difficult to doubt the truth of this explana- 
 tion ; for if the vein which cuts through the other be subsequent to 
 the fissure in which itself lies, it must also be subsequent to the vein 
 which that fissure divides. The occasional complication of the 
 problem by the number of intersections does not at all change its 
 nature. 
 
 Appearances at the Crossing. It is sometimes observed that 
 the vein which upon this theory is the oldest, suffers a particular 
 kind of accident at its junction with the other. It is divided 
 into several branches on one or both sides of the cross veins, and 
 these branches enclose portions of the neighbouring rocks. There 
 is some difficulty in this case, however it be considered, but the 
 coincidence of this splitting of a vein with the crossing of another 
 vein may often be only accidental ; for such splitting frequently occurs 
 in a wide vein, far from any cross course, as in a fault. 
 
 The fissures which have received the mineral veins are in most 
 cases accompanied by slips or dislocations of the strata in a vertical 
 direction, and the veins are of course subject to the same accidents of 
 displacement. When two veins cross, and both are vertical, the lines 
 of bearing of the two portions of the displaced vein must remain 
 coincident after the fracture. If the divided vein be not vertical, its 
 separated portions will have their lines of direction parallel, but not 
 coincident ; and in any horizontal plane they will appear to have 
 sustained a lateral movement. 
 
 Thus in the diagram (fig, 79), & ' * 
 
 the cross vein a and the divided I /^ \\ 
 
 vein b are both vertical; but the ^ II // *" Jff- 
 
 divided vein c is inclined in the " *"'* I // \\ 
 
 direction of the arrows, and its 
 
 apparent lateral displacement is 
 
 really due to a vertical movement. Fi g . 79- 
 
 If two divided veins are inclined 
 
 in opposite directions, and be dislocated by the same cross vein, they 
 
 will appear to have moved laterally in opposite directions, as c and d. 
 
 Were we to include the cases of the inclined cross veins, and also 
 those where the inclination of these veins varies both in amount and 
 direction, the results would become too complicated for explanation 
 without mathematical symbols ; and we must, besides, remember that 
 the displacement of the strata is really very seldom in a vertical direc- 
 tion, but generally accomplished by an angular movement from some 
 fixed point, or round a virtual centre. 
 
 Geographical Relation of Veins. Werner long ago indicated that 
 mining districts are almost entirely confined to the vicinity of moun- 
 tains or elevated land, because in these situations the rocks were most 
 
4 o8 DISTRIBUTION AND DIRECTION OF VEINS. 
 
 dislocated by slips, and divided by fissures at the period of their eleva~ 
 tion. It is not the absolute height of the ground, but the circumstance 
 of its having been much exposed to subterranean convulsion, infiltra- 
 tion, and denudation, that determines the prevalence of mineral veins. 
 The rich mines of Cornwall are in comparatively low situations, but 
 they are all in the vicinity of elevated rocks which have been heated. 
 
 There appears to be no limit either of height above or depth below 
 the sea, which defines the productiveness of veins, though in some 
 countries like America, higher situations are most favourable. 
 
 It is sometimes found that the contents of a vein vary with the 
 depth, without any particular geological conditions ; as, for instance, 
 in Cornwall copper is prevalent in the mines at greater depths than 
 tin ; and in the slate tract of Cumberland veins which bear lead near 
 the surface yield copper at a depth. In other cases there appears a 
 peculiar determination of the metallic ingredients to particular situa- 
 tions. The mines about Ecton yield copper; those of Derbyshire 
 and the Pennine chain generally yield lead, but toward the eastern 
 and western limits of the district, copper becomes less uncommon. 
 
 The length of a vein or fissure is perhaps hardly in any case cer- 
 tainly known ; because, when it ceases to be worth working, it is for 
 all the ordinary purposes of mining said to be dying out, or cut out, 
 or ended. The richest veins are productive for limited lengths, but 
 the fissures which they fill may be, and often are, extended far beyond 
 the spaces occupied by metallic impregnations. Some of them are 
 known to extend, and to be productive for many miles in the Harz, in 
 Cornwall, and in the North of England. The width is various in 
 different veins, but generally nearly constant in the same vein. A 
 width of twenty feet is very unusual. Most veins are less than six 
 feet wide. 
 
 Directions of Mineral Veins. The most general direction of the 
 great dykes and faults in the North of England may perhaps be 
 defined to be nearly east and west. Eut this is much more certainly 
 true with respect to the mineral veins of the limestone districts of 
 Weardale, Allendale, Alston Moor, and all the mining districts of 
 Yorkshire ; and it is equally recognised in the Primary tracts of 
 Cumberland, Westmoreland, and Lancashire. This is so general a 
 fact, that the east-and-west veins are called right-running veins, while 
 the few which range more nearly north and south are called cross 
 courses. These latter are seldom rich in metal. They often cut 
 through and shift the right-running veins laterally, as both of them 
 shift the strata vertically. There is often to be observed a sort of 
 compensation in the dislocating effects of veins. In Weardale most 
 of the veins throw down to the south, while the parallel courses in 
 Allendale and Alston Moor throw down to the north. The lead veins 
 of Flintshire and Cardiganshire have the same east-and-west direction, 
 and so have those of Mexico. 
 
 The lodes and veins of Cornwall are most generally east-and-west 
 veins, or nearly so ; and these are the oldest veins in that district, 
 being traversed by oblique veins and by cross course elvans and 
 
RELATIVE ANTIQUITY OF VEINS. 409 
 
 flukans. But all the east-and-west lodes are not of the same age, the 
 tin being older than the copper ; neither are all the east-and-west tin 
 veins of one age, for those that underlie to the north are generally 
 traversed by those that underlie southwards. These curious generalisa- 
 tions are not to be overthrown by particular discordances ; and they 
 are certainly supported by analogous though less varied occurrences 
 in other countries. 
 
 Directions of Veins of Different Antiquity. The general order 
 of their dates may be thus expressed : 
 
 1. Oldest, east-and-west tin veins underlying to the north. 
 
 2. East-and-west tin veins underlying to the south. 
 
 3. East-and-west copper veins generally east to south. 
 
 4. Oblique or contra copper veins, generally east 30 to 45 south. 
 
 5. Cross courses not metalliferous, north and south. 
 
 6. Copper lodes of more recent date and lead veins. 
 
 7. Cross flukans or clay dykes nearly north and south. 
 
 8. Slides in all directions, but generally east and west. 
 
 The porphyritic and other dykes called elvan courses are very 
 generally divided by the veins, and seem to be of greater antiquity. 
 Werner observed this geographical relation of mineral veins, and stated 
 the two following cases. In the mining district of Freiberg are two 
 classes of veins very different from one another. One of these classes 
 consists of veins which run from north to south. The veins of this 
 group contain lead glance, black blende, iron, copper, and arsenical 
 pyrites, quartz, and brown spar. This is the oldest vein-formation. 
 The second class of veins, which always traverse the former, and are 
 never crossed by them, contains lead glance, radiated pyrites, heavy 
 spar, fluor spar, and quartz. They strike between the sixth and ninth 
 hours of the mining compass (east to south-east). 
 
 The mining district of Ehrenfriedesdorf contains veins of tin 
 and silver glance. The tin veins are always traversed by the silver. 
 The direction of the first is between the sixth and ninth hour (east 
 and south-east), that of the last from the ninth to the third hour (south- 
 east, south, south-west). 
 
 Direction of Mining Districts. There is observed in some mining 
 districts another remarkable relation of metalliferous veins to geo- 
 graphical lines. Though in the North of England the most frequent 
 direction of the veins lies east and west, the mining districts seem 
 rather to be ranged in lines from north to south. The nature of this 
 relation will be more easily understood if we add that both in Corn- 
 wall and in Cardiganshire, where the veins are also most frequently 
 east and west, lines of greater productiveness range nearly north 
 and south across the bearing of the veins. These curious notices 
 suggest the inquiry whether the lines of productiveness are dependent 
 on any principal axis of dislocation or strain, or on the occurrence 
 of cross courses. The former case seems to be indicated by the 
 phenomena in the North of England. Perhaps the latter may be 
 more applicable to Cornwall. 
 
 Connection of Fissures and Main Joints. It is certain that in 
 
410 RELATION OF METALS TO ROCKS. 
 
 the limestone dales of the North of England mineral veins are some- 
 times directed along the master joints of the rocks, also that in the 
 slate tracts of the Craven district veins and dislocations range along 
 the cleavage planes of the slates. Dr. Boase noticed the same thing 
 in the slate tract in Cornwall, and such observations might be multi- 
 plied. Mechanical considerations would have led us to anticipate this 
 result ; for the main joints and cleavage planes are often the lines of 
 least resistance, and yield more easily to infiltration than other parts 
 of a stratum, or 1 3 any eruptive or depressing force. 
 
 Relation of Mineral Veins to the Rocks which enclose them. 
 The relation of mineral veins to the rocks which enclose them is very 
 little understood. It is difficult to distinguish clearly between the 
 accidental and the necessary association of the phenomena of veins 
 and rock masses. 
 
 Metalliferous veins occur more or less frequently in every class 
 of rocks, and equally in limestone, in argillaceous slate and shale, in 
 quartz and sandstone rocks, and in rocks of mingled ingredients ; in 
 uniform slates, and fragmentary millstone grit, in granular sandstone, 
 compact limestone, and crystallized limestone and granite ; in sedi- 
 mentary grits and shales, in porphyries, basalts, and metamorphic 
 conglomerates. The existence of mineral veins in a rock is therefore 
 wholly independent of the particular chemical and mineral ogical 
 nature and proximate origin of that rock ; nor, when due allowance 
 is made for the relative prevalence of the different kinds of rocks, 
 does there appear any reason to admit that any preference or more 
 frequent occurrence of metalliferous veins in rocks of particular kinds 
 can be traced, except in particular districts. There yet remains the 
 inquiry whether certain metals are specially associated with or related 
 to particular sorts of rocks. 
 
 Diffusion of Pyrites, &c. Hardly any substance is more abundant 
 in the mineral kingdom than iron pyrites ; it occurs both in veins and 
 disseminated crystals or concretions; and, in one or other of these 
 states, it is associated with almost every known rock. It occurs dis- 
 seminated in limestone of various kinds, as crystalline limestone, 
 carboniferous limestone, and chalk ; in clay slates, shales, clays ; and 
 basalt. Veins containing iron pyrites traverse rocks of as great diver- 
 sity. Copper pyrites is not disseminated through so many rocks as 
 iron pyrites, but it occurs in veins which traverse limestone, sandstone, 
 and shale, clay slate, mica schist, granite, &c. Ores 'of manganese 
 are also very generally diffused through rocks of very different kinds. 
 The converse is true. In one and the same kind of rock occur 
 veins of copper, lead, silver, and tin. 
 
 Relations of Metals to Slate and Granite. The tin veins of 
 Cornwall sometimes pass through clay slate and granite ; they produce 
 ores in both. A vein that has been productive of copper ore in the 
 clay state, passing into the granite, becomes richer, or, what is more 
 remarkable, furnishes ores of the same metal differently mineralized. 
 If we pursue it farther into the granite, the produce of metal is 
 frequently found to diminish. A change of ground is looked upon 
 
CONDITIONS OF PRODUCTIVENESS. 411 
 
 by miners as affording reason to expect an alteration for better or 
 wor.-e, suggesting secretion of minerals from the rocks. 
 
 Metalliferous Veins in Silesia. Remarkable instances of this 
 relation are given by Von Dechen. 1 The numerous veins which 
 cross the steeply-inclined strata of greywacke in the Liegen district, 
 are metalliferous in narrow bands parallel to the inclined beds of 
 slate. The veins of the Kupferberg, in Silesia, bear ore only in 
 the hornblende schist, and are impoverished in mica schist. At 
 Joachimsthal the mica schist is traversed by quartzose porphyry in 
 veins, which, as well as the contiguous rock, hold pyrites in mica 
 slate. The rothegang of Elias consists of loam, and holds only 
 uranite ; where it runs between mica schist and a porphyry vein, and 
 where it traverses the latter, its substance is a red hornstone, and it 
 bears vitreous silver, native silver, arsenical cobalt, bismuth glance, 
 kupfernickel, arsenic, and bismuth ; but red silver, elsewhere abun- 
 dant, is entirely wanting. 
 
 Condition of Veins in North of England. In the lead veins 
 of the north of England, which are situated in the Carboniferous 
 limestone tract, a singular dependence is observed between the con- 
 tents of the vein and the nature of the adjacent rock. The vein 
 divides limestones, sandstones, and shales, and these are brought 
 variously into apposition by the dislocations which accompany almost 
 all the veins. The vein is sometimes productive of lead ore under 
 every case of apposition in rocks. Where limestone, or schist, or 
 solid sandstone forms the walls, its productiveness is at the maximum, 
 but generally it is contracted in breadth and impoverished in its 
 metallic contents, wherever it is included between walls of shale ; and 
 even where only one side is occupied by shale, the same effect is fre- 
 quently observed. It would appear that the impoverishing influence 
 of the shale is referable to mechanical causes. In the same way as 
 the shales in a coal-pit swell out from the undisturbed parts to fill 
 the excavated vacuities, so we may conceive them to have expanded 
 into the natural fissure ; this will account for the contraction of the 
 vein. In the process of crystallisation, to which all the contents of a 
 vein are subject, it seems conformable to analogy to suppose that the 
 permanent walls of limestone and gritstone would permit a more early 
 growth of sparry and metallic crystals than the crumbling edges of 
 shale ; a supposition, perhaps, confirmed by the occasional mixture of 
 shale in the sparry mass of a vein, where it is " nipped," as the miner 
 says, in beds of shale. 
 
 Quantity of Lead Ore from Different Beds of Limestone. 
 From some or all of these causes it happens in the north of England 
 that certain limestones are very much more productive than the 
 others ; in different mining districts, different limestones are thus 
 favourably distinguished, but in the country of Alston Moor, Tees- 
 dale, arid Swaledale, the uppermost thick limestone is by far the most 
 rich in lead. 
 
 Upon the whole there is no sufficient evidence to show that the 
 1 "De la Beche's Manual," German Trans., 594. 
 
412 THE WALLS OF VEINS. 
 
 local production of metallic substances is in any special manner de- 
 pendent upon the chemical or mineralogical composition, or the 
 circumstances of the formation of the adjacent rocks, though in many 
 instances we observe the aggregation of the substances in the vein to 
 have been influenced by peculiar conditions of the including rocks. 
 
 Alteration of Substance of Walls of a Vein. The walls or 
 cheeks which form the more or less definite boundaries of the vein 
 are in some instances highly indurated, and very often fissured, so 
 as to break parallel to the vein ; in other cases certain sorts of rock 
 (as clay slate, both in Cornwall and Germany) are greatly softened, 
 and even converted to clay, along one or both sides of a vein. 
 Werner mentions the decomposition of f elspathic and hornblendic rocks 
 for a fathom from the vein. We have also witnessed the fact of lime- 
 stone, usually a blue or grey crinoidal rock, burnt, as the miners term 
 it; that is, converted to a brown granular crystalline rock in Tees- 
 dale. Another remarkable effect in the walls is the production of 
 slickenside, so long known in the mines of Derbyshire, which are 
 situated in limestone, and tilled with fluoric and barytic spars, and 
 yield lead ; in those of Cornwall, which are in killas, and with a 
 matrix of quartz, and yield copper ; in the magnesian limestone of 
 Yorkshire, where copper or lead lines the limestone cheeks ; and in 
 the faults of the coal system of Yorkshire, where neither spar nor 
 metallic matters are common. These and many other occurrences of 
 rubbed surfaces along planes of fissures speak a clear language, and 
 prove to the fullest conviction the mechanical movement of the sides 
 of the fissure upon one another or upon the contained substances. 
 The groovings of the surfaces, thus produced by rubbing, indicate, of 
 course, the line of the movement ; the circumstance that the polished 
 faces are partially covered by lead ore, copper ore, &c., as the nature 
 of the vein is, proves, moreover, that the movement was, in such 
 cases, posterior to the introduction of the whole or a part of the 
 mineral impregnation, so that the same fissure has been, in such 
 cases, the plane of more than one convulsive movement. 
 
 Eelation of Veins to the Different Ages of Rocks. It is in 
 the palaeozoic rocks, and in the metamorphic and igneous rocks 
 associated with them, that all the veins in Great Britain are worked. 
 In a few instances veins of small value, producing lead and copper, 
 pass through the magnesian limestone ; but not a single example is 
 known of a true metallic vein in the Oolitic, Cretaceous, or Tertiary 
 strata. The connection of metallic veins with the older rocks is 
 not an accidental coincidence, but a constantly recurring pheno- 
 menon; and the absence of such veins from the newer strata in 
 England cannot be accounted for by any circumstances of the geo- 
 graphical position of these strata ; for both around the metalliferous 
 slates of Cumberland and limestones of Derbyshire the trias is exten- 
 sively spread, and yet not one lead or copper vein occurs in it. Any 
 one who should confine his attention to the British Isles might infer 
 that the causes of the production of mineral veins had been almost 
 wholly inactive ever since the Carboniferous epoch ; and as a general 
 
SYSTEMS OF MINERAL VEINS. 413 
 
 expression this may apply to the continent of Europe, though both 
 in the Pyrenees, and around the central granitic tract of France, 
 metalliferous veins apparently originating in these rocks, traverse 
 strata of the oolitic and cretaceous systems. 
 
 Association of Veins and Dykes. It must here be remarked, that 
 both in Great Britain and throughout Europe intrusive veins and 
 basaltic dykes are in the same manner abundant in the primary and 
 rare in the secondary and tertiary strata. This is one of many general 
 analogies tending to substantiate the opinion that rock veins and 
 dykes, and metalliferous veins, ften form two parallel series of 
 products developed during the same geological periods by the same 
 general causes, acting under different circumstances upon different 
 materials. The disruptions by which igneous rocks were placed in 
 contact with secondary and tertiary strata must have been first expe- 
 rienced by the older strata, from beneath which the disturbing force 
 originated ; hence it is easy to understand why the primary rocks are 
 so universally, and the secondary and tertiary strata so partially 
 affected by dykes and enriched with mineral treasures. 
 
 Modern Veins. As an example of veins of more recent date, we 
 may quote Von Dechen's notice of the veins of Joachimsthal. In 
 this case the dykes of basalt which divide the mica slate are them- 
 selves cut through by mineral veins. These dykes are variously 
 connected with great overlying masses of basalt which break into the 
 Brown Coal formation. It is therefore evident that the silver, arsenic, 
 and cobalt ores have been thrown into the veins at a later epoch than 
 that of the brown coal tertiary deposit at the foot of the Bohemian 
 Erzgebirge. 
 
 Werner's Eight Systems. Werner distinguished eight principal 
 systems of mineral veins in the mining field of Freiberg. The first 
 and oldest produces abundance of argentiferous lead glance. It con- 
 sists of coarse granular lead glance with from one and a half to two 
 and a half ounces of silver per quintal, common arsenical pyrites, 
 black blende in large grains, common iron and hepatic pyrites, some- 
 times a little copper pyrites, and a little sparry ironstone. The vein- 
 stones are chiefly quartz, sometimes a little brown spar, rarely calc 
 spar. These circumstances occur most generally in veins ranging from 
 north to south. 
 
 The second yields lead very rich in silver. It contains lead glance 
 in large and small grains ; black blende in small grains ; iron and 
 hepatic pyrites, and a little arsenical pyrites. In addition, dark-red 
 silver ore, brittle silver ore, white silver glance, plumose antimony 
 ore. The veinstones chiefly quartz, with much brown spar and often 
 calc spar. The veins range south and south-west. 
 
 The third yields lead glance with one ounce of silver per quintal, 
 much iron pyrites, a little black blende, and red iron ochre. Vein- 
 stones are quartz, sometimes with chlorite mixed and surrounded with 
 clay. The veins range north and south. 
 
 The fourth yields lead glance with one-fourth to three-fourths of 
 an ounce of silver per quintal, radiated pyrites, and sometimes brown 
 
4H RELATION OF MINERAL VEINS TO IGNEOUS ACTION. 
 
 blende. The veinstones are heavy spar, fluor spar, a little quartz, and 
 rarely calc spar. The veins range east and west. To this system 
 Werner boldly referred the veins of Derbyshire, the Harz, and also 
 those of Gisloif in Scania ! 
 
 The fifth consist of native silver, silver glance, and glance cobalt, 
 sometimes with grey copper ore, lead glance rich in silver, fine-grained 
 brown blende, and sparry ironstone. The veinstone is heavy spar in a 
 state of disintegration, and fluor spar. It always occurs in the intersec- 
 tions of the first and fourth systems. Its directions are north and 
 south, and east and west. It sometimes is found even in the middle 
 of the westerly veins. 
 
 The sixth contains native arsenic and light red silver ore ; w r ith a 
 little orpiment, copper nickel, glance cobalt, native silver, lead glance, 
 iron pyrites, and sparry ironstone. The veinstones are heavy spar, 
 green fluor spar, calc spar, and a little brown spar. This system 
 occurs in the intersections of the fourth and fifth systems, or in the 
 middle of veins. 
 
 The seventh is of red ironstone, with a little iron glance, quartz, 
 and heavy spar. Occurs in the upper parts of veins. 
 
 The eighth and newest is of copper pyrites, mountain green, mala- 
 chite, and red and brown iron ochre, with a little quartz and fluor 
 spar. 
 
 Relation of Mineral Veins to the Local Centres of Igneous Action. 
 
 Our investigations lead directly to the inquiry, how far the geogra- 
 phical occurrence of metalliferous veins is connected, as that of rock 
 dykes is known to be, with the evolution of igneous rocks ? 
 
 Evidence on this subject can be obtained in two ways : first, 
 by comparing metalliferous and non-metalliferous districts of old 
 strata in their geographical relation to igneous rocks and convulsions ; 
 secondly, by examining the relation to igneous agency of the locally 
 metalliferous newer strata. 
 
 In Older Rocks. The older rocks are not by any means univer- 
 sally stored with metalliferous veins any more than with dykes. 
 Large tracts in the slate rocks of Devonshire are nearly devoid 
 of metals, but near the granitic masses of Cornwall they are abund- 
 antly supplied with veins. In Wales the slate rocks yield copper and 
 lead chiefly along the western borders of the Principality, where the 
 local centres and axes of elevation are situated. Amid the Cumbrian 
 lakes lead and copper veins adjoin the granitic, hypersthenic, and 
 syenitic axes of Carrock, Skiddaw, High Pike, &c. They occur near 
 the porphyries of Helvellyn and Old Man, but the greater portion of 
 the slates, far removed from the foci of disturbance, are devoid of mineral 
 treasures. 1 
 
 In Scotland metallic veins adjoin the granitic nucleus of Strontian. 
 
 The Mining tracts of the Harz, the Erzgebirge, Hungary, Brit- 
 tany, and other localities are convulsed by disruption and diversified 
 1 See Postlethwaite. 
 
DYKES AND MINERAL VEINS IN ENGLAND. 415 
 
 by the intrusion of granitic and porphyritic rocks ; the Ardennes 
 mountains, which yield few veins, develop hardly any igneous 
 rocks. 
 
 The carboniferous limestone tracts of Mendip, Derbyshire, and 
 Flintshire, of Wharfdale, Swaledale, and Alston Moor, have been 
 shaken to pieces by many convulsions, and they are very rich in 
 lead and zinc ; but the greater part of the Yorkshire and Northumber- 
 land limestones, affected by only one or a few general elevations, 
 are poor in metal. 
 
 In Newer Rocks. The newer rocks are metalliferous only in the 
 vicinity of the foci of their disturbance, as round the central granite 
 of France, near the igneous masses of the Pyrenees and the Alps ; in 
 all which places the metallic ores are PO related to the igneous rocks 
 that they occur only in a narrow zone at the junction of the igneous 
 and the altered stratified rocks. 1 
 
 Conclusions on this Subject. We must not shut our eyes to 
 some decided differences between the situations of dykes and veins. 
 For instance, the Island of Arran is traversed by hundreds of dykes 
 of basalt, porphyry, and pitchstone, but metallic veins are almost 
 unknown there. Alston Moor is dissected like a map by veins of 
 lead ore, but very few whin dykes occur there ; on the contrary, in 
 Northumberland and Durham whin dykes abound in the coal tracts 
 where lead is hardly known. It is, besides, too remarkable a thing 
 to be overlooked, that south of Durham barely a solitary whin dyke or 
 porphyry dyke is known through the metalliferous tracts of York- 
 shire, Derbyshire, Somersetshire, and Flintshire. This contrast is 
 the more remarkable in the country about the sources of the Tyne 
 and Tees, because there basalt has been erupted in vast quantity, and 
 at its eastern termination appears related to several dykes of great 
 extent. This mass of basalt is traversed by the veins in the same 
 manner as the limestone is, and we may accept the conclusion that 
 both are due to heat, but that the vein marks a later or collateral 
 stage, in which there was less heat and more water, acting more 
 slowly. 
 
 Mining. At the present day we find in California and Aus- 
 tralia the very same modes of working alluvial deposits which were 
 practised in Gallicia for gold, and the Cassiterides for tin in the 
 days of Pliny and Strabo and Herodotus, 2 and perhaps in equally 
 ancient times in those hyperborean regions, the Ural mountains, which 
 still furnish so much gold to Europe. Probably many great mountain 
 chains, full of quartzose and metamorphic rocks, yielded gold in the 
 earlier ages of the world ; though none of the rivers washed it out 
 in such abundance as the streams of Lydia. Not much of the gold 
 and silver of the ancient world was obtained by mines properly so 
 called, at least in Europe, till the heroic spirit was replaced by great 
 commercial activity. Then the Athenians dug silver ores from 
 Laurion, the Carthagenians and Komans obtained silver and gold 
 
 1 Observations of Dufrenoy, Von Buch, &c. 
 - Hist. Nat., many notices. 
 
416 THE ART OF THE MINER. 
 
 from Spain ; the Romans separated silver from the lead ores of 
 Derbyshire and the north of England. Peculiar mining customs, 
 not yet extinct in Derbyshire and Cornwall, bear testimony to the 
 high antiquity and foreign source of the art of mining, as established 
 in these countries ; while throughout the North of England such terms 
 as groove, and sump, and toadstone 1 betray the later influence of 
 German workmen. 
 
 As in all other departments of geology, practical knowledge of the 
 phenomena discussed must be gained by examination of the struc- 
 ture of some of the districts which have been mentioned. Many 
 public museums, especially that of the Royal School of Mines, contain 
 examples of the modes of occurrence of the several ores, models of 
 lodes, and illustrations of various facts connected with mining dis- 
 tricts, by studying which a useful preliminary practical knowledge 
 may be gained, which will facilitate practical work in the field and 
 underground. The Jermyn Street Museum also contains a valuable 
 collection of models of mining appliances. 
 
 The art of the miner, founded on long experience, is now gradu- 
 ally acquiring the aspect of applied science. To bring it fairly within 
 the circle of inductive philosophy to give it more exact laws, based 
 on a surer classification of phenomena is an object of the highest 
 import for humanity. On the command which man has acquired over 
 the various properties of metallic matter has depended much of his 
 civilisation and a large part of his power over the forces of nature. As 
 this command is extended, these forces may be still more completely 
 brought within the direction of the human mind. To do this, science 
 must be carried into the mines, and miners brought into the class- 
 rooms of the professors of chemistry, geology, mineralogy, and 
 mechanics. Practice thus becomes method, and experience is 
 exalted to theory. 
 
 1 Groove is scarcely altered German for a mine ; sump, a shaft below level, is 
 clearly from sump/en, a German verb to sink ; toadstone, in which the metallic vein 
 is unfruitful, is todstein, German for dead or unproductive rock. 
 
( 417 ) 
 
 CHAPTER XXIII. 
 
 THE CHIEF MINEKAL DEPOSITS IN BRITAIN. 
 
 Gold. 
 
 GOLD is always native, always alloyed with silver, and contains 
 minute quantities of copper and iron. Iron pyrites almost always con- 
 tains gold. Gold usually occurs in quartz veins, which are sometimes 
 in granite, and sometimes in schists and the older primary rocks. 
 Much gold has been obtained from the eastern slope of the Ural, a 
 little from the western slope, and from the Siberian governments of 
 Tomsk, Yenisseisk, and Irkutsk. The veins at Beresow are in 
 granite. The Austrian gold mines are on the western borders of 
 Transylvania at Zalathna, where the gold is combined with tellurium. 
 At Schemnitz and Kremnitz, gold is obtained from silver. The 
 sands of the Rhine between Bale and Mannheim, especially about 
 Strassbourg, have long yielded gold. In France the river Arriege 
 derives its name from " Aurigera." French gold is chiefly obtained 
 from lead veins. The only quartz vein in France worked for gold 
 is that of La Gardette in the department of Isere. The sands of the 
 Douro and Tagus have both been washed for gold, but at the 
 present day the only auriferous streams in the peninsula are the 
 rivers Sil and Salor. Auriferous pyrites is found in the Auzasca 
 valley near Monte Rosa ; and gold is worked in the Serpentine near 
 Genoa. 1 
 
 Gold in Wales. The British gold mines were formerly of some 
 importance. The Romans worked a gold mine at Gogofaw, west of 
 Llandovery, in South Wales, where gold occurs in a quartz vein, 
 cutting the Arenig rocks. A more important gold region lies on the 
 north side of the river Mowddach, in Merionethshire ; and from 
 1854 to 1866, 12,800 ounces of gold were obtained in this region, 
 chiefly from the Vigra and Clogau mines. The auriferous veins run 
 east and west, and the gold was most abundant, in quartz, at the 
 points where the veins cross other mineral lodes, especially those con- 
 taining ores of silver, copper, and lead. "Welsh gold is of a pale 
 colour, owing to its being alloyed with silver ; and this circumstance 
 somewhat reduces the value of the metal. 
 
 1 Concerning the distribution of gold and other metals, reference may be made 
 to Whitney, " Mineral Wealth of the United States ; " and John Arthur Phillips 
 on Ore Deposits. 
 
 VOL. I. 2 D 
 
418 GOLD, SILVER, AND COPPER. 
 
 In Scotland l gold lias been found in no inconsiderable quantity 
 among the hills of Dumfriesshire, where gold-mining employed hun- 
 dreds of men in the reign of James Y. of Scotland, and yielded a 
 large amount of metal And among the metamorphic rocks of Suther- 
 land gold occurs, and has been from time to time successfully worked 
 by washing the superficial gravels. 
 
 In Ireland. In county Wicklow gold is well known to occur ; 
 and at one time, in 1795-96, the Balin Valley brook, a tributary of 
 the Ovoca, yielded about ^14,000 worth of gold. All the mines were 
 shallow placer mines less than 50 feet deep. 
 
 Silver. A considerable vein of silver was found at Hilderston in 
 Linlithgow ; and in 1715 a valuable silver vein was worked in the 
 Ochills, between Middlehill and Woodhill. 
 
 There is now no silver mine in Britain, but a large amount of silver 
 is obtained from lead mines. Some of these occur in the Llandeilo 
 rocks, and some in the Carboniferous limestone. The highest yield of 
 silver is from the great Laxey mine, in the Isle of Man. Other im- 
 portant silver lead mines occur in Cornwall, Devonshire, Cardigan, 
 Montgomery, Shropshire, Westmoreland, Yorkshire, Durham, and 
 Northumberland. 
 
 Copper. 
 
 British Copper Mines. The greater number of copper mines are 
 in Cornwall. The copper mostly occurs in lodes, which run within 
 twenty-five degrees of due east and west. Other lodes run from north- 
 east to south-west, and from north-west to south-east. The productive 
 strip of country stretches along the course of the granite masses, form- 
 ing a belt about fifteen miles wide, between Dartmoor and the Scilly 
 Isles. On the south-east of the granite of St. Austell are the Fowey 
 copper mines, and round Redruth and Penrhyn there is abundance of 
 copper ore. Many mines contain copper ore in the upper part and tin 
 ores below. The copper ores traverse both the granite and overlying 
 slates. The prevailing ores of Cornwall are the bi-sulphide and sul- 
 phide of copper, together with some amount of red oxide, carbonates, 
 and silicates. Arsenical pyrites abound in the Tavistock district of 
 Devonshire. In Cheshire, at Alderly Edge, copper occurs" in the 
 Lower Keuper Sandstone as blue and green carbonates. A simi- 
 lar deposit, also exhausted, occurs at Gardistori, near West Felton in 
 Shropshire. 
 
 1 Native gold attracted attention in Scotland at an early period. In 1153 
 David I. granted to the Abbey of Dunfermline a tythe of all the gold he should 
 obtain from Fife and Forthrif. The Scotch Parliament in 1424 granted the crown 
 all the gold mines in Scotland. Those of Crawford Moor were discovered in the 
 reign of James IV. ; and in the reign of James V. much of the gold coinage was 
 minted from native gold. For a long period the mines were worked by Germans 
 and Dutchmen. About 1578 gold was found abundantly in Henderland Moor 
 in Ettrick Forest, and from time to time a little has been met with down to 
 recent years. 2 
 
 a "Early Records relating to Mining in Scotland," by R. W. Cochrane- Patrick 
 of Woodside, 1878. 
 
THE ORIGIN OF TIN. 419 
 
 The carboniferous limestone of the Parys mountain in Anglesea 
 has long been productive of copper ; and other mines in this for- 
 mation have been worked at Slanymyneck Hill in Shropshire, near 
 the Great Ormes Head, near Hartington, on the borders of Stafford- 
 shire and Derbyshire ; while there is an accumulation of bog copper 
 on the west of Khobell-fawr, near Dolgelly. Lodes of sulphide and 
 blue and green carbonate occur in the neighbourhood of Snowdon, 
 Beth-gelert, and Moel Hebog. The lead mines near Aberystwith are 
 charged with copper. In Ireland important mines occur in Avoca in 
 "Wicklow. Copper pyrites is the chief British ore. 
 
 The Coniston copper mines of Cumberland are in Cambrian rocks. 
 They are principally copper pyrites found in quartz lodes running east 
 and west. 1 Similar deposits are found in Wicklow, and it is in rocks 
 of similar age that we find the native copper of Lake Superior, and 
 the Rio Tinto district in the south-west of Spain. The Devonian 
 rocks are nowhere richer in copper than in Cornwall and Devonshire. 
 
 In many of the Cornish mines the copper pyrites is associated 
 with iron pyrites, and about Redruth copper succeeds tin in the 
 deeper part of the lode. In the west of Cornwall the grey sulphide 
 is associated with specular iron ore. 
 
 Foreign Copper Mines. Copper is widely distributed in Europe, 
 chiefly in the Primary rocks. The Kupferschiefer of Thuringia is 
 worked as an ore of copper about Mansfeld in Prussia, where the rock 
 yields fifty pounds of copper to the ton. The Permian strata of Russia 
 are also copper-bearing, especially between Perm and Kazan. The 
 richer mines near Katherineburg are on the east side of the Ural, and 
 consist of lodes of copper pyrites with malachite in the upper part. 
 At Chessy near Lyons, the blue carbonate of copper was formerly 
 obtained from the base of the Permian sandstone. The principal 
 Italian copper ore is the sulphide found at several places in Tus- 
 cany, in serpentine and gabbro ; but the greatest copper-producing 
 districts of the world are in Chili between the 25th and 3oth paral- 
 lels, where the ore is found in volcanic rocks of Cretaceous age. 2 
 
 Tin. 
 
 DaubreVs Views on the Origin of Tin. Tin veins differ from 
 those of other metals in the ore being an oxide ; the gangue consists 
 of quartz, the tin is diffused in the quartz, and associated with wol- 
 fram and arsenical pyrites, besides which there occur various silicates, 
 fluorides, and borates, including lepidolite, topaz, pycnite, tourmaline, 
 and axinite. Apatite, a phospho-fluoride of lime, is common, and 
 accompanied by other phosphates. All over the world there is the 
 same association of tin with silica, fluor, boron, phosphorus, and 
 arsenic. M. Daubree urges that fluorine has been the chief agent, 
 not only in forming these minerals, but in bringing the tin to the 
 positions in which it is found. 
 
 1 See Postlethwaite : "Mines and Mining in the Lake District," 1877. 
 
 ~ For many facts relating to Copper we are indebted to H. Bauerman, F.G.S. 
 
420 THE GREAT FLAT LODE OF CORNWALL. 
 
 Fluoride of tin at high temperatures is a stable compound, and 
 may have been brought up from great depths in this combination. 
 Oxide of tin is always associated with tourmaline, the existence of 
 which is assumed to be connected with a volatile fluoride of boron. 
 In fact, all the minerals would originally have been fluorides, and 
 have gradually acquired their present combinations. Oxide of tin 
 occurs in the crystalline form of felspar, which is sometimes also 
 replaced by tourmaline. Oxide of tin has been formed experimen- 
 tally by Daubre"e, by heating together perchloride of tin and steam, 
 as well as by dissolving the perchloride of tin in a current of dry 
 carbonic acid. 
 
 Eutile is similarly obtained artificially by heating together steam 
 and the perchloride of titanium, from which it is inferred that titanic 
 acid has resulted from the decomposition of fluoride of titanium, 
 which is associated with the fluoride and chloride of phosphorus and 
 boron. On such a view the ores would be comparable to deposits 
 formed around volcanic vents. 
 
 Apatite is produced by passing a current of perchloride of phos- 
 phorus over caustic lime heated to redness, when the perchloride is 
 absorbed and decomposed. In a like manner, topaz is formed by the 
 action of fluoride of silica on pure alumina ; and corundum has been 
 obtained by the action of volatile metallic fluorides reacting 011 
 oxides. 1 Such an interpretation has the merit of accounting for the 
 minerals associated with tin. 
 
 Tin Lode at Redruth. Around the granite hill of Cam Brae, 
 about a mile south-west of Redruth, are many famous tin mines con- 
 nected with the Great Flat Lode. The lode in the eastern part of 
 the Wheal Uny strikes a little north of west, and being only inclined 
 46 south, is much flatter than most of the tin Veins in Cornwall, 
 which have an average inclination of 70. The leader varies from 
 eighteen inches wide to two inches wide. Its walls have a smooth 
 slickenside character. The lode which lies on the under side of the 
 leader is a compact schorl rock with veins of quartz, cassiterite, 
 chlorite, and iron pyrites. On the upper side of the leader the capel 
 is compact schorl rock. The greyback or black granite is not separ- 
 ated by any line of demarcation from the lode, and the capel is 
 similarly inseparable from the killas. Though this is the section at 
 the 130 fathoms level, it varies a good deal as to the relations of 
 lode to the leader. At a greater depth (175 fathoms) the leader 
 becomes a copper lode two to four feet wide. In some of the other 
 mines the leader becomes a quartz vein. The lode where it contains 
 tin is a mass of schorl rock from four to fifteen feet wide, and one to 
 three per cent, of cassiterite distributed in little grains or strings. 
 The capel is also a schorl rock poor in tin ore, with its constituent 
 minerals arranged in layers, thus indicating, and by its graduation 
 into the adjacent rocks, that the lode itself is only a more altered 
 portion of the granite or killas in which it occurs. Killas is fre- 
 quently altered into schorl rock, sometimes becoming tourmaline 
 1 Daubrde, "Geologie Expcrimentale, " Premiere partie, 1879, 
 
THE TIN LODES OF CORNWALL. 421 
 
 schist and sometimes capel ; hence the Great Flat Lode, being a 
 band of altered rock, is different from a vein as commonly understood, 
 and might be termed a tabular stock work. This deposit yields 
 more than one-eighth of the tin ore of Cornwall. 1 
 
 Tin near Helston. Close to the turnpike road leading from 
 Penryn to Helston, and about four miles from the latter town, are the 
 tin deposits of the East Wheal Lovell. These lodes are usually very 
 narrow, sometimes only a couple of inches thick ; but in places rich 
 deposits of tin occur on both sides of the vein. There is a leader or 
 little vein of quartz, less than half an inch thick, running through the 
 granite, and on each side of it a quantity of tin stuff which is a 
 mixture of quartz, mica, cassiterite, pyrites, and other minerals. 
 There is no wall separating the tinny mass from the granite. Some- 
 times the tin was more developed on one side of the leader than on 
 the other. The main mass of the East Wheal Lovell extends like a 
 pipe from the forty fathom level to the no fathom level. Dr. 
 Eoster remarks that there are other pipes and bunches of tin ore, 
 similar to those which occur in the St. Ives district ; but they differ 
 in the absence of the mineral tourmaline. Concerning their origin 
 he observes that the leader was evidently an open fissure, so that 
 after the solidification of the granite, cracks were formed in it through 
 which vapours or solutions ascended, decomposed the felspar, and 
 deposited tin stone and other minerals among the constituents of the 
 granite. 2 
 
 Tin Lode of St. Columb. Another important localit} 7 for tin is 
 Park of Mines, three miles south of St. Columb. The nearest 
 granite is a portion of the great mass north of St. Austell, which 
 approaches within three quarters of a mile. Many little veins run 
 from north to south through the clay slate, varying in thickness up 
 to a quarter of an inch. They occasionally enlarge to one or two 
 inches and contain cassiterite. Near these insignificant little strings, 
 lenticular masses of tin stone occur in the planes of bedding of the 
 clay slate. They are termed the east-and-west lodes, but correspond 
 to the deposits which when horizontal are termed floors in Cornwall, 
 and flats in the north of England. The only minerals associated with 
 the cassiterite are schorl, quartz, and kaolin. The clay slate near 
 the boundary of the tin layers is usually stained red by oxide of 
 iron. 3 
 
 Tin of St. Agnes. In the St. Agnes district, Dr. Le Xeve 
 Eoster has described several tin lodes. One of these, known as the 
 Pink lode in the Penhalls Mine, east of Trevaunance Cove, extends 
 thirty-five degrees north of west in the clay slate. The leader 
 consists of quartz, cassiterite, chlorite, iron pyrites, and pieces of 
 capel, and has a width of from four to fifteen inches. The rock on 
 each side is termed capel, when by the infiltration of mineral solu- 
 tions it is changed from a soft slaty rock to a compact mass of quartz 
 
 1 Dr. Le Neve Foster, Q. J. G. S., vol. xxxiv. p. 640. 
 
 2 Trans. Roy. Geol. Soc. Cornwall, vol. ix. part II., 1876. 
 
 3 Foster, Miners' Association of Cornwall and Devon Report, 1875. 
 
422 LE NEVE FOSTER ON TIN LODES. 
 
 and schorl, arranged in the original bedding of the killas with veins 
 of quartz, and intersected with strings of cassiterite and chlorite. 
 
 The capel is usually on both sides of the leader, but sometimes on 
 one side only. It is never more than two feet thick. The deposit 
 of tin ore appears to have been subsequent to the formation of the 
 capel. The Wheal Kitty is a similar mine. The Wheal Coates is 
 especially interesting as yielding pseudomorphs of cassiterite after 
 orthoclase, showing how the metallic matter has replaced the felspar. 
 The Cligga promontory consists of granite intersected by more or less 
 parallel metallic veins running 20 to 30 north of east. Veins of 
 quartz run through the granite, and on each side of the quartz is a 
 band of greisen which consists of quartz and mica, though it often 
 contains schorl, gilbertite, and a little tin stone. There is no plane 
 of separation between the greisen and the granite as there is between 
 the greisen and the quartz. The cliff is intersected with a countless 
 number of little veins, which stand out because the greisen is more 
 durable than the granite. 1 
 
 The Wheal Mary Ann. The lode at Wheal Mary Ann runs a 
 few degrees east of true north through the killas. In one working 
 the killas is lined with cab, which is a sort of chalcedony; next 
 succeeds crystallised quartz of the ordinary kind, which is lined with 
 galena ; and, finally, the centre is occupied by chalybite or spathose 
 iron. Sometimes the cab contains fragments of killas and galena, 
 while vitreous quartz fills up the centre part of the lode. Elsewhere 
 the cab on the eastern wall is traversed by crystallised quartz, then 
 succeeds a band of vitreous quartz, and, finally, on the foot wall a 
 breccia of killas and cab cemented with galena and calc spar. This 
 condition of the lode is explained by Dr. Foster, who supposes the 
 formation of the fissure as a slight fault, in the cavities of which the 
 cab was deposited, cementing the fragments of the rock into a breccia 
 on the foot walls. After this the fissure was reopened, the new 
 fracture sometimes passing along the middle of the cab and sometimes 
 cutting across it, and then quartz, galena, chalybite, and calc spar were 
 successively deposited in the open spaces. 2 
 
 Stockworks near Bodmin. The term stockwork has been 
 adopted from the German, to designate large masses of rock impreg- 
 nated with metallic ores, or intersected with mineral veins which 
 cross each other in all directions, or at short distances apart. The 
 tin stockworks of Cornwall occur in the clay slate, granite, and 
 elvans. 
 
 Such mines are found at "Wheal Prosper, near Bodmin, where a 
 distance of 800 yards of killas, thirty yards wide, is crossed by mul- 
 titudes of little veins, rarely more than one-eighth of an inch thick, 
 and it only contains three pounds of ore to the ton of rock. The 
 Mulberry mine in the same neighbourhood exhibits similar pheno- 
 
 1 " Remarks on some Tin Lodes in the St. Agnes District," Le Neve Foster, 
 Trans. Roy. Geol. Soc. Cornwall, vol. ix. part III., 1877. 
 
 2 Le Neve Foster, " The Lode at Wheal Mary Ann," Trans. Roy. Geol. Soc. 
 Cornwall, vol. ix. part I., 1875. 
 
DISTRIBUTION OF LEAD VEINS. 423 
 
 mena. At Carrigan in the north, in the Hensbarrow granite tin 
 mines occur in the finest mass of greisen in Cornwall. Dr. Foster 
 believes the greisen to be merely altered granite. 1 
 
 Tin in New South Wales, &c. The granite in New South 
 Wales is regarded by Mr. Ulrich as forming a vast stockwork near 
 the township of Inverell, and the quantity of tin lying on the surface 
 in that district is estimated at twenty-five times the annual produce 
 of Cornwall. 
 
 Immense quantities are obtained from Banca and Billiton. 2 
 
 The mines of Chili and Peru also yield a large quantity of 
 tin. 
 
 Lead. Lead occurs in veins chiefly in the newer Primary rocks. 
 Galena is the most abundant ore, but carbonate and phosphate of 
 lead are both met with in some quantity. 
 
 In the Cambrian district of Shelve in Shropshire, towards the 
 borders of Montgomeryshire, the lead mines have been worked at 
 intervals since the time of the Romans. The mineral veins are 
 almost entirely in the Arenig and Llandeilo rocks. In the north- 
 west of Montgomeryshire mines also occur at Llangynog in Arenig 
 and Llandeilo rocks. In the Llanidloes district the veins have a 
 general east-and-west direction. 
 
 When the Carboniferous limestone appears in North Wales, in 
 Flint and Denbigh, ores of lead are found between Flint, Ilolywell, 
 and Llangollen. The general direction of the veins is east and west. 
 
 In Cardiganshire the lodes chiefly occur in anticlinal folds, the 
 corresponding synclinal curves being usually barren. There are 
 many mines on the south side of the estuary of the Dovey along the 
 course of the Rheidol, between Plynlimmon and Lampeter, and on 
 the east side of Plynlimmon. In this district the lodes range from 
 E.N.E. to W.S.W. In Carnarvonshire the lead district lies between 
 Capel Curig, Bettws-y-Coed, and Trefriw. 
 
 In Cornwall the most important are at West Chiverton, near Camel- 
 ford, and near Laureath. Lead is limited to the eastern side of the 
 county ; the direction of the lode is east and west at West Chiverton. 
 Herodsfoot mine, near Laureath, has the lode nearly north and south. 
 The number of lead mines in Devonshire is small. They chiefly occur 
 in the Ilfracombe slates, in the neighbourhood of Combe Martin, and 
 at Frank mills, near Exeter. In the North of England, Cumberland, 
 Northumberland, Durham, and Westmoreland, all yield large quan- 
 tities of lead. The mines at Alston Moor belong to the governors of 
 Greenwich Hospital ; the Weardale mines belong to the Ecclesiastical 
 Commissioners ; but among the more interesting are those of Allen 
 Dale, which belong to the family of Mr. W. B. Beaumont. Nearly 
 all these mines occur along the line of the Carboniferous limestone of 
 the Pennine chain, in the portion which lies above the Great Whin 
 Sill. In Swaledale, in Yorkshire, the ore chiefly occurs in the Main 
 limestone, and extends over an area fifteen miles long by six broad, 
 
 1 Le Neve Foster, "Tin Stockworks in Cornwall," Q. J. G. S., vol. xxxiv., 
 p. 654. 2 Reyer, Banka. 
 
424 THE LEAD OF ALSTON MOOR. 
 
 between Wensleydale on the south and Teesdale on the north ; 
 the lodes all run east and west, though in the south-east of the 
 district there are a few lodes which run from north-east to south- 
 west. South of the Swale, the ore frequently lies in caverns in the 
 limestone rather than in true lodes. 
 
 The lead veins in Alston Moor occur in fissures, where the strata 
 are sometimes dislocated 200 feet. On the downthrow side of the 
 fault the limestone is compact with scarcely a parting ; on the up- 
 throw side, the limestone is broken up with joints, often filled with 
 clay. The joints are most numerous near to the surface ; most 
 numerous in limestones, and least numerous in shales. The veins are 
 grouped by Mr. Wallace into three kinds. The first run between 
 K 60 E. and S. 60 E. The second class includes the veins running 
 north and south. Both these classes of veins are intersected by 
 small veins, some of which run S. 55 E. and others S. 55 W. 
 magnetic. These veins contain lead above, and lead and copper 
 below. The great sulphur vein, as it is termed, is not less than 300 
 feet wide at Crossgill. It is filled with quartz and iron pyrites, and 
 has the aspect of being formed by a great number of parallel fissures. 
 It runs from N. W. to S.E. At Cashburn it consists of quartz, and 
 forms low round hills ; it is supposed to be posterior in date to the 
 cross veins of Alston Moor. It is parallel to a whin dyke. Among 
 the east-and-west veins are the Browngill and Benty Field veins. The 
 Benty Field ramifies into weak strings, but at Dryburn the strings 
 unite and form a wide vein, which dislocates the strata 60 feet; 
 and this is the character of many of the veins of this district, though 
 the throw is usually much less. Xext to the Browngill, Fletcheras 
 vein is the strongest in this district. The cross veins are of all 
 magnitudes the Carrs vein has a dislocation of 260 feet, but the 
 throw varies with the vein and is sometimes only a few inches. The 
 contents of the cross veins are generally softer and less compact than 
 those of the E. and W. veins. It is held by Mr. Wallace that the 
 cross veins were either anterior to or contemporary with the veins 
 that run E. and W. 
 
 Lead ore is found mixed with quartz, carbonate and sulphate of 
 lime, carbonate and sulphide of iron, fluor spar, baryta, &c. The 
 richest deposits are in the upper part of the mountain limestone ; 
 but in the lower part of the mountain limestone, which is traversed 
 by augite andesites, there is very little lead ore. The deposits of 
 lead ore in the veins are generally divided from each other in this 
 district by shale. The Kampgill vein is uniformly wide, and well 
 filled with vein minerals. 1 
 
 The mining district of Derbyshire lies between Buxton and 
 Castleton on the north, and Cromford and Wirksworth on the south. 
 It is about thirty miles long by twelve miles broad, and is most 
 productive on the eastern side. In this district the richest deposits 
 occur when the strata form synclinal folds. 
 
 1 Wallace on Lead Deposits of Alston Moor. 
 
ZINC, BISMUTH, AND NICKEL. 425 
 
 Mr. J. D. Kendall urges that the veinstones of all the veins in 
 Cumberland are part of the original rock in- which the metallic matter 
 has been deposited 1 from chemical solutions, which removed the 
 /soluble rock-substance along a joint, and filled the spaces with ores. 
 
 In the Isle of Man there are eleven lead mines, the most famous 
 of which is the Great Laxey. Galena yields from 60 to 75 per cent, 
 of metallic lead. The quantity of silver in the lead varies at different 
 times in the same mine; 12 ounces to the ton is common, and 35 
 ounces of silver to the ton of galena is exceptional. 
 
 In Scotland the Leadhill mines in Lanarkshire were formerly 
 among the most important ; and in Ireland an important lead mine 
 occurs at Luganure, in County Wicklow. 
 
 Zinc is obtained from lodes, some of which occur in the older 
 Cambrian rocks, others in the Carboniferous limestone. The mines 
 which yield the largest quantity of zinc, also yield ores of lead. 
 Thus the West Chiverton in Cornwall, the Van mines of Montgomery- 
 shire, and the Great Laxey of the Isle of Man, are among the most 
 important ; but the Minera in Denbighshire, the Talargoch in Flint- 
 shire, the Woodend mine near Threlkeld, and Force Crag mine, Cum- 
 berland, and other mines in Shropshire, Devonshire, Cardiganshire, 
 Caernarvonshire, and Eadnorshire, all have local importance. The 
 chief ores are sulphide termed blende, carbonate termed calamine, 
 and silicate of zinc. The production is decreasing. Blende yields 40 
 to 47 per cent, of metallic zinc. 
 
 Bismuth. Native bismuth is found near Redruth in Cornwall, at 
 Carrack Fell in Cumberland, and at Alloa near Stirling. The 
 sulphide of bismuth is also met with in Cumberland and Cornwall; 
 and the oxide and carbonate are found at St. Agnes in Cornwall ; 
 but the supply of bismuth in our own country is too small for it to 
 have any importance as an ore. 
 
 Of Mercury there is no British ore. Mr. J. A. Phillips remarks 
 that the ores are found in the most ancient as well as in the most 
 modern rocks. Some of the principal veins are in the Silurian 
 strata, while the deposits of New Almaden, in California, are of 
 Cretaceous age. 
 
 Arsenic is obtained from the ores of tin in Cornwall and 
 Devon. 
 
 Antimony. A little sulphide of antimony is found in Cornwall, 
 but the chief supply is from Borneo. 
 
 Nickel occurs in small quantity in this country, chiefly in the 
 Pengelley and St. Austell Consol's mines in Cornwall, and at the 
 Bathgate silver mine in Scotland. Nickel is found associated with 
 iron at Voel Hiradig, Cwm in Flintshire, to the amount of about 2 -3 
 per cent. 
 
 Cobalt, in the form of cobalt glance, is found in the Botallack 
 mine ; and smaltine and cobalt bloom are also found in Corn- 
 wall. 
 
 1 Manch. Geol. Soc., April 1884. 
 
426 
 
 DISTRIBUTION OF BRITISH IRON ORES. 
 
 Distribution of the Chief Iron Ores in Britain. 
 
 Locality. 
 
 Cornwall at Restormel, 
 near Lostwithiel 
 
 Devonshire. 
 
 Somersetshire 
 
 Cumberland and North 
 
 Lancashire 
 Northumberland 
 Durham 
 South Yorkshire 
 Derbyshire 
 Staffordshire 
 Shropshire . 
 Warwickshire 
 North Wales 
 South Wales 
 Monmouthshire 
 Gloucestershire 
 Scotland . 
 Ireland 
 
 Lincolnshire 
 North Yorkshire 
 
 Oxfordshire 
 
 Northamptonshire 
 Wealden district 
 Wiltshire . 
 Lincolnshire 
 
 Ireland 
 
 Kind of Ore. 
 In Primary Rocks. 
 
 Hematite and Gothite ; and iron pyrites. 
 
 Hematite and magnetite, at Brixham and Haytor 
 
 mine. 
 Hematite and chalybite, in Brendon and Mendip 
 
 Hills. 
 Hematite, limonite, and chalybite, in Carboniferous 
 
 limestone. 
 
 Clay ironstone or chalybite, chiefly in the coal mea- 
 sures. 
 
 Limonite in Forest of Dean coal-field. 
 Clay ironstone. 
 Clay ironstone. 
 
 In Secondary Rocks. 
 
 Lower lias. 
 
 Middle lias, &c. , of Cleveland district. 
 
 Lias (Vale of Evenlode and Vale of Cherwell), In- 
 ferior Oolite (at Stow-nine-churches) ; the Neo- 
 comian sands of Shotover Hill are not worked. 
 
 Inferior Oolite. 
 
 Wealden. 
 
 Coralline Oolite, oolitic ore at Westbury. 
 
 Neocomian oolitic ore at Seend in Wilts ; Tealby, &c. 
 
 In Tertiary Rocks. 
 Aluminous hematite of Antrim. 
 
 Iron. Iron ores in Britain are widely distributed in the Carboni- 
 ferous and Secondary strata. In the Carboniferous limestone they 
 chiefly occur in the form of red oxide or hematite, which is extensively 
 worked in the district of Furness and about Cleator Moor in Cumber- 
 land. But in Somerset in the Brendon Hills, and in Devonshire on the 
 eastern borders of Dartmoor, and in Cornwall near Lostwithiel and 
 Wadebridge, there are important mines of red or brown hematite. In 
 the coal measures, the iron ores are regularly interstratified, and form 
 what are known as clay iron ore, which is a carbonate of iron. 
 Important beds are the black band ironstone of Staffordshire, Lan- 
 cashire, and Scotland, which lie near the top of the upper coal mea- 
 sures. Ironstone beds also occur below the thick coal of South 
 Staffordshire, and on the same horizon in Warwick and North Wales. 
 
IRON OF CORNWALL AND DEVON. 427 
 
 The South "Wales iron is chiefly in the Gannister beds. In the 
 Cleveland district of Yorkshire, extending south of the river Tees, 
 the middle lias contains important beds of clay ironstone. 
 
 In the north-west of Lincolnshire, a similar ironstone occurs in 
 the lower part of the lower Lias ; and in Northamptonshire the 
 Northampton sands, corresponding to the lower part of the inferior 
 oolite, have yielded rich deposits of iron ore about Wellingborough, 
 Crauford, and Woodford. 
 
 Iron in Cornwall Most of the iron in Cornwall occurs between 
 Padstow on the north and the neighbourhood of St. Austell on the 
 south. Among the more important of these lodes are the Pawton 
 mines near Padstow. The ore is partly red hematite mixed with 
 limonite and spathose carbonate. The Perran lode has similar ore, 
 but has a direction 35 north of west. 
 
 Many other localities in Cornwall are known to yield good ores of 
 iron, and among the more important are Boscarne, west of Bodmin ; 
 Tregorne, west of Mulberry Hill, and the neighbourhood of Gram- 
 pound, where magnetic ore occurs. 
 
 The iron lodes of Cornwall mostly run to the west of north, and 
 contain hematite or limonite. An important mine occurs at Lost- 
 withiel, and extends for if miles; the ore is chiefly the hydrous 
 oxide termed Goethite. Near to the surface the carbonate of iron is 
 altered into limonite. The ore yields thirty to fifty per cent, of 
 metal. 
 
 Iron in Devonshire. The Hay tor iron mines on the eastern 
 border of Dartmoor are an example of the occurrence of thick beds of 
 magnetite, interstratified with altered shales and sandstones of Car- 
 boniferous age. The rock near the iron ore is always charged with 
 hornblende, and sometimes almost made up of actinolite. 
 
 The beds of iron ore are, according to Dr. Le Neve Foster, ten feet, 
 fourteen feet, and six feet thick in three of the four beds. The 
 strike of the beds is south-east. The iron ore is thought to have 
 been originally in a condition like that of Cleveland ore, and to have 
 been subsequently altered into magnetite. Near the surface the 
 magnetite is converted into ochre by atmospheric decomposition. 
 Actinolite and garnets almost invariably accompany deposits of 
 magnetite. 1 
 
 Magnetite iron ore worked at Haytor, near Islington, is a com- 
 pact black granular substance. Iron ore occurs at Smallacombe in the 
 form of limonite. Other lodes of brown hematite are found at 
 Hennock, Buckfastleigh, and North Moulton. At the Frank Mills 
 lead mine the lead ore is succeeded by carbonate of iron containing a 
 large amount of manganese. 
 
 Red hematite and limonite are found in the Plymouth limestone, 
 near Brixham ; the ore is mostly compact, and includes some kidney 
 ore ; on analysis it yields from forty-four to sixty-three per cent, of 
 metal. 
 
 Somersetshire Iron Ores. In Somersetshire iron ores occur at 
 
 1 Dr. Le Neve Foster on Haytor Iron Mine. Q. J. G. S., vol. xxxi. p. 28. 
 
428 IRON IN THE CARBONIFEROUS LIMESTONE. 
 
 Eisen hill, and in the Brendon hills. The ore is partly carbonate of 
 iron, rich in manganese and partly brown oxide. These ores are found 
 in the Morte slates which belong to the middle of the Devonian rocks. 
 The ore chiefly occurs in veins and pockets. The produce is about 
 thirty-four per cent, of metallic iron, and nine to ten per cent, of man- 
 ganese. Hematite iron ores are also worked at Yatton, Windford, 
 and in the Mendip Hills. 
 
 On Exmoor iron ores are found near Combe Martin, and in 
 the Valley of the Exe, and north-east of Simon's Bath. It is usually 
 a brown oxide. Some of the lodes are from nine to twenty-five feet 
 thick. 
 
 Iron Ores in the North of England. The iron ores of the 
 North of England are chiefly obtained from the Carboniferous lime- 
 stones of "Weardale, Alston Moor, Haydon Bridge, Whitehaven, and 
 Ulverstone. They consist partly of carbonate of iron and partly of 
 hematite. The clay ironstone or argillaceous carbonate is contained 
 in some of the bands of shale in a similar way to the ores of the coal 
 measures. This ore is sometimes known as ballstones. Brown iron 
 ore sometimes forms regular layers in the rocks round Alston Moor, 
 and sometimes the brown iron ore occurs in the lead veins of Alston 
 Moor, instead of fluor spar and quartz. Thus the lode of Eodderup 
 Fell, where it crosses the valley of the Tyne, is a vein of brown iron 
 ore sixteen to twenty feet thick, and similar veins occur in other 
 parts of the same district. The carbonate of iron or sparry ore also 
 appears abundantly in the lead veins, and is termed the rider or vein 
 stone ; and near Weardale, about Stanhope, the veins are so " rid- 
 ered " with this ore in interlacing strings, that the rock has been 
 absolutely carried away. The brown peroxide of iron is frequently 
 mixed with the sparry ore ; its existence is attributed to the decom- 
 position of the carbonate of iron. 
 
 This is the ore which is worked in Carinthia, Styria, and Siegen. 
 The carbonate of iron frequently invests previously-formed crystals of 
 fluor spar and galena. The hematite or sesqui-oxide of iron is chiefly 
 worked at Whitehaven and Ulverstone. 
 
 Cumberland Hematite. The Cumberland hematite deposits 
 occur in granite, Skiddaw slate, the Borrowdale series, Coniston 
 rocks, and Carboniferous strata. In the Eskdale granite, the 
 principal workings are near the Boot and the King of Prussia. The 
 direction of the vein is north and south. The ore occurs in 
 bunches on each side of a leader. This deposit is usually earthy, 
 and sometimes contains decomposed felspar. Hematite has only 
 been worked in the Skiddaw slate near Knockmurton, on one of the 
 hills near Ennerdale Lake. Some of the veins run north-west, 
 others north-east, and are quite as irregular as at Eskdale. 
 
 In the Coniston limestone, hematite is found at Water Blean, 
 near Millour. The ore is free from mixture with the associated 
 rock, and occurs in veins from a few inches to nine feet thick. 
 The most important deposits of the district are in the Carboniferous 
 limestone, and these present several different forms, occurring some- 
 
HEMATITE IN CUMBERLAND AND LANCASHIRE. 429 
 
 times in the manner of beds, and sometimes in the manner of veins, 
 and occasionally occupying depressions on the surface of the limestone, 
 immediately beneath the drift. Examples of the bed-like deposits 
 occur at the top of the Carboniferous limestone, as at Frizington, at 
 Winder Gill, and at Bigrigg. Sometimes the floor is uneven, some- 
 times the roof is uneven, and the floor is often nearly all silica, 
 forming a substance called by the miner's " whirlstone." 
 
 Other limestone bands are similarly more or less replaced by 
 hematite. The vein-like deposits are shown by Mr. Kendall's map 
 to lie in lines of fault. Among these are the Salter Hall mines and 
 the Eskelt mines. Of the superficial dish-like deposits, one of the 
 most important occurs at Hod barrow, and in Furness deposits of this 
 kind are common. Some of the vein-deposits, like those of Salter 
 Hall and the Cleator district, run north and south, while others, as at 
 Bigrigg and Frizington, run east to west. 1 
 
 The percentage of peroxide of iron varies with the percentage of 
 silica, so that the ore is poorer as it is more siliceous. In the upper 
 coal measures hematite is found at Millyeat, near Frizington. The 
 formation of the ore is usually dated as prior to the Permian period, 
 because hematite fragments occur in the lower Permian breccia. 
 
 Mr. E. W. Binney, F.R.S., described hematite as interstratified 
 in the lower coal, and there is no doubt that occasionally coal 
 plants are mineralised with hematite. 
 
 In Furness the chief ores are the hard compact purple blue ore, 
 lined with kidney-like concretions and spar, similar to those which 
 occur in the Whitehaven district. This ore specially occurs at 
 Lindal Moor, where it is worked for a thousand yards in a direction 
 running north-west to south-east, in a vein which varies from a few 
 inches to over ninety yards in width. A similar deposit is worked at 
 the Stank mines, and the Yarlside mines. One of the most charac- 
 teristic features of the hematite deposits is the way in which they 
 are interstratified with the surrounding limestone and shale ; but 
 this condition is always due to replacement of the limestone in the 
 planes of bedding by iron ore. It is impossible to speak with any 
 certainty concerning the source of this or other ores ; but although 
 the hypothesis of emanations of vapour and solutions of metallic 
 matter from the deep-seated regions of the earth offer a simple ex- 
 planation, it is probable that we may with more confidence attribute 
 the iron deposits to the insoluble residue left, when limestones are 
 denuded by solution, and attribute their occurrence in veins to in- 
 filtration from above, slowly and during long periods of time. The 
 chemical processes involved in making hematite out of carbonate 
 of iron, are certainly consistent with the ordinary processes of 
 denundation. 2 
 
 Veins of ironstone in the Carboniferous limestone of South 
 Wales extend from near Pontypool to Lydstep Point in Pernbroke- 
 
 1 J. D. Kendall, " Hematite in the Silurians," Q. J. G. S., vol. xxxii. p. 180. 
 
 2 J. D. Kendall, " Hematite Deposits of Cumberland and of Furness," Proc. 
 North of England Institute of Mining Engineers, vol. xxxi., 1882. 
 
430 CLAY IRONSTONE OF THE COAL MEASURES. 
 
 shire. They have long been worked in the Taff Valley, where the 
 iron ore occurs in nearly vertical fissures. 
 
 Forest of Dean Iron Ores. The iron ore of the Forest of 
 Dean occurs in the Carboniferous limestone, and locally in the Mill- 
 stone grit. It is brown hematite or limonite, and occurs in irregular 
 cavities in the rock termed Churns. It is sometimes worked 750 feet 
 below the surface. The iron ore deposits are about 100 yards 
 below the coal measures. The ore varies in appearance from a close- 
 grained black to a spongy black ore, or an earthy ore which on 
 exposure becomes red. The richer ore yields 63 per cent, of 
 metallic iron, the poorer ore about 23 per cent. Some other deposits 
 occur in the Pennant sandstone at Frampton Cotterell, near Bristol, 
 and many other localities in Gloucestershire. 
 
 Iron Ores in the Coal Measures. In the coal measures the 
 iron ore is the argillaceous carbonate of iron which occurs in grey, 
 brownish, or black lumps and nodules, and is found in all the coal- 
 bearing districts, though sometimes the quantity is too small for 
 profitable commercial extraction. 
 
 In the Northumberland and Durham district, on the western side 
 of the coal-field at Consett and Tow Law, the ironstone is in close 
 proximity to the coal. The lowest seam is four to six feet thick. 
 
 In the Yorkshire Coal Field the clay ironstone is largely worked 
 in the Bradford district, where some of the best iron in Britain is 
 made. The South Yorkshire ironworks extend from Leeds by 
 Barnsley to Sheffield and Kotherham. The best beds of ironstone 
 intervene between the Barnsley Thick Coal and the Silkstone, ranging 
 through about a thousand feet of the coal measures. The line is 
 traced southward in Derbyshire, from the valley of the Don to the 
 extreme south of the coal measures, but the bands of nodules are 
 more irregular than in Yorkshire. 
 
 In Derbyshire the ironstones occur on the same horizon as in 
 Yorkshire, between the Barnsley coal which is known as the Top 
 Hard, and the Silkstone, called the Black Shale. The bands which 
 contain the ironstone nodules are known as rakes, the more important 
 of which are the cement rake of Alfreton, the pinder park rake of 
 Staveley, the brown rake of Butterley, the black rake, the dogtooth 
 rake at Staveley, the nodule rake, and the black shale rake, and the 
 striped rake. These beds have a total thickness of 1600 feet. The 
 coal and ironstone together are about 400 feet thick. The dogtooth 
 rake at Chesterfield is an important mine, in which some of the beds 
 are made up of the shells of the bivalve genus Anthracosia. In the 
 Lancashire coal-field clay ironstone occurs, but the beds are thin and of 
 little value. 1 
 
 A little ironstone is raised in Nottinghamshire, and in Warwick- 
 shire. Excellent clay ironstone is found in the Bedworth district near 
 Coventry. 
 
 In Shropshire there are many beds of ironstone, distinguished by 
 local names, such as Chance Pennystone, Blackstone, Yellowstone, 
 1 " Iron Ores of Great Britain," Part i., W. W. Smythe, 1856. 
 
THE IRON OF THE BLACK COUNTRY. 431 
 
 Blue Flats, White Flats, Pennystone and Black Flats. The ore yields 
 about 35 to 38 per cent, of metallic iron, the nodules are usually 
 brownish grey, often have a septarian structure filled with crystals of 
 sulphate of baryta. Occasionally the iron contains minute traces of 
 other metals. It is chiefly worked at Lillieshall, Madeley Court, and 
 Madeley Wood, Ketley, Wombridge, and Packmoor. 
 
 In North Staffordshire the chief ironstone bands are the Eed 
 Shag, which has a reddish-brown fracture, and occurs at Hanley and 
 Newcastle, the Gatter Mine at Hanley, the Eed Mine, Cannel Mine, 
 Pennystone, and Deep Mine. The percentage of metallic iron varies 
 from about 32 to 40 in the different beds. 
 
 In South Staffordshire Dudley is the centre of the iron manu- 
 facture, which covers an area of about 50 square miles. The district, 
 commonly known as the Black Country, extends from Wolverharnpton 
 by Bloxwick to Walsall, from Walsall to Halesowen, to Stourbridge, 
 and from Stourbridge to Wolverharnpton. The district includes seven 
 important towns and sixteen large villages. The quantity of ironstone 
 is probably greater than is to be found in any similar area within the 
 same vertical space. Professor Jukes enumerates fifteen principal 
 bands, which vary from two or three feet to thirty or forty feet in 
 thickness, though the ironstones usually occur in dark clay called 
 clunch or clod, which forms much of the thickness of the bed. Thus 
 the Gubbin and Balls ironstone is seven feet thick, but only contains 
 two feet of ironstone divided by three beds of clod. The Balls have 
 a septarian structure, the septa being lined with iron pyrites, and 
 sometimes with galena and blende. 1 
 
 North Wales. Argillaceous iron ore occurs in the Euabon coal-field 
 at Trefechan in about six or seven bands. In Flintshire the iron is 
 found in the Carboniferous limestone near the Talargoch lead-mine, and 
 contains a little cobalt and nickel. 
 
 The South Wales iron ores were formerly mined on the outcrop 
 by means of races or streams of water directed on the ore, and then 
 the ironstone was converted into oxide of iron and smelted. The 
 lower coal measures contain the bulk of the beds of iron ore and coal. 
 They extend over the whole coal-field ; the yield of iron is richest in 
 the east, and in the west the beds are thicker and poorer. The beds 
 are extremely numerous, but rarely of any great thickness. The bed 
 known as the Three-quarter Balls in the eastern part of the basin is 
 about three feet thick. The iron beds are well worked in the Ebbw 
 valley. 
 
 Irish Carboniferous Iron Ore. The Leinster coal-field in the 
 south yields carbonate of iron, but most of the ironstone, which occurs 
 in nodules and concretionary masses, is found in the northern district of 
 Leitrim, Tyrone, and Antrim. 
 
 In County Wicklow Mr. Tichborne has described a vein of mag- 
 netic oxide near Kilbride, which is two to three miles long, and some- 
 times six feet wide. Limonite occurs in the mineral veins at Cronebane. 
 
 1 Jukes, " Iron Ores of Great Britain," Part ii., Mem. Geol. Surv., 1858. 
 
432 IRON IN THE LIAS. 
 
 Oxides of iron more or less siliceous are found in the Cambrian rocks 
 of Counties Longford and Cavan, but chiefly in the district between 
 Granard and Carrick-on-Shannon. 
 
 Scotch Iron Ores. In Scotland most of the iron ores occur in the 
 coal measures of Ayrshire, Lanark, and the country between the Firths 
 of Forth and Clyde. The seams of ironstone vary in thickness from 
 a few inches to two feet, and are mostly blackband clay ironstone, 
 which yields 30 to 40 per cent, of metal, and is similar to the ores of 
 North Staffordshire. Iron oxide ores are worked in small quantity 
 in Shetland, Aberdeen, at Auchencairn in Kirkcudbrightshire, at 
 Garleton in Haddingtonshire, and Whytock and Muirkirk in Ayr- 
 shire. The hematite at Auchencairn is similar to the kidney ore of 
 Whitehaven. 
 
 Permian Iron. 
 
 In the Permian beds iron ore occurs in hollows or basins, as at 
 Mwyndy, near Llantrissant. Sir Henry De la Beche recorded the 
 occurrence of hematite un conformably covering the coal measures near 
 Llanharran, in Glamorganshire. 
 
 Liassic Iron Ores. 
 
 Cleveland Ironstone. The Cleveland ironstone is well seen in 
 the Yorkshire coast between Old Peak, south of Whitby, and Middles- 
 borough-on-Tees. It extends in a basin with several minor undula- 
 tions, descending below the sea-level north of Robin Hood's Bay, and 
 reappearing at Kettleness, and between that place and Staithes 
 another synclinal fold sinks the ironstone below the sea-level. The 
 best section is to be seen at Boulby. The iron ore continues to rise 
 in the cliff, till at Huntcliffe it retires from the coast. It is then 
 followed inland by way of Arncliffe, extending through the whole of the 
 Cleveland Hills, and turning south-east, it disappears under the oolites. 
 Usually the several seams of ironstone are parted by shale. They are 
 frequently oolitic, and contain Pecten cequivalvis, Avicula cynipes, 
 and other Lias fossils. 1 They thicken towards the north and ulti- 
 mately unite, so that in the Eston Hills the ironstone has an aggre- 
 gate thickness of 18 feet. The thick beds, however, are not always 
 the most valuable, for though the ore at Grosmont has a thickness 
 of nearly 14 feet, only 6 feet 6 inches of it are worth extracting, the 
 remainder being interstratified with seams of shale. The ore yields 
 30 per cent, of iron, and has much the aspect of a sandstone. Mr. 
 Bewick believes that the ironstone covers an area of 420 square miles. 
 He estimates the average yield of every acre at 20,000 tons, and the 
 total supply of the district at 4,820,659,200 tons, an amount which 
 would supply 200 blast furnaces for 680 years, and enable each to turn 
 out 200 tons of pig iron in a week. 2 
 
 1 Mem. Geol. Surv., " Iron Ores of Great Britain," Part i., W. W. Smythe, 1856. 
 
 2 Jos. Bewick, " Geol. Treatise on the District of Cleveland," 1861. 
 
IRON FROM THE OOLITES. 433 
 
 In Lincolnshire iron ore is worked in the Lower Lias, on a lower 
 horizon than the Cleveland ironstone. The mining district lies be- 
 tween the Trent, Humber, and Ancholrne. The ore is sometimes 27 
 feet thick. It is spread about the village of Scunthorpe. The yield 
 is 27 or 28 per cent, of metal. Another band known as the Pecten 
 ironstone is about 4 feet thick, and occurs in the Middle Lias. 
 
 In North Lincolnshire, Frodingham is the centre of the iron 
 district. The ironstone is covered with drift sand. 
 
 In Oxfordshire the Marlstone has been worked for iron ore at 
 Fawler, Aynho, and near Banbury ami Woodstock. The ore is especi- 
 ally developed in the valley of the Cher well. It varies from 10 to 15 
 feet in thickness, and abounds in the Khynclionella tetrahedra. It is 
 often dark olive green, and yields about 32 per cent of metal. 
 
 Above the Upper Lias is the magnetic ironstone known as the 
 Dogger bed. In the Kosedale Abbey mines the band is 20 feet 
 thick, and probably corresponds to the Northampton sand. This ore 
 contains about 65 per cent, of oxide of iron, equal to 45 per cent, of 
 metal * 
 
 Iron Ores from the Oolites. 
 
 The Iron Ore of the Northampton Sand. The Eomans worked 
 the Northampton sand of the Inferior Oolite near Oundle, and the 
 Norman castle of Rockingham is said to have been built for the pro- 
 tection of the iron furnaces of Kockingham Forest. The works, long 
 discontinued, were resumed about 1860. The ore at the surface is 
 hydrated peroxide of iron, but when quarried at some depth, each 
 block has a nucleus of compact impure carbonate of iron, blue or grey, 
 while deeper still the entire mass is carbonate of iron. This mineral 
 replaces the carbonate of lime of fossils, though the greater part of 
 the ore is free from fossils. The ore occurs in lenticular patches 
 at Holt, Desbro, and near Stamford. Sometimes it forms the whole 
 thickness of the formation, sometimes it is entirely absent. It con- 
 tains grains of quartz, which in some beds are invested with hydrated 
 peroxide of iron. Certain beds have the whole mass made up of 
 oolitic grains -J^ to yj^ inch in diameter. On analysis the ores give 
 about 30 to 75 per cent, of sesquioxide of iron, or 60 to 80 per cent, 
 of carbonate. 2 
 
 Beds of ironstone from 4 to 18 feet thick or more occur in North- 
 amptonshire, while at Cogenhoe and Woodford the thickness is 20 feet. 
 The stone at Cogenhoe yields 42 per cent, of metallic iron. At Gay- 
 ton near Blisworth the ironstone is 10 to 14 feet ; at Easton, Neston, 
 near Towcester, from 6 inches to 3 feet ; near Northampton, 1 2 to 
 14 feet ; at Daston, near Northampton, 16 feet. It is largely worked at 
 "Wellingborough, where it is upwards of 1 2 feet thick ; and here the 
 ore yields 53 per cent, of metallic iron. 
 
 1 Meade, " Coal and Iron Industries," p. 368. 
 
 2 Judd, Mem. Geol. Surv., " Geology of Rutland." 
 VOL. I. 2 E 
 
434 WEALDEN IRON. 
 
 In former years ironstone was worked near Peterborough in the 
 Blisworth clay. 
 
 Iron Ore in the Coralline Oolite. At Westbury in Wiltshire an 
 oolite iron ore occurs in the Upper Calcareous grit. It is about 1 5 
 feet thick, and extends by Warminster to the north of Devizes and 
 through Steeple Ashton into Oxfordshire. The Westbury ore is known 
 locally as Thrown and green. The former yields about 42 per cent, of 
 iron, the latter 38 per cent. 
 
 At Abbotsbury oolitic iron ore is found in the upper part of the 
 coral rag, but is not at present worked. 
 
 Wealden Iron Ore, 
 
 The iron ore of the Weald has played an important part in 
 English history, having been a chief source of iron from the time 
 of Henry III. down to the first quarter of this century, when the 
 works ceased to be remunerative owing to the failure of fuel. The 
 common ore is clay ironstone, which occurs in nodules and thin 
 beds towards the bottom of the Wadhurst clay. Another bed of 
 shelly calcareous ironstone, serving both for ore and flux, occurs a few 
 feet above the Ashdown sand. The ore is termed "myne." It was 
 worked as late as 1857-58 in the Ashdown Sand near Wadhurst, and 
 the ore was then sent to Staffordshire to be smelted. It usually 
 yields about 35 per cent, of iron, varying between 25 to 40 per cent. 
 The Ashburnham iron was remarkable for its toughness ; cannon 
 were cast from it ; it was forged into weapons of war and most of 
 the older ornamental ironwork of London exhibits evidence of its 
 malleable quality. Iron ore is still worked in the Lower Wealden beds 
 of the Lower Boulonnais; the ore is chiefly raised at Ferques and 
 near Boulogne ; it is there a hydrated peroxide, which yields 34 per 
 cent, of iron. 1 
 
 Neocomian Iron Ore. A brown oolitic iron ore occurs in the 
 Middle Neocomian at Tealby near Market-Kasen. 
 
 Occasionally ironstone has been extracted from the Upper Neo- 
 comian beds near Leighton-Buzzard in Bedfordshire, where it occurs 
 in massive brown nodules. 
 
 At Seend in Wiltshire iron ore occurs in the Lower Greensand. 
 It is quarried in open workings, similar to those at Westbury. It 
 yields 45 per cent, of metallic iron, and is a hydrous brown oxide. 
 
 In many localities the Neocomian sands are rich in brown iron 
 ore, but deficiency of fuel has rendered them of no commercial value. 
 
 Tertiary Iron Ores. 
 
 The Bagshot Sands at Hengistbury Head near Christchurch con- 
 tain several bands of nodular ironstone. The ironstone masses are 
 several feet in length, and have accumulated in great quantity on the 
 
 1 Topley, Mem. Geol. Surv., "Geology of the Weald," 1875. 
 
BASALTIC IRON ORE OF ANTRIM. 435 
 
 beach. This ore was formerly collected and smelted, but of late 
 years the works have been suspended. 
 
 On the north-west coast of the Isle of Wight near Yarmouth a 
 clay ironstone was formerly collected to the amount of several hundred 
 tons a year. 
 
 Iron Ore of Antrim. In County Antrim interstratified with the 
 basalt are a number of ferruginous bands, which occur above each other 
 like seams of coal. They are well seen in the cliffs near Down Hill, and 
 usually consist of a ferruginous clay called " bole," with an underlying 
 layer of lithomarge. The most important bed, which has been worked 
 commercially, and is used in the Cumberland iron furnaces, is seen at 
 Port-Moon, immediately under the lower tier of basaltic columns. It 
 consists of pisolitic iron ore of brown or reddish colour ; it contains 
 a good deal of fossil wood replaced by limonite. The thickness of the 
 bed is about two feet. Beneath the pisolite is six feet of bole, a 
 yellowish red ochre containing nodules of basalt, and this rests upon 
 twenty-five feet of lithomarge. The pisolitic ore yields from 40 to 60 
 per cent, of metallic iron. The bole contains about half this amount 
 of iron, while in the lithomarge the quantity of iron is too small to 
 render it of any commercial value. There can be no doubt that the 
 iron of this district has been derived from the decomposition of the 
 basalt, and in this connection it is interesting to bear in mind the 
 large percentage of iron that sometimes occurs in basaltic rocks, such 
 as those of the Sandwich Islands. 
 
 The tertiary iron ore of Antrim extends along the coast from 
 Cushendall to Carrickfergus, and for a distance inland, which varies 
 from two to ten miles. 
 
 Important mines are situate at Glenarm, Carinlough, Cargan, 
 Xewton Crommellin, Glenariff, and Irish Hill near Ballyclare. 1 
 
 European Iron Ores. 
 
 Spain. The larger part of the foreign ores of iron imported into 
 this country is derived from Spain. It is found in the Carboniferous 
 limestone near Bilbao, ranging as far as 19 miles north-west of that 
 town. The principal ore is brown hematite with some spathic iron. 
 It is quarried at the surface, and is worked chiefly in the Somorrostro 
 district. Other iron mines occur near Carthagena, Santander, Granada, 
 and in Catalonia. 
 
 Portugal. In Portugal lodes of magnetic oxide of iron or magne- 
 tite occur in the Serra dos Monges in Alentejo. Other ores occur at 
 Moncorvo and at Quadmaril in Tras os Montes. 
 
 France. The iron ores of France are chiefly contained in the 
 lower secondary rocks. The most important deposit extends from 
 Luxembourg through Lorraine to beyond Nancy. It occurs in the 
 Upper Lias. Other beds of the same age are found near Privas and 
 La Voulte in Ardeche. 
 
 1 Iron Ores of Antrim, J. D. Kendall, Proc., " North of England Institute of 
 Mining Engineers," voL xxx., 1880. 
 
436 IRON ORES OF EUROPE. 
 
 Italy. The spathous iron and limonite of Lombardy are well 
 known from the Lakes Como and Iseo, and the Bergamask moun- 
 tains, occurring in the former localities in gneiss, and at the latter in 
 Trias. Elba is rich in red hematite and specular iron. 
 
 Greece. Iron ores of Greece are chiefly obtained from the island 
 of Seriphos. They are partly magnetic ore and partly brown hema- 
 tite. 
 
 Norway. The iron ore of Norway is chiefly magnetic. It occurs 
 in mica schist and hornblende schist, extending along the coast in the 
 neighbourhood of Arendal and Kragero. 
 
 Sweden. The ores of Sweden are partly contained in gneiss. 
 The most important are in the neighbourhood of Dannemora and 
 Taberg, where the ore contains titanium and vanadium. 
 
 Iron ores are widely distributed over Western Russia, especially 
 in Finland. 
 
( 437 ) 
 
 PALAEONTOLOGY. 
 
 CHAPTER XXIV. 
 ELEMENTARY IDEAS IN PALEONTOLOGY. 
 
 PALEONTOLOGY is the history of the succession of life on the earth. 
 It begins with a remote past, when the great groups of organisms were 
 already characterised, and many surviving genera were in existence. 
 It passes over long ages of past time, distinguished by ordinal groups 
 of animals long since extinct. It follows the steps of geological his- 
 tory, and demonstrates how faunas and floras have been preserved by 
 becoming adapted to the circumstances which succeeding ages produced. 
 And it teaches how the existing distribution of life has been evolved 
 during the past history of the earth. We thus learn that species are 
 mutable ; that their distribution is limited in space and time ; that 
 they are thrown off from a genus, like leaves from a tree, and that the 
 genus survives by developing characters which repeatedly obliterate 
 its specific identity. The alterations which time elaborates fre- 
 quently affect the more vital structures, so that the genera in the 
 older rocks are replaced by allied genera which vary, partly by 
 increased growth of some parts with decreased growth in others. 
 Every type of life passes through a series of changes comparable to 
 those in the life of an individual, and species, genera, families, and 
 orders all grow old, lose their vitality, and disappear from the life that 
 survives. Other species and groups under new circumstances put on 
 an amazing vitality, and increase at the expense of surrounding life, 
 and become dominant in regions of the earth and periods of time. 
 These are the abundant demoid types which are termed characteristic 
 fossils, for their abundance is such that strata are easily recognised by 
 them. Every formation has its demoid types, which in the Primary 
 rocks are generally brachiopods, in secondary clays are oysters, and 
 in the limestones are frequently ammonites, or terebratuke. The 
 dwindling or asthenoid species, by failing with different successive 
 periods of time, have a value of another kind in associating rocks 
 together in groups. These two types of life are the most interesting 
 
438 PHYSIOLOGICAL ORIGIN OF SPECIES. 
 
 in every stratum, for they give a facies to the fauna though the species 
 may be numerically few. 
 
 Origin of Species. The philosophy of palaeontology was long re- 
 tarded by the views held concerning the nature of species, and although 
 it was considered that any number of varieties might be thrown off, 
 all the varieties were imagined to be infertile, or to revert in a state of 
 nature to the original type ; and it was not until the publication by Mr. 
 Charles Darwin, in 1859, of his work on " The Origin of Species," that 
 naturalists began to conceive of the possibility of species coming into 
 existence under laws which w^ere closely associated with their gradual 
 extinction. The two great principles which Mr. Darwin recognised 
 as aiding the development of life were " the struggle for existence," 
 and " the survival of the fittest," meaning by the struggle for exist- 
 ence the obvious fact that more individuals of all races are produced 
 than can survive ; and that those selected by Nature to survive are 
 the individuals or races which have physical or mental advantages 
 over the others ; and that, by continuous variation, a divergence might 
 in this way be produced which would result in the elaboration of new 
 species. The origin of species, however, is not a simple process, and 
 the cause of the structural variation is here left unexplained, or is 
 assumed to be a matter of accident. This logical defect in Mr. Dar- 
 win's original argument retarded for a time the acceptation of his views, 
 and led to a belief that the causes of evolution might admit of some 
 elaboration. We had arrived, in I862, 1 at the conclusion that, how- 
 ever important an agent in change the struggle for existence may be, 
 the fundamental active principle in evolution is physiological causation, 
 partly dependent upon external influences which govern the condi- 
 tions of food supply, but more dependent upon circumstances which 
 govern digestion, nutrition, and elimination of waste products and 
 unused structures ; and we still believe that too much attention cannot 
 be given to the physiological aspects of evolution. 
 
 Thus, where animals are fixed on the sea-bed, and derive their 
 food supplies from currents, or where animals are dependent on the 
 vibratile motions of cilise, it is difficult always to realise the condi- 
 tions of a struggle for existence, or the nature of endowments which 
 would characterise a favoured race. But from the physiological point 
 of view the problem is simpler, for all variation is ultimately a matter 
 of organic dialysis, in which the action of the dialysing membrane of 
 a tissue or organ varies with its own nutrition and physical conditions, 
 and hence elaborates or eliminates more or less of a tissue, or modifies 
 the tissue so as to vary the sum of the animal structures under 
 the conditions of geological change. The principle which would 
 account for the origin of species must be equally applicable to all 
 types of life ; and because there is nothing in common between all 
 types of life except the modes of nutrition of the tissues, the func- 
 
 1 Camb. Phil. Soc., March 17, 1862, "Researches on the Homologies of the 
 Bivalve Mollusca, and therein of the Law of the Variation of Forms and the 
 Nature of Genera." This memoir was only printed in abstract in the Cambridge 
 Chronicle. 
 
GENESIS OF GENERA. 439 
 
 tions of the several organs, their dependence on each other, and the 
 functions of the sum of the organs, we are compelled to recognise 
 variation in the work performed by the whole of the organs, as the 
 originating cause for that structural variation by which species are 
 distinguished from each other. As an organ ceases to perform work, 
 owing to the work being 110 longer necessary, or performed by some 
 other part of the body, so there comes to be not merely change in the 
 organs concerned, but in all other organs which are related to them, 
 and often in the characters and habits of the organism as a whole. 
 Just as in the individual, a certain degree of growth takes place with 
 increased work, and a corresponding atrophy follows diminished work, 
 so in wild active races bones are found to develop strong ridges and 
 crests, and muscles acquire long ligaments, which are unknown in 
 domesticated tribes ; 1 and therefore nothing is needed to produce an 
 infinity of variation in a type but ever-progressing atrophy and hyper- 
 trophy of its constituent structures, consequent on changed nutrition 
 of the structures, which has resulted from the work they have had to 
 perform. Side by side with variation in internal vital organisation, 
 variation of another kind has gone on simultaneously, which has been 
 due to the manner in which the changing conditions of existence have 
 modified the purely muscular and skeletal elements of an animal. 
 The directions of simultaneous variation may thus be numerous. If 
 the variation progresses too rapidly for the organism to become adapted 
 to the change, the modification is designated disease ; but such varia- 
 tion, though leading to extinction of races, probably has little to do 
 directly with the origin of species. The direction of physiological 
 variation is always towards increased complexity of structure, but the 
 direction of variation under external influences is often towards in- 
 creased simplicity of structure. 2 
 
 Genus and Species. Whatever other classification may be 
 adopted, every fossil, like every living plant and animal, must be 
 referred to its genus and species, and we need to have clear ideas of 
 the nature of the facts indicated by these terms. A genus is an 
 organic variation from a group of structures such as is comprised 
 under the natural history term Ordinal group, and is thrown off with 
 distinctive characters which perpetuate the Order, in a direction which 
 its structure and the animal's habits determine to be the direction of 
 chief variation. Thus, among the bivalve shells termed Lamelli- 
 branchiata, the key to many generic modifications is found in the 
 mode in which the development and arrangement of the digestive 
 organs govern the position of the adductor muscles. In those types 
 with a single central muscle, like the oyster, scallop, lima, the digestive 
 canal is comparatively short and simple in its folds, and the axis of 
 growth of the shell is at right angles to the axis of the digestive 
 canal. But as the canal becomes more complicated, its change of posi- 
 tion results in changed position of the adductor, and the development 
 of a second adductor muscle, when the digestive tube becomes forced 
 
 1 Darwin : " Variation of Plants and Animals under Domestication." 
 
 2 " Origin of the Vertebrate Skeleton," Annals and Mag. Nat. Hist., November 
 1866, and April and July 1872. 
 
440 
 
 PECTEN, LIMA, AND AVICULA. 
 
 into an organ which is termed the foot. The shape of the shell follows 
 that of the animal, and becomes circular, oval, and more and more 
 elongated transversely as the foot and digestive and respiratory organs 
 become more modified in form. We may perhaps illustrate the nature 
 of these modifications by examples. The common scallop is referred to 
 the genus Pecten. The characters which are common to all species 
 of Pecten are the definition of the genus, which the palaeontologist 
 needs to remember ; they are three. First, a straight hinge-line ; 
 secondly, two lateral processes of shell called ears, at the sides of the 
 central ligament of the hinge ; and third, the axis of greatest growth 
 at right angles to the central line of the hinge. No matter what 
 other characters the shell might have, if it had 
 these it would be a Pecten. The other characters 
 would serve to distinguish species. They would 
 be first, the relative length and breadth of the 
 shell ; second, the degree of inflation of its 
 valves ; third, the form and development of 
 the ears; fourth, size; fifth, the external con- 
 dition of the surface of the shell, which might 
 be smooth, marked with many kinds of radiat- 
 ing ornament, or concentric ornament, or can- 
 cellate ornament, &c. ; so that it is scarcely 
 conceivable that any external specific character should approximate 
 a pecten towards another genus. Yet a modification of one of the 
 generic characters would permit of that character being associated 
 with the whole series of secondary modifications of growth which we 
 agree to designate specific. Thus, if while the hinge-line remains 
 straight, as in Pecten, it is shortened so that the ears become smaller, 
 and the axis of growth is inclined anteriorly, so that the anterior 
 margin of the shell is straight or nearly so, and the posterior margin 
 convexly rounded, a generic type results, which is 
 indicated by the name Lima. Similarly, if the 
 hinge-line is elongated by extending the ears, and 
 the axis of growth is inclined posteriorly, the 
 genus Avicula is defined, and it too may undergo 
 variation, which is practically infinite, by modifica- 
 tion of ornament and form. From Avicula many 
 other genera may be derived ; thus, by still further elongation of the 
 hinge-line and increased obliquity of the axis of growth of the shell, the 
 ears become entirely obliterated, forming the genus Pinna. There are 
 some shells which, instead of growing only at the occludent margin, 
 also grow at the hinge margin ; and if we conceive of a form of Avi- 
 cula in which the ears are small, and in which the shell is marked by 
 fine radiating ribbing which reproduces itself in the hinge, as may be 
 seen in thin shells like some species of Modiola or Crenella, then the 
 step may easily be conceived by which a transition is made from Avi- 
 cula to the genus Area, characterised by its straight denticulated hinge- 
 line and transverse and inflated mode of growth. Area, through some 
 of its fossil representatives, like Macrodon of the Great Oolite, repro- 
 duces the hinge-teeth of the genus Avicula, and Area, by losing its ear- 
 
 Fig. 81. Avicula. 
 
GENERA WHICH LIVE IN FRESH WATER. 
 
 441 
 
 like appendages, acquires a circular outline and a crescent-shaped den- 
 ticulate hinge-line, and becomes the genus Pectunculus. Isoarca is 
 another modification in which the shell grows most rapidly at the 
 anterior margin, so that the beaks recede from each other and become 
 spiral, and the shell is globular. Pecten, by thickening the lower 
 valve at the hinge, gives rise to the genus Spondylus ; and Spondylus, 
 by losing the thickening and the ear-like appendages of the hinge, but 
 retaining the interlocking denticles of the hinge, becomes the genus 
 Plicatula. We might in this way show how all the genera of shells 
 are related to each other, and may well have been derived from a few 
 recognisable types by growth in some directions and atrophy in 
 others. But our object is rather to exhibit the nature of generic 
 characters, so that the eye of the student may be educated to discern 
 the characters which follow a plan, and thus discriminate genera, 1 
 recognise their affinities, and identify species. 
 
 Fresh- water Deposits. The great divisions of strata into fresh- 
 water and marine deposits are recognised by fossils. Some fresh- water 
 strata certainly were deposited during the Primary period, especially 
 among the Coal-Measures ; but with the exception of Anodonta Jukesii, 
 found in the Upper Old Red Sandstone of Kiltorkan, in Kilkenny, 
 they yield no fresh- water fossil referable to an existing genus. But in 
 the Secondary and Tertiary periods, not only are all the fossils which 
 occur in British fresh-water beds species of existing genera, but the 
 genera are all of the same types as still live in fresh waters in the 
 British area. Among GASTEROPODA the characteristic genera are 
 
 Fig. 82. Plunorbis. 
 J'lanorbis. 
 VaLvata. 
 
 Fig. 83 Paludina. 
 Paludinn. 
 
 Limnaa. 
 
 Physa. Valvata. Neritina. 
 
 The characteristic LAMELLIBRANCHIATA are 
 
 A ncylus. 
 
 Unio. 
 
 Fig. 84. Cyrena. 
 Anodonta. 
 Cydas. 
 
 " Identification of Fossil Bivalve Shells," Geologist, Feb. 1864. 
 2 Several marine types have fresh-water representatives. Thus the marine 
 Mytilus is represented by Dreissena, the marine A rat by fresh-water species in 
 the Ganges and South-east Asia. 
 
442 
 
 MOLLUSC A OF SHALLOW SEAS. 
 
 These genera are found in the Secondary Rocks in the Purbeck 
 and Wealden Beds ; and in the Lower Tertiary lacustrine beds of the 
 Me of Wight, known as Headon and Bembridge Beds; and in the 
 valley gravels. 
 
 Fresh- water deposits often contain pulmonate land gasteropoda, 
 such as Helix, Bulimus, Pupa, Clausilia, Cyclostoma ; together with 
 land mammals, birds, reptiles, and laud plants. 
 
 The absence of fossil species of the following groups 
 is to be expected in fresh-water strata : Foraminifera, 
 Sponges, Corals, Echinoderms, Cephalopod Mollusca, 
 Brachiopoda, and Bryozoa. All these groups are typi- 
 Fig. 85. Helix, cally marine ; though foraminifera live in the Thames 
 at Gravesend, and Spongilla is characteristic of fresh- 
 water ponds. 
 
 Estuarine and Brackish- water Deposits. There are some genera 
 which are typical of the mouths of rivers, and such forms may be 
 termed estuarine ; but whether they have always existed under such 
 conditions, may well be doubted. Among the more re- 
 markable of such genera may be mentioned Potamides, 
 Cerithium, Melania, Melanopsis, Auricula, Nerita, Mya, 
 Scrobicularia, Cyrena ; and a predominance of such types 
 in a stratum would justify us in affirming its estuarine 
 origin. Examples of strata showing such conditions may 
 be seen in the Woolwich and Reading beds between Upnor 
 and Dulwich, and certain beds of the fresh- water tertiary 
 series of the Isle of Wight. But towards the mouths of 
 rivers the marine shells become modified in form and 
 regularity of growth and ornament, by the influence of 
 fresh water, so that varieties are developed which, in 
 their divergence, at first suggest to the collector a multitude- 
 Such variation is well known in the Caspian, and Sea of 
 Aral, and Black Sea, at the mouths of rivers, especially among species 
 of Cardium. It is strikingly shown in the estuarine part of the Red 
 Crag, known as the Norwich Crag, in which the Fusus antiquus and 
 Purpura lapillis are singularly variable. 
 
 Shallow-water Deposits. The geologist will always judge from 
 physical evidence of the conditions of deposition of a stratum. Con- 
 glomerates and grits must be shallow-water deposits. Sandstones are 
 likely to be accumulated in shallow water, and no evidence can over- 
 ride the presence in them of false bedding and ripple-mark. But 
 mineral character alone is not always proof of nearness to shore of a 
 stratum, for that may change with the nature of the coast wasted. 
 At the present day no one would hesitate to affirm low-water condi- 
 tions for a collection of living British shells which included 
 
 of species. 
 
 Purpura. 
 
 Patella. 
 
 Cardium. 
 
 Nassa. 
 
 Haliotis. 
 
 Tapes. 
 
 Trochus. 
 
 Pecten. 
 
 Solen. 
 
 Littorina. 
 
 Mytilus. 
 
 Pholas, 
 
DEEP-SEA LIFE. 443 
 
 We might enlarge the list by taking from other waters species of 
 
 Conus. Planaxis. Emarginula. Modiola. 
 
 Pleurotoma. ColumbeUa. Chiton. Area. 
 
 Mitra. Pisania. Butta. Chama. 
 
 Murex. Pinna. Mactra, &c. 
 
 which indicate in most cases low-water conditions. If, then, these 
 genera occur in association in an ancient deposit, we are required to 
 believe, if we affirm shallow-water conditions for the stratum, that 
 the changes in level of the ancient sea-bed went on so slowly that the 
 life gradually accommodated itself to change, finding the conditions 
 of existence easier by following the coast-line than by remaining fixed 
 on the sea-bed. This migration of life is implied in the recognition 
 of an ancient sea margin or shallow ocean by characteristic fossils. 
 
 Marine Deposits of the Open Ocean. The deep waters of the 
 ocean, formerly thought to be barren, were proved by the researches of 
 Dr. Carpenter, Dr. Gwyn Jeffreys, Sir Wyville Thompson, 1 and Pro- 
 fessor Sars, to be rich in life, though Dr. Wallich, in his account of 
 the voyage of the Bulldog, was the first to make known the occur- 
 rence of deep-water foraminifera, shells, and echinoderms in the 
 northern seas. Dr. Carpenter observes that the abundance and variety 
 of animal life on a bottom on which the temperature is at least 2 F. 
 below the freezing point of fresh water, is a fact which has all the 
 interest of surprise ; and it is scarcely less remarkable that the forms 
 of molluscs, echinoderms, and sponges, which seem to be characteristic 
 inhabitants of this cold area, should attain a very considerable size. 
 The deep-sea exploration has yielded many genera previously supposed 
 to be extinct, and many types allied to extinct genera of the secondary 
 strata. Thus a crinoid from a depth of 2435 fathoms belongs to the 
 extinct apiocrinite type, and is associated with an urchin allied to 
 the flexible Echinothuria of the chalk. The vitreous sponges, like 
 Holtenia, recall fossil sponges of the Upper Greensand and Chalk ; 
 while the foraminifera and polycystinae present a similar interest. Of 
 the foraminifera, Globigerina and Orbulina live on the surface of the 
 ocean, and like the pteropods only reach the deeper waters after death, 
 when they are mixed with heavier-shelled forms which live on the 
 ocean floor. 
 
 Extinction of Species. Species are most easily exterminated by 
 the struggle for existence. They drop away one by one, first by 
 diminution in numbers, then by limitation of the inhabited area, till 
 they cease to be met with in geological time. This process is similar 
 to that which has been recorded in human history, by which so many 
 birds have disappeared from the earth. Not to mention the moas, 
 still found with the feathers attached and with moa eggs buried in 
 Maori graves, there are the well-authenticated instances of the dodo 
 of Mauritius, and the solitaire of Keunion ; and Professor Alfred 
 Newton 2 records, among other extinct Mascarene birds, in addition 
 
 1 "Depths of the Sea ;" and "Voyage of the Challenger in the Atlantic." 
 
 2 Encyc. Brit., article " Birds." 
 
444 EXTINCTION OF SPECIES NOW IN PROGRESS. 
 
 to the crested parrot, two other parrots, a dove, a large coot, an abnor- 
 mal starling, a ralline bird incapable of flight, and many others. The 
 Antilles have in the same way lost in the last century many macaws 
 and other species; while the garefowl, Alca impinnis, has disap- 
 peared along with the Labrador duck and the Philip Island parrot, 
 Nestor productus. Evidence of the process of extinction may be 
 traced in our own country, in the changed and more limited distri- 
 bution of the crane, spoonbill, capercailzie, bustard, &c. And if we 
 go back to the remains preserved in beds of peat, we add to our 
 British fauna the pelican, wolf, brown bear, roebuck, Bos pri mi- 
 genius, Bos frontosus, the Irish elk, and beaver, which were certainly 
 contemporary with man. Species have a life like individuals ; some 
 insects which live but a single season have been kept alive for many 
 years by preventing them from experiencing the change of temperature 
 under which reproduction and death occur. And so species by existing 
 under similar conditions may undergo little or no change, and survive 
 through long periods of geological time ; but as a rule, a species, how- 
 ever dominant it may become for a time, by immunity from enemies, 
 or organisms causing disease which have not become accustomed to 
 their new companion, sooner or later fails in physiological vitality if 
 it is exposed to the ordinary variable conditions of climate ; and then 
 disease attacks the species with fatal energy. Many instances might 
 be recorded of the wholesale death of mammals from drought, and 
 of fishes from poisoned water, which would parallel anything met with 
 among the strata. 
 
 Sudden Destruction of Marine Animals. The extraordinary 
 abundance of fossils in some deposits, and the evidences of the sudden 
 death of others, do not imply any wholesale extinction of life, but 
 rather such destruction as occurs from time to time by natural agencies. 
 Professor Rupert Jones, E.RS., records several such phenomena. 
 Thus lobsters have been killed in the boxes in which they are kept 
 at the bottom of the sea at Stornoway by a heavy fall of rain in the 
 night. The octopus has been similarly cast on shore in immense 
 numbers after the passage to the sea of floods of snow water. The 
 great rains of the south-west monsoon kill thousands of fishes- and 
 other sea animals on the coast of India. Star-fishes and other marine 
 animals have been found killed on the south of Tobago by the fresh 
 waters of the Orinoco. Even the Yenisei in flood kills the deep-sea 
 fish of the Arctic Ocean. The fishes of the Jordan are similarly killed 
 when carried into the Dead Sea. The heat from volcanic eruptions 
 has killed fishes in all parts of the globe. A bank of crabs three 
 feet high was thrown up after an earthquake-shock in the Bay 
 of Payta in Peru. Storms sometimes kill millions of oysters, and 
 storms are probably the most destructive agents affecting fishes. 
 Occasionally the sea-bed is seen covered for acres with the dead sand- 
 lance and other fishes. Sometimes an epidemic affects fishes, and 
 salmon die in great numbers, and may even die under the influence of 
 unusual heat. By drying up the coast in estuaries, eels have been 
 killed in hot seasons on the coast of "France in sufficient numbers to 
 
CAUSE OF THE SUCCESSION OF LIFE IN TIME. 445 
 
 produce fever. A ferruginous spring entering a river may kill the 
 fish for miles. 1 Such and such-like conditions may account for the 
 appearances of sudden mortality which are familiar to the geologist, 
 but the extinction of species even locally is a phenomenon of another 
 order, and is in most cases to be attributed to the upheaval and 
 depression of land. 
 
 PalaBontological Laws. Pictet summarised palaeontological belief 
 in the following laws : i. All species have had a limited geological 
 duration. 2. The contemporary species of a geological fauna in any 
 one locality in most cases appeared simultaneously, and disappeared in 
 the same manner. 3. The differences between living and extinct 
 faunas are in proportion to the geological antiquity of the extinct 
 fauna. 4. The diversity of animal organisation has increased with 
 the duration of geological time. 5. The highest types of life have a 
 comparatively recent origin. 6. The order of appearance on the earth 
 of the different types of life often recalls the phases of embryological 
 development. 7. The existence of a type upon the earth is uninter- 
 rupted from its first appearance to its extinction. 8. The ancient 
 distribution of life shows that the distribution of temperature has 
 changed. 9. The geographical distribution of species found in the 
 strata was more extended than the range of species of existing faunas. 
 10. Fossil animals were constructed on the same plan as existing 
 animals, and their lives manifested the same physiological functions. 2 
 
 Succession of Life in Time. Just as the earth's surface at the 
 present day is inhabited by groups of animals and plants, as distinct 
 from each other as are the faunas of past ages of geological time, so 
 every era, the earth being divided up into masses of land and 
 water, must always have had its life characterised by differences in 
 the generic groups of species in different regions, which amounted to 
 provinces, comparable to those which have been established upon the 
 distribution of living animals and plants. Hence the fauna or flora 
 of a geological formation is nothing but a natural history life pro- 
 vince, which has become preserved in sediments, which enclosed 
 organic remains in the positions in which they once lived, and are now 
 found. And every life province, whether on land or in the sea, 
 is only a geological fauna or flora which is now becoming fossilised. 
 Formerly there was a belief that the physical conditions which caused 
 the deposition of a new mineral sediment affected the whole surface 
 of the earth, or were of the nature of " catastrophes," and that many 
 successive creations replaced the life which was supposed to have been 
 periodically exterminated. Such ideas of catastrophe and creation 
 find no support in the observations which geologists have accumu- 
 lated. On the other hand, Lyell and Darwin, and the uniformitarian 
 school, by supposing that life, when it was once placed upon the 
 earth, had become gradually and successively modified from age to 
 age, appealed to " breaks in time " and the " imperfection of the 
 geological record," as means of accounting for differences which were 
 
 1 Geol. Mag., vol. ix. p. 533. 
 
 2 "Traite de Paisontologie," tome i. 1853. 
 
446 MIGRATION OF LIFE IN GEOLOGICAL TIME. 
 
 found in the faunas of successive rocks. A slight difference in the 
 life in an overlying deposit was thought to imply a moderate denu- 
 dation, or a short interval in deposition. Great changes were held 
 to imply great denudation, and vast gaps utterly unrepresented by 
 strata. We escape from both of these hypotheses in better geogra- 
 phical knowledge of the distribution of life. For, although we must 
 appeal on the one hand to great upheavals and depressions of land, 
 which are essentially catastrophes in their effects, and have no choice 
 but accept the evidences of evolution concerning the variability of 
 organic types, and must allow that many of these are lost to us 
 through denudation of strata during the elevation and depression of 
 ancient lands, yet nothing but the succession of life provinces, super- 
 imposed vertically upon each other, can account for the succession of 
 life which the fossiliferous strata disclose. 1 
 
 Migration of Life Provinces. When life is traced back in time 
 in any part of the earth, the geographical distribution of species is 
 found to differ from the existing distribution. Thus, in the peat of 
 the fen district of the eastern counties, several mammals occur fossil- 
 ised, such as the Bos primigenius, the Bos frontosus, and the Cervus 
 megaceros, which are now extinct, and these are associated with the 
 brown bear, wolf, beaver, roebuck, and other species which have long 
 since become extinct in these islands, or more limited in range. Simi- 
 larly we may compare the mammals of the peat with those found in 
 the valley gravels, with the result that some species of the peat, like 
 the Bos primigenius and the Cervus megaceros, are still met with, 
 associated with hippopotamus, lion, rhinoceros, elephants, and other 
 types which present a remarkable correspondence to the mammal life 
 of Northern Africa. We can only explain this succession of life in 
 the same district on the hypothesis of migration from some neigh- 
 bouring district, and must seek back for the origin of the fauna of the 
 gravel in the Newer Tertiary strata. We may not be justified in 
 inferring that the gravel fauna reached North Africa by migration 
 from Europe, though it is highly probable that the upheaval of 
 Northern Europe which produced the glacial period necessitated a 
 migration of the crag fauna southward, and that the mammalia re- 
 turned again to the north as the land resumed its ordinary vegetation 
 on bemg again depressed. Another instance which might be adduced 
 as tending to show how the limits of mammalian provinces of life 
 have varied in past geological time may be found in the types by 
 which the Crag deposits of the eastern counties show connections with 
 the Oriental region. Not to mention the mastodons, elephants, rhino- 
 ceroses, which have Eastern affinities, there is a tapir in the Crag com- 
 parable to the Malayan tapir, and the Hyaenarctos, which closely 
 resembles the same genus in the Siwalik rocks of Northern India. 
 This migration of mammal life is in harmony with the migration of 
 plant life. 
 
 At the present day Japan belongs to a different natural history 
 
 1 "Laws which have determined the Distribution of Life and of Rocks," 
 Annals and Mag. Nat. Hist., December 1867. 
 
MODE OF ORIGIN OF A FAUNA. 447 
 
 province from India, but it was not always so ; for the Stegodon 
 clifti, Stegodon insignis, Elephas namadicus, and Elephas primi- 
 genius, which are fossils found in the Siwalik and Narbadda deposits, 
 also occur in Japan, presumably of the same age. 1 
 
 The Siwalik fossils comprise far more African types than are found 
 living in India, and they also include many genera which are met with 
 in the older Tertiary rocks of Europe. And just as we are entitled to 
 infer that such genera as Hyopotamus and Anthracotherium and Ace- 
 rotherium. which occur in the Miocene of Europe, migrated eastward 
 to from part of the Siwalik fauna, so we may conclude that the giraffe 
 and ostrich, which were Siwalik types of life in the Pliocene period, 
 migrated from India to Africa, and therefore we must look to this 
 ancient geological connection for an explanation of the partial com- 
 munity of life between the existing Ethiopian and Oriental regions. 
 There is no element in a fauna, so far as we are aware, in any period 
 of geological time which does not prove this migration to have been 
 always in progress. 
 
 Origin of Faunas. Suppose, by way of hypothesis, that we have 
 to deal with a world in which the surface may be divided into natural 
 history provinces, denned like the spaces on a chess-board, where 
 the value of the moves of the several pieces is different, and that 
 after the life has accumulated on a given area, K, upheaval affects 
 the sea-bed of an adjacent region along a line A, B. The result of 
 this upheaval is to displace the life from the region S, which adjoined 
 the line A, B, and cause it to migrate to the region R ; it thus becomes 
 mixed with the life in B. But since the sea-bed at R has itself 
 undergone change, some of its life has migrated away, some species 
 have been exterminated by the altered circumstances, and similarly 
 the immigrating group loses some of the forms which were character- 
 istic of it when occupying the region S. These losses may be desig- 
 nated X. Hence a new fauna is compounded out of pre-existing 
 groups, and does not correspond with that which would be obtained 
 by simply mixing them. And just as some species of plants and 
 animals taken to new regions increase to an unheard-of extent, 
 as the Anacharis increased in our own country, the thistle in New 
 Zealand, the rabbit in South Australia, so in the history of life every 
 province becomes enriched by species which were previously unknown, 
 unimportant, or rare, that under their new geographical conditions 
 come to be dominant in number of individuals, and often in variation. 
 This gain to the province may be termed Y. Similarly, if a change 
 in the amount of elevation of the axis A, B, at one end of the axis 
 towards A, comes to lay bare the sea-bed in the region T, and causes 
 the life of this region to be superimposed upon the spot where the life 
 R was fossilised, and where the life R + S - X + Y was superimposed 
 upon it, then a third fauna will succeed, which may differ from the 
 second as much as the second differs from the first, which must have 
 been compounded out of older groups, and must itself undergo the 
 loss indicated by X, and acquire the facies indicated by Y. In this 
 1 R. Lydekker : " Palseontologia Indica." 
 
448 HOW FAR STRATA MAY BE KNOWN BY FOSSILS. 
 
 hypothesis we have supposed the conditions of the sea-bed to remain 
 essentially the same in the locality R ; but it is obvious that depres- 
 sion might produce in that region deep-sea conditions, and cause the 
 influx of a deep-sea fauna, or that elevation might develop laminarian 
 or littoral conditions, so that the variations of the fauna which are 
 possible, without seriously affecting the distribution of land and water 
 on the earth's surface, are more numerous than might at first sight 
 appear. Similarly, when a land emerges from the sea, the life which 
 covered that area is displaced, and the rising land divides the marine 
 fauna, so as to cause the group to be pushed apart as the island 
 enlarges into a continent, until by following the littoral or laminarian 
 zones which offered the conditions of least resistance to their move- 
 ment, it has happened that the fauna, which was originally temperate, 
 has in part become arctic ; in another part has become tropical ; while 
 temperate provinces are found in temperate latitudes. If a land sur- 
 face becomes enlarged, the life diffuses itself, so that the species are 
 few relatively to the area ; but if the land is undergoing depression, 
 then the fauna and flora become rich, because the area of land 
 decreases without any corresponding decrease in the variety of life. 
 The whole doctrine of the identification of strata by means of organic 
 remains, rests upon the assumption that the strata identified were 
 deposited in the same natural history province. If strata are defined 
 to be of the same age from their fossils, it can only be because the 
 old life province can be traced continuously from one point to the 
 other. But great practical difficulties result from the imperfection 
 of our knowledge of the past distribution of life ; for though 95 per- 
 cent, of the shells of the Ked Crag are still living, a much smaller 
 percentage of British shells will be found in the fauna of Portugal ; 
 so that contemporary fauna*, even when separated by moderate dis- 
 tances, may have less in common than faunas of different geological 
 ages which are superimposed on each other. 
 
 Identification of Strata by Fossils. As taught by William Smith, 
 the identification of strata by fossils was a matter of observation. He 
 used fossils as an aid in tracing the strata through England. The 
 results, as displayed in his books and maps, were so remarkable, that 
 the principle, which in his hands was sound, came to be regarded as 
 a universal law, though no conditions were imposed on its application. 
 The British strata were taken as types, and British fossils found in dis- 
 tant places spread William Smith's names for the beds over Europe. 
 And before long the great divisions of geological time, thus marked out 
 conveniently, came to be recognised over distant parts of the globe, and 
 their ages were determined by their fossils. This was not a scientific use 
 of fossils, for it ignored the origin and distribution of species. It led to 
 the theo-geological conception of successive creations and extinctions 
 of life on the earth, which marked off the range of species exactly. 
 But when it came to be seen, by detailed examination of strata, that 
 there was no such homogeneous group of life in a period of time, but 
 that the life changed all through a formation, approximating to the 
 older life in the bottom beds, and to newer life in the top beds, we 
 
MANY FAUNAS OF ONE AGE. 449 
 
 reached the conception of a continuous succession of life on the earth, 
 modified in successive periods through all past time. Still this did 
 not invalidate the doctrine of the identification of strata lay fossils, 
 though geologists began to find from experience, and taught that some 
 species lost their identity more rapidly than others, and that species 
 have different values in identifying strata. After Edward Forbes had 
 marked out the limits of life in zones of depth, and in his " Natural 
 History of the European Sea " had traced the limits and life of exist- 
 ing natural history provinces, and in his discussion of the geological 
 relations of the British fauna and flora, had traced the geographical 
 and geological antecedents of existing life, 1 only an effort of the 
 imagination was needed to set the existing life provinces in motion 
 with the upheaval and depression of land, and see that the succession 
 of life in time implied migration, and therefore that no flora or fauna 
 was ever universal. 2 This discovery limited the idea of identification 
 of strata by fossils ; for it implied that similar fossils could only 
 prove the continuity of a stratum when the deposit took place in the 
 same zoological province. The moment we travel beyond that central 
 part of the province where its typical species and genera abound, so 
 soon do we find a greater change in life than the geologist would 
 expect in a stratum. It is impossible to run a line along the sea-bed 
 from Britain north to Spitzbergen, or south to the Azores, south- 
 west to the West Indies, or west to Labrador, without passing over 
 groups of organisms which if fossilised would not tend to establish 
 contemporaneous deposition by community of life. Among many 
 groups of animals there are species which are world-wide ; but the 
 wider the range in space the longer is the duration in time, and there- 
 fore the less the value in identifying a stratum ; and in existing seas 
 a geographical interval means a change iii*fauna. So must it have 
 been in all past time ; otherwise there could have been no succession 
 of life in time. It may have been that life provinces in the sea were 
 as numerous in primary or secondary ages as they are now ; in any 
 case we must appeal to them in explanation of the successive changes 
 of life in the geological formations of any one district ; and we cannot 
 imagine such geographical changes as would produce succession of strata, 
 without the elaboration of life provinces at the same time. If we recog- 
 nise this ancient arrangement of life, then it follows that the life of 
 several geological formations existed simultaneously in the same sea, 
 otherwise they could not have been superimposed. That is to say, 
 the life which we term Silurian, Devonian, Carboniferous, must in its 
 main elements have been contemporaneous in horizontal succession in 
 the oceans of the ancient world, otherwise it could not have been 
 accumulated in vertical succession in the strata. But if these faunas 
 were contemporaneous, the fossils cannot identify different limited 
 periods of time in widely separated localities. 
 
 Homotaxis. De la Beche 3 was the first to recognise the difficulties 
 
 1 Mem. Geol. Survey, vol. i. 1846. 
 
 2 Annal. and Mag. Nat. Hist. Dec. 1867. 
 
 3 "Researches in Theoretical Geology," p. 261. 1834. 
 VOL. I. 2 F 
 
450 ONE FAUNA OF MANY AGES. 
 
 in the way of identifying strata by fossils, for he observes that if the 
 coasts on two sides of a sea are so situate that the one is subject to oscil- 
 lations, rising above or falling beneath the general ocean level, while 
 the other remained firm, we should obtain different results. In the 
 one case there would be the exuviae characteristic of a marine deposit 
 with unchanged mineralogical structure, while, in the other, there 
 might be great variety in the life, or a destruction of species which 
 continued to exist unchanged in the other situation, so that there 
 would be little or no resemblance in the organic contents of contem- 
 poraneous rocks. Similar views were long afterwards restated by Mr. 
 Herbert Spencer in an anonymous article entitled " Illogical Geology," 
 since reprinted. 1 
 
 Subsequently, in 1862, Professor Huxley 2 discussed the question 
 in an address to the Geological Society, and after demonstrating that 
 geology can only prove a local order of succession, goes on to state that 
 for areas of moderate extent no practical inconvenience is likely to result 
 from assuming the corresponding strata to be synchronous. But the 
 moment the geologist has to deal with the large areas or completely 
 separated deposits, then the mischief of confounding that homotaxis, 
 or " similarity of arrangement," which can be demonstrated, with 
 synchrony or identity of date, for which there is not a shadow of 
 proof, under the one common term of contemporaneity, becomes incal- 
 culable, and proves the constant source of gratuitous speculations. 
 Hence 3 it is possible that similar or even identical faunas and floras, 
 in two different localities, may be of extremely different ages, if the 
 term age is used in its proper chronological sense. 
 
 Barrande's Colonies. Edward Forbes enunciated the doctrine 
 that when a species is once extinct it never reappears. But the dis- 
 appearance of a species from a stratum succeeding that in which it 
 is found, does not, however, imply extinction. Barrande conceived 
 of the idea that assemblages of species, forming the fauna of a sea-bed, 
 might migrate away in one age, and return again to the same district 
 in a later period of time. This first appearance of the same fauna he 
 terms a colony. The Cambrian rocks of Bohemia exhibit many examples 
 of such colonies. The reality of these colonies has been attacked and 
 defended. The great difficulty in accepting them is not so much the 
 inherent improbability of the repetition of the same life group in the 
 same district, as the universal absence of such a phenomenon in the 
 newer strata. This assumed repetition reminds us of the old assertion 
 on which the name Lower Silurian was first instituted, that the fossils 
 found in the Upper Silurian also characterised the Lower Silurian. 
 This repetition existed, and is now well known to have been due to 
 inversion of the strata, so that the Lower Silurian were the Upper 
 Silurian. Mr. J. E. Marr has similarly suggested that the colonies of 
 Barrande are due to the same strata being repeated by inversion and 
 contortion. 
 
 Persistent Types of Life. In the succession of life exhibited 
 
 1 Essays. 2 Huxley Address, Geol. Soc. 1862. 
 
 3 Huxley Address, Geol. Soc. 1870. 
 
PERSISTENCE OF GENERA IN TIME. 451 
 
 by the strata, nothing probably is more astonishing than the evidence 
 afforded of the way in which the principal types of life have persisted 
 on the earth. This correspondence between the ancient life and the 
 existing life was first enunciated in recent times by Professor Huxley. 1 
 It has been shown that the Lepidodendron of the coal-measures in all 
 essential details of structure is closely allied to the living Lycopodium. 2 
 Species of Araucaria are known from the oolites which also con- 
 tain species of Pinites, while the cretaceous rocks yield species of 
 Cedrus, pandanaceous fruits, Juglans (walnut), and a multitude of 
 other genera which still exist, and are met with throughout the 
 Tertiary period. This history is repeated by almost every group of 
 animals. The foraminifera of the Carboniferous period, together 
 with many extinct types, include existing genera (see p. 479). Among 
 the alcyonarians, the fossil Hdiolites makes a remarkable approxima- 
 tion to the surviving Heliopora. The great existing divisions of 
 sponges date back in time to the oldest rocks ; while the Mollusca are 
 for the most part represented at the present day by genera in which 
 the generic characters have remained unchanged through all geological 
 time. The Nautilus ranges through all strata. The Loligo com- 
 menced with the Lias and lives on to the present day. Among 
 gasteropods, Patella, D&ntalium, Trochus, Natica, Chemnitzia, are 
 common genera which all date back at least as far as the middle of 
 the Primary period. The bivalve shells tell the same story, Area, 
 Avicula, Lucina, Cardium, Pinna, Pecten, are all types which date 
 from the Primary epoch. Among brachiopods, Crania, Discina, 
 Lingula, Rhynclionella, and Terebratula are genera met with plentifully 
 in early Primary rocks. Among Annulose animals the Carboniferous 
 rocks have yielded scorpions, spiders, centipedes, and representatives 
 of the chief groups of insects and Crustacea. Nor are the vertebrata 
 an exception to this general law of the antiquity of life, for the oldest 
 fishes belonged to living groups, and the Elasmobranch and Ganoid 
 groups of the Palaeichthyes date from before the middle of the Palaeo- 
 zoic period. The Amphibia at their first appearance present a higher 
 type in some respects than any which now survive. The Crocodilia, 
 Chelonia, and Lacertilia, date from the secondary epoch ; and in the 
 older Tertiary rocks serpents are found, differentiated as in existing 
 groups. The few slight indications of birds found in the Cambridge 
 Greensand, combine in one type characters of divers, grebes, and 
 penguins ; while even the Mammalia in their oldest forms show, as 
 far as can be judged from a few bones in the lower oolites, and a few 
 teeth from the Trias, the essential organisation of existing marsupials. 
 We therefore discover that on the hypothesis of evolution, geological 
 history does not carry us back appreciably towards the origin of the 
 great divisions of organic nature ; or even towards the origin of the 
 chief generic groups. Hence it becomes rather a matter for wonder 
 that the changes in life should have been so slight with the successive 
 physical circumstances which have changed the distribution of land 
 
 1 Royal Institution: Friday, June 3, 1859. 
 
 2 Carruthers, Royal Institution, April 16, 1869. 
 
452 LOCAL SUCCESSION OF SIMILAR TYPES. 
 
 and water, and brought new lands into existence in geological time. 
 For they have so changed the distribution of life on the earth as to 
 entomb and fossilise successively thirty or more distinct faunas in 
 the sediments laid down upon the same area of sea -bed. Pro- 
 fessor Huxley at first, perhaps, showed excessive caution in the 
 statement " There may, or there may not, have been a progressive 
 development of animal and vegetable life ; but the palaeontological 
 evidence before us does not justify the assertion that any proof of such 
 progressive change, if it ever occurred, exists;" 1 but subsequently,' 
 in elucidating the evolution of the horse 2 and other animals, demon- 
 strated that the discoveries of paleontologists have justified an aban- 
 donment of this negative attitude. 
 
 Ancient Types of Echinoderms now Living. Cidaris is the 
 oldest living genus reaching back to the Trias. At the present day 
 its. habitat is littoral. Among genera which survive from the Jurassic 
 rocks are Echinobrissus, which is littoral, and Hemipedina and 
 Pygaster, which are found iu deeper water. The genera which live 
 on from the chalk number 13. Of these 8 are now found in the 
 littoral zone including Leiocidaris, Echinus, Echinocyamus, Fibu- 
 laria, Rhynchopygus, Nucleolites, Hemiaster. The range of some of 
 these genera in depth is less restricted. Thus the laminarian zone 
 includes Salenia, Cottaldia, Echinus, Echinocyamus, Fibidaria, Cono- 
 clypeus, Catopygus, Hemiaster, and Periaster ; while the deep sea 
 yields Salenia, Echinus, Echinocyamus, Fibidaria, and Hemiaster, 
 which are all Cretaceous genera. It is highly probable that the con- 
 ditions of depth in which Echinoderms live have changed with geolo- 
 gical time, for at the present day we might quote as representatives 
 of littoral conditions most of the Cidaridae, Diademadae, Triplechinidae, 
 Temnopleuridae, Nucleolitida?,, Echinonse, and Palseostominse. Lami- 
 narian conditions are represented by the Salenidae, Galeritidse, and 
 many of the Fibularina and Nucleolitidse. 3 The families of the Abys- 
 sal Ocean are the Ananchytidae and Echinothuridse. Therefore, in- 
 ferences as to the conditions of life of an Echinoderm fauna, like that 
 of the Upper Chalk, need to be very diffident. 
 
 Local Persistence of Types. One of the most interesting pro- 
 blems, and at the same time one of the most difficult of explanation, 
 is the persistence in the same locality of types which have undergone 
 remarkable change ; for it seems to be associated with absence of com- 
 petition from more highly organised races. This succession is most 
 striking among the mammalia of the more recent deposits as compared 
 with living groups. Thus in South America the Edentates reach their 
 maximum development, and it is among the muds of the pampas that 
 we find the gigantic Megatherium and Mylodon, Megalonyx and Sceli- 
 dotherium, representing the living sloths. In the same way the living 
 
 1 Catalogue of a collection of fossils in the Museum of Practical Geology, with 
 an explanatory introduction by Thomas H. Huxley and Robert Etheridge, 
 1865. 
 
 2 Address Geol. Soc. 1870. 
 
 3 Neumayr : " Neuen Jahrbuch fur Min." 1882. 
 
CAN CLIMATE BE INFERRED FROM FOSSILS? 453 
 
 armadillos are represented by extinct gigantic Glyptodons, in which 
 the armour is free from joints, and the living Auchenia is represented 
 by the larger Macrauclienia. The monkey, Protopithecus, which 
 occurs with them, is of the American platyrhine group. 1 
 
 Similarly, in Australia the Marsupials reach their maximum ; and 
 here also gigantic forms occur fossil, and represent most of the groups. 
 Thylacoleo is considered to represent a marsupial carnivore ; Dipro- 
 todon is termed a marsupial pachyderm. Wombats occur as large as 
 tapirs, and there were gigantic kangaroos and large phalangers. It is 
 remarkable that the living Australian lizard Moloch appears to be 
 represented in a fossil state by a gigantic prototype, Megalania. In 
 New Zealand there are found in a barely fossil condition the remains 
 of multitudinous gigantic birds, the most familiar of which is the 
 Dinornis, and these may be regarded as represented at the present day 
 by the surviving ratitse of the Australian province. All giant types 
 appear to have had a short duration in time. 
 
 Climatal Conditions of Ancient Seas. Any attempt to estimate 
 climatal conditions in past ages of the earth's history must rest upon 
 physical evidence. Ice-scratched stones, glaciated rocks, and boulder 
 clay may prove conditions of great cold, but we are acquainted with 
 no physical evidence that would demonstrate heat as a climatic con- 
 dition of the earth. There is nothing in the condition of the skeleton 
 of the musk ox that would indicate the conditions of extreme cold 
 under which the species lives, and no one examining the arctic seals 
 in a fossil state could affirm that they had not lived in tropical waters. 
 It is equally impossible to recognise climatic conditions from structure, 
 collocation of species, or other evidence, for the abundant tertiary flora 
 of Greenland, with its many sub-tropical types, has shown that the 
 distribution of land and water may enable plants as well as animals 
 to flourish in latitudes where their existence would not be suspected 
 from the present distribution of life. Edward Forbes laid down the 
 great doctrine that " every species is controlled by its own peculiar 
 laws, and no acquaintance with one species of a genus, however exten- 
 sive and accurate, warrants us in predicting concerning any other 
 species, even though very striking resemblances may prevail ; for the 
 truths of zoology forbid us to reason concerning the species of a genus 
 in the same manner as we do with the individuals of a species." 
 Many genera are world-wide in distribution, and the habits, food, and 
 climate of one species are quite dissimilar from those of other species. 
 If, then, one recent species cannot indicate the climate under which 
 another species lived, least of all can rare and surviving recent species 
 tell anything of the climatic adaptability of extinct fossil forms. We 
 need scarcely recall attention to the inferences which were drawn 
 when the valley gravels of England first yielded the remains of 
 elephant, rhinoceros, hippopotamus, lion, and associated species, in 
 days before the glacial period was recognised. Such species were 
 almost everywhere affirmed to indicate for Britain a warmer climate 
 in the time when the fossils lived. Subsequently, when the same 
 1 Owen's "Palaeontology," 1862. 
 
454 GENERA INDEPENDENT OF CLIMATE. 
 
 mammoth was found frozen in Arctic ice, the conclusion veered round ; 
 so that the animals, previously supposed to be sub-tropical, were 
 believed to have lived under colder conditions than the present 
 British climate. Such an example may warn us that, although the 
 Nipa fruit of the London clay of Sheppey is of the same genus as the 
 Kipa of the Ganges and Irrawaddy, and although associated in both 
 cases with crocodiles and fresh-water turtles, in a sediment which 
 includes many genera of shells found in eastern seas, we can no more 
 infer a sub-tropical climate for the plant life or reptiles or shells of the 
 London clay, than we can for the mammals of the gravel, which are 
 often inseparable as species, from the similar assemblage of mammals 
 now inhabiting the North of Africa. There has been a tendency to 
 hasty generalisation in the matter of climate, as inferred from palseonto- 
 logical evidence. 
 
 It has often been pointed out that there is a correspondence 
 between the marsupial mammals of the Stonesfield slate and asso- 
 ciated cestraciont fishes, plants, and shells like Trigonia, with similar 
 forms now living in the Australian region ; and the conclusion has 
 been drawn that the British fossil fauna and flora of Stonesfield lived 
 under like conditions of climate. For such a view to be sustained, it 
 must be held, either that the Australian life is a survival of a creation 
 which was once universal, and of which a part was fossilised at 
 Stonesfield, or else that the life from Stonesfield migrated through 
 the equatorial region to the antipodes. Both views are equally unten- 
 able ; the former being disposed of by the occurrence of Cretaceous 
 and Tertiary strata in Australia, and the latter by the fact that, though 
 it is just possible that elevation of land might preserve equable con- 
 ditions between Britain and the antipodes, so that the equator should 
 be passed without the experience of an equatorial climate, yet this 
 hypothesis would require that the marine life should similarly and 
 simultaneously migrate along a sea-bed, depressed so deep in the 
 equatorial region that the climate was still temperate. 
 
 It is difficult;, when dealing with living species in the newer geolo- 
 gical formations, to be sure that we do not attach too much importance 
 to the facies of the faunas of the Coralline Crag and the Middle Glacial 
 sands, as indicative of climate. For although the Pyrula, Cassidaria, 
 Valuta, Lingula, &c., of the Crag, suggest southern seas, and though 
 the same area came to be occupied subsequently by an assemblage of 
 shells which is now arctic, it is quite possible that climate may have 
 had nothing to do with the succession, but that it may have been a 
 continuous though slow migration of life southward, consequent upon 
 continued elevation of land to the north. 1 
 
 Existing Distribution of Life. Since existing plants and animals 
 are the surviving descendants of fossilised forms which in bygone time 
 were distributed to different regions of the world from those which 
 their descendants occupy, it is only natural to find that the great 
 geographical groups of life on land do not coincide with present 
 arrangements of land and water. In fact, the present distribution 
 1 Cambridge Phil. Soc., March 3, 1862. 
 
LIFE SUPERIMPOSED IN ZONES OF DEPTH. 455 
 
 depends, first, upon the distribution in past time ; and, secondly, 
 upon the great physical changes which brought the known masses of 
 land into existence. Edward Forbes long since recognised the law 
 that life diffuses itself in the main under climatic conditions. 1 We 
 may say that life diffuses itself in the direction of least resistance, 
 which term may conveniently indicate comparatively uniform con- 
 ditions of life. The influence of climate is obvious, because it implies 
 similar physiological action for the parts of an organism and similar 
 conditions of nutrition, as well as similar food. And the low forms of 
 life may find similar climatic conditions in the deep waters of the 
 tropics, to those which are presented by the shallow waters of the 
 arctic seas ; so that all over the world the life of the deep ocean forms 
 as Sir Wyville Thomson observed, one natural history province so 
 far as its lower organisms are concerned. How far this uniformity 
 may be the consequence of the existence of deep-sea currents, diffus- 
 ing the cold water and the germs of life within it, may perhaps be 
 worth consideration. In like manner, above the surface of the earth 
 there is a zone of comparatively uniform climatic conditions, which 
 ascends from the poles higher and higher above the sea, following 
 the snow-line, until, at about 16,000 feet of elevation in the tropics, 
 the life is in the main the same in kind as that within the arctic 
 circle. These aspects of climate are complicated, however, by varia- 
 tions in pressure of air and water, and this difference of pressure has 
 a value in elaborating structures by inducing modified functions ; 
 so that the term alpine is not always synonymous with arctic, 
 just as the deep ocean has yielded many forms of life which are 
 unknown in the polar waters. The ordinary conception of climate, 
 however, as indicated by limits of latitude, is exhibited in the distri- 
 bution of many groups of organisms like reef-building corals. There 
 are probably no conditions of climate to which life may not adapt itself, 
 or be compelled to adapt itself, by physical circumstances. Perhaps 
 the influence of coast-lines is one of the most important influences 
 tending to modify the distribution of marine life in directions which 
 are different, because not only is the water aerated by tidal action near 
 to shore, but the conditions of existence are similar at similar depths. 
 Edward Forbes observed the superposition of distinct faunas on the 
 same shore. He defined by the term littoral all such animals as lived 
 between tide-marks ; laminarian zone was the home of the laminaria 
 and similar sea-weeds, in water reaching from low tide to fifteen 
 fathoms, where in our own seas the larger number of vegetable-feeding 
 mollusca, as well as the carnivorous mollusca like buccinum, nassa, 
 and natica, abound. Below this is the coralline zone, where the 
 ordinary sea-weeds give place to nullipores and horny zoophytes, where 
 the great scallop replaces the oyster of the laminarian zone, where 
 the common gasteropods are buccinum, fusus, pleurotoma, natica, 
 aporrhais, fissurella, emarginula, pileopsis, eulima, and chemnitzia ; 
 the bivalves are lima, area, nucula, astarte, venus, artemis, corbula, 
 thetis. But the muddy or sandy character of the sea-bed has an 
 1 See Keith Johnston's Physical Atlas. 
 
456 GEOGRAPHICAL DISTRIBUTION OF STROM BUS. 
 
 influence of its own in governing the distribution of life, at least in 
 the littoral zone, and it is well known that genera like litorina, 
 patella, purpura, fissurella, and trochus are found on rocky coasts 
 cardium, tdlina, solen, cerithium, on sands ; while mytilus is a lover 
 of gravel, and mi/a, lutraria, pullastra, frequent muddy shores. It is 
 probable that mollusca, like fishes, migrate, for frequently large num- 
 bers of dead shells are dredged without a trace of living animals. 
 
 Existing Distribution of Genera. Any one who would become 
 a palaeontologist must be familiar with the geographical distribution 
 of existing genera, and to this end should indicate as far as possible 
 upon blank maps the homes of the species of each genus as they are 
 examined. For, only when the geographical areas occupied by species 
 are known, and considered in relation to their several distinctive char- 
 acters, is it possible to estimate the geological circumstances to which 
 the distribution is due, or the antecedent specific characters from 
 which the attributes of living species have been evolved. The study 
 of genera may thus sometimes aid in discovering to us changes in the 
 distribution of land by which the species have become distributed 
 over the world. And often when the genera of a family are compared 
 together, and compared with their fossil representatives, the mode of 
 origin of the genera, as well as the migrations and modifications which 
 species have undergone, are suggested with great probability. We 
 may illustrate this aspect of palseontology by reference to the Gas- 
 teropod genus Strombus, which dates back to the Cretaceous period, 
 is imperfectly known from the Tertiary rocks, and is widely dis- 
 tributed in existing seas. This genus includes some large species in 
 the West Indies, such as are commonly carved for cameos, and a 
 group of small species distinctive of the Philippine Islands, besides 
 the ordinary widely-distributed types. Some species of Strombus ap- 
 proach towards the allied genus Pteroceras, and others towards the 
 allied genus Kostellaria ; and among the Lower and Middle Tertiary 
 rocks of Europe we find the blending of generic characters is more 
 marked, and species occur which show that some of the areas from 
 which Strombus and Rostellaria are now absent, were inhabited by 
 those genera in comparatively late geological periods of time. So 
 that we are compelled to believe that the uplifting of existing 
 land has divided the species, and gathered them in affiliated groups 
 in definite localities. The story of the Strombs, or any family, 
 would take too much space to tell in detail here, and we can only 
 indicate the general distribution of species. First : In the Red Sea 
 are Strombus gallus, S. fasciatus, S. Ruppelli, and S. lineatus. 
 Strombus tricornis is common to the Red Sea and the Antilles ; 
 Strombus elegans is common to the Red Sea and Philippine Islands ; 
 and S. gibberulus has a wide range from the Red Sea by the 
 East Indies, Philippine Islands, and Moluccas to the Society Islands. 
 On the coasts of India and Ceylon are found Strombus Lamarcldi 
 and S. SiUbaldii ; but S. fissurella and S. canarium range to the 
 Philippines, while S. plicatus, with the last-named species, range 
 from India to the Moluccas. The Philippine Islands yield Strom- 
 
PROVINCES OF MOLLUSCA. 
 
 bus alatus, also characteristic of the Gulf of Mexico. 
 floridus, which ranges to the Society Islands, S. Luhuanus, 
 mutabilis, are common to the Philippine Islands and Moluccas, 
 while the typical Philippine species comprise S. succinctus, S. pul- 
 chellus, S. vittatus, S. bulbulus, S. crispatus, S. dentatus, S. epidro- 
 mis, S. samarensis, S. guttatus, S. minimus, S. Isabella, S. labiosus, 
 S. laciniatus, S. marginatus, S. leutiginosus, S. latissimus, S. papilio. 
 The Strombus auris-Diance is common to Malacca and some other 
 parts of the coast of Southern Asia. S. rugosus ranges north to 
 the Korea. S. Japonicus is peculiar to Japan. Strombus urceus 
 ranges from the Philippines to the north of Australia, while S. 
 variabilis ranges to Australia on the one hand, and .Zanzibar on 
 the other. Both these regions are small Stromb centres ; the 
 former includes, in addition to the two species already named, 
 S. fusiformis, S. deformis, S. Australis, and S. Campbelli. New 
 Zealand has the Strombus Novce-Zealandice. On the Zanzibar coast, 
 in addition to the S. variabilis is the S. columba, which probably 
 ranges to India and the Philippines. The Mauritius yields S. Mauri- 
 tianus, and Bourbon has S. cylindricus. There are at least three 
 species at the Society Islands. Strombus granulatus of the Galo- 
 pagos, and the S. gracilior of Panama, are common to St. Helena. 
 The West Indian types include, besides species already mentioned, 
 Strombus Peruvianus, S. galeatus, S. pugilis, S. gigas, S. bubonius, 
 also found at the Cape Verd Islands, S. accipitrinus, and S. bitubercu- 
 latus. 1 Thus the geographical areas of the groups of species are highly 
 suggestive of antecedent physical circumstances governing distribution 
 of faunas, and the student may bring this knowledge to bear in 
 palseontological studies. 
 
 S. P. Woodward's Provinces of Marine Life. The distribution 
 of marine life in provinces justifies a recognition of tropical, tempe- 
 rate, and arctic zones. In so far as these are defined by the mollusca, 
 we are still indebted for definition of the provinces to the work of the 
 late Dr. S. P. Woodward. 2 He recognised eighteen marine provinces, 
 of which the Indo-Pacific, West African, Caribbean, Panamic, and 
 Peruvian may be accounted tropical, unless the Peruvian be included 
 in the southern temperate with the Patagonian, South African, and 
 Australian regions. In the northern parts of the world, warm tempe- 
 rate and cold temperate faunas are recognised. Thus in the Pacific 
 the Calif ornian and Japonic are warm temperate, and the Aleutian 
 cold temperate, while in the Atlantic the Gulf weed divides the trans- 
 atlantic province of the United States seaboard from the Lusitanian 
 province, which extends into the Mediterranean; the colder temperate 
 area reaches from Nova Scotia across the Atlantic, and widens out 
 with the Gulf Stream to the north of Norway, forming the provinces 
 which Forbes and S. P. Woodward termed Boreal and Celtic. There 
 remains an Antarctic province, which is named Magellanic, which 
 includes the southern part of South America ; and a large Arctic pro- 
 
 1 See Lovell Reeve : " Conchologia Iconica." 
 3 "Manual of the Mollusca," 1851-1856. 
 
458 NORTH TEMPERATE PROVINCES. 
 
 vince, which extends southward with the Arctic currents to Iceland 
 and Newfoundland in the Atlantic and the Aleutian Isles in the 
 Pacific. These provinces may be taken to represent at least as many 
 survivals from faunas which have been fossilised in seas neighbouring 
 to those in which they now exist. Dr. S. P. Woodward placed upon 
 a map some facts in the distribution of genera, which, from a philo- 
 sophical point of view, are more valuable than the enumeration of 
 species ; and to this reference should be made. We now enumerate 
 a few of the typical genera l of the several provinces, grouped as 
 (i) Arctic, (2) Temperate, (3) Tropical. 
 
 Arctic Province. Includes Buccinum, Chrysodomus, Trophon, 
 Admete, Trichotropis, Velutina, Lacuna, Astarte, Cyprina, which are 
 perhaps most characteristic of the Old World ; while the Arctic part 
 of the New World includes Margarita, Crenella, Rhynchonella, Tere- 
 bratella, Yoldia, Glycimeris, Solecurtus, Crepidula, Scalaria, Natica. 
 
 Magellanic Province. This comprises many of the Arctic types, 
 showing that the Arctic and Antarctic life are really continuous, but 
 exist as deep-sea species in the intervening temperate and tropical 
 areas. Among the Arctic genera common to the Magellanic province 
 may be mentioned Chrysodomus, Trophon, Trichotropis, Margarita, 
 Rhynchonella, Crenella, and Astarte, to which may be added Voluta, 
 Modiolarca, and Mytilus. 
 
 Cold Temperate Zone. The Boreal and Celtic Provinces repre- 
 sent a large part of the life which is influenced by the Gulf Stream. 
 This circumstance accounts for the narrowness of its seaboard on the 
 Canadian coast and its great expansion northward from the mouth of 
 the English Channel. There is much to be said in favour of the sub- 
 division of this region into Scandinavian, Germanic, and Celtic, and 
 any sub-division must recognise the fact that the upheaval of Scandi- 
 navia distributed life in a different direction to that produced by the 
 elevation of the Alpine axis. Among the typical genera of the nor- 
 thern part of the province are Terebratulina, Waldheimia, Trivia, 
 Cryptodon, Panopea, with Crania, Astarte, Cyprina. The southern 
 or Celtic part includes Patella, Litorina, Cardium, Tellina, Donax, 
 Ostrea, Pecten, Thetis, Aporrhais. 
 
 Aleutian Region. The corresponding Aleutian region in the 
 Pacific presents life which may be regarded as only separated from 
 that of the British seas by the elevation of the intervening land. It 
 comprises the genera Panopsea, Crepidula, Chrysodomus, Purpura, 
 Buccinum, Pleurotoma, Haliotis. 
 
 Warm Temperate Zone. The Calif ornian Region gives a good 
 example of the life of the warm temperate region reaching almost 
 as far south as the tropic of Cancer. Among the genera are Chiton, 
 Patella, Haliotis, Fissurella, Cypricardia, Waldheimia. 
 
 Japonic Province. The Japanese islands and Corea among others 
 comprise the genera Astarte, Panopea, Conus, Terebra, Murex, 
 
 1 Such genera may be examined in the Natural History Collection of almost 
 any public museum, but the student would do well to obtain examples of genera 
 for further study. 
 
SOUTH TEMPERATE PROVINCES. 459 
 
 Cyprea, Haliotis, Pecten, Nucula, Crassatella, Diplodonta, Isocardia, 
 Artemis. In latitude and in life this region represents the Lusi- 
 tanian province. 
 
 Lusitanian Province. This area includes the Bay of Biscay, the 
 coast of Portugal, Mediterranean, and north-west coast of Africa. 
 Among its characteristic genera are Conus, Pleurotoma, Terebra, 
 Cassis, Triton, Vermetus, Solarium, Turbo, Haliotis, Spondylus, 
 Avicula, Chama, Crassatella, Cardita, Cytherea, and Columbella. 
 Among characteristic Mediterranean genera are Fasciolaria, Sili- 
 quaria, Clavagella, Thecidium, Cassidaria. The fauna includes many 
 minor types, especially in the islands of the north-east of Africa and of 
 the Mediterranean. 
 
 The Trans-Atlantic Province. On the opposite side of the Atlantic, 
 extending southward from New England to Florida, is a fauna which 
 varies with latitude, but comprises Conus, Oliva, Fasciolaria, Avicula, 
 Artemis, Lutraria in the south, and Nassa, Columbella, Eanella, Sca- 
 laria, Calyptrsea, Bulla, Area, and Solemya. 
 
 Southern Temperate Zone. The warmer temperate fauna is repre- 
 sented south of the tropic of Capricorn, though on the Peruvian coast 
 of America it reaches northward to near the equator. This correspon- 
 dence may be taken to indicate that the life is not really severed by 
 the intervening faunas of tropical regions, but only descends in the 
 intervening areas to greater depths. 
 
 The Patagonian Province. This fauna has presumably been sepa- 
 rated from the Peruvian province by the upheaval of South America, 
 and many characteristic living species are found far inland in raised 
 beaches. Among the characteristic genera are Oliva, Voluta, Tere- 
 bra, Scalaria, Natica, Chiton, Solen, Lutraria, Nucula, Leda, Cytherea, 
 Corbula, Pinna, Mytilus, Litthodomus, Pecten, Ostrea, besides some 
 forms like Plicatula, Lucina, and Venus, of which species range to the 
 Antilles. 
 
 South African Province. The fauna of South Africa has much in 
 common with that of the Indian Ocean, but very few species range as 
 far north as the Red Sea, and fewer still to Senegal. The character- 
 istic genera comprise Terebratella, Chiton, Patella, Fissurella, Crepi- 
 dula, Trochus, Littorina, Phasianella, Tumtella, Pleurotoma, Typhis, 
 Triton, Nassa, Columbella, Mitra, Voluta, Cyprea, ' Conus, most of 
 which are represented by several species. 
 
 Australian Province. The Australian province includes the 
 southern half of Australia, Tasmania, and New Zealand. Among the 
 characteristic types are Struthiolaria, Trigonia, Chamostrea, Myadora, 
 Myochama, Crassatella, Cardita, Cypricardia, Oliva, Conus, Voluta, 
 Fasciolaria, Pandora, Terebratella, "Waldheimia, and Crania. Ehyn- 
 chonella is common to the Arctic Seas, and Panopea is not known 
 nearer than Japan. Haliotis is common with Litorina on the shores 
 of South Australia, while New Zealand yields Voluta, Strombus, 
 Triton, Conus, Oliva, Cyprea, &c. 
 
 Peruvian Province. Includes the Pacific coast of South America, 
 .from Callao to Valparaiso. The genera comprise Chiton, Patella, 
 
460 TROPICAL PROVINCES OF MOLLUSCS. 
 
 Trochita, Crepidula, Fissurella, Tornatella, Litorina, Cancellaria, Murex, 
 Triton, Colunibella, Oliva, Purpura, Mitra, Pholas, Solen, Mactra, 
 Venus, Crassatella, Nucula, Leda. 
 
 Tropical Zone. There remains the large tropical region, which we 
 imagine to be to some extent superimposed upon the temperate life. 
 
 The Panamic Province. Here, at the mouths of the rivers, are 
 the Potamides, Potamomya, Area, Cyrena, Auricula, while the Litorinae 
 climb trees. The shells of the shore include Columbella, Cyprsea, 
 Mitra, Purpura, Cassis, Conus, Strombus, Pleurotoma, Hipponyx, 
 Fissurella ; while among shells of deeper range are many species of 
 Murex, Monoceros, Oliva, Cancellaria, Lingula, Discina, Spondylus, 
 Cardium, Venus, Artemis, Pandora, Pholas. The narrow land of 
 Central America and Mexico makes a complete separation between 
 this region and that on the American side of the Atlantic. 
 
 The Caribbean Province includes the Gulf of Mexico, West 
 Indian Islands, and South American coast as far as Rio. It is a 
 region fringed with coral growth. The characteristic genera include 
 Strombus, Murex, Triton, Fasciolaria, Cancellaria, Terebra, Cassis, 
 Columbella, Voluta, Oliva, Conus, v Cyprea, Litorina, Turritella, 
 Fissurella, Nerita, Calyptnea, Crepidula, Patella, Chama, Lucina, 
 Venus, Cytherea, Artemis, Lima, Area, Yoldia, Crassatella, Phola- 
 domya. 
 
 The West African Province includes the coast within the tropics. 
 Its genera comprise Strombus, Triton, Oliva, Purpura, Murex, Terebra, 
 Mitra, Pleurotoma, Clavatula, Conus, Natica, Cyprsea, Vermetus, Ceri- 
 thium, Turritella, Haliotis, Nerita, Area, Cardium, Artemis, Cytherea, 
 Card ita. 
 
 Indo- Pacific Province. This is an enormous area ranging between 
 Australia and Japan and Easter Island in the Pacific and the east 
 coast of Africa. This also is a region of coral reefs. A large number 
 of the characteristic genera occur fossil in the Lower Tertiary 
 strata of Europe. Among the tropical genera are Nautilus, Pteroceras, 
 Rimella, Rostellaria, Seraphs, Turbinella, Ancillaria, Magilus, 
 Neritopsis, Hemipecten, Placuna, Malleus, Vulsella, Cucullaea, 
 Hippopus, Tridacna, Cypricardia, Anatina. Among types which are 
 common to the American coast of the Pacific are Conus, Pleuro- 
 toma, Harpa, Mitra, Pyrula, Imperator, Lingula, and Discina. The 
 difference between the Mediterranean fauna and that of the Red Sea 
 is marked by some of the genera that we have named, and the 
 abundance in the Red Sea of species of others, such as Conus, Cyprea, 
 Mitra, Cerithium, Pinna, Chama, and Circe. 
 
 Relation of Living to Fossil Faunas. If we compare these 
 faunas with each other, it would be impossible to predicate from any 
 principle recognisable in structure that some were Arctic and others 
 tropical. They rather suggest the grouping met with in different 
 geological formations ; and it is only when we begin palaeontological 
 studies from the point of view of such faunal distribution, and trace 
 its origin back to the development of existing continents, which 
 laid the newest strata bare, and drove marine life from off them, 
 
PROVINCES OF TERRESTRIAL LIFE. 461 
 
 that we are able to attack the problem of identification and correla- 
 tion of strata by means of the life they contain. It must be the 
 palaeontologist's endeavour to know the history of life, genus by 
 genus, and fauna by fauna, accurately tracing distribution in time, 
 and distribution in space, as it varied with the succession of geolo- 
 gical ages. Hence, every fossil has a value as a member of a fauna, 
 infinitely more important than its value as a characteristic species 
 of a stratum ; and the aim of the geologist in collecting fossils from 
 strata is more a demonstration of the history and evolution of life, 
 than the division of geological time into successive epochs, which have 
 no meaning except as recording the physical history of life on the earth. 
 
 The student finds life provinces superimposed upon each other in 
 the sequence of the strata, and soon discovers that these life provinces 
 varied in past time with geographical distribution in a way similar to 
 their variation on the earth's surface now ; and in proportion to the 
 fulness of knowledge of this succession and variation of life, so will 
 be our success in converting the vertical succession of life preserved 
 in the strata back into the horizontal distribution of geographical pro- 
 vinces in the seas which geological formations represent. We have 
 no hesitation in affirming that the task is now within the student's 
 grasp ; for life provinces were clearly defined in the earliest epochs of 
 geological history which have been discovered. 
 
 Sclater's Natural History Provinces. To Dr. P. L. Sclater 
 belongs the honour of having been the first to map out the division 
 of land life into six great provinces. He recognised, as others have 
 done before and since, the predominant influence of climate in giving 
 a similar facies to life in the same latitude. Dr. Sclater's provinces 
 are named Palaearctic, Nearctic, Neotropical, Ethiopian, Oriental, and 
 Australian. These divisions do not coincide with the existing divi- 
 sions of land, for Mexico and the West Indies are grouped with South 
 America; the Ethiopian region only comprises Africa, south of the 
 equator ; while the Oriental region, in addition to the Malay Archi- 
 pelago, takes in Southern China, India, and the intervening countries ; 
 while the Australian region includes New Zealand, Polynesia, New 
 Guinea, and other islands northward. 1 
 
 Arctic Region. It is convenient to recognise, as does Joel Asaph 
 Allen, 2 a small arctic realm which unites the Palaearctic and Nearctic 
 regions. This area is characterised by the polar bear, 3 polar hare, 
 musk ox, arctic fox, lemmings, narwhal, white whale, and walrus. It 
 shares with the colder temperate region many seals, the reindeer, 
 moose, and some fur-bearing carnivora and rodents. 
 
 Palceardic Region. The Palaearctic region, or northern zone of 
 the Old World, includes Europe and Iceland, the Azores, Canaries, and 
 Cape Verd Islands, Africa north of the Sahara, Asia as far as Affghan- 
 
 1 Wallace : " Geographical Distribution of Animals." 
 
 2 Bull U. S. Geol. and Geog. Surv. Territ., vol. iv. p. 313. 
 
 3 It maybe useful to examine these types whenever possible in the Natural 
 History Collections of the British Museum, or other museums ; and in Zoological 
 gardens. 
 
462 TEMPERATE LAND LIFE. 
 
 isia.ii and north of the Himalayas, and the northern half of China and 
 Japan. This vast region, though distinguishable into warmer tem- 
 perate and colder temperate portions, is essentially one province, char- 
 acterised by the life ranging through it from the Pacific to the Atlantic 
 Ocean. Most of the familiar animals of Europe range through 
 Northern Asia ; and although the distribution of wild animals has been 
 much modified by man, the evidences of ancient distribution which 
 are furnished by the valley gravels of the north of Europe, enable us 
 to recognise a community between the North African life and the 
 European life which man has exterminated. Among the more familiar 
 mammals in this region may be named horses, asses, sheep, goats, 
 camels, the fallow deer, red deer, roebuck, ibex, chamois, addax ante- 
 lope, saiga antelope, hedgehog, mole, shrews, water shrew, desman, 
 vole, mole-rat, beaver, squirrel, and hare, with occasional monkeys, 
 flying foxes, bears, and the hyrax. There are 107 genera of mam- 
 mals in the region, 36 of which are peculiar to it. The Mediterranean 
 province includes 60 genera, the Manchurian province 65 genera, 
 while 50 genera are common to the two distant regions. Eighty 
 genera occur in the warm temperate portion, 50 in the cold temperate 
 portion ; 1 5 are universally distributed. The only family of birds 
 peculiar to the province is that represented by the bearded titmouse. 
 Of the 323 genera of birds, 128 are common to the Neartic region, 
 and 51 genera are common to the Indian and Ethiopian regions. 
 Thirty-seven genera are peculiar to the region, which may be grouped 
 as warblers, babblers, fly-catchers, finches, buntings, starlings, crows, 
 woodpeckers, sand-grouse, grouse, pheasants, duck, crane, plover, and 
 snipe. The reptiles and amphibia include vipers, the giant salamander, 
 the land salamander and proteus ; while among fresh- water fishes there 
 are salmon, trout, carp, pike, perch, sturgeon, and lampreys. 
 
 The Nearctic Region. As compared with the corresponding region 
 of the Old World, it is found that, while there is a similar richness in 
 the cold temperate region, the warm temperate region of North 
 America is poorer in life, in harmony with its smaller geographical 
 extent. It contains a total of 72 genera of mammals, of which 
 23 are peculiar to the region. But the correspondence between the 
 Palaearctic and Nearctic regions is found chiefly in the north tem- 
 perate parts, and is indicated by such animals as dog, wolf, wolverine, 
 sable, weasel, otter, bear, seal, deer, sheep, bison, shrew, squirrel, 
 vole, hare ; while the characteristic genera include urocyon, the 
 skunks, cariacus, mazama, antilocapra, scalops, scapanus, neosorex, 
 blarina, cynomys, haplodon, zapus, geomys. Only one family of birds, 
 the chamyaeidae, is peculiar. The Nearctic birds include 330 genera, 
 of which 24 are peculiar to the region, among which are gulls, ducks, 
 snipe, woodpeckers, pigeons, falcons, crows, &c. Among the reptiles 
 is the salt-water terrapin and the genus kinosternon, though some 
 species range to Central America, and include the alligator, terrapin, 
 chelydra, the platypeltis of Fitzinger, and the Mississippi alligator. 
 Among fishes there are fewer cyprinoids than in the Palaearctic region. 
 Sticklebacks are as numerous as in Europe, and the pike is well repre- 
 
TROPICAL LAND LIFE. 463 
 
 sented. Sturgeons and lampreys occur with the ganoid fishes amia 
 and lepidosteus. The latter is found fossil in Europe. 
 
 The Neotropical Region is limited towards the north by the 22d 
 parallel of north latitude. It includes the Galapagos Islands, the 
 Falkland Islands, and West Indies. Mr. Allen subdivides it into two 
 parts, tropical and temperate. The southern temperate area includes 
 the family of Cliinchillidae, the genera Auchenia, Chlamadophorus, 
 Myopotamus, Octodon, Ctenomys, &c. The spectacled bear is con- 
 fined to it. It is divided into Andean and Pampean portions. The 
 tropical part of the region includes three districts Central American, 
 Antillean, and Brazilian. This region includes all the American 
 monkeys, howlers, woolly monkeys, spider monkeys, the capuchins, 
 the marmosets, all the American edentata, such as the ant-eaters, 
 armadilloes, and sloths, many of the American felidse, and many rodents 
 and bats. The birds are more than half passerine. The chief group 
 is the humming-birds, which comprises 120 genera, of which only 5 
 are found in the Nearctic region. Other characteristic birds are the 
 rheas, tanageis, the piculules, the ant-thrushes, together with toucans, 
 trumpeters, screamers, &c. The region is rich in reptiles. There are 
 the great Galapagos tortoise, the matamata, and hydromedusa; the 
 jacare. Among lizards, the ameiva, cnemidophorus, phrynosoma, 
 teius, heloderma, iguanas, the marine amblorhynchus ; and rattle- 
 snakes, which, however, also range farther north. The fishes all belong 
 to nine families, and mainly to the Sihmdse (54 genera), Characinidaa 
 (40 genera), Chromides, and electric eels. There is a close relation- 
 ship to the Ethiopian fishes ; and one Indian species occurs which is 
 not found in Africa. 
 
 Ethiopian Region. All Africa, south of the Sahara, and Mada- 
 gascar, are grouped as the Ethiopian region. Mr. Allen would unite 
 with it the Indian region, but although a connection of a remarkable 
 kind between these regions is evidenced by the range of many genera 
 of mammals, birds, &c., we are disposed to regard the connection as 
 one rather belonging to the later Tertiary time than to the existing 
 order of nature. The African region, compact as it is, includes eastern, 
 western, and southern sections. The following types are characteristic 
 of the African region : The aard wolf (Proteles), hippopotamus, 
 rhinoceros, the wart hogs (Phacochcerus), giraffes, hyrax, the lemurs, 
 golden moles, jumping shrews, African river shrews, lophiomys, and 
 the Cape anteater. Besides these may be mentioned the anthropoid 
 apes, gorilla and chimpanzee, a multitude of antelopes, and various 
 rodents. In the eastern province there are twelve peculiar genera ; 
 thirty-nine are restricted to Ethiopia, but range over more than one 
 division of the province ; thirty also occur in the Indian region. The 
 proportion of Indian species is as well marked in the south and west. 
 The birds are as distinctive; and, like the mammals, indicate a close affi- 
 nity with Indian types. The ostrich helps to bridge over the geographical 
 interval between the African and Indian regions, and the soft tortoises 
 of the Old World are chiefly divided between Africa and the Indian 
 region. The true crocodiles of both provinces may be referred to the 
 
464 LIFE OF THE ORIENTAL REGION. 
 
 same genus. The monitors are best known from their African repre- 
 sentatives, hut range over India, some, indeed, reaching to Australia ; 
 and the chameleons, through having their home in Madagascar, and 
 spreading over Africa to the south of Spain, are found in the south 
 of India and Ceylon, reaching to Singapore. Fishes, like the other 
 groups, pass from the African region by the valley of the Nile to 
 Syria and South- Western Asia. Dr. Gunther states that of the 
 39 groups of fresh-water fishes 15 are represented in the African 
 region, though the number of species at present known is small. The 
 best represented families are the Siluroids, Cyprinoids, Mornugridse, 
 Characinidae, and Chromides. Many of the Siluroid genera, such as 
 Clariina, Silurina, and Bagrina, are common to India, and a few to 
 South America. One species of the genus Arius is common to Indian 
 and African rivers, which is the case with the Cyprinoid fish Discog- 
 nathus. Of ganoid types tropical Africa yields the Lepidosiren and 
 Polypterus. The affinity with South American fishes is chiefly marked 
 by the Chromides and Characinidae, though for the most part the 
 generic groups are distinct. 
 
 Oriental or Indian Region. This region includes India, China 
 south of the Yang-tse-Kiang, the Malay Peninsula, the Malay Archi- 
 pelago, Philippine Islands, and the islands as far as " Wallace's line," 
 which excludes Lombok, Celebes, and the islands south and east of 
 them, but includes the island of Bali. It is continued north-east up 
 the Strait of Macassar. In this region, in the continental part of 
 the area, there are 94 genera of mammals, of which 43 are peculiar to 
 the Indian region, and 28 common to the African region. The 
 Malayan or insular part of the province comprises 83 genera, of which 
 25 are restricted to the province, 52 are otherwise restricted to the 
 Indian region, and 29 are common to India and Africa. ' Each of the 
 larger islands has one or two types peculiar to itself. Among the 
 characteristic genera are the monkeys Hylobates, and Semnopithecus 
 and Macacus ; Paradoxurus, many deer, a few antelopes, chevrotains 
 of the genus Tragulus, rhinoceros and elephants, civets, tigers, leopards, 
 bats and flying foxes, the scaly anteaters, and a tapir. In common 
 with Africa are the manis, porcupines, buffaloes, elephants, civet; 
 while many genera, like bears, deer, goat, pig, hedgehog, mole, shrew, 
 are common to the Palaearctic region. The birds include in the con- 
 tinental area 116 genera, of which 32 are common to the Palseartic 
 region, though of course the community of genera does not extend to 
 species. Among the common types are fowls and pheasants, pigeons, 
 parrots, hornbills, woodpeckers, and sun-birds, the parrots and wood- 
 peckers becoming more numerous in the insular part of the province. 
 Besides the reptiles mentioned as common to the Ethiopian region, 
 there are the shield-tailed snakes, the thorn-tailed uromastix, the 
 dragons, the gavial ; and here, as in the Ethiopian region, salamanders 
 are wanting. The fresh-water fishes of India are found extending 
 through Persia to the Tigris. Twelve families are represented ; the 
 species constitute two-sevenths of the known fresh- water species, of 
 which Siluroids comprise 200 species, and Cyprinoid fishes about 330 
 
LIFE OF THE AUSTRALIAN REGION. 465 
 
 species. Barbels are common to the Palsearctic region, as are the 
 Mountain Barbels (Schizothorax). The Siluroid genera Clarias and 
 Heterobranchus are common to Africa and India. The genus Syni- 
 branchus is common to India and tropical America. 
 
 Australian Region The Australian region is limited to the north 
 by " Wallace's line," a strait 15 miles wide. The mammals are chiefly 
 found in Australia, Tasmania, New Guinea, and neighbouring islands. 
 They are mostly marsupials and monotremes, with bats and small 
 rodents. The monotremes comprise the Platypus, Echidna, and 
 Trachyglossus. The marsupials are almost limited to this region. 
 They parallel most of the orders of placcntal mammals the wombat, 
 Phascolomys, representing the Rodents, the Bandicoot Perameles and 
 Myrmecobius being Insectivores ; the grass-eaters being paralleled by 
 kangaroos and kangaroo-rats ; while the Tasmanian types, Thylacinus 
 and /Sarcophilus, with Dasyurus, represent carnivora; and the Pla- 
 langers and Phascolarctos have been termed marsupial monkeys. 
 Among birds there are nearly twenty families peculiar to the region. 
 These include honeysuckers, thickheaded shrikes, caterpillar-eaters, 
 flower-peckers, and swallow-flycatchers ; while the weaver birds, more- 
 porks, kingfishers, and pigeons attain their maximum development in 
 the province. There are many Ratitse. Birds of paradise are found in 
 the Papuan and Moluccan areas. In Australia are many parrots, bower 
 birds, lyre birds ; while the New Zealand region is chiefly remarkable 
 for its living kiwi and extinct moas. The Australian lizards include 
 the frilled lizard Chlamydosaurus, and the species of Moloch, which 
 closely resemble Phrynosoma, the horned lizard of Mexico. The 
 Hatteria is distinctive of New Zealand. The fishes are few, mostly 
 of Indian types in the tropical area, with Ceratodus for the most 
 interesting of its genera. In the south there are no fishes in the Cod 
 family, while New Zealand has 6 genera. 
 
 Method of Palseontological Work. Dumont, one of the greatest 
 of physical geologists of the last generation, had a strong belief that 
 palaeontology was superfluous for geological work. It may be that it 
 has sometimes been used with too much dogmatism and too little 
 learning, but it is as certainly a science as physical geology, and 
 absolutely inseparable from it. Every induction of palseontology is 
 based upon evidence from the purely physical conditions of strata. 
 Thus we meet with the genus Cyprina in the Lower Greensand, 
 represented say by the species Cyprina angulata ; we find it again 
 in the Thanet sands, represented by Cyprina Morrisi and Cyprina 
 planata, though these range on into the London clay ; we meet with 
 it next in the Cyprina Islandica of the crag, which is also a sandy 
 deposit ; and we find this species widely distributed in the seas of 
 the north of Europe and North America, so that we are justified in 
 inferring that Cyprina has always lived along a line of coast, and pro- 
 bably in water of no great depth. But although we may examine 
 every genus in this way, and compare the species so as to satisfy 
 ourselves concerning their descent from each other, and to determine 
 the physical conditions under which they lived, the chief business of 
 
 VOL. I. 2 G 
 
466 PHYSICAL GEOLOGY AND PALEONTOLOGY. 
 
 the palaeontologist is with faunas as a whole. And he can only dis- 
 cover whence they came and whither they went in the ages of geolo- 
 gical history, by first knowing the physical history of given geological 
 deposits, and then considering the ways in which the changes in 
 neighbouring seas modified the life in the sea which the geological 
 stratum represents. 
 
 We may illustrate these more complex studies by reference to the 
 Lower Tertiary strata. First, we find that, in tracing the Cretaceous 
 beds eastward, we come upon evidences of ancient shores and land 
 conditions. The Maestricht strata, though formed in shallower water, 
 may be no newer than the Norfolk chalk, and the chalk of the Danish 
 Archipelago and Scania may be essentially a prolongation of the same 
 deposit under conditions nearer to land ; but at Aix-la-Chapelle the 
 cretaceous sands, which may well be of chalk age, contain the rich 
 flora of an ancient land. Hence, then, without going further for other 
 evidence of a like kind, we may conclude that in certain parts of 
 Europe lacustrine and littoral strata were formed contemporaneous 
 with the English chalk. Now, if we believe these cretaceous islands 
 to have existed throughout the Chalk period, any change of level 
 which once more brought sediments into the seas where the chalk was 
 formed, must be supposed to have enlarged those islands, and have 
 made them a source for sands, such as constitute the oldest of English 
 Tertiary strata. 
 
 The lowest of our Tertiary beds is named Thanet sands. JSTinety- 
 nine per cent, of the sand, according to Dr. Sorby, is granitic sand ; 
 the one per cent, is derived from chalk flints. Hence, we must pro- 
 bably look to find a range of crystalline rocks as the source of the 
 Thanet sands. We next notice the area over which the stratum is 
 distributed. It is not found west of a line drawn from St. Alban's 
 to Leatherhead. It is thinner on the northern outcrop of the London 
 basin than on the southern outcrop, where the thickness is 90 feet 
 at Canterbury, Erith 75 feet, Charlton 55 feet, Bank of England 40 
 feet, Battersea 1 7 feet, Leatherhead 3 feet. If we cross the Channel 
 the thickness is 90 feet at Calais, and in Belgium the stratum is well 
 represented by the Landanien inferieur, which becomes somewhat 
 coarser in its eastward extension. There is hence a strong probability 
 that, as the sand was derived from a granitic source, it would be suc- 
 ceeded, farthest from the source in horizontal direction, by a clay. If 
 the antecedent circumstances of the Tertiary rocks justify us in looking 
 to Central Europe for the rocks which yielded the Thanet sands, then 
 we should expect, when the sands thin away westward, to find that 
 the clay which replaces them would be of the same age, that is to 
 say, that a clay would rest upon the chalk in the western part of the 
 London basin and in the Hampshire basin. To the Thanet sands 
 succeed the Woolwich and Beading beds ; they are the Landanien 
 supe"rieure of Belgium. They are marine sands in East Kent, but 
 from Rochester westward give evidence of nearness to land in the 
 mixture of fresh-water shells like Neritina, with estuarine types like 
 Melania and Cerithium, with marine genera like Ostrea ; and at Lewis- 
 
RELATION OF STRATA TO SUCCESSION OF LIFE. 467 
 
 ham fossil leaves in good preservation indicate nearness to land. This 
 estuarine condition is found as far south as Newhaven and Arundel, 
 and extends into France, indicating that an estuary occupied much of 
 the region which is now the Weald, and to this source we are disposed 
 to attribute the constant change in mineral character of the unfossili- 
 ferous sands and mottled clays which form the western part of the 
 deposit. Hence we should not regard the marine condition of the 
 beds as graduating into the estuarine, lacustrine, and fresh-water con- 
 dition of the deposit, but rather that sediments derived from different 
 sources, and bringing different materials, overlapped each other. The 
 succeeding Oldhaven beds are most instructive on this point, for in 
 East Kent they consist of sands with small flint pebbles ; and as the 
 beds are followed westward to Blackheath, the perfectly rounded flint 
 pebbles reach a thickness of 40 feet. There can be no doubt that the 
 chalk was denuded to form these pebbles, and that the rounding of 
 the pebbles furnished much flint sand, such as occurs irregularly in 
 the Woolwich beds at Reading. There is therefore reason to think 
 that the Oldhaven beds are an extension eastward of the upper part 
 of the strata termed Woolwich and Reading series in the Hampshire 
 basin, and that they were accumulated, as Mr. Whitaker suggests, as 
 a shoal at some distance from shore, but, as we believe, at the mouth 
 of an estuary, which came through France by way of Rheims. Next 
 succeeds the London clay. This brings us, as Professor Prestwich 
 first pointed out, a succession of new faunas, which are at least four 
 in number, in the eastern part of the London basin, but which are not 
 separated from each other in the same manner in the western part of 
 the Hampshire basin. The superposition of the London clay on the 
 Oldhaven beds indicates, first, a depression of land, which removed 
 from the British region the evidences of shallow-water deposits ; but 
 we find them persisting longest towards the south in the Bognor rock 
 and lower London clay sands, showing that it was from the French 
 direction that the underlying sands and pebble beds had been derived. 
 After the old land of Central Europe had thus become modified, and 
 sent down to the sea its mammals, birds, emydians, and crocodiles to 
 become mixed with the sea-serpents and chelonian reptiles of the 
 shore, the process of depression continued, bringing an ever-different 
 assemblage of species into the British area of the London clay sea ; 
 and then upheaval brought back again the crocodiles and fruits, until, 
 after a second depression and renewed upheaval, the old river, now 
 grown larger or brought nearer, swept a luxuriant flora into the clay 
 of what is now Sheppey and Belgium. The upheaval continuing, 
 brought back once more the condition of dry land to the south, super- 
 imposing sands upon the London clay, in which were fossilised the 
 leaves of such trees as yielded the fruits of Sheppey. These physical 
 conditions, here stated in brief abstract, not only govern the distribu- 
 tion of life, so as to explain why the fossils change with the successive 
 strata, but serve to demonstrate the relative differences of distribution 
 in depth of marine life preserved in the successive strata. The Thanet 
 sands yield a littoral fauna, but the fauna of the London clay is that of 
 
468 GEOGRAPPIICAL DISTRIBUTION OF PLANTS. 
 
 a rather deeper sea. If sands are superimposed upon the London clay, 
 they cannot be expected to bring back again the fauna of Thanet 
 sands, any more than a clay newer than the London clay could be 
 expected to repeat the London clay fauna, for the reasons which have 
 been shadowed forth in stating the laws of upheaval of land. The 
 palaeontologist may suspect physical changes which the geologist has 
 no proof of, but more frequently his task is to demonstrate the events 
 which stratigraphical conditions exemplify ; and in any case he must 
 work out the physical history of the deposits before co-ordinating the 
 faunas. 
 
 Distribution of Plants. If plants are to be used as an instru- 
 ment for research by which to investigate physical mutations and 
 revolutions in the distribution of land and water, it is impossible to 
 neglect their present geographical distribution. For important as are 
 temperature, moisture, and soil in governing the diffusion of plant life, 
 when it has reached a region of land, it is to geological changes upon 
 the earth's surface that we look for the only agent which was capable 
 of severing floras by the upheaval of mountains and deserts, or by the 
 depression of seas ; while the upheaval of land alone blends floras 
 which seas previously divided. The changed distribution of plant 
 life which the strata make known, lays before us some of the stages 
 by which the existing floras have been distributed, denned, and 
 evolved. 
 
 There are a few water plants like Potamogeton and Ceratophyllum 
 demersum which range almost from pole to pole. Other species, like 
 the Pteris aquilina, range from Lapland and the upper limits of culti- 
 vation to the banks of the Amazon. Few genera have a world- wide 
 range; but among the most widely distributed are Lotus, Rubus, 
 Plantago, Typha, Oxalis, Nasturtium, Gnaphalium, and Senecio. But 
 for the most part the distribution of plants is limited by the distribu- 
 tion of land, and denned by climate, so that zones, such as are named 
 tropical, temperate, and arctic, may be characterised by floras. 
 
 Bentham l suggests that the great botanical and zoological regions 
 probably coincide in general disposition, but that the plant regions are 
 older than the animal regions. There is first a northern type, secondly 
 a tropical type, thirdly a southern type. The northern type is espe- 
 cially marked by its needle-leaved conifers, Arnentacese, Eanun- 
 culacese, Cruciferae, and Trifolieas. It spreads over Europe, Northern 
 and Central Asia, and a part of North America, but has long been 
 divided by the Atlantic and Pacific Oceans. Asia has preserved 
 American types in Japan, Manchuria, and the Himalayas, occasionally 
 with identical species, but chiefly with representative species. 
 Astragalus is equally common in both continents. But other genera, 
 like Aster, Flox, Solanum, Eupatorium, are more numerous in 
 America than in Eastern Asia, dwindling towards Europe. On the 
 other hand, the European and Asiatic genera of CruciferaB, Caryo- 
 phyllea3, Lotese, and UmbelliferaB, have but few representatives in 
 America. In the pre-glacial period the northern flora extended far 
 1 See all Bentham's addresses to the Linnaean Society, especially 1869. 
 
ARCTIC AND NORTHERN FLORAS. 469 
 
 north in America, and was driven southward as the glacial cold came 
 on. The northern flora is found in the mountains of Tropical Asia 
 as far south as India ; in the Abyssinian and Cameroon Mountains in 
 Africa ; in the Andes, to the extreme south of America ; and in a less 
 striking degree in the mountains of New Zealand, Tasmania, and 
 Victoria. This large flora has been divided by climatic zones into the 
 Alpino- Arctic, the cool temperate, and the Mediterraneo-Caucasian or 
 warm temperate floras. 1 
 
 The Arctic Flora is very uniform. The birch is cultivated at 
 Reykjavik, but the summer is too short for its growth in Greenland ; 
 yet, owing to the influence of the continental mass of land, trees 
 flourish in Siberia as far north as 67 30'. On the eastern border of 
 the White Sea, lat. 65, peonies reach a large size, and aconites grow 
 equally well in the peninsula of Kola, lat. 64 ; while on the seashore 
 the smallest willows and herbs are found. Owing to the cold, the 
 roots of Arctic plants do not penetrate deep, but spread horizontally ; 
 thus the Valerian capitata and Salix lanata root differently in Kussia 
 and the Arctic regions. The fossil plants of the Arctic regions can in 
 no way be regarded as the progenitors of those now met with, for all 
 existing Arctic plants are stunted and have relatively large leaves. 
 Iceland possesses 450 vascular plants ; the west coast of Greenland, 
 323 ; Spitzbergen, 83.2 
 
 The Northern Flora. In the western part of the Old World the 
 northern and tropical floras are widely severed by the great mountain 
 barrier which runs east and west, as well as by the African and 
 Arabian deserts. No tropical forms of vegetable life appear to have 
 crossed these northern barriers. In Central Asia the floras come 
 nearer together, and the Himalayan chain alone parts these types of 
 life, a few representatives of the tropical group extending into the 
 warmer valleys on the one side just as the northern life is found ex- 
 tending on the other side. In the extreme east this mountain chain 
 declines, so that there is no boundary dividing the two floras, and the 
 change from one to the other is gradual, so that in the Chino- 
 Japanese region the northern and southern types are blended, and 
 they form a distinct flora which has much in common with the 
 American flora. Thus the genera of eastern North America, Illicium, 
 Schizandra, Menispermum, Caulophyllum, Thermopsis, Podophyllum, 
 Amphicarpaea, Apios, Penthorum, Hamamelis, Dieroilla, Phryma, 
 Pyrularia, are represented in North-eastern Asia. 
 
 In Europe the growth of wood begins in May and ends in August, 
 but in Siberia the larch has leaves for ten weeks only. On the Alps 
 it grows to a greater height than any other tree. The beech marks 
 climatic conditions in a striking way by its northern distribution. In 
 Norway it reaches latitude 59, passes the Swedish coast near Gothen- 
 burg, and crosses the continent from near Konigsberg to Podolia. It 
 
 1 Grisebach : "Vegetation der Erde," 1872. 
 
 2 Many of the plants mentioned may be studied in any botanical garden, but 
 the collections at Kew should be consulted. 
 
470 FLORA OF NORTH AMERICAN FOREST REGION. 
 
 reappears in the Crimea and the Caucasus, but is absent from Russia, 
 except in the western provinces. 
 
 The flora of France passes by gradual changes to that of Kams- 
 chatka, which reproduces closely the plant life of Northern and 
 Central Europe. In Kamschatka the birch is dominant, and 
 there, as in Northern Scandinavia, there is a belt of fir and larch 
 in the middle of the peninsula, while the same species of berry- 
 bearing plants occur in the forests; and the marshes contain the same 
 dwarf willows. Thirty per cent, of European species of vascular 
 plants occurs in Siberia, while only 15 per cent, of the indigenous 
 plants of the N.E. of the United States are identical with those of 
 Europe. The northern limit of the birch nearly coincides with trees 
 with acicular leaves. In Siberia the larch advances beyond this limit, 
 for it begins to grow at a lower temperature than other leafy trees. 
 The growth of wheat coincides with limit of the oak, which forms 
 a large forest belt in Russia from the Gulf of Finland towards the 
 Steppes, but the Ural Mountains, low as they are, stop it from pene- 
 trating further. Grisebach thinks it doubtful whether the forest 
 areas of the Palsearctic region can be sub-divided into restricted natural 
 floras. 
 
 North American Forest Region. According to Dr. Asa Gray, 1 
 the true divisions of the forest region are those formed by the 
 Atlantic, the Yabloni Mountains, the Pacific, and the Sierra 
 Nevada of North America. Of forest trees Europe has 7 genera 
 and 17 species Coniferous; Non-Coniferous, 26 genera and 68 
 species. But Europe wants the characteristic types of the great 
 Appalachian land, now forming the American Atlantic States. It 
 has no Magnolia, Liriodendron, Asimina, Negundo, or ^Esculus. 
 It wants the characteristic .leguminous trees known as locusts, 
 honey locusts, Gymnocladus and Cladrastis. It has no Nyssa, 
 or Liquidambar ; no tree referable to the Ericaceae ; no Bumelia, 
 Catalpa, Sassafras, Osage Orange, Hickory, or Walnut ; and no 
 ConifersB referable to the hemlock spruce, Arbor vitse, Taxodium, 
 or Torreya. Most of these types have lived in Europe in past 
 Tertiary periods of geological time. The Miocene flora, as Unger 
 demonstrated, has remarkable resemblances to that of North America. 
 This led him to argue that the sunken island of Atlantis of Plato 
 might have been a geological reality, and although we are not 
 pledged to this hypothesis when the elevation of land to the north of 
 Europe or between America and Asia offers more probable alternatives, 
 we still must admit that the existing forest vegetation of North 
 America is a survival of the Miocene forests. This American 
 flora occurs fossil round the North Pole, in Greenland, Iceland, 
 Spitzbergen, and includes with the Magnolias, Hickories, Sassafras, 
 and Southern Cypress, Californian trees like the Sequoias, and others, 
 now peculiar to Japan and China, like the Ginko trees, associated 
 with pines, maples, poplars, birches, and lindens, which Asa Gray 
 accepts as ancestors of the American temperate flora. The vegetation 
 1 Bull. U.S. Geol., and Geogr. Survey Territ., vol. vi. p. I. 
 
FLORA OF THE TROPICAL ZONE. 471 
 
 of the Pacific side of the country is altogether different from the 
 Atlantic side. The forest trees of the Rocky Mountain region 
 include about 50 species, many of which are tree-like shrubs. 
 Reaching into the Mexican region are the Yucca brevifolia, the giant 
 Cactus, a species of Pinus, Arbutus, Fraxinus, Platanus, and 
 Quercus. A desert willow, Chilopsis, fringes the watercourses. A 
 Texas mulberry, Morus, extends into Arizona. Among conifers the 
 yellow pine, Pinus contorta, covers large areas. It is associated with 
 various other pines, such as the nut pine, fox-tail pine, white pine, 
 the Douglas spruce, and a species of Juniper known as red cedar. 
 The mountain mahogany, Cercocarpus, is peculiar to the great basin, 
 but the ash-leafed maple, which yields sugar from the sap, reaches 
 to Canada and New England. The poplars, generally known as 
 Cotton-woods and Balsam poplars, rather belong to the arid regions, 
 where streams issue from the mountains. Oaks are conspicuously 
 absent. 
 
 Tropical Zone. The tropical type of life was separated at an an- 
 cient period into American and Asiatic portions. It is characterised by 
 its gigantic Monocotyledons and Arborescent Polypetalse. 1 Some of the 
 palms of the Amazon have a height of one to two hundred feet, and 
 some, like Raphia and Maximiliana, have leaves more than 50 feet long. 
 The distinctive features of tropical forests are the tall trunks, with a 
 crown of foliage shutting out the light, and descending aerial roots 
 like buttresses. The silk-cotton trees are thus characterised. Below 
 the tall forest trees is a growth of lower trees, rising to 40 or 50 
 feet, and below these dwarf palms, tree ferns, and herbaceous ferns. 
 The ground is sometimes carpeted with club mosses and small 
 flowering plants. The palms include creepers like Calamus, which 
 grows to a length of thousand feet, and yields the rattan. The rattan 
 canes abound in the Malay Archipelago. One of them yields the 
 dragon's blood ; another palm yields the sago ; the Arenga or sugar 
 palm, and the Areca are both Malayan types. Other characteristic 
 tropical forms of plant life are mangroves, bamboos, screw pines, 
 bananas, arums, sensitive mimosas, and among orchids the Oneidiums 
 of the flooded Amazon, the Cselogynes of the swamps, and the 
 Cattleyas, of the drier forest ; the ginger-worts, which produce 
 ginger, cardamoms, grains of paradise and turmeric, begonias, vanilla, 
 and a multitude of ferns. Among the characteristic trees many have 
 the trunk covered with flowers, such as the cocoa tree, and Polyalthea, 
 one of the custard apples in Borneo. Tropical trees yield the drugs 
 toulu, camphor, benzoin, catechu, cajuput oil, ' gamboge, quinine, 
 angostura bark, quassia, the urari poison, and the upas poison. 
 Among spices are cloves, cinnamon, and nutmeg. Among fruits^ 
 brazil nuts, tamarinds, guavas, cocoa, bread-fruit, the avocado pear, 
 custard apple, durian, mango, mangosteen, soursop, and papaw, which 
 are all exogenous. Among the tropical exogenous woods are 
 mahogany, teak, sandal wood, and trees which yield log wood, brazil 
 wood, sappan wood, iiidia rubber, gutta percha, gum tragacanth, gum 
 1 Wallace, "Tropical Nature," 1878 ; and Henfrey's "Botany," by Masters. 
 
472 FLORA OF THE SOUTH TEMPERATE ZONE. 
 
 damar, copal, and lac. The abundance of ferns is so great that 
 several localities might be mentioned with 250 to 300 species in each. 
 
 Tropical Africa is a well-defined botanical region, separated from 
 the Mediterranean region by the Sahara desert, and divided from the 
 Cape region by the dry district north of the Gariep. It is only 
 along the eastern side of the continent that the tropical African flora 
 becomes blended with the South African flora, just as there is a Euro- 
 pean character in the high mountain vegetation of Abyssinia and the 
 Cameroons. The tropical American races found on the coast region of 
 Western Africa are more frequently identical species than repre- 
 sentative species. It is especially to be remarked that the tropical 
 African flora interchanges many species with continental India, 
 but while this has long been admitted for regions north of the 
 equator, there are many distinct African types south of it, which 
 have representatives in Madagascar, Ceylon, Malacca, the Malay 
 Archipelego, and Australia. North Australia contains Mangroves, 
 Palms, Screw-pines, Bamboos, and numerous Orchids, and the Arau- 
 carias reach to just within the tropic. 
 
 Southern Zone. The Southern type of vegetation shows its original 
 continuity by being abundant in Kestiaceae, Protacese, Ericaceae, single- 
 leaved Papilionacese and Diosmeae, as well as by the exclusive abundance 
 of Myrtaceae in the Australian portion. This southern region comprises 
 four well-marked floras first the Andean, with Fuchsia, Calceolaria, 
 Gautheria, Ourisia, which range along the whole chain, and penetrate 
 through Western America, and even reach Eastern Asia on the north, 
 while on the south the Andean genera, Caltha, Drimys, Colobanthus, 
 Acsena, Eueryphia, Fuchsia, Gunnera, Azorella, Huanaca, Crantzia, 
 Abrotanella, Pernethya, Gaultheria, Ourisia, are, represented in New 
 Zealand, Tasmania, or Victoria. 
 
 A warmer southern temperate vegetation is found in Mexico and 
 California and in the Argentine States, and this type is prolonged 
 into Southern Africa and Australia. The Mexican or California 
 Microlotus, Hoffmanseggia, Strombocarpus, Melasma, Chorizanthe, 
 Oxytheca, are represented in extra-tropical South America. Asa 
 Gray and Hooker enumerate about 90 genera, with almost half the 
 species common to the two regions. The Australian flora is another 
 type which has some connection with New Zealand on the one hand, 
 and extends northward into Borneo and the Indian Archipelago on 
 the other. The tropical African genera, Cadaba, Cochlospermum, Poly- 
 carpea, Adansonia, Melhania, Zygophyllum, Cassise pictse, Pterolobium, 
 Erythropha3lum are represented in Australia. Australia yields more 
 than 400 species of Eucalyptus and many leafless Acacias ; it abounds 
 in Proteaceae. A heath of the genus Apacris is abundant. The Aca- 
 cias and Eucalyptus are absent from New Zealand, where tree-ferns, 
 lycopods, and mosses abound, and the Kawri pine, Dammara aus- 
 tralis, is an important forest tree. Yet the New Zealand flora has 
 much in common with Australia; and 76 genera and 89 species are 
 said to be common to New Zealand and South America. 
 
 The South African flora is remarkably rich and distinct, showing 
 
FLORAS OF THE OLDER STRATA. 473 
 
 some connection with Australia on the one hand, and with extra 
 tropical South America on the other. "Western Europe has its Erica, 
 GenistesB, Lobelia, and Gladiolus, genera characteristic of the Cape, 
 while other European groups appear to have diverged from South 
 African stocks, but hardly any species cross the Khine or the Rhone, 
 or extend beyond Britain into Scandinavia. 
 
 Succession of Plant Life. Geological history presents, in so far 
 as it is at present known, three types of vegetation, which correspond 
 with epochs of time. First the Primary flora, mainly cryptogamic 
 and coniferous; secondly the Jurassic flora, including cryptogams, 
 coniferae, and cycads ; thirdly the Cretaceous and Tertiary floras in 
 which the living types of exogens and endogens preponderate. The 
 oldest known plants in Europe are described by Dr. Hicks from 
 the slates of Corwen, under the name Berwynia : it is supposed 
 to be Lycopodiaceous. Slightly older land -plants in the United 
 States have been named Glyptodendron. The Silurian rocks are 
 poor in plant remains, and the well-known Pachytheca from the 
 Ludlow bone bed is almost the only type they yield. But with the 
 Devonian age a remarkable group of coniferous trees with a central 
 pith is found, together with tree ferns, Lycopodiaceous plants of the 
 Lepidodendron type, Sigillaroid trees, and numerous Equisetacese ; 
 and this flora presents no essential modification during the Carboni- 
 ferous Period, so that, from the point of view of land vegetation, the 
 Devonian and Carboniferous rocks are united together by a character 
 which divides them from the Lower Primary series. 1 It is not till 
 we come to the Trias that a new life-province appears, as indicated 
 by the existing type of solid-hearted coniferae, some of which were 
 closely allied to Araucaria, mixed with a varied succession of Cyca- 
 daceous plants. Coniferse and cycads include 9 of the 1 2 genera, and 
 1 1 species of the flora of the Lower Lias ; but the Inferior Oolite, 
 especially in the estuarine shales and sandstones of the Yorkshire 
 coast, yields a richer flora, comprising 41 genera and 130 species, 
 among which are 53 species of ferns, 23 cycads, and 7 types of 
 conifers. A cycad is found in the Oxford clay, and Yatesia, 
 Bucklandia, and other cycads are associated with Araucarites. 
 A species of Pinites is found in the Kimmeridge clay. But the 
 Jurassic flora, though well preserved in some Indian and Australian 
 localities, is still too imperfectly known to permit of it having more 
 than a stratigraphical interest, for it is impossible to infer from it 
 either the descent of the plants or their geographical distribution. 
 
 With the Cretaceous Period, however, and the discoveries made at 
 Aix-la-Chapelle by Dr. Debey, 2 we meet with a new phase of plant 
 life. Pandanus had already yielded a representative in the Lincoln- 
 shire limestone, and other Pandanaceous fruits had been found in the 
 Neocomian beds, \vhere Mr. Carruthers has also described the fruits 
 of conifers. The Gault, of Folkestone, has yielded cedars, and a species 
 of Sequoites and Pinites. The Cretaceous floras are remarkable for 
 
 1 For enumeration of these floras see vol. ii. of this work. 
 
 2 See Lyell, "Elements of Geology," Sixth Ed., 1865, p. 330. 
 
474 CRETACEOUS AND TERTIARY FLORAS. 
 
 their diversity. This is especially seen on comparing the plants of 
 this epoch from the following localities Greenland, Neidershoena in 
 Saxony, Moletin in Moravia, Quedlinburg and Blankenburg in the 
 Hartz, Halden in Westphalia, the sands of Aix, the Senonien of 
 Bausset, the Santonien of Fuveau in France, the North American 
 cretaceous flora of Nebraska, 1 and the Gelinden beds of Belgium, 
 which latter make a transition to the Tertiary. 
 
 Thus the existence of floras well defined from each other is per- 
 haps more evident in the Cretaceous period than in any other epoch of 
 time. The Aix flora is said to include some 200 species, of which 
 67 are cryptogams, chiefly ferns, including species of Gleichenia, 
 Lygodium, Asplenium, and 10 other existing genera. There are numer- 
 ous coniferae, some hardly separable from Sequoia, species of Araucaria, 
 rare cycads, and Pandanus, while the familiar forms of the oak, fig, 
 and walnut are associated with several genera of Myrtacese, and 
 between 60 and 70 species of Protaceae, some of which belong to 
 the living genera Dryandra, Grevillia, Hakea, Banksia, Persoonia, 
 all Australian, and Leucospermum, now found at the Cape of Good 
 Hope. The American genera found in the Dacotah group are less 
 numerous. Professor Leo Lesquereux enumerates Liquidambar, Salix, 
 Populus, Betula, Myriea, Celtis, Quercus, Ficus, Platanus, Laurus, 
 Sassafras, Cinnamonum, Diospiros, Aralia, Magnolia, Leriodendron, 
 Menispermum, Negundo or Acer, Paliurus, Rhus or Juglans, and 
 Primus, an assemblage which includes all the essential arborescent 
 types of the existing American flora, except those marked by serrate 
 or doubly-serrate leaves. Very few of the species in these rocks have 
 the leaves serrate. No one could recognise a great break in time 
 between the Cretaceous and Tertiary rocks on the evidence of such 
 floras as these. 
 
 The Lower Tertiary floras elaborated by linger, Heer, Ettingshau- 
 sen, and Mr. J. S. Gardner, F.G.S., are full of interest, as showing the 
 change in geographical distribution which took place with altered phy- 
 sical circumstances. We may illustrate this by citing the floras of 
 the London clay of Sheppey, and the immediately succeeding Bagshot 
 sands of Alum Bay. The Sheppey flora is chiefly known from the 
 fruits, that of Alum Bay chiefly from leaves. The Sheppey flora 
 comprises 72 genera and 200 species, of which 7 genera are 
 Gymnosperms, 18 genera Monocotyledons, and 43 Dicotyledons. 
 What at the present day would be termed a sub-tropical character, 
 is indicated by plants referable to the natural orders Musaceae, Pan- 
 danse, Cichonaceae, Loganiaceae, Sapotaceae, Ebenaceae, Biittneriaceas, 
 Sapindaceae, &c. Among the genera of the London clay are Sequoia, 
 Pinus, and Salisburia among conifers. The Agave, Sarsaparilla, Ban- 
 ana, Cardamoms, and Nipa which includes many species, are all found 
 on the north of Sheppey. There are about 15 palms, including 
 species of Areca, Iriartea, Livistona, Sabal, Chamaerops, Trinax. The 
 Oak and Walnut, Liquidambar, Laurel, Nyssa, and Euphorbiophyllum 
 
 1 U.S. GeoJ. Surv. Territ., vols. vi. and vii. Lesquereux, Cret. and Tert. 
 Floras. 
 
FLORAS OF SHEPPEY AND ALUM BAY. 475 
 
 are all present, with Cinchonidium, Strychnos, and Solanites. The 
 Ebony is represented by species of Diospyros. Magnolia, Lotos, and 
 Victoria Water Lilies, Melons, representatives of the Mallows, Soap- 
 worts, Eucalyptus, Cotoneaster, Almond, Plum, and a multitude of 
 leguminous plants, do not exhaust the list. When we compare this 
 flora with that of the Alum Bay clays, 25 genera are found to be in 
 common. The newer deposit is the richer of the two, including 
 representatives of 116 genera, and 274 species. Among the types in 
 common are Conifer, Sequoia, the Palm, Sabal, Oak, Convolvulus, 
 Walnut, Laurel, Cinchonidium, Magnolia, Lotus, Maple, Plum, 
 Almond, and the Australian Gum-tree Eucalyptus. 
 
 The types characteristic of the Alum Bay beds are more numer- 
 ous Oaks, Elms, the Beech, many species of Ficus, Santalum, which 
 yields the sandal wood, and Nyssa, the American sour gum tree. 
 Laurels abound, the Daphne and Aristolochia occur ; the Oleacese have 
 representatives of the Olive and Ash, and the Sapotacese are more 
 numerous than in Sheppey. The Verbena group, Convolvulus, Heaths, 
 Aralia, Saxifrages, white Water-lily, Spindlewood, Buckthorn and 
 Vines, Sumach, Pistacio, and many other interesting types are found, 
 besides numerous leguminous forms, among which Cassia is particu- 
 larly abundant. The Acacia and Mimosites are both represented. 1 
 
 This flora has much in common with the Middle Tertiary of Green- 
 land, Iceland, Spitzbergen, and Central Europe. And at the present 
 day, in passing from the Mediterranean to the Levant, Caucasus, 
 and Persia, we find representatives of Miocene genera such as 
 ChamEerops, Platanus, Liquidambar, Pterocarya, Juglaris, &c. Other 
 Middle Tertiary genera may be traced through the Himalayas, and 
 towards China ; and the Tulip tree. Liriodendron, common in the 
 Miocene time, is found in Central China as well as in North 
 America. Some Miocene genera, like Astragalus, are common to 
 both continents. Others, like Eupatorium, Aster, Flox, Solanum, 
 are more numerous at the present day in America than in 
 Eastern Asia, and diminish westward, reaching Europe in single 
 species. 
 
 Coal. Wherever vegetation has accumulated in swampy localities 
 necessary for its preservation, coal has been formed, and hence coal is 
 of every geological age. Its formation in the Carboniferous period, 
 and generally, was analogous to the growth of peat. Intercepted 
 drainage killed the forest trees in districts experiencing a temperate 
 climate, and, as in the English fens or Irish bogs, the stumps of 
 forest trees are found beneath the vegetable growth, which was itself 
 a soil for plants of many kinds, now imperfectly preserved. Spores 
 of coniferous trees furnished bituminous bands. Peat, like coal, 
 alternates with beds of clay. 2 
 
 1 Ettingshausen : Proc. Roy. Soc., vol. xxix. p. 388, and vol. xxx. p. 228. 
 
 2 Geological Magazine, vol. iii., Nov. 1866. 
 
( 476 ) 
 
 CHAPTER XXV. 
 
 THE SUCCESSION OF LIFE IN CLASSES AND ORDERS OF ANIMALS. 
 
 The Constituents of Faunas. When we turn from the theoretical 
 or inductive aspect of paleontology to its stratigraphical facts, the 
 duration in time of species and genera is found to be different in the 
 several groups of organisms. This may be a defect in our classification 
 by which characters of different chronological value have to be made 
 use of in distinguishing animal types in the several orders and classes. 
 But it may also be a consequence of the struggle for existence by 
 which the varying collocation of representatives of different groups of 
 animals favours the extinction of some, the unchanged survival of 
 others, and the varied development of special types of life in definite 
 periods of time. Absence of enemies, such as indicates to us undis- 
 turbed nutrition, is calculated to result in large physical growth and 
 abundant increase of individuals. And then the incoming of multi- 
 tudinous enemies upon races which have not been inured to the con- 
 flict is likely to exterminate such groups. Professor Owen has specu- 
 lated on the way in which the advent of a higher type of life 
 modifies the physical structure of a lower type, and is disposed to 
 attribute the changes which converted the ancient Teleosaurian type of 
 crocodile into the modern type of crocodile, as exemplified in larger 
 forelimbs and other distinctive characters of the skeleton, to the ad- 
 vent of mammalia, which furnished a new kind of prey. 1 When the 
 higher and lower types existed under similar conditions and as com- 
 petitors, the advent of the higher type, however it might stimulate 
 specialisation in the skeleton, would probably tend to arrest organic 
 development in the less specialised groups of lower organic grade. We 
 are disposed to look to this principle for an interpretation of the 
 circumstance that fishes and amphibians of the Primary rocks, and the 
 reptiles of the Secondary rocks, when compared with their nearest 
 living allies, approximate to higher types, and present a more varied 
 development than the surviving groups of the same class. Similarly, 
 the Tetrabranchiata have given place to the Dibranchiata ; and the 
 Entomostraca have not the dominance in later times which they had 
 in the Primary period. We are therefore disposed to urge that time 
 will not be misspent which is used in analysing a fauna into its 
 elements, in tracing the history of the representatives of the several 
 
 1 Q. J. G. S., vol. xxxiv. p. 426. 
 
THE MOST ANCIENT SPONGES. 477 
 
 Orders and Classes of which it is composed, and weighing the influ- 
 ence of these elements of faunas upon each other, in the light of such 
 knowledge as is available of the zoological functions of similar groups 
 of organisms in existing natural history provinces or regions. 
 
 We now present a brief abstract of the succession of life on the 
 earth in geological time, with a view to direct attention to some of the 
 principal elements in fossil groups of life. No attempt at numerical 
 estimates of individuals or species is made; and attention is only 
 drawn to some of the commoner generic types in the more abundant 
 Orders and Classes in which the palaeontologist may study the main 
 facts of the succession of life. 
 
 Many of the genera mentioned are only to be found in great public 
 collections like the museums of Universities, Colleges, and the British 
 Museum ; which are to the palaeontologist what a lexicon is to the 
 student of a language, and must be used for reference. In any case 
 it is to be desired that, before endeavouring to grasp the grand pro- 
 blems of distribution, evolution, and succession of life which the strata 
 unfold through their physical geology and their fossils, some such 
 elementary study should be gone through as is implied in the practical 
 examination of the generic types in the chief natural history groups 
 which we now enumerate. 
 
 Spongia. Sponges which occur fossil are necessarily siliceous or 
 calcareous. The siliceous sponges are chiefly hexactinellid, having 
 six-rayed spiculae, or lithistid, having four-rayed spiculae or an irregular 
 spicular structure. The spiculse are usually united together, and in 
 the lithistid sponge the mesh is generally dense. Siliceous spiculae 
 may be replaced by calcite, peroxide of iron, and iron pyrites. Sponges 
 are chiefly known from sandstones and limestones. They are com- 
 paratively rare in the Primary rocks, from which calcareous sponges 
 are unknown, if we except Peronella, which appears in the Devonian, 
 but is otherwise found only in the Secondary strata. The Monacti- 
 nellid sponges are represented in the Primary rocks, though the fossil 
 genera are few. 
 
 The oldest known sponge is Protospongia ; it belongs to the Hexac- 
 tinellid group and is found in the Menevian rocks. Other primary 
 hexactinellid sponges are species of Astylospongia, Palceomanon, Bra- 
 chiospongia, Dictyophyton, Plectoderma, Protachilleum, and the Lyssa- 
 kine Hexactinellid genera Astriovpongia, Hyalostelia, Holasterella, arid 
 Amphispongia. The Tetractinellids are represented in the Carbonifer- 
 ous rocks by the species of Geodia and Pachastrella. The Lithistid 
 sponges of the Primary strata comprise species of Doryderma, Hindia, 
 and Aulocopium. The MonactineHid sponges include Climacospongia, 
 Lasiocladia, Haplistion. 
 
 With the Secondary rocks the Monactinellid sponges become more 
 numerous ; the living genus Spongilla occurs in the Purbeck limestone, 
 but most of the types of this group are Cretaceous, among which may 
 be mentioned Dirrhopalicm, Acanthomphis, and Cliona ; the latter 
 ranging through the Tertiary deposits. The Tetractinellid sponges 
 are chiefly Cretaceous ; and among the genera which appear in the 
 
478 SUCCESSION IN TIME OF GENERA OF SPONGES. 
 
 Upper Chalk are Opliiraphidites, Ttthyopsis, Stelleta, Geodia, Thenia, 
 and Pachastrella. The Triassic rocks have only yielded calcareous 
 sponges, among which are Colospongia, Enoplocoelia, Celyphia, &c., 
 chiefly known from the St. Cassian beds ; but some genera, like 
 Eudia, range through the Jurassic, and some, like Peronella, through 
 the Secondary rocks. Other Lower Secondary genera are Corynella, 
 which sends species to the Upper Greensand, Eusiphonella, Lym- 
 norea, Inobolia, Stellispongia, Diaplectia. The Upper Secondary rocks 
 yield Synopella, Oculospongia, Masmostoma, Raphidonema, Phare- 
 trospongia, and Pachytilodia. 
 
 The Lithistid sponges are numerous in the Jurassic rocks of the 
 continent of Europe, though almost unrepresented in this country. 
 Among characteristic genera are Cnemidiastrum, Hyalotragos, Platy- 
 clionia Placonella, Melonella. The Neocomian period is barren, with 
 the exception of a species of Mastosia. The Upper Greensand yields 
 Chenendopora, which is also represented in the Chalk. Indeed, many 
 genera are common to these formations, such as Seliscothon, Jereica, 
 Doryderma, PJiymatella, Trachysycon, Siphonia, 1 Jerea. Other genera 
 characteristic of the Upper Greensand are Holodictyon, Pachypoterion, 
 Nematinion, Hdllirlwa, Kalpinella, and Rhopalospongia. Among the 
 genera limited to the Upper Chalk are Verruculina, Scytalia, Stachy- 
 spongia, Pachinion, Heterostinia, Aulaxinia, Callopegma, Nelumbia, 
 Bolospongia, Thecosiphonia, Thamnospongia, Pholidodadia, Ragadinia, 
 Plinthocella, and PJiymaplectia. 
 
 The Hexactinellid sponges are largely represented in the Jurassic 
 strata of the Continent. Among them are Tremadictyon, Sphenaulax, 
 Platyteichisma, Trochobolus, Cypellia, Stauroderma, and Porospongia. 
 Of genera common to the Oolites and newer Secondary series may be 
 mentioned Craticularia (which ranges up to the Chalk), Sporadopyle 
 (which ranges to the Neocomian), Verrucoccelia, and Toulminia (which 
 range to the Chalk). Plocoscyphia ranges from the Neocomian to the 
 Chalk. The Upper Greensand is characterised by some peculiar types, 
 and contains others which range to the Chalk. Among the former are 
 Brachiospongia, Eubrochus, and Sderokalia. Among the genera which 
 appear first in the Upper Greensand and range to the Chalk are Lepto- 
 phragma, Guettardia, and Ophrystoma. Genera limited to the Chalk 
 comprise Strepliinia (found in the Grey Chalk), Pleurostoma, Aphro- 
 lidllistes, Ventriculites, Rhizopoterion, Sporadoscinia, Plolyblastidium, 
 CephaUtes, Camerospongia, Callodictyon, Diplodictyon, Cceloptycliium, 
 and Stauractinella. 
 
 A few sponges have been described from Tertiary strata, but among 
 the Miocene genera of Lithistids Siphonia, Jerea, Chenenopora, and 
 Astrocladia reappear. 2 
 
 Dr. Hinde regards Eeceptaculites, Ischadites, and Sphaerospongia 
 of the Silurian and Devonian rocks as Hexactinellid sponges. 
 
 1 Sollas, Q. J. G. S., vol. xxxiii. 
 
 2 M any species of British sponges are figured by Dr. Hinde in his catalogue of 
 the fossil sponges in the Geological department of the British Museum, 1883, 
 which see. See also Zittel : Handbuch der Palceontologie (Miinchen u. Leipzig). 
 
GEOLOGICAL ANTIQUITY OF FORAMINIFERA. 479 
 
 Foraminifera. The Foraminifera are widely distributed in exist- 
 ing seas, occur at all depths, and in all oceans. Their living sub- 
 stance, though sometimes yellow or red, is of a white-of-egg-like 
 texture, is termed sarcode, and secretes shells, which are mostly micro- 
 scopic, and in many types are perforated for the passage of retractile 
 threads termed " pseudopodia." The group is subdivided into three 
 sections ; first, the PORCELLANEA or Imperforata, which consists of 
 solid white shell tissue ; secondly, the ARENACIA, which are built up 
 of grains of sand and other particles blended with their calcareous 
 coating ; third, the HYALINA or Perforata, which comprises the great 
 majority of fossil forms. All three groups are closely connected by 
 intermediate types. 
 
 The only Foraminifer yet determined in the Pre-Cambrian rocks 
 is the large incrusting Eozoon. There are none certainly identified 
 from the Cambrian rocks ; but with the Silurian strata we find the 
 existing genera Dentalina y Lagena, Nodosaria, and Textularia, which 
 all belong to the Hyaline section. The majority of Primary types 
 date from the Carboniferous period. Among such genera which still 
 survive are the porcellanous Cornuspira, the arenaceous Saccammina, 
 Lituola, and Trochominina, and the hyaline Spiroplecta, Valvulina, 
 Planorbulina, Pulvinulina, Calcarina, Amphistegina, and Nummu- 
 lites. Several of these types are at present imperfectly known in the 
 Secondary strata. Among genera which are peculiar to the Carbon- 
 
 Fig. %j.Nummulit'.s (Lower Tertiary species). 
 
 iferous period are Stacheia, Archcediscus, and Fusulina, which last is 
 also of Permian age. 
 
 With the Trias other recent types come in, such as Miliola, 
 Nubecularia, Dactylopora, Webbina, Bulimina, Lingulina, Frondicu- 
 laria, Marginulina, Vaginulina, Cristettaria, Planularia, Flabellina, 
 Polymorphina, and Globigerina. 1 With the Lias appear the por- 
 cellanous Orbitolites and the Hyaline Orbulina. With the Oolites 
 come in Verneuilina and Spirillina ; the only new Ncocomian genus 
 is Operculina. The Upper Greensand introduces Patellina, while in 
 the Chalk the new forms all belong to the Hyaline group. They 
 comprise Spiroplecta, Chrysalidina, Allomorphma, Ramulina, Pullenia, 
 Sphceroidina, Discorbina, Cymballopora, Rotalia, and Polystomella. 
 The extinct genera of the Secondary period belong to the Arenaceous 
 
 1 This genus has been quoted from Carboniferous rocks. 
 
480 GRAPTOLITES. 
 
 group. They include Involutina (found in the Lias and lower Oolite). 
 Endoihyra (found in the Lias), and Parkeria (in the Upper Green- 
 sand). Hauerina commences in the Gault and survives to the Middle 
 Tertiary ; Orbitoides, which resembles a Nummulite, commences in the 
 Chalk and survives to the Middle Tertiary; in which the Triassic 
 Fldbellina also appears to becomes extinct. Other extinct Tertiary 
 genera are Loftusia and Fabularia, of the Lower Tertiary. Elipsodina 
 characterises the Upper Tertiary. 
 
 Among the Foraminifera which appear with the Lower Tertiary 
 rocks are Vertebralina, Pe?ieroplis, Cheilostomella, Uvigerina, Tino- 
 porus, and Heterostegina. Some other genera appear with the Middle 
 and Upper Tertiaries. 1 
 
 Hydrozoa. The Hydrozoa are for the most part unknown in a 
 fossil state, though several jelly-fish are beautifully preserved in the 
 lithographic slate of Solenhofen, and are to be seen in the Munich 
 Museum. There are a few other forms like the Palceocoryne, which is 
 found attached to Fenestella in the Carboniferous rocks of Scotland. 
 
 The entire group of graptolites is fossil in the Cambrian and Silu- 
 rian strata. They have an axis, with cells arranged on one side or 
 both sides, and these axes are variously related to each other by modes 
 of growth. The simplest form is Graptolithus, or Monograptus, which 
 has a row of cells on one side of the axis. Didymograptus resembles 
 two individuals of Monograptus diverging from a common origin ; 
 Tegragraptus is the similar divergence of four ; and Dichograptus the 
 divergence of more than four graptolite-like forms from a common 
 centre. When the Graptolite grows in a spiral, with cells on one 
 side, it becomes Rastrites ; when it branches in a tree-like form, it 
 becomes Dendrograptus, with which may perhaps be connected the 
 ancient Oldhamia of the lower Cambrian, and the Dictyonema of the 
 Cambrian, Silurian, and Devonian rocks. 
 
 Diplograptus has a row of cells on each side of a straight axis like 
 two simple graptolites back to back. Occasionally a Diprionodont 
 form divides into two Monoprionodont branches, as in Dicranograptus. 
 In another modification, four Graptolites diverge from a common axis, 
 like two specimens of Diplograptus crossing at right angles ; this type 
 is termed Phyllograptus 2 
 
 Hydrocorallina. Louis Agassiz had referred the living Millepora 
 to the Hydrozoa. This genus forms a large part of the growth of 
 existing coral reefs, is represented in the Tertiary rocks by Axopora, 
 and in the chalk by Porosphcera. Professor Moseley, F.R.S., has 
 shown the allies of Stylaster to be hydroid ; 3 it is a Miocene fossil. 
 
 In near association with these forms, Stromatopora of the Devonian 
 rocks is sometimes grouped ; and the Silurian Ldbechia has similar 
 affinities. 
 
 1 T. Rupert Jones, Catalogue of Foraininifera in British Museum. See also 
 Carpenter, Jones, and Parker, " Introduction to the Study of the Foraminifera : " 
 Ray Society. 
 
 2 See Zittel, " Handbuch der Palaeontologie." Lapworth Geol. Mag., 1873 
 and 1876. 3 Moseley, Proc. Royal Soc., 1876. 
 
FOSSIL ACTING ZO A. 
 
 481 
 
 Actinozoa. The allies of the sea anemones are grouped into three 
 orders, named ZOANTHARIA, KUGOSA, and ALCYONARIA. 
 
 Alcyonarians are composite, and have the pinnate tentacles in 
 multiples of four ; they are represented by the living Alcyonium, 
 which occurs fossil in the Red Crag. The sea pens are represented 
 by Graphularia in the London clay ; arid Pavonaria in the Chalk. 
 The Gorgonias are chiefly of Tertiary age ; but the living Isis is found 
 in the Cretaceous rocks. The Maestricht beds yield Moltlda ; and the 
 living Mopsea is first found in the London clay. The living genus 
 Corallum dates from the Jurassic rocks. In this order of late years 
 have been placed many genera of the 
 Primary period, among which may be 
 named Aulopora, Syringopora, rang- 
 ing from Cambrian to Carboniferous, 
 Halysites, Theda, Lyellia, Propora 
 of the Silurian, Helioiites^ and Plas- 
 mopora of the Silurian and Devonian, 
 and the Devonian Thecostegites. The 
 living genus Heliopora dates from 
 the Cretaceous rocks, where it is as- 
 sociated with Stylophyllum, Professor 
 Duncan suggests that the Favositidae, and many of the corals formerly 
 grouped with the Tabulata, belong to this type. 
 
 Rugosa is an extinct group of corals, confined to the Primary rocks. 
 The septa are in multiples of four ; and this is expressed in Haeckel's 
 name for the group, Tetracoralla. The vertical septa are combined with 
 transverse tabulae ; frequently one of the principal septa is undeveloped, 
 and a fossette exists in its place. The corallites of the compound coral 
 are never connected by true ccenenchyma, being often connected by 
 their septa or their walls, or they may be connected by lateral processes. 
 
 The larger number of corals referred to this group belong to the 
 
 Fig. 88. Heliolites. 
 
 Fig. 89. Strombodes. 
 
 Fig. go.Cystiphyllum. 
 
 Lower Primary Rocks, or Cambrian and Silurian periods. Among 
 such are Duncanella, Acanthocydus, Calophyllum, Cyathophylloides, 
 
 VOL. I. 2 H 
 
4 82 
 
 DISTRIBUTION OF RUGOSE CORALS. 
 
 Fig. gi.^Lithostrotioii. 
 
 Streptelasma, Palwophyllum, Siphoncuis, Favislella, Stauria, Micro- 
 plasma, Fletcheria, Omphyma, Darwinia, and the operculate corals 
 Goniophyllum and Rhizophyllum. But many genera range up to the 
 Devonian. Among such are Anisopliyllum, Ptychophyllum, Chono- 
 phyllum, Hallia, Aidacophyllum, Erido- 
 phyllum, Acervularia, Spongophyllum, 
 Strombodes, and Cystiphyllum, which has 
 the septa more or less rudimentary, so that 
 the coral is built up of vesicular tissue (tig. 
 90). Among peculiar Devonian genera are 
 Jlydrophyllum, Combophyllum, Microcy- 
 clus, Barypliyllum, Metriophyllum, Cras- 
 pedophyll'um, Pachyphylhim, and the oper- 
 culate Calceola. Several genera range 
 from the Lower Primary to the Carbon- 
 iferous, and among these are Petraia, 
 Cyathaxonia, Amplexus, Zaphrentis, Cya- 
 thopht/Uum, Diphyphyllum, C lisiophyllum, 
 which has a central tabulate structure dis- 
 tinct from the external structure, and 
 Strepliodes. Genera which range from 
 the Devonian to the Carboniferous include 
 Lophophyllum, Campophyllum, and Phil- 
 lipsastrcea. Carboniferous genera include 
 Menophyllum, Phryganophyllum, Pentaphyllum, Trochophyllum, Ca- 
 ninia, Lithostrotion, Koninckophyllum, Lonsdalia, Chonaxis, Dibu- 
 nophyllum, Ci/dophi/llum, Aulopliyllum, and Michelinia. Polyccelia 
 ranges through the Primary period. 
 
 The Hexacoralla, or Zoantharia, comprises two principal groups ; 
 the Sclerobasica, which are compound organisms, not known prior 
 to the Tertiary rocks ; and the Sclerodermata, which includes the 
 majority of corals, both recent and fossil. This latter group has been 
 divided by Professor Martin Duncan, F.R.S., into Perforata, Fungida, 
 and Aprosa. 1 
 
 Perforata is a type with the walls of the corallum porous or re- 
 ticulate. It is largely developed at the present day, and comprises 42 
 genera and 5 sub-genera. In the Palaeozoic rocks it is represented by 
 Favositipora, which is still living. Prisciturlen, Stylarcea, Calostylis, 
 and Somphopora are Lower Palaeozoic genera. Protarcea is Silurian 
 and Devonian, while Palceacis is Carboniferous. There is no recorded 
 Jurassic genus. The Cretaceous rocks yield Cyclobacia and Adino- 
 acis, Litharcea, which ranges to the Middle Tertiary, and Stephano- 
 phyllia, which stills survives. Dendracis and Stereopsammia are 
 found only in the Lower Tertiary ; and the following genera com- 
 mence in the Lower Tertiary, and still survive Ballanophyllia, En- 
 psammia, Endopacliys, Dendrophyllia, Madrepora, Astrceopora and 
 Porites. The Middle Tertiary yields Lobopsammia, and the following 
 
 1 For Classification of the Madreporaria and Characters of the Genera, see 
 Duncan : Journal, Linn. Soc., Zoology, vol. xviii., Nos. 104-106, 1884. 
 
FOSSIL MADREPORARIA. 483 
 
 existing genera: Terbinaria, Rlwdarcea, Alveopora. Thecopsammia 
 dates from the Upper Tertiary. 
 
 Fungida is a group of corals placed by Professor Duncan be- 
 tween the Perforata and the Aporosa, characterised by having pro- 
 cesses termed " synapticulae," which cross the spaces between the 
 septa and cost. It has no Palaeozoic representative. In the Trias 
 the genera are Astroeomorpha and mphalophyllia. Microsolena is 
 Triassic and Jurassic. Thamnastrcea commences in the Trias, and 
 survives till the Middle Tertiary. The Jurassic genera include Epi- 
 streptophyllum, Clausastrcea, Phragmatoseris, Gonioseris, Thamnoserlt, 
 Protoseris, Anabacia, Genabacia, Haplaria, Thamnarcea, Diplarcea, 
 Disarcea, Di?norpharcea, Mycetaroea. Latimceandrarcea is of Corallian. 
 age. Crateroseris is Portlandian. The genera common to the Jurassic 
 and Cretaceous rocks comprise Podoseris, Comoseris, Leptophyllia , 
 Cydoliies. Among Cretaceous genera are Mesomorpha, Dimorphastrcea, 
 titylomoeandrina, Micrabacia, Gyroseris, Placoseris, Asteroseris, Micro- 
 set-is, Episeris, Polyphylloseris. 
 
 Dimorphoccenia is Neocomian, and T iirbinoseris ranges from the 
 Lower Greensand to the Lower Tertiary. Cyathoseris also survives to 
 the Lower Tertiary, and Trochoseris and Cycloseris are Cretaceous 
 genera still living. Among Lower Tertiary types may be mentioned 
 Polyarcea, Pseudastrcea, Pironastrcea, Reussastrcea, Elliptoseris, Zittelo- 
 fungia, and Pratzia. Packyseris and Siderastrcea commence in the 
 Lower Tertiary, and still survive. Diaseris and Agaricia are surviv- 
 ing genera, which date from the Middle Tertiary. 
 
 Aporosa. This great group of the Madreporaria has the hard 
 structures solid and imperforate, and the spaces between the septa arc 
 usually open, but may be closed by plates or tabulae. This group is 
 but poorly represented in the Primary rocks, the only genera (accord- 
 ing to Professor Duncan) being Battersbyia in the Devonian, and 
 Heterophyllia in the Carboniferous limestone. The Trias is charac- 
 terised by Coccophyllum, Iioilocoenia, and Elysastrcea, which range to 
 the Infra Lias. Goniocora and Convexastrcea are common to the Trias 
 and Oolites. Stylina and Adelastrcea extend from the Trias to the 
 Cretaceous. Calamophyllia, Thecosmilia, fsastrcKa, and Latimoeandra 
 range from the Trias to the Tertiary rocks ; and Plerastrcea, Astro- 
 ccenia, and Montlivaltia are Triassic genera, which still exist. Tro- 
 chocyathus survives from the Lias. The genera from the Jurassic 
 rocks include C t/athocoenia, Discocyathus, Eukelia, Dendrohelia, Plesios- 
 milia, Pleurosmilia, Blastosmilia, Axosmilia, Placophyllia, *Donacos- 
 milia, Aplosmilia, /Stiboria, Phyllogyra, Stibastroea, Latipkyllia, 
 Pkytogra, Lepidophyllia, Diplothecastrcea, and Stylohelia. Jurassic 
 genera which range to the Cretaceous rocks include Enallohdia, 
 Prokelia, Latusastrcea, Stylosmilia, Rhipidogyra, Pachygyra, Bary- 
 pkyllia, Placoccenia, and Cyathophora. Diplocoenia ranges from the 
 Oolites to the ISTeocomian. Trochosmilia, Demorphophyllia, and 
 Stylocoenia are genera of the Oolites, which survive till the Tertiary 
 period ; and the following genera from Jurassic rocks are found in 
 existing seas, viz., Cladocora, Euphyllia, Mceandrina, SympHyllia, 
 
4 8 4 
 
 FOSSIL APOROSE CORALS. 
 
 Ulophyllia, Favia, Heliastrcea, Rhymastrcea, and Stephanocoenia. 
 The Neocomian genera of Aporosa are Holocystis, Acanthocoenia, 
 Pentaccenia, and Bracltycyathus. Dasmia commences in the Neo- 
 comian, arid survives till the Lower Tertiary. The Cretaceous 
 rocks abound in Aporosa: among the genera are Onchotrochus, 
 Cydocyathus, Baryhelia, Diblasus, SynJielia, Diploctenium, Peplo- 
 utmlia, Pleurocora, Rhabdocora, Hexasmilia, Dactylosmilia, Hymen- 
 ophyllia, Glyphophyllia, Eugyra, Meandrastrcea, Stelloria, Aspidiscus, 
 Stenosmilia, Phyllastrcea, Placophoria, Stylastrcea, Psammophora, 
 Heteroccenia, Elasmocoenia, Haldonia, Placastrcea, and Dictyophyllia. 
 Cretaceous genera -which survive to the Lower Tertiary period are 
 Smilotrochus, Stylocyathiis, Placosmilia, Dendrosmilia, Stylocora, 
 Barysmilia, Columnastrcea, PJiyllocoenia, and Holoccenia. While 
 among genera which first appear in the Cretaceous rocks and still 
 survive are Caryophyllia, Lophosmilia, Diploria, Leptoria, Myceto- 
 pliyllia, Phrogyra, and Hydnophora. 
 
 Several genera of corals are peculiar to the Lower Tertiary, such 
 as Ceratophyllia, Circophyllia, Pattalophyllia, Stylangia, Desmodadia, 
 Cyathromorplia, Areacis, Anisocoenia, Narcissastrcea, Aploccenia, 
 Heterogyra. But many genera which still survive date from the 
 Lower Tertiary period. Among these are Flabellum, Sphenotrochut, 
 Placocyathus, Platycyathus, Leptocyathus, Paracyatlms, Ceratotrochus, 
 Amphelia, Oculina, Stylophwa, Asterosmilia, Asterangia, Solenastrcea, 
 and Plesiastrcea. The Middle Tertiary genera comprise Astrohelia, 
 J/aplohelia, Lithopliyllia, Bathangia, Cladangia, Teleiophyllia, Mon- 
 ticulastrcea,Favoidea, D'Archiardia, Antillastrcea, and Diplothecastrcea. 
 
 Genera which commence in the Middle Tertiary, and still survive, 
 include Conocyathus, Deltocyathus, CoBnocyatlius, Lopliolielia, Diclw- 
 coenia, Echinopora, and Desmopliyllum. In the Upper Tertiary 
 Eusmilia is found. 
 
 Asteroidea. The true star-fishes are characterised by having the 
 digestive and ovarian organs contained within the arms. An attempt 
 has been made to classify star-fishes by the arrangement of the am- 
 bulacra! organs into those with four rows, those with two rows, and 
 those in which ambulacral plates separate the pores ; but none of these 
 groups are natural, and the only scientific division of star-fishes fur- 
 nishes no palseontological characters. 
 
 Fig. 92. Palceaster. 
 
 The oldest fossil star-fishes are found in the Cambrian rocks, Palce- 
 aster, Urasterella, and Palceasterina being common to the Cambrian 
 
GEOLOGICAL HISTORY OF STAR-FISHES. 485 
 
 and Silurian. Among the genera peculiar to the Silurian, and chiefly 
 of Wenlock and Ludlow age, are Palaeodiscus, Lepidaster, Trichotaster, 
 Rhopalocoma, Bdellacoma, and Paloeocoma. The Devonian star-fishes 
 are Aspidosoma, Archasterias, HeliantJiaster, Xenaster. The Carbo- 
 niferous limestone has yielded no peculiar genera except Schoenaster, 
 and the forms named Calliaster and Gribellites. The Triassic types 
 are Pleur aster and Trichasteropsis. The Lias yields Astropeden, Gonio- 
 discus, Luidia, Solaster, and Uraster, which still survive, together with 
 Tropidaster and Plumaster ; Sphceraster is Jurassic. The living genus 
 Oreaster, common in the Chalk, commences in the Upper Oolites. 
 Coulonia is Neocomian, and the living genus, lihopia, is first found 
 in beds of that age. Arthraster is peculiar to the Chalk. Astro- 
 c/onium and Stellaster are first found in the Cretaceous Rocks, Very 
 little is known at present of Tertiary star-fishes. 1 
 
 Ophiuroidea. The Brittle stars are a remarkable group of Echino- 
 derms, having the viscera excluded from the arms, which suggest much 
 the same relation to the Cystideans and Crinoids that ordinary star- 
 fishes exhibit to Echinoidea. They comprise two principal types : first, 
 that of Eurt/alus, and secondly, that of Ophioderma. Both these groups 
 are known in a fossil state ; the former, by Eudadia from the Ludlow 
 
 Fig. 93. Ophioderma. 
 
 rocks of England, and Onychaster from the Carboniferous rocks of 
 America. Professor Quenstedt has indicated a representative of the 
 group in the Lias. 
 
 The Ophiuroid star-fishes are represented in the Primary rocks by 
 Protaster, which is of Silurian and Devonian age in Europe. Toeniaster 
 is of Cambrian age in Canada, and Eugaster is an American-Devonian 
 genus. The existing genus Ophioderma dates from the Muschelkalk, 
 and is well known in the English Lias. Aspidura is a characteristic 
 Triassic genus. The Oolites yield Ophiurella and Geocoma. Ophio- 
 glypha dates from the Lias, and is believed to include species which 
 have previously been referred to many genera. 
 
 Crinoidea. The Crinoids, or sea-lilies, are an order of Echino- 
 derms in which a jointed column supports a more or less complex cup 
 1 See Wright in publications of the Palseontographical Society. 
 
486 CRINOIDS IN PRIMARY STRATA. 
 
 or calyx, surrounded by well-developed arms, which recall the arms 
 of a star-fish, or the branching arms of Euryalus. Some types have the 
 calyx covered with plates, and then the base of the calyx is commonly 
 formed by five plates; the genera with a single basal plate usually have 
 the calyx open above (fig. 99^). Crinoids are numerous in the Primary 
 rocks, and less common in the Secondary strata. They are now verging 
 on extinction, though several genera still survive, such as Pentacrinus, 
 Conocrinus, Bathycrinus, Hyocrinus, and Holopus. 
 
 The oldest known genus is Dendrocrinus from the Tremadoc 
 rocks. The Llandeilo beds yield Glt/ptocrinus, Cyathocrinus, JKJiodo- 
 crinus, and Adinocrinus, the last three surviving till the Carboniferous. 
 In America Crinoids are more numerous in the Cambrian period, and 
 comprise Hybocrinus, Anomalocrinus, Homocrinus, Cupalocrinus. 
 
 The Wenlock period is remarkably rich in Crinoids, especially in 
 the families represented by Cyathocrinus, which ranges to the Per- 
 mian ; TaxocrinuSj Iclithyocrinus, Cheirocrimcs, Habrocrinus, and Pote- 
 
 Fig. 94. Cyathocrinus. Fig. 95. Taxocrinus. 
 
 riocrinus, which range to the Carboniferous, and such peculiar genera 
 us Crotalocrinus, Carpocrinus, Briarocrinus, Dimerocrinus, Periecho- 
 crinus, Corymlocrinus, and Pisocrinus. Several genera, like Euca- 
 lyptocrinuS) survive to the Devonian. The Devonian Crinoids are 
 also numerous. They comprise Cupressocrinus, Symbathocrinus, RMpa- 
 docrimts, Grasterocoma, Culicocrinus, Dolatocrinus. In the Carbon- 
 iferous limestone Crinoids are particularly abundant, sometimes 
 forming the entire mass of the rock. A large number of species be- 
 long to such genera as Poteriocrinus, Platycrinus, and Actinocmnus, or 
 to sub-genera grouped under these types. Other genera characteristic 
 of this period are Woodocrinus, Agassizocrinus, Dichocrinus, Ollo- 
 crinus, Graphiocrinus, Onychocrinus. In the Dyas, or Permian, the 
 only Crinoid is Cyathocrinus, already mentioned. 
 
 The Trias is characterised by Encrinus, which belongs to the arti- 
 culate sub-order, in which all the secondary Crinoids find a place. 
 In this formation Pentacrinus appears for the first time, though it 
 attains its most remarkable development in the Lias. Cotylederma is 
 Liassic. 
 
 The Jurassic genera comprise Eugeniocrinus, Triacrinus, Phyllo- 
 
CRINOIDS IN SECONDARY STRATA. 
 
 487 
 
 crinus, Plicatocrinus, Apiocrinus, Miller icrinus, Solanocrinus ; to- 
 gether with Antedon, Actinometra, and Saccocoma, 1 the former of 
 which still survive, and the last ranges to the Chalk. 
 
 Fig. 96. Encrinus. Fig. 97. Pentacrinus. 
 
 Another cretaceous genus is Torynocrinus. Bourguetocrinus (fig. 
 98) ranges to the Tertiary. Marsupites and Uintacrinus are also cre- 
 taceous. The Tertiary Crinoids almost all belong to existing genera. 
 
 Fig. 99. Apiocrinus. Fig. 100. Torynocrinus,- 
 
 Cystoidea are an extinct group of Echinoderms limited to the 
 Primary rocks. They have a more or less globular body formed of 
 
 1 See P. H. Carpenter, Q. J. G. S., vol. xxxvi.-viii. 
 , 2 An. Nat. Hist., March 1866. 
 
4 88 EXTINCT ORDERS OF ECHINODERMATA. 
 
 plates ; it is supported upon a short jointed column like that of the 
 Crinoids (which is absent in Protocrmus), and in many genera arms 
 are developed which are comparable to those of Crinoids. There 
 are arnbulacral furrows, which are sometimes forked, and peculiar 
 organs termed " pectinated rhombs." 
 
 The oldest known genera are Protoci/stites, of the Menevian beds, 
 and Macrocystella, which is also of Cambrian age. Other genera 
 occur in the primordial rocks of Bohemia and the Fichtelgebirge, 
 and PalcBOcystites is found in strata of the same 
 age in Canada. By far the larger number of Cys- 
 tidians is found in the Middle and and Upper 
 Cambrian rocks of the Continent. Echinosphcerites 
 occurs in the British Llandeilo rocks, but in Russia 
 the following genera also occur Sphceonites, Caryo- 
 cystites, CystoWastus, Asteroblastus, Blastoidocrinus, 
 and Mesites. Several genera occur in Sweden, such 
 as EucystiSj Holocystites, Glyptosphcerites, Glypto- 
 cystites, Gomphocystites, and Lepadocrinus. Bo- 
 hernia yields Trochocystites and RhombifercL, while 
 from the Canadian rocks many other genera are 
 described, such as Pleurocystites, Comarocystites, Amygdalocytes, 
 Malocystites, &c. The true Silurian rocks contain Ateleocystites and 
 Lepadocrinus. Both date from the Cambrian rocks, and the former 
 survives the Devonian. These, with Primocystites and Ecliino- 
 encrinus, are well-known Dudley fossils. In North America, Caryo- 
 crintts, Callocystites, Holocystites are also found of this age. The 
 Devonian strata yield Tiaracrinus and Echinocystites, which is also 
 found in the Silurian rocks of America. Agelacrimis, which com- 
 mences in the Cambrian, survives to the Carboniferous, in which 
 Lepadocrinus and"the embryonic crinoid Hypocrinus x are also found. 
 After the Carboniferous period Cystidians are unknown. 
 
 Blastoidea. The Blastoidea is an extinct order limited to the 
 Primary rocks closely related to the Crinoidea and Cystoidea, which 
 it resembles in having a jointed column. Its test is made up of 
 polygonal plates ; there are large ambulacral areas but no arms. Mr. 
 Herbert Carpenter considers the type genus Pentremites to be absent 
 from the British rocks. The group first appears in the Silurian strata 
 of Tennessee, but is chiefly characteristic of the Devonian and Car- 
 boniferous periods. Pentremites, Nucleocrinus, and Eleutherocrinus 
 are characteristic of the Devonian rocks, but Granatocrinus and 
 Codonaster are common to the Devonian and Carboniferous. 
 
 Echinoidea. The ancient sea urchins of the Primary rocks are 
 mostly distinguished by the shell being composed of more than 
 twenty rows of plates, with five or ten perforated plates in the apical 
 disc. These characters have been thought sufficient by Zittel to 
 separate the Palceechinoidea from the Euechinoidea, in which the 
 skeleton is formed of ten rows of ambulacral plates in five pairs, 
 alternating with five pairs of interambulacral plates, and the apical 
 1 See P. H. Carpenter, Q. J. G. S., vol. xxxviii. 
 
GEOLOGICAL HISTORY OF SEA-URCHINS. 489 
 
 disc has never more than four or five perforated plates. The Palae- 
 echinoidea comprise the Cystocidaris, formerly described as Echino- 
 cystites by Wyville Thomson. It is found in the Silurian rocks. 
 Bothriocidaris is a genus from the Cambrian of Russia, in which there 
 are only fifteen rows of plates in the test, one interambulacral row 
 dividing four rows of ambulacral plates. 
 The Perischoechinidce are distinguished by 
 having more than two rows of plates in each 
 interambulacral area. The group is almost 
 entirely of Carboniferous age, and includes 
 such genera as Lepidocentrus and Xeno- 
 cidaris from the Devonian, Pholidocidaris, 
 Perischodomus, Rhoechinus, Palcechinus, 
 which has a Silurian species ; Me/onites, Fi s- io2.-Patecfti*. 
 
 Oligoporus, Protoechinus, Archceocidaris, 
 
 Lepidocidaris, Lepidecliinus are common to Carboniferous and Devo- 
 nian, and Eocidaris is Permian. 
 
 The ordinary sea-eggs are divided into the regular and irregular 
 types. The regular forms, usually termed " Endocyclica," commence 
 with the Secondary rocks. Cidaris dates from the Trias. This genus 
 has been subdivided into many sections, some of which have a 
 pala3ontological value. Thus Rhabdocidaris is characteristic of the 
 secondary rocks ; Diplocidaris and Polycidaris belong to the Oolites. 
 TemnocMaris is found in the Upper Chalk. Echinothuria is a flexible 
 sea-urchin, in which the plates overlap like those of the oral disc of 
 a Cidaris. It is only known from the Chalk, and is represented at 
 the present day by Phormosoma and Asthenosoma, or Calvaria. The 
 Salenia type commences in the Lias with Acrosalenia, which survives 
 to the Neocomian. It becomes accompanied in the Upper Oolites 
 by Pseudosalenia and in the Upper Greensand by Goniophoi^us and 
 Heterosalenia, and the still surviving genera Salenia and Peltastes. 
 The great family of the Diadematida3 commences in the Trias with 
 Hypodiadema, is succeeded in the Lias by Diademopsis and Micro- 
 diadema. In the Lower Oolites appear Pseudodiadema, which ranges 
 to the Chalk, and Hemipediita, which ranges from the Oolites to the 
 Neocomian. Pedina, Hemipygus, Glypticus, Acropeltis are character- 
 istic of the Upper Oolites. Hemicidaris and Acrocidaris both range 
 to the Neocomian, and Hemicidaris is also found in the Chalk ; under 
 the name of Hypodiadema it is known from the Permian and Trias. 
 The Neocomian genera, which are common to the Jurassic rocks, in- 
 clude Magnosia and Pseudocidaris. Cyplwsoma is common to the 
 Neocomian and Cretaceous ; and one species is found in Japanese 
 seas. Among cretaceous genera are Codtopsis, Echinocyphus, Leio- 
 soma, Leiocyphus, Glyphocyphus, and Heterodiadema. The Tertiary 
 rocks yield several genera, among which Ecliinopsis and Hebertia are 
 in the Lower Tertiary, and others, like Temnechinus, Temnopleurus, 
 and the Cretaceous genus Cottaldia, survive to existing seas. Coelo- 
 pleurus is a Lower Tertiary type, which still survives. 
 
 The Echinidse appear in the Lower Oolites with Ecliinodiadema, 
 
490 
 
 THE IRREGULAR SEA-URCHINS. 
 
 Pseudopedina, Stomechinus, Polycyphus, of which only Stomechinus 
 survives to the Neocomian, in which the living genus Psammecliinus 
 is found, together with Pedinopsis, Gflyptechinus, and Codechinus. 
 The cretaceous types are Microped.ma and Diplotagina. The Lower 
 Tertiary genera are Echinopedina and Leiopedina, while Echinus and 
 many other surviving genera date from this period. 
 
 The irregular sea-urchins called Exocylica, in which the two poles 
 are not opposite, have a similar range in time. Among them Pygaster 
 commences in the Lias, and ranges to existing seas, accompanied by 
 Holectypus as far as the Neocomian. Pileus characterises the Upper 
 Oolites, Discoidea ranges from the Neocomian to the Chalk, Galerites 
 is typically Cretaceous. Conoclypus commences in the Chalk, and 
 lias one existing representative. 
 
 The Euclypeasters commence in the Cretaceous beds with Eclii- 
 nocyamus, which characterises the Tertiary, and is still living. It is 
 associated with Fibularia. Lower Tertiary genera include Sismon- 
 dia, Scutellina, and Lenita. Clypeaster and Lagamum first appear 
 in the Lower Tertiary. 
 
 The Scutelline urchins commence with the Tertiary, and are 
 represented by the extinct genus Mortonia, and by species of Scutella, 
 Amphiope. We then pass to the urchins in which an elongated form 
 replaces the hitherto circular base, and in which both poles depart more 
 or less from the central position. The Cassidulidce present two types 
 first, that of Galeropygus, which appears in the Lias, and ranges 
 through the Oolites, in which it is accompanied by Hyboclypus, 
 Galerodypus, and Infradypeus. Pyrina commences in the Lower 
 Oolite, and survives to the Lower Tertiary. 
 Echinoneus is found in the Middle Tertiary, 
 and still exists. The closely allied group of 
 Ecliinolampidce commences with Clypeus in the 
 Lower Oolite, and in the Middle Oolite are 
 found Nucleolites and Pygurus, which range up 
 to the Neocomian, unless the living genus 
 Fig 103. Nucleolites Nucleolites prolong it to existing seas. . Cato- 
 pygus is a Neocomian type, which is still 
 
 living. Cassidulus commences in the Cretaceous, and is found in the 
 Tertiary. Other Tertiary genera are Echinolampas and Rhynchopyyun, 
 which are still living. Collyrites appears 
 with the Lower Oolite, and survives to the 
 Xeocomian. The Ananchytidce begin in the 
 Neocomian with Holaster, which survives 
 to the Middle Tertiary. Other Cretaceous 
 genera are Cardiaster, Infidaster, and Hemi}h 
 neustes. Anancliytes survives from the Chalk 
 to the Middle Tertiary. The Spadangida; 
 are represented in the Neocomian by Heter- 
 aster, Toxaster, and Enalaster. The Creta- 
 ceous rocks yield Hemiaster, Epiaster, Micraster, and Prenaster. The 
 Lower Tertiary are characterised by Macropmustes, Peripneustes, 
 
 Fig. 104. Auanchytes. 
 
SUCCESSION OF CRABS AND LOBSTERS. 491 
 
 Cydaster, while many genera still survive, such as Schizaster, Brls- 
 sus, Eupatus, Spatangm, &C. 1 
 
 Crustacea. The Crustacea enumerated by Dr. Henry Woodward 
 as British fossils comprise 197 genera, of which 108 are referred to 
 the Malacostraca. The stalk-eyed group includes 53 genera, and the 
 sessile-eyed group 53, of which 51 are trilobites. 2 
 
 The history of the highest group, or crabs properly so called, is 
 very imperfectly known. It commences in the Great Oolite with the 
 genus Prosopon, which ranges to the Coral Rag. Palceinachus is 
 another Oolitic genus found in the Forest Marble ; but with these 
 exceptions, most crabs from Secondary Strata belong to the Cretaceous 
 beds ; they are of small size. Diaulax, Etyus, Aurithopsis are common 
 to the Gault and Upper Greensand. Necroscarcinus ranges from the 
 Gault to the Chalk marl. Palceocorystes ranges from the Gault to the 
 Lower Tertiaries. The distinctive Upper Greensand genera are Cypho- 
 notus, Eucorystes, Hemiono, Plagiophthalmus, Tracliynotus, and Xau- 
 thosia. Platypodia is peculiar to the Lower Chalk. There is a con- 
 siderable fauna of crabs from the London clay, comprising Cycloco- 
 rystes, Campylostoma, Goniochele, Gonioxypoda, Litoricola, Necrozius, 
 CEdisoma, Plagiolophus, Portunites, Rachiosoma, Xanthilites, and 
 Xanthopsis. 2 * The Anura are only known from two genera Homo- 
 lopsis from the Upper Greensand and Gault, and Dromolites from the 
 London clay. 
 
 The Lobster tribe is first known in the Coal-measures, where it is 
 represented by the genera Anthrapalcemon and Palceocrangon. The 
 Lias yields JEg&r^ Eryon, Palinurus, Tropifer, Scaplwus, Pseudo- 
 glyphcea, and Penceus. Eryma is common to the Lias and Lower 
 Oolites. Glyphcea ranges from the Lias to the Upper Greensand. 
 Mecochirus is found in the Oxford and Kimmeridge clays. Callia- 
 nassa ranges from the Kimmeridge clay to existing seas. Astacodes 
 and Astacus are quoted from the Speeton clay. Meyeria is common to 
 the Speeton clay and Lower Greensand. Mithracites is found in the 
 Lower Greensand. HoplopaTia ranges from the Lower 
 Greensand to the London clay. Scyllaridia extends 
 from the Gault to the London clay. Mesocragnon is 
 peculiar to the Gault; Phlyctisoma is peculiar to the 
 Upper Greensand. The Chalk yields Enoplodytia. In 
 the London clay are found Archceocarabus, Mitkracia, 
 Thenops, Trachysoma. The Stomopods are only repre- 
 sented by Pygowplialus in the Coal-measures. The Iso- 
 pods commence with Prcearturus in the Old Red Sand- 
 stone. Palcega is from the Chalk. The living Parasitic 
 genus Bopyrus dates from the Upper Greensand. Archwoniscux, 
 which resembles the living Sphseroma, is peculiar to the Purbeck 
 
 1 Wright : " British fossil Echinodermata : " Palteontographical Society. 
 
 2 See Salter and Woodward : Chart of Fossil Crustacea. For the general 
 structure of the several ordinal groups reference maybe made to Huxley's "Ana- 
 tomy of Invertebrates and Vertebrates." 
 
 3 Bell in Palseontographical Society. See also Dana : " Geographical Distribu- 
 tion of Crustacea." 
 
492 
 
 GEOLOGICAL SUCCESSION OF TRILOBITES. 
 
 beds ; but is represented in the Lower Tertiary of Garnet Bay by 
 Eosphceroma. 1 
 
 Trilobites. Trilobites are the most distinctive fossils of Primary 
 rocks. Like other Crustacea, they go through a metamorphosis, and 
 develop their characters as the test is shed. 2 They are mostly confined 
 to the Cambrian and Silurian strata. The only genera which exist 
 in the Carboniferous rocks are Bracliymetopus, Griffithides, and Phil- 
 lipsia. Bronteus is characteristic of the Devonian ; Cheirurus ranges 
 from the Tremadoc to the Devonian ; Harpes has a similar range ; and 
 Phacops and Homalonotus extends from the Arenig to the Devonian. 
 Dcdmannia is the only genus peculiar to the true Silurian rocks, though 
 Proetus and several genera survive to them. Thus, Acidaspis ranges 
 from the Llandeilo to the Ludlow ; Ampyx from the Tremadoc to the 
 Ludlow ; Calymene from the Arenig to the Ludlow ; Cyphapsis from 
 the Llandeilo to the Ludlow ; Eucrinurus from the Bala to the Lud- 
 low. A few genera reach no farther upward than the Wenlock rocks, 
 and are wanting in the Ludlow. Among such are Illcenus, which 
 commences in the Arenig ; and Lichas and Sphcerexochus, which com- 
 mence in the Llandeilo. Trinuneleus stops short of the Upper Lland- 
 overy, and begins with the Arenig. Among the Bala forms are 
 Amphion, Cybele, Staurocephalus, and Stygina. 
 
 Fig. 106. Illamus. Fig. 107 Lichas. 
 
 Fig. 108. Trinucleus. 
 
 The Llandeilo period includes Eccoptocliile and Cyplioniscus, It 
 has, in common with the Arenig rocks, JEglina and Barrandia. 
 Peculiar to the Arenig are Dionide and Placoparia. The Tremadoc 
 trilobites include Psiloceplialus, Niobe, Neseuretus, and Angelina. 
 Ogygia commences in the Tremadoc, and ranges to the Llandeilo ; 
 Asaplws commences in the Tremadoc, and ranges to the Bala beds. 
 Several genera are common to the Lingula Flags and Menevian beds, 
 such as Paradoxides, Holoceplialina, and Anopolenus. Among Mene- 
 
 1 See H. Woodward, Q. J. G. S., May 1879. 
 
 2 C. G. Walcot, Bulletin of the Museum of Comparative Anatomy at Harvard 
 College, vol. viii., No. 10, 1881. Barrande, " System e Silurien de la Boheme." 
 Salter and Woodward in Palseontographical Society, &c. 
 
HISTORY OF SESSILE-EYED CRUSTACEA. 
 
 493 
 
 vian genera are Arionellus, Carausia, Erinnys. Olenus and Microdiscus 
 characterise the Lirigula Flags, Dikelocephaliis ranges from the Lingula 
 
 Fig. no. Agnostus. 
 
 Fig. 109 Asaphus. 
 
 Pig. in. Prestwicliia. 
 
 Flags to the Tremadoc. The Harlech grits yield Plutonia, Pdlceopyye, 
 and Agnostus, which latter ranges to the Llandeilo rocks. 
 
 Xiphosura. The King Crals 
 commence with Neolimulus in 
 the Ludlow rocks ; Prestwicliia 
 (fig. in) and Bellinurus are 
 found in the Coal-measures ; but 
 the group is absent from the 
 newer English strata, though Li- 
 multis is well represented in the 
 lithographic slate of Solenhofen. 
 
 Merest omata. 1 The Mero- 
 stomata comprise an allied 
 group of Crustacea, in which 
 the abdominal segments re- 
 main separated as among trilo- 
 bites, but the appendages round 
 the inouth are developed after 
 the plan of the King Crabs. 
 Hemiaspis is found in the Wen- 
 lock and Ludlow, Slimonia in 
 the Ludlow rocks, Stylonunts 
 and Pterygotus in the Ludlow 
 and Old Ked Sandstone, while Eu- Restored by - Dr Henry woodward, F.R.S. 
 
 ryptUTUS ranges from the Ludlow eyes ; 6. post-oral plate; c. antennulse ; d. mi- 
 , ,1 ri T -r T j_ tennse ; e. mandibles; f. maxillae ; q. swirn- 
 
 to the Carboniferous limestone. ming f ee t ; 1-12 segmeuts, 13 teison 
 
 The Phyllopods are mostly 
 
 small Crustacea with a carapace often formed of two parts. They 
 are characteristic fossils of the Primary rocks. Hymenocaris is found 
 
 13 
 
 Fig. 112. Pterygotus. 
 by Dr. Henrj 
 
 Henry Woodward : Palseontographical Society. 
 
494 
 
 FOSSIL PHYLLOPODS AND OSTRACODS. 
 
 Fig. 113. Hymenocaris. 
 
 Fig. 114. 
 Beyrichia 
 (enlarged). 
 
 in the Lingula flags, Lingulocaris in the Tremadoc slates, Cariocaris 
 in the Arenig. Ceratiocaris ranges from the Tremadoc to the Car- 
 boniferous, Peltocaris from the Llandeilo to 
 the Wenlock, Dithyrocaris from the Lud- 
 low to the Carboniferous, Leaia is charac- 
 teristic of the British Carboniferous strata, 
 and the living genus Estlieria is found in 
 the Devonian and all newer strata, espe- 
 cially Triassic. 
 
 The Ostracods are bivalve Crustacea of 
 small size, which are especially numerous 
 in the Primary rocks. Among the oldest 
 
 types, Leperditia extends from the Harlech rocks to the Carboniferous. 
 Beyrichia ranges from the Arenig to the Coal, Primitia from the 
 Llandeilo to the Carboniferous, ^Echmina and Thlipsura 
 ^^ are "Wenlock ; Cytherellina is also Silurian. Moorea ranges 
 !||gj? from the Ludlow to the Lias, Entomis from the Silurian to 
 ^-"^ the Carboniferous, Entomidella is Menevian. The Carbon- 
 iferous period has many peculiar genera. Among them 
 are Carbonia, Cyprella, Cypridella, Cypridellina, Entomo- 
 conchus, Off a, Mhombina, and Sulcuna. But among other 
 genera which commence in the Carboniferous and survive to the 
 present time, may be mentioned Bairdia, Candona, Ct/pripina, Philo- 
 medes, and Polycope. Kirkbia is common to 
 the Carboniferous and Permian, Cytherideis 
 commences in the Permian, and still sur- 
 vives. Cythere is a living genus, which begins 
 in the Lias. Cypridea characterises the 
 Purbeck and Wealden. Cytheridea and 
 Cytherura begin in the Gault, and still sur- 
 vive ; Cytherella and Macrocypris are found 
 in the Chalk and Tertiary. The recent genus 
 Loxoconcha begins in the Crag. 
 The fossil Cirripedes belong to but few types. Turrilepas char- 
 acterises the Wenlock limestone ; Scalpellwn survives from the Lower 
 Greensand, Pollicipes from the Rhsetic beds ; Verruca survives from 
 the Chalk, Lauricula is peculiar to the Chalk, Coronula is found in 
 the Red Crag, while Balanus survives from the Headon beds of the 
 Isle of Wight. 1 
 
 Vermes. The large group of worms is imperfectly represented in 
 the strata. Many are only known from their jaws, and they have 
 been for the most part described by Dr. G. J. Hinde in various 
 members of the Primary rocks. Other tubicolous types occur with 
 them, such as the Cambrian Conchicolites, Spirorbis, and Ortonia, 
 which range from the Cambrian to the Carboniferous. Cornulites, 
 Serpulites, and Trachyderma are Silurian. Genicularia is Jurassic. 
 The living Serpula commences in the Trias, Terebella in the Lias, 
 Ditrupa in the Cretaceous. Several errant worms are recogisable in 
 1 H. Woodward, Catalogue of Crustacea in the British Museum, 1876. 
 
 Fig. 115. Cypridea. 
 
SUCCESSION OF GENERA OF BRYOZOA. 
 
 495 
 
 the lithographic slate of Bavaria, and have been named Lumbriconerites, 
 Ischyr acanthus, Meringosoma, Ctenosolex, and Eunicites, the last being 
 found also at Monte Bolca. Numerous genera have been described 
 from rocks of all ages, but many of them are of uncertain affinity, 
 such as Scolithus, Arenicola, Histioderma, Ortonia, Scolicoderma, &c., 
 which are well-known fossils of the Cambrian rocks. 
 
 Bryozoa. Bryozoa are compound organisms, which form colonies, 
 and often encrust other organisms. They are found in the Primary 
 rocks represented by genera which for the most part are extinct. 
 Phyllopora, Polypora, Fenestella range through the period. Ptilodictya 
 and Penuiretipora are com- 
 mon to Silurian and Devo- 
 nian. Pcetniopora is De- 
 vonian, Hippotha and Sto- 
 matopora commence in the 
 Silurian, and still exist ; 
 Montwulipora ranges from 
 the Silurian to the Car- 
 boniferous. The Carbon- 
 iferous genera are Choetetes, 
 Archimedes, Ptilopora, Cos- 
 cmium, and Ichthyorhachis. 
 In the Permian is found 
 Acanthocladia. Ceriopora 
 is a Palaeozoic genus well 
 known in the Secondary 
 strata. Many genera com- 
 mence in the Jurassic epoch, 
 and still survive. Among 
 them are Diastopora, Bere- 
 nicia, Defrancia, Reptotu- 
 bigera, Spiropora, &c., while 
 Heteropora, Terelellaria, and 
 some others are common to 
 the Jurassic and Cretaceous periods. The Cretaceous types comprise 
 Hornera, Idmonea, Tubulipora, Discosparsa, Frondipora, Fasiculipora, 
 Vincidaria, Myriozoum, Cupularia, Lunulites, Retepora, Flusterella, 
 Eschara, Meinbranopora, which still survive. Nodelict, Multelia, 
 Osculipora, Truncatula, Cellulipora, and Semimultisparsa are peculiar 
 to the Chalk. Alveolaria, though characteristic of the Crag, com- 
 mences in the Chalk. Among Tertiary types which survive are 
 Cellepora, Cumulipora, and Eutalopora. Buslda is characteristic of 
 the Oligocene. 1 
 
 Brachiopoda. Bvachiopods are symmetrical bivalve shells, which 
 are equal-sided, but usually inequivalve. In most genera the valves 
 are locked together; the larger valve is perforated by a foramen, 
 giving passage to a ligament, by which the shell is attached; and 
 
 1 The Bryozoa of the Crag by Busk has been published by the Palaeonto- 
 graphical Society. 
 
 Fig. 1 16. Fencstclla. Fig. 117. Monticulipora. 
 
496 
 
 FOSSIL GENERA OF BRACHIOPODA. 
 
 Fig. 118. Spirifera. 
 
 several genera are characterised by the internal shelly plates, loops, 
 and spirals (see fig. 118) which support the breathing organs. Most 
 of the genera are extinct and confined to the Primary rocks, though 
 
 several primary genera range up to the 
 lower Oolites. We first enumerate 
 some of the extinct genera. Spiri- 
 fera .and. Atrypa range through tlie 
 Primary to the Trias, Orthis and 
 Strophomena only range up to the 
 Carboniferous, Obolus and Siphonotrda 
 are Cambrian and Silurian, Untites 
 is Devonian, and Stringocephalus, 
 Merista, and Pentamerus are Silurian 
 and Devonian ; Chonetes ranges from Cambrian to Permian, and Pro- 
 ducta and Stroplialosia from Devonian to Permian. Several genera 
 extend up to the Trias; among them are Cyrtia, ftetzia, 
 which both date from the Silurian ; and Davidsonia, which 
 dates from the Devonian. Other genera range up to the 
 Lias ; among them are Leptcena and Athyris. Koninckia is 
 confined to the Trias ; Suessia is confined to the Upper 
 Lias. Spiriferina extends from the Devonian to the Lower 
 Oolite, Zellania extends from the Lias to the Great Oolite. 
 F Sa"J'~~ Ki n ff ena > Magas, Trigonosemus, and Lyra are Cretaceous. 
 
 Of genera which date from the older primary rocks and 
 still survive, the most important are Lingula, Crania, Discina, and 
 Rhynchonella. Terebratula survives from the Devonian, Waldheimia 
 from the Trias. Thecidium first appears in the Carboniferous, Tere- 
 bratella in the Lias, Argiope in the Inferior Oolite ; the living Tere- 
 hratulma is found in the Oxfordian rocks. 1 
 
 Fig. 120. Rhynchonella. 
 
 Fig. 121. Terebratula. 
 
 Fig. 122. Terebratula. 
 
 At the present day Brachiopods live in all depths of water and in 
 all seas. 
 
 Lamellibranchiata. The unsymmetrical bivalve shells vary 
 greatly in form. Occasionally some species of Pecten and a few other 
 genera have the right and left sides of the shell equal and similar, 
 and although the right and left valves of the shell are commonly 
 equal, yet in many genera, such as Ostrea, Corbula, and the common 
 scallop, the valves are unequal. As among the other large groups of 
 
 1 See Davidson, " Fossil Brachiopoda " (Palaeontographical Society) ; and 
 Zittel, "Handbuch d. Palseontologie. " 
 
ANCIENT GENERA OF LAMELLIBRANCHIATA. 497 
 
 Mollusca, several of the existing genera date back to the Primary 
 period. An attempt is sometimes made to separate the Primary from 
 the Secondary species of the same genus, and although this is often 
 legitimate from the point of view of classification, we prefer in this 
 summary to use generic names in the large synthetic sense of the 
 older naturalists. 
 
 The Primary rocks are characterised by the extinct genera of bi- 
 valves, Pterincea, Ambonychia, Modiolopsis, Lyrodesma, Cardiomorpha, 
 Cardiola, Conocardium, Megalodon, Anthracosia, Eurydesma, Pachy- 
 dornus, Poseidonomya, Myalina, Sol&niya. A few genera range into 
 the Secondary Rocks, like Poseidonomya, which reaches the Trias. 
 Pleurophorus and Axinus also reach the Trias. 
 
 Ostrea is said to commence in the Carboniferous rocks, but 
 abounds in the Secondary and all newer strata. 
 
 Among the genera characteristic of the Secondary rocks are 
 Inoceramus, which ranges to the Chalk and may begin in the Silurian ; 
 Aucella, which begins in the Permian and ranges to the Gault ; Myo- 
 roncha commences in the Permian and ranges up to the Middle 
 Tertiary. Monotis and Myophoria are Triassic. Sphcera ranges from 
 the Trias to the Lower Tertiary. Opis is limited to the Secondary 
 Strata, as are Exogyra, and Gryphcea. Cardinia, Goniomya, and 
 Vnicardium extend through the Lias and Oolites. Tancredia charac- 
 terises the Lias and Lower Oolites. Hippopodium is Liassic. Ptero- 
 perna, Macrodon, and Pachyrisma are distinctive of the Lower Oolites. 
 Protocardium ranges from the Lower Oolites to the Chalk ; Ceromya 
 
 Fig. 123. Ostrea. Fig. 124. Protocardium. Fig. 125. Iiioceramus. 
 
 and Cercomya range from the Lower Oolites to the Upper Greensand. 
 Discerns and Isodonta are distinctive of the Middle Oolites. Mono- 
 pleura and Requinia range from the Neocomian to the Upper Green- 
 sand. These genera have no recent representatives. 
 
 The living genera which may be considered to have representa- 
 tive species that date from the Primary rocks are Avicula, Area, 
 Cypricardia, Lucina, Cardium, Pinna, Pecten ; Mytilus does not 
 appear till the Permian. With the Trias Lima, Plicatula, Pema, 
 Nucula, Trigonia, Cyprina, Isocardia, Cardita, and Corbis come in. 
 Among genera which appear with the Lias are Astarte, Mactra, 
 Pholodomya, Teredo, Leda. The Lower Oolites first make us ac- 
 quainted with Anomia, Litlwdomus, Limopsis, Venus, Tellina, Cor- 
 bula, Panopea, Anatina, Thracia, and Gastrochwna. There are no 
 new genera introduced in the Middle or Upper part of the Oolites, 
 
 VOL. I. 21 
 
498 
 
 LAMELLIBRANCHIATA OF TERTIARY AGE. 
 
 The Wealden beds first being in Unio, Cydas, and Cyrena. The 
 Neocomian seas introduced Pedunculus, Spondylus, Crassatella, Thetis, 
 Mesodesma, and Solecurtus. The Upper Greensand introduces Cre- 
 
 Fig. 126. Cardium. 
 
 Fig. 127. Trigoiiia. 
 
 nella, Chama, Capsula, Machaera, Clavagella. The Chalk makes 
 Vulsella known. The following genera come in the Lower Tertiary 
 period Nucinella, Lithocardium, Cryptodon, Diplodonta, Pythina, 
 Petricola, Psammobia, Sanguinolaria, Semele, 
 Syndosmya, Donax, Solen, Potamomya, Pandora, 
 Pholas, Teredina, and Cardilia. The Middle 
 Tertiary makes known Artemis, Trigona, Luci- 
 nopsis, Tapes, Venerupis, Lutraria, Gastrana, 
 Mya, Glycimeris, Yoldia, Solenella, Tridacna. 
 Very few genera appear in the Upper Tertiary 
 strata which were not previously known. The 
 Fig. 128. Phoiadomya. range in time of genera is always being carried 
 further back, and needs to be considered by the 
 student as affecting the locality or district under consideration ; 
 for the object of all collections of fossils is to demonstrate the local 
 geographical distribution of life in geological time. The use of a 
 summary consists in the evidence it furnishes of the change of life 
 in time, by indicating epochs when new genera made their appear- 
 ance. It is only, however, after comparison of the later forms with 
 the types with which they have family affinities, that we recognise 
 the evolution which they represent. 1 
 
 Gasteropoda. So far as the palaeontologist is concerned, the soft 
 Gasteropoda which have no shells may be disregarded, and we may 
 define the class palaeontologically as characterised by having the body 
 more or less perfectly contained in a shell which usually consists of 
 one piece, with the aperture often closed by a horny or shelly oper- 
 culum. The shell may be tubular, as in Dentalium, conical, as in 
 Patella ; but more commonly exhibits some degree of spiral growth. 
 Comparatively few genera are absolutely extinct. Murchisonia is 
 
 1 See "Structural and Systematic Conchology," by George W. Tryon, Phila- 
 delphia, 1882. Also Woodward's "Manual of' the Mollusca," of which it is 
 a new edition. 
 
GEOLOGICAL SUCCESSION OF GASTEROPODS. 499 
 
 characteristic of the Primary rocks, Loxonema and Euomplialus range 
 through the Primary to the Trias, Holopea is Cambrian, Macrocheilus 
 is limited to the Devonian and Carboniferous rocks, Nerinoea ranges 
 from the Inferior Oolite to the Upper Chalk, Trocho- 
 toma extends from the Lias to the Coral Eag, Rimula ex- 
 tends from the Great Oolite to the Coral Rag. 
 
 The existing genera which are known to commence in 
 the older Primary rocks are not very numerous ; the oldest 
 are Turbo, Chemnitzia, Patella, Chiton, and Pleurotomaria. 
 With the Devonian rocks Natica, Trochus, Dentalium, and 
 Phasianella begin. Calyptrea and Fissurella may date 
 from the Carboniferous, and Rissoa appears with the Permian strata. 
 With the Secondary strata Emarginula and Cerithium commence 
 in the Trias. The Lias contains the oldest known species of Ptero- 
 ceras, Aporrhais, Nerita, and Pileopsis. Fusus is first known from 
 the Bath Oolite, Scalaria from the Coral Rag, Melania, Paludina, and 
 Valvata are known first from the Wealden, Rostellaria, Pyrula, Tur- 
 ritella, and Vermetus from the Neocomian ; while the Chalk brings 
 in Strombus, Fasciolaria, Gancellaria, Dollum, Conus, Plurotoma, 
 Voluta, Mitra, Cyprcea, Phorus, and Hipponyx. In the Lower Ter- 
 
 Fig. 130. Voluta 
 (Lower Tertiary type). 
 
 Fig. 131. Voluta. 
 (Upper Tertiary type). 
 
 tiary stratxi fossil species are met with of Seraphs, Murex, Typhis, 
 Ranella, Triton, Terebra, Nassa, Purpura, Cassis, Cassidaria, Oliva, 
 Ancittaria, Volvaria, Ovulum, Potamides, Melanopsis, Solarium, Neri- 
 tina, Crepidula. In the Middle Tertiary the genera Turbinella, 
 Haliotis, and Litorina are found for the first time. 1 
 
 Cephalopoda. Cephalopoda include a large number of extinct 
 
 1 Monographs of the British Tertiary Mollusca by Searles Wood have been 
 published by the Palaeontographical Society. 
 
500 EXTINCT GENERA OF TETRABRANCHIATA. 
 
 genera. The Tetrabranchiata is the older group, comprising the 
 animals which dwell in chambered shells, and have a siphuncle 
 running through the chambers. This group is represented at the 
 present day by the Nautilus. The genera are chiefly distinguished 
 from each other by the mode of the growth of the shell, the character 
 of the margin of the septa, and the position of the siphuncle. These 
 differences are all connected with the development of the reproductive 
 organs, and positions of the muscle which holds the animal in the 
 shell. Two principal types are known, represented by the Ammonite, 
 which has the septa folded ; and the Nautilus, which has the septa 
 simple. The Nautilus dates from the Cambrian strata, and is met 
 with in all subsequent deposits. It becomes most specialised in 
 the form of the shell in the Carboniferous limestone, and in folding of 
 septa in the Palaeozoic genus Clymenia y and the Tertiary group Aturia, 
 found in the London clay : Discites ranges from Cambrian to Car- 
 boniferous ; Cryptoceras is Devonian and Carboniferous ; Temno- 
 cheilus and Trematodiscus are both Carboniferous ; Hercoglossa is 
 Cretaceous and Lower Tertiary ; Cinonia is confined to the Lower 
 Tertiary. Lituites and Trochoceras are both lower Palaeozoic genera ; 
 Gomphoceras, Phragmoceras, and Cyrtoceras are genera ranging from 
 the Cambrian to the Carboniferous ; Orthoceras ranges from Cambrian 
 to Trias, and includes multitudes of species. Gyroceras and Gonia- 
 tites range from the true Silurian to the Trias ; Rliabdoceras is Trias- 
 
 Fig. 132. Nautilus. Fig. 133. Goniatites. Fig. 134. Ammonites. 
 
 sic ; and though Ceratites is chiefly known from the Trias, it ranges 
 from the Devonian to the Chalk. 
 
 The genus AMMONITES has been divided into sub-genera, distin- 
 guished by folding of the septa and form of the shell. Among them 
 are Sageceras, which is Permian and Triassic ; Arcestes, Didymites, 
 Lobites, Pinacoceras, Ptychites, Trachyceras, and Tropites, which are 
 exclusively Triassic ; JEgoceras is Triassic and Liassic ; A maltheus, 
 Lytoceras, Pliylloceras are Triassic, Jurassic, and Cretaceous ; Arietites, 
 Harpoceras, ^Ekotraustes, Oppelia, Peltoceras, Stephanoceras, Simo- 
 ceras, &c., are Jurassic; Avpidoceras, Cosmoceras, Haploceras, and 
 Perisphinctes are Jurassic and Cretaceous ; and Acanlhoceras, Olcoxte- 
 phanus, Schlceubachia, Stoloczkaia, &c., are Cretaceous. 
 
 The genera which differ from Ammonites in their uncoiled mode of 
 
DIBRANCHIATE CEPHALOPODA. 
 
 501 
 
 growth of the shell comprise Ancyloceras and Helioceras, which range 
 from the Inferior Oolite to the Chalk ; Crioceras and Toxoceras ex- 
 tend from Neocomian to Upper Greensand ; Hamites, Ptychoceras, and 
 Baculites range from the Xeocomian to the Chalk, while Turrilites 
 ranges from the Gault to the Chalk, in which it is most developed. 
 
 The Tetrabranchiata are therefore of great stratigraphical value ; 
 and the large number of fossil species invests them with importance, 
 for it would be easy to make a Table of strata in which each marine 
 bed was distinguished by well-characterised and easily-recognised 
 Cephalopods. 
 
 The Dibranchiata are especially characteristic of existing seas; 
 they are naked Cephalopods, which have an internal shell or pen, an 
 inkbag about the mouth, and only eight or ten arms, which are pro- 
 vided with suckers or horny hooks. No representative of the group 
 has been found in the Primary rocks ; it begins with the Lias, where 
 the living genus Loligo first appears. Teudopsis, Belotheuthis, Geo- 
 theusis, and Plesiotheusis are all Liassic genera which range into the 
 Lower Oolites. Leptotheuthis is found in the Oxford clay and at 
 Solenhofen. Ommastrephes ranges from the Oxford clay to existing 
 seas ; and Enoplotheuthis from the Oolite to existing seas. 
 
 The most interesting genera of Dibranchiates are the Belemnites, 
 which are almost entirely Secondary. Belemnites range from the Lias 
 to the Chalk, though a belemnite-like fossil has been found 
 in the Tertiary of South Australia. Xipoteuthis is a genus 
 with a fusiform phragmocone found in the Lias. Belem- 
 noteutliis characterises the Oxford and Kimmeridge Clays. 
 Conoteuthis ranges from the Neocomian to the Upper Green- 
 sand. Belemnitella is limited to the Upper Greensand and 
 Chalk. Sepia has survived from the Oxford clay ; and the 
 allied Belemnosis and Beloptera are found in the London 
 Clay and Bracklesham beds. 
 
 Fishes. Although existing fishes are divided into four 
 sub-classes, Palseichthyes, Teleostei, Cyclostomata, and Lep- 
 tocardii, only the two former can be recognised in a fossil 
 state. The Palaeichthyes, which comprise the majority of 
 the fishes found in the older rocks, are divided into five 
 orders, the Dipnoi or mud fishes, the Ganoidei, the Holo- 
 cephala or Chimseras, the Plagiostomata or sharks and 
 rays, and the Chondrostei or sturgeons. Palaeichthyes have 
 a spiral valve to the intestine. Dr. Glinther remarks that 
 they stand to the Teleostei in the same relation as the 
 Marsupialia to the Placentalia among mammals. This view 
 is perhaps favoured by Alexander Agassiz's embryologicai 
 work, 1 which has shown that Teleostean fishes in an early 
 stage of development have the tail symmetrical, without 
 either the homocercal or the heterocercal modification (fig. 136), such as 
 is seen in most of the Ganoids of the Old Red Sandstone and in the 
 
 1 Young, " Stages of some Osseous Fishes," Proc. Americ. Acad., vol. xiii., xiv. , 
 xvii. 
 
 Fig. 135. 
 
 Belernni- 
 
 tella. 
 
502 THE MOST ANCIENT FISHES. 
 
 living Ceratodus. The sharks are one of the most ancient groups of 
 fishes ; at least one of the oldest known fishes from the Ludlow bone 
 bed, Onchus, is a shark. It is succeeded in the Devonian rocks by Dime- 
 
 Heterocercal. Homocercal. 
 
 Fig. 1 36. -Fish tails. 
 
 racanthus and H&macanthus. In the Carboniferous rocks succeed Ora- 
 canthus, Gyracanthzts, Tristychius, Astroptychius, Ptychacanthus, and 
 Sphenacanthus. In the Coal-measures are found such types as Clada- 
 canthus, Leptacanthus, and Gyropristis. Leptacanthus ranges from the 
 Coal to the Oolite. The Trias yields some peculiar genera like 
 Nemacanthus and Liacanthus ; while Aster acanthus, Myriacanthus, 
 and Pristacanthus are known from the Oolites. These are a few of 
 the genera founded upon fish spines. Several existing genera of 
 sharks are actually or closely represented in the strata, thus Corax, 
 closely allied to the Blue Shark, is Cretaceous, and Tertiary. Car- 
 charias is found in the Chalk, in which Hemipristis and Galeocerdo 
 occur. The Lamna family, which comprises the living porbeagles, 
 first appears with Carcharopsis in the Carboniferous period. Carcha- 
 rodon, which yields teeth in the Crag five inches long, is not known 
 to attain a greater length than forty feet, though teeth as large as the 
 crag teeth occur at the bottom of the Pacific : it is recorded from the 
 Maestricht beds and ranges through the Tertiary. The genus Lamna, 
 which includes Oxyrliina, is abundant in the Cretaceous and dates back 
 to the Wealden. Among allied fossil types are Splienodus from the 
 Oolites ; Gomphodus and Ancistrodon from the Chalk. The grey 
 sharks, Notidanus, are represented in the Brown Jura, and in this 
 country are found in the Upper Greensand and London Clay. JEllopos 
 is found in the Solenhofen Slate. The dog-fishes are well represented 
 in the Secondary strata, and are known as Scylliodus, Palceoscyllium, 
 and Pristiurus, which is a well-known European genus. The Hybo- 
 donts are all extinct. Cladodus, in the Devonian and Carboniferous, 
 is the oldest representative of the group. Hybodus is said to occur in 
 the Carboniferous and in the Tertiary, but is characteristic of the 
 Secondary Rocks. The Cestraciodonts appear with Ctenoptyclnus 
 in the Devonian, Psammodus, Cocliliodus, and Polyrhizodus in the 
 Carboniferous ; Strophodus and Acrodus in the Oolites ; and Ptychodus 
 in the Chalk. The picked-dogs are represented in the Lias by Palceo- 
 spinax, but the genus Spinax does not date back further than the 
 Secondary Rocks. 
 
 OrtUacanthus of the Carboniferous may have been a monk fish ; 
 
SUCCESSION OF FOSSIL PAL&ICHTHES. 
 
 503 
 
 the Oolitic Thaumas belong to this type. Squaloraja of the Lias 
 represents the living Pristiophorus. The Rays are represented by the 
 saws of Pristis in the London clay and newer rocks. Spathobatis 
 
 Dorsal spine. 
 
 Fig. 137.- 
 
 Tooth. 
 
 -Hvbodus. 
 
 is a ray from the Oolites. The torpedoes appear in the genus Cyclo- 
 batis in the Cretaceous of Lebanon, though the genus Torpedo is first 
 found in Monte Bolca. Arthropterus from the Lias was a true ray, 
 and ray spines are met with in the Coal and in Crag. The sting- 
 rays are numerous in the Lower Tertiary ; the eagle ray Myliobatis, 
 with the allied genera ^Etobatis and Rhinoptera, characterise the 
 Lower Tertiary, and Zygobatis is found in the Crag. The Chimaeras 
 appear in the Devonian, according to Dr. Newberry, with Rhynclw- 
 dus. Ganodus is found in the Lower Oolites. Ischyodus ranges from 
 the Lias to the Chalk. Edaphodon is Cretaceous, but is represented 
 in the Tertiary. Elasmodus and Psaliodus are Tertiary. Callorhyn- 
 chus, a living type, is found in the Cretaceous of New Zealand. 
 
 The Ganoid fishes have been divided into eight sub-orders. The 
 Placodermi are extinct ; they range from the Lower Ludlow to the 
 Carboniferous, but are characteristic of the Old Red Sandstone, and 
 comprise Ptericlithys, Coccosteus, Dinichthys, Cephalaspis, and the 
 allied forms Pteraspis, Scaphaspis, Auchenaspis, &c. Another sub- 
 order, distinguished by carrying large spines, is represented in the Old 
 Red Sandstone and Carboniferous rocks by Acantliodes and Cheira- 
 canthus. The Dipnoi are represented by the existing Ceratodus, which 
 is found in the Trias and Lower Oolites, but is unknown in newer 
 strata. Closely allied are the extinct Dipterus and Heliopus of the 
 Devonian ; Plianeropleuron belongs to the same rocks. The sturgeons 
 are not known prior to the London Clay, though Polyodon is repre- 
 sented in the Lias by Chondrosteus. The Polypteroid fishes comprise 
 
 Fig. 138. Polypterns (living), 
 a. pectoral fin ; 6. ventral fin ; c. anal fin. 
 
 the Saurodiptemni, including the Old Red Sandstone Osteolopis and 
 Diplopterus, and the Carboniferous Megalichthys. The Coelacanthi 
 
504 
 
 FOSSIL GANOID FISHES. 
 
 include Ccdacanthus from the Coal, Macropoma from the Kimmeridge 
 Clay and the Chalk, Rldzodus, &e. The genera allied to Holoptychius 
 include Glyptolepis, Dendrodus, Glyptopomus, Gyroptichius, and other 
 
 Fig. 139. Osteolepis restored. 
 . pectoral ; b. ventral ; c. anal; d, e. dorsal. 
 
 Fins : a. pectoral 
 
 Devonian and Carboniferous genera. The Pycnodonts were ganoids ; 
 they are represented by Pleurolepis and Homceolepis from the Lias, and 
 many genera of the Secondary rocks, such as Gyrodus, Microdon, 
 
 Fig. 140. Holoptychius (fragment of jaw). 
 
 Pycnodus, and Misodon, some of which range to the Tertiary. The 
 type represented by the living Lepidosteus is a large one. Lepidostens 
 itself is a Lower Tertiary genus. The Sauroid fishes are characteristic 
 of the Lias and Oolites, and comprise such genera as Semionotus, 
 Euynathus, Pholidophorus, Pachycormus, &c., Tetragonolepis from the 
 Lias. Lepidotus ranges from the Lias to the Chalk. Aspidorhynchus 
 characterises the Middle Secondary strata. The allies of Palceoniscus 
 range between the Old Red Sandstone and the Lias. CMerolepis, Cos- 
 moptychius, PalceoniseMS, Amblypterus, and Pygopterus are characteristic 
 Primary genera, while the Lias yields other forms like Centrolepis and 
 Cosmolepis. The allies of Platysomus are confined to the Carbonifer- 
 ous and Permian strata. The living American genus Amia is the 
 type of the last section of the Ganoids. Leptolepis characterises the 
 Lias and Oolites ; and Caturus, well known from Solenhofen, is found 
 in the Chalk. 
 
 The Teleostean fishes are divided into six Orders the Acanthop- 
 terygii, Pharyngognathi, Anacanthini, Physostomi, Lophobranchii, 
 and Plectognathi. 
 
 The Acanthopterygii are well represented in a fossil state. The 
 Perch is represented at Oeningen ; the Bass, Lalrax, the genus Ser- 
 
SUCCESSION OF ACANTHOPTERYGIAN FISHES. 505 
 
 ranus, and many other living genera, occur in the Tertiary of Monte 
 Bolca, where the perch of the Nile and Ganges, Lates, is also found. 
 The Squamipinnes are also well represented at Monte Bolca, and in the 
 Calcaire Grossier of the Paris basin by Holacanthus, Pomacanthus, 
 Ephippium, Scalophagus of the Indian seas, and Toxotes. The sea- 
 breams, though found in the Cretaceous of Lebanon in the living 
 genera Sargus and Pagellus, are known in the older Tertiary from 
 extinct genera, Sparnodus Sargodon, &c. The genus Scorpcena is 
 found in the Lower Tertiary of Oran, Beryx dates back to the Chalk, 
 and many allied genera occur in Cretaceous rocks ; while Holocentrum 
 and Myripristis are allies from the Tertiary of Monte Bolca. Sword- 
 fishes occur in the Chalk, and are represented in the London clay and 
 Lower Tertiary by the extinct genus Ccelorhynchus. The TrichiuridaB 
 are represented in the Cretaceous rocks by Enchodus, and Anenchelum 
 occurs with other typical genera in the Lower Tertiary. The schists 
 of Glarus yield Palceorhynchus, and Hemirliynchus is found in the 
 Paris basin. Acantliurus and Naseus are both represented in Monte 
 Bolca. The Horse-Mackerel group is represented in Cretaceous rocks by 
 Platax, Vomer, &c. ; but many other living types occur at Monte Bolca 
 and in newer Tertiary beds. Among extinct genera are Semiophorm 
 and Pseudovomer. The John Dory, Zeus, is found in Tertiary rocks, 
 GoniognatJius is a fossil of Sheppey, and Mene occurs at Monte Bolca. 
 
 The Scomberoid fishes are not known prior to the Lower Tertiary, 
 in which the mackerel, Scomber, and tunny, Thynnus, are common. 
 The Eocene schists of Glarus yield extinct genera, Isurus and Pal- 
 imphyes. The Trachinidae, carnivorous bottom-feeding fish, are repre- 
 sented in the Lower Tertiary rocks by Callipteryx. Two or more 
 gurnards, Trigla, and fishes closely allied to the Miller's Thumbs, 
 Coitus, are found in the Lower Tertiary. The flying gurnard, Dady- 
 lopterus, is represented in the Chalk of Lebanon by Petalopteryx. 
 Gobies first appear in the Chalk ; the Blennies are doubtfully repre- 
 sented in the Lower Tertiaries of Monte Bolca, but the Barracudas 
 are represented in the Chalk and London clay by Hypsodon, Portheus, 
 and Saurocephalus ; and Sphyrcena is common in the Lower Tertiary. 
 
 The living Atherina is represented in the Monte Bolca beds by 
 Mesagaster ; the grey mullets do not occur prior to the Tertiary 
 period. The great marine Sticklebacks are another group which 
 appear with the Lower Tertiary, Fistularia and Aulostoma being found 
 at Monte Bolca and Glarus. The remarkable AmpTiisile, sheathed in 
 dorsal armour, is another Monte Bolca fossil. 
 
 The Pharyngognathi have the pharyngeal bones in the gullet 
 blended together. They include four families. The Pomacentridae 
 are represented at Monte Bolca by Odonteus, allied to the living 
 Heliastes. The Wrasses are represented by many labroid genera in 
 the Lower Tertiary, such as Egertonia, from the London Clay ; Pliyl- 
 lodus and other types occur in the Miocene, while the genus Labrus 
 is found in the Swiss molasse. 
 
 The Anacanthini or soft-finned fishes comprise the Cod tribe and 
 the flat fishes. The Ganoid group is not abundant in a fossil state. 
 
5o6 FOSSIL ACANTHOPTERYGIAN FISHES. 
 
 The hake Merluccius, and fishes allied to the cod, are found in the 
 London Clay. Paloeogadus and like forms occur in the schists of 
 Glarus. The Pleuronectidse are carnivorous fishes, dating from the 
 Tertiary period. A species of turbot, Rhombus, is found at Monte 
 Bolca, and soles occur near Ulm. 
 
 The Physostomi comprises fishes with all the fin rays jointed, 
 though the first rays of the dorsal and pectoral fins are sometimes 
 ossified ; it includes thirty-one families. The Siluroid fishes cer- 
 tainly date from the Cretaceous period, though they become charac- 
 teristic of the Tertiary rocks. The deep-sea Scopelidae are represented 
 by the so-called Osmeroides from Lebanon, and other genera allied to 
 Saurus from Comen in Istria and the Middle Tertiary of Licata in 
 Sicily. The Carps appear in the Middle Tertiary deposits of (Eningen 
 and the Lignites of Bonn and Bilin, and are mostly referable to exist- 
 ing genera, Cyprinus, Tincta, Leuciscus, Rhodeus, Cobitis. Cyprinodon 
 is found at Aix in Provence and the Middle Tertiary of Germany. 
 The Scombresocidse are represented at Monte Bolca by ffolosteus, and 
 in some Middle Tertiary beds by the gar pike, Belone. The true pike, 
 Esox, dates from the (Eningen beds. The Osmeroides of the Chalk 
 is allied to the smelt, and is associated with Aulolepis, Tomognathus, 
 and Acrognathus, which probably belong to the salmon family. 
 
 The herrings are represented in the Gault by ThrissopaUr ; in the 
 Chalk by Opisthopteryx, while Clupea, Engraulis the anchovy, and 
 Chanos of the Indian seas are found in the Lower Tertiary associated 
 with many extinct genera. The Hoplopleuridas is an extinct family, 
 first known in the Chalk, and extending into the Tertiary, represented 
 by Saurorhamphus, Pelargorhynckus, <fec. Finally, the Eels are 
 represented in the London Clay by Khynchorkinus, and in the Monte 
 Balca beds occur the living Anguilla, and the tropical Ophichthys. 
 
 The Lophobranchiate order is represented by the Tertiary genus 
 Solenorhynckus, the true pipe fish Syngnathus, and the sea-horse 
 Calamastoma, found at Monte Bolca. 
 
 The order Plectognathi comprises fishes with rough scales, or 
 spiny ossifications in the skin, and few imperfectly ossified vertebrae. 
 The Scleroderm family is represented by Glyptocephalus of Sheppey, 
 which resembles the living file-fish, Batistes ; the box-fish, Ostracion, 
 occurs at Monte Bolca ; and the schists of Glarus yield Acanthoderma 
 and Acanthopleurus. The Gymnodonts are represented in a fossil state 
 by a Diodon at Monte Bolca, Enneodon from Monte Postale, and 
 Trigonodon from the Middle Tertiary of Turin. 1 
 
 If the Lampreys occur in a fossil state, they are only known from 
 their dental plates. Organs which closely resemble the teeth of living 
 Cyclostomata are frequently met with in the Cambrian, Silurian, and 
 Devonian rocks, and are known as Conodonts. 
 
 Amphibia. The amphibia are a group which exemplify the evo- 
 lution of animals in the circumstance that they begin life as fishes, 
 
 1 See Pictet : "Traite de Paleontologie," Tom. II., Gunther's "Study of Fishes, " 
 and monographs by E. Ray Lankester, and by Traquair, in Palseontographical 
 Society's Publications. 
 
EXTINCT AMPHIBIA. 
 
 507 
 
 pass through a metamorphosis, and become air-breathers." The chief 
 characters are the two occipital condyles to the skull, the absence of 
 sternal ribs, and the naked skin. The group as known at the pre- 
 sent day includes three sections first Saurobactrachia or Urodela, 
 which comprises the Proteidea and Salamandridea ; secondly, the 
 Gymnophiona, which includes the Caeciliidse ; and thirdly, the 
 Anura, which comprises frogs, toads, and their allies. The extinct 
 Labyrinthodontia have the external skeleton much developed, especi- 
 ally on the ventral side of the body. Professor Cope has united the 
 Labyrinthodontia with the Ganocephala and Microsauria in one group, 
 which he names Stegocephali. "With one or two exceptions the fossil 
 types belong to this group, and are found between the Carboniferous 
 rocks and the Trias. Among the Carboniferous genera l in Europe are 
 Anthracosaurus, Loxoma, Bratrachiderpeton, Pteroplax, P holiday aster, 
 Ichthyerpeton, Pholiderpeton, Lepterpeton, Urocordylus, Erprtocephalus, 
 Keraterpeton, Megalerpeton, Ophiderpeton, Dolicosoma ; Protriton and 
 Pleuronura are found in France. Archcegosaurus is common in Ger- 
 many. In North America these organisms are as well developed. In 
 Nova Scotia Den<lrerpeton,Hylonom us, Hylerpeton,J3aphetes, andJiWm- 
 rus occur, and in the coal-field of Ohio Amphibamus, Brachydectes, 
 Eupdor, Eiinithorax, &c. Several of these types range to the Permian, 
 from which Fritsch 2 has described a large fauna in Bohemia. It in- 
 cludes the genera Branchiosaurus, Sparodus, Hylonomus, Dawsonia, 
 Melanerpeton, J)olichosoma, Ophiderpeton, Palceosiren, Adenoderma, 
 Urocordi/lus, Keraterpeton, Limnerpeton, Hyloplesion, Seeleya, Ricno- 
 don, Orthocosta, Microbrackis, &c. 
 
 Fig. 141. Branchiosaurus salamandroides (Fritsch),3 twice the natural size. 
 
 Dr. Fritsch describes Branchiosaurus as resembling the Earth- 
 
 1 See C. L. Miall : Rep. Brit. Assoc. 1873, 1874. 
 
 2 "Fauna der Gaskohle, und der Kalksteine, der Permformation Bohmens." 
 Prague. 
 
 3 From Fritsch's "Fauna der Gaskohle," &c. Prag. 1879. Electrotypes of the 
 specimens figured have been published. 
 
508 
 
 THE SKULL OF BRANCHIOSA URUS. 
 
 salamanders, especially the young forms, in possessing gills ; and in 
 the broad head rounded in front, the short thick well-developed 
 extremities terminating in digits, and in the rudder-like tail which 
 strongly suggests the larval forms of living Urodela. The skin was 
 dense, and its impression is preserved and shown in the figure. The 
 form of the skull will be gathered from the figure, 142, in which 
 Professor Fritsch has restored the upper surface of the skull of 
 tiranchiosaurus, enlarged six times. The lettering indicates the same 
 
 Fig. 142. 
 
 bones as in the skull of Dolichosoma. There are about 20 vertebrae 
 in the neck and back, and 21 in the tail. 
 
 Dolichosoma longissimus (Fritsch) may serve as another type 
 of amphibian structure. It is described by Professor Fritsch as dis- 
 tinguished by having the ribs twice as long as the vertebrae. In form 
 it closely resembles the whip-snake (DendropMs). The fragment 
 found measures 60 cm., and indicates for the entire animal a length 
 of about a metre. An impression of the skin is preserved ; enlarged 
 45 times it shows a fine granular structure, so that if any scales existed 
 they are not preserved. 
 
 The eyes are placed rather behind the middle line of the skull, 
 and are separated by an interspace of two-thirds their diameter. The 
 parietal foramen (Pa) lies far behind the orbits. The anterior nares 
 cannot be distinguished. The nasal bones (N) are anchylosed toge- 
 
THE SKULL OF DOLICHOSOMA. 
 
 509 
 
 ther, and forked behind. The premaxillaries (im) resemble those of 
 Siren lacertina. The maxillary (ms) is a strong bone extending to 
 the anterior extremity of the snout, and to the middle of the orbit; 
 
 Fig. 143- 
 Dolichosoma longissimum, upper surface of skull restored, enlarged six times. 
 
 it carries about 15 smooth teeth, which are curved backward: there 
 may have been two rows. The frontal (F) and parietal (Pa) bones 
 are blended into one mass. The frontal terminates in front in three 
 processes. The epiotic bone (Ep) terminates at its outer angle in a 
 knob-like swelling. The teeth of the lower jaw, 20 in number, were 
 
5io 
 
 THE SKELETON OF DOLICHOSOMA. 
 
 smaller than those in the upper jaw. There are indications of a 
 branchial skeleton extending to the sixteenth vertebra. One hundred 
 and fifty vertebrae are preserved, but there may well have been fifty 
 more. In the neck the ribs are simple, in the body they are compli- 
 cated, and in the tail they are absent. The dorsal vertebras have a 
 depressed elongated quadrate form, with a ridge in the position of 
 the neural spine ; the neural arch is constricted in the middle, and 
 terminates at its four corners in zygapophyses. The transverse pro- 
 cesses are compressed, directed downward and outward, and given off 
 
 Fig. 144. 
 
 Ddichosoma longissimum (Fr.), restoration half natural size. From the Coal 
 
 of Nyran. 
 
 from the lower margin of the front of the centrum. The centrum 
 has a circular cup at each end, and the cones unite, so that the noto- 
 chord was persistent. The form of this vertebra may be compared 
 
FOSSIL CAINOSAURIA. 511 
 
 with that of Epicrium glutinosum. Each vertebra has a slightly 
 different shape. The tail vertebrae are smaller and shorter, and have 
 weak zygapophyses, and are perforated in the middle of the side as 
 though for the passage of an intervertebral nerve. The ribs of the 
 neck at the sixteenth vertebra are twice as long as the centrum. The 
 dorsal ribs show towards the proximal end two peculiar processes. 
 The proximal end is bent at nearly a right angle to the body of the 
 rib ; the processes occur at the flexure, one is dorsal, and the other 
 has a ventral direction. These processes present some analogy to the 
 uncinate processes on the ribs of birds. 
 
 Other Permian types in this country are Dasyceps and Lepidoto- 
 saurus. In Russia, Zygosaurus and Melosaurus. The Trias yields 
 Mastodonsaurus, Capitosaurus, Trematosaurus, Metopias, Labyrintho- 
 don, Diadetognathus ; from Australia, Bothriceps ; from South Africa, 
 Micropholis ; from India, Gonioglyptas and Pachygonia ; and Bra- 
 chyops, which is said to be Jurassic. 
 
 The modern batrachians are only found in the Tertiary strata. 
 They include species of Rana, Palceobatrachus, Asphcerion, Latonia, 
 Pelophilus, Pelceophrt/nos, and the Urodelians are the great Andreas, 
 Triton and Orthophya. Fossil amphibians of the Trias, like Masto- 
 donsaurus, attain to a larger size than living types, and the ancient 
 amphibia are in no sense the ancestors of the living amphibia ; so 
 there is no evidence of evolution of existing reptilia from fossil reptilia, 
 because the surviving reptiles are less specialised or degraded forms. 
 Types become degraded because higher organisms in the existing con- 
 dition of nature restrict the conditions of their activity, in much the 
 same way that the higher branches of the human race affect aboriginal 
 races. 
 
 Fossil Reptiles. The Secondary Eocks have often been termed 
 the Age of Reptiles. 
 
 The reptilia are divided into two sections ; first, Cainosauria, 
 which comprises the Lacertilia and Ophidia ; and, secondly, Palceo- 
 sauria, which includes Rhynchocephalia, Chelonia, Crococfilia, Plesio- 
 sauria, Anomodontia, Ichthyosauria, and Dinosauria. The Caino- 
 sauria attain their maximum development in the existing life of the 
 world, but all the groups of the Palseosauria (except, perhaps, the 
 Chelonia) have their chief development in the Secondary rocks. 
 
 CAINOSAURIA. Ophidia. Very little is known of serpents in a 
 fossil state. The oldest serpent yet known is the sea-serpent of the 
 London Clay and Bracklesham beds, Palceophis, which reached a 
 length of from twelve to twenty feet. From Hordwell the genus 
 Paleryx is obtained, which closely resembles Eryx. A poisonous 
 serpent has been described from Salonica under the name Laophis. 
 Coluber is known from the schists of ^Eningen. 
 
 Pylhonomorpha. Under this name Professor Cope indicated Mosa- 
 saurus and its allies. Professor Owen regards these animals as a family 
 of the Lacertilia. The Lacertilia might certainly be subdivided into 
 several sub-orders, and there can be no reason for hesitating to admit 
 Mosasanrus to at least the same rank. The name Pythonomorpha is 
 
512 EXTINCT REPTILES OF THE LIZARD TYPE. 
 
 taken merely to indicate the elongated form of these marine animals, 
 which had the anterior limbs expanded into fins like those of Chelo- 
 nians or Cetaceans. Mosasaurus in this country is found in the 
 
 Fig. 145. Head of Mosasaurus. 
 
 Upper Chalk ; it is best known from the Maestricht beds. The 
 genera recognised in the Niobrara group of North America are Platy- 
 carpus, Clidastes, Leiodon, Sironectes. 
 
 The Dolickosauria is another well marked sub-order distin- 
 guished by having many vertebrae in the neck ; the intervertebral 
 articulation includes a zygosphene ; the limbs are small. The genera 
 Dolichosaurus and Saurospondylus are both from the Chalk. 
 
 The HomoBosaurifi * is a group of Lizards with biconcave vertebrae 
 which ranges from the Trias to the Solenhofen Slates, and includes 
 Telerpeton ' 2 in the former, and Sapkceosaurus and Homceosaurus 3 in 
 the latter. 
 
 Lacertilia. The typical Lizards are represented in the Tertiary 
 strata of Europe by Iguana and Lacerta ; and some fragments have 
 been referred to Scincus and to Anguis. 
 
 PAL^EOSAURIA. Rhynchoce pit alia. This group, now almost ex- 
 tinct, has for its sole surviving representative the Hatteria of New 
 Zealand. This animal is often classed with the lizards, which it 
 resembles in form and size. It, however, has so many characters in 
 common with the older crocodilia and their allies, that it is conveniently 
 grouped with the Palseosauria rather than with the Cainosauria. Pro- 
 colophon, and many similar animals from Cape Colony regarded as 
 Triassic, may be placed in this group. Professor Huxley would place 
 here the British Triassic Hyperodapeton and Rhynchosaurus. Pro- 
 fessor Cope has referred many American genera to this type. 
 
 Chelonia. Chelonians, so far as is known, retain the carapace 
 and plastron, more or less developed in all fossil forms. The 
 Chelonia have been divided into three principal groups first, Dema- 
 tochelyidae, in which the carapace is not developed, but represented by 
 a tesselated bony skeleton within the skin, such as is seen in the 
 
 1 Huxley : " Anatomy of Vertebrates," 1871. 
 
 2 Huxley : Q. J. G. S., vol. xxiii. 
 
 3 Von Meyer : " Fauna der Vorwelt." 
 
EXISTING ORDERS OF PAL&OSA URIA. $13 
 
 leathery turtle Spharyis and the fossil Psepliophorus ; second, the 
 Peltochelyidae, including the Trionychidae, in which there are no 
 horny scutes, but a granular bony skeleton, like that of the Dermato- 
 chelyidre, is superimposed upon the bony skeleton of the carapace and 
 plastron ; third, the Aspedochelyidae, which comprises turtles, emy- 
 dians, and tortoises, distinguished by symmetrical horny scutes cover- 
 ing the bony carapace. 
 
 We might expect the simple shield of Sphargis to be the oldest, 
 then theoretically it should precede Trionyx ; but Sphargis is only 
 known from the Upper Tertiary, and Trionyx dates back to the Creta- 
 ceous of the United States, The majority of Chelonian fossils belong 
 to the Emydian type. Among tortoises may be mentioned the gigantic 
 ColossodielySj of the Siwalik beds, and many species of Testiido. 
 Professor Cope records Pmtostega, the type of a family in which the 
 large dermal bones are separate from the ribs. Toxoclielys belongs to 
 the marine chelonia. In Europe, true marine chelonians are best 
 known from the Chalk and London Clay in .this country, though they 
 are said to date back to the Purbeck. The Emydians comprise, 
 among other types, Pelobatochelys and Enaliochelys from the Kimme- 
 ridge Clay, which also yields PlesiorJtelys, and, according to Sauvage, 
 Platemys, The Upper Jurassic rocks also yield Thalassemys, Tropid- 
 emys, CraspedocJielys, which have not been recognised in this country. 
 Eurysternon is characteristic of the Solenhofen slate, Tretosternon and 
 Pleurosternon characterise the Purbeck and Wealden beds, Chitra- 
 cepltalus is found in the Wealden of Belgium, which also yields Pelto- 
 chelys. Protemys is found in the Lower Greensand. Rhinochelys and 
 Trachydermochelys characterise the Cambridge Greensand. In the 
 Tertiary rocks of the Continent are found Chelydra, Trachaspis, 
 Aplwlidemys. Pulceochelys. There are many forms referred to Emys, 
 such as most of those which occur in the London Clay x and the Middle 
 Tertiary of the Continent. 
 
 Crocodilia.^ The antecedent group from which a type has been 
 modified is often to be detected in the vertebral characters of the tail. 
 Thus in birds the caudal vertebras are often bi-concave, and a similar 
 character is found in the tails of crocodiles. Hence, just as birds have 
 existed with bi-concave vertebras, so also have crocodiles. The Croco- 
 dilia are hence often divided into two groups, viz., the Teleosauria, 
 comprising the genera of the older Secondary rocks with flat or concave 
 intervertebral articulation, and the Eucrocodilia, comprising the Cre- 
 taceous, Tertiary, and existing procoelous types. 
 
 The teleosaurs 3 were preceded by Belodon or Phytosaurus of the 
 Trias. Tdeosaurus has a form of head like the gavial of the Ganges ; 
 but the posterior nares do not extend so far back. The genus has 
 been divided into several sub-genera. Mystriosaurus hast he skull flat- 
 and the eyes directed upward ; it is found in the Lias. Macrospon- 
 dylus only differs from Mystriosurus in the greater length of the ver- 
 
 1 Palaeont. Soc. 
 
 2 See W. K. Parker, F.R.S. : Trans. Zoological Soc., vol. xi., part 9, " On .Struc- 
 ture and Development of the Skull in Crocodilia." 
 
 3 E. E. Deslongschamps : " Notes Palaeontologiques," 1863-1869. 
 VOL. I. 2 K 
 
514 PLESIOSAURIA. 
 
 tebrae and the proportions of the limbs, which the palaeontologist does 
 not regard as generic characters. Pelagosaurus is distinguished by the 
 interspace between the eyes being greater than the diameter of the eye ; 
 it is characteristic of the Lias. The name Teleosaurtis has been 
 reserved for the crocodiles of the Inferior and Bath Oolite, which 
 have large and approximating orbits. The Teleosaurs of the Kimme- 
 ridge Clay have the eyes lateral, and have been referred to the genus 
 Steneosaurus. Gnathosaurus of the lithographic slate is a crocodile 
 with lateral eyes, of doubtful affinity. In the Purbeck and Wealdon 
 beds some crocodiles occur which in cranial characters make a transi- 
 tion towards the modern type. They comprise Goniopholu, Petro- 
 suchus, and Theriosuchus. 
 
 The Eucrocodilia or Proccelia appear for the first time in the Cam- 
 bridge Greensand, Gosau beds, and Greensand of New Jersey. They 
 are met with in the London Clay and Lower and Middle Tertiaries 
 of Europe. The gavial is found in the Bracklesham beds, and the alli- 
 gator in the Headon beds. 
 
 Plesiosauria. The Plesiosauria comprise a variety of saurians. 
 They include two principal groups the Nothosauria of the Trias and 
 the Plesiosauria which range from the Rhsetic beds to the Chalk. 
 These groups are linked together by the triassic genus Neusticosaurus, 
 which exhibits a land animal in process of acquiring the aquatic limbs 
 of the plesiosaui. 1 At first sight, plesiosaurs offer many characters in 
 common with chelonians, such as the forms of the pelvic bones and 
 pectoral bones, and even in the shape of the larger-limb bones, espe- 
 cially as seen in the Nothosauria. But as the anatomy is examined 
 in detail, there are many suggestions of affinity to crocodiles. All 
 plesiosaurs have the palate closed, and the teeth in distinct sockets, 
 the eyes midway in the side of the head, usually a long neck, the cer- 
 vical ribs occasionally have the articulation double, frequently single. 
 Elasmosaurus has no interclavicle. The terrestrial plesiosaurs become 
 greatly modified, and apparently develop into the Anomodontia. The 
 British fossil genera are at present only defined in a few instances ; 
 they include the large-headed form with triangular teeth and short 
 cervical vertebrae named Pliosaurus, ranging from the Kimmeridge 
 Clay to the Neocomian beds. In the Oxford Clay there is a long- 
 necked type called Murcenosaurus, with an imperfectly developed pec- 
 toral arch, and in the Kimmeridge Clay another Elasmosaurian type 
 named Colymbosaurus. There is a stiff-backed plesiosaur called Stcreo- 
 saurus in the Cambridge Greensand, and Polyptychodon is a genus of 
 the Greensand and Chalk. All these genera are remarkable for the 
 uniform character of the vertebrae, which are moderately cupped (ex- 
 cept in Stereosaurus, where they are flat), and in the dorsal region have 
 the ribs supported upon the neural arch. 2 
 
 Anomodontia. The Anomodontia have the palate more open 
 
 1 See Monographs on Sauropterygia and Ichthyopterygia by R. Owen, F.R.S., 
 published by Palseontographical Society. 
 
 2 Dr. Liitken regards Lariosaurus as a Plesiosaur. See his valuable " Skild- 
 ringer af Dyrelivet i Fortid og Nutid." Kjobenhavn. 1880., 
 
ICHTHYOSA URIA . 515 
 
 than in Plesiosaurus. They are divided into three sub-orders, viz., 
 the Dicynodontia, distinguished by two tusks in the position of 
 canine teeth ; the Cynodontia, which have teeth anterior and pos- 
 terior to the canines on the mammalian plan, so that Sir Richard 
 Owen has termed them " Theriodonts ; " and, thirdly, the Crypto- 
 dontia, in which the jaws are edentulous, and teeth, when present, 
 are palatal. This group is chiefly known from South African fossils. 
 Rkynchosaurus and H ~ yperodapedon from the British Trias have been 
 referred by Sir R. Owen to the Cryptodontia. Placodus may be placed 
 here. Many of these Anomodont fossils present so remarkable a re- 
 semblance to the lowest types of mammalia in various parts of the 
 skeleton, that the reptiles seem to foreshadow the higher group. 1 
 
 IchtJiyosauria. The Ichthyosauria have a type of skull which 
 differs from that of Dinosaurs chiefly in the enormous prenarial 
 elongation of the snout, and the less perfect ossification of the skull. 
 The squamose overlapping of the bones is also seen in fishes and 
 Cetaceans, and is connected with aquatic habit. The biconcave con- 
 dition of the vertebral column is closely paralleled in some Dinosaurs 
 from South Africa. The double-headed articulation for the rib is a 
 character only found in association with well-developed ribs in croco- 
 diles and higher vertebrata. The circumstance that the articulation 
 for the ribs descends in position from the neck to the pelvis is asso- 
 ciated with marine habit, and the absence of organs ministering to 
 locomotion on land. The scapular arch is in many respects Dino- 
 saurian ; the pelvic arch is so feebly developed as only to suggest a 
 generalised reptilian type. Many species of Ichthyosaurus were vivi- 
 parous. 2 Few genera have at present been defined. A genus of the 
 Oxford clay has been termed Ophthalmosaurus, in which there are 
 three bones in the forearm, and the clavicle and interclavicle arc 
 united by suture into one mass. A femur from the Cambridge Green- 
 sand has been referred to a genus Cetarthrosaurus, and is regarded as 
 Ichthyosaurian. Professor Marsh has described an American toothless 
 ichthyosaurus named Sauranodon, which, in other respects, closely re- 
 sembles the European type. There is no evolution of the Ichthyo- 
 saurs at present known comparable to that of Dinosaurs. The oldest 
 species are found in the Rhsetic beds, the newest in the Chalk. The 
 remains found in the London Clay are probably derived fossils. 
 
 Dinosauria. The whole of the Dinosauria, so far as is at present 
 known, were land animals, and their remains are most abundant in 
 those formations which give evidence of near proximity to land, such 
 as Trias, Wealden, and Greensand, though they are represented in 
 almost all Secondary deposits. It is probable that Protorosaurus, 
 found in the Permian, and best known as the fossil monitor of Thur- 
 ingia, must be included in the Dinosaurian order. The Trias has 
 yielded (especially from the Bristol conglomerate) remains of Theco- 
 dontosaurus, Palceosaurus, and Teratosaurus ; but the finest mate- 
 rials of this age are found in Wiirtemberg, and belong to the genus 
 
 1 See Catalogue of the South African Reptilia in the British Museum. By 
 R. Owen, F.R.S. 2 Kept. Brit. Assoc., Swansea, 1880. 
 
5i6 HISTORY OF THE DINOSAURIA. 
 
 Zanclodon, distinguished by having only two vertebrae in the sacrum. 
 The Lias has j'ielded the well-armoured genus Scelit/osaurus, and some 
 undescribed forms. Megalosaurns, though best known from the 
 Stonesfield slate and Wealdeii beds, appears from teeth to date back 
 to the Lias, and survive to the Gosau beds, which are of the age of 
 the Upper Greensand. Cetiosaurus abounds in the Forest Marble, 
 Kimmeridge Clay, and Wealden beds. Cryptosaurus is found with 
 Megalosaurus in the Oxford Clay. Priodontognathus probably belongs 
 to the Calcareous grit. Kimmeridge Clay genera include Omosaurus 
 and Gigantosaurus. The Wealden beds are characterised by lyuanodon, 1 
 perfectly preserved in Belgium, which ranges down to the Kimmeridge 
 Clay, and perhaps upward to the Upper Greensand. With it occur 
 Hylceosaurus, Hypsilosphodon^ Pelorosaurus, Vectisaurtts, Polacanthus, 
 and Ornithopsis. Craterosaurus is found in the Neocomian Sands. 
 The Upper Greensand of Cambridge includes such genera as Acantho- 
 polis, Macrurosaurus, Anoplosaurus. Syngonosaurus,&n<\. Eucercosaurus? 
 The Gosau fauna includes 4 the genera Mochlodon Struthiosaurus, 
 Cratceomus, Ornithomerus, Doratodon Rhadinosaurus, Oligosaurus, and 
 Hoplosaurus. Pleuropeltus may possibly prove to be another Dinosaur 
 allied to Polacanthus. The most recent dinosaurs found in Europe are 
 from the Maestricht beds. They are referred to the genera Orthomerus 
 and Megalosaurus. 
 
 The American Dinosauria are numerous. Their exact age is uncer- 
 tain. Some are Cretaceous ; others are regarded as Jurassic. Professor 
 Marsh has greatly subdivided the group, 5 as in the following provisional 
 classification : 6 
 
 SUB-CLASS DINOSAUKIA. 
 I. Order SAUROPODA. 
 
 Family Atlantosauridse : Atlantosaurits, Apatosaurus, Bronto- 
 
 saurus, Camarasaurus, Dystrophceus. 
 Family Diplodocidas : Diplodocus. 
 Family Morosauridae : Morosaurus, Cttiosaurus, Oniithopsis, 
 
 Emamerotus. 
 
 II. Order STEGOSAURIA. 
 
 Family Stegosauridae : Stegosaurus, Diracodon Omosaurus. 
 Family Scelidosauridae : Scelidosaurus, Acanthopholis, Cratceomits, 
 ffylceosaurus, Polacant/tus. 
 
 III. Order ORNITHOPODA. 
 
 Family Camptonotidae : Camptonotus, Laosaurus, Nanosaurus^ 
 Hypsiiopkodon. 
 
 1 Dollo, Bulletin du Mus^e Royal d'Histoire Nat. de Belgique. 
 
 2 Hulke, Phil. Trans. Royal Soc., Part III., 1882; loc. cit., Part III.. 1881 ; 
 and numerous papers in Q. J. G. S. 
 
 3 Q. J. G. Soc., vol. xxxv. 4 Quart. Jour. Geol. Soc., vol. xxxvii. 
 
 5 See Marsh, in American Journal of Science, vols. xiv.-xxvii. Cope, United 
 States Geol. Survey of the Territories, vol. ii. 1875, &c. 
 
 6 American Journal of Science, vol. xxiii., January 1882. 
 
THE ORNITHOSAURIA. 517 
 
 Family Ignanodontidae : lyuanndon, Pdorosaurus, Vectisaurus. 
 Family Hadrosauridae : Hadrosaurus, Agathaumus, Cionodon, 
 Diclonius. 
 
 IV. Order THEROPODA. 
 
 Family Megalosauridae : Megalosaurus, Allostiurus, Ccelosaurns, 
 
 Crensaurus, Lcelaps. 
 
 Family Zanclodontidae : Zanclodon, Teratosaurus. 
 Family Amhisauridae : Amphisaurus, Bathygnathus, Clepsysaurus, 
 
 Palceosawus, Tkecodontosaurus. 
 Family Labrosauridae : Labrosaurus. 
 
 V. Sub-Order C^ELURIA. 
 Family Caeluridae : Celurus. 
 
 VI. Sub-Order COMPSOGNATHA. 
 Family Compsognathidae : Compsognathus. 
 
 VII. Order HALLOPODA. 
 Family Hallopodidse : Hallopus. 
 
 Dinosaurs are to be regarded as modified Crocodiles. The living 
 Croeodilia and the Dinosauria both probably are descended from the 
 Teleosauria. The Teleosaurs which have the articular surfaces of the 
 vertebrae flat or slightly concave, make a slight approximation to 
 Dinosaurs in other parts of the skeleton, and from this Teleosaurian 
 basis all the Dinosaurs may have been evolved by differences of func- 
 tion developed in the several parts of the body. Some of the genera 
 are probably naked, others, like Potacanthus, were (as the Rev. Darwin 
 Fox, its discoverer, first pointed out) sheathed in armour only com- 
 parable to the carapace of a tortoise or armadillo. Some genera, like 
 Iguanodon, had the tail enormously developed ; while in others, 
 like some Dinosaurs from the Cambridge Greensand, the tail was 
 almost wanting. Some genera were of the ordinary quadruped type, 
 others had a kangaroo-like form and motion. But the most singular 
 structural modification is seen in Oniithopsis and allied genera, in 
 which the vertebrae are excavated by pneumatic cavities comparable 
 to those similarly placed in birds. We should class the Ichthyosauria 
 as an aquatic order in the sub-class Dinosauria, and somewhat sim- 
 plify Prof. Marsh's classification. 
 
 Ornithosauria. The secondary strata yield between the Lias and 
 the Chalk a remarkable group of flying animals which combined 
 characters of reptiles with those of birds. They were devoid of 
 feathers, and no covering to the skin of any kind has been found. 
 They mostly possessed teeth, better developed than those of the 
 birds from the Secondary rocks ; though Ornithostoma from the Cam- 
 bridge Greensand, and some American types, like Pteranodon, were 
 toothless, like existing birds. Perhaps if the toothless forms had 
 been found first, the avian nature of these animals would never have 
 been doubted. The tail was often long; the vertebrae usually, if 
 
5 i8 
 
 THE ARCH&OPTERYX. 
 
 not always, have the cup in front and ball behind, as in living croco- 
 diles and most lizards and all serpents, but all the other bones are 
 bird-like in their plan. The wing is only comparable with the wing 
 of a bird, though a digit was elongated to stretch the wing membrane 
 to the same area as feathers extend the wing of the bird. Ornithosaurs 
 could walk on two feet or four feet, but most were aquatic animals, 
 probably divers, living on fish. Air-chambers prolong the breathing 
 organs into the limb bones, as in living birds. The English genera are 
 JJimorpJtodon in the Lias, Rhampliorhynchus and Dolicorhampkus in 
 the Stonesfield slate ; some Pterodactyles occur in the Kimmeridge 
 clay. Doratorhynchus is a genus of the Purbeck beds, and Ornitho- 
 cheirus, distinguished by having prehensile teeth directed forward 
 from the front of the snout, ranges from the Wealden beds to the 
 Chalks, and is especially numerous in the Cambridge Greensand. 
 Omithocheirus probably had but three digits in the fore-limb. 1 The 
 Ornithosaurs include the Pterodactylia and Pterosauria. 
 
 Aves. Birds have been divided into three groups : Saururise, in- 
 
 C. 
 
 Fig. 146. Head of Archaeopteryx. 
 
 A. orbit; B. ant-orbital vacuity ; C. external nasal aperture ; *cl. sclerotic circle ; p.f. frontal 
 bones; n. nasal bones; im. premaxillary ; I. lachrymal; m. maxillary; p.p.nt. palatine 
 plate of the maxillary ; p. palatine bone ; pt. pterygoid bone ; qu. quadrate bone ; mi. 
 lower jaw ; p.pa. post articular process of the mandible ; h. hyoid bones. 2 
 
 dicated by the Archaeopteryx ; the Ratitae, including all Struthious 
 birds ; and Carinatae, which comprise birds of flight. 
 
 The Archseopteryx is remarkable for having the skull formed upon 
 
 1 See Ornithosauria. 8vo, 1870. Jour. Linn. Soc. Zool., vol. xiii. Annal< 
 Nat. Hist., Jan. 1871. 
 
 - See W. Dames : " Ueber Archaeopteryx, Palaeontologische Abhandlungen, 
 Zweiter Band," Heft 3, Berlin, 1884, for an admirable discussion of this fossil, and 
 an excellent figure. See also R. Owen : Phil. Trans. Royal Soc. 1863. 
 
BIRDS FROM CRETACEOUS ROCKS. 519 
 
 the bird plan, with teeth in the premaxillary and maxillary bones. 
 The teeth were in distinct sockets, and appear to have resembled in 
 plan the teeth of Mosasaurus. The tail is elongated as in some 
 ornithosaurs, and the character from being previously unknown 
 among birds has an ordinal value: it suggests the name Saururiae. 
 The digits of the fore limb are distinct from each other ; they are 
 three in number, and each terminates in a claw. These characters 
 are differences from existing birds, but although often taken to 
 indicate a resemblance to ornithosaurs, there is nothing reptilian in 
 the skeleton of the Archseopteryx, except that it is less specialised 
 than the skeleton in existing groups of birds. Feathers in all respects 
 similar to the feathers of existing birds were developed. We are 
 indebted to Professor Dames for the figure of the head. The only 
 specimens known are in the Royal Museum at Berlin, and the British 
 Museum ; they are from the Solenhofen Slate. 
 
 An American Jurassic bird has been described by Marsh under the 
 name Laopteryx. It is rather larger than a blue heron, but only an 
 imperfectly-preserved skull is described. 
 
 Cretacaous birds were first discovered in the Upper Greensand of 
 Cambridge. They are known from fragments, which however proved 
 that some of the vertebrae are bi-concave in the back, that the avian 
 form of articulation exists in the neck, that the head resembles the 
 divers, that the tibia is like that of the grebes, while the pelvis has 
 some characters in common with the penguins. These birds have been 
 referred to the genus Enaliornis ; but a larger bird existed with them, 
 in which the extremities of the long bones were not ossified. In 
 America Professor Marsh has described, under the name Odontor- 
 nithes, two genera of toothed birds. One of large size named Hesper- 
 omis had a very reptilian type of brain. Its teeth are like the teeth 
 of Arcliceopteryx ; the sternum has no keel ; its scapular arch is 
 small, and the fore limb is represented by a rudimentary humerus. 
 The pelvic arch is not struthious ; it is only anchylosed to the sacrum 
 in the acetabular region. The acetabulum is closed by bone. The 
 nearest approach to the legs and feet of Hesperornis is seen in the 
 genus Podiceps. The bird is regarded as having been aquatic, inca- 
 pable of flight, with habits comparable to those of the loon. The 
 animal is classed as a carnivorous swimming ostrich. We should 
 rather say a carnivorous natatorial bird, which had not evolved or lost 
 carinate characters. The resemblances of classificational value are all 
 with the water-birds. The resemblances to the ostrich show the 
 direction of the parent stock from which water-birds became evolved. 
 
 Ichthyornis is so named because the articular ends of the vertebrae are 
 bi-concave as in fishes. The bird had the wings well developed. Professor 
 Marsh remarks that it can be interpreted by supposing that certain parts 
 have become highly specialised in the direction of the evolution of re- 
 cent birds, while others have been derived with but little change from 
 reptilian, or even a more lowly ancestry. We should have preferred 
 to say that all the structures which functional conditions have made 
 distinctive of birds are well marked, while some other characters, like 
 
520 THE OLDEST KNOWN MAMMALIA. 
 
 the intervertebral articulation, which varies in the caudal region of 
 existing birds, remain embryonic. We should explain in the same 
 way the ligamentous union of the jaws ; and regard the presence of 
 teeth as an embryonic character lost in living birds, much as teeth are 
 lost in the adult whalebone whales. Hence we refer Ichthyornis 
 to the natatorial group of birds. Professor Marsh has described 
 many other cretaceous birds under the names Apatornis, Baptornis, 
 Graculavus, Laornis, Palceotringa, and Telmatornis. 1 
 
 The fossil birds of the Tertiary rocks of Europe have been described 
 in detail by Professor Alphonse Milne-Edwards, 2 but the most remark- 
 able of these remains occur in the Lower Tertiary beds of the Paris 
 basin, from which they have been described by Professor Victor 
 Lemoine, who figures Gastoniis, uptromit t and lienicornis. Gas- 
 tornis was a struthious bird, which may have stood 7 or 8 feet high. 
 
 Struthious birds found in our own Tertiary strata are represented 
 in the London Clay by Dasornis, imperfectly known from the skull, 
 which is considered to resemble the Dinornis of New Zealand. Frag- 
 mentary remains indicate the kingfisher Halcyornis, the vulture 
 Lithornis, besides birds like the heron and sea-swallows. The 
 albatross is represented by an allied genus, Argillorni*, and Odontap- 
 teryx is a bird with some affinities to the duck tribe, which had long 
 bony teeth on the jaw, which was probably covered with horn. A 
 struthious bird referred to the genus Macornis is found in the Headon 
 beds of the Isle of Wight, and some other birds are known from the 
 Hempstead beds and the Eracklesham beds. 
 
 Evolution and Succession of Mammalia. The oldest mammals 
 are chiefly known from teeth. Molar teeth of Microlestes, from the 
 Rhsetic beds in this country, and Keuper beds of the Trias of Germany, 
 are shown by the premolar teeth, named Hypsiprymnopsis, to so closely 
 resemble Plagiaulax of the Purbeck beds, and the living kangaroo-rat 
 Hypsiprymnus, that we may affirm descent without recognised modi- 
 fication of plan. Under the name Neoplagiaulax, Dr. Lemoine has de- 
 scribed a near ally of Plagiaulax from the Lower Tertiary rocks. One of 
 the most interesting of recent discoveries is the skull of the South Afri- 
 can Triassic mammal, named by Sir Richard Owen 2'ritylodon, which 
 has three longitudinal rows of cusps on the molars, closely resembling 
 the Triglyphus of Fraas from the Trias of Stuttgart. Professor Cope 
 has found a representative of the type in the Tertiary beds of North 
 America, which is named Polymastodon. 3 The lower jaws of mam- 
 mals from the Stonesfield slate, termed Amphitherium and Pkascolo- 
 therium, with strong marsupial characters, combine many characters 
 of the Insectivora, and are associated with a humerus and femur which 
 indicate a generalised Insectivorous type, modified from a Monotreme 
 stock in the direction of the marsupial plan. 4 The only mammal from 
 
 1 Odontornithes : Extinct toothed birds of North America, by O. C. Marsh. 
 
 2 " Recherches anatomiques et paldontologiques 1'histoire des oiseaux fossiles 
 de la France." 4to. 1867-74. 
 
 ;i Cope : "The Tertiary Marsupial ia, " American Naturalist, July 1884. 
 4 Q. J. G. S., August 1879. 
 
THE EVOLUTION OF MAMMALIA. 521 
 
 Stonesfield with teeth of placental type is Stereoynathus. The affinities 
 of the numerous mammals from the Purbeck beds, Spalacotherium, Tri- 
 conodon, Galestes, Plagiaulax t &c. t are apparently marsupial. 1 There are 
 
 Fig. 147. Phascolotherium. Lower jaw enlarged. 
 
 no herbivorous marsupials in the Tertiary rocks ; and Professor Gaudry 2 
 urges that this group would be at a disadvantage in the struggle for ex- 
 istence, as compared with higher herbivorous mammals, from the way 
 in which the movements of the parent are controlled by the helpless 
 condition of the young, and that this disadvantage might have led to 
 their extermination in the European Tertiary area. Carnivorous 
 marsupials, which so closely resemble the existing opossums that it is 
 not easy to separate them by generic characters, are found in the 
 older Tertiaries of Paris, Auvergne, and Vaucluse. These Carnivorous 
 marsupials of Insectivorous type are termed by Professor Cope Creo- 
 donta. 3 Hycenodon and Pterodon, somewhat intermediate between the 
 placental and implacental types in their dentition, are inferred by 
 Gaudry to have been marsupials from the form of the axis vertebra. 
 Professor Huxley regards them as connecting the Garni vora and In- 
 sectivora. This intermediate character is found in other genera, such 
 as Palceonictes and Cynohamodon, from the Phosphorites of Quercy. 
 Ardocyon is an omnivorous mammal, resembling the marsupial bears. 
 This blending of characters in Tertiary genera is intelligible on the 
 hypothesis that implacental mammals of the Secondary period of geo- 
 logical time become ultimately developed into the insectivorous placen- 
 tal type ; and in this way Professor Gaudry interprets the resemblances 
 to marsupials seen in some fossil carnivora and in living lemurs. 
 
 There is no fossil evidence as to the parentage of Cetacea. Palceo- 
 cetus Sedffwicki 4 is recorded as a fossil from the Secondary rocks on 
 the evidence of its mineral condition, which is that of fossils from the 
 Secondary clays. Zeuglodon occurs in the Barton clay, Squalodon in 
 the Miocene rocks and crag, Balenoptera dates from the Headon beds ; 5 
 Halitherium, from the Crag and Middle Tertiary, is intermediate 
 between the Dugong and Lamantin ; Pugmeodon is intermediate 
 between the Lamantin and Halitherium. 
 
 1 R. Owen : " Purbeck Mammalia : " Palseontographical Society. 
 
 2 See Gaudry : " Enchainments du Monde Animal," one of the most important 
 modern contributions to Palaeontology, in which most of the following facts con- 
 cerning mammals may be found. 3 "American Naturalist," 1884. 
 
 4 Geol. Mag., Feb. 1865. p. 54. & Q. J. G. S., vol. xxxvii. 
 
522 EVOLUTION OF DEER. 
 
 The Pachyderms form a large group, in which extinct fossil 
 genera have marked resemblances to living species. Thus the Rhino- 
 ceros was preceded by Acerotherium, Palceotherium, and Paloplo- 
 therium, arid differences are found in the form of the skull and the 
 dentition. Acerotherium is essentially a Rhinoceros without horns, 
 and consequently with smaller nasal bones. Fossil species of Rhino- 
 ceros show all intermediate conditions of this region of the skull. 
 The American species of Acerotherium have three pairs of incisor 
 teeth, as in Palceotherium ; and this number is preserved in the 
 extinct Rhinoceros sivalemis of India, though in other species the 
 incisors become lost. Tapirs are traced back by the American genus 
 Hyrachyus to Lophiodon in the Eocene. Pigs of the genus Sits 
 appear in the Middle Miocene. Sus Lockarti is nearly allied to Hyo- 
 therium, and this genus is closely allied to Palceochcerus, which 
 resembles the peccary of South America. Palceochwrus passes to 
 Choeropotamus, and this genus is closely related to Dichobune of the 
 Headon beds. Pachyderms are the prevalent type in the older 
 Tertiaries of Europe. 
 
 The oldest known Ruminants are Xiphodon, Dichodon, and Am- 
 phim.eryx. Most of the older American Ruminants possess, like 
 Xiphodon, some characters of Pachyderms. This is exemplified by 
 the French genera Gelocus and Dremotherium. All the chief ruminant 
 types are found in the Upper Miocene. The older genera, such as 
 Oreodon, are allied to Anoplotherium, and without horns. The first 
 antelopes have the horns very small ; and horns appear to have de- 
 veloped gradually, becoming relatively large in Antelope recticornis 
 of the Lower Pliocene. The antlers of Ruminants also developed 
 gradually in geological time, just as they grow in complexity during 
 life in existing deer. In the Middle Miocene the genus Dicroceras 
 has antlers with two prongs. In the Upper Miocene deer first appear 
 in which the antler has three prongs ; and it is only in Pliocene and 
 Post-Tertiary deposits that antlers attain their full complexity, though 
 since the extinction of types like Cervus Sedgwicld of the Forest Bed, 
 and the Cervus megaceros, no greater development of antler has 
 occurred. Since the older fossil antlers are always attached to the 
 skull, they have been regarded as permanent appendages in those 
 types. When they began to become deciduous, at first only the upper 
 portion of the antler was shed, long pedestals remaining attached to 
 the frontal bones. The portion shed gradually increased in length. 
 
 The earlier Ruminants, Oreodon, Dichodon, and Xiphodon, have 
 canines and incisors in the upper jaw like Pachyderms. It is, how- 
 ever, impossible to discover from study of the teeth what the Pachy- 
 derm type was from which the Ruminants were evolved. In the 
 digits of the limbs existing genera show types in which a transition 
 may be traced from those with four metapodial bones to those in 
 which there is but one ; and similarly, by tracing the ancestral forms 
 of specialised living genera, a like series of modifications is met with, 
 until in the older genera the metapodial bones are all separated. The 
 steps by which this evolution was brought about are, according to 
 
EVOLUTION OF THE HORSE. 523 
 
 Gaudry (i.) The first, second, and fifth metatarsals and first cunei- 
 form bone are carried backward; (2.) the posterior proximal part of 
 the third metatarsal is enlarged to sustain the cuneiform ; (3.) the first 
 and second cuneiform elements of the tarsus, the second and fifth 
 metatarsals, and the trapezius become very small; (4.) various ele- 
 ments of the metacarpus and metatarsus are blended. 
 
 The ancestry of the Horse offers some difficulties which impose 
 caution, for one line of descent for the Old World horses is suggested 
 by European specimens, while Professor Marsh has developed another 
 line of descent for the horses of America. In Europe the horse is 
 traced from Paloplotlierium by way of Pachynolyphus to Anchitherium, 
 which is allied by its teeth to Palceotherium, though its limbs ap- 
 proximate to the more slender forms of Paloplothvrium and Hipparion, 
 which last is little more than a horse with two small supplemental 
 digits, one on each side of the hoof. Monstrosities of the living horse 
 sometimes have a supplemental digit developed with small hoof, such 
 as is found in the Miocene Hipparion : Professor Marsh has figured 
 the eight-hoofed Cuban horse, in which there is one such supplemental 
 digit on each foot. Such a horse as Equus stenonis of the Pliocene, in 
 which the teeth closely approximate to Hipparion, is regarded as the 
 immediate ancestor of the Old World horse. In the characters of the 
 extremities of the limbs, Acerotherium of the Middle Miocene diverges 
 considerably from the type of the existing horse, for it has three large 
 nearly equal digits, with a small one on the outside. PalceotJterium 
 makes a nearer approximation, for it has three digits, of which the 
 middle one is slightly the largest. In Paloplotherium the lateral 
 digits and metapodial bones are much reduced in size ; while in 
 Hipparion the reduction is carried a stage further, and the middle 
 digit is proportionately enlarged. 
 
 The American ancestors of the horse are of similar type to those 
 which are known in Europe. Eohippw had four well-developed toes, 
 and the rudiment of another on each forefoot, and three toes behind. 
 It was no larger than a fox. It is found near the base of the Eocene, in 
 beds which yield Coryphodon. A little higher up in the Eocene is the 
 Orohippus, which was of similar size, but had four toes in front and 
 three behind, the middle one being the stouter. At the top of the 
 Eocene is a third equine animal, termed by Professor Marsh Epihippus. 
 It differed from its predecessor more in its teeth than in the digits. 
 At the base of the Miocene is the Mesohippus, which was about as 
 large as a sheep. It had three well-developed toes, and the splint of 
 another on each forefoot, and three toes behind. In newer beds the 
 animal is termed Mioliippus. It has the splint bone of the outer 
 digit reduced to a short rudiment, and the middle digit is becoming 
 relatively stout. It may be compared with the Ancliitlierium. In 
 the Pliocene beds is a three-toed horse as large as a donkey, termed 
 Protohippus, which may be compared with Hipparion, and, like its 
 European representative, differs from the older horse ancestors in the 
 wavy complexity of the tooth enamel. Newer sti'l is a near ally of the 
 modern horse termed Pliohippus, which has lost the lateral digits, and 
 
524 EVOLUTION OF THE CAMEL. 
 
 is distinguished chiefly by the teeth from the true horses which occur 
 fossil in the most recent beds. 1 This series renders it likely that 
 closely allied genera may descend by parallel lines from a distant 
 parent stock. 
 
 What Professors Huxley and Marsh have done for the evolution 
 of the Perissodactyla by tracing the history of the horse, Professor 
 Cope has attempted for the Artiodactyla in the history of camels. 
 The oldest American representative of the group is Poebrotherium 
 of the Lower Miocene. This genus has four incisor teeth, and 
 has the metapodial bones distinct throughout life. In Protolabis 
 the upper incisor teeth also remain well marked. This genus is 
 associated in the Upper Miocene with Procamelus. in which there is 
 first a marked retardation in development of the first incisor which 
 leads to its atrophy ; and secondly, an acceleration of ossification, by 
 which a canon bone is formed by the blending of the metapodial bones, 
 though they long remain separate. A further step in the change is 
 seen in Pliauchenia, in which the number of premolar teeth is reduced 
 from four to three, and the first and second incisors are lost. In 
 Camelus the premolars are further reduced to two, and in Auclienia to 
 one , though even this type shows its affinity with Poebrotherium by 
 retaining a separate condition of the elements of the canon bone during 
 a portion of foetal life. Procamelus was about the size of a llama, 
 with the head and muzzle more elongated, limbs more slender, and 
 neck shorter. 2 The giraffes have similarly been traced by Lydekker 
 back to the Sivatheres, in which a shortening of the neck is associated 
 with the development of palmate horns. 
 
 Thus both Artiodactyla and Perissodactyla are traced back to 
 simpler types of ungulata, and Gaudry urges that they result from 
 the necessity that the foot, when applied to the ground, should sup- 
 port the pressure of the body evenly. In the Perissodactyla the 
 fibula rests directly against the tibia, instead of articulating with the 
 calcaneum, as in the Artiodactyla. This arises from the external digit 
 becoming rather more slender than the internal digit, and leads to the 
 articulation of the heel-bone or calcaneum with the cuboid-bone 
 becoming smaller, and this causes an attenuation of the fourth digit ; 
 so that the whole weight of the animal is thrown on the astragalus, 
 which is carried by the naviculare, which is itself carried by the third 
 cuneiform bone, which supports the third or middle digit of the foot. 
 Hence it would follow that the astragalus of the odd-hoofed mammals, 
 which are flattened in foot, is modified from the type of the even-hoofed 
 mammals, and this conclusion is supported by the other tarsal bones. 
 Therefore, though the modern classification into Artiodactyla and 
 Perissodactyla is valuable as an evidence of evolution, the classifica- 
 tion becomes less marked as ungulates are traced back in geological 
 time. 
 
 North and South America have yielded several extinct orders of 
 Mammals defined by Owen, Marsh, and Cope. 
 
 1 Mareh, Amer. Jour. Sci., vol. xvii. p. 497. 
 
 8 Rep. U. S. Geog. Surveys West of looth Merid., part ii. vol. iv., 1877. 
 
EVOLUTION OF CARNIVORA. 525 
 
 Elephants, Mastodon and Dinotlierium, which date from the Mio- 
 cene period, have furnished no palaeontological evidence of their 
 parentage, and in many other groups the evidence is imperfect. 
 
 Edentates lived in Europe in the Eocene and Miocene periods, 
 though, in America, they are only known from the newest deposits. 
 Macrotherium was an enormous animal of the Middle Miocene, which 
 had relatively small hind limbs, and claws drawn back towards the 
 metacarpals, so as not to impede progression on the ground. It might 
 have been a climber. Ancylotherium^ from Pikermi, is intermediate 
 between climbing and walking edentates. Rodents occur in the 
 Headon and Bembridge beds ; porcupines in the Upper Miocene of 
 Greece ; hares in the Pliocene of Auvergne ; beavers in the Upper 
 Miocene, and the extinct genera only differ from those now living in 
 minor characters. Hedgehogs are found in the Miocene of Auvergne, 
 moles in the same strata, in the Rhine, and at the foot of the Pyrenees. 
 Shrews are also Miocene, plesiosorex being so generalised as to present 
 some characteristics of a hedgehog. 
 
 The Carnivora are fully represented in a fossil state. Pseudcdurus 
 is a primitive cat, which commenced in the Upper Eocene phosphorites 
 of France ; and there is a regular succession of Felidae through the 
 Miocene and Pliocene periods, though the true cats are most numerous 
 in the newest Tertiary. It is only in the Smilodon of Buenos Ayres 
 that we have evidence that the claws become retractile. There may 
 be some reason for thinking that carnivorous animals are descended 
 from the herbivora, since bears most nearly resemble the pachyderms. 
 The Miocene genus Amphicyon had the most striking characteristics 
 of dogs, yet was plantigrade like a bear, was probably able to climb, 
 and resembled bears in minor dental characters. The genus Hyce- 
 narctos has an inner row of denticles to the teeth, and thus is inter- 
 mediate between bears and Amphicyon. In the same way Cynodon, 
 from the phosphorites of Quercy, bridges over the gap between the 
 civets and dogs. Hyaenas are in like manner related to civets, Icty- 
 tkerium being a modified civet with four digits like hyaena, and it pro- 
 duced similar coprolites ; while Hycenictis is a modified hyaena, closely 
 approaching Ictytherium. The martens of the Miocene period approach 
 the civets, but not so closely as to break down the distinction between 
 the two groups. 
 
 The oldest Lemur is Ccenopithecus, from the Eocene. Several 
 lemurs from the Eocene of Quercy show affinities with the ungulata 
 which suggest a common origin for lemurs and several Eocene pachy- 
 derms. So marked are these characters, that the lemurine genus 
 Adapts has sometimes been arranged with the pachyderms, while the 
 lemur Aphelotherium was formerly placed near to that group. 
 
 The earliest Apes present affinities with pachyderms, which are 
 marked in the genus Ceboclicerus, from the lignites from Debruge. 
 Hyracotherium has some ape-like characters. The teeth of Oreopithecus, 
 from the Miocene of Italy, show resemblances to the pachyderm Clion- 
 rdpotamus. True apes from the Middle Miocene include Semnopithec.us, 
 from the Siwalik Hills and Montpellier. Mesopithecus t from Pikermi,, 
 
526 SUCCESSION OF LIFE IN TIME. 
 
 appears to have been a walker rather than a climber, and intermediate 
 between Semnopithecus and the Macaques. The anthropomorphous 
 apes include PliopitTiecus and Drt/opithecus. The latter is of a very 
 high type, but differs from man in many points, but especially in the 
 large size of the canines in the males. 
 
 Professor Gaudry considers that the doctrine of evolution would 
 justify us in regarding a European deposit as Eocene if the mammals 
 were numerous but unlike those now living, and unassociated with true 
 ruminants, solipeds, proboscidians, and apes. In the older Miocene 
 we should expect to find living genera rare, marsupials disappearing, 
 pachyderms passing into horses, and the incoming of true ruminants. 
 In the newer Miocene marsupials would be lost ; but ruminants, 
 horses, whales, edentates, elephants, carnivora, and apes are represented 
 by numerous individuals and many genera, some of which diverge 
 from living forms. In the Pliocene period the genera are mostly still 
 living, though the species are extinct. 
 
 Summary. If we endeavour to summarise the conclusions which 
 the succession of life on the earth indicates, the most important 
 generalisation is no doubt the fact that the life now existing is sub- 
 stantially the same as life has been in all the past ages of time. The 
 combination of the different groups of organisms is of like character, 
 though the genera and species have varied. There is no trace of 
 a beginning. There is evolution, but it is only the evolution of 
 genera and of ordinal groups, and not of classes. 
 
 It is chiefly by means .of extinct families and orders that strata are 
 characterised, and the periods of past time separated from each other ; 
 but when we bear in mind what the circumstances are which are 
 causing extinction at the present day, we may doubt whether a classi- 
 fication so made is the best possible. Its method is unphilosophical. 
 At least of equal importance with the occurrence of extinct types is 
 the first appearance as elements in a fauna of genera and orders which 
 still survive ; for both are connected with the changed distribution 
 of land and water which time has developed. The first appear- 
 ance of organisms as a characteristic feature in a fauna, would divide 
 the strata differently from the extinct types, and would show how 
 local are all the phenomena of the succession of life. Many groups 
 of organisms which still survive appear plentifully in the Cretaceous 
 rocks, so that a palseontological division might be drawn on the 
 evidence of plants and fishes and many intermediate groups of organ- 
 isms, which link the Lower Greensand with strata below, and the Gault 
 with strata above. The Trias is sharply cut off from the Lias above, 
 and from Permian rocks below. The Primary period is certainly 
 divided into two by a gap in succession of species between the upper- 
 most beds of the Silurian and the lower part of the Devonian, which 
 is not less marked than the other great changes in life, such as divide 
 the Secondary and Tertiary rocks, or the Trias and Lias. 
 
 The names Palaeozoic, Mesozoic, and Cainozoic, therefore, do not 
 represent completely palaeontological facts, and the divisions which 
 they indicate are artificial when studied in the light of the groups of 
 
USE OF FOSSILS IN CLASSIFICATION OF STRATA. 527 
 
 animals composing the several faunas. The Sponges give no indication 
 of the larger divisions of time ; the Foraminifera introduce their new 
 types gradually, so that we look to the Carboniferous rocks, the Trias, 
 and the Chalk as furnishing the majority of existing genera. Among 
 corals the Alcyonarians are scantily developed, yet date back to the 
 older Primary rocks. The Rugosa are chiefly, though not exclusively, 
 of Primary age ; the Sclerobasic corals are not known prior to the 
 Tertiary period ; and the Perforata, which are common corals at the 
 present day, date from the Cambrian rocks"; the Aporosa are more 
 numerous in the newer rocks than in the Primary period. The sea- 
 urchins would tend to unite Secondary and Tertiary rocks together, 
 while some urchin types show a remarkable connection between the 
 Cretaceous and Tertiary periods. The Crinoids are an asthenoid group 
 most numerous in the Primary period ; but otherwise have little value 
 in stratigraphical classification. The living groups of Crustacea do not 
 suggest any of the existing divisions of the strata, since the higher 
 forms are chiefly known from the Lias, the Cambridge Greensand, and 
 the London Clay. The Lamellibranchiata furnish many surviving 
 types in the Primary rocks, especially the Carboniferous ; others become 
 known with the Trias, Lias, Neocomian, Cretaceous, and Lower and 
 Middle Tertiary. The Gasteropods commence gradually, one or two' 
 with a formation, though they are most numerous in the Carboniferous, 
 Lias, and Chalk, until the Lower Tertiary introduces the majority of 
 living forms. Hence there are nine or ten great palseontological divi- 
 sions of British strata. 
 
 Paleontology has often been regarded merely as the aid which a 
 naturalist contributes to the work of the stratigraphical geologist. 1 
 But in addition to this work which it was at first called upon to per- 
 form, palaeontology has a more important role in the future history of 
 science, in demonstrating the steps in the evolution and succession of 
 faunas ; and on this basis its evidence must always be important in 
 forming a useful geological classification of strata. 2 It also contributes 
 important evidence of physical changes which took place in adjacent 
 regions. But the physical and palseontological evidences rarely coin- 
 cide ; so that for some time to come Stratigraphical classifications 
 should be made independently, first upon the evidences of the Physi- 
 cal History of a Region, and secondly upon its Succession of Life. 
 
 1 Twenty-five years ago we began to give effect to a different view by arrang- 
 ing the collections in the Woodwardian Museum, in the University of Cambridge, 
 in the local faunas in which they were collected, since it was thus possible to trace 
 the migration of life step by step as it became diffused on the earth in conse- 
 quence of physical changes. This aspect of palaeontology was originally suggested 
 by the study of Count Munster's and other palseontological collections in the 
 Woodwardian Museum, and its importance has since been enforced by studies of 
 many collections in different parts of Europe. 
 
 2 It is many years since Mr. R. A. C. Godwin-Austen wrote " The artificial 
 scale of formations which still figures in elementary treatises, more particularly 
 with respect to Secondary Geology, represents an order of superposition and lines 
 of separation which are both untrue as well with respect to the mineral masses as 
 the forms they contain, the result of too hasty generalisation of local phenomena ;" 
 but notwithstanding some improvements the criticism remains still just. 
 
52 8 GEOLOGICAL RESEARCH. 
 
 The two methods may eventually be united, but it can only be by 
 discovering the physical conditions which limited, determined, and 
 changed the mineral characters of the strata, and changed the dis- 
 tribution of fauna and flora in the area which the strata occupy. 1 
 
 CONCLUSION. 
 
 There is but one safe road in geology, and that is practical 
 familiarity with facts. If we have succeeded in our elementary task 
 of unfolding the origin of strata, and stating the ways in which their 
 origin is bound up with the origin of igneous rocks and the succession 
 of life on the earth, we shall have failed altogether in our purpose if 
 the reader has not step by step tested both exposition and theory by 
 familiar acquaintance with Nature. Thus practical knowledge is 
 readily gained as it is needed ; and facts and ideas easily become the 
 student's own when seen or interpreted by himself. There can now 
 remain no need to indicate lines of further work to be followed, for 
 the student's chief needs henceforth are hammer, handbag, and note- 
 book in connection with the subject of this volume. 
 
 We may, however, mention a few authors of the earlier period of 
 Geological Research, whose every work may be profitably read, for 
 they may long influence the future development of geological science. 
 In physical geology there has been no greater master of description 
 than John Macculloch ; and Sir Henry de la Beche, W. D. Conybeare, 
 and Adam Sedwick, knew better what to observe than most men. In 
 volcanic geology, every writing of George Poulett Scrope is worthy of 
 study ; and in the philosophy of igneous rocks, Henry Clifton Sorby is 
 the pioneer of the modern methods of research. Robert Alfred Cloyne 
 Godwin-Austen laid the foundation of philosophical work among the 
 strata; and Ed ward Forbes has been the greatest teacher in palaeontology. 
 It is by familiarity with the whole life-work of such men as these, 
 their associates, colleagues, pupils, and successors, that the student may 
 hope to assimilate the methods of thought which they used so well, 
 and aid in advancing the science. The days of apprenticeship in science 
 have ended, but it is still possible to study carefully every writing 
 of the Great Masters in the country which they have made classical. 
 
 No study will be more profitable than the examination of a classical 
 district with the original exposition of its geological structure ; or the 
 study of a type of life with a description in which matured experience 
 guides the use of facts which elucidate the fossil form. When the clift- 
 sections have been examined, and drawn, and measured, and the student 
 returns from disentangling complicated inversion of mountain ranges, 
 he turns from Nature to books, books of detail in which other observers 
 record their impressions of similar phenomena, in countries which may 
 be compared with the area of his studies. The "Journal of the Geo- 
 logical Society of London," the " Geological Magazine," and publica- 
 tions of the Geological Survey contain the more important contributions 
 to British Geology which are made from year to year, but even more 
 1 Annals and Mag. Nat. Hist., Dec. 1867. 
 
EVOLUTION OF THE PRESENT FROM THE PAST. 529 
 
 essential for research are the Catalogue of the Library of the Geological 
 Society, Ormerod's Index to the Publications of the Geological Society, 
 the Royal Society's Catalogue of Scientific Papers, and the publica- 
 tions of the Palseontographical Society. The literature which bears 
 on work thus discovered and assimilated, but not before Nature has 
 been first read with unbiassed powers, we turn from books to Museums ; 
 and there see the formations divested of sediment, with their life in 
 the cabinets as on the old sea-beds ; and so journey on from one 
 museum to another, from country to country, comparing rocks and 
 faunas, until the fossil life becomes migratory like that of this exist- 
 ing world of ours, and the conditions of its existence and modification 
 are seen. 
 
 Thus the mind becomes saturated with a conception of the order 
 of Nature, from which it is found that in all ways the past contains 
 within itself the interpretation of the existing world. By using this 
 illumination which the geologist has to give, the studies of the geo- 
 grapher and the naturalist are divested of their isolated character, and 
 take places in a kosmos. In earlier times it was necessary, as a pre- 
 liminary, to show how the present order of Nature is analogous to that 
 exhibited in the strata, and that work was well done by Hutton and 
 Lyell ; but such comparison is only a step in grouping facts, explains 
 nothing but the processes of Nature, and leaves the sequence and 
 method of her work unexplored. The existing natural history of the 
 earth may render its past history intelligible, but it does not account 
 for its Existence, its course of Evolution, or its Facts ; these are the 
 inheritance of the Geologist. The progress of Earth's history has been 
 from the past to the present, and the function of the geologist is to 
 demonstrate how the existing world perpetuates the past, and has 
 been evolved from it in all ways. The great departments of geology 
 have been surveyed already ; but the fabric of geological science has 
 yet to be constructed. Facts lie to hand in all parts of the earth, and 
 need only to be organised and vitalised by law to become luminous 
 with their history. As this process takes form it will become clearer 
 that Geological history is an Evolution, in which deposition of strata, 
 elaboration of igneous products and ore deposits, and succession and 
 distribution of life, are mutually dependent phenomena, which exem- 
 plify the persistence and action of physical laws that govern the earth 
 without variableness. 
 
 VOL. I. 2 L 
 
INDEX. 
 
 ABERTSTWITH, grey grit of, 95. 
 
 Abich, 3, 193. 
 
 Acanthopterygii, 504. 
 
 Accessory minerals in mica schist, 376. 
 
 in trachyte, 266. 
 
 Acid granite of Croghan Kinshela, 246. 
 
 Acidic rocks of North of Ireland, 314. 
 
 Acidic volcanic rocks, 35. 
 
 Acklington dyke, 339. 
 
 Actinolite, 25. 
 
 Actinozoa, orders of, 481. 
 
 Adams, Professor J. C., 11. 
 
 ^Eolian rocks, 107. 
 
 Agassiz, Alex., on young osseous fishes, 
 
 501. 
 Louis, on antiquity of coral reefs, 
 
 115. 
 Age of an upheaval, 332. 
 
 of igneous rocks, 228. 
 
 Agnostus, figure of, 493. 
 Agricola, 3. 
 Ailsa Craig, 251. 
 Aim of this work, 8. 
 Airey, Sir G. B., 9. 
 Aix, flora of, 474. 
 A line, 16. 
 Albite, 23. 
 
 in Leinster granite. 245. 
 
 in Scotch rocks, 362. 
 
 Alcyonaria, 481. 
 
 Aldabaran, 17. 
 
 Alderley edge, copper of, 418. 
 
 Aleutian province, 458. 
 
 Allen, Joel Asaph, on Arctic region, 461. 
 
 Alloys, 400. 
 
 Allport on basalts, 278. 
 
 on Brent Tor, 302. 
 
 on carboniferous dolerites, 304. 
 
 on diorites, 304. 
 
 on greenstones, 280. 
 
 on the Wolf Kock, 251. 
 
 on the Wrekin, 291. 
 Alps, gneiss of, 394. 
 valleys in, 149. 
 
 Alston Moor, dislocations of, 328. 
 
 lead of, 411, 424. 
 
 rivers of, 153. 
 
 Alternation of beds, 89. 
 Alum, 46. 
 
 Alum Bay, 46. 
 
 sands of, 97. 
 
 flora, 475. 
 
 America, metamorphic rocks of, 395. 
 American andesites, 261. 
 
 augite andesites, 275. 
 
 birds, 519. 
 
 dacites, 263. 
 
 dinosaurs, 516. 
 
 diorites, 226. 
 
 forest region, 470. 
 
 gneiss, 395. 
 
 granite, 220. 
 
 Jurassic bird, 519. 
 
 marsupials, 520. 
 
 propylite, 256. 
 
 rhyolites, 272. 
 
 trachytes, 265. 
 
 Ammonites, divisions of, 500. 
 
 figure of, 500. 
 
 Amphibole in polarised light, 253. 
 
 Anacanthini, 505. 
 
 Analogy of distant deposits, 68. 
 
 of mineral veins and trap dykes, 80. 
 
 Analyses of albite, 362. 
 
 of American andesites, 261. 
 
 basalts, 282. 
 
 dacites, 261. 
 
 diorite, 256. 
 
 propylite, 256. 
 
 rhyolite, 272. 
 
 trachyte, 265. 
 
 of amphibole, 369. 
 
 of andesine, 363. 
 
 of andesite, 299. 
 
 of anorthite, 364. 
 
 of augite andesite, 259. 
 
 of basalt, 277. 
 
 of biotite, 365. 
 
 of chlorite, 367. 
 
 of clays, 206. 
 
 of felsite porphyry, 270. 
 
 of f oyaite, 224. 
 
 of garnets, 369. 
 
 of granite, 231, 238, 243, 244, 246. 
 
 of haughtonite, 366. 
 
 of labradorite, 363. 
 
 of latrobite, 364. 
 
 of lepidomelane, 365. 
 
 of leucite basalt, 284. 
 
 of magarodite, 365. 
 
 of muscovite, 365. 
 
 of oligoclase, 363. 
 
 of orthoclase, 262. 
 
 of phonolite, 251. 
 
532 
 
 INDEX. An-Be 
 
 Analyses of pyroxene, 368. 
 
 of quartz-hornblende-andesite, 259. 
 
 of rhyolite, 211. 
 
 of sandstones, 210. 
 
 of schists and shales, 285. 
 
 of slates, 225. 
 
 of syenite, 225. 
 
 Ananchytes, figure of, 490. 
 Ancestry of the horse, 523. 
 Ancient and modern volcanic pheno- 
 mena, 168. 
 
 seas, climatal conditions of, 453. 
 Andesine, 23. 
 
 in Scotland, 363. 
 
 Audesite, 40. 
 
 and propylite compared, 262. 
 
 Andesites, 258. 
 
 of the Cheviots, 298. 
 
 Angle of rest of sediments, 331. 
 Anglesey, 129. 
 
 metamorphic rocks of, 381. 
 
 serpentine of, 312. 
 
 Anomodontia, 514. 
 
 Anorthite, 23. 
 
 Ansted on solfataras, 192. 
 
 on bitumen in springs, 193. 
 
 Anticlinal dip, 64. 
 
 valleys, 142. 
 
 Antiquity of the earth, 12. 
 
 Antrim, iron ores of, 51, 435. 
 
 Apatite, artificial formation of, 420. 
 
 Apennines, serpentine of, 288. 
 
 Apes, fossil, 525. 
 
 Apiocrinus, figure of, 487. 
 
 Apophylite, 28. 
 
 Aporosa, Duncan on succession of, 483. 
 
 Aragonite organisms, 106. 
 
 Arans, 294. 
 
 Area, origin of, 440. 
 
 Archaean rocks of America, 396. 
 
 Archaeoniscus, figure of, 491. 
 
 Arctic flora, 469. 
 
 glaciers, 150. 
 
 province, 458. 
 
 region, 461. 
 
 Ardnamurchan, 318. 
 
 Ardtun Head, leaf -beds of, 317. 
 
 Arenig volcanoes, 294. 
 
 rocks, 340. 
 
 Argyll, Duke of, on Ardtun Head, 317. 
 
 on lakes, 136. 
 
 Arnaboll series, 386; 
 
 Arran, general features of, 240. 
 
 Arrangement of materials in lakes, 161. 
 
 of rocks at the surface, 61. 
 
 Art of the miner, 416. 
 Arthur's Seat, 305. 
 
 Artificial groups of strata, Godwin- 
 Austen on, 527. 
 
 manure, 47. 
 
 Asaphus, figure of, 493. 
 Ash-deposits on the ocean floor, 183. 
 Asosan, crater of, 187. 
 Aspect of basalt, 283. 
 
 of lava, 186. 
 
 Asphalt of the Dead Sea, 193. 
 Aspedochelyidae, 513. 
 
 Assynt series, 386. 
 
 Asteroidea, 484. 
 
 Asthenoid types, 437.' 
 
 Atacama, salt plains of, 133. 
 
 Atmospheric denudation, 143. 
 
 Atoll, 116. 
 
 Atwood, Melville, on cinnabar and gold 
 
 of Colusa, 397. 
 Augite, 24. 
 
 in metamorphic rocks, 367. 
 
 Augite-andesite, 40, 259, 274. 
 
 of Ben Shiant, 319. 
 
 dykes, 339. 
 
 Augite-syenite, 223. 
 Australian province, 459. 
 
 region, 465. 
 
 Avicula, figure and characters of, 440. 
 Aymestry, limestone, 109. 
 Ayr, porphyrites of, 299. 
 Ayrshire, Permian volcanoes of, 311. 
 serpentine of, 313. 
 
 BADDELEY on distribution of gold mines, 
 398. 
 
 Bagshot sands, 46, 100. 
 
 Baier. 3. 
 
 Baikal Lake, 135. 
 
 Baily, W. H., on flora of Ballypallidy, 
 317. 
 
 Baku petroleum wells, 193. 
 
 Bala limestone, phosphatite of, 103. 
 
 oolitic, 109. 
 
 Banff granite, 238. 
 
 Baring Islands, shells above sea-level in, 
 326. 
 
 Barmouth grits. 94. 
 
 Barnsley coalfield, iron of, 430. 
 
 Barrande's colonies, 450. 
 
 Barrier reef, 416. 
 
 Bars at the mouth of rivers, 165. 
 
 Basalt, 41, 277. 
 
 in Arran, 304. 
 
 in relation to dolomite, 286. 
 
 of the Clyde basin, 309. 
 
 of the Forth basin, 308. 
 
 of North of Ireland, 315. 
 
 of Eowley Hills, 303. 
 
 Basalt and augite andesite compared, 
 280. 
 
 Basaltic iron ore, 435. 
 
 Base in igneous rocks defined, 254. 
 
 Basic granite of Ardara, 243. 
 
 volcanic rocks, 35. 
 
 Bass Rock, A. Geikie on, 308. 
 
 Bath, oolites near, 71. 
 
 Baths of Leuk, 197. 
 
 Bauerman, H., on copper, 419. 
 
 Bawdon Castle syenite, 233. 
 
 Beaumont, E. de, on European convul- 
 sions, 345. 
 
 Bedding of gneiss, 372. 
 
 Bedford Level, rivers of, 159. 
 
 Bedfordshire chalk hills, 5. 
 
 Beer Head, flints in chalk, 111. 
 
 Belemnitella, 501. 
 
 Bell on Crustacea, 491. 
 
 Ben Cruachan, 335. 
 
INDEX. Be-Ce 
 
 533 
 
 Ben Lomonrl, Nicol on, 384. 
 Ben More grit, 386. 
 Ben Nevis, 335. 
 Ben Sarsta, 318. 
 Ben Shiant, 319. 
 Beresite, 217. 
 Bermuda limestones, 48. 
 Bertrich hot springs, 197. 
 Bethesda, slate of, 101. 
 Bewick on Cleveland iron, 432. 
 Beyrichia, figure of, 494. 
 Biotite, 26, 365. 
 
 granite, 221. 
 
 Birds, fossil, 518. 
 
 Bischoff on hot springs, 197. 
 
 Bismuth, 425. 
 
 Bittadou, Bonney on felsite of, 302. 
 
 Bivalves of the primary rocks, 497. 
 
 Black country, iron of the, 431. 
 
 micas, 366. 
 
 Blankenburg, red rain at, 18. 
 Blastoidea, 488. 
 
 Blocs perches in Pass of Llanberis, 150. 
 Blue elvans, 301. 
 
 Boase, Dr., on veins in master joints, 
 410. 
 
 on granite of Cornwall, 229. 
 
 Bodmin Moor granite, 230. 
 
 stock works, 422. 
 Bog copper ore, 419. 
 Bombs from volcanoes, 184. 
 Bonney, 38, 233. 
 
 on Anglesey serpentine, 313. 
 
 on Ayrshire serpentines, 313. 
 
 on Corriegills, 305. 
 
 on felsites of Bettws-y-Coed, 296. 
 
 on joint structure, 42. 
 
 on Lake district minettes, 237. 
 
 on Iherzolite, 287. 
 
 on the Lizard, 312. 
 
 on pikrite, 310. 
 
 on rhyolite of Carnarvonshire. 292. 
 
 on schists of South Devon, 381. 
 
 of the Lizard, 380. 
 
 on serpentines west of Genoa, 289. 
 
 on Tuscan serpentines, 289. 
 Boracic acid, 196. 
 
 Boreal province, 458. 
 
 Borrowdale series, volcanic rocks of, 295. 
 
 Boulder clay, 52. 
 
 Boulder clay sand, 97. 
 
 Bourguetocrinus, figure of, 487. 
 
 Brachiopoda, 495. 
 
 Bracklesham beds, glauconite of, 98. 
 
 Branchiosaurus, figure of, 507, 508. 
 
 Brazil wood diorite, 234. 
 
 Breadalbane Highlands, 384. 
 
 Breaks in primary succession, 340. 
 
 Brent Tor, volcano of, 302. 
 
 British clays, 46. 
 
 coast, 1.19. 
 
 convulsions, 344. 
 
 strata, breaks in succession of, 340. 
 
 table of chief, 55. 
 
 Brittany, metamorphic rocks of, 392. 
 Brixham, hematite of, 427. 
 Brocken, 4. 
 
 Brogger and Reusch on giants' kettles, 
 
 154. 
 Bronzite, 25, 312, 289. 
 
 in basalt, 279. 
 
 Brooke and Miller, 22. 
 
 Brown, Dr. R., on depression in Disco 
 
 Bay, 327. 
 
 Brunton dyke, 339. 
 Bryozoa, 495. 
 
 Buch on Vesuvian lavas, 200. 
 Buusen, 15. 
 
 on geysers, 195. 
 
 on ground basalt, 184. 
 
 Burntisland, volcanoes of, 309. 
 Busk, G., on Bryozoa, 495. 
 
 CADGWITH serpentine, 312. 
 Cainosauria, 511. 
 Calcite, 21. 
 
 organisms, 106. 
 
 Calderon Snout, " whin sill " of, 336. 
 
 Caledonian gneiss, 385. 
 
 Calloway on Hebridean gneiss, 385. 
 
 on the Wrekin, 292. 
 
 Cambrian slates, 101. 
 
 volcanoes, 293. 
 
 Camel, Cope on the, 524. 
 Campsie Fells, 308, 309. 
 
 Cape Cornwall, granite vein of, 250. 
 
 Wrath, gneiss of, 355. 
 
 Capel, 420. 
 Carara marble, 31. 
 Carboniferous dolerites, 303. 
 
 foraminifera, 479. 
 
 limestone, 48. 
 
 limestone oolitic, 109. 
 
 sandstone, 96. 
 
 volcanoes, 303. 
 
 Cardigan, lodes in anticlinals, 423. 
 Cardium, figure of, 498. 
 Caribbean province, 460. 
 Carlingford granites, 244. 
 Cam Menelez granite, 230. 
 Carnivora, 525. 
 Carnsore granite, 247. 
 Carpathians, slate of, 394. 
 Carpenter, W. B., on foraminifera, 100. 
 480. 
 
 on life of the deep sea, 443. 
 
 P. H., on antedon, 487. 
 
 on hypocrinus, 488. 
 
 on pentremites, 488. 
 
 on echinoidea, 488. 
 
 Carrickfergus, 21. 
 
 Carruthers on lepidodendron, 451. 
 
 on pandanaceous fruits, 473. 
 
 Caspian, 135. 
 
 petroleum springs of, 193. 
 
 Catania lava streams, 185. 
 Causes of earthquakes, 324. 
 Cavendish, 10. 
 
 Sir T., 12. 
 
 Caves, 153. 
 
 Cayenne, 13. 
 
 Central France, metamorphic rocks of, 
 
 393. 
 gneiss of the Highlands, 384. 
 
534 
 
 INDEX. Ce Cr 
 
 Cephalopoda, 499. 
 Cetacea, fossil, 521. 
 Chabasite, 28. 
 Chalk, 111. 
 
 of Antrim, 28. 
 
 foraminifera, 479. 
 
 headlands, 123. 
 Challenger observations on glauconite, 
 
 at Furnas, 194. 
 
 Changes of level in land, 325. 
 Characteristics of rhyolite, 271. 
 Charnwood Forest, upheaval of, 332. 
 Chelonia, 512. 
 
 Chemical composition of granite, 203, 
 215. 
 
 metamorphism, 359. 
 
 relations of metals, 402. 
 
 Chert in sandstones, 99. 
 
 Chesil Bank, 124. 
 
 Chiastolite slate, 102. 
 
 Chile River, 184. 
 
 China clay in Bagshot sands, 100. 
 
 Chines, 151. 
 
 Chlorite, 26. 
 
 Chlorites, 366. 
 
 Christiansand, foliation at, 360. 
 
 Chronological terms, 74. 
 
 Cinnabar, 398. 
 
 Cirripedes, 494. 
 
 Classification by fossils, 527. 
 
 Clay, 21, 46, 99, 100. 
 
 its deposition, 50. 
 
 on sand, 53. 
 
 ironstone of the coal measures, 430. 
 Cleavage, 32, 84, 
 
 in contorted slates, 86. 
 
 in lias shales, 86. 
 
 Sedgwick on, 33. 
 
 Cleat, 358. 
 
 Cleveland dyke, 338. 
 
 ironstone, ,432. 
 
 Cliffs, tertiary, 125. 
 
 Clifton Ward, analyses of Cambrian 
 slates, 225. 
 on igneous lake rocks, 235. 
 
 Climatal conditions of ancient seas, 453. 
 
 Clovelly, contortion at, 65. 
 
 Coal, analogy to peat, 475. 
 
 measures, iron ores of, 430. 
 
 Coast-lines, 118. 
 
 Cobalt, 425. 
 
 Coccoliths, 106. 
 
 Cochraue, Patrick, on mining in Scot- 
 land, 418. 
 
 (Jock field and Armathwaite dyke, 338. 
 
 < 'old temperate zone, 458. 
 
 Cole, Grenville, on Trachylyte, 281. 
 
 Collateral effects of upheaval, 349. 
 
 Colonies, doctrine of, 450. 
 
 Columnar structure, 42. 
 
 Comparison of granite and clays, 205. 
 
 of Lake granite and Scotch granite, 
 238. 
 
 of syenite with Cambrian slates, 
 
 225. 
 Composition of strata, 86. 
 
 Comstock lode, 256. 
 
 Conclusion, 528. 
 
 Concomitants of volcanic energy, 321. 
 
 Concretions in andesite, 259. 
 
 in granite, 218. 
 
 Cones arranged in lines, 188. 
 Coniston copper mines, 419. 
 Conodonts, 506. 
 Constituents of faunas, 476. 
 Contact metamorphism by granite, 220. 
 Contemporaneous veins, 406. 
 Contents of strata, 69. 
 Continuity of strata, 65. 
 Contorted schist, 31. 
 strata, 76. 
 Convulsion, epochs of, 79. 
 
 relation of igneous rocks to, 333. 
 
 Cope on American reptiles, 516. 
 
 on amphibia, 507. 
 
 on pythonomorpha, 511. 
 
 on tertiary marsupialia, 520. 
 
 Copper, 418. 
 
 in Europe, 419. 
 
 Coral reefs, 113. 
 Coralline crag, 104. 
 
 limestone, 48. 
 
 Cordas, 3. 
 
 Cornish copper, 418. 
 
 granites, 93, 229. 
 
 Cornwall, dykes of, 336. 
 
 iron of, 427. 
 
 veins in, 249. 
 
 volcanic rocks of, 252, 300. 
 
 Corriegills, 305. 
 
 Corstorphine Hill, Allport on gabbro of, 
 
 306. 
 
 Cortez range, quartz propylite of, 257. 
 Coseguina ash eruption, 183. 
 Cotta, 22, 35. 
 
 on intrusive gneiss, 373. 
 
 on succession of lavas, 201. 
 
 Cottonwood sandstone compared with 
 
 granite, 205. 
 Crabs, fossil, 491. 
 Craters of Italy, 187. 
 Crater lakes, 136. 
 Craven fault, 78, 334. 
 Craven, vertical limestone of, 76. 
 Crayford, 47. 
 Creep, 376. 
 Creetown granite compared with Prague 
 
 slate, 205. 
 
 Creodonta, E. D. Cope on, 521. 
 Cretaceous aporosa, 484. 
 - birds, 519. 
 
 bryozoa, 495. 
 
 crinoids, 487. 
 
 flora, 473. 
 
 Crianlorich, foliation at, 360. 
 
 Crinoidea, 485. 
 
 Crocodilia, 513. 
 
 Croll, J., 15. 
 
 Crossfell, strata in, 72. 
 
 Crossing of veins, 407. 
 
 Crozet Islands, glauconite of, 98. 
 
 Crustacea, 491. 
 
 Cryptocrystalline texture, 253. 
 
INDEX. Cr-Dr 
 
 535 
 
 Crystalline limestone, 377. 
 
 in Scotland, 389. 
 
 Crystals on sand, 94. 
 Cuchullin Hills of Skye, 320. 
 Cumberland haematite, 428. 
 Cumbrian Mountains, dykes of, 335. 
 Cuvier on revolutions of the globe, 75. 
 Cwm, Arthur, 149. 
 Cyathocrinus, figure of, 486. 
 Cypridea, figure of, 494. 
 Cyprina, history of, 465. 
 Cyrena, figure of, 441. 
 Cystiphyllum, figure of, 481. 
 Cystoidea, 487. 
 
 DACITE, 39, 263. 
 Didmahoy, dolerite of, 306. 
 Dames, W., on archaeopteryx, 519. 
 Dana, 22. 
 
 on Crustacea, 491. 
 
 on life of corals, 114. 
 
 on veinstones, 250. 
 
 on volcanoes without cones, 188. 
 
 Dartmoor, 50. 
 
 granite, 230. 
 
 Darwin, 31, 33. 
 
 on classification of coral reefs, 114. 
 
 on crystals in Galapagos lava, 209. 
 
 on earthquakes, 322. 
 on gypsum, 105. 
 
 on growth of reefs, 107. 
 
 on oolite now forming, 48. 
 
 on origin of species, 438. 
 
 on plains in the Andes, 133. 
 
 on South American Coast, 50. 
 
 on water-formed rocks, 45. 
 Daubeny, 2. 
 
 on earthquakes, 321. 
 
 volcanic hypothesis, 201. 
 Daubree on zeolites, 29. 
 on origin of ores, 399. 
 
 on origin of tin, 419. 
 
 on travelled sand, 93. 
 
 on veinstones, 250. 
 
 on water as affecting volcanic rocks, 
 
 177. 
 
 Davidson on fossil brachiopoda, 496. 
 
 J )avy, volcanic hypothesis, 201. 
 
 Debey on flora of Aix-la-Chapelle, 473. 
 
 Dechen (Von), on granite veins at Mouse- 
 hole, 250. 
 
 on metalliferous veins, 411. 
 Decline of volcanic activity, 191. 
 Decomposition of granite, 217. 
 Deer, evolution of, 522. 
 Definition of geology, 1. 
 
 De la Beche on identification of strata, 
 
 450. 
 
 on schists of South Devon, 381. 
 
 on a Trias volcano, 313. 
 
 Delaunay, 11. 
 
 Deltas, 163. 
 
 Demoid types, 437. 
 
 Denbigh grit, 95. 
 
 Density of the earth, 9. 
 
 Denudation of carboniferous limestone, 
 
 147. 
 
 Denudation of Culver Cliff, 47. 
 
 of volcanic regions, 198. 
 
 Deposition simulating upheaval, 331.' 
 
 Depth at which volcanoes originate, 178. 
 
 Derbyshire, carboniferous limestone do- 
 lomite, 109. 
 
 Dermatochelyidse, 512. 
 
 Desaguardero, table-land of, 132. 
 
 Descending streams, 153. 
 
 Deslongchamps, E. E., on teleosauria, 
 513. 
 
 Destruction of marine animals, 444. 
 
 Dettingen, basalts of, 280. 
 
 Devonian grits, 95. 
 
 limestones, 109. 
 
 slates, 100. 
 
 volcanoes of Cornwall, 300. 
 
 Devonshire, iron of, 427. 
 
 Diabase, 40. 
 
 A. Geikie on, 309. 
 
 Diallage, 25. 
 
 rock, relation of serpentine to, 28S. 
 
 Diameter of the earth, 11. 
 
 Dibranchiata, 501. 
 
 Differences between dykes and veins, 
 399. 
 
 Dinosauria, 515. 
 
 Diorite, 40, 225. 
 
 and propylite compared, 256. 
 
 of the Lakes, 237. 
 
 of the Virginia range, 226. 
 
 of Warwickshire, 304. 
 
 Dip, 63. 
 
 Directions of veins, 408. 
 
 Dislocation, proofs of, 330. 
 relative age of, 78. 
 
 Disseminated veins, 404. 
 
 Distinction of stratified and un stratified 
 rocks, 68. 
 
 Distribution of basalt, 281. 
 
 of British iron ores, 426. 
 
 of dykes, 335. 
 
 of earthquakes, 324. 
 
 of elvans, 232. 
 
 of gabbro, 227. 
 
 of gneiss and mica schist, 379. 
 
 of mollusca, 457. 
 
 of plants, 468. 
 
 of quartz augite andesite, 275. 
 
 Disturbance of the strata, 330. 
 
 Ditroite, 224. 
 
 Devitrification, 254. 
 
 D line, 16. 
 
 Dolerite, 40. 
 of Kowley, 278. 
 
 Doleritic rocks, 280. 
 
 Dolichosauria, 512. 
 
 Dollo on Iguanodon, 516. 
 
 Dolomite, 21. 
 
 Domite, 40. 
 
 Donegal, granites of, 242. 
 
 Downthrow, 77. 
 
 Downton sandstone, 46. 
 
 Drachenfels trachyte, 39. 
 
 Drake (Sir F.), 12. 
 
 Drew on Fans, 158. 
 
 Droitwich, 21. 
 
536 
 
 INDEX. Dr Fo 
 
 Drumadoon, 305. 
 
 Dufrenoy, age of mefcamorphism in Py- 
 renees, 393. 
 Dumfries, gold of, 418. 
 
 Permian, volcanoes of, 311. 
 
 Duucan, Prof. P. M., on Favosites, 481. 
 
 on Madreporaria, 482. 
 
 Dunite, 27, 287. 
 
 Durocher's hypothesis of magmas, 169. 
 
 Dyer, W. T. Thistleton, on algae, 194. 
 
 Dykes, 77, 334. 
 
 Dymoke, sands of, 96. 
 
 EARTH-MOVEMENT IN BRITAIN, 347. 
 Earthquakes, 321. 
 
 causes, 324. 
 
 distribution, 324. 
 
 effects, 324. 
 
 motion, 323. 
 
 origin, 323. 
 
 velocity of waves, 322. 
 
 East Wheal Lovell, Foster on, 421. 
 
 Eccles, sand dunes of, 125. 
 
 Echinoderms, changed habit of, 452. 
 
 Echinosphaerites, 488. 
 
 Eddystone, gneiss of, 381. 
 
 Edentates in Europe, 525. 
 
 Edinburgh district, volcanoes of, 308. 
 
 Effects of earthquakes, 324. 
 
 Eigg, lavas of, 319. 
 
 Eklogite, 27. 
 
 Elaeolite syenite, 223. 
 
 Elements forming rocks, 21. 
 
 Elevation and depression of land, 325. 
 
 Elevation of Norway, Lyell on, 171. 
 
 Elie ruby, 369. 
 
 Elie de Beaumont, hypothesis of, 346. 
 
 Klvanite, 37. 
 
 El vans and granites compared, 231. 
 
 Enaliornis, 519. 
 
 Eucelius, 3. 
 
 Encroachments of the sea, 165. 
 
 Encrinus, figure of, 487. 
 
 Eudocyclica, 489. 
 
 English Channel, 122. 
 
 Ennerdale syenite, 236. 
 
 Enville, Permian breccias of, 150. 
 
 Epochs, 74. 
 
 of convulsion, 79. 
 
 of upheaval, 332. 
 
 Erosion, simulating fracture, 330. 
 
 Erratics in Secondary rocks, 151. 
 
 Eruption, sequence of events in an, 181. 
 
 Eruptions, why intermittent, 178. 
 
 Eruptive hot springs, 194. 
 
 Eruptive power of volcanoes, 177. 
 
 Erythraean sea, 2. 
 
 Eskdale granite, 235. 
 
 Estuarine deposits, mollusca of, 442. 
 
 Ethiopian resrion, 463. 
 
 Etna, 2, 154/ 
 
 lava streams from 185. 
 
 Ettingshausen, Baron C., on floras of 
 
 Alum Bay and Sheppey, 475. 
 Eucrocodilia, 513. 
 Eulysite, 287. 
 European andesites, 260. 
 
 European basin, mountains of, 352. 
 convulsions, Elie de Beaumont on, 
 
 345. 
 
 iron ores, 435. 
 
 syenites, 222. 
 
 Evidence of extinct volcanoes, 290. 
 Evolution of igneous rocks, 207. 
 
 of mammalia. 520. 
 
 Excavated lakes, 136. 
 
 Excavating action of streams, 153. 
 
 Exchange of beds, 89. 
 
 Existing distribution of life, 454. 
 
 Exmoor, iron ores of, 428. 
 
 Exocyclica, 490. 
 
 Experimental formation of igneous rocks, 
 
 213. 
 
 Extinct geysers, 196. 
 volcanoes, 179. 
 Extinction of species, 443- 
 
 FABRICIUS, 3. 
 
 Fairbairn, Sir "VV., on atmospheric pres- 
 sure, 328. 
 
 Falls of St. Anthony, 156. 
 
 False bedding, 73. 
 
 Fans, 157. 
 
 Farrid Head, serpentine of, 367. 
 
 Faroe Isles, 28. 
 
 Faults, 76, 78, 333. 
 
 relation of, to axes of elevation, 334. 
 
 Felsite, 37. 
 
 porphyry, 270. 
 
 Felspar basalts, 278. 
 
 magnetites, A. Geikie on, 310. 
 
 in metamorphic rocks, 361. 
 
 in rhyolite, 272. 
 
 Felspars, 22, 361. 
 
 Fenestella, figure of, 495. 
 
 Ferrite, 255. 
 
 Fife, volcanoes of, 309. 
 
 Figure of the earth, 10. 
 
 Fisher, Osmond, on elevation of moun- 
 tains, 171. 
 
 on internal heat, 169. 
 
 Fissure eruptions, 189. 
 
 Fissures, direction of, 183. 
 
 Fitton, geology of Hastings, 166. 
 
 Flat veins, 403. 
 
 Flexures in mica schist, 375. 
 
 Flight of meteorites, 18. 
 
 Flint, 21. 
 
 in fissures, 112. 
 
 Flints, 111. 
 
 Fluxion structure in volcanic rocks, 254. 
 
 Focus of elevation, 348. 
 
 Foliation, 31. 
 
 Foraminifera, antiquity of, 479. 
 
 Foraminiferal limestone, 48. 
 
 Forbes, David, on foliated limestone, 
 360. 
 
 on South America, 105. 
 
 on table-laud in the Andes, 132. 
 
 Edward, on climate and distribu- 
 tion, 455. 
 
 on distribution of life, 449. 
 
 on extinction of species, 450. 
 
 on inferences from species, 453. 
 
INDEX. Fo-Gu 
 
 537 
 
 Forest of Dean iron ores, 430. 
 region of North America, 470. 
 
 Forschammer on elements in the sea, 
 182, 390. 
 
 Foster, Dr. Le Neve, on tin, 420, 
 on Haytor, 427. 
 
 Fouque & Levy, on formation of igneous 
 rocks, 213. 
 
 Fowey copper mines, 418. 
 
 Fox, Rev, Darwin, on polacanthus, 517. 
 
 Fox's, Mr. , electro - magnetic experi- 
 ments, 83. 
 
 Foyaite, 223. 
 
 Fraas, Oscar, on triglyphus, 520. 
 
 Fracastorio, 5. 
 
 Fractures and dislocations of strata, 333. 
 
 Fractures of the Weald, 329. 
 
 France, gold in, 417. 
 
 granite in, 220. 
 
 iron in, 435. 
 
 Fraunhofer, 15. 
 
 Freiberg gneiss, 23. 
 
 mining district of, 413. 
 
 Fresh-water deposits, 441. 
 
 limestones, 49. 
 
 Fringing reefs, 114. 
 
 Fritsch, Anton, on amphibia, 507. 
 
 Frost, action on rocks, 146. 
 
 Fuchs, Karl, on volcanic distribution, 
 190. 
 
 Fucus vesiculosus as a source of metals, 
 399. 
 
 Fundamental gneiss, 373. 
 
 Fungida, Duncan, on distribution of, 
 483. 
 
 Furnas, springs of, 194. 
 
 Furness, kidney iron ore of, 429. 
 
 GABBRO, 40, 227. 
 
 - of Mull, 317. 
 Galloway granite, 240. 
 Galway granite, 247. 
 Ganges delta, 166. 
 Ganoid fishes, 503. 
 
 Gardner, J. S., on tertiary floras. 474. 
 Garnets, 26, 369. 
 
 developed by basalt, 338. 
 Gash veins, 403. 
 Gasteropoda, 498. 
 
 in fresh-water deposits, 441. 
 
 succession of, 498. 
 
 Gaudry on succession of mammals, 526. 
 
 Enchainements de Monde Animal, 
 
 521. 
 
 on the struggle for existence, 521. 
 Gefle, raised beach-mark at, 326. 
 Geikie, A., on Ailsa Craig, 251. 
 on Eigg, 319. 
 
 on foliation in volcanic ashes, 381. 
 
 on Forth volcanic rocks, 308. 
 
 on old red sandstone volcanic 
 
 rocks, 299. 
 
 on Permian volcanoes, 311. 
 
 on pikrite, 310. 
 
 James, on diallage at Pinbain, 367. 
 
 Genera, existing distribution of, 4o6. 
 Genesis of genera, 440. 
 
 Genus and species, 439. 
 Geological age of gneiss, 373. 
 
 of granite, 221. 
 
 evidence of nature of volcanic heat, 
 
 172. 
 
 publications, 528. 
 
 Germany, granite in, 220. 
 Gesner, 3. 
 
 Geysers of Iceland, 194. 
 
 Giant's Causeway, 28, 43, 316. 
 
 kettles, 154. 
 
 Gibraltar, sea bottom at, 163. 
 
 Glaciation of Anglesey, 129. 
 
 Glaciers, 148. 
 
 Glauconite, 21, 98. 
 
 Glen Roy, 387. 
 
 Sannox, granite of, 242. 
 
 Shee, 384. 
 
 Tilt, granite veins in, 249. 
 
 Globigerina, 48. 
 
 Glyders, Rutley on felstone of, 296. 
 
 Gneiss, its origin, 370. 
 
 resting on strata, 374. 
 
 Goat Fell, 241. 
 
 Godwin- Austen, R. A. C., on artificial 
 
 classifications, 527. 
 ou lacustrine deposits, 138. 
 
 Gogofaw, gold mines of, 417. 
 
 Gold, 417. 
 
 Goniatites, figure of, 500. 
 
 Gorge, its excavation, 155. 
 
 of the Rhine, 156. 
 
 Gosau fauna, 516. 
 
 valleys of secondary age, 142. 
 
 Gradation of beds, 89. 
 
 Graham Island, 180. 
 
 Grampians, Judd on, 239. 
 
 southern slope of, 383. 
 
 Granite, geological age of, 221. 
 
 in Scotland, 238. 
 
 of Anglesey, 234. 
 
 of Cornwall, 229. 
 
 of the Isle of Man, 391. 
 
 porphyry, 37. 
 
 veins, 219, 248. 
 
 Granites, their origin, 203. 
 
 Granitic rocks of the Lake District, 235. 
 
 texture, 33. 
 
 Grauitite, 216. 
 
 of South of Ireland, 392. 
 
 Graphic granite, 217. 
 
 Graptolites, 480. 
 
 Gray, Dr. Asa, on forest regions, 470. 
 
 Great masters in geology, 528. 
 
 Greenland, depression of, 327. 
 
 Green slates of Cumberland, 101. 
 
 Gregory, Watts, experiments on basalts, 
 82. 
 
 Greisen, 36, 422. 
 
 Grenville series, 396. 
 
 Grey gneiss, 373. 
 
 Grit, 45. 
 
 Groby syenite, 233. 
 
 Ground mass, 254. 
 
 Groups of British strata, 74. 
 
 Guiscardi on fossils ejected from Vesu- 
 vius, 179. 
 
538 
 
 INDEX. Gu-In 
 
 Gumbel on Iherzolite, 287. 
 Giinther, Dr., on palaeichthyes, 501. 
 Gypsum, 21, 105. 
 
 HADDINGTON district, volcanoes of, 
 
 308. 
 
 Hade, 77. 
 
 Haematite of Cumberland, 428. 
 Hahn, Otto, 19. 
 Hamilton, 2. 
 
 on serpentines of Tuscany, 289. 
 
 on Tuscan springs, 196. 
 
 Sir w., on Vesuvius, 188. 
 
 Haplite, 36, 217. 
 
 Hardman on rhyolite of Tardree, 211, 
 
 315. 
 
 Harlech grits, 46. 
 Hartley dyke, 339. 
 Harz Mountains, 4. 
 Harzburg Forest, 25. 
 Haughton, S., on Carlingford syenite, 
 
 205. 
 
 on Donegal granites, 243. 
 
 on eruptive granites, 247. 
 
 on sranite of Cornwall, 229. 
 
 on Ross of Mull, 239. 
 
 and Bonney on minette, 237. 
 
 Hayden on geysirs, 195. 
 Haytor iron mines, 427. 
 Headlands, 123. 
 Hebbnrii dyke, 339. 
 Hebridean gneiss, 385. 
 Hebrides, Inner, 130. 
 
 strike of rocks in, 382. 
 
 Heddle on Hebridean gneiss, 385. 
 
 on inclusions in granite, 372. 
 
 on minerals in metamorphic rocks, 
 
 361. 
 
 on serpentines, 367. 
 
 Heim on elevation in the Grisons, 352. 
 Heliolites, figure of, 481. 
 Helix, figure of, 442. 
 Helston, tin near, 421. 
 Helvellyn rhyolite, 236. 
 Hennock, dolerites of, 302. 
 Hensborough granite, 230. 
 Henslow on metamorphism, 359. 
 
 on dykes at Plas Newydn, 338. 
 
 Herodotus, 2. 
 Herschel, Sir W., 17. 
 
 Sir J., on volcanic energy, 202. 
 
 Hertfordshire pudding-stone, 97. 
 Hesperornis, 519. 
 Heterocercal tail, figure of, 502. 
 Hetts dyke, 339. 
 Heulandite, 28. 
 Hexacoralla, 482. 
 Hexactinellid sponges, 478. 
 Hicks, Dr. H., on Berwynia, 473. 
 
 on St. David's, 291, 385, 396. 
 
 High force, columnar basalt, of, 337. 
 
 Green dyke, 339. 
 
 Higher orders succeed lower, 476. 
 
 Highlands, mica schist in, 383. 
 
 Hill and Bonney on Charnwood Forest, 
 
 233, 293. 
 Hinde, Dr. G. J., on sponges, 478. 
 
 Hinde, Dr. J. G., on receptaculites, 478. 
 
 on vermes, 495. 
 
 Hochstetter on geysers, 196. 
 Holopea, figure of, 499. 
 Holoptychius, figure of, 504. 
 Homceosauria, 512. 
 Homotaxis, 449. 
 Hope series, 386. 
 
 Hopkins, W., on pressure of steam, 328. 
 theory of elevation, 177, 328. 
 
 Hordwell Cliff, sands of, 97. 
 Horizontal sequence of rocks, 51. 
 Hornblende, 25, 369. 
 
 andesite, 261. 
 
 granite, 221. 
 
 slates in Scotland, 388. 
 
 syenite, 222. 
 
 Hornblendic schists like diorite, 380. 
 Horse, Marsh and Huxley on the, 523. 
 Hot springs, 196. 
 Hudleston on serpentine, 312. 
 Huggins, W., 17. 
 
 Hughes, M 'Kenny, on glauconite, 98. 
 on gnarled schists, 381. 
 
 Hulke, J. W"., on dinosaurs, 516. 
 Hull on Mayo granite, 248. 
 Humboldt on bombs, 185. 
 
 on earthquakes, quoted, 321. 
 
 on gneiss of the Andes, 394. 
 
 on volcanic mud streams, 184. 
 
 Humboldtilite at the Lowther Iron 
 
 Works, 203. 
 
 Humidity, wasting influence of, 145. 
 Hungarian dacite, 263. 
 Hunstanton Carstone, 97. 
 Hunt, Sterry, on eruptive rocks, 202. 
 
 on veinstones, 250. 
 
 Huronian series, 396. 
 Button, work of, 529. 
 Huxley, Prof. T. H., 44. 
 
 on homotaxis, 450. 
 
 on lacertilia, 512. 
 
 on persistent types of life, 451. 
 
 text-books of, 491. 
 
 Hyalomelane, 281. 
 Hybodus, figure of, 503. 
 Hydrocorallina, Prof. Moseley on, 480. 
 Hydrozoa, 480. 
 Hymenocaris, figure of, 494. 
 Hypersthene, 24. 
 Hypothesis of magmas, 169. 
 
 ICEBERGS, 151. 
 Iceland, 28. 
 
 geysers of, 194. 
 Ichthyornis, 519. 
 Ichthyosauria, 515. 
 Ideal section, 35. 
 
 Identification of strata by fossils, 448. 
 Idocrase, 27. 
 Igneous evolution, 201. 
 Igneous rocks related to sandstones, 211. 
 Illsenus, figure of, 492. 
 Imperfection of the geological record, 
 
 445. 
 
 Inclination of strata, 63. 
 Inclined strata, not so deposited, 331. 
 
INDEX. In-Li 
 
 539 
 
 Inclusions in granite, 218. 
 
 Indo-Pacific province, 460. 
 
 Inductive geology, 2. 
 
 Influence of radiated heat on the earth's 
 
 surface, 170. 
 Ingleborough, 71. 
 
 Ingredients of mechanical strata, 88. 
 Inlier, 67. 
 
 Inoceramus, figure of, 497. 
 Internal arrangement of rocks, 62. 
 
 heat of the earth, 169. 
 
 structure of rocks, 81. 
 Intersection of veins, 407. 
 Intrusive basalt of Teesdale, 337. 
 Inundations, their effects, 148. 
 lona, crystalline marbles of, 389. 
 Ireland, granites of, 242. 
 
 Iron, 426. 
 
 in Coal-measures, 430. 
 
 in snow, 18. 
 
 in the Lias, 432. 
 
 ores of the Oolites, 433. 
 
 pyrites, 21. 
 
 slags compared with lavas, 203. 
 
 Irvine on Galloway granite, 240. 
 
 Islands, 127. 
 
 Isle of Man, metamorphic rocks of, 391. 
 
 Isle of Wight, 127. 
 
 Isolated granites of Leinster, 246. 
 
 Italian philosophers, 5. 
 
 Ivy Bridge, black slate of, 102. 
 
 JAPONIC province, 458. 
 
 Jeffreys, Dr. Gwyii, on deep-sea life, 
 
 443. 
 Joints, 81. 
 
 causes of, 82. 
 in granite, 217. 
 
 mineral veins in, 425. 
 
 Joint-structure, 42. 
 
 Jones, Prof. Rupert, on destruction of 
 life, 444. 
 
 on foraminifera, 480. 
 Jorullo, 180. 
 
 Judd, Prof., on Arthur Seat, 306. 
 
 on Ben Nevis, 297. 
 
 on cones in the Eifel, 179. 
 
 on felsites and granites, 211. 
 
 on hypothesis of interior of the 
 earth, 170. 
 
 on iron ore of Northampton sand, 
 
 433. 
 
 on Mull, 317. 
 
 on relation of felsites to granites, 
 176. 
 
 on tachylyte, 280. 
 
 on the elevation of the Gram- 
 pians, 120. 
 
 Jukes, Prof., on river-valleys, 329. 
 on Staffordshire iron, 431. 
 
 on volcanic rocks of Dudley, 303. 
 Jupiter, 16. 
 
 Jupiter Serapis, temple of, 327. 
 Jurassic crinoids, 487. 
 
 KANE, Dr., on change of level in Arctic 
 lands, 326. 
 
 Kaolin, manufacture of, 100. 
 
 Kiiskeller, 43. 
 
 Katakekaumene, 2. 
 
 Keilhau, Prof., on upheaval of North 
 
 Cape, 326. 
 Kendall on haematite, 428. 
 
 on iron ores of Antrim, 435. 
 
 on veins in joints, 425. 
 
 Kentish rag, 110. 
 
 Kentmann, 3. 
 
 Kilauea, 181. 
 
 Killerton Park, volcano of, 314. 
 
 Killiecraukie, garnets of, 369. 
 
 Kinahan on Galway granite, 247. 
 
 on raised sea margins, 327. 
 
 King, Clarence, on volcanic rocks, 203, 
 
 208. 
 
 King-crabs, 493. 
 Kirchoff, 15. 
 Kronstedt, 3. 
 Kupferberg, ores of, 411. 
 
 LAACH, 27. 
 Labradorite, 23, 363. 
 
 rocks, 40. 
 
 Labrador series, 396. 
 Ladakh, fans at, 158. 
 Lacertilia, 512. 
 Lacustrine Limestones, 163. 
 
 strata, 137. 
 
 Lake Lherz, 25. 
 Lakes, 134. 
 Lameliibranchiata, 496. 
 
 in fresh-water deposits, 441. 
 
 Lamination, 72. 
 Lancerote oolite, 48. 
 Land gasteropoda, 442. 
 
 relation of secondary strata to, 54. 
 
 Land's End granite, 230. 
 
 Landslips, 126. 
 
 Lankaster, E. Kay, on fossil fishes, 506. 
 
 Laopteryx, O. C. Marsh on, 519. 
 
 Lap worth on graptolites, 480. 
 
 Largo, volcanic neck of, 311. 
 
 Lariosaurus, Dr. Liitken on, 514. 
 
 Lateral pressure, 172. 
 
 Laumonite, 28. 
 
 Lava streams, 185. 
 
 Lead, 423. 
 
 Lebour, Prof. , on Northumberland dykes, 
 
 339. 
 
 Le Conte, Joseph, on lava floods, 190. 
 Lee, J. E., on coast of Antrim, 315. 
 Lehmann, 3. 
 Leibnitzian theory, 3. 
 Leinster granites, 245. 
 Lemoine, Victor, on tertiary birds, 520. 
 on Neoplagiaulax, 520. 
 
 Lemurs, 525. 
 Lepidomelane, 365. 
 Leucite, 23. 
 Leucite basalt, 283. 
 Lherzolite, 287. 
 Lias, 46. 
 Lias sand, 96. 
 Liassic iron ores, 432. 
 Lichas, figure of, 492. 
 
540 
 
 INDEX. Li-Mo 
 
 Lickey Hills, 95. 
 
 Life, existing distribution of, 454. 
 
 Limestone, 47, 87. 
 
 Lincolnshire iron ore, 437. 
 
 Linear arrangement of volcanoes, 176. 
 
 Linnaeus, 3, 326. 
 
 Liulithgow district, volcanoes of, 308. 
 
 Liparite, Roth on, 296. 
 
 Liskeard, concretions in slate of, 102. 
 
 Lister, 3, 6, 71. 
 
 Lithistid sponges, 478. 
 
 Lithostrotion figure, 482. 
 
 Little Knott, Pikrite of, 310. 
 
 Lizard, 25. 
 
 schists, 280. 
 
 serpentine of, 312. 
 
 Llanberis slale, 101. 
 
 Llandovery beds, 46. 
 
 Llautrissant, iron of, 431. 
 
 Lloyd, Eev. H., on pebbles in granite, 
 
 245. 
 
 Llwyd, 6. 
 
 Lobsters, fossil, 491. 
 Local persistence of types, 452. 
 Loch Awe, slates of, 101. 
 Loch Coruisk, gabbro of, 320. 
 Loch Earn, Nicol on contorted rocks of, 
 
 384. 
 
 Loch Etive granite, 239. 
 Loch Ken granite, 240. 
 Loch Maree, 30. 
 Lophobranchiate fishes, 506. 
 Low plains, 134. 
 
 valleys in, 141. 
 
 Lowenburg, augite andesite of, 275. 
 
 Luudy, Isle of, 129. 
 
 Lusitanian province, 459. 
 
 Lusus naturae, 5. 
 
 Liitken, Dr. C., on Lariosaurus, 514. 
 
 Luxulianite, 36. 
 
 Lyddeker on distribution of mammalia, 
 
 447. 
 Lyell, 2, 47. 
 
 on excavation by rivers, 154. 
 
 on Niagara, 156. 
 
 quoted, 164. 
 
 work of, 529. 
 
 MACCULLOCH, JOHN, 528. 
 
 on Ben Cruachan, 335. 
 
 Macroscopic texture, 253. 
 
 Magas, figure of, 496. 
 
 MagellanJF., 12. 
 
 Magellanic province, 458. 
 
 Magnesia in magnesian limestones, 51. 
 
 mica, 26, 365. 
 
 Magnetic intensity, lines of, 349. 
 Magendie on granite veins at Mousehole, 
 
 249. 
 Mallet, 3. 
 
 on earthquakes, 322. 
 
 theory of volcanic heat, 171. 
 
 Malvern, Weulock limestone oolitic, 109. 
 
 Man, Isle of, 128. 
 
 Manganese in the Atlantic, 100, 399. 
 
 Marbles of Sutherland, 390. 
 
 Marine sediments, 45. 
 
 Marr, J. E., on Barrande's colonies, 450. 
 
 Mars, 16. 
 
 Marsh, O. C., on cretaceous birds, 520. 
 
 on classification of dinosaurs, 516. 
 
 on sauranodon, 515. 
 
 Martindale, met.nphoric rocks of, 379. 
 Maskelyne, 9. 
 Master-joints, 83. 
 
 relation of veins to, 410. 
 
 Mayhill sandstone, 95. 
 
 Mayo granite, 248. 
 
 Meade on iron in the Dogger bed, 432. 
 
 Mechanical deposits, 87. 
 
 Melania, figure of, 442. 
 
 Mendip Hills, upheaval of, 332. 
 
 Merostomata, H. Woodward on, 493. 
 
 Metamorphic granites, 34. 
 
 origin of igneous rocks, 170. 
 
 Metamorphism, 356. 
 
 Meteorites, 18. 
 
 Meyer, Hermann von, on fossil reptiles, 
 
 512. 
 
 Miall, C. L., on Amphibia, 507. 
 Miascite, 39, 224. 
 Miask, 23. 
 Mica, 25, 237. 
 
 in metamorphic rocks, 364. 
 
 in polarised light, 253. 
 
 in rhyolite, 272. 
 
 in sandstones, 50. 
 
 schist, 374. 
 
 syenite, 39, 223. 
 
 Michell, 5. 
 
 on earthquakes, 322. 
 
 Microliths, 255. 
 
 Mid-Britain, 121. 
 
 Migration of life provinces, 446. 
 
 Miliola, 48. 
 
 Millet seed-beds, 96. 
 
 Millstone grit, 45. 
 
 origin of, 88. 
 
 Milne-Edwards, Alphonse, on birds, 520. 
 Milne on Asosan, 187. 
 Mineral composition of granite, 214. 
 constituents of metamorphic rocks, 
 
 361. 
 
 deposits in Britain, 417. 
 
 matter in veins, 400. 
 
 veins, 397. 
 
 Minerals of aqueous rocks, 21. 
 
 in gneiss, 372. 
 
 in meteorites, 18. 
 
 in propylite, 257. 
 Mineralogical school, 3. 
 Mines of Saxony, 4. 
 Minette, 39, 237. 
 Mining, 415. 
 
 Minor cones on Etna, 187. 
 Modern veins, 413. 
 Modes of occurrence of granite, 220. 
 Moel Hebog, synclinal of, 64. 
 Mofettes, 197. 
 Moffat, slates of, 101. 
 Moisture, wasting influence of, 145. 
 Molecular metamorphism, 358. 
 Montalban series, 396. 
 Mont Blanc, axes of cleavage on, 358. 
 
INDEX. Mo-Ow 
 
 54i 
 
 Mont Dore, fractures of, 329. 
 
 Monte Nuovo, 328. 
 
 Monte Sooima, 187. 
 
 Monticulipora, 495. 
 
 Monumental stones, 146. 
 
 Moore, Charles, on volcanic rocks at 
 
 Shepton Mallet, 302. 
 Moreton, 6. 
 Morne Mountains, 23. 
 Moro, 3. 
 
 Morpeth dyke, 339. 
 Moseley on fresh-water algae in hot 
 
 springs, 194. 
 Mountain Cork, 314. 
 Mountain ranges, study of, 351. 
 Mountains, effect on strata, 80. 
 Mount Sorel granite, 233. 
 Mount's Bay, dolerites of, 300. 
 Mourn e granite, 243. 
 Mud streams from volcanoes, 184. 
 
 of the Hebrides, 317. 
 Mud volcanoes, 192. 
 Mull, volcano of, 317. 
 Muscovite, 26. 
 Museums, 529. 
 
 use of, 477. 
 
 Myne of the Weald, 434. 
 
 NANTWTCH, 21. 
 
 Naphtha springs, 193. 
 
 Native iron in basalt, 279. 
 
 Natrolite, 28. 
 
 Nature of deposits in gulfs, 164. 
 
 and origin of gneiss. 370. 
 
 of volcanic energy, 168. 
 
 Nautilus, figure of, 500. 
 
 Nearctic region, 462. 
 
 Nebular hypothesis, 17. 
 
 Necropolis hill, dolerite of, 309. 
 
 Neocomian iron ore, 434. 
 
 Neotropical region, 463. 
 
 Nepheline, 23. 
 basalt, 285. 
 
 in polarised light, 253. 
 
 Neumayr on ancient Echinoderms, 452. 
 
 Neusticosaurus, 514. 
 
 Nevadite, 270. 
 
 Newry granites, 244. 
 
 Newton, Prof. Alfred, on Mascarene 
 birds, 443. 
 
 Newton, Sir L, 13. 
 
 New Zealand geysers, 196. 
 
 Nicholson on Borrowdale rocks, 295. 
 
 Nickel, 425. 
 
 Nicol, Prof., on southern slope of Gram- 
 pians, 383. 
 
 Niedermendig, 42. 
 
 Nile delta, 166. 
 
 Ninety-fathom dyke, 333. 
 
 Nipped veins, 411. 
 
 Noachian deluge, 5. 
 
 Non-metallic minerals, 400. 
 
 North American syenite, 222. 
 
 Northampton sand, iron ore of, 433. 
 
 North and West Britain, coasts of, 
 119. 
 
 North Cornwall, lavas of, 301. 
 
 North-east of Ireland, granites of, 243. 
 Northern flora, 469. 
 North of England, iron ores of, 428. 
 mineral veins in, 415. 
 
 North of Ireland, mica slate of, 391. 
 North Wales, dykes of, 336. 
 North-west Highlands, gneiss of, 385. 
 Norway, fossil-bearing schists of, 394. 
 Nottinghamshire coal strata, 5. 
 Nucleolites, figure of, 490. 
 Nucleus of oolitic grains, 48. 
 Nullipores, 49. 
 Nummulina, 48. 
 Nummulites figured, 479. 
 
 OBERSTEIN, 29. 
 
 Obsidian, 37. 
 
 Ocean level, possible changes in, 349. 
 
 Ochills, 299. 
 
 Odontornitb.es, O. C. Marsh on, 520. 
 
 Oldest crinoid, 486. 
 
 cystoidian, 488. 
 
 mammals, 520. 
 
 sponge, 477. 
 
 star-fishes, 484. 
 
 Old red sandstone, 46. 
 
 volcanoes, 297. 
 
 Oldham on earthquakes, 178. 
 Oligoclase, 23, 363. 
 
 in polarised light, 253. 
 
 Olivine, 27. 
 
 diallage pikrite, 286. 
 
 enstatite, 287. 
 
 in basalt, 278. 
 
 in polarised light, 253. 
 
 Oolite, 47, 110. 
 
 Oolitic foraminifera, 479. 
 
 grains in recent limestones, 109. 
 
 group, 54. 
 
 Opacite, 255. 
 
 Open ocean, life of, 443. 
 
 Ophidia, 511. 
 
 Ophioderma, 485. 
 
 Ophiuroidea, 485. 
 
 Orbit of the earth, 14. 
 
 Orbulina, 48. 
 
 Ord of Caithness, 248. 
 
 Oreval, gabbro core of, 319. 
 
 Oriental region, 464. 
 
 Origin of faunas, 447. 
 
 of mineral veins, 397. 
 
 of species, 438. 
 
 Ornithosauria, 517. 
 Orthoclase, 23, 362. 
 
 in polarised light, 252. 
 
 rocks (quartz-free), 38. 
 
 Qsteolepis, figure of, 504. 
 
 Ostracods in primary rocks, 494. 
 
 Ostrea, figure of, 497. 
 
 Ottawa series, 396. 
 
 Outcrop, 66. 
 
 Outlier, 67. 
 
 Overlap, 79. 
 
 Owen, Sir R., on advent of a higher 
 
 type life, 476. 
 on archaeopteryx, 518. 
 
542 
 
 INDEX. Ow-Pr 
 
 Owen, SirR., on ichtbyosaurs and pie- 
 siosaurs, 514. 
 
 on local persistence of types, 452. 
 
 on South African reptilia, 515. 
 
 on tritylodon, 520. 
 
 Oxfordshire irou ore, 432. 
 Oxland on silver in sinter, 397. 
 
 PACHYDERMS, evolution of, 522. 
 Palarctic flora, 470. 
 
 region, 461. 
 
 Palseaster, figure of, 484. 
 Palsechinus, figure of, 489. 
 Palseontological laws, 445. 
 
 work, method of, 465. 
 
 Palaeontology, 437. 
 
 Palaeosauria, 512. 
 
 Palissy, 6. 
 
 Palraieri, 3. 
 
 Paludina, figure of, 441. 
 
 Panamic province, 460. 
 
 Parallel mountain chains, 353. 
 
 Parker, W. K., on foraminifera, 480. 
 
 on crocodilia, 513. 
 
 Parys mine, 403. 
 
 Patagonian province, 459. 
 
 Pea grit, 48. 
 
 Pebble beds indicate upheaval, 332. 
 
 Pebbly river-beds, 165. 
 
 Pecteii, figure and characters of, 440. 
 
 Pegmatite, 217. 
 
 Pelolithic group, 54. 
 
 Peltochelyida), 513. 
 
 Penig, 24. 
 
 Peninsulas pointing south, 353. 
 
 Pennant, 46. 
 
 Pennyghent, vertical strata of, 76. 
 
 Pentacrinus, figure of, 487. 
 
 Pentland Hills, 299. 
 
 Perennial springs, 152. 
 
 Perforata, geological succession of, 482. 
 
 Peridotite, 286. 
 
 Perlite, 37. 
 
 Permian sandstone, 96. 
 
 volcanoes, 311. 
 
 Persistent types of life, 450. 
 
 Peruvian province, 459. 
 
 Petit Puy de Dome, augite andesite of, 
 
 275. 
 
 Petroleum springs, 193. 
 Petrological microscope, 253. 
 Pharyngognathi, 505. 
 Phascolotherium, figure of, 521. 
 Phene on travertine at Hierapolis, 108. 
 Phillips, John, on Glen Tilt, 249. 
 
 on hypothesis of upheaval, 330. 
 
 on Salisbury Crag, 30. 
 
 J. A., analyses of sandstones, 210. 
 
 on bubbles in quartz, 229. 
 
 on concretions in granite. 218. 
 
 on Devonian volcanoes, 300. 
 
 on elvans, 232. 
 
 on granite of Cornwall, 229. 
 
 on mineral history of sandstones, 
 
 92. 
 
 397. 
 
 on mineral veins now forming, 
 
 Phillips, J. A., on the Wolf Hock, 251. 
 
 "W., 22. 
 
 Plilogopite, 26. 
 Pholadomya, 498. 
 Phonolite, 39, 268. 
 
 of the Wolf Rock, 251. 
 
 Phosphatite, 21, 47, 103. 
 
 origin of, 104. 
 
 Phyllopods in primary rocks, 493. 
 Physical and palaeontological breaks, 
 340. 
 
 history of tertiary strata, 466. 
 
 Physiological origin of species, 438. 
 
 Physostomi, 506. 
 
 Pictet on palseontological laws, 445. 
 
 traite de paleontologie, 506. 
 
 Pietra di Sarto, 289. 
 Pipe veins, 403. 
 Pikrite, 286. 
 
 at Little Knott, 237. 
 
 of Scotland, 310. 
 
 Pitch Lake of Trinidad, 193. 
 Pitchstone, 37. 
 Plagioclase, 22, 363. 
 
 granite, 221. 
 
 rocks (quartz free), 40. 
 
 Planorbis, figure of, 441. 
 
 Plant life, succession of, 473. 
 
 Plants, distribution of, 468. 
 
 Plas Newydd, Henslow on garnets at, 
 
 338. 
 
 Plaster-of-Paris, 46. 
 Plectognathi, 506. 
 Plesiosauria, 514. 
 
 Plombieres, zeolites in brickwork, 29. 
 Plot, 6. 
 
 Pl} r mouth limestone, 48. 
 P<> delta, 166. 
 
 Pocillopora as a source of metals, 399. 
 Polacanthas, Rev. D. Fox on, 517. 
 Polarised light, colours of minerals in, 
 
 252. 
 
 Poly mastodon, Cope on, 520. 
 Polypterus, figure of, 503. 
 Porites, 107. 
 Porphyrite, 38, 310. 
 
 of the Cheviots, 298. 
 
 Portrush prismatic lava, 316. 
 Portsoy, serpentine of, 313, 389. 
 Portus Valesize, 161. 
 Postlethwaite on mines in the Lake dis- 
 trict, 419. 
 Potash mica, 26. 
 Potstone of Scotland, 389. 
 Potstones in chalk, 112. 
 Pre-Cambrian volcanoes, 291. 
 Prehnite, 28. 
 
 Preserving power of the soil, 145. 
 Pressure in metamorphism, 357. 
 
 involved in rock-construction, 173. 
 
 Prestwichia, figure of, 493. 
 Prestwich on the Chesil Bank, 124. 
 
 on denudation of the Thames basin, 
 
 152. 
 
 on relation of volcanoes to springs, 
 
 191. 
 Prevost, Constant, on upheaval, 170. 
 
INDEX. Pr-St 
 
 543 
 
 Primary crinoids, 486. 
 
 foramiuifera, 479. 
 
 rocks, palaeontological breaks in, 
 
 340. 
 sea-urchins, 489. 
 
 sponges, 477. 
 Principle of stratification, 68. 
 Procoelia, 514. 
 
 Productiveness of mineral veins, 411. 
 
 Proportions of sands, clays, and. lime- 
 stones, 89. 
 
 Propylite, 255. 
 
 Protocardium, figure of, 497. 
 
 Protogine, 36, 216. 
 
 Provinces of land life, 461. 
 
 of marine life, 457. 
 
 Psammolithic group, 54. 
 
 Pterodactylia, 518. 
 
 Pterosauria, 518. 
 
 Pterygotus, restored by Dr. Woodward, 
 493. 
 
 Pumice-stone, 37. 
 
 Purbeck, elevation of Isle of, 329. 
 mammalia, R. Owen on, 521. 
 
 Puy de la Piquette, 28. 
 
 Pyramid Lake, trachyte of, 266. 
 
 Pyrenees, metamorphic rocks of, 392. 
 
 Pyrites, diffusion of, 410. 
 
 Pyroxene in polarised light, 253. 
 
 Pythonomorpha, 511. 
 
 QUARTZ, 21. 
 
 augite andesite, 275. 
 -T diorite, 226. 
 
 dolerite, 39. 
 
 felsite of Corriegills, 305. 
 
 orthoclase rocks, 36. 
 
 in polarised light, 252. 
 
 in rhyolite, 271. 
 
 plagioclase rocks, 39. 
 
 porphyry, 37. 
 
 propylite, 39, 257. 
 
 rock, 376. 
 
 of Anglesea, 381. 
 
 in Scotland, 388. 
 
 trachyte of Steve's Ridge, 267. 
 
 Quartzite of the Central Highlands, 384. 
 Quenastite, 39. 
 
 RAIN channels, 147. 
 
 Raised sea-beaches, 327. 
 
 Rake veins, 403. 
 
 Ramazzini, 6. 
 
 Ramsay (Sir A.), on Arran, 240. 
 
 on cleavage, 32. 
 
 on the granite of Anglesea, 234. 
 
 on Holyhead, 381. 
 
 on pisolitic iron, 51. 
 
 on Welsh volcanoes, 294. 
 
 Rate of flow of lava, 186. 
 Ray, 6. 
 
 Reconstructed deposits, 52. 
 Red clay in the deep sea, 183. 
 
 of the Atlantic, 100. 
 Red rain, 18. 
 Redruth, tin lode at, 420. 
 Red Sea, coral reefs in, 115, 
 
 Reduced pressure iuvolved in liquefac- 
 tion, 177. 
 
 Relation of hot springs to mineral veins, 
 198. 
 
 of living to fossil faunae, 460. 
 
 of metals to rocks, 410. 
 
 of plutonic and volcanic rocks, 209. 
 
 Reptiles, fossil, 511. 
 
 Reusch on fossil-bearing schists, 394. 
 
 Reusch and Brogger ou giants' kettles, 
 154. 
 
 Reykiadal, 194. 
 
 Rhabdoliths, 106. 
 
 Rhone, 161. 
 
 Rhynchocephalia, 512. 
 
 Rhynchonella, figure of, 496. 
 
 Rhyolite, 37. 
 
 of Mont Dore compared with sand- 
 stone, 211. 
 
 of Fairy Crag, 236. 
 
 of Mount Richthofen, 273. 
 
 Von Richthofen on, 269. 
 
 Rib of ore, 401. 
 
 Richer, 13. 
 
 Richthofen on fissure eruptions, 189. 
 
 on hypothesis of five magmas, 170. 
 
 on propylite, 255. 
 
 on Tertiary lavas, 200. 
 
 Rider, 401. 
 
 Riesengebirge, metamorphic rocks of, 
 394. 
 
 Rimbombo, 181. 
 
 Ripple drift in mica schist, 375. 
 
 Riviera, serpentines of, 289. 
 
 Rivers without lakes, 156, 161. 
 
 Roches moutonnees, 149. 
 
 Rocks formed of coral, 113. 
 
 Rock-salt, 21. 
 
 Rocks, unstratified, 69. 
 
 Rogers on earthquakes, 322. 
 
 Rolled masses in mineral veins, 403. 
 
 Roman bathg, 29. 
 
 Roscoe on spectrum analysis, 16. 
 
 Rosen au, 259. 
 
 Rosenbusch, 38. 
 
 petrological microscope, 253. 
 
 on volcanic rocks, 255, 260, 264, 
 
 268, 285. 
 
 Rotation of the earth, 12. 
 
 Roth, Justus, on felsite porphyry, 270. 
 
 Rowley Hills, 303. 
 
 Ruabon coal-field, iron of the, 431. 
 
 Rugosa, 481. 
 
 Rurn, volcano of, 319. 
 
 Ruminants, Gaudry on evolution of, 522. 
 
 Russia, granite in, 220. 
 
 Rutile, artificial formation of, 420. 
 
 Rutley, 22. 
 
 on Brent Tor, 302. 
 
 on Skomer Island, 297. 
 
 on the Glyders, 296. 
 
 on volcanic rocks near Bristol, 302. 
 
 Rydal, green slates of, 101. 
 
 SABINE, General, on Amazon mud, 165. 
 
 St. Agnes, tin of, 421. 
 
 St. Andrews, volcanoes of, 339. 
 
544 
 
 INDEX. St-Sp 
 
 St. Austell granite, 230. 
 
 St. Davids, pre-Cambrian volcano of, 
 
 291. 
 
 St. Gothard, 23. 
 St. Helena, 48. 
 St. Johns quartz felsite, 236. 
 St. Kilda, structure of, 320. 
 St. Paul, 24. 
 Salisbury Crag, 308. 
 
 Suiter and "Wood ward on Crustacea, 491. 
 Sand, 45, 92. 
 
 size of grains, 93. 
 
 dunes, 125. 
 
 glacier, 107. 
 
 Sanidine trachyte, 38. 
 
 Santa Cruz. 45. 
 
 Santiago, plain of, 133. 
 
 Santorin, augite andesite of, 275. 
 
 Saponites, 366. 
 
 Sartorius, 3. 
 
 Saussure, 3. 
 
 Scalpa, serpentine of, 389. 
 
 Sceuery of metamorphic rocks, 387. 
 
 of millstone grit, 81. 
 
 Schaffhausen falls of the Rhine, 156. 
 Scheibner on Foyaite, 224. 
 Schiehallion, 9. 
 Schist, 31. 
 Schorl granite, 36. 
 
 of the West of Eugland, 229. 
 
 Scilla, 5. 
 
 Scilly Islands, granite of, 230. 
 
 Sclater's natural history provinces, 461. 
 
 Scomberoid fishes, 505. 
 
 Scotland, gneiss in. 382. 
 
 Scott. R. H., on Loch Etive granite, 
 
 239. 
 
 Scrope, 2. 
 Scrope Paulett, on water as a volcanic 
 
 agent, 177. 
 Scur of Eigg, 320. 
 Scutelline sea-urchins, 490. 
 Searles Wood on tertiary mollusca, 499. 
 Seaton dyke, 339. 
 
 Sea-water as a source of metals, 399. 
 Secondary lamellibranchiata, 497. 
 
 sea-urchins, 489. 
 
 sponges, 477. 
 
 strata, table of, 54. 
 
 palseontological breaks in, 341. 
 
 Sedgwick, Prof. A., on cleavage, 32. 
 
 on Teesdale, 336. 
 Sedimentary terms, 74. 
 Seend, iron ore of, 434. 
 Seismology, 321. 
 Selenite, 46. 
 
 Semi-crystalline texture, 254. 
 Septaria, 47, 102. 
 Sequence of earth-movement in Britain, 
 
 347. 
 Serpentine, 288. 
 
 of Anglesey, 312. 
 
 of the Lizard, 312. 
 
 in Scotland, 388. 
 
 Sevenoaks stone, 97. 
 
 Shallow-water deposits, mollusca of, 
 
 442. 
 
 "Shannon," coral growth on British 
 
 ship, 115. 
 Sliap granite, 235. 
 Sharks, fossil, 502. 
 Sharpe on foliation, 360. 
 Shell limestone, 49. 
 Shell Ness, 49. 
 Shelve, lead mines of, 423. 
 Sheppey Flora, 474. 
 Shirleywich, 21. 
 Shore, 124. 
 
 Shropshire coalfield, iron of, 430. 
 Siberian limestones, 88. 
 Silesia, metalliferous veins of, 411. 
 Silver, 418. 
 
 relation of propylite to, 256. 
 
 Silvestri on Etna, 188. 
 
 Simeto River, excavation by, 154. 
 
 Simultaneous origin of water-formed 
 
 rocks, 49. 
 Sirius, 17. 
 
 Skiddaw granite, 235. 
 Skull of archaeopteryx, 518. 
 
 branchiosaurus, 508. 
 
 Dolichosoma, 509, 510. 
 
 Mososaurus, 512. 
 
 Skye, volcanoes of, 320. 
 Slate, 32, 46, 100. 
 Slickensides, 77. 
 Smith, William, 7, 65. 
 
 on identification of strata, 448. 
 
 Smyth, Admiral, on mouth of Rhone, 
 
 164. 
 
 Smyth, Warrington, on iron ores, 430. 
 Soda granites, 245. 
 Solfataras, 192. 
 Solidity of the earth, 11. 
 Sollas, W. J., on Siphonia, 478. 
 Somersetshire, iron ores of, 427. 
 Sorby, Dr. H., on cleavage, 32. 
 
 on metamorphism, 359. 
 
 on mineral history of strata, 92. 
 
 on oolite, 48. 
 
 on quartz crystals, 36. 
 
 on temperature of rock-formation, 
 
 174. 
 Source of lime, 51. 
 
 of metals, 398. 
 
 South African province, 459. 
 South Devon, schists of, 381. 
 South-east Britain, coast of, 122. 
 South of Ireland, metamorphic rocks of, 
 
 391. 
 
 South Wales coal-field, iron of, 431. 
 Southern temperate flora, 472. 
 
 zone, mollusca of, 459. 
 
 Spain, granite in, 220. 
 Specific gravity, 9. 
 
 heat of limestone, 390. 
 
 Speeton, 6. 
 
 Spencer, Herbert, on illogical geology, 
 
 450. 
 
 Sphene, 27. 
 
 Spirifera, figure of, 496. 
 Spongia, 477. 
 
 Spongilla in the Purbeck, 477. 
 Springs, 151. 
 
INDEX. Sp-Un 
 
 545 
 
 Springs, relation of volcanoes to, 191. 
 Spurs from mountain-chains, 354. 
 Staffa, 318. 
 
 Staffordshire coal-fields, iron of, 431. 
 Start Point, schists of, 381. 
 Statuary limestone with garnets, 390. 
 Steam, its relation to upheaval, 327. ^ 
 
 in volcanic eruptions, 182. 
 
 Steno, 3. 
 Stenzelberg, 259. 
 Stilbite, 28. 
 
 Stiperstones, sand of, 95. 
 Stirling, volcanoes of, 308. 
 Stockworks, 404. 
 Stonesfield Slate, 47. 
 Strabo, 2. 
 Strata, contorted, 76. 
 
 defined by Playfair, 71. 
 
 disturbed, 75. 
 
 interposed, 71. 
 
 originally level, 75. 
 
 table of, 55. 
 
 vertical, 76. 
 
 Stratification, 61. 
 
 valleys of, 139. 
 
 Striated boulders, 150. 
 
 Strike, 65. 
 
 Strings, 403. 
 
 Strokr, 194. 
 
 Strombodes, figure of, 481. 
 
 Stromboli, 178. 
 
 Strombus, distribution of, 456. 
 
 Strontian, 29. 
 
 granite, 239. 
 
 Structural metamorphism, 357. 
 Study of volcanic rocks, 252. 
 Sub-serial denudation, rate of, 143, 166. 
 Succession of continents, 355. 
 
 of life in time, 445. 
 of plant-life in time, 473. 
 Successive deposition in veins, 401. 
 Sulphur caves of Corinth, 192. 
 
 on Aran Mowddwy, 103. 
 
 Summary, 526. 
 
 Superposition of faunas, Edw. Forbes 
 on, 455. 
 
 of strata, 63. 
 
 Surface of a volcano, 186. 
 Swab, 3. 
 Syenite, 38. 
 
 of Ailsa Craig, 251. 
 
 Syenitic granite, 36, 216. 
 Symes on Mayo granite, 248. 
 Synclinal dip, 64. 
 valleys, 141. 
 
 TABLELANDS, 131. 
 
 valleys in, 140. 
 
 Tachylyte, 280. 
 
 Taconian series, 396. 
 
 Talc, 25. 
 
 Talcose slates in Scotland, 388. 
 
 Tardree, rhyolite of, 315. 
 
 Taxocrinus, figure of, 486. 
 
 Teall on the Cheviots, 298. 
 
 on dykes, 339. 
 
 Teesdale, basalt of, 336. 
 VOL. I. 
 
 Teesdale, metamorphism in, 360. 
 
 Teleosauria, 513. 
 
 Temperature of earth's surface, 14. 
 
 of lava, 185. 
 
 Temple's comet, 18. 
 Terebratula, figures of, 496. 
 Terminal moraines, 150. 
 Tertiary birds, 520. 
 
 foraminifera. 480. 
 
 iron ores, 434. 
 
 sponges, 478. 
 
 strata, physical breaks in, 343. 
 
 volcanic rocks, 314. 
 
 Tetrabranchiata, 500. 
 Tetracoralla, 481. 
 Texture of gneiss, 371. 
 
 of igneous rocks, 253. 
 
 Thickness of basalt streams, 282. 
 Thomson, Sir W., 11. 
 
 on deep-sea life, 443, 455. 
 
 on echinoderms, 489. 
 
 Thomsonite, 28. 
 
 Tin, 419. 
 
 Tiree, crystalline limestone of, 378. 
 
 Tiverton, volcano of, 314. 
 
 Tobermory, 41. 
 
 Topley on inclined deposition, 331. 
 
 Tor Cross, slates of, 381. 
 
 Tornidneon, granite veins of, 248 
 
 Torpedoes, fossil, 503. 
 
 Torre del Greco overwhelmed, 185. 
 
 Torridon sandstone, 386. 
 
 Torynocrinus, figure of, 487. 
 
 Tourmaline, 27. 
 
 granite, 217. 
 
 Trachyte, 264. 
 
 porphyry, 37. 
 
 Transatlantic province, 459. 
 Transition from gneiss to granite, 
 
 371. 
 
 Transporting power of streams, 156. 
 Traquair on fossil fishes, 506. 
 Trees in volcanic ash, 304. 
 Triassic crinoids, 486. 
 
 foraminifera, 479. 
 
 sands, 96. 
 
 Trigonia, figure of, 498. 
 Trilobites, British, 492. 
 Trinidad pitch lake, 193. 
 Trinucleus, figure of, 492. 
 Tropical zone, flora of, 471. 
 
 mollusca of, 460. 
 
 Trossachs, slate rocks of, 384. 
 
 Tryon, G. W., on mollusca, 498. 
 
 Tylas, 3. 
 
 Tyndall, Dr. J., on cleavage, 32. 
 
 Tynemouth dyke, Teall on, 339. 
 
 Types of basalt in North America, 
 
 281. 
 
 UDDEVALLA, 326. 
 Unconformity, 78. 
 
 inferred from fossils, 341. 
 
 Uncrystalline texture, 254. 
 United States, granite of, 220. 
 Unkel, 27. 
 Unstratified rocks, 69. 
 
 2 M 
 
546 
 
 INDEX. Up-Zo 
 
 Upheaval as affecting volcanic outbursts, 
 
 175. 
 
 forming lakes, 135. 
 
 in the North of Europe, 326. 
 
 Upper Greensand foraminifera, 479. 
 
 Uralite, 25. 
 
 Uses of granite, 36. 
 
 Ussher on raised beaches of Devon and 
 
 Cornwall, 327. 
 
 VALLEYS, 138. 
 
 Vallisneri, 6. 
 
 Varieties of amphibole, 369. 
 
 of granite, 216. 
 
 of pyroxene, 368. 
 
 Veins, age of, 405. 
 
 in granite, 34. 
 
 relation to igueous action, 414. 
 
 Veinstones, 250. 
 
 Velocity of fluid lavas, 186. 
 
 Verde Antico, 23. 
 
 Verde di Prato, 289, 
 
 Vermes, 495. 
 
 Verschoyle, Archdeacon, on trap dykes, 
 
 329. 
 Vertical sequence of rocks, 52. 
 
 strata, 76. 
 
 Vesuvius, 3, 187. 
 
 fossils ejected from, 179. 
 
 Vicary on volcanic rocks of Exeter, 313. 
 Virginia Mountains, propylite of, 256. 
 Viridite, 255. 
 Volcanic ashes, foliation in, 382. 
 
 rocks of North Wales, 293. 
 
 Volcano, its structure, 181. 
 Volcanoes near the sea, 190. 
 
 without cones, 188. 
 
 Voluta, figures of, 499. 
 
 WADHURST clay, iron ore of, 433. 
 Wallace on lead of Alston Moor, 424. 
 
 A. R. , on tropical plants, 471. 
 
 Wallerius, 3. 
 
 Wallich, Dr., on flint, 112. 
 
 on voyage of the " Bulldog," 443. 
 
 Wall on Trinidad, 193. 
 Walls of the vein, 412. 
 Warm temperate zone, 458. 
 Wastdale granite, 235. 
 Waste of cliffs, 126. 
 
 of rocks, 144. 
 
 Waterfalls, 154. 
 Waterformed rocks, 44. 
 Weald, elevation of, 329. 
 Wealden iron ore, 433. 
 
 sands, 97. 
 
 Weaver on Irish metamorphic rocks, 392. 
 Wellingborough iron ore, 433. 
 Werner, 3. 
 
 Werner, his eight systems of veins, 413. 
 West African province, 460. 
 Westbury iron ore, 431. 
 West Chiverton, lead of, 423. 
 West of Scotland, rocks of, 383. 
 Wheal Mary Ann, Le Neve Foster on, 
 
 422. 
 
 Wheal Clifford hot spring, 197. 
 Whitaker on river- valleys of the Weald, 
 
 330. 
 
 White micas, 365. 
 Wicca Pool, slate of, 102. 
 Wicklow, gold of, 418. 
 Wiltshire, Portland sand of, 97. 
 Winchell on Falls of St. Anthony, 156. 
 Wind, a denuding agent, 143. 
 Wolf Kock, phonolite of, 251. 
 Wolkenburg, 262. 
 
 Woodward, Dr. H., on Crustacea, 491. 
 H. B., on volcanic rocks of the 
 
 Mendips, 302. 
 
 J., 6. 
 
 Dr. S. P., Manual of the mollusca, 
 
 498. 
 
 provinces of marine life, 457. 
 
 Woolhope, anticlinal of, 64. 
 
 Work of rivers, 160. 
 
 Wrekin, pre-Cambrian volcano of, 291. 
 
 Wright on British echinoderms, 491. 
 
 on star-fishes, 485. 
 
 Wiinsch, on trees in volcanic ashes, 305. 
 Wyoming, leucite hills of, 284. 
 
 XlPHOSURA, 493. 
 
 YELLOWSTONE GEYSERS, 195. 
 Yorkshire coalfield, iron of, 430. 
 Ouse, 157. 
 
 ZEOLITES, 27. 
 Zellerthal, 25. 
 Zinc, 425. 
 Zircon, 27. 
 
 syenite, 38, 224. 
 
 Zirkel, 22. 
 
 on augite andesite, 275. 
 
 on basalt, 277, 284. 
 
 on classification of igneous rocks, 
 
 212. 
 
 on gneiss, 396. 
 
 on propylite, 262. 
 
 on rhyolites, 270. 
 
 on texture of igneous rocks, 253. 
 
 on trachyte, 266. 
 
 Zittel on sponges, 478. 
 
 Handbuch d. Palseontologie, 480, 
 
 496. 
 
 Zostera marina as a source of metals, 
 399. 
 
 BY BALLANTYNE, HANSON AND CO. 
 EDINHURGH AND LONDON. 
 
MANUAL OF GEOLOGY. 
 
 PART I. 
 PHYSICAL GEOLOGY AND PALEONTOLOGY. 
 
 BY PROFESSOR SEELEY, F.R.S. 
 
 PART II. 
 
 STRATIGRAPHICAL GEOLOGY AND PALEONTOLOGY. 
 
 BY ROBERT ETHERIDGE, F.R.S. 
 
 Each Part can be had separately. 
 
PART II. 
 STRAT1GRAPHICAL GEOLOGY AND PALEONTOLOGY, 
 
 BY 
 
 ROBERT ETHERIDGE, F.R.S., 
 
 ASSISTANT-KEEPER, GEOLOGICAL DEPARTMENT, BRITISH MUSEUM J 
 
 LATE PALAEONTOLOGIST TO THE GEOLOGICAL 
 
 SURVEY OF GREAT BRITAIN. 
 
 Tfflitb verg IRumerous 'Cables anb plates* 
 

YC 21 336 
 
 UNIVERSITY OF CALIFORNIA