THE LIBRARY 
 
 OF 
 
 THE UNIVERSITY 
 OF CALIFORNIA 
 
 LOS ANGELES 
 
 The RALPH D. REED LIBRARY 
 
 DEPARTMENT OF GEOLOGY
 
 A LIBRARY OF 
 UNIVERSAL LITERATURE 
 
 IN F O U R PARTS 
 
 Comprising Science, Biography, Fiction 
 and the Great Orations 
 
 PART ONE SCIENCE 
 
 Text-Book of Geology 
 
 (PART ONE) 
 
 BY 
 
 SIR ARCHIBALD GEIKIE, F.R.S. 
 
 NEW YORK 
 
 P. F. COLLIER AND SON 
 M C MI 
 
 29
 
 PRESS OF 
 P. F. COLLIER &SON 
 
 ALL RIGHTS RESERVED
 
 Geology 
 Library 
 
 A LIBRARY OF 
 UNIVERSAL LITERATURE 
 
 SCIENCE 
 
 VOLUME TWENTY-NINE
 
 BOARD OF EDITORS 
 
 SCIENCE 
 
 ANGELO HEILPRIN, author of "The Earth and Its Story," etc.; 
 
 Curator Academy of Natural Sciences of Philadelphia. 
 JOSEPH TORRE Y, JR., Ph.D., Instructor in Chemistry in Harvard 
 
 University. 
 RAY STANNARD BAKER, A.B., author of "The New Prosperity," 
 
 etc.; Associate Editor of McClure's Magazine. 
 
 BIOGRAPHY 
 
 MAYO W. HAZELTINE, A.M., author of "Chats About Books," etc.; 
 
 Literary Editor of the New York Sun. 
 JULIAN HAWTHORNE, author of "Nathaniel Hawthorne and His 
 
 Wife," "History of the United States," etc. 
 CHARLES G. D. ROBERTS, A.B., A.M., author of "A History of 
 
 Canada"; late Professor of English and French Literature, 
 
 King's College. 
 
 FICTION 
 RICHARD HENRY STODDARD, author of "The King's Bell," etc.; 
 
 Literary Editor of the New York Mail and Express. 
 HENRY VAN DYKE, D.D., LL.D., author of "Little Rivers," etc.; 
 
 Professor of English Literature at Princeton University. 
 THOMAS NELSON PAGE, LL.D., Litt.D., author of "Red Rock," etc. 
 
 ORATIONS 
 
 HON. HENRY CABOT LODGE, A.B., LL.B., author of "Life of Daniel 
 Webster," etc.; U. S. Senator from Massachusetts. 
 
 HON. JOHN R. PROCTOR, President U. S. Civil Service Commission. 
 
 MORRIS HICKEY MORGAN, Ph.D., LL.D., Professor in Latin, Har- 
 vard University.
 
 SIR ARCHIBALD GEIKIE 
 
 SIR ARCHIBALD GEIKIE was born at Edinburgh on De- 
 cember 28, 1835. He is of French descent on the maternal 
 side. He was educated at the High School and University 
 of his native city, but has since been made a D.C.L. by 
 Oxford, a D.Sc. by Cambridge and Dublin, and LL.D. 
 by Edinburgh and St. Andrews. He entered the Geolog- 
 ical Survey in 1855, and became successively Director of 
 the Geological Survey of Scotland, Murchison Professor 
 of Geology and Mineralogy iu Edinburgh University, 
 Director of the Museum of Practical Geology in Jermyn 
 Street, London, Foreign Secretary of the Eoyal Society, 
 President of the Geological Society and President of the 
 British Association. He is a Correspondent of the Institute 
 of France, of the Lincei, Eome, of the Academies of Berlin, 
 Vienna, Belgium, Stockholm, Turin, Munich, Christiania, 
 Gottingen, and so forth. He has been the recipient of 
 innumerable medals. Among his numerous publications 
 we have selected for reproduction the "Text-Book of Geol- 
 ogy," which has been translated into almost every Euro- 
 pean language. ^ 
 
 734004
 
 PREFACE TO THE THIRD EDITION 
 
 THE present edition of this Text-book has been entirely 
 revised, and in some portions recast or rewritten, so as to 
 bring it abreast of the continuous advance of Geological 
 Science. The additions made to the text, which extend to 
 every branch of the subject, increase the volumes by about 
 270 pages. Care has been taken to preserve a characteristic 
 feature of former editions by inserting references to the 
 more important memoirs and papers, where the student will 
 find fuller information than can be given in a Text-book. 
 
 While the book was passing through the press I received 
 from my friend Professor Zirkel the first volume of the new 
 edition of his great text-book of Petrography, but too late 
 to avail myself of its assistance. I can only now recom- 
 mend it as an indispensable part of the outfit of every 
 serious student of the petrographical section of Geology. 
 
 In the revision of the stratigraphical portion of this 
 work I have been assisted with suggestions and informa- 
 tion by my colleagues, Mr. Topley, Mr. H. B. Woodward, 
 Mr. E. T. Newton, and Mr. C. Reid, to whom my best 
 thanks are due. 
 
 MUSEUM, JERMYX STREET, 
 1st August 1893.
 
 FROM THE PREFACE TO THE FIRST EDITION 
 
 THE method of treatment adopted in this Text-book 
 is one which, while conducting the class of Geology in 
 the University of Edinburgh, I have found to afford the 
 student a good grasp of the general principles of the science, 
 and at the same time a familiarity with and interest in de- 
 tails of which he is enabled to see the bearing in the general 
 system of knowledge. A portion of the volume appeared 
 in the autumn of 1879 as the article " Geology" in the 
 Encyclopaedia Britannica. My leisure since that date has 
 been chiefly devoted to expanding those sections of 
 the treatise which could not be adequately developed in 
 the pages of a general work of reference. 
 
 While the book will not, I hope, repel the general 
 reader who cares to know somewhat in detail the facts 
 and principles of one of the most fascinating branches of 
 natural history, it is intended primarily for students, and 
 is therefore adapted specially for their use. The digest 
 given of each subject will be found to be accompanied by 
 references to memoirs where a fuller statement may be 
 sought. It has long been a charge against the geologists 
 of Great Britain that, like their countrymen in general, 
 they are apt to be somewhat insular in their conceptions, 
 even in regard to their own branch of science. Of course, 
 specialists who have devoted themselves to the investiga- 
 tion of certain geological formations or of a certain group 
 of fossil animals, have made themselves familiar with 
 (6)
 
 FROM THE PREFACE TO THE FIRST EDITION 7 
 
 what has been written upon their subject in other coun- 
 tries. But I am afraid there is still not a little truth in 
 the charge, that the general body of geologists here is but 
 vaguely acquainted with geological types and illustrations 
 other than such as have been drawn from the area of the 
 British Isles. More particularly is the accusation true in 
 regard to American geology. Comparatively few of us have 
 any adequate conception of the simplicity and grandeur of 
 the examples by which the principles of the science have 
 been enforced on the other side of the Atlantic. 
 
 Fully sensible of this natural tendency, I have tried to 
 keep it in constant view as a danger to be avoided as far 
 as the conditions of my task would allow. ID a text-book 
 designed for use in Britain, the illustrations must obviously 
 be in the first place British. A truth can be enforced much 
 more vividly by an example culled from familiar ground 
 than by one taken from a distance. But I have striven to 
 widen the vision of the student by indicating to him that 
 while the general principles of the science remain uniform, 
 they receive sometimes a clearer, sometimes a somewhat 
 different, light from the rocks of other countries than our 
 own. If from these references he is induced to turn to the 
 labors of our fellow workers on the Continent, and to share 
 my respect and admiration for them, a large part of my 
 design will have been accomplished. If, further, he is led 
 to study with interest the work of our brethren across 1 the 
 Atlantic, and to join in my hearty regard for it and for 
 them, another important section of my task will have been 
 fulfilled. And if in perusing these pages he should find 
 in them any stimulus to explore nature for himself, to 
 wander with the enthusiasm of a true geologist over the 
 length and breadth of his own country, and, where oppor-
 
 8 FROM THE PREFACE TO THE FIRST EDITION 
 
 tunity offers, to extend his experience and widen his sym- 
 pathies by exploring the rocks of other lands, the remaining 
 and chief part of my aim would be attained. 
 
 The illustrations of Fossils in Book VI. have been 
 chiefly drawn by Mr. George Sharman; a few by Mr. B. 
 N. Peach, and one or two by Dr. B. H. Traquair, F.B.S., 
 to all of whom my best thanks are due. The publishers 
 having become possessed of the wood-blocks of Sir Henry 
 de la Beche's "Geological Observer," I gladly made use 
 of them as far as they could be employed in Books III. 
 and IV. Sir Henry's sketches were always both clear and 
 artistic, and I hope that students will not be sorry to see 
 some of them revived. They are indicated by the letter 
 (). The engravings of the microscopic structure of rocks 
 are from my own drawings, and I have also availed myself 
 of materials from my sketch-books. The frontispiece is 
 a reduction of a drawing by Mr. W. H. Holmes, whose 
 pictures of the scenery in the Far West of the United States 
 are by far the most remarkable examples yet attained of 
 the union of artistic effectiveness with almost diagram- 
 matic geological distinctness and accuracy. Captain Button, 
 of the Geological Survey of the United States, furnished me 
 with this drawing, and also requested Mr. Holmes to make 
 for me the cation-sections given in Book VII. To both of 
 these kind friends I desire to acknowledge my indebtedness.
 
 CONTENTS 
 
 INTRODUCTION ........... 13 
 
 BOOK I 
 COSMICAL ASPECTS OF GEOLOGY, 21 
 
 I. RELATIONS OF THE EARTH IN THE SOLAR SYSTEM ... 23 
 II. FORM AND SIZE OP THE EARTH ....... 31 
 
 III. MOVEMENTS OF THE EARTH IN THEIR GEOLOGICAL RELATIONS . 34 
 
 1. Rotation, 342. Revolution, 363. Precession of the Equinoxes, 
 37-4. Change in the obliquity of the Ecliptic, 385. Stability 
 of the Earth's Axis, 386. Changes of the Earth's Centre of 
 Gravity, 437. Results of the Attractive Influence of Sun and 
 Moon on the Geological Condition of the Earth, 468. Climate 
 in its Geological Relations, 49. 
 
 BOOK II 
 
 GEOGNOSY: AN INVESTIGATION OF THE MATERIALS OF 
 THE EARTH'S SUBSTANCE 
 
 PART I. A GENERAL DESCRIPTION OF THE PARTS OF 
 THE EARTH 
 
 I. THB ENVELOPES ATMOSPHERE AND HYDROSPHERE ... 62 
 1. The Atmosphere, 63 2. The Oceans, 66. 
 
 II. THB SOLID GLOBE OR LITHOSPHERE ...... 74 
 
 1. The Outer Surface, 742. The Crust, 873. The Interior or 
 Nucleus, 89; Evidence of Internal Heat, 92; Irregularities in 
 the downward Increment of Heat, 95; Probable Condition 
 of the Earth's Interior, 994. Age of the Earth and Measures 
 of Geological Time, 107. 
 
 (9)
 
 10 CONTENTS 
 
 PART II. AN ACCOUNT OF THE COMPOSITION OP THE EARTH'S 
 CRUST MINERALS AND ROCKS 
 
 L GENERAL CHEMICAL CONSTITUTION OF THE CRUST . . .112 
 II. ROOK-FORMING MINERALS 119 
 
 III. DETERMINATION OF BOOKS 146 
 
 i. Megascopic Examination, 146 ii. Chemical Analysis, 157 iii. 
 Chemical Synthesis, ICO-iv. Microscopic Investigation, 101. 
 
 IV. GENERAL OUTWARD OR MEGASCOPIC CHARACTERS OF ROCKS . 172 
 
 1. Structure, ITS 2. Composition, 186-8. State of Aggregation, 
 1874. Color and Lustre, 189-6. Feel and Smell, 101-6. 
 Specific Gravity, 1987. Magnetism, 19(2 
 
 V. MICROSCOPIC CHARACTERS OF ROCKS 192 
 
 1. Microscopic Elements of Bocks, 1948. Microscopic Struc- 
 tures of Rocks, 808. 
 
 VL CLASSIFICATION OF ROCKS 218 
 
 VII. A DESCRIPTION OF THE MORE IMPORTANT ROCKS OF THE EARTH'S 
 
 CRUST 223 
 
 i. SEDIMENTARY 223 
 
 A. Fragmented, (Clastic) 223 
 
 1. Gravel and Sand Rocks (Psamrnites) 224 
 
 2. Clay Rocks (Pelites) 233 
 
 3. Volcanic Fragmented Rocks (Tuffs) 238 
 
 4. Fragmental Rocks of Organic Origin 243 
 
 (1) Calcareous, 243-(2) Siliceous, 247 (8) Phosphatic, 848 (4) Car- 
 bonaceous, 249 (5) Ferruginous, 265. 
 
 B. Crystalline, including Rocks formed from Chemical 
 
 Precipitation 258 
 
 ii. MASSIVE, ERUPTIVE, IGNEOUS 269 
 
 i. Acid Series, 272 ii. Intermediate Series, 284 iii. Basic Series, 298. 
 
 iii. SCHISTOSE (METAMOEPHIC) 303 
 
 1. Argillites, 8092. Quartz-Bocks, 310-3. Pyroxene-Bocks, 818-4. 
 Hornblende-Bocks, 314-5. Garnet-Bocks, 815-3. Epidote-Bocks, 
 315-7. Chlorite-Bocks, 315-8. Talc-Bocks, 8159. Olivine- 
 Bocks, or Peridotites, 316-10. Pelsitoid-Bocks, 81611. Quartz- 
 and Tourmaline-Bocks, 81712. Quartz- and Mica-Bocks, 317 
 18. Quartz- and Felspar-Bocks, 81914. Quartz-, Felspar-, and 
 Mica-Bocks, 32015. Quartz-, Felspar-, and Garnet-Bocks, 
 322-16. Felspar- and Mica-Bocks, 822 Composition of some 
 Schistose Bocks, 323.
 
 CONTENTS 11 
 
 BOOK III 
 
 DYNAMICAL GEOLOGY, 324 
 
 PART L HYPOGENE ACTION: AN INQUIRY INTO THE GEOLOGI- 
 CAL CHANGES IN PROGRESS BENEATH THE SURFACE 
 OP THE EARTH, 326 
 
 L VOLCANOES AND VOLCANIC ACTION 
 
 1. Volcanic Products 327 
 
 1. Gases and Vapors, 3802. Water, 386-8. Lava, 8884. Frag- 
 mentary Materials, 340. 
 
 2. Volcanic Action 344 
 
 Active, Dormant, and Extinct Phases, 344 Sites of Volcanic 
 Action, 346 Ordinary Phase of an Active Volcano, 349 
 Conditions of Eruption, 849 Periodicity of Eruptions, 852 
 General Sequence of Events in an Eruption, 854 Fissures, 
 355 Explosions, 360 Showers of Dust and Stones, 868 Lava- 
 streams, 370 Elevation and Subsidence, 895 Torrents of 
 Water and Mud, 396 Effects of the closing of a Volcanic 
 Chimney Sills and Dikes, 398 Exhalations of Vapors and 
 Gases, 899-Geysers, 402 Mud- Volcanoes, 407. 
 
 3. Structure of Volcanoes 409 
 
 i. Volcanic Cones, 409 Submarine Volcanoes, and Volcanic 
 Islands, 434 ii. Fissure (Massive) Eruptions, 432. 
 
 4. Geographical and Geological Distribution of Volcanoes . . 439 
 
 5. Causes of Volcanic Action 447 
 
 II. EARTHQUAKES . 459 
 
 Amplitude of Earth-Movements, 461-Velocity, 461-Duration, 468 
 Modifying Influence of Geological Structure, 463 Extent of 
 Country affected, 466 Depth of Source, 466-Geological Effects, 
 468-Distribution, 478 Origin, 474. 
 
 III. SECULAR UPHEAVAL AND DEPRESSION 478 
 
 Upheaval, 482 Subsidence, 489 Causes of Upheaval and Depres- 
 sion of Land, 494.
 
 
 TEXT-BOOK OF GEOLOGY 
 
 INTRODUCTION 
 
 GEOLOGY is the science which investigates the history 
 of the Earth. Its object is to trace the progress of our 
 planet from the earliest beginnings of its separate existence, 
 through its various stages of growth, down to the present 
 condition of things. It unravels the complicated processes, 
 involving vast geographical revolutions, by which each 
 continent and country has been built up, tracing out the 
 origin of their materials and the manner in which their 
 existing outlines have been determined. It likewise fol- 
 lows into detail the varied sculpture of mountain and 
 valley, crag and ravine. 
 
 Nor does this science confine itself merely to changes 
 in the inorganic world. Geology shows that the present 
 races of plants and animals are the descendants of other 
 and very different races that once peopled the earth. It 
 teaches that there has been a progress of the inhabitants, 
 as well as one of the globe on which they have dwelt; that 
 each successive period in the earth's history, since the intro- 
 duction of living things, has been marked by characteristic 
 types of the animal and vegetable kingdoms; and that, how 
 imperfectly soever they may have been preserved or may 
 be deciphered, materials exist for a history of life upon the 
 
 (13)
 
 14 TEXT-BOOK OF GEOLOGY 
 
 planet. The geographical distribution of existing faunas 
 and floras is often made clear and intelligible by geological 
 evidence; and in a similar way, light is thrown upon some 
 of the remoter phases in the history of man himself. 
 
 A subject so comprehensive as this must require a wide 
 and varied basis of evidence. One of the characteristics of 
 geology is to gather evidence from sources which, at first 
 sight, seem far removed from its scope, and to seek aid 
 from almost every other leading branch of science. Thus, 
 in dealing with the earliest conditions of the planet, the 
 geologist must fully avail himself of the labors of the as- 
 tronomer. Whatever is ascertainable by telescope, spec- 
 troscope, or chemical analysis, regarding the constitution 
 of other heavenly bodies, has a geological bearing. The 
 experiments of the physicist, undertaken to determine con- 
 ditions of matter and of energy, may sometimes be taken 
 as the starting-point of geological investigation. The work 
 of the chemical laboratory forms the foundation of a vast 
 and increasing mass of geological inquiry. To the botanist, 
 the zoologist, even to the unscientific, if observant, traveller 
 by land or sea, the geologist turns for information and 
 assistance. 
 
 But while thus culling freely from the dominions of 
 other sciences, geology claims, as its peculiar territory, the 
 rocky framework of the globe. In the materials composing 
 that framework, their composition and arrangement, the 
 processes of their formation, the changes which they have 
 individually undergone, and the grand terrestrial revolu- 
 tions to which they bear witness, lie the main data of 
 geological history. It is the task of the geologist to group 
 these elements in such a way that they may be made to 
 yield up their evidence as to the march of events in the
 
 INTRODUCTION 15 
 
 evolution of the planet. He finds that they have in large 
 measure arranged themselves in chronological sequence 
 the oldest lying at the bottom and the newest at the top. 
 Eelics of an ancient sea -floor are overlain with traces of 
 a vanished land-surface; these are in turn covered by the 
 deposits of a former lake, above which once more appear 
 proofs of the return of the sea. Among these rocky records, 
 too, lie the lavas and ashes of long-extinct volcanoes. The 
 ripple left upon a sandy beach, the cracks formed by the 
 sun's heat upon the muddy bottom of a dried-up pool, 
 the very imprint of the drops of a passing rain-shower, 
 have all been accurately preserved, and often bear witness 
 to geographical conditions widely different from those that 
 exist where such markings are now found. 
 
 But it is mainly by the remains of plants and animals 
 imbedded in the rocks that the geologist is guided in un- 
 ravelling the chronological succession of geological changes. 
 He has found that a certain order of appearance charac- 
 terizes these organic remains; that each successive group 
 of rocks is marked by its own special types of life; that 
 these types can be recognized, and the rocks in which they 
 occur can be correlated, even in distant countries, where no 
 other means of comparison are available. At one moment, 
 he has to deal with the bones of some large mammal scat- 
 tered through a deposit of superficial gravel, at another 
 time, with the minute foraminifers and ostracods of an 
 upraised sea-bottom. Corals and crinoids, crowded and 
 crushed into a massive limestone on the spot where they 
 lived and died, ferns and terrestrial plants matted together 
 into a bed of coal where they originally grew, the scattered 
 shells of a submarine sand-bank, the snails and lizards that 
 left their mouldering remains within a hollow tree, the in-
 
 16 TEXT-BOOK OF GEOLOGY 
 
 sects that have been imprisoned within the exuding resin 
 of old forests, the footprints of birds and quadrupeds, or 
 the trails of worms left upon former shores these, and 
 innumerable other pieces of evidence, enable the geologist 
 to realize in some measure what the vegetable and animal 
 life of successive periods has been, and what geographical 
 changes the site of every land has undergone. 
 
 It is evident that to deal successfully with these 
 varied materials, a considerable acquaintance with differ- 
 ent branches of science is desirable. The fuller and more 
 accurate the knowledge which the geologist has of kindred 
 branches of inquiry, the more interesting and fruitful will 
 be his own researches. From its very nature, geology 
 demands on the part of its votaries wide sympathy with 
 investigation in almost every branch of natural science. 
 Especially necessary is a tolerably large acquaintance with 
 the processes now at work in changing the surface of the 
 earth, and of at least those forms of plant and animal life 
 whose remains are apt to be preserved in geological deposits, 
 or which, in their structure and habitat, enable us to realize 
 what their forerunners were. 
 
 It has often been insisted upon that the Present is the key 
 to the Past; and in a wide sense this assertion is eminently 
 true. Only in proportion as we understand the present, 
 where everything is open on all sides to the fullest inves- 
 tigation, can we expect to decipher the past, where so much 
 is obscure, imperfectly preserved, or not preserved at all. 
 A study of the existing economy of nature ought evidently 
 to be the foundation of the geologist's training. 
 
 While, however, the present condition of things is thus 
 employed, we must obviously be on our guard against the 
 danger of unconsciously assuming that the phase of nature's
 
 INTRODUCTION 17 
 
 operations which we now witness has been the same in 
 all past time; that geological changes have taken place, 
 in former ages, in the manner and on the scale which we 
 behold to-day, and that at the present time all the great 
 geological processes, which have produced changes in past 
 eras of the earth's history, are still existent and active. 
 Of course, we may assume this uniformity of action, and 
 use the assumption as a working hypothesis. But it ought 
 not to be allowed a firmer footing, nor on any account be 
 suffered to blind us to the obvious truth that the few cen- 
 turies, wherein man has been observing nature, form much 
 too brief an interval by which to measure the intensity of 
 geological action in all past time. For aught we can tell, 
 the present is an era of quietude and slow change, compared 
 with some of the eras that have preceded it. Nor can we 
 be sure that when we have explored every geological proc- 
 ess now in progress, we have exhausted all the causes of 
 change which, even in comparatively recent times, have 
 been at work. 
 
 In dealing with the Geological Record, as the accessible 
 solid part of the globe is called, we cannot too vividlj 
 realize that, at the best, it forms but an imperfect chroni- 
 cle. Geological history cannot be compiled from a full and 
 continuous series of documents. Owing to the very nature 
 of its origin, the record is necessarily from the first frag- 
 mentary, and it has been farther mutilated and obscured by 
 the revolutions of successive ages. Even where the chroni- 
 cle of events is continuous, it is of very unequal value in 
 different places. In one case, for example, it may present 
 us with an unbroken succession of deposits, many thou- 
 sands of feet in thickness, from which, however, only a few 
 meagre facts as to geological history can be gleaned. In
 
 18 TEXT-BOOK OF GEOLOGY 
 
 another instance, it brings before us, within the compass 
 of a few yards, the evidence of a most varied and com- 
 plicated series of changes in physical geography, as well 
 as an abundant and interesting suite of organic remains. 
 These and other characteristics of the geological record 
 will become more apparent and intelligible to the student 
 as he proceeds in the study of the science. 
 
 In the present volumes the subject will be distributed 
 under the following leading divisions: 
 
 1. The Gosmical Aspects of Geology. It is desirable to 
 realize some of the more important relations of the earth 
 to the other members of the solar system, of which it forms 
 a part, seeing that geological phenomena are largely the 
 result of these relations. The form and motions of the 
 planet may be briefly touched upon, and attention should 
 be directed to the way in which these planetary movements 
 influence geological change. The light cast upon the early 
 history of the earth by researches into the composition of 
 the sun and stars deserves notice here. 
 
 2. Geognosy An Inquiry into the Materials of the Earth's 
 Substance. This division describes the constituent parts of 
 the earth, its envelopes of air and water, its solid crust, 
 and the probable condition of its interior. Especially, it 
 directs attention to the more important minerals of the 
 crust, and the chief rocks of which that crust is built up. 
 In this way, it lays a foundation of knowledge regarding 
 the nature of the materials constituting the mass of the 
 globe, whence we may next proceed to investigate the proc- 
 esses by which these materials are produced and altered. 
 
 8. Dynamical Geology embraces an investigation of the 
 operations which lead to the formation, alteration, and dis- 
 turbance of rocks, and calls in the aid of physical and
 
 INTRODUCTION 19 
 
 chemical experiment in elucidation of these operations. 
 It considers the nature and operation of the processes that 
 have determined the distribution of sea and land, and have 
 molded the forms of the terrestrial ridges and depressions. 
 It further investigates the geological changes which are in 
 progress over the surface of the land and floor of the sea, 
 whether these are due to subterranean disturbance, or to 
 the effect of operations above ground. Such an inquiry 
 necessitates a careful study of the existing economy of 
 nature, and forms a fitting introduction to the investiga- 
 tion of the geological changes of former periods. This 
 and the previous section, including most of what is 
 embraced under Physical Geography and Petrogeny or 
 Geogeny, will here be discussed more in detail than is 
 nsual in geological treatises. 
 
 4. Geotectonic, or Structural Geology the Architecture of 
 the Earth. This section of the investigation, applying the 
 results arrived at in the previous division, discusses the 
 actual arrangement of the various materials composing 
 the crust of the earth. It proves that some have been 
 formed in beds or strata, whether by the deposit of sedi- 
 ment on the floor of the sea, or by the slow aggregation of 
 organic forms, that others have been poured out from sub- 
 terranean sources in sheets of molten rock, or in showers of 
 loose dust, which have been built up into mountains and 
 plateaus. It further shows that rocks originally laid down 
 in almost horizontal beds have subsequently been crum- 
 pled, contorted, dislocated, invaded by igneous masses from 
 below, and rendered sometimes crystalline. It teaches, 
 too, that wherever exposed above sea-level, they have been 
 incessantly worn down, and have often been depressed, so 
 that older lie buried beneath later accumulations.
 
 20 TEXT-BOOK OF GEOLOGY 
 
 5. Palceontological Geology. This branch of the subject 
 deals with the organic forms which are found preserved in 
 the rocks of the crust of the earth. It includes such ques- 
 tions as the manner in which the remains of plants and 
 animals are entombed in sedimentary accumulations, the 
 relations between extinct and living types, the laws which 
 appear to have governed the distribution of life in time 
 and in space, the nature and use of the evidence from 
 organic remains regarding former conditions of physical 
 geography, and the relative importance of different genera 
 of animals and plants in geological inquiry. 
 
 6. Stratigraphical Geology. This section might be called 
 Geological History, or Historical Geology. It works out 
 the chronological succession of the great formations of the 
 earth's crust, and endeavors to trace the sequence of events 
 of which they contain the record. More particularly, it 
 determines the order of succession of the various plants 
 and animals which in past time have peopled the earth, 
 and thus, by ascertaining what has been the grand march 
 of life upon the planet, seeks to unravel the story of the 
 earth as made known by the rocks of the crust. Further, 
 by comparing the sequence of rocks in one country with 
 that of those in another, it furnishes materials for enabling 
 us to picture the successive stages in the geographical evo- 
 lution of the various portions of the earth's surface. 
 
 7. Physiographical Geology, starting from the basis of 
 fact laid down by Stratigraphical geology regarding former 
 geographical changes, embraces an inquiry into the history 
 of the present features of the earth's surface continental 
 ridges and ocean basins, plains, valleys, and mountains. It 
 investigates the structure of mountains and valleys, com- 
 pares the mountains of different countries, and ascertains
 
 COSMIC AL ASPECTS OF GEOLOGY 21 
 
 the relative geological dates of their upheaval. It explains 
 the causes on which local differences of scenery depend, 
 and shows under what very different circumstances, and 
 at what widely separated intervals, the varied contours, 
 even of a single country, have been produced. 
 
 BOOK I 
 
 COSMICAL ASPECTS OF GEOLOGY 
 
 BEFOEE geology had attained to the position of an 
 inductive science, it was customary to begin all 
 investigations into the history of the earth by pro- 
 pounding or adopting some more or less fanciful hypoth- 
 esis, in explanation of the origin of our planet or of the 
 universe. Such preliminary notions were looked upon as 
 essential to a right understanding of the manner in which 
 the materials of the globe had been put together. To the 
 illustrious James Hutton (1785) geologists are indebted, if 
 not for originating, at least for strenuously upholding the 
 doctrine that it is no part of the province of geology 
 to discuss the origin of things. He taught them that in 
 the materials from which geological evidence is to be com- 
 piled there can be found "no traces of a beginning, no 
 prospect of an end." In England, mainly to the influence 
 of the school which he founded, and to the subsequent rise 
 of the Geological Society (1807), which resolved to collect 
 facts instead of fighting over hypotheses, is due the dis- 
 appearance of the crude and unscientific cosmologies of 
 previous centuries. 
 
 But there can now be little doubt that in the reaction
 
 22 TEXT-BOOK OF GEOLOGY 
 
 against the visionary and often grotesque speculations of 
 earlier writers, geologists were carried too far in an oppo- 
 site direction. In allowing themselves to believe that 
 geology had nothing to do with questions of cosmogony, 
 they gradually grew up in the conviction that such ques- 
 tions could never be other than mere speculation, interest- 
 ing or amusing as a theme for the employment of the fancy, 
 but hardly coming within the domain of sober and inductive 
 science. Nor would they soon have been awakened out of 
 this belief by anything in their own science. It is still true 
 that in the data with which they are accustomed to deal, 
 as comprising the sum of geological evidence, there can 
 be found no trace of a beginning, though there is ample 
 proof of constant, upward progression from some invisible 
 starting-point. The oldest rocks which have been discov- 
 ered on any part of the globe have possibly been derived 
 from other rocks older than themselves. Geology by itself 
 has not yet revealed, and is little likely ever to reveal, 
 a portion of the first solid crust of our globe. If, then, 
 geological history is to be compiled from direct evidence 
 furnished by the rocks of the earth, it cannot begin at the 
 beginning of things, but must be content to date its first 
 chapter from the earliest period of which any record has 
 been preserved among the rocks. 
 
 Nevertheless, though, in its usual restricted sense, geol- 
 ogy has been, and must ever be, unable to reveal the 
 earliest history of our planet, it no longer ignores, as mere 
 speculation, what is attempted in this subject by its sister 
 sciences. Astronomy, physics and chemistry have in late 
 years all contributed to cast much light on the earliest 
 stages of the earth's existence, previous to the beginning 
 of what is commonly regarded as geological history. What-
 
 COSMIC AL ASPECTS OF GEOLOGY 23 
 
 ever extends oar knowledge of the former conditions of our 
 globe may be legitimately claimed as part of the domain 
 of geological inquiry. If Geology, therefore, is to continue 
 worthy of its name as the science of the earth, it must take 
 cognizance of these recent contributions from other sciences. 
 It can no longer be content to begin its annals with the 
 records of the oldest rocks, but must endeavor to grope its 
 way through the ages which preceded the formation of any 
 rocks. Thanks to the results achieved with the telescope, 
 the spectroscope, and the chemical laboratory, the story 
 of these earliest ages of our earth is every year becoming 
 more definite and intelligible. 
 
 I. BELATIOXS OF THE EARTH IN THE SOLAR SYSTEM 
 
 As a prelude to the study of the structure and history 
 of the earth, some of the general relations of our planet to 
 the solar system may here be noticed. The investigations 
 of recent years, showing the community of substance be- 
 tween the different members of that system, have revived 
 and have given a new form and meaning to the well-known 
 nebular hypothesis of Kant, Laplace and W. Herschel, 
 which sketched the progress of the system from the state 
 of an original nebula to its existing condition of a central 
 incandescent sun with surrounding cool planetary bodies. 
 According to this hypothesis, the nebula, originally diffused 
 at least as far as the furthest member of the system, began 
 to condense toward the centre, and in so doing threw off 
 or left behind successive rings. These, on disruption and 
 further condensation, assumed the form of planets, some- 
 times with a further formation of rings, which in the case 
 of Saturn remain, though in other planets they have broken 
 up and united into satellites.
 
 24 TEXT-BOOK OF GEOLOGY 
 
 Accepting this view, we should expect the matter com* 
 posing the various members of the solar system to be every- 
 where nearly the same. The fact of condensation round 
 centres, however, indicates probable differences of density 
 throughout the nebula. That the materials composing the 
 nebula may have arranged themselves according to their 
 respective densities, the lightest occupying the exterior, 
 and the heaviest the interior of the mass, is suggested by 
 a comparison of the densities of the various planets. These 
 densities are usually estimated as in the following table, 
 that of the earth being taken as the unit: 
 
 Density of the Sun 0-25 
 
 Mercury 1-12 
 
 Venus..- 1-03 
 
 Earth 1-00 
 
 Mars 0-70 
 
 Jupiter 0-24 
 
 Saturn 0'13 
 
 Uranus 0-17 
 
 Neptune. 0'16 
 
 It is to be observed, however, that "the densities here 
 given are mean densities, assuming that the apparent size 
 of the planet or sun is the true size, i.e. making no allow- 
 ance for thousands of miles deep of cloudy atmosphere. 
 Hence the numbers for Jupiter, Saturn and Uranus are 
 certainly too small, that for the sun much too small." 1 
 Taking the figures as they stand, while they do not indi- 
 cate a strict progression in the diminution of density, they 
 state that the planets near the sun possess a density about 
 twice as great as that of granite, but that those lying toward 
 the outer limits of the system are composed of matter as 
 light as cork. Again, in some cases, a similar relation has 
 been observed between the densities of the satellites and 
 
 1 Prof. Tait, MS. note.
 
 COSMIC 'AL ASPECTS OF GEOLOGY ^ 
 
 their primaries. The moon, for example, has a density 
 little more than half that of the earth. The first satellite 
 of Jupiter is less dense, though the other three are said to 
 be more dense than the planet. Further, in the condition 
 of the earth itself, a very light gaseous atmosphere forms 
 the outer portion, beneath which lies a heavier layer of 
 water, while within these two envelopes the materials form- 
 ing the solid substance of the planet are so arranged that 
 the outer layer or crust has only about half the density of 
 the whole globe. 
 
 According to the hypothesis now under consideration 
 it is conceived that, in the gradual condensation of the 
 original nebula, each successive mass left behind repre- 
 sented the density of its parent shell, and consisted of 
 progressively heavier matter." The remoter planets, with 
 their low densities and vast absorbing atmospheres, may 
 be supposed to consist of metalloids, like the outer parts 
 of the sun's atmosphere, while the interior planets are no 
 doubt mainly metallic. The rupture of each planetary ring 
 would, it is thought, raise the temperature of the resultant 
 nebulous planet to such a height as to allow the vapors 
 to rearrange themselves by degrees in successive layers, or 
 rather shells, according to densities. And when the planet 
 gave off a satellite, that body might be expected to possess 
 the composition and density of the outer layers of its 
 primary. 8 
 
 For many years, the only evidence available as to the 
 
 2 On the origin of Satellites, see the researches of Prof. G. H. Darwin, Phil. 
 Trans. (1879) clxx. p. 535. Proc. Roy. Soc. xxx. p. 1. 
 
 3 Lockyer in Prestwich's Inaugural Lecture, Oxford, 1875, and in Man- 
 chester Lectures, Why the Earth's Chemistry is as it is. Readers interested 
 in the historical development of geological opinion will find much suggestive 
 matter bearing on the questions discussed above, in De la Beche's "Researches 
 in Theoretical Geology," 1834 a work notably in advance of its time. 
 
 GEOLOGY Vol. XXIX 2
 
 26 TEXT-BOOK OF GEOLOGY 
 
 actual composition of other heavenly bodies than our own 
 earth was furnished by the meteorites, or fallen stars, which 
 from time to time have entered our atmosphere from plan- 
 etary space, and have descended upon the surface of the 
 globe. 4 Subjected to chemical analysis, these foreign 
 bodies show considerable diversities of composition; but 
 in no case have they yet revealed the existence of any 
 element not already recognized among terrestrial materials. 
 They have been classified in three groups: Siderites, com- 
 posed chiefly of iron; Siderolites, consisting partly of iron 
 and partly of various stony materials ; and Aerolites, formed 
 almost entirely of such stony minerals. Twenty-four of our 
 elements have been detected in meteorites. Those most 
 commonly found are iron, nickel, phosphorus, sulphur, 
 carbon, oxygen, silicon, magnesium, calcium and alumin- 
 ium. Less frequent or occurring in smaller quantities are 
 hydrogen, nitrogen, chlorine, lithium, sodium, potassium, 
 titanium, chromium, manganese, cobalt, arsenic, antimony, 
 tin and copper. These various elements occur for the most 
 part in a state of combination. The iron exists as an alloy 
 with nickel, the phosphorus is combined with nickel and 
 iron, the silicon is combined with oxygen and various bases. 
 A few of the elements occur in a free state. Thus hydrogen 
 and nitrogen are found as occluded gases and carbon as 
 
 4 On meteorites consult Partsch, "Die Meteoriten," Vienna, 1843. Rose, 
 Abhand. konigl. Akad. Berlin, 1863. Rammelsberg, "Die Chemische Natur 
 der Meteoriten," 1870-9. Tschermak, Sitzb. Akad. "Wissen. Vienna (1875), 
 Ixxi. ; "Die Mikroskopische Beschaffenheit der Meteoriten," Stuttgart, 1885. 
 Daubree, "Etudes Synthetiques de Geologie Experimentale, " 1879; Compt. 
 rend. cvi. (1888), 1671-1682 (compare Amer. Journ. Sci. xlii. [1891], p. 413). 
 S. Meunier, "Le Ciel Geologique," 1871; "Meteorites," 1884. Brezina tmd 
 Coheu, "Die Structur und Zusammensetzung der Meteoreisen," Stuttgart, 1886. 
 W. Flight, Geol. Mag. 1875, Pop. Sci. Rev. new ser. i. p. 390. Proe. Roy. Soc. 
 xxxiii. p. 343. A. W. Wright, Amer. Journ. ser. 3, xi. p. 253; xii. p. 165. 
 L. Fletcher, "An Introduction to the Study of Meteorites," British Museum 
 Catalogue, 1886.
 
 COSMIC AL ASPECTS OF GEOLOGY 27 
 
 graphite, rarely as diamond. Of combinations of elements 
 in meteorites some, not yet recognized among terrestrial 
 minerals, comprise alloys of iron and nickel and various 
 sulphides and silicates. But others have been identified 
 with well-known minerals of the earth's crust, including 
 olivine, enstatite and bronzite, diopside and augite, horn- 
 blende, anorthite and labradorite, magnetite and chromite, 
 etc. There is likewise a carbonaceous group of meteorites 
 containing carbon, both amorphous and as black diamond, 
 also combined with hydrogen and oxygen, and in some 
 cases combustible, with a bituminous smell. Some iron 
 meteorites contain a large proportion of occluded hydro- 
 gen, nitrogen, or carbonic oxide, occasionally as much as 
 six times the volume of the meteorite itself. 
 
 Various theories have been propounded as to the origin 
 or source of those bodies which come to our planet from 
 space. But at present we possess no satisfactory basis of 
 fact on which to speculate. Whether these stones belong 
 to the solar system, or, as seems more probable, reach us 
 from remoter space, they prove that some at least of the 
 elements and minerals with which we are familiar extend 
 beyond our planet. 
 
 But, in recent years, a far more precise and generally 
 available method of research into the composition of the 
 heavenly bodies has been found in the application of the 
 spectroscope. By means of this instrument, the light 
 emitted from self-luminous bodies can be analyzed in 
 such a way as to show what elements are present in their 
 intensely hot luminous vapor. When the light of the in- 
 candescent vapor of a metal is allowed to pass through 
 a properly arranged prism, it is seen to give a spectrum 
 consisting of transverse bright lines only. This is termed
 
 28 TEXT-BOOK OF GEOLOGY 
 
 a radiation -spectrum. Each element appears to have its 
 own characteristic arrangement of lines, which in general 
 retain the same relative position, intensity and colors. 
 Moreover, gases and the vapors of solid bodies are found 
 to intercept those rays of light which they themselves 
 emit. The spectrum of sodium-vapor, for example, shows 
 among others two bright orange lines. If therefore white 
 light, from some hotter light-source, passes through the 
 vapor of sodium, these two bright lines become dark lines, 
 the light being exactly cut off which would have been 
 given out by the sodium itself. This is called an absorption- 
 spectrum. 
 
 From this method of examination, it has been inferred 
 that many of the elements of which our earth is composed 
 must exist in the state of incandescent vapor in the atmos- 
 phere of the sun. Thirty-two metals have been thus iden- 
 tified, including aluminium, barium, manganese, lead, cal- 
 cium, cobalt, potassium, iron, zinc, copper, nickel, sodium 
 and magnesium. These elements, or at least substances 
 which give the same groups of lines as the terrestrial ele- 
 ments with which they have been identified, do not occur 
 promiscuously diffused throughout the outer mass of the 
 sun. According to Mr. Lockyer's first observations, they 
 appear to succeed each other in relation to their respective 
 densities. Thus the coronal atmosphere which, as seen in 
 total eclipses, extends to so prodigious a distance beyond 
 the disk of the sun, consists mainly of subincandescent 
 hydrogen and another element which may be new. Beneath 
 this external vaporous envelope lies the chromosphere, 
 where the vapors of incandescent hydrogen, calcium and 
 magnesium can be detected. Further inward the spot-zone 
 shows the presence of sodium, titanium, etc. ; while still
 
 COSM1CAL ASPECTS OF GEOLOGY 29 
 
 lower, a layer (the reversing layer) of intensely hot vapors, 
 lying probably next to the inner brilliant photosphere, gives 
 spectroscopic evidence of the existence of incandescent iron, 
 manganese, cobalt, nickel, copper, and other well-known 
 terrestrial metals. 6 
 
 It is to be observed, however, that in these spectroscopic 
 researches the decomposition of the elements by electrical 
 action was not considered. The conclusions embodied in 
 the foregoing paragraph have been founded on the idea that 
 the lines seen in the spectrum of any element are all due to 
 the vibrations of the molecules of that element. But Mr. 
 Lockyer has suggested that this view may after all be but a 
 rough approximation to the truth; that it may be more ac- 
 curate to say, as a result of the facts already acquired, that 
 there exist basic elements common to calcium, iron, etc., 
 and to the solar atmosphere, and that the spectrum of each 
 body is a summation of the spectra of various molecular 
 complexities which can exist at different temperatures, the 
 simplest only being found in the hottest part of the sun. 8 
 
 The spectroscope has likewise been successfully applied 
 by Mr. Huggins and others to the observation of the fixed 
 stars and nebulas, with the result of establishing a similarity 
 of elements between our own system and other bodies in 
 
 6 On spectroscopic research as applied to the sun, see Kirchhoff and Bunsen, 
 "Researches on Solar Spectrum," etc., 1863; Angstrom, "Eecherches sur le 
 Spectre normal du Soleil"; Lockyer, "Solar Physics," 1873, and "StudieS in 
 Spectrum Analysis" (International Series), 1878; Huggins and Miller, Proc. 
 Roy. Soc. xii., Phil. Trans. 1864; Roscoe's "Spectrum Analysis," with au- 
 thorities there cited. An ingenious theory to account for the conservation of 
 solar energy was suggested by the late Sir C. W. Siemens (Proc. Roy. Soc. 
 xxxiii. (1881) p. 389). It requires the presence of aqueous vapor and carbon 
 compounds in stellar space, which are dissociated and drawn into the solar 
 photosphere, where they burst into flame with a large development of heat, and 
 then passing into aqueous vapor and carbonic anhydride or oxide, flow to the 
 solar equator whence they are projected into space. 
 
 8 See also the opposite views of Dewar and Liveing, Proc. Roy. Soc. xxx. 
 p. 93, and H. W. Vogel, Nature, xxvii. p. 233.
 
 80 TEXT-BOOK OF GEOLOGY 
 
 sidereal space. In the radiation-spectra of nebulae, Mr. 
 Huggins finds the hydrogen lines very prominent; and he 
 conceives that they may be glowing masses of that element. 
 Prof. Tait has suggested, on the other hand, that they are 
 more probably clouds of stones frequently colliding and 
 thus giving off incandescent gases. Sir William Thomson 
 (now Lord Kelvin) favors this view, which is further amply 
 supported by spectroscopic observations. Among the fixed 
 stars, absorption-spectra have been recognized, pointing to 
 a structure resembling that of our sun, viz. an incandescent 
 nucleus which may be solid or liquid or of very highly com- 
 pressed gas, but which gives a continuous spectrum and 
 which is surrounded with an atmosphere of glowing vapor. 7 
 Those stars which show the simplest spectra are believed to 
 have the highest temperature, and in proportion as they cool 
 their materials will become more and more differentiated 
 into what we call elements. The most brilliant or hottest 
 stars show in their spectra only the lines of gases, as hydro- 
 gen. Cooler stars, like our sun, give indications of the 
 presence, in addition, of the metals magnesium, sodium, 
 calcium, iron. A still lower temperature is marked by the 
 appearance of the other metals, metalloids, and compounds. 8 
 The sun would thus be a star considerably advanced in the 
 process of differentiation or association of its atoms. It con- 
 tains, so far as we know, no metalloid except carbon, and 
 possibly oxygen, nor any compound, while stars like Sirius 
 show the presence only of hydrogen, with but a feeble pro- 
 portion of metallic vapors; and, on the other hand, the red 
 stars indicate by their spectra that their metallic vapors have 
 
 7 Huggins, Proc. Roy. Soc. 1863-66, and Brit. Aaaoc. Lecture (Nottingham, 
 1866); Huggins and Miller, Phil. Trans. 1864. 
 
 8 Lockyer, Comptes rendus, Dec. 1873.
 
 COSMIC AL ASPECTS OF GEOLOGY 31 
 
 entered into combination, whence it is inferred that their 
 temperature is lower than that of our sun. 
 
 More recently, however, another view of the evolution 
 of stars has been propounded by Mr. Lockyer. He con- 
 ceives that all self-luminous cosmical bodies are composed 
 either of swarms of meteorites, or of masses of vapor pro- 
 duced by collisions of meteorites: that stars, comets and 
 nebulas are only different phases of the same series of 
 changes; that where the temperature of a star is increasing 
 the star consists of a meteor-swarm, which by constant col- 
 lision of its individual meteorites is gradually being vapor- 
 ized by heat; and that after volatilization cooling sets in and 
 the vapor finally condenses into a globe. 9 
 
 II. FORM AND SIZE OF THE EARTH 
 
 Further confirmation of some of the foregoing views as 
 to the order of planetary evolution is furnished by the form 
 of the earth and the arrangement of its component materials. 
 
 That the earth is an oblate spheroid, and not a perfectly 
 spherical globe, was discovered and demonstrated by New- 
 ton. He even calculated the amount of ellipticity long be- 
 fore any measurement had confirmed such a conclusion. 
 During the present century numerous arcs of the meridian 
 have been measured, chiefly in the northern hemisphere. 
 From a series made by different observers between the l#ti- 
 tudes of Sweden and the Cape of Good Hope, Bessel ob- 
 tained the following data for the dimensions of the earth: 
 
 Equatorial diameter . . . 41,847,192 feet, or 7925-604 miles 
 
 Polar diameter 41,707,314 " 7899-114 " 
 
 Amount of polar flattening . 139,768 " 26-471 " 
 
 "The Meteoritic Hypothesis," 1890.
 
 32 TEXT-BOOK OF GEOLOGY 
 
 The equatorial circumference is thus a little less than 
 25,000 miles, and the difference between the polar and equa- 
 torial diameters (nearly 26* miles) amounts to about *ioth of 
 the equatorial diameter. 10 More recently, however, it haa 
 been shown that the oblate spheroid indicated by these meas- 
 urements is not a symmetrical body, the equatorial circum- 
 ference being an ellipse instead of a circle. The greater 
 axis of the equator lies in long. 8 15' W. a meridian pass- 
 ing through Ireland, Portugal, and the northwest corner of 
 Africa, and cutting off the northeast corner of Asia in the 
 opposite hemisphere." 
 
 The polar flattening, established by measurement and 
 calculation as that which would necessarily have been as- 
 sumed by an originally plastic globe in obedience to the 
 movement of rotation, has been cited as evidence that the 
 earth was once in a plastic condition. Taken in connection 
 with the analogies supplied by the sun and other heavenly 
 bodies, this inference appeared to be well grounded." More 
 recently, however, it has been contended that even in a truly 
 solid body a polar flattening might be developed under the 
 influence of rotation." 
 
 Though the general spheroidal form of our planet, and 
 
 19 Herechel, "Astronomy," p. 186. 
 
 11 A. R. Clarke, Phil. Mag. August, 1878; Encyclopedia Britannica, 9th 
 edit. x. 172. 
 
 18 It was opposed by Mohr ("Gescbichte der Erde," p, 472), who, adopting 
 a suggestion long ago made by Playfair, endeavored to show that the polar flat- 
 tening can be accounted for by greater denudation of the polar tracts, exposed 
 as these have been by the heaping up of the oceanic waters toward the equator 
 in consequence of rotation. He dwelt chiefly on the effects of glaciers in lower- 
 ing the land, but as Pfaff has pointed out, the work of erosion is chiefly per- 
 formed by other atmospheric forces that operate rather toward the equator than 
 the poles ("Allgemeine Geologie als exacte Wissenschaft, " p. 6). Compare 
 Naumann, Neues Jahrb. 1871, p. 250. Nevertheless, Mohr undoubtedly re- 
 called attention to a conceivable cause by which, in spite of polar elevation or 
 equatorial subsidence, the external form of the planet might be preserved. 
 
 18 See in particular the papers by Mr. C. Chree. Phil. Mag. 1891, pp. 238 
 and 342.
 
 COSMIC AL ASPECTS OF GEOLOGY 33 
 
 probably the general distribution of sea and land, are refer- 
 able to the early effects of rotation on a fluid or viscous 
 mass, it is certain that the present details of its surface-con- 
 tours are of comparatively recent date. Speculations have 
 been made as to what may have been the earliest character 
 of the solid surface, whether it was smooth or rough, and 
 particularly whether it was marked by any indication of the 
 existing continental elevations and oceanic depressions. So 
 far as we can reason from geological evidence, there is no 
 proof of any uniform superficies having ever existed. Most 
 probably the first formed crust broke up irregularly, and 
 not until after many successive corrugations did the surface 
 acquire stability. Some writers have imagined that at first 
 the ocean spread over the whole surface of the planet. But 
 of this there is not only no evidence, but good reason for be- 
 lieving that it never could have taken place. As will be 
 alluded to in a later page, the preponderance of water in the 
 southern hemisphere seems to indicate some excess of den- 
 sity in that hemisphere. This excess can hardly have been 
 produced by any change since the materials of the interior 
 ceased to be mobile; it must therefore be at least as ancient 
 as the condensation of water on the earth's surface. Hence 
 there was probably from the beginning a tendency in the 
 ocean to accumulate in the southern rather than in the 
 northern hemisphere. 
 
 That land existed from the earliest ages of which 'we 
 have any record in rock-formations, is evident from the 
 obvious fact that these formations themselves consist in 
 great measure of materials derived from the waste of land. 
 When the student, in a later part of these volumes, is pre- 
 sented with the proofs of the existence of enormous masses 
 of sedimentary deposits, even among some of the oldest geo-
 
 34 TEXT-BOOK OF GEOLOGY 
 
 logical systems, he will perceive how important mast have 
 been the tracts of land that could furnish such piles of 
 detritus. 
 
 The tendency of modern research is to give probability 
 to the conception, first outlined by Kant, that not only in 
 our own solar system, but throughout the regions of space, 
 there has been a common plan of evolution, and that the 
 matter diffused through space in nebulae, stars and planets 
 is substantially the same as that with which we are familiar. 
 Hence the study of the structure and probable history of the 
 sun and the other heavenly bodies comes to possess an evi- 
 dent geological interest, seeing that it may yet enable us to 
 carry back the story of our planet far beyond the domain of 
 ordinary geological evidence, and upon data not less trust- 
 worthy than those furnished by the rocks of the earth's 
 crust. 
 
 III. MOVEMENTS OF THE EARTH IN THEIR GEOLOGICAL 
 KELATIONS 
 
 We are here concerned with the earth's motions in so far 
 only as they materially influence the progress of geological 
 phenomena. 
 
 1. Rotation. In consequence of its angular momentum 
 at its original separation, the earth rotates on its axis. The 
 rate of rotation has once been much more rapid than it now 
 is (p. 46). At present a complete rotation is performed in 
 about twenty-four hours, and to it is due the succession 
 of day and night. So far as observation has yet gone, this 
 movement is uniform, though recent calculations of the in- 
 fluence of the tides in retarding rotation tend to show that 
 a very slow diminution of the angular velocity is in prog- 
 ress. If this be so, the length of the day and night will
 
 COSMICAL ASPECTS OF GEOLOGY 35 
 
 slowly increase until finally the duration of the day and 
 that of the year will be equal. The earth will then have 
 reached the condition into which the moon has passed rela- 
 tively to the earth, one-half being in continual day, the 
 other in perpetual night. 
 
 The linear velocity due to rotation varies in different 
 places, according to their position on the surface of the 
 planet. At each pole there can be no velocity, but from 
 these two points toward the equator there is a continually 
 increasing rapidity of motion, till at the equator it is equal 
 to a rate of 507 yards in a second. 
 
 To the rotation of the earth are due certain remarkable 
 influences upon currents of air circulating either toward the 
 equator or toward the poles. Currents which move from 
 polar latitudes travel from parts of the earth's surface 
 where the velocity due to rotation is small, to others where 
 it is great. Hence they lag behind, and their course is 
 bent more and more westward. An air current, quitting 
 the north polar or north temperate regions as a north wind, 
 is deflected out of its course, and becomes a northeast wind. 
 On the opposite side of the equator, a similar current set- 
 ting out straight for the equator, is changed into a south- 
 east wind. Hence, as is well known, the Trade-winds have 
 their characteristic westward deflection. On the other 
 hand, a current setting out northward or southward from 
 the equator, passes into regions having a less velocity' due 
 to rotation than it possesses itself, and hence it travels on 
 in advance and appears to be gradually deflected eastward. 
 The aerial currents, blowing steadily across the surface of 
 the ocean toward the equator, produce oceanic currents 
 which unite to form the westward flowing Equatorial 
 current.
 
 36 TEXT-BOOK OF GEOLOGY 
 
 It has been maintained by Von Baer, 14 that a certain 
 deflection is experienced by rivers that flow in a meridi- 
 onal direction, like the Volga and Irtisch. Those travel- 
 ling poleward are asserted to press upon their eastern rather 
 than their western banks, while those which run in the op- 
 posite direction are stated to be thrown more against the 
 western than the eastern. When, however, we consider 
 the comparatively small volume, slow motion, and contin- 
 ually meandering course of rivers, it may reasonably be 
 doubted whether this vera causa can have had much effect 
 generally in modifying the form of river channels. 
 
 2. Revolution. Besides turning on its axis, the globe 
 performs a movement round the sun, termed revolution. 
 This movement, accomplished in rather more than 365 
 days, determines for us the length of our year, which is, 
 in fact, merely the time required for one complete revolu- 
 tion. The path or orbit followed by the earth round the 
 sun is not a perfect circle but an ellipse, with the sun 
 in one of the foci, the mean distance of the earth from 
 the sun being 92,800,000, the present aphelion distance 
 94,500,000, and the perihelion distance 91,250,000 miles. 
 By slow secular variations, the form of the orbit alter- 
 nately approaches to and recedes from that of a circle. 
 At the nearest possible approach between the two bodies, 
 
 14 "Ueber ein allgemeines Gesetz in der Gestaltung der Flussbetten. " Bull 
 Acad. St. Petersbourg, ii. (1860). See also Ferrel on the motion of fluids and 
 solids relatively to the earth's surface, Camb. (Mass.) Math. Monthly, vols. i. 
 and ii. (1859-60); Bulk, Z. Deutsch. Geol. Ges. xxxi. (1879) p. 224. The River 
 Irtisch is said in flowing northward to have cut so much into its right bank that 
 villages are gradually driven eastward, Demiansk having been shifted about a 
 mile in 240 years (Nature, xv. p. 207). But this may be accounted for by local 
 causes. See an excellent paper on this subject with special reference to the 
 regime of some rivers in northern Germany, by F. Klockmann, Jahrb. Preuss. 
 Geol. Landesanst. 1882; also E. Bunker, Zeitsch. fur die gesammten Nattirwis- 
 senschaften, 1876, p. 463; G. K. Gilbert, Amer. Journ. Sci. xxvii. (1884) p. 427.
 
 COSMIC AL ASPECTS OF GEOLOGY 37 
 
 owing to change in the ellipticity of the orbit, the earth is 
 14,368,200 miles nearer the sun than when at its greatest 
 possible distance. These maxima and minima of distance 
 occur at vast intervals of time." The last considerable 
 eccentricity took place about 200,000 years ago, and the 
 previous one more than half a million years earlier. Since 
 the amount of heat received by the earth from the sun is 
 inversely as the square of the distance, eccentricity may 
 have had in past time much effect upon the climate of the 
 earth, as will be pointed out further on ( 8). 
 
 3. Precession of the Equinoxes. If the axis of the earth 
 were perpendicular to the plane of its orbit, there would be 
 equal day and night all the year round. But it is really 
 inclined from that position at an angle of 23 27' 21". Hence 
 our hemisphere is alternately presented to and turned away 
 from the sun, and, in this way, brings the familiar alterna- 
 tion of the seasons. Again, were the earth a perfect sphere, 
 of uniform density throughout, the position of its axis of 
 rotation would not be changed by attractions of external 
 bodies. But owing to the protuberance along the equato- 
 rial regions, the attraction chiefly of the moon and sun 
 tends to pull the axis aside, or to make it describe a 
 conical movement, like that of the axis of a top, round 
 the vertical. Hence each pole points successively to dif- 
 ferent stars. This movement, called the precession of the 
 equinoxes, in combination with another smaller movement, 
 due to the attraction of the moon (called nutation), com- 
 pletes its cycle in 21,000 years, the annual total advance 
 of the equinox amounting to 62". At present the winter in 
 the northern hemisphere coincides with the earth's nearest 
 
 15 See Croll'a "Climate and Time," chaps, iv. xix.
 
 88 TEXT-BOOK OF GEOLOGY 
 
 approach to the sun, or perihelion. In 10,500 years hence 
 it will take place when the earth is at the furthest part of 
 its orbit from the sun, or in aphelion. This movement 
 may have had great importance in connection with former 
 secular variations in the eccentricity of the orbit ( 8). 
 
 4. Change in the Obliquity of the Ecliptic. The angle 
 at which the axis of the earth is inclined to the plane 
 of its orbit does not remain strictly constant. It oscillates 
 through long periods of time to the extent of about a degree 
 and a half, or perhaps a little more, on either side of the 
 mean. According to Dr. Croll, 1 ' this oscillation must have 
 considerably affected former conditions of climate on the 
 earth, since, when the obliquity is at its maximum, the polar 
 regions receive about eight and a half days' more of heat 
 than they do at present that is, about as much heat as lat. 
 76 enjoys at this day. This movement must have aug- 
 mented the geological effects of precession, to which refer- 
 ence has just been made, and which are described in 8. 
 
 5. Stability of the Earth's Axis. That the axis of the 
 earth's rotation has successively shifted, and consequently 
 that the poles have wandered to different points on the sur- 
 face of the globe, has been maintained by geologists as the 
 only possible explanation of certain remarkable conditions 
 of climate, which can be proved to have formerly obtained 
 within the Arctic Circle. Even as far north as lat. 81 45', 
 abundant remains of a vegetation indicative of a warm 
 climate, and including a bed of coal 25 to 30 feet thick, 
 have been found in situ." It is contended that when these 
 plants lived, the ground could not have been permanently 
 
 16 Croll, Trans. Greol. Soc. Glasgow, ii. 177. "Climate and Time," chap. xxv. 
 11 Fielden and Heer, Quart. Journ. Qeol. Soc. Nov. 1877.
 
 COSMIC AL ASPECTS OF GEOLOGY 39 
 
 frozen or covered for most of the year with thick snow. In 
 explanation of the difficulty, it has been suggested that the 
 north pole did not occupy its present position, and that the 
 locality where the plants occur lay in more southerly lati- 
 tudes. Without at present entering on the discussion of 
 the question whether the geological evidence necessarily 
 requires so important a geographical change, let us con- 
 sider how far a shifting of the axis of rotation has been a 
 possible cause of change during that section of geological 
 time for which there are records among the stratified rocks. 
 From the time of Laplace," astronomers hare strenu- 
 ously denied the possibility of any sensible change in the 
 position of the axis of rotation. It has been urged that, 
 since the planet acquired its present oblate spheroidal form, 
 nothing but an utterly incredible amount of deformation 
 could overcome the greater centrifugal force of the equato- 
 rial protuberance. It is certain, however, that the axis of 
 rotation does not strictly coincide with the principal axis 
 of inertia. Though the angular difference between them 
 must always have been small, we can, without having re- 
 course to any extramundane influence, recognize two causes 
 which, whether or not they may suffice to produce any 
 change in the position of the main axis of inertia, undoubt- 
 edly tend to do so. In the first place, a widespread up- 
 heaval or depression of certain unsymmetrically arranged 
 portions of the surface to a considerable amount would tend 
 to shift that axis. In the second place, an analogous result 
 might arise from the denudation of continental masses of 
 land, and the consequent filling up of sea- basins. Lord 
 Kelvin (Sir William Thomson) freely concedes the physical 
 
 18 Mecanique Celeste," tome v. p. 14.
 
 40 TEXT-BOOK OF GEOLOGY 
 
 possibility of such changes. "We may not merely admit," 
 he says, "but assert as highly probable, that the axis of 
 maximum inertia and axis of rotation, always very near one 
 another, may have been in ancient times very far from their 
 present geographical position, and may have gradually 
 shifted through 10, 20, 30, 40, or more degrees, without 
 at any time any perceptible sudden disturbance of either 
 land or water." 19 But though, in the earlier ages of the 
 planet's history, stupendous deformations may have oc- 
 curred, and the axis of rotation may have often shifted, 
 it is only the alterations which can possibly have occurred 
 during the accumulation of the stratified rocks, that need 
 to be taken into account in connection with the evidence of 
 changes of climate during geological history. If it can be 
 shown, therefore, that the geographical revolutions neces- 
 sary to shift the axis are incredibly stupendous in amount, 
 improbable in their distribution, and not really demanded 
 by geological evidence, we may reasonably withhold our 
 belief from this alleged cause of the changes of climate dur- 
 ing the periods of time embraced by geological records. 
 
 It has been estimated by Lord Kelvin "that an elevation 
 of 600 feet, over a tract of the earth's surface 1000 miles 
 square and 10 miles in thickness, would only alter the 
 position of the principal axis by one-third of a second, or 
 84feet."** Professor George Darwin has shown that, on 
 the supposition of the earth's complete rigidity, no redis 
 tribution of matter in new continents could ever shift the 
 pole from its primitive position more than 8, but that, if 
 its degree of rigidity is consistent with a periodical readjust- 
 
 19 Brit. Assoc. Rep. (1876), Sections, p. 11. 
 
 20 Trans. Geol. Soc. Glasgow, iv. p. 313. The situation of the supposed 
 area of upheaval on the earth's surface is not stated.
 
 COSMIC AL ASPECTS OF GEOLOGY 41 
 
 rnent to a new form of equilibrium, the pole may have 
 wandered some 10 or 15 from its primitive position, or 
 have made a smaller excursion and returned to near its 
 old place. In order, however, that these maximum effects 
 should be produced, it would be necessary that each ele- 
 vated area should have an area of depression corresponding 
 in size and diametrically opposite to it, that they should 
 lie on the same complete meridian, and that they should 
 both be situated in lat. 45. With all these coincident 
 favorable circumstances, an effective elevation of aJo of 
 the earth's surface to the extent of 10,000 feet would shift 
 the pole 11 J'; a similar elevation of would move it 1 46"; 
 of , 1 , 3 17'; and of J, 8 4J'. Mr. Darwin admits these to 
 be superior limits to what is possible, and that, on the 
 supposition of intumescence or contraction under the re- 
 gions in question, the deflection of the pole might be reduced 
 to a quite insignificant amount." 
 
 Under the most favorable conditions, therefore, the pos 
 sible amount of deviation of the pole from its first position 
 would appear to have been too small to have seriously in- 
 fluenced the climates of the globe within geological history. 
 If we grant that these changes were cumulative, and that 
 the superior limit of deflection was reached only after a 
 long series of concurrent elevations and depressions, we 
 must suppose that no movements took place elsewhere to 
 counteract the effect of those about lat. 45 in the two hemi- 
 spheres. But this is hardly credible. A glance at a geo- 
 graphical globe suffices to show how large a mass of land 
 exists now both to the north and south of that latitude, 
 especially in the northern hemisphere, and that the deepest 
 parts of the ocean are not antipodal to the greatest heights 
 
 81 Phil. Trans. Nov. 1876.
 
 42 TEXT-BOOK OF GEOLOGY 
 
 of the land. These features of the earth's surface are of 
 old standing. There seems, indeed, to be no geological 
 evidence in favor of any such geographical changes as 
 could have produced even the comparatively small dis- 
 placement of the axis considered possible by Professor 
 Darwin. 
 
 In an ingenious suggestion, Sir John Evans contended 
 that, even without any sensible change in the position of 
 the axis of rotation of the nucleus of the globe, there might 
 be very considerable changes of latitude due to disturbance 
 of the equilibrium of the outer portion or shell by the up- 
 heaval or removal of masses of land between the equator 
 and the poles, and to the consequent sliding of the shell 
 over the nucleus until the equilibrium was restored. 22 
 Subsequently he precisely formulated his hypothesis as 
 a question to be determined mathematically;" and the 
 solution of the problem was worked out by the Rev. J. F. 
 Twisden, who arrived at the conclusion that even the large 
 amount of geographical change postulated by Dr. Evans 
 could only displace the earth's axis of figure to the extent 
 of less than 10' of angle, that a displacement of as much as 
 10 or 15 could be effected only if the heights and depths 
 of the areas elevated and depressed exceeded by many times 
 the heights of the highest mountains, that under no circum- 
 stances could a displacement of 20 be effected by a transfer 
 of matter of less amount than about a sixth part of the 
 whole equatorial bulge, and that even this extreme amount 
 would not necessarily alter the position of the axis of 
 figure." 
 
 Proc. Roy. Soc. xv. (1867), p. 46. 43 Q. J. Geol. Soc. xxxii. (1876), p. 62. 
 24 Q. J. Geol. Soc. xxxiv. (1878), p. 41. See also E. Hill, Geol. Mag. v. 
 (2d ser.) pp. 262, 479. 0. Fisher, op. cit. pp. 291, 551.
 
 COSMIC 'AL ASPECTS OF GEOLOGY 43 
 
 Against any hypothesis which assumes a thin crust in- 
 closing a liquid or viscous interior, weighty and, indeed, 
 insuperable objections have been urged. It has been sug- 
 gested, however, that the almost universal traces of present 
 or former volcanic action, the evidence from the compressed 
 strata in mountain regions that the crust of the earth must 
 have a capacity for slipping toward certain lines, the great 
 amount of horizontal compression of strata which can be 
 proved to have been accomplished, and the secular changes 
 of climate notably the former warm climate near the north 
 pole furnish grounds for inquiry whether the doctrine of 
 a fluid substratum over a rigid nucleus, which has been 
 urged by several able writers, would not be compatible 
 with mechanical considerations, and whether, under these 
 circumstances, changes in latitude would not result from 
 unequal thickening of the crust." This question of the 
 internal condition of the globe is discussed at p. 89. 
 
 6. Changes of the Earth's Centre of Gravity. If the 
 centre of gravity in our planet, as pointed out by Herschel, 
 be not coincident with the centre of figure, but lies some- 
 what to the south of it, any variation in its position will 
 affect the ocean, which of course adjusts itself in relation 
 to the earth's centre of gravity. How far any redistribu- 
 tion of the matter within the earth, in such a way as to 
 affect the present equilibrium, is now possible, we cannot 
 tell. But certain revolutions at the surface may from time 
 to time produce changes of this kind. The accumulation 
 of ice which, as will be immediately described ( 8), is 
 believed to gather round one pole during the maximum 
 
 0. Fisher, Geol. Mag. 1878, p. 552, "Physics of the Earth's Crust,' 
 2d Edition 1889.
 
 44 TEXT-BOOK OF GEOLOGY 
 
 of eccentricity, will displace the centre of gravity, and, 
 as the result of this change, will raise the level of the 
 ocean in the glacial hemisphere. 1 " The late Dr. Croll esti 
 mated that, if the present mass of ice in the southern hemi- 
 sphere is taken at 1000 feet thick extending down to lat. 60, 
 the transference of this mass to the northern hemisphere 
 would raise the level of the sea 80 feet at the north pole. 
 Other methods of calculation give different results. Mr. 
 Heath put the rise at 128 feet; Archdeacon Pratt made 
 it more; while the Rev. O. Fisher gave it at 409 feet." 
 Subsequently, in returning to this question, Dr. Croll re- 
 marked "that the removal of two miles of ice from the 
 Antarctic continent [and at present the mass of ice there 
 is probably thicker than that] would displace the centre of 
 gravity 190 feet, and the formation of a mass of ice equal 
 to the one-half of this, on the Arctic regions, would carry 
 the centre of gravity 95 feet further; giving in all a total 
 displacement of 285 feet, thus producing a rise of level at 
 the north pole of 285 feet, and in the latitude of Edinburgh 
 of 234 feet." A very considerable additional displacement 
 would arise from the increment of water to the mass of the 
 ocean by the melting of the ice. Supposing half of the two 
 miles of Antarctic ice to be replaced by an ice-cap of similar 
 extent and one mile thick in the northern hemisphere, the 
 other half being melted into water and increasing the mass 
 of the ocean, Dr. Croll estimated that from this source an 
 extra rise of 200 feet would take place in the general ocean 
 level, so that there would be a rise of 485 feet at the north 
 
 26 Adhemar, "Revolutions de la Mer," 1840. 
 
 21 Croll, in Reader for 2d September, 1865, and Phil. Mag. April, 1866; 
 Heath, Phil. Mag. April, 1869; Pratt, Phil. Mag. March, 1866; Fisher, Reader, 
 10th February, 1866.
 
 COSMIC AL ASPECTS OF GEOLOGY 45 
 
 pole, and 434 feet in the latitude of Edinburgh." An inter- 
 mittent submergence and emergence of the low polar lands 
 might be due to the alternate shifting of the centre of 
 gravity. 
 
 To what extent this cause has actually come into opera- 
 tion in past time cannot at present be determined. It has 
 been suggested that the "raised beaches," shore-lines 
 (strand-linien), or old sea- terraces, so numerous at various 
 heights in the northwest of Europe, might be due to the 
 transference of the oceanic waters, and not to any subter- 
 ranean movement, as generally believed. Had they been 
 due to such a general cause, they ought to have shown 
 evidence of a gradual and uniform decline in elevation 
 from north to south, with only such local variations as 
 might be accounted for by the influence of masses of high 
 land or other local cause. No such feature, however, has 
 been satisfactorily established." On the contrary, the levels 
 of the terraces vary within comparatively short distances. 
 Though numerous on both sides of Scotland, they disap- 
 pear further north among the Orkney and Shetland islands, 
 although these localities were admirably adapted for their 
 formation and preservation. 80 The conclusion may be 
 drawn that the "raised beaches" cannot be adduced as 
 evidence of changes of the earth's centre of gravity, but 
 are due to local and irregularly acting causes. (See Book 
 III. Part I. Section iii. 1, where this subject is more 
 fully discussed.) 
 
 28 Croll, Geol. Mag. new series, i. (1874), p. 347; "Climate and Time," 
 chaps, xxiii. and xxiv. and postea, p. 286. Consult also Fisher, Phil. Mag. 
 xxxiv. (October, 1892), p. 337. 
 
 ** The student ought, however, to consult Prof. Suess' Antlitz der Erde for 
 the arguments in favor of an opposite opinion. 
 
 30 Nature, xvi. (1877), p. 415.
 
 46 TEXT-BOOK OF GEOLOGY 
 
 7. Results of the Attractive Influence of Sun and Moon 
 on the Geological Condition of the Earth. Many speculations 
 have been offered to account for supposed former greater 
 intensity of geological activity on the surface of the globe. 
 Two causes for such greater intensity may be adduced. In 
 the first place, if the earth has cooled down from an original 
 molten condition, it has lost, in cooling, a vast amount of 
 potential geological energy. It does not necessarily follow, 
 however, that the geological phenomena resulting from in- 
 ternal temperature have, during the time recorded in the 
 accessible part of the earth's crust, been steadily decreasing 
 in magnitude. We might, on the contrary, contend that 
 the increased resistance of a thickening cooled crust may 
 rather have hitherto intensified the manifestations of subter- 
 ranean activity, by augmenting the resistance to be over- 
 come. In the second place, the earth may have been once 
 more powerfully affected by external causes, such as the 
 greater heat of the sun, and the greater proximity of the 
 moon. That the formerly larger amount of solar heat re- 
 ceived by the surface of our planet must have produced 
 warmer climates and more rapid evaporation, with greater 
 rainfall and the important chain of geological changes 
 which such an increase would introduce, appears in every 
 way probable, though the geologist has not yet been able 
 to observe any indisputable indication of such a former 
 intensity of superficial changes. 
 
 Prof. Darwin, in investigating the bodily tides of viscous 
 spheroids, has brought forward some remarkable results 
 bearing on the question of the possibility that geological 
 operations, both internal and superficial, may have been 
 once greatly more gigantic and rapid than they are now. 81 
 
 31 Phil. Trans. 1879, parts i. and ii.
 
 COSMIC AL ASPECTS OF GEOLOGY 47 
 
 He assumes the earth to be a homogeneous spheroid and to 
 have possessed a certain small viscosity, 82 and he calculates 
 the internal tidal friction in such a mass exposed to the at- 
 traction of moon and sun, and the consequences which these 
 bodily tides have produced. He finds that the length of 
 our day and month have greatly increased, that the moon's 
 distance has likewise augmented, that the obliquity of the 
 ecliptic has diminished, that a large amount of hypogene 
 heat has been generated by the internal tidal friction, and 
 that these changes may all have transpired within compara- 
 tively so short a period (57,000,000 years) as to place them 
 quite probably within the limits of ordinary geological his- 
 tory. According to his estimate, 46,300,000 years ago the 
 length of the sidereal day was fifteen and a half hours, the 
 moon's distance in mean radii of the earth was 46 - 8 as com- 
 pared with 60-4 at the present time. But 56,810,000 years 
 back, the length of a day was only 6| hours, or less than a 
 quarter of its present value, the moon's distance was only 
 nine earth's radii, while the lunar month lasted not more 
 than about a day and a half (1-58), or tV of its present dura- 
 tion. He arrives at the deduction that the energy lost by 
 internal tidal friction in the earth's mass is converted into 
 heat at such a rate that the amount lost during 57,000,000 
 years, if it were all applied at once, and if the earth had the 
 specific heat of iron, would raise the temperature of the 
 whole planet's mass 1,760 Fahrenheit, but that the distribu- 
 tion of this heat-generation has been such as not to interfere 
 
 83 The degree of viscosity assumed is such that "thirteen and a half tons to 
 the square inch acting for twenty-four hours on a slab an inch thick displaces 
 the upper surface relatively to the lower through one-tenth of an inch. It is 
 obvious," says M'r. Darwin, "that such a substance as this would be called 
 a solid in ordinary parlance, and in the tidal problem this must be regarded as 
 a very small viscosity." Op. cit. p. 631.
 
 48 TEXT-BOOK OF GEOLOGY 
 
 with the normal augmentation of temperature downward 
 due to secular cooling, and the conclusion drawn therefrom 
 by Sir William Thomson. Mr. Darwin further concludes 
 from his hypothesis that the ellipticity of the earth's figure 
 having been continually diminishing, "the polar regions 
 must have been ever rising and the equatorial ones falling, 
 though as the ocean followed these changes, they might 
 quite well have left no geological traces. The tides must 
 have been very much more frequent and larger, and accord- 
 ingly the rate of oceanic denudation much accelerated. The 
 more rapid alternation of day and night* 8 would probably 
 lead to more sudden and violent storms, and the increased 
 rotation of -the earth would augment the violence of the 
 trade-winds, which in their turn would affect oceanic cur- 
 rents." 3 * As above stated, no facts yet revealed by the 
 geological record compel the admission of more violent 
 superficial action in former times than now. But though 
 the facts do not of themselves lead to such an admission, 
 it is proper to inquire whether any of them are hostile to it. 
 It will be shown in Book VI. that even as far back as early 
 Palaeozoic times, that is, as far into the past as the history 
 of organized life can be traced, sedimentation took place 
 very much as it does now. Sheets of fine mud and silt were 
 pitted with rain drops, ribbed with ripple-marks, and fur- 
 rowed by crawling worms, exactly as they now are on the 
 shores of any modern estuary. These surfaces were quietly 
 buried under succeeding sediment of a similar kind, and this 
 for hundreds and thousands of feet. Nothing indicates vio- 
 lence; all the evidence favors tranquil deposit." If, there- 
 
 83 According to his calculation, the year 57,000,000 of years ago contained 
 1300 days instead of 365. M Op. cit. p. 532. 
 
 35 Sir R. Ball (Nature, xxv. 1881, pp. 79, J03), starting from Prof. Darwin's
 
 COSMIC AL ASPECTS OF GEOLOGY 
 
 fore, Mr. Darwin's hypothesis be accepted, we must con- 
 clude either that it does not necessarily involve such violent 
 superficial operations as he supposes, or that even the oldest 
 sedimentary formations do not date back to a time when the 
 influence of increased rotation could make itself evident in 
 sedimentation, that is to say, on Mr. Darwin's hypothesis, 
 the most ancient fossiliferous rocks cannot be as much as 
 57,000,000 years old. 
 
 8. Climate in its Geological Relations. In subsequent 
 parts of this work data will be given from which we learn 
 that the climates of the earth have formerly been consider- 
 ably different from those which at present prevail. A con- 
 sideration of the history of the solar system would of itself 
 suggest the inference that, on the whole, the climates of 
 early geological periods must have been warmer. The sun's 
 heat was greater, probably the amount of it received by the 
 earth was likewise greater, while there would be for some 
 time a sensible influence of the planet's own internal heat 
 upon the general temperature of the whole globe." Al- 
 though arguments based upon the probable climatal neces- 
 sities of extinct species and genera of plants and animals 
 
 data, pushed his conclusions to such an extreme as to call in the agency of tides 
 more than 600 feet high in early geological times. In repudiating this applica- 
 tion of his results, Mr. Darwin (Nature, xxv. p. 213) employs the argument 
 I have here used from the absence of any evidence of such tidal action in the 
 geological formations, and from the indication, on the contrary, of tranquil 
 deposit. 
 
 * Lord Kelvin (Sir William Thomson) believes that the hypothesis that ter- 
 restrial temperature was formerly higher by reason of a hotter sun "is rendered 
 almost infinitely probable by independent physical evidence and mathematical 
 calculation." (Trans. Geol. Soc. Glasgow, v. p. 238.) Prof. Tail, however, has 
 suggested, that the former greater heat of the sun may have raised such vast 
 clouds of absorbing vapor round that luminary as to prevent the effective amount 
 of radiation of heat to the earth's surface from being greater than at present; 
 while on the other hand, a similar supposition may be made with reference to 
 the greater amount of vapor which increased solar radiation would raise to be 
 condensed in the earth's atmosphere. "Recent Advances in Physical Science," 
 1876, p. 174. 
 
 GEOLOGY Yol. XXIX 3
 
 50 TEXT-BOOK OF GEOLOGY 
 
 must be used with extreme caution, it may be asserted with 
 some confidence that from the vast areas over which Palgeo- 
 zoic mollusks have been traced, alike in the eastern and the 
 western hemispheres, the climates of the globe in Palaeozoic 
 time were probably more uniform than they now are. There 
 appears to have been a gradual lowering of the general tem- 
 perature during past geological time, accompanied by a ten- 
 dency toward greater extremes of climate. But there are 
 propfs also that at longer or shorter intervals cold cycles 
 have intervened. The Glacial Period, for example, pre- 
 ceded our own time, and in successive geological formations 
 indications, of more or less value, have been found that 
 suggest if they do not prove a former prevalence of ice in 
 what are now temperate regions." 
 
 Various theories have been proposed in explanation of 
 such alternations of climate. Some of these have appealed 
 to a change in the position of the earth's axis relatively to 
 the mass of the planet (ante, 5). Others have been based 
 on the notion that the earth may have passed through hot 
 and cold regions of space. Others, again, have called in the 
 effects of terrestrial changes, such as the distribution of land 
 and sea, on the assumption that elevation of land about the 
 poles must cool the temperature of the globe, while eleva- 
 tion round the equator would raise it. 88 But the changes of 
 temperature appear to have affected the whole of the earth's 
 surface, while there is not only no proof of any such enor- 
 mous vicissitudes in physical geography as would be re- 
 quired, but good grounds for believing that the present ter- 
 
 31 Consult a suggestive paper by the late Dr. M. Neumayr, Nature, xlii. (1890) 
 p. 148. 
 
 38 In Lyell's "Principles of Geology," this doctrine of the influence of geo- 
 graphical changes is maintained.
 
 COSMICAL ASPECTS OF GEOLOGY 51 
 
 restrial and oceanic areas have remained, on the whole, on 
 the same sites from very early geological time. Moreover, 
 as evidence has accumulated in favor of periodic alterna- 
 tions of climate, the conviction has been strengthened that 
 no mere local changes could have sufficed, but that secular 
 variations in climate must be assigned to some general and 
 probably recurring cause. 
 
 By degrees, geologists accustomed themselves to the be- 
 lief that the cold of the Glacial Period was not due to mere 
 terrestrial changes, but was to be explained somehow as the 
 result of cosmical causes. Of various suggestions as to the 
 probable nature and operation of these causes, one deserves 
 careful consideration change in the eccentricity of the 
 earth's orbit. Sir John HerscheP* pointed out many years 
 ago that the direct effect of a high condition of eccentricity 
 is to produce an unusually cold winter, followed by a corre- 
 spondingly hot summer, in the hemisphere whose winter 
 occurs in aphelion, while an equable condition of climate 
 at the same time prevails on the opposite hemisphere. But 
 both hemispheres must receive precisely the same amount 
 of solar heat, because the deficiency of heat, resulting from 
 the sun's greater distance during one part of the year, is 
 exactly compensated by the greater length of that season. 
 Sir John Herschel even considered that the direct effects 
 of eccentricity must thus be nearly neutralized. 40 As 
 a like verdict was afterward given by Arago, Humbolch, 
 and others, geologists were satisfied that no important 
 change of climate could be attributed to change of eccen- 
 tricity. 
 
 The late Dr. James Croll, as far back as the year 1864, 
 
 39 Trans. Geol. Soc. vol. iii. p. 293 (2d series). 
 
 40 "Cabinet Cyclopaedia," sec. 315; "Outlines of Astronomy," sec. 368.
 
 62 TEXT-BOOK OF GEOLOGY 
 
 made an important suggestion in this matter, and subse- 
 quently worked out an elaborate development of the whole 
 subject of the physical causes on which climate depends. 41 
 He was good enough to draw up the following abstract of 
 them for former editions of the present work. 
 
 "Assuming the mean distance of the sun to be 92,400,000 
 miles, then when the eccentricity is at its superior limit, 
 07775, the distance of the sun from the earth, when the 
 latter is in the aphelion of its orbit, is no less than 99,584,100 
 miles, and when in the perihelion it is only 85,215, 900'miles. 
 The earth is, therefore, 14,368,200 miles further from the 
 sun in the former than in the latter position. The direct 
 
 N. Winter Solstice in Aphelion N. Winter Solstice in Perihelion 
 
 Fig. 1. Eccentricity of the Earth's Orbit in Relation to Climate 
 
 heat of the sun being inversely as the square of the distance, 
 it follows that the amount of heat received by the earth in 
 these two positions will be as 19 to 26. The present eccen- 
 tricity being -0168, the earth's distance during our northern 
 winter is 90,847,680 miles. Suppose now that, from the 
 precession of the equinoxes, winter in our northern hemi- 
 sphere should happen when the earth is in the aphelion of 
 its orbit, at the time that the orbit is at its greatest eccen- 
 
 41 Phil. Mag. xxviii. (1864), p. 121. His researches will be found in detail 
 in his volume "Climate and Time," 1875, and his later work "Discussions on 
 Climate and Cosmology."
 
 COSMIC AL ASPECTS OF GEOLOGY 53 
 
 tricity; the earth would then be 8,736,420 miles further from 
 the sun in winter than it is at present. The direct heat of 
 the sun would therefore, during winter, be one-fifth less and 
 during summer one-fifth greater than now. This enormous 
 difference would necessarily affect the climate to a very 
 great extent. Were the winters under these circumstances 
 to occur when the earth was in the perihelion of its orbit, 
 the earth would then be 14,368,200 miles nearer the sun in 
 winter than in summer. In this case the difference between 
 winter and summer in our latitudes would be almost annihi- 
 lated. But as the winters in the one hemisphere correspond 
 with the summers in the other, it follows that while the one 
 hemisphere would be enduring the greatest extremes of sum- 
 mer heat and winter cold, the other would be enjoying per- 
 petual summer. 
 
 "It is quite true that, whatever may be the eccentricity 
 of the earth's orbit, the two hemispheres must receive equal 
 quantities of heat per annum; for proximity to the sun is 
 exactly compensated by the effect of swifter motion. The 
 total amount of heat received from the sun between the two 
 equinoxes is, therefore, the same in both halves of the year, 
 whatever the eccentricity of the earth's orbit may be. For 
 example, whatever extra heat the southern hemisphere may 
 at present receive per day from the sun during its summer 
 months, owing to greater proximity to the sun, is exactly 
 compensated by a corresponding loss arising from the short- 
 ness of the season; and, on the other hand, whatever defi- 
 ciency of heat we in the northern hemisphere may at present 
 have per day during our summer half-year, in consequence 
 of the earth *s distance from the sun, is also exactly compen- 
 sated by a corresponding length of season. 
 
 "It is well known, however, that those simple changes 
 in the summer and winter distances would not alone produce 
 a glacial epoch, and that physicists, confining their attention 
 to the purely astronomical effects, were perfectly correct in 
 affirming that no increase of eccentricity of the earth's orbit 
 could account for that epoch. But the important fact was 
 overlooked that, although the glacial epoch could not result 
 directly from an increase of eccentricity, it might neverthe- 
 less do so indirectly from physical agents that were brought 
 into operation as a result of an increase of eccentricity. 
 The following is an outline of what these physical agents 
 were, how they were brought into operation, 'and the way 
 in which they may have led to the glacial epoch.
 
 54 TEXT-BOOK OF GEOLOGY 
 
 "With the eccentricity at its superior limit and the win- 
 ter occurring in the aphelion, the earth would, as we have 
 seen, be 8,736,420 miles further from the sun during that 
 season than at present. The reduction in the amount of 
 heat received from the sun, owing to his increased distance, 
 would lower the midwinter temperature to an enormous ex- 
 tent. In temperate regions the greater portion of the moist- 
 ure of the air is at present precipitated in the form of rain, 
 and the very small portion which falls as snow disappears 
 in the course of a few weeks at most. But in the circum- 
 stances under consideration, the mean winter-temperature 
 would be lowered so much below the freezing-point that 
 what now falls as rain during that season, would then fall 
 as snow. This is not all; the winters would then not only 
 be cooler than now, but they would also be much longer. 
 At present the winters are nearly eight days shorter than the 
 summers; but with the eccentricity at its superior limit and 
 the winter solstice in aphelion, the length of the winters 
 would exceed that of the summers by no fewer than thirty- 
 six days. The lowering of the temperature and the length- 
 ening of the winter would both tend to the same effect, viz. 
 to increase the amount of snow accumulated during the win- 
 ter; for, other things being equal, the longer the snow-ac- 
 cumulating period, the greater the accumulation. It may 
 be remarked, however, that the absolute quantity of heat 
 received during winter is not affected by the decrease in the 
 sun's heat, for the additional length of the season compen- 
 sated for this decrease." As regards the absolute amount 
 of heat received, increase of the sun's distance and length- 
 ening of the winter are compensatory, but not so in regard 
 to the amount of snow accumulated. The consequence of 
 this state of things would be that, at the commencement 
 of the short summer, the ground would be covered with the 
 winter's accumulation of snow. Again, the presence of so 
 much snow would lower the summer temperature, and pre- 
 vent to a great extent the melting of the snow. 
 
 "There are three separate ways whereby accumulated 
 masses of snow and ice tend to lower the summer tempera- 
 ture, viz. : 
 
 "First, By means of direct radiation. No matter what 
 the intensity of the sun's rays may be, the temperature of 
 
 42 When the eccentricity is at its superior limit, the absolute quantity of heat 
 received by the earth during the year is, however, about one three-hundredth 
 part greater than at present. But this does not affect the question at issue.
 
 COSMICAL ASPECTS OF GEOLOGY 55 
 
 enow and ice can never rise above 32. Hence, the pres- 
 ence of snow and ice tends by direct radiation to lower the 
 temperature of all surrounding bodies to 32. In Green- 
 land, a country covered with snow and ice, the pitch has 
 been seen to melt on the side of a ship exposed to the direct 
 rays of the sun, while at the same time the surrounding air 
 was far below the freezing-point; a thermometer exposed to 
 the direct radiation of the sun has been observed to stand 
 above 100, while the air surrounding the instrument was 
 actually 12 below the freezing-point. A similar experience 
 has been recorded by travellers on the snow-fields of the 
 Alps. These results, surprising as they no doubt appear, 
 are what we ought to expect under the circumstances. Per- 
 fectly dry air seems to be nearly incapable of absorbing 
 radiant heat. The entire radiation passes through it almost 
 without any sensible absorption. Consequently the pitch 
 on the side of the ship may be melted, or the bulb of the 
 thermometer raised to a high temperature by the direct rays 
 of the sun, while the surrounding air remains intensely cold. 
 The air is cooled by contact with the snow-covered ground, 
 but is not heated by the radiation from the sun. 
 
 "When the air is charged with aqueous vapor, a similar 
 cooling effect also takes place, but in a slightly different 
 way. Air charged with aqueous vapor is a good absorber 
 of radiant heat, but it can only absorb those rays which 
 agree with it in period. It so happens that rays from snow 
 and ice are, of all others, those which it absorbs best. The 
 humid air will absorb the total radiation from the snow 
 and ice, but it will allow the greater part of, if not nearly 
 all, the sun's rays to pass unabsorbed. But during the 
 day, when the sun is shining, the radiation from the snow 
 and ice to the air is negative; that is, the snow and ice 
 cool the air by radiation. The result is, the air is cooled 
 by radiation from the snow and ice (or rather, we should 
 say, to the snow and ice) more rapidly than it is heated by 
 the sun; and as a consequence, in a country like Greenland, 
 covered with an icy mantle, the temperature of the air, even 
 during summer, seldom rises above the freezing-point. 
 Snow is a good reflector, but as simple reflection does not 
 change the character of the rays, they would not be ab- 
 sorbed by the air, but would pass into stellar space. Were 
 it not for the ice, the summers of North Greenland, owing 
 to the continuance of the sun above the horizon, would be 
 as warm as those of England ; but instead of this, the Green-
 
 56 TEXT-BOOK OF GEOLOGY 
 
 land summers are colder than our winters. Cover India 
 with an ice sheet, and its summers would be colder than 
 those of England. 
 
 " Second, Another cause of the cooling effect is that the 
 rays which fall on snow and ice are to a great extent re- 
 flected back into space. But those that are not reflected, 
 but absorbed, do not raise the temperature, for they disap- 
 pear in the mechanical work of melting the ice. For what- 
 soever may be the intensity of the sun's heat, the surface 
 of the ground will be kept at 82 so long as the snow and 
 ice remain unmelted. 
 
 "Third, Snow and ice lower the temperature by chilling 
 the air and condensing the vapor into thick fogs. The great 
 strength of the sun's rays during summer, due to his near- 
 ness at that season, would, in the first place, tend to produce 
 an increased amount of evaporation. But the presence of 
 snow-clad mountains and an icy sea would chill the atmos- 
 
 Ehere and condense the vapor' into thick fogs. The thick 
 )gs and cloudy sky would effectually prevent the sun's 
 rays from reaching the earth, and the snow, in consequence, 
 would remain unmelted during the entire summer. In fact, 
 we have this very condition of things exemplified in some 
 of the islands 01 the Southern Ocean at the present day. 
 Sandwich Land, which is in the same parallel of latitude 
 as the north of Scotland, is covered with ice and snow the 
 entire summer; and in the island of South Georgia, which 
 is in the same parallel as the centre of England, the per- 
 petual snow descends to the very sea-beach. Captain Sir 
 James Eoss found the perpetual snow at the sea-level at 
 Admiralty Inlet, South Shetland, in lat. 64; and while 
 near this place the thermometer in the very middle of 
 summer fell at night to 23 F. The reduction of the sun's 
 heat and lengthening of the winter, which would take place 
 when the eccentricity is near to its superior limit and the 
 winter in aphelion, would in this country produce a state 
 of things perhaps as bad as, if not worse than, that which 
 at present exists in South Georgia and South Shetland. 
 
 "The cause which above all others must tend to produce 
 great changes of climate, is the deflection of great ocean cur- 
 rents. A high condition of eccentricity tends, we have seen, 
 to produce an accumulation of snow and ice on the hemi- 
 sphere whose winters occur in aphelion. The accumulation 
 of snow, in turn, tends to lower the summer temperature, 
 cut off the sun's rays, and retard the melting of the snow.
 
 COSMICAL ASPECTS OF GEOLOGY 57 
 
 In short, it tends to produce, on that hemisphere, a state 
 of glaciation. Exactly opposite effects take place on the 
 other hemisphere, which has its winter in perihelion. 
 There the shortness of the winters, combined with the 
 high temperature arising from the nearness of the sun, 
 tends to prevent the accumulation of snow. The general 
 result is that the one hemisphere is cooled and the other 
 heated. This state of things now brings into play the 
 agencies which lead to the deflection of the Q-uli Stream 
 and other great ocean currents. 
 
 "Owing to the great difference between the temperature 
 of the equator and the poles, there is a constant flow of air 
 from the poles to the equator. It is to this that the trade- 
 winds owe their existence. Now, as the strength of these 
 winds will, as a general rule, depend upon the difference 
 of temperature that may exist between the equator and 
 higher latitudes, it follows that the trades on the cold 
 hemisphere will be stronger than those on the warm. When 
 the polar and temperate regions of the one hemisphere are 
 covered to a large extent with snow and ice, the air, as we 
 have just seen, is kept almost at the freezing-point during 
 both summer and winter. The trades on that hemisphere 
 will, of necessity, be exceedingly powerful; while on the 
 other hemisphere, where there is comparatively little snow 
 or ice, and the air is warm, the trades will consequently be 
 weak. Suppose now the northern hemisphere to be the cold 
 one. The northeast trade-winds of this hemisphere will far 
 exceed in strength the southeast trade-winds of the southern 
 hemisphere. The median line between the trades will con- 
 sequently lie to a very considerable distance to the south 
 of the equator. We have a good example of this at the 
 present day. The difference of temperature between the two 
 hemispheres at present is but trifling to what it would be in 
 the case under consideration; yet we find that the southeast 
 trades of the Atlantic blow with greater force than the 
 northeast trades, sometimes extending to 10 or 15 N. lat., 
 whereas the northeast trades seldom blow south of the 
 equator. The effect of the northern trades blowing across 
 the equator to a great distance will be to impel the warm 
 water of the tropics over into the Southern Ocean. But this 
 is not all; not only would the median line of the trades be 
 shifted southward, but the great equatorial currents of the 
 globe would also be shifted southward. 
 
 "Let us now consider how this would affect the Gulf
 
 58 TEXT-BOOK OF GEOLOGY 
 
 Stream. The South American continent is shaped some- 
 what in the form of a triangle, with one of its angular 
 corners, called Cape St. Roque, pointing eastward. The 
 equatorial current of the Atlantic impinges against this 
 corner; but as the greater portion of the current lies a little 
 to the north of the corner, it flows westward into the Gulf 
 of Mexico and forms the Gulf Stream. A considerable 
 portion of the water, however, strikes the land to the south 
 of the cape, and is deflected along the shore of Brazil into 
 the Southern Ocean, forming what is known as the Brazilian 
 current. Now, it is obvious that the shifting of the equa- 
 torial current of the Atlantic only a few degrees to the south 
 of its present position a thing which would certainly take 
 place under the conditions which we have been detailing 
 would turn the entire current into the Brazilian branch, and 
 instead of flowing chiefly into the Gulf of Mexico, as at 
 present, it would all flow into the Southern Ocean, and the 
 Gulf Stream would consequently be stopped. The stoppage 
 of the Gulf Stream, combined with all those causes which 
 we have just been considering, would place Europe under 
 a glacial condition, while at the same time the temperature 
 of the Southern Ocean would, in consequence of the enor- 
 mous quantity of warm water received, have its temperature 
 (already high from other causes) raised enormously. And 
 what holds true in regard to the currents of the Atlantic 
 holds also true, though perhaps not to the same extent, of 
 the currents of the Pacific. 
 
 "If the breadth of the Gulf Stream be taken at 50 miles, 
 its depth at 1000 feet, its mean velocity at 2 statute miles 
 an hour, the temperature of the water when it leaves the 
 gulf at 65, and the return current at 40 F., 48 then, the 
 (quantity of heat conveyed into the Atlantic by this stream 
 is equal to one-fourth of all the heat received from the sun 
 by that ocean from the Tropic of Cancer to the Arctic 
 Circle." From principles discussed at considerable length 
 
 48 Sir Wyville Thomson states that in May, 1873, the "Challenger" expedi- 
 tion found the Gulf Stream, at the point where it was crossed, to be about sixty 
 miles in width, 100 fathoms deep, and flowing at the rate of three knots per 
 hour. This makes the volume of the stream one-fifth greater than the above 
 estimate. 
 
 44 The quantity of heat conveyed by the Gulf Stream for distribution is equal 
 to 77,479,650,000,000,000,000 foot-pounds per day. The quantity received 
 from the sun by the North Atlantic is 310,923,000,000,000,000,000 foot-pounds. 
 "Climate and Time," chap. ii.
 
 COSMIC 'AL ASPECTS OF GEOLOGY 59 
 
 in 'Climate and Time' it is shown that, but for the Gulf 
 Stream and other currents, London would have a mean 
 annual temperature 40 lower than at present. 
 
 "But there is still another cause which must be noticed 
 a strong undercurrent of air from the north implies an 
 equally strong upper current to the north. Now if the 
 effect of the undercurrent would be to impel the warm 
 water at the equator to the south, the effect of the upper 
 current would be to carry the aqueous vapor formed at the 
 equator to the north ; the upper current, on reaching the 
 snow and ice of temperate regions, would deposit its moist- 
 ure in the form of snow; so that it is probable that, not- 
 withstanding the great^ cold of the glacial epoch, the quan- 
 tity of snow falling m the northern regions would be 
 enormous. This would be particularly the case during 
 summer, when the earth would be in the perihelion and 
 the heat at the equator great. The equator would be the 
 furnace where evaporation would take place, and the snow 
 and ice of temperate regions would act as a condenser. 
 
 "The foregoing considerations, as well as many others 
 which might be stated, lead to the conclusion that, in order 
 to raise the mean temperature of the globe, water should be 
 placed along the equator, and not land, as was contended 
 by Sir Charles Lyell and others. For if land be placed at 
 the equator, the possibility of conveying the sun's neat from 
 the equatorial regions by means of ocean currents is pre- 
 vented. ' ' * 
 
 The astronomical theory in explanation of former great 
 differences of terrestrial climate has recently been illustrated 
 and enforced by Sir Robert Ball, who, while strengthening 
 the general arguments in its favor, especially insists upon 
 the existence of an important law in the distribution of 
 solar heat on the earth's surface, which he thinks has been 
 hitherto overlooked. He remarks that the original state- 
 
 45 That climate, however, may be considerably affected by changes, such as 
 are known to have taken place in the distribution of land and sea, must be 
 frankly conceded. This has been recently cogently argued by Mr. Wallace 
 in his "Island Life," 1880. Mr. Croll's views, summarized above, have been 
 adversely criticised by Prof. Newcombe, for whose papers and Dr. Croll's re- 
 plies see Amer. Journ. Science, 1876, 1883, 1884, and the work by the latter 
 writer, "Discussions on Climate and Cosmology," already referred to.
 
 60 TEXT-BOOK OF GEOLOGY 
 
 ment of Sir John Herschel that the heat received by the 
 earth from the sun is equally divided between the winter 
 and summer seasons has given rise to an entirely erroneous 
 impression. Although "it is certainly true that during the 
 summer in one hemisphere the heat received on the whole 
 earth is equal to the heat received on the whole earth dur- 
 ing the ensuing winter on the same hemisphere," yet on 
 any given hemisphere almost twice as much heat can be 
 demonstrated to be received during summer as during 
 winter. 4 ' The law is thus stated: "Of the total amount 
 of heat received from the sun on a hemisphere of the 
 earth' in the course of a year, 63 per cent is received 
 during the summer, and 37 per cent is received during 
 the winter." 47 It is obvious that while, under the opera- 
 tion of this law, the total amount of heat received and 
 the ratio of its distribution between summer and winter 
 would remain unchanged, enormous differences in terres- 
 trial climate might result according as the seasons varied 
 in length with changes in the eccentricity of the orbit. 
 
 Inter-Glacial Periods. Allusion has already 
 been made to the accumulating evidence that changes of 
 climate have been recurrent, and to the deduction from 
 this alternation or periodicity that they have probably 
 been due to some general or cosmical cause. Dr. Croll 
 ingeniously showed that every long cold period arising 
 in each hemisphere from the circumstances sketched in 
 the preceding pages, must have been interrupted by sev- 
 eral shorter warm periods. 
 
 "When the one hemisphere," he says, "is under glacia- 
 tion, the other is enjoying a warm and equable climate. 
 
 46 "The Cause of an Ice Age," London, 1891, p. 120. 41 Ibid. p. 90.
 
 COSMIC AL ASPECTS OF GEOLOGY 61 
 
 But, owing to the precession of the equinoxes, the condi- 
 tion of things on the two hemispheres must be reversed 
 every 10,000 years or so. When the solstice passes the 
 aphelion, a contrary process commences; the snow and 
 ice gradually begin to diminish on the cold hemisphere 
 and to make their appearance on the other hemisphere. 
 The glaciated hemisphere turns by degrees warmer, and 
 the warm hemisphere colder, and this continues to go on 
 for a period of ten or twelve thousand years, until the 
 winter solstice reaches the perihelion. By this time the 
 conditions of the two hemispheres have been reversed; 
 the formerly glaciated hemisphere has now become the 
 warm one, and the warm hemisphere the glaciated. The 
 transference of the ice from the one hemisphere to the other 
 continues as long as the eccentricity remains at a high 
 value. It is probable that, during the warm inter-glacial 
 periods, Greenland and the Arctic regions would be com- 
 paratively free from snow and ice, and enjoying a temperate 
 and equable climate. ' '
 
 62 TEXT-BOOK OF GEOLOGY 
 
 BOOK II 
 
 GEOGNOSY 
 
 AN INVESTIGATION OF THE MATERIALS OF THE EARTH'S SUBSTANCE 
 
 PART I. A GENERAL DESCRIPTION OF THE PARTS 
 OF THE EARTH 
 
 A DISCUSSION of the geological changes which our 
 planet has undergone ought to be preceded by a 
 study of the materials of which the planet consists. 
 This latter branch of inquiry is termed Geognosy. 
 
 "Viewed in a broad way, the earth may be considered as 
 consisting of (1) two envelopes an outer one of gas (atmos- 
 phere), completely surrounding the planet, and an inner one 
 of water (hydrosphere), covering about three-fourths of the 
 globe; and (2) a globe (lithosphere), cool and solid on its 
 surface, but possessing a high internal temperature. 
 
 I. The Envelopes Atmosphere and Hydrosphere 
 
 It is certain that the present gaseous and liquid enve- 
 lopes of the planet form only a portion of the original mass 
 of gas and water with which the globe was invested. Fully 
 a half of the outer shell or crust of the earth consists of oxy- 
 gen, which, there can be no doubt, once existed in the at- 
 mosphere. The extent, likewise, to which water has been 
 abstracted by minerals is almost incredible. It has been 
 estimated that already one-third of the whole mass of the 
 ocean has been thus absorbed. Eventually the condition of 
 the planet will probably resemble that of the moon a globe 
 without air, or water, or life of any kind.
 
 GEOGNOSY 68 
 
 1. The Atmosphere. The gaseous envelope to which 
 the name of atmosphere is given, extends to a distance 
 of perhaps 500 or 600 miles from the earth's surface, pos- 
 sibly in a state of extreme tenuity to a still greater 
 height. But its thickness must necessarily vary with 
 latitude and changes in atmospheric pressure. The layer 
 of air lying over the poles is not so deep as that which 
 surrounds the equator. 
 
 Many speculations have been made regarding the chemi- 
 cal composition of the atmosphere during former geological 
 periods. There can indeed be no doubt that it must origi- 
 nally ha.ve differed very greatly from its present condition. 
 Besides the abstraction of the oxygen which now forms fully 
 a half of the outer crust of the earth, the vast beds of coal 
 found all over the world, in geological formations of many 
 different ages, doubtless represent so much carbon -dioxide 
 (carbonic acid) once present in the air. According to Sterry 
 Hunt, the amount of carbonic acid absorbed in the process 
 of rock-decay, and now represented in the form of carbo- 
 nates in the earth's crust, probably equals two hundred 
 times the present volume of the entire atmosphere. 1 The 
 chlorides in the sea, likewise, were probably carried down 
 out of the atmosphere in the primitive condensation of 
 aqueous vapor. It has often been stated that, during the 
 Carboniferous period, the atmosphere must have been 
 warmer and with more aqueous vapor and carbon-diox- 
 ide in its composition than at the present day, to admit 
 of so luxuriant a flora as that from which the coal-seams 
 were formed. There seems, however, to be at present no 
 method of arriving at any certainty on this subject. 
 
 1 Brit. Aasoc. Rep. 1878, Sects, p. 544.
 
 64 TEXT-BOOK OF GEOLOGY 
 
 As now existing, the atmosphere is considered to be nor- 
 mally a mechanical mixture of nearly 4 volumes of nitrogen 
 and 1 of oxygen (N79-4, O20-6), with minute proportions of 
 carbon-dioxide and water- vapor and still smaller quantities 
 of ammonia and the powerful oxidizing agent, ozone. 
 These quantities are liable to some variation according to 
 locality. The mean proportion of carbon-dioxide is about 
 3-5 parts in every 10,000 of air. In the air of streets and 
 houses the proportion of oxygen diminishes, while that of 
 carbon- dioxide increases. According to the researches of 
 Angus Smith, very pure air should contain not less than 
 20-99 per cent of oxygen, with 0-030 of carbon-dioxide; but 
 he found impure air in Manchester to have only 20-21 of 
 oxygen, while the proportion of carbon-dioxide in that city 
 during fog was ascertained to rise sometimes to 0-0679, and 
 in the pit of the theatre to the very large amount of 0-2734. 
 As plants absorb carbon-dioxide during the day and give it 
 off at night, the quantity of this gas in the atmosphere oscil- 
 lates between a maximum at night and a minimum during 
 the day. During the part of the year when vegetation is 
 active, it is believed that there is at least 10 per cent more 
 carbonic acid in the air of the open country at night than in 
 the day.* Small as the normal percentage of this gas in the 
 air may seem, yet the total amount of it in the whole atmos- 
 phere probably exceeds what would be disengaged if all the 
 vegetable and animal matter on the earth's surface were 
 burned. 
 
 The other substances in the air are gases, vapors, 
 and solid particles. Of these by much the most impor- 
 tant is the vapor of water, which is always present, but 
 
 9 Prof. G. F. Armstrong. Proc. Roy. Soc. xxx. (1880), p. 343.
 
 GEOGNOSY 65 
 
 in very variable amount according to temperature.' It 
 is this vapor which chiefly absorbs radiant heat. 4 It con- 
 denses into dew, rain, hail, and snow. In assuming a 
 visible form, and descending through the atmosphere, it 
 takes up a minute quantity of air, and of the different sub- 
 stances which the air may contain. Being caught by the 
 rain, and held in solution or suspension, these substances 
 can be best examined by analyzing rain-water. In this way, 
 the atmospheric gases, ammonia, nitric, sulphurous, and 
 sulphuric acids, chlorides, various salts, solid carbon, inor- 
 ganic dust, and organic matter have been detected. The 
 fine microscopic dust so abundant in the air is no doubt for 
 the most part due to the action of wind in lifting up the finer 
 particles of disintegrated rock on the surface of the land. 
 Volcanic explosions sometimes supply prodigious quantities 
 of fine dust. There is probably also some addition to the 
 solid particles in the atmosphere from the explosion and dis- 
 sipation of meteorites on entering our atmosphere. To the 
 wide diffusion of minute solid particles in the air great im- 
 portance in the condensation of vapor has recently been 
 assigned. (Book III. Part II. Section ii.) 
 
 The comparatively small, but by no means unimportant, 
 proportions of these minor components of the atmosphere 
 are much more liable to variation than those of the more 
 essential gases. Chloride of sodium, for instance, is, as 
 might be expected, particularly abundant in the air bor- 
 dering the sea. Nitric acid, ammonia, and sulphuric acid 
 
 * A cubic metre of air at the freezing-point can hold only 4-871 grammes of 
 water- vapor, but at 40 C. can take up 50-70 grammes. One cubic mile of air 
 saturated with vapor at 35 C. will, if cooled to 0, deposit upward of 140,000 
 tons of water as rain. Eoscoe and Schorlemmer's "Chemistry," L p. 452. 
 
 4 See Tyndall's researches which established this important function of the 
 aqueous vapor. of the atmosphere, and their confirmation by meteorological 
 observation. S. A. Hill, Proc. Roy. Soc. xxxiii. 216, 435.
 
 66 TEXT-BOOK OF GEOLOGY 
 
 appear most conspicuously in the air of towns. The or- 
 ganic substances present in the air are sometimes living 
 germs, such as probably often lead to the propagation of 
 disease, and sometimes mere fine particles of dust derived 
 from the bodies of living or dead organisms. 6 
 
 As a geological agent, the atmosphere effects changes by 
 the chemical reactions of its constituent gases and vapors, 
 by its varying temperature, and by its motions. Its func- 
 tions in these respects are described in Book III. Part. 11. 
 Section i. 
 
 2. The Oceans. Bather less than three-fourths of the 
 surface of the globe (or about 144,712,000 square miles) are 
 covered by the irregular sheet of water known as the Sea. 
 Within the last twenty years, much new light has been 
 thrown upon the depths, temperatures, and biological con- 
 ditions of the ocean-basins, more particularly by the "Light- 
 ning," "Porcupine," "Challenger," "Tuscarora," "Blake," 
 "Gazelle" and other expeditions fitted out by the British, 
 American, German and Norwegian Governments." It has 
 been ascertained that few parts of the Atlantic Ocean ex- 
 ceed 3000 fathoms, the deepest sounding obtained there 
 being one taken about 100 miles north from the island of 
 St. Thomas, which gave 3875 fathoms, or rather less than 
 44 miles. The Atlantic appears to have an average depth 
 
 B The air of towns is peculiarly rich in impurities, especially in manufactur- 
 ing districts, where much coal is used. These impurities, however, though of 
 serious consequence to the towns in a sanitary point of view, do not sensibly 
 affect the general atmosphere, seeing that they are probably in great measure 
 taken out of the air by rain, even in the districts which produce them. They 
 possess, nevertheless, a special geological significance, and in this respect, too, 
 have important economic bearings. See on this whole subject, Angus Smith's 
 "Air and Rain," and the account of Rain in Book III. Part II. Sect. ii. 
 
 6 See Wyville Thomson, "The Depths of the Sea," 1873; "The Atlantic," 
 1877; "Report of 'Challenger' Expedition," especially the forthcoming volumes 
 giving a summary of results; A. Agassiz, "Three Cruises of the 'Blake,' " 1888; 
 "Den Norske Nordhavs-Expedition," 1876-78.
 
 GEOGNOSY 67 
 
 in its more open parts of from 2000 to 3000 fathoms, or from 
 about 2 to 3\ miles. In the Pacific Ocean H.M. Ship "Chal- 
 lenger" got soundings of 3950 and 4475 fathoms, or about 
 4\ and rather more than 5 miles. Since then the TJ. S. Ship 
 "Tuscarora" obtained a still deeper sounding (4655 fathoms), 
 to the east of the Kurile Islands. This is the deepest abyss 
 yet found in any part of the ocean. But these appear to 
 mark exceptionally abysmal depressions, the average depth 
 being, as in the Atlantic, between 2000 and 3000 fathoms. 
 We may therefore assume, as probably not far from the 
 truth, that the average depth of the sea is about 2500 fath- 
 oms, or nearly 3 miles. Its total cubic contents will thus 
 be about 400 millions of cubic miles. 
 
 With regard also to the form of the bottom of the great 
 oceans, much additional information has recently been ob- 
 tained. Over vast areas in the central regions, the sea-floor 
 appears to form great plains, with comparatively few in- 
 equalities, but with lines of submarine ridges, comparable to 
 chains of hills or mountains on the land. Recent sound- 
 ings, however, taken at short distances, have revealed, in 
 parts of the Atlantic that were supposed to be deep and 
 with a tolerably uniform bottom, submarine peaks rising to 
 within 50 fathoms from the surface. 7 A vast central ridge 
 has also been traced down the length of this ocean, from 
 which a few lonely peaks rise above sea-level the Azores, 
 St. Paul, Ascension, and Tristan d'Acunha. In the Pacific 
 Ocean, the lines of coral-islands appear to rise on subma- 
 rine ridges, having a general northwesterly and southeast- 
 erly trend. It is significant that the islands which thus 
 appear far from any large mass of land are either coral-reefs 
 
 1 "Times," 7th Dec. 1883. [J. Y. Buchanan.]
 
 TEXT-BOOK OF GEOLOGY 
 
 or of volcanic origin, and contain none of the granites, 
 schists and other ordinary continental rocks. St. Helena 
 and Ascension in the Atlantic, and the Friendly and Sand- 
 wich Islands in the Pacific Ocean are conspicuous examples. 
 
 Another important result of recent deep-sea research is 
 the determination of the relation of mediterranean seas to 
 the main ocean. These basins, such as the North, Mediter- 
 ranean, and Black Seas, the Gulf of Mexico, Caribbean Sea, 
 Baffin's Bay, Hudson's Bay, Sea of Okhotsk, and Chinese 
 Sea, belong rather to the continental than the oceanic areas 
 of the earth's surface. An elevation of a few hundred fathoms 
 would convert most of them into land, with here and there 
 deep water-filled basins. 
 
 A question of high importance in geological inquiry is 
 the form of the surface of the sea or what is usually called 
 the sea-level. It has been generally assumed that this sur- 
 face is stable and uniform and nearly that of an ellipsoid of 
 revolution, owing its equilibrium to the force of gravity on 
 the one hand and the centrifugal force of rotation on the 
 other. But in recent years this conception has been called 
 in question both by physicists and geologists. Observations 
 as well as calculations haye shown that the attraction exer- 
 cised by masses of land raises the level of the adjacent sea, 
 and attempts have been made to determine how far the de- 
 formation thus caused departs from the mean of the theoret- 
 ical ellipsoid of revolution. According to Bruns, a conti- 
 nent may cause a difference of more than 3000 feet between 
 the actual level of the sea and that of the ellipsoid. But 
 the results of such calculations will greatly depend on the 
 assumption on which they start as to the nature of the 
 earth's crust. R. S. Woodward has calculated that if 
 the continent of Europe and Asia be supposed to be sim-
 
 GEOGNOSY 69 
 
 ply a superficial aggregation of matter with a density as 
 great as the parts under the sea, the elevation of sea-level 
 at the centre of the continent due to attraction would amount 
 to about 2900 feet, but that, if the continental mass be as- 
 sumed to imply a defect of density underneath it, the eleva- 
 tion of the sea at the centre of the continent due to attrac- 
 tion would be only about 10 feet. 8 This subject is further 
 considered in Book III. Part I. Section iii. 
 
 The water of the ocean is distinguished from ordinary 
 terrestrial waters by a higher specific gravity, and the pres- 
 ence of so large a proportion of saline ingredients as to 
 impart a strongly salt taste. The average density of sea- 
 water is about 1*026, but it varies slightly in different parts 
 even of the same ocean. According to the observations of 
 J. Y. Buchanan during the "Challenger" expedition, some 
 of the heaviest sea-water occurs in the pathway of the trade- 
 winds of the North Atlantic, where evaporation must be 
 comparatively rapid, a density of 1 '02781 being registered. 
 Where, however, large rivers enter the sea, or where there 
 is much melting ice, the density diminishes; Buchanan 
 found among the broken ice of the Antarctic Ocean that it 
 had sunk to 1 -02418. A series of soundings taken during 
 the "Vega" expedition in the Kara Sea (lat. 76 18', long. 
 95 30' E.) gave a progressive increase of salinity from 1-1 
 at the surface to 3 '4 at 30 fathoms, the surface being fresh- 
 ened by the water poured into the sea by the Siberian 
 rivers. I0 
 
 The greater density of sea-water depends, of course, 
 
 8 Bruns, "Die Fignr der Erde," Berlin, 1876; R. S. Woodward, Bull. U. S. 
 Geol. Surv. No. 48, p. 85 (1888). 
 
 * Buchanan, Proc. Roy. Soc. (1876), vol. xxiv. 
 
 10 0. PeUersson, "Vega-Expeditionena Yetenskapliga lakttagelser, " vol. ii 
 Stockholm, 1883.
 
 TO TEXT-BOOK OF GEOLOGY 
 
 upon the salts which it contains in solution. At an early 
 period in the earth's history, the water now forming the 
 ocean, together with the rivers, lakes and snowfields of 
 the land, existed as vapor, in which were mingled many 
 other gases and vapors, the whole forming a vast atmosphere 
 surrounding the still intensely hot globe. Under the enor- 
 mous pressure of the primeval atmosphere, the first con- 
 densed water might have had a temperature little below the 
 critical one. 11 In condensing, it would carry down with it 
 many substances in solution. The salts now present in 
 sea- water are to be regarded as principally derived from 
 the primeval constitution of the sea, and thus we may infer 
 that the sea has always been salt. It is probable, however, 
 that, as in the case of the atmosphere, the composition of 
 the ocean-water has acquired its present character only after 
 many ages of slow change, and the abstraction of much 
 mineral matter originally contained in it. There is evi- 
 dence, indeed, among the geological formations that large 
 quantities of lime, silica, chlorides and sulphates have in 
 the course of time been removed from the sea. 11 
 
 But it is manifest also that, whatever may have been 
 the original composition of the oceans, they have for a vast 
 section of geological time been constantly receiving mineral 
 matter in solution from the land. Every spring, brook and 
 river removes various salts from the rocks over which it 
 moves, and these substances, thus dissolved, eventually 
 find their way into the sea. Consequently sea-water ought 
 to contain more or less traceable proportions of every 
 
 " Q. J. Geol. Soc. xxxvi. (1880), pp. 112, 117. Fisher, "Physics of Earth's 
 Crust," 2d edit. p. 148. 
 
 >* Sterry Hunt supposed that the saline waters of North America derive their 
 mineral ingredients from the sediments and precipitates of the sea in which the 
 Palaeozoic rocks were deposited. "Geological and Chemical Essays," p. 104.
 
 GEOGNOSY 71 
 
 substance which the terrestrial waters can remove from 
 the land in short, of probably every element present in 
 the outer shell of the globe, for there seems to be no con- 
 stituent of the earth which may not, under certain circum- 
 stances, be held in solution in water. Moreover, unless 
 there be some counteracting process to remove these mineral 
 ingredients, the ocean-water ought to be growing, insensibly 
 perhaps, salter, for the supply of saline matter from the 
 land is incessant. It has been ascertained indeed, with 
 some approach to certainty, that the salinity of the Baltic 
 and Mediterranean is gradually increasing." 
 
 The average proportion of saline constituents in the 
 water of the great oceans far from land is about three and 
 a half parts in every hundred of water. 14 But in inclosed 
 seas, receiving much fresh water, it is greatly reduced, 
 while in those where evaporation predominates it is corre- 
 spondingly augmented. Thus the Baltic water contains 
 from one-seventh to nearly a half of the ordinary propor- 
 tion in ocean-water, while the Mediterranean contains some- 
 times one-sixth more than that proportion. Forchhammer 
 has shown the presence of the following twenty-seven ele- 
 ments in sea- water: oxygen, hydrogen, chlorine, bromine, 
 iodine, fluorine, sulphur, phosphorus, nitrogen, carbon, 
 
 18 Paul, in Watts's "Dictionary of Chemistry," v. p. 1020. For a detailed 
 study of the Eastern Mediterranean, see the Reports of a Commission, Denksch. 
 Akad. Wiss. Vienna, 1892 et seq. 
 
 14 Dittmar's elaborate researches on the samples of ocean water collected by 
 the "Challenger" expedition show that the lowest percentage of salts obtained 
 was 3-301, from the southern part of the Indian Ocean, south of lat. 66, while 
 the highest was 3 -737, from the middle of the North Atlantic, at about lat. 23. 
 Some valuable results from observations on the waters of the North Atlantic are 
 given by H. Tornoe and L. Schmelck in the Report of the Norwegian North- 
 Atlantic Expedition, 1876-78. The average proportion of salts was found to 
 be from 3-47 to 3*51 per cent, the mean quantities of each constituent as esti- 
 mated being as follows: CaCO,, 0-002; CaSo 4 , 0-1395; MgS0 4) 0-2071; MgCl t , 
 0-3561; KC1, 0-0747; NaHC0 8 , 0-0166; NaCl, 2-682.
 
 72 
 
 TEXT-BOOK OF GEOLOGY 
 
 silicon, boron, silver, copper, lead, zinc, cobalt, nickel, 
 iron, manganese, aluminium, magnesium, calcium, stron- 
 tium, barium, sodium, and potassium. 15 To these may be 
 added arsenic, lithium, caesium, rubidium, goldj and prob- 
 ably most if not all of the other elements, though in pro- 
 portions too minute for detection. The chief constituents 
 have been determined by Dittmar to be present in the pro- 
 portions shown in the first column of the subjoined tables. 
 Assuming them to occur in the combinations shown in the 
 second column, they are present in the average ratios 
 therein stated: 14 
 
 Chlorine 56-292 
 
 Bromine 0-188 
 
 Sulphuric acid, SO 3 6-410 
 
 Carbonic acid, C0 r 0-152 
 
 Lime, CaO 1-676 
 
 Magnesia, MgO 6-209 
 
 Potash, KO 1-332 
 
 Na,0 41-234 
 
 Subtract Basic Oxygen equiv- ) , . . Q , 
 
 alent to the Halogens [ l 
 
 Total Salts 100-000 
 
 II 
 
 Chloride of sodium 77-768 
 
 Chloride of magnesium. 10-887 
 
 Sulphate of magnesia 4-737 
 
 Sulphate of lime. 3-600 
 
 Sulphate of potash 2-466 
 
 Bromide of magnesium. 0-217 
 
 Carbonate of lime 0-346 
 
 Total Salts 100-000 
 
 Sea-water is appreciably alkaline, its alkalinity being due 
 to the presence of carbonates, of which carbonate of lime is 
 one. 17 In addition to its salts it always contains dissolved 
 
 ls Forchhammer, Phil. Trans, civ. p. 205. According to Thorpe and Morton 
 (Chem. Soc. Journ. xxiv. p. 507), the water of the Irish Sea contains in summer 
 rather more salts than in winter. In 1000 grammes of the summer water of the 
 Irish Sea they found 0-04754 grammes of carbonate of lime, 0-00503 of ferrous 
 carbonate and traces of silicic acid. For exhaustive chemical investigations re- 
 garding the chemistry of ocean-water consult Dittmar, in vol. i. "Physics and 
 Chemistry," Report of Voyage of the "Challenger," 1884; also the "Chem- 
 istry" part of the Report of the Norwegian North-Atlantic Expedition, 1876- 
 1878. 
 
 16 Dittmar, op. cit. p. 203 et seq. For further reference to the chemistry of 
 sea-water, especially in connection with the action of marine organisms, see 
 posted, p. 484. 
 
 17 Dittmar, op. cit. p. 200
 
 GEOGNOSY 73 
 
 atmospheric gases. From the researches conducted during 
 the voyage of the "Bonite'" in the Atlantic and Indian 
 Oceans, it was estimated that the gases in 100 volumes of 
 sea- water ranged from 1-86 to 3-04, or from two to three 
 per cent. From observations made during the "Porcupine" 
 cruise of 1868, it was ascertained that the proportion of 
 oxygen was greatest in the surface water, and least in the 
 bottom water. The dissolved oxygen and nitrogen are 
 doubtless absorbed from the atmosphere, the proportion 
 so absorbed being mainly regulated by temperature. Ac- 
 cording to Dittmar's recent determinations, a litre of sea- 
 water at C. will take up 15-60 cubic centimetres of nitro- 
 gen and 8-18 of oxygen, while at 30 C. the proportions 
 sink respectively to 8'36 and 4-17. He regards the carbonic 
 acid as occurring chiefly as carbonates, its presence in the 
 free state being exceptional. During the voyage of the 
 "Challenger," Buchanan ascertained that the proportion 
 of carbonic acid is always nearly the same for similar tem- 
 peratures, the amount in the Atlantic surface water, between 
 20 and 25 C., being 0-0466 gramme per litre, and in the 
 surface Pacific water 0*0268; and that sea-water contains 
 sometimes at least thirty times as much carbonic acid as 
 an equal bulk of fresh water would do. 18 A supposed 
 greater proportion of carbonic acid in the deeper and 
 colder waters of the ocean has been suggested as the majn 
 cause of the disappearance of the larger and more delicate 
 calcareous pelagic organisms from abysmal deposits, these 
 forms being more readily attacked and carried away in 
 
 18 Proc. Roy. Soc. xxiv. According to Mr. Tornoe (Norwegian North- Atlantic 
 Expedition, 1876-78, "Chemistry") most of the carbonic acid of sea-water is in 
 combination with soda as bicarbonate of soda. See his memoir for an estimate 
 of the proportion of air in sea-water; also J. T. Buchanan, Nature, xxv. p. 386. 
 Dittmar, op. cit. p. 209. 
 
 GEOLOGY Vol. XXIX 4
 
 74 TEXT-BOOK OF GEOLOGY 
 
 solution; but according to Dittmar, even alkaline sea- 
 water, if given sufficient time, will take up carbonate of 
 lime in addition to what it already contains." Another 
 of the constituents of sea-water is diffused organic matter, 
 derived from the bodies of dead plants and animals, and no 
 doubt of great importance as furnishing food for the lower 
 grades of animal life.* 
 
 II. The Solid Globe or Lithosphere 
 
 "Within the atmospheric and oceanic envelopes lies the 
 inner solid globe. The only portion of it which, rising 
 above the sea, is visible to us, and forms what we term 
 Land, occupies rather more than one-fourth of the total 
 superficies of the globe, or about 52,000,000 square miles- 
 
 1. The Outer Surface. The land is placed chiefly in 
 the northern hemisphere and is disposed in large masses, 
 or continents, which taper southward to about half the dis- 
 tance between the equator and the south pole. No adequate 
 cause has yet been assigned for the present distribution of 
 the land. It can be shown, however, that portions of the 
 continents are of extreme geological antiquity. There is 
 reason to believe, indeed, that the present terrestrial areas 
 have on the whole been land, or have, at least, never been 
 submerged beneath deep water, from the time of the earliest 
 stratified formations; and that, on the other hand, the 
 ocean-basins have always been vast areas of depression. 
 This subject will be discussed in subsequent pages. 
 
 In the New World, the continental trend is approxi- 
 
 19 Dittmar, op. cit. p. 222. 
 
 90 Different estimates have been made of the proportion of organic matter. 
 According to the researches of L. Schmelck (Norwegian North-Atlantic Expedi- 
 tion, 1876-78, Part. ix. p. 4), the proportion is 0-0025 gramme in 100 c.c. of
 
 GEOGNOSY 75 
 
 mately north and south; in the Old World, though less 
 distinctly marked, it ranges on the whole east and west. 
 The intimate relation which may be observed between this 
 general trend and the direction of mountain chains, is best 
 exhibited by the American continent. Europe and Africa 
 may be considered as forming, with Asia, the vast conti- 
 nental mass of the Old World. The existing severance of 
 Africa and Europe is of comparatively recent date. On the 
 other hand, Europe and Asia were not always so continuous 
 as at present. But even where the continents of the Old 
 World are separated by sea, the intervening hollows, though 
 now covered by ocean-water, must be regarded as essentially 
 part of the continental areas. Asia is linked with Australia 
 by a chain of islands. The great contrast between the Asi- 
 atic and Australian faunas, however, affords good grounds 
 for the belief that, at least for an enormous period of time, 
 Asia and Australia have been divided by an important 
 barrier of sea. 
 
 While any good map of the globe enables us to see at a 
 glance the relative positions and areas of the continents and 
 oceans, most maps fail to furnish any data by which the 
 general height or volume of a continent may be estimated. 
 As a rule, the mountain-chains are exaggerated in breadth, 
 and incorrectly indicated, while no attempt is made to dis- 
 tinguish between high plateaus and low plains. In North 
 America, for example, a continuous shaded ridge is placed 
 down the axis of the continent, and marked "Kooky Moun- 
 tains," while the vast level or gently rolling prairies are left 
 with no mark to distinguish them from the maritime plains 
 of the Eastern and Southern States. In reality there is no 
 such continuous mountain -chain. The so-called "Rocky 
 Mountains" consist of many independent and sometimes
 
 76 
 
 TEXT-BOOK OF GEOLOGY 
 
 widely separated ridges, having a general meridional trend, 
 and rising above a vast plateau, which is itself 4000 or 5000 
 feet in elevation. It is not these intermittent ridges which 
 really form the great mass of the land in that region, but the 
 widely extended lofty plateau, or rather succession of pla- 
 teaus, which supports them. In Europe, also, the Alps 
 form but a subordinate part of the total bulk of the land. 
 If their materials could be spread out over the continent, it 
 has been calculated that they would not increase its height 
 more than about twenty-one feet." 
 
 Attempts have been made to estimate the probable aver- 
 age height which would be attained if the various inequali- 
 ties of the land could be levelled down. Humboldt esti- 
 mated the mean height of Europe to be about 671, of Asia 
 1132, of North America 748, and of South America 1151 
 feet. 88 Herschel supposed the mean height of Africa to be 
 1800 feet." These figures, though based on the best data 
 available at the time, are no doubt much under the truth. 
 In particular, the average height assigned to North Amer- 
 ica is evidently far less than it should be; for the great 
 plains west of the Mississippi Valley reach an altitude of 
 about 5000 feet, and serve as the platform from which the 
 mountain ranges rise. The height of Asia also is obviously 
 
 SI M. De Lapparent ("Trait* de G^ologie, " 3d edit. p. 57) gives the follow- 
 ing estimate of relative heights and areas, the area below sea-level being taken 
 as 0-6 of the whole. 
 Zone I. (from sea-level to 200 metres) covers 34-7 % of the terrestrial surface 
 
 II. 
 III. 
 IV. 
 
 V. 
 VI. 
 VII. 
 
 200 
 
 500 ' 
 1000 ' 
 2000 ' 
 3000 ' 
 
 500 
 1000 
 
 3000 
 4000 
 
 above 4000 
 
 "Aise Centrale," torn. i. p. 168. 
 
 21-6 
 21-4 
 14-2 
 3-7 
 2-1 
 1-7 
 
 99-4 " 
 3 "physical Geography," p. 119.
 
 GEOGNOSY 77 
 
 much greater than this old estimate. Gr. Leipoldt has com- 
 puted the mean height of Europe to be 296-838 metres 
 (973-628 feet). 84 Prof. A. De Lapparent makes the mean 
 height of the land of the globe 2120 feet, and estimates the 
 mean height of Europe to be 958 feet, Asia 2884, Africa 
 1975, North America 1952, and South America 1762." Dr. 
 John Murray computes these heights as follows: Europe 
 939, Asia 3189, Africa 2021, North America 1888, South 
 America 2078, Australia 805 feet, general mean height of 
 land 2252 feet. 88 It is of some consequence to obtain as 
 near an approximation to the truth in this matter as may 
 be possible, in order to furnish a means of comparison be- 
 tween the relative bulk of different continents, and the 
 amount of material on which geological changes can be 
 effected. 
 
 The highest elevation of the surface of the land is the 
 summit of Mount Everest, in the Himalaya range (29,000 
 feet); the deepest depression not covered by water is that of 
 the shores of the Dead Sea (1300 feet below sea-level). 
 There are, however, many subaqueous portions of the land 
 which sink to greater depths. The bottom of the Caspian 
 Sea, for instance, lies about 3000 feet below the general sea- 
 level. The vertical difference between the highest point 
 of the land and the maximum known depth of the sea is 
 56,932 feet or nearly 11 miles. 
 
 There are two conspicuous junction- lines of the land 
 with its overlying and surrounding envelopes. First, with 
 
 24 "Die MittlereHoheEuropas," Leipzig, 1874. In this work the mean height 
 of Switzerland is put down as 1299-91 metres; Spanish peninsula, 700-60; Aus- 
 tria, 517-87; Italy, 517-17; Scandinavia, 428-10; Prance, 393 -84; Great Britain, 
 217-70; German Empire, 213-66; Russia, 167-09; Belgium, 163-36; Denmark 
 (exclusive of Iceland), 35-20; the Netherlands (exclusive of Luxemburg and the 
 tracts below sea-level), 9 '61. 
 
 25 "TraLte"," p. 56. 26 Scottish Geog. Mag. iv. (1888), 23.
 
 78 TEXT-BOOK OF GEOLOGY 
 
 the Air, expressed by the contours or relief of the land. 
 Second, with the Sea, expressed by coast-lines. 
 
 (1.) Con tours or Belief of the Land. While 
 the surface of the land presents endless diversities of de- 
 tail, its leading features may be generalized as mountains, 
 table-lands, and plains. 
 
 Mountains. The word "mountain" is, properly speak- 
 ing, not a scientific term. It includes many forms of ground 
 utterly different from each other in size, shape, structure, 
 and origin. It is popularly applied to any considerable emi- 
 nence or range of heights, but the height and size of the ele- 
 vated ground so designated vary indefinitely. In a really 
 mountainous country the word would be restricted to the 
 loftier masses of ground, while such a word as hill would 
 be given to the lesser heights. But in a region of low or 
 gently undulating land, where any conspicuous eminence 
 becomes important, the term mountain is lavishly used. 
 In Eastern America this habit has been indulged in to 
 such an extent, that what are, so to speak, mere hum- 
 mocks in the general landscape, are dignified by the 
 name of mountains. 
 
 It is hardly possible to give a precise scientific definition 
 to a term so vaguely employed in ordinary language. 
 When a geologist uses the word, he must either be con- 
 tent to take it in its familiar vague sense, or must add some 
 phrase defining the meaning which he attaches to it. He 
 finds that there are three leading and totally distinct types 
 of elevation which are all popularly termed mountains. 1. 
 Single eminences, standing alone upon a plain or table-land. 
 This is essentially the volcanic type. The huge cones of 
 Vesuvius, Etna, and Teneriffe, as well as the smaller ones 
 so abundant in volcanic districts, are examples of it. There
 
 GEOGNOSY 79 
 
 occur, however, occasional isolated eminences that stand up 
 as remnants of once extensive rock-formations. These have 
 no real analogy with volcanic elevations, but should be 
 classed under the next type. The remarkable buttes of 
 Western America are good illustrations of them. 2. Groups 
 of eminences connected at the sides or base, often forming 
 lines of ridge between divergent valleys, and owing their 
 essential forms not to underground structure so much as to 
 superficial erosion. Many of the more ancient uplands, 
 both in the Old World and the New, furnish examples of 
 this type, such as the Highlands of Scotland, the hills of 
 Cumberland and Wales, the high grounds between Bohemia 
 and Bavaria, the Laurentide Mountains of Canada, and the 
 Green and White Mountains of New England. 3. Lines of 
 lofty ridge rising into a succession of more or less distinct 
 summits, their general external form having relation to an 
 internal plication of their component rocks. These linear 
 elevations, whose existence and trend have been determined 
 immediately by subterranean movement, are the true moun- 
 tain-ranges of the globe. They may be looked upon as the 
 crests of the great waves into which the crust of the earth 
 has been thrown. All the great mountain-lines of the world 
 belong to this type. 
 
 Leaving the details of mountain-form to be described in 
 Book VII., we may confine our attention here to a few of 
 the more important general features. In elevations of the 
 third or true mountain type, there may be either one line or 
 range of heights, or a series of parallel and often coalescent 
 ranges. In the Western Territories of the United States, 
 the vast plateau has been, as it were, wrinkled by the uprise 
 of long intermittent ridges, with broad plains and basins be- 
 tween them. Each of these forms an independent moun-
 
 80 TEXT-BOOK OF GEOLOGY 
 
 tain-range. In the heart of Europe, the Bernese Oberland, 
 the Pennine, Lepontine, Ehaetic, and other ranges form one 
 great Alpine chain or system. 
 
 In a great mountain-chain, such as the Alps, Himalayas, 
 or Andes, there is one general persistent trend for the suc- 
 cessive ridges. Here and there, lateral offshoots may di- 
 verge, but the dominant direction of the axis of the main 
 chain is generally observed by its component ridges until 
 they disappear. Yet while the general parallelism is pre- 
 served, no single range may be traceable for more than a 
 comparatively short distance; it may be found to pass in- 
 sensibly into another, while a third may be seen to begin 
 on a slightly different line, and to continue with the same 
 dominant trend until it in turn becomes confluent. The 
 various ranges are thus apt to assume an arrangement en 
 echelon. 
 
 The ranges are separated by longitudinal valleys, that is, 
 depressions coincident with the general direction of the 
 chain. These, though sometimes of great length, are rela- 
 tively of narrow width. The valley of the Ehone, from the 
 source of the river down to Martigny, offers an excellent 
 example. By a second series of valleys the ranges are 
 trenched, often to a great depth, and in a direction transverse 
 to the general trend. The Ehone furnishes also an example 
 of one of these transverse valleys, in its course from Mar- 
 tigny to the Lake of Geneva. In most mountain regions, 
 the heads of two adjacent transverse valleys are often con- 
 nected by a depression or pass (col, joch). 
 
 A large block of mountain ground, rising into one or 
 more dominant summits, and more or less distinctly denned 
 by longitudinal and traverse valleys, is termed in French a 
 massif a word for which there is no good English equiva-
 
 GEOGNOSY 81 
 
 lent. Thus in the Swiss Alps we have the massifs of the 
 Grlarnisch, the Todi, the Matterhorn, the Jungfrau, etc. 
 
 Very exaggerated notions are common regarding the 
 angle of declivity in mountains. Sections drawn across 
 any mountain or mountain-chain on a true scale, that is, 
 with the length and height on the same scale, bring out the 
 fact that, even in the loftiest mountains, the breadth of base 
 is always very much greater than the height. Actual ver- 
 tical precipices are less frequent than is usually supposed, 
 and even when they do occur, generally form minor inci- 
 dents in the declivities of mountains. Slopes of more than 
 80 in angle are likewise far less abundant than casual tour- 
 ists believe. Even such steep declivities as those of 38 or 
 40 are most frequently found as talus-slopea at the foot of 
 crumbling cliffs, and represent the angle of repose of the 
 disintegrated debris. Here and there, where the blocks 
 loosened by weathering are of large size, they may accumu- 
 late upon each other in such a manner that for short dis- 
 tances the average angle of declivity may mount as high as 
 65. But such steep slopes are of limited extent. Declivi- 
 ties exceeding 40, and bearing a large proportion to the 
 total dimensions of hill or mountain, are always found to 
 consist of naked solid rock. In estimating angles of incli- 
 nation from a distance, the student will learn by practice 
 how apt is the eye to be deceived by perspective and to ex- 
 aggerate the true declivity, sometimes to mistake a horizon- 
 tal for a highly inclined or vertical line. The mountain 
 outline shown in Fig. 2 presents a slope of 25 between a 
 and 5, of 45 between b and c, of 17 between c and c?, of 40 
 between d and c, and of 70 between e and /. At a great 
 distance, or with bad conditions of atmosphere, these might 
 be believed to be the real declivities. Yet if the same an-
 
 82 TEXT-BOOK OF GEOLOGY 
 
 gles be observed in another way (as on a cottage roof at B), 
 we may learn that an apparently inclined surface may really 
 ^ be horizontal (as from a to b and from c to c), and 
 that by the effect of perspective, slopes may be 
 made to appear much steeper than they really are." 
 Much evil has resulted in geological research 
 from the use of exaggerated angles of slope in 
 sections and diagrams. It is therefore desirable 
 that the student should, from the beginning, 
 accustom himself to the drawing of outlines as 
 
 
 
 \ 
 
 Pig. 2. Angles of Slope where the eye may be deceived by per- 
 spective. (After Ruskin.) A, Mountain outline; B, The same 
 outline as shown by cottage roof 
 
 nearly as possible on a true scale. The accom- 
 panying section of the Alps by De la Beche 
 (Fig. 3) is of interest in this respect, as one of the 
 earliest illustrations of the advantage of con- 
 structing geological sections on a true scale as 
 to the relative proportions of height and length." 
 
 87 Mr. Ruskin has well illustrated this point. See "Modern 
 Painters," vol. iv. p. 183, whence the illustrations in the text 
 are taken. 
 
 28 ' 'Sections and Views, illustrative of Geological Phenomena," 
 1830. Geol. Observer, p. 646.
 
 GEOGNOSY 83 
 
 Table-lands or Plateaus are elevated regions of flat or un- 
 dulating country, rising to heights of 1000 feet and upward 
 above the level of the sea. They are sometimes bordered 
 with steep slopes, which descend from their edges, as the 
 table-land of the Spanish peninsula does into the sea. In 
 other cases, they gradually sink into the plains and have no 
 definite boundaries; thus the prairie-land west of the Mis- 
 souri slowly and imperceptibly ascends until it becomes a 
 vast plateau from 4000 to 5000 feet above the sea. Occa- 
 sionally a high table-land is encircled with lofty mountains, 
 as in those of Quito and Titicaca among the Andes, and that 
 of the heart of Asia; or it forms in itself the platform on 
 which lines of mountains stand, as in North America, where 
 the ranges included within the Rocky Mountains reach ele- 
 vations of from 10,000 to 14,000 feet above the sea, but not 
 more than from 5000 to 10,000 feet above the table-land. 
 
 Two types of table-land structure may be observed. 
 
 1. Table-lands consisting of level or gently undulated 
 sheets of rock, the general surface of the country corre- 
 sponding with that of the stratification. The Kocky Moun- 
 tain plateau is an example of this type, which may be called 
 that of Deposit, for the flat strata have been equably up- 
 raised nearly in the position in which they were deposited. 
 
 2. Table-lands formed out of contorted, crystalline, or other 
 rocks, which have been planed down by superficial agents. 
 This type, where the external form is independent of geo- 
 logical structure, may be termed that of Erosion. The 
 fjelds of Norway are portions of such a table-land. In 
 proportion to its antiquity, a plateau is trenched by run- 
 ning water into systems of valleys, until in the end it may 
 lose its plateau character and pass into the second type of 
 mountain-ground above described. This change has largely
 
 84 TEXT-BOOK OF GEOLOGY 
 
 altered the ancient table-land of Scandinavia, as will be 
 illustrated in Book VII. 
 
 Plains are tracts of lowland (under 1000 feet in height) 
 which skirt the sea-board of the continents and stretch in- 
 land up the river valleys. The largest plain in the world 
 is that which, beginning in the centre of the British Islands, 
 stretches across Europe and Asia. On the west, it is 
 bounded by the ancient table-lands of Scandinavia, Scot- 
 land and Wales on the one hand, and those of Spain, France 
 and Germany on the other. Most of its southern boundary 
 is formed by the vast belt of high ground which spreads 
 from Asia Minor to the east of Siberia. Its northern mar- 
 gin sinks beneath the waters of the Arctic Ocean. This 
 vast region is divided into an eastern and western tract by 
 the low chain of the Ural Mountains, south of which its 
 general level sinks, until underneath the Caspian Sea it 
 reaches a depression of about 3000 feet below sea-level. 
 Along the eastern sea-board of America lies a broad belt 
 of low plains, which attain their greatest dimensions in the 
 regions watered by the larger rivers. Thus they cover 
 thousands of square miles on the north side of the Gulf of 
 Mexico, and extend for hundreds of miles up the valley of 
 the Mississippi. Almost the whole of the valleys of the 
 Orinoco, Amazon and La Plata is occupied with vast plains. 
 
 From the evidence of upraised marine shells, it is certain 
 that large portions of the great plain of the Old World com- 
 paratively recently formed part of the sea-floor. It is like- 
 wise probable that the beds of some inclosed sea-basins, 
 such as that of the North Sea, have formerly been plains 
 of the dry land. 
 
 It is obvious, from their distribution along river- valleys, 
 and on the areas between the base of high grounds and the
 
 GEOGNOSY 85 
 
 sea, that plains are essentially areas of deposit. They are 
 the tracts that have received the detritus washed down from 
 the slopes above them, whether that detritus has originally 
 accumulated on the land or below the sea. Their surface 
 presents everywhere loose sandy, gravelly, or clayey forma- 
 tions, indicative of its comparatively recent subjection to 
 the operation of running water. 
 
 (2.) Coast-lines. A mere inspection of a map of the 
 globe brings before the mind the striking differences which 
 the masses of land present in their line of junction with 
 the sea. As a rule, the southern continents possess a more 
 uniform unindented coast-line than the northern. It has 
 been estimated that the ratios between area and coast-line 
 among the different continents, stand approximately as in 
 the following table: 
 
 ( Europe has 1 geographical mile of coast-line to 143 sq. m. of surface 
 
 Northern \ North America '' 
 
 ( Asia including the islands 
 Africa 
 
 ( Africa 
 
 < South Americ 
 
 ( Australia 
 
 Southern < South America 
 
 265 
 469 
 895 
 434 
 332 
 
 In estimating the relative potency of the sea and of the 
 atmospheric agents of disintegration, in the task of wearing 
 down the land, it is evidently of great importance to take 
 into account the amount of surface respectively exposed 
 to their operations. Other things being equal, there is 
 relatively more marine erosion in Europe than in North 
 America. But we require also to consider the nature of 
 the coast-line, whether flat and alluvial, or steep and rocky, 
 or with some intermediate blending of these two characters. 
 By attending to this point, we are soon led to observe such 
 great differences in the character of coast-lines, and such 
 an obvious relation to differences of geological structure, 
 on the one hand, and to diversities in the removal or de-
 
 86 TEXT-BOOK OF GEOLOGY 
 
 posit of material, on the other, as to suggest that the present 
 coast-lines of the globe cannot be aboriginal, but must be 
 referred to the operation of geological agents still at work. 
 This inference is amply sustained by more detailed investi- 
 gation. While the general distribution of land and water 
 must undoubtedly be assigned to terrestrial movements 
 affecting the solid globe, the present actual coasts of the 
 land have chiefly been produced by local causes. Head- 
 lands project from the land because, for the most part, they 
 consist of rock which has been better able to withstand the 
 shock of the breakers. Bays and creeks, on the other hand, 
 have been cut by the waves out of less durable materials. 
 Again, by the sinking of land, ranges of hills have become 
 capes and headlands, while the valleys have passed into 
 the condition of bays, inlets, or fjords. By the uprise of 
 the sea-bottom, tracts of low alluvial ground have been 
 added to the land. Hence, speculations as to the history 
 of the elevation of the land, based merely upon inferences 
 from the form of .coast-lines as expressed upon ordinary 
 maps, to be of real service, demand a careful scrutiny of 
 the actual coast-lines, and an amount of geological investi- 
 gation which would require long and patient toil for its 
 accomplishment. 
 
 Passing from the mere external form of the land to the 
 composition and structure of its materials, we may begin 
 by considering the general density of the entire globe, com- 
 puted from observations and compared with that of the 
 outer and accessible portion of the planet. Eeference has 
 already been made to the comparative density of the earth 
 among the other members of the solar system. In inquiries 
 regarding the history of our globe, the density of the whole
 
 GEOGNOSY 87 
 
 mass of the planet, as compared with water the standard 
 to which the specific gravities of terrestrial bodies are re- 
 ferred is a question of prime importance. Various methods 
 have been employed for determining the earth's density. 
 The deflection of the plumb-line on either side of a moun- 
 tain of known structure and density, the time of oscillation 
 of the pendulum at great heights, at the sea-level, and in 
 deep mines, and the comparative force of gravitation as 
 measured by the torsion balance, have each been tried with 
 the following various results: 
 
 Plumb-line experiments on Schichallien (Maskelyne and Playfair) gave 
 
 as the mean density of the earth 4-713 
 
 Do. on Arthur's Seat, Edinburgh (James) 5-316 
 
 Pendulum experiments on Mont Cenis (Carlini and Giulio) 4-950 
 
 Do. in Harton coal-pit, Newcastle (Airy) 6'565 
 
 Torsion balance experiments (Cavendish, 1798) 5-480 
 
 Do. do. (Reich, 1838) 5-49 
 
 Do. do. (Baily, 1843) 5-660 
 
 Do. do. (Corou and Bailie, 1872-73) 5-50-5-56 
 
 Though these observations are somewhat discrepant, we 
 may feel satisfied that the globe has a mean density neither 
 much more nor much less than 6 -5; that is to say, it is five 
 and a half times heavier than one of the same dimensions 
 formed of pure water. Now the average density of the 
 materials which compose the accessible portions of the 
 earth is between 2 '5 and 3; so that the mean density of 
 the whole globe is about twice as much as that of its outer 
 part. We might, therefore, infer that the inside consists 
 of much heavier materials than the outside, and conse- 
 quently that the mass of the planet must contain at least 
 two dissimilar portions an exterior lighter crust or rind, 
 and an interior heavier nucleus. But the effect of pressure 
 must necessarily increase the specific gravity of the interior, 
 as will be alluded to further on. 
 
 2. The Crust. It was formerly a prevalent belief that
 
 88 TEXT-BOOK OF GEOLOGY 
 
 the exterior and interior of the globe differed from each 
 other to such an extent that, while the outer parts were 
 cool and solid, the vastly more enormous inner intensely 
 hot part was more or less completely liquid. Hence the 
 term "crust" was applied to the external rind in the usual 
 sense of that word. This crust was variously computed to 
 be ten, fifteen, twenty, or more miles in thickness. In the 
 accompanying diagram (Fig. 4), for example, the thick line 
 forming the circle represents a relative thickness of 100 
 miles. There are so many proofs of enormous and wide- 
 spread corrugation of the 
 materials of the earth's 
 outer layers, and such 
 abundant traces of former 
 volcanic action, that geol- 
 ogists have naturally re- 
 garded the doctrine of a 
 thin crust over a liquid in- 
 terior as necessary for the 
 explanation of a large 
 class of terrestrial phe- 
 
 Fig. 4. Supposed Crust of the Earth, _, 
 
 100 Miles thick nomena. For reasons 
 
 which will be afterward given, however, this doctrine 
 has been opposed by eminent physicists, and is now aban- 
 doned by most geologists. Nevertheless the term "crust" 
 continues to be used, apart from all theory regarding the 
 nucleus, as a convenient word to denote those cool, upper 
 or outer layers of the earth's mass in the structure and 
 history of which, as the only portions of the planet accessi- 
 ble to human observation, lie the chief materials of geolog- 
 ical investigation. The chemical and mineral constitution 
 of the crust is fully discussed in later pages (p. 112 et seq.}.
 
 GEOGNOSY 89 
 
 3. The Interior or Nucleus. Though the mere outside 
 skin of our planet is all with which direct acquaintance can 
 be expected, the irregular distribution of materials beneath 
 the crust may be inferred from the present distribution of 
 land and water, and the observed differences in the amount 
 of deflection of the plumb-line near the sea and near moun- 
 tain-chains. The fact that the southern hemisphere is 
 almost wholly covered with water, appears only explica- 
 ble, as already remarked, on the assumption of an excess 
 of density in the mass of that half of the planet. The 
 existence of such a vast sheet of water as that of the 
 Pacific Ocean is to be accounted for, says Archdeacon 
 Pratt, by the presence of "some excess of matter in the 
 solid parts of the earth between the Pacific Ocean and 
 the earth's centre, which retains the water in its place, 
 otherwise the ocean would flow away to the other parts of 
 the earth." " The same writer points out that a deflection 
 of the plumb-line toward the sea, which has in a number 
 of cases been observed, indicates that "the density of the 
 crust beneath the mountains must be less than that below 
 the plains, and still less than that below the ocean-bed. 34 
 Apart, therefore, from the depressions of the earth's sur- 
 face, in which the oceans lie, we must regard the internal 
 density, whether of crust or nucleus, to be somewhat irreg- 
 ularly arranged there being an excess of heavy materials 
 in the water- hemisphere, and beneath the ocean-beds as 
 compared with the continental masses. 
 
 It has been argued from the difference between the 
 specific gravity of the whole globe and that of the crust, 
 
 99 "Figure of the Earth," 4th edit. p. 236. 
 
 30 Op. cit. p. 200. See also Herschel, "Phys. Geog." 13; O. Fisher, 
 Cambridge Phil. Trans, xii. part ii. ; "Physics of the Earth's Crust," p. 75. 
 Phil. Mag. July, 1886. Faye, Comptes reudus, cii. (1886), p. 651.
 
 TEXT-BOOK OF GEOLOGY 
 
 that the interior must consist of heavier material, and may 
 be metallic. But the effect of the enormous internal press- 
 ure, it might be supposed, should make the density of the 
 nucleus much higher, even if the interior consisted of 
 matter which, on the surface, would be no heavier than 
 that of the crust. In fact, we might, on the contrary, argue 
 for the probable comparative lightness of the substance 
 composing the nucleus. That the total density of the 
 planet does not greatly exceed its observed amount, may 
 indicate that some antagonistic force counteracts the effect 
 of pressure. The only force we can suppose capable of so 
 acting is heat, though to what extent this counterbalancing 
 takes place is still unknown. It must be admitted that we 
 are still in ignorance of the law that regulates the com- 
 pression of solids under such vast pressure as must exist 
 within the earth's interior. We know that gases and va- 
 pors may be compressed into liquids, sometimes even into 
 solids, and that in the liquid condition another law of com- 
 pressibility begins. We know also from experiment that 
 some substances have their melting-point raised by press- 
 ure. 81 It may be that the same effect takes place within 
 the earth; that pressure increasing inward to the centre of 
 the globe, while augmenting the density of each successive 
 shell, may retain the whole in a solid condition, yet at 
 temperatures far above the normal melting-points at the 
 surface. Hence, on this view of the matter, it is conceiv- 
 able that the difference between the density of the whole 
 globe and that of the crust may be due to pressure, rather 
 than to any essential difference of composition. Laplace 
 proposed the hypothesis that the increase of the square of 
 
 31 Under a pressure of 792 atmospheres, spermaceti has its melting-point 
 raised from 51* to 80 '2, and wax from 64 -5 to 80 '2.
 
 GEOGNOSY 91 
 
 the density is proportional to the increase of the pressure, 
 which gives a density of 8-23 at half the terrestrial radius 
 and of 10 '74 at the centre. From another law proposed by 
 Prof. Darwin, the density at half the radius is only 7-4, 
 but thence toward the centre increases rapidly up to infin- 
 ity." Dr. Pfaff believes that the mean terrestrial density 
 of 5*5 is not incompatible with the notion that the whole 
 globe consists of materials of the same density as the rocks 
 of the crust." It is possible that the gases dissolved in the 
 hot magma of the nucleus, with their very high tension, 
 may counteract the effects of compression and thus reduce 
 density. 
 
 Analogies, in the solar system, however, as well as the 
 actual structure of the rocky crust of the globe, suggest 
 that heavier metallic ingredients possibly predominate in 
 the nucleus. If the materials of the globe were once, as 
 they are believed to have been, in a liquid condition, they 
 would then doubtless be subject to internal arrangement, in 
 accordance with their relative specific gravities. We may 
 conceive that, as in the case of the sun, as well as of the 
 solar system generally (ante, p. 25), there would be, so long 
 as internal mobility lasted, a tendency in the denser ele- 
 ments of our planet to gravitate toward the centre, in the 
 lighter to accumulate outside. That a distribution of this 
 nature has certainly taken place to some extent, is evident 
 from the structure of the envelopes and crust. It is what 
 might be expected, if the constitution of the globe resem- 
 bles, on a small scale, the larger planetary system of which 
 
 32 See Fisher "Physics of Earth's Crust," 2d edit. chap. ii. Legendre sup- 
 posed that the density being 2-5 at the surface, it is 8-5 at half the length of the 
 radius and 11-3 at the centre. More recently E. Roche calculated these densi- 
 ties to be 2'1, 8-5 and 10-6 respectively. 
 
 33 "Allgemeine Geologie als exacte Wissenschaf t, " p. 42.
 
 92 TEXT-BOOK OF GEOLOGY 
 
 it forms a part. The existence even of a metallic interior 
 has been inferred from the metalliferous veins which tra- 
 verse the crust, and which are commonly supposed to 
 have been filled from below. 
 
 Evidence of Internal Heat. In the evidence 
 obtainable as to the former history of the earth, no fact 
 is of more importance than the existence of a high temper- 
 ature beneath the crust, which has now been placed beyond 
 all doubt. This feature of the planet's organization is made 
 clear by the following proofs: 
 
 (1.) Volcanoes. In many regions of the earth's surface, 
 openings exist from which steam and hot vapors, ashes and 
 streams of molten rock, are from time to time emitted. The 
 abundance and wide diffusion of these openings, inexplica- 
 ble by any mere local causes, must be regarded as indicative 
 of a very high internal temperature. If to the still active 
 vents of eruption, we add those which have formerly been 
 the channels of communication between the interior and 
 the surface, there are perhaps few large regions of the 
 globe where proofs of volcanic action cannot be found. 
 Everywhere we meet with masses of molten rock which 
 have risen from below, as if from some general reservoir. 
 The phenomena of active volcanoes are fully discussed in 
 Book III. Part I. 
 
 (2.) Hot Springs. Where volcanic eruptions have ceased, 
 evidence of a high internal temperature is still often to be 
 found in springs of hot water which continue for centuries 
 to maintain their heat. Thermal springs, however, are not 
 confined to volcanic districts. They sometimes rise even 
 in regions many hundreds of miles distant from any active 
 volcanic vent. The hot springs of Bath (temp. 120 Fahr.) 
 and Buxton (temp. 82 Fahr.) in England are fully 900 

 
 GEOGNOSY 93 
 
 miles from the Icelandic volcanoes on the one side, and 
 1100 miles from those of Italy and Sicily on the other. 
 
 (3.) Borings, Wells and Mines. The influence of the 
 seasonal changes of temperature extends downward from 
 the surface to a depth which varies with latitude, with 
 the thermal conductivity of soils and rocks, and perhaps 
 with other causes. The cold of winter and the heat of 
 summer may be regarded as following each other in suc- 
 cessive waves downward, until they disappear along a limit 
 at which the temperature remains constant. This zone of 
 invariable temperature is commonly believed to lie at a 
 depth of somewhere between 60 and 80 feet in temperate 
 regions. At Yakutsk in Eastern Siberia (lat. 62 N.), 
 however, as shown in a well-sinking, the soil is perma- 
 nently frozen to a depth of about 700 feet. 34 In Java, on 
 the other hand, a constant temperature is said to be met 
 with at a depth of only 2 or 3 feet." 
 
 It is a remarkable fact, now verified by observation all 
 over the world, that below the limit of the influence of 
 ordinary seasonal changes the temperature, so far as we yet 
 know, is nowhere found to diminish downward. It always 
 rises; and its rate of increment never falls much below the 
 average. The only exceptional cases occur under circum- 
 stances not difficult of explanation. On the one hand, the 
 neighborhood of hot-springs, of large masses of lava, or of 
 other manifestations of volcanic activity, may raise the 
 subterranean temperature much above its normal condi- 
 tion; and this augmentation may not disappear for many 
 thousand years after the volcanic activity has wholly 
 ceased, since the cooling down of a subterranean mass of 
 
 34 Helmereen, Brit. Assoc. Rep. 1871, p. 22. See vol. for 1886, p. 271. 
 45 Junghuhn's "Java," ii. p. 771.
 
 &t TEXT-BOOK OF GEOLOGY 
 
 lava must necessarily be a very slow process. Lord Kelvin 
 has even proposed to estimate the age of subterranean 
 masses of intrusive lava from their excess of temperature 
 above the normal amount for their isogeotherms (lines of 
 equal earth-temperature), some probable initial temperature 
 and rate of cooling being assumed. On the other hand, the 
 spread of a thick mass of snow and ice over any consider- 
 able area of the earth's surface, and its continuance there 
 for several thousand years, would so depress the isogeo- 
 therms that, for many centuries afterward, there would 
 be a fall of temperature for a certain distance downward. 
 At the present day, in at least the more northerly parts 
 of the northern hemisphere, there are such evidences of 
 a former more rigorous climate, as in the well-sinking 
 at Yakutsk just referred to. 88 Lord Kelvin (Sir W. Thom- 
 son) 87 has calculated that any considerable area of the 
 earth's surface covered for several thousand years by 
 snow or ice, and retaining, after the disappearance of 
 that frozen covering, an average surface temperature 
 of 13 C., "would during 900 years show a decreasing 
 temperature for some depth down from the surface, and 
 3600 years after the clearing away of the ice would still 
 show residual effect of the ancient cold, in a half rate of 
 augmentation of temperature downward in the upper strata, 
 gradually increasing to the whole normal rate, which would 
 be sensibly reached at a depth of 600 metres." 
 
 Beneath the limit to which the influence of the changes 
 of the seasons extends, observations all over the globe, and 
 
 36 Professor Prestwich (Inaugural Lecture, 1875, p. 45) has suggested that 
 to the more rapid refrigeration of the earth's surface during this cold period, and 
 to the consequent depression of the subterranean isothermal lines, the alleged 
 present comparative quietude of the volcanic forces is to be attributed, the inter- 
 nal heat not having yet recovered its dominion in the outer crust. 
 
 31 Brit. Assoc. Reports, 1876, Sections, p. 3. 

 
 GEOGNOSY 95 
 
 at many different elevations, give a rate of increase of tem- 
 perature downward, or "temperature gradient," which has 
 been usually taken to be 1 Fahr. for every 50 or 60 feet 
 of descent, this computation being based especially on ob- 
 servations in deep mines and borings. Professor Prestwich 
 concluded from a large series of observations collated by 
 him, that the average increment might be taken at 1 Fahr. 
 for every 45 feet." Observations taken in the extraordi- 
 narily deep boring at Schladebach, near Diirrenberg, 
 showed that in a depth of 5736 feet the average rise of 
 temperature was 1 Fahr. for every 65 feet. 89 According 
 to data collected by a Committee of the British Associa- 
 tion, the average gradient appears to be 1 Fahr. for every 
 64 feet, or i of a degree per foot. 
 
 Isogeotherms near the surface follow approximately the 
 contours of the surface, but are flatter than these, and "their 
 flattening increases as we pass to lower ones, until at a con- 
 siderable depth they become sensibly horizontal planes. 
 The temperature gradient is consequently steepest beneath 
 gorges and least steep beneath ridges. ' ' * 
 
 Irregularities in the Downward Incre- 
 ment of Heat. While there is everywhere a progres- 
 sive increase of temperature downward, its rate is by no 
 means uniform. The more detailed observations which 
 
 * Proc. Roy. Soc. xli. (1885), p. 55. 
 
 39 Brit. Assoc. 1889. Report of Underground Temperature Committee. 
 
 40 J. D. Everett, Brit. Assoc. 1879, Sections, p. 345. Compare also the elab- 
 orate observations made in the St. Gotuard Tunnel, F. Stapff, "Rapports, Con- 
 seil Fed. St. Golhard," vol. viii., and "Geologische Durchschnitte des Gothard 
 Tunnels" ; "Etude de I'Influence de la Chaleur de 1'Interieur de la Terre," etc., 
 Eevue. Univ. Mines, 1879-80. Min. Proc. N. England Inst. Mining-Mechan. 
 Engin. xxxii. (1883), p. 19. "Reports of Committee on Underground Tempera- 
 ture," Brit. Assoc. Rep. from 1868 onward, with summary of results in the vol- 
 ume for 1882. A voluminous and valuable collection of data bearing on this 
 subject was compiled by Prof. Prestwich and is published in Proc. Roy. Soc. 
 Xli. (1885), p. 1.
 
 96 
 
 TEXT-BOOK OF GEOLOGY 
 
 have been made in recent years have brought to light the 
 important act that considerable variations in the rate of 
 increase take place, even in the same bore. The tempera- 
 tures obtained at different depths in the Eose Bridge colliery 
 shaft, Wigan, for instance, read as in the following columns: 
 
 Depth in 
 Yards 
 
 558 . .. 
 
 Temperature 
 (Fahr.) 
 
 78 
 
 746 
 
 Tem^rat^ 
 89 
 
 605 
 
 80 
 
 761 
 
 90| 
 
 630 
 
 83 
 
 775 
 
 
 663 
 
 85 
 
 783 
 
 92 
 
 671 
 
 86 
 
 800 
 
 93 
 
 679 
 
 87 
 
 806 .. 
 
 .934 
 
 734 
 
 ....884 
 
 815.... 
 
 ....94 
 
 At La Chapelle, in an important well made for the water- 
 supply of Paris, observations have been taken of the tem- 
 perature at different depths, as shown in the subjoined 
 
 table: 41 
 
 pthin 
 letres 
 
 100 
 
 Temperature 
 (tfahr.) 
 59-5 
 
 Depth in 
 
 Metres 
 
 500 
 
 Temporal! 
 (rfahr.) 
 72-6 
 
 200 
 
 .. 61'8 
 
 600 
 
 75-0 
 
 300 
 
 65*5 
 
 660 
 
 76-0 
 
 400 
 
 ....69-0 
 
 
 
 In drawing attention to the foregoing temperature- 
 observations at the Hose Bridge colliery the deepest 
 mine in Great Britain Prof. Everett points out that, as- 
 suming the surface temperature to be 49 Fahr., in the 
 first 558 yards, the rate of rise of temperature is 1 for 
 67-7 feet; in the next 257 yards it is 1 in 48-2 feet; in 
 the portion between 605 and 671 yards a distance of only 
 198 feet it is 1 in 33 feet; in the lowest portion of 432 
 feet it is 1 in 54 feet. 48 When such irregularities occur 
 in the same vertical shaft, it is not surprising that the 
 average should vary so much in different places. 
 
 41 Brit. Assoc. Eep. 1873, Sections, p. 254. 
 
 42 Brit. Assoc. Rep. 1870, Sections, p. 31.
 
 GEOGNOSY 9 
 
 There can be little doubt that one main cause of these 
 variations is to be sought in the different thermal conduc- 
 tivities of the rocks of the earth's crust. The first accurate 
 measurements of the conducting powers of rocks were made 
 by the late J. D. Forbes at Edinburgh (1837-1846). He 
 selected three sites for his thermometers, one in "trap- 
 rock" (a porphyrite of Lower Carboniferous age), one in 
 loose sand, and one in sandstone, each set of instruments 
 being sunk to depths of 8, 6, 12 and 24 French feet from 
 the surface. He found that the wave of summer heat 
 reached the bulb of the deepest instrument (24 feet) on 
 4th January in the trap-rock, on 25th December in the 
 sand, and on 8d November in the sandstone, the trap-rock 
 being the worst conductor and the solid sandstone by far 
 the best. 48 
 
 As a rule, the lighter and more porous rocks offer the 
 greatest resistance to the passage of heat, while the more 
 dense and crystalline offer the least resistance. The resist- 
 ance of opaque white quartz is expressed by the number 
 114, that of basalt stands at 273, while that of cannel coal 
 stands very much higher at 1638, or more than thirteen 
 times that of quartz. 44 
 
 It is evident also, from the texture and structure of most 
 rocks, that the conductivity must vary in different direc- 
 tions through the same mass, heat being more easily con- 
 ducted along than across the "grain," the bedding, and the 
 other numerous divisional surfaces. Experiments have been 
 made to determine these variations in a number of rocks. 
 Thus the conductivity in a direction transverse to the divi- 
 
 43 Trans. Roy. Soc. Edin. xvi. p. 211. 
 
 44 Herschel and Lebour (British Association Committee on Thermal Conduc- 
 tivities of Rocks), Brit. Assoc. Rep. 1876, p. 69. The final Report is in the vol 
 for 1881. 
 
 GEOLOGY Vol. XXIX 6
 
 98 TEXT-BOOK OF GEOLOGY 
 
 sional planes being taken as unity, the conductivity parallel 
 with these planes was found in a variety of magnesian schist 
 to be 4-028. In certain slates and schistose rocks from cen- 
 tral France, the ratio varied from 1 : 2-56 to 1 : 8-952. 
 Hence in such fissile rocks as slate and mica-schist, heat 
 may travel four times more easily along the planes of cleav- 
 age or foliation than across them. 4 * 
 
 In reasoning upon the discrepancies in the rate of in- 
 crease of subterranean temperatures, we must also bear in 
 mind that convection by percolating streams of water must 
 materially affect the transference of heat from below." Cer- 
 tain kinds of rock are more liable than others to be charged 
 with water, and, in almost every boring or shaft, one or 
 more horizons of such water-bearing rocks are met with. 
 The effect of interstitial water is to diminish thermal resist- 
 ance. Dry red brick has its resistance lowered from 680 to 
 405 by being thoroughly soaked in water, its conductivity 
 being thus increased 68 per cent. A piece of sandstone has 
 its conductivity heightened to the extent of 8 per cent by 
 being wetted. 47 Q, 
 
 Mallet contended that the variations in the amount of in- 
 crease in subterranean temperature are too great to permit 
 us to believe them to be due merely to differences in the 
 transmission of the general internal heat, and that they 
 point to local accessions of heat arising from transforma- 
 tion of the mechanical work of compression, which is due 
 
 45 Report of Committee on Thermal Conduclivities of Rocks, Brit. Assoc. Rep. 
 1875, p. 61. Jannettaz, Bull. Soc. Geol. France (April-June, 1874), ii. p. 264. 
 This observer has carried out a series of detailed researches on the propagation 
 of heat through rocks which will be found in Bull. Soc. Geol. France, tomes 
 i.-ix. (3d series). 
 
 48 In the great bore of Sperenberg (4172 feet, entirely in rock-salt, except 
 the first 283 feet) there is evidence that the water near the top is warmed 4^ 
 Pahr. by convection. Brit. Assoc. 1882, p. 78. 
 
 47 Herschel and Lebour, Brit. Assoc. Rep. 1875, p. 68.
 
 GEOGNOSY 99 
 
 to the constant cooling and contraction of the globe. 48 But 
 it may be replied that these variations are not greater than, 
 from the known divergences in the conductivities of rocks, 
 they might fairly be expected to be. 
 
 Probable Condition of the Earth's Inte- 
 rior. Various theories have been propounded on this sub- 
 ject. There are only three which merit serious considera- 
 tion. (1.) One of these supposes the planet to consist of a 
 solid crust and a molten interior. (2.) The second holds 
 that, with the exception of local vesicular spaces, the globe 
 is solid and rigid to the centre. (3.) The third contends 
 that while the mass of the globe is solid, there lies a liquid 
 substratum beneath the crust. 
 
 1. The arguments in favor of internal liquidity may be 
 summed up as follows, (a.) The ascertained rise of tem- 
 perature inward from the surface is such that, at a very 
 moderate depth, the ordinary melting-point of even the most 
 refractory substances would be reached. At 20 miles the 
 temperature, if it increases progressively, as it does in the 
 depths accessible to observation, must be about 1760 Fahr. ; 
 at 60 miles it must be 4600, or far higher than the fusing- 
 point even of so stubborn a metal as platinum, which melts 
 at 3080 Fahr. 4 * (6.) All over the world volcanoes exist 
 from which steam and torrents of molten lava are from time 
 to time erupted. Abundant as are the active volcanic vents, 
 they form but a small proportion of the whole which have 
 been in operation since early geological time. It has beea 
 inferred, therefore, that these numerous funnels of comma- 
 
 48 "Volcanic Energy," Phil. Trans. 1876. 
 
 49 But Lord Kelvin (Sir W. Thomson) has shown that if the rate of increase 
 of temperature is taken to be 1 for every 51 feet for the first 100,000 feet, it 
 will begin to diminish below that limit, being only 1 in 2550 feet at 800,000 
 feet, and then rapidly lessening. Trans. Boy. Soc. Edin. xxiii. p. 163.
 
 100 TEXT-BOOK OF GEOLOGY 
 
 nication with the heated interior could not have existed and 
 poured forth such a vast amount of molten rock, unless thej 
 drew their supplies from an immense internal molten nu- 
 cleus, (c.) When the products of volcanic action from dif- 
 ferent and widely-separated regions are compared and ana- 
 lyzed, they are found to exhibit a remarkable uniformity of 
 character. Lavas from Vesuvius, from Hecla, from the 
 Andes, from Japan, and from New Zealand present such 
 an agreement in essential particulars as, it is contended, can 
 only be accounted for on the supposition that they have all 
 emanated from one vast common source. 60 (c?.) The abun- 
 dant earthquake-shocks which affect large areas of the globe 
 are maintained to be inexplicable unless on the supposition 
 of the existence of a thin and somewhat flexible crust. 
 These arguments, it will be observed, are only of the na- 
 ture of inferences drawn from observations of the present 
 constitution of the globe. They are based on geological 
 data, and have been frequently urged by geologists as sup- 
 porting the only view of the nature of the earth's interior, 
 supposed by them to be compatible with geological evi- 
 dence. 
 
 2. The arguments in favor of the internal solidity of the 
 earth are based on physical and astronomical considerations 
 of the greatest importance. They may be arranged as fol- 
 lows: 
 
 (a.) Argument from precession and nutation. The prob- 
 lem of the internal condition of the globe was attacked as 
 far back as the year 1839 by Hopkins, who calculated how 
 far the planetary motions of precession and nutation would 
 be influenced by the solidity or liquidity of the earth's into- 
 
 See D. Forbes, Popular Science Review, April, 1869.
 
 GEOGNOSY 101 
 
 rior. He found that the processional and nutational move- 
 ments could not possibly be as they are, if the planet con- 
 sisted of a central core of molten rock surrounded with a 
 crust of twenty or thirty miles in thickness; that the least 
 possible thickness of crust consistent with the existing 
 movements was from 800 to 1000 miles; and that the whole 
 might even be solid to the centre, with the exception of 
 comparatively small vesicular spaces filled with melted 
 rock." 
 
 M. Delaunay" threw doubt on Hopkins' views, and sug- 
 gested that, if the interior were a mass of sufficient viscos- 
 ity, it might behave as if it were a solid, and thus the 
 phenomena of precession and nutation might not be af- 
 fected. Lord Kelvin (Sir W. Thomson), who had already 
 arrived at the conclusion that the interior of the globe must 
 be solid, and acquiesced generally in Hopkins' conclusions, 
 remarked that the hypothesis of a viscous and quasi-rigid 
 interior "breaks down when tested by a simple calculation 
 of the amount of tangential force required to give to any 
 globular portion of the interior mass the precessional and 
 nutational motions which, with other physical astronomers, 
 M. Delaunay attributes to the earth as a whole.*' He held 
 the earth's crust down to depths of hundreds of kilometres 
 to be capable of resisting such a tangential stress (amounting 
 to nearly ith of a gramme weight per square centimetre) as 
 would with great rapidity draw out of shape any plastic sub- 
 stance which could properly be termed a viscous fluid, and 
 
 51 Phil. Trans. 1839, p. 381; 1840, p. 193; 1842, p. 43; Brit. Assoc. 1847. 
 
 52 In a paper on the hypothesis of the interior fluidity of the globe, Comptes 
 rendus, July 13, 1868. Geol. Mag. v. p. 507. See also H. Hennessy, Comptes 
 rendus, March 6, 1871, Geol. Mag. viii. p. 216. Nature, xv. p. 18. 0. Fisher, 
 'Physics of the Earth's Crust," 2d Edition, 1889. 
 
 53 Nature, February 1, 1872.
 
 102 TEXT-BOOK OF GEOLOGY 
 
 he concluded "that the rigidity of the earth's interior sub- 
 stance could not be less than a millionth of the rigidity of 
 glass without very sensibly augmenting the lunar nineteen- 
 yearly nutation.' 4 
 
 In Hopkins' hypothesis he assumed the crust to be in- 
 finitely rigid and unyielding, which is not true of any ma- 
 terial substance. Lord Kelvin subsequently returning to 
 the problem, in the light of his own researches in vortex- 
 motion, found that, while the argument against a thin crust 
 and vast liquid interior is still invincible, the phenomena of 
 precession and nutation do not decisively settle the question 
 of internal fluidity, as Hopkins, and others following him, 
 had believed, though the solar semi-annual and lunar fort- 
 nightly nutations absolutely disprove the existence of a thin 
 rigid shell full of liquid. If the inner surface of the crust 
 or shell were rigorously spherical, the interior mass of sup- 
 posed liquid could experience no processional or nutational 
 influence, except in so far as, if heterogeneous in composi- 
 tion, it might suffer from external attraction due to non- 
 sphericity of its surfaces of equal density. But "a very 
 slight deviation of the inner surface of the shell from per- 
 fect sphericity would suffice, in virtue of the quasi-rigidity 
 due to vortex-motion, to hold back the shell from taking 
 sensibly more precession than it would give to the liquid, 
 and to cause the liquid (homogeneous or heterogeneous) and 
 the shell to have sensibly the same precessional motion as if 
 the whole constituted one rigid body."" The problem pre- 
 sented by the precession of a viscous spheroid has more re- 
 cently been discussed by Prof. George Darwin, who arrives 
 at results nearly the same as those announced by Lord Kel- 
 
 54 Loc. ctt. p. 258. 
 
 55 Lord Kelvin (Sir W. Thomson), Brit. Assoc. Rep. 1876, Sections, p. 5.
 
 GEOGNOSY 103 
 
 vin regarding the slight difference between the precession of 
 a fluid and a rigid spheroid." 
 
 The assumption of a comparatively thin crust requires 
 that the crust shall have such perfect rigidity as is possessed 
 by no known substance. The tide-producing force of the 
 moon and sun exerts such a strain upon the substance of 
 the globe, that it seems in the highest degree improbable 
 that the planet could maintain its shape as it does unless the 
 supposed crust were at least 2000 or 2500 miles in thick- 
 ness.* 7 That the solid mass of the earth must yield to this 
 strain is certain, though the amount of deformation is so 
 slight as to have hitherto escaped all attempts to detect it." 
 Had the rigidity been even that of glass or of steel, the de- 
 formation would probably have been by this time detected, 
 and the actual phenomena of precession and nutation, as 
 well as of the tides, would then have been very sensibly 
 diminished. M The conclusion is thus reached that the mass 
 of the earth "is on the whole more rigid certainly than a 
 continuous solid globe of glass of the same diameter." 6 * 
 
 (ft.) Argument from the tides. The phenomena of the 
 oceanic tides show that the earth acts as a rigid body either 
 solid to the centre, or possessing so thick a crust (2500 miles 
 or more) as to give to the planet practical solidity. Lord 
 Kelvin remarks that "were the crust of continuous steel and 
 500 kilometres thick, it would yield very nearly as much as 
 if it were India-rubber to the deforming influences of cen- 
 trifugal force, and of the sun's and moon's attractions." It 
 would yield, indeed, so freely to these attractions "that it 
 
 56 Phil. Trans. 1879, Part 2, p. 464. 
 
 61 Lord Kelvin, Proc. Roy. Soc. April, 1862. 
 
 58 See Association Francaise pour 1'Avancement des Sciences, v. p. 281. 
 
 59 Lord Kelvin, loc. cit. 
 
 60 Ibid. Trans. Roy. Soc. Edin. xxiii. p. 157.
 
 104 TEXT-BOOK OF QEOLOQY 
 
 would simply carry the waters of the ocean up and down 
 with it, and there would be no sensible tidal rise and fall of 
 water relatively to land." Prof. Gr. II. Darwin, in the series 
 of papers already referred to, has investigated mathemati- 
 cally the bodily tides of viscous and semi-elastic spheroids, 
 and the character of the ocean tides on a yielding nucleus.** 
 His results tend to increase the force of Sir "W illiam Thom- 
 son's argument, since they show that "no very considerable 
 portion of the interior of the earth can even distantly ap- 
 proach the fluid condition," the effective rigidity of the 
 whole globe being very great. 
 
 (c.) Argument from relative densities of melted and solid 
 rock. The two preceding arguments must be considered 
 decisive against the hypothesis of a thin shell or crust cov- 
 ering a nucleus of molten matter. It has been further 
 urged, as an objection to this hypothesis, that cold solid 
 rock is more dense than hot melted rock, and that even if 
 a thin crust were formed over the central molten globe it 
 would immediately break up and the fragments would sink 
 toward the centre." Recent experiments show that diabase 
 (of density 8*017) contracts nearly 4 per cent on solidifica- 
 tion, and that the resulting homogeneous glass has a density 
 of only 2-717. 64 As has been already pointed out, the spe- 
 cific gravity of the interior is at least twice as much as that 
 of the visible parts of the crust. If this difference be due, 
 not merely to the effect of pressure, but to the presence in the 
 interior of intensely heated metallic substances, we cannot 
 
 61 Lord Kelvin, Brit. Assoc. Rep. 1876, Sections, p. 7. 
 
 62 Phil. Trans. 1879, Part 2. See also Brit. Assoc. Rep. 1882, Sects, p. 473. 
 
 63 This objection has been repeatedly urged by Lord Kelvin. See Trans. 
 Roy. Soc. Edin. xxiii. p. 157; and Brit. Assoc. Rep. 1870, Sections, p. 7. 
 
 64 C. Barus, Phil. Mag. 1893, p. 174. It is nevertheless true that, from a 
 cause merely mechanical, pieces of the original cold rock, though so much 
 denser, will iloat for a time on the melted material. Ib. p. 189.
 
 GEOGNOSY 105 
 
 suppose that solidified portions of such rocks as granite and 
 the various lavas could ever have sunk into the centre of the 
 earth, so as to build up there the honey-combed cavernous 
 mass which might have served as a nucleus in the ultimate 
 solidification of the whole planet. If the earliest formed 
 portions of the comparatively light crust were denser than 
 the underlying liquid, they would no doubt descend until 
 they reached a stratum with specific gravity agreeing with 
 their own, or until they were again melted." 
 
 3. Hypothesis of a liquid substratum between a solid nucleus 
 and the crust. Since the early and natural belief in the 
 liquidity of the earth's interior has been so weightily op- 
 posed by physical arguments, geologists have endeavored 
 to modify it in such a way as, if possible, to satisfy the re- 
 quirements of physics, while at the same time providing an 
 adequate explanation of the corrugation of the earth's crust, 
 the phenomena of volcanoes, etc." The hypothesis has 
 been proposed of "a rigid nucleus nearly approaching the 
 size of the whole globe, covered by a fluid substratum of no 
 great thickness, compared with the radius, upon which a 
 crust of lesser density floats in a state of equilibrium." The 
 nucleus is assumed to owe its solidity to "the enormous 
 pressure of the superincumbent matter, while the crust owes 
 its solidity to having become cool. The fluid substratum is 
 
 M See D. Forbes, Geol. Mag. vol. iv. p. 435. The evidence for the internal 
 solidity of the earth is criticised by Dr. M. E. Wadsworth in the American 
 Naturalist, 1884. 
 
 64 See Dana in Silliman's Journal, iii. (1847), p. 147. Amer. Journ. Science 
 (1873). The hypothesis of a fluid substratum has been advocated by Shaler. 
 Proc. Bost. Nat. Hist. Soc. xi. (1868), p. 8. Geol. Mag. v. p. 511. J. Le Conte, 
 Amer. Journ. Sci. 1872, 1873. 0. Fisher, Geol. Mag. v. (new series), pp. 291 
 and 551. "Physics of the Earth's Crust," 1883. [This author in his second 
 edition modifies this view.] Hill, Geol. Mag. v. (new series), pp. 262, 479. The 
 idea of a viscous layer between the solidifying central mass and the crust was 
 present in Hopkins' mind. Brit. Assoc. 1848, Reports, p. 48.
 
 106 TEXT-BOOK OF GEOLOGY 
 
 not under sufficient pressure to be rendered solid, and is 
 sufficiently hot to be fluid, being probably more viscous in 
 its lower portion through pressure and likewise passing into 
 a viscous state in its upper parts through cooling, until it 
 joins the crust."' 7 The contraction and consolidation of this 
 substratum are assumed as the explanation of the plication 
 which the crust has certainly undergone. 
 
 It must be admitted that the widespread proofs of great 
 crumpling of the rocks of the crust present a difficulty, for 
 they indicate a capability of yielding to strain such as has 
 been supposed impossible in a globe possessing on the 
 whole the rigidity of steel or glass. But this difficulty 
 may be more formidable in appearance than in reality. 
 The earth must certainly possess sjich a degree of rigidity 
 as to resist tidal deformation. Prof. Darwin has calculated 
 the limiting rigidity in the materials of the earth which is 
 necessary to prevent the weight of mountains and continents 
 from reducing them to the fluid condition or else cracking, 
 and has found that these materials must be as strong as 
 granite 1000 miles below the surface, or else much stronger 
 than granite near the surface." But high rigidity, that is, 
 elasticity of form, is not contradictory of plasticity. Even 
 bodies like steel may, under suitable stress, be made to flow 
 like butter (see postea, Book III. Part I. Sect. iv. 3). 
 While, therefore, the earth may possess as a whole the 
 rigidity of steel, there seems no reason why, under suffi- 
 cient strain, the outer portions may not be plicated or 
 even reduced to the fluid condition. It is important "to 
 distinguish viscosity, in which flow is caused by infini- 
 tesimal forces, from plasticity, in which permanent distor- 
 
 61 Fisher, "Physics of Earth's Crust," 1st edit. p. 269. 
 68 Proc. Roy, Soc. 1881, p. 432.
 
 GEOGNOSY 107 
 
 tion or flow only sets in when the stresses exceed a certain 
 limit." 69 
 
 In speculating on the plication of the earth's crust, we 
 ought not to forget that, from the earliest times, the exist- 
 ing continental regions seem to have specially suffered from 
 the efforts of the planet to adjust its external form to its 
 diminishing diameter and lessening rapidity of rotation. 
 They have served as lines of relief from the strain of com- 
 pression during many successive epochs. It is along their 
 axial lines their long dominant mountain-ranges, that we 
 should naturally look for evidence of corrugation. Away 
 from these lines of weakness the ground has been upraised 
 for thousands of square miles without plication of the rocks, 
 as in the instructive region of the Western Territories of 
 North America. Nor is there any proof that corrugation 
 takes place beneath the great oceanic areas of subsidence. 
 
 It appears highly probable that the substance of the 
 earth's interior is at the melting-point proper for the press- 
 ure at each depth. Any relief from pressure, therefore, 
 may allow of the liquefaction of the matter so relieved. 
 Such relief is doubtless afforded by the corrugation of 
 mountain-chains and other terrestrial ridges. And it is 
 in these lines of uprise that volcanoes and other manifes- 
 tations of subterranean heat actually show themselves. 
 
 4. Age of the Earth and Measures of Geological Tipie. 
 The age of our planet is a problem which may be at- 
 tacked either from the geological or physical side. 
 
 1. The geological arguments rest chiefly upon 
 the observed rates at which geological changes are being 
 effected at the present time, and is open to the obvious 
 preliminary objection that it assumes the existing rate of 
 
 69 Prof. Darwin in a letter to the author, 9th January, 1884.
 
 108 TEXT-BOOK OF GEOLOGY 
 
 change as the measure of past revolutions an assumption, 
 however, which may be erroneous, for the present may be 
 a period when all geological events march forward more 
 slowly than they used to do. The argument proceeds on 
 data partly of a physical and partly of an organic kind, 
 (a.) The physical evidence is derived from such facts as 
 the observed rates at which the surface of a country 
 is lowered by rain and streams, and new sedimentary de- 
 posits are formed. These facts will be more particularly 
 dwelt upon in later sections of this work. If we assume 
 that the land has been worn away, and that stratified 
 deposits have been laid down, nearly at the same rate as 
 at present, then we must admit that the stratified portion 
 of the crust of the earth must represent a very vast period 
 of time. 70 (6.) On the other hand, human experience, so far 
 as it goes, warrants the belief that changes in the organic 
 world proceed with extreme slowness. Yet in the stratified 
 rocks of the terrestrial crust we have abundant proof that 
 the whole fauna and flora of the earth's surface have passed 
 through numerous cycles of revolution species, genera, 
 families, orders, appearing and disappearing many times 
 in succession. On any supposition, it must be admitted 
 that these vicissitudes in the organic world can only have 
 
 10 Dr. Croll put this period at not less, but possibly much more, than 60 mil- 
 lion years. Dr. Haughton gives a much more extended period. Estimating the 
 present rate of deposit of strata at 1 foot in 8616 years, assuming the former 
 rate to have been ten times more rapid, or 1 foot in 861 '6 years, and taking the 
 thickness of the stratified rocks of the earth's crust at 177,200 feet, he obtains 
 a minimum of 200,000,000 years for the whole duration of geological time: "Six 
 Lectures on Physical Geography," 1880, p. 94. Dr. Haughton has also pro- 
 posed another geological measure of past time, based upon the assumed effects 
 of continental upheaval (Proc. Roy. Soc. xxvi. (1877), p. 534). But Prof Dar- 
 win has shown it to be inadmissible. (Op. cit. xxvii. (1878), p. 179.) For vari- 
 ous opinions regarding geological measures of time see J. Phillips, Brit. Assoe. 
 1864: Croll, Phil. Mag. 1868: T. McK. Hughes, Proc. Roy. Inst. Great Britain, 
 March 24, 1876: Dupont, Bull. Acad. Roy. Belgique, viii. (1834): T. Mellard 
 Reade, Quart. Journ. Geol. Soc. 1888, p. 291.
 
 GEOGNOSY 109 
 
 been effected with the lapse of vast periods of time, though 
 no reliable standard seems to be available whereby these 
 periods are to be measured. The argument from geological 
 evidence indicates an interval of probably not much less 
 than 100 million years since the earliest forms of life ap- 
 peared upon the earth, and the oldest stratified rocks began 
 to be laid down. 
 
 2. The physical argument as to the age of our 
 planet is based by Lord Kelvin upon three kinds of evi- 
 dence : (1) the internal heat and rate of cooling of the earth ; 
 (2) the tidal retardation of the earth rotation ; and (3) the 
 origin and age of the sun's heat. 
 
 (1.) Applying Fourier's theory of thermal conductivity, 
 he pointed out as far back as the year 1862, that in the 
 known rate of increase of temperature downward beneath 
 the surface, and the rate of loss of heat from the earth, we 
 have a limit to the antiquity of the planet. He showed, from 
 the data available at the time, that the superficial consolida- 
 tion of the globe could not have occurred less than 20 mil- 
 lion years ago, or the underground heat would have been 
 greater than it is; nor more than 400 million years ago, 
 otherwise the underground temperature would have shown 
 no sensible increase downward. He admitted that very 
 wide limits were necessary. In subsequently discussing 
 the subject, he inclined rather toward the lower than the, 
 higher antiquity, but concluded that the limit, from a con- 
 sideration of all the evidence, must be placed within some 
 such period of past time as 100 millions of years. He would 
 now restrict the time to about 20 millions. 71 
 
 11 Trans. Roy. Soc. Edin. xxiii. p. 157. Trans. Geol. Soc. Glasgow, iii. p. 25. 
 "Popular Lectures and Addresses," 2d edit. (1891), p. 397. Prof. Tail reduces 
 the period to 10 or 15 millions. "Recent Advances in Physical Science," 
 p. 167.
 
 110 TEXT-BOOK OF GEOLOGY 
 
 (2.) The reasoning from tidal retardation proceeds on the 
 admitted fact that, owing to the friction of the tide-wave, 
 the rotation of the earth is retarded, and is therefore slower 
 now than it must have been at one time. Lord Kelvin 
 contends that had the globe become solid some 10,000 
 million years ago, or indeed any high antiquity beyond 
 100 million years, the centrifugal force due to the more 
 rapid rotation must have given the planet a very much 
 greater polar flattening than it actually possesses. He ad- 
 mits, however, that though 100 million years ago that force 
 must have been about 3 per cent greater than now, yet 
 "nothing we know regarding the figure of the earth and 
 the disposition of land and water would justify us in say- 
 ing that a body consolidated when there was more centrif- 
 ugal force by 3 per cent than now, might not now be in all 
 respects like the earth, so far as we know it at present." " 
 
 (3.) The third kind of evidence leads to results similar 
 to those derived from the two previous lines of reasoning. 
 It is based upon calculations as to the amount of heat that 
 would be available by the falling together of masses from 
 space, which gave rise by their impact to our sun, and the 
 rate at which this heat has been radiated. Assuming that 
 the sun has been cooling at a uniform rate, Prof. Tait con- 
 cludes that it cannot have supplied the earth, even at the 
 present rate, for more than about 15 or 20 million years. 7 * 
 Lord Kelvin also believes that the sun's light will not last 
 more than 6 or 6 millions of years longer. 7 * 
 
 TO Trans. Geol. Soc. Glasgow, iii. p. 16. Prof. Tait, in repeating this argu- 
 ment, concludes that, taken in connection with the previous one, "it probably 
 reduces the possible period which can be allowed to geologists to something less 
 than 10 millions of years." "Recent Advances," p. 174. Compare Newcombe, 
 "Popular Astronomy," p. 505. 
 
 Op. cit. p. 174. M "Popular Lectures," etc. p. 397.
 
 GEOGNOSY HI 
 
 There can be no doubt that the demands of the earlier 
 geologists for an unlimited duration of past time, for the 
 accomplishment of geological history, were extravagant and 
 unnecessary. But it may be questioned how far the recent 
 limitation of time proposed from physical considerations 
 are really founded on well-established facts. The argument 
 from the geological record in favor of a much longer period 
 than physicists are disposed to concede is so strong that 
 one is inclined to believe that these writers have overstated 
 their case. The evidence from the nature of the sedimen- 
 tary rocks, and from the succession of organic remains in 
 these rocks, appears to me to demand an amount of time 
 not far short of the hundred millions of years originally 
 granted by Lord Kelvin." 
 
 PART II. AN ACCOUNT OF THE COMPOSITION OP THE 
 EARTH'S CRUST MINERALS AND BOCKS 
 
 The earth's crust is composed of mineral matter in va- 
 rious aggregates included under the general term Bock. 
 A rock may be denned as a mass of matter composed of 
 one or more simple minerals, having usually a variable 
 chemical composition, with no necessarily symmetrical ex- 
 ternal form, and ranging in cohesion from mere loose debris 
 up to the most compact stone. Granite, lava, sandstone, 
 limestone, gravel, sand, mud, soil, marl and peat, are all 
 recognized in a geological sense as rocks. The study of 
 rocks is known as Lithology, Petrography or Petrology. 
 
 It will be most convenient to treat 1st, of the general 
 chemical constitution of the crust; 2d, of the minerals of 
 
 15 I have touched on this question in my Presidential Address to the British 
 Association 1892. But aee a paper by Mr. Clarence King, Amer. Journ. Set 
 xiy. (1893).
 
 112 TEXT-BOOK OF GEOLOGY 
 
 which rocks mainly consist; 3d, of the methods employed 
 for the determination of rocks; 4th, of the external char- 
 acters of rocks; 5th, of the internal texture and structure 
 of rocks; 6th, of the classification of rocks; and 7th, of 
 the more important rocks occurring as constituents of the 
 earth's crust. 
 
 i. General Chemical Constitution of the Crust 
 Direct acquaintance with the chemical constitution of 
 the globe must obviously be limited to that of the crust, 
 though by inference we may eventually reach highly prob- 
 able conclusions regarding the constitution of the interior. 
 Chemical research has discovered that some sixty-four 1 
 simple or as yet undecomposable bodies, called elements, 
 in various proportions and compounds, constitute the ac- 
 cessible part of the crust. Of these, however, the great 
 majority are comparatively of rare occurrence. The crust, 
 go far as we can examine it, is mainly built up of about 
 sixteen elements, which may be arranged in the two follow- 
 ing groups, the most abundant bodies being placed first in 
 each list: 
 
 MetaMoids At. Wt. 
 
 Oxygen 15-96 
 
 Silicon 28-00 
 
 Carbon 11-97 
 
 31-98 
 
 irogen 1-00 
 
 ....35-37 
 
 Phosphorus 30-96 
 
 Fluorine.... ....19-10 
 
 Metals At. Wt. 
 
 Aluminium. 27-30 
 
 Calcium 39.90 
 
 Magnesium. 23-94 
 
 Potassium , 39-04 
 
 Sodium 22-99 
 
 Iron 55-90 
 
 Manganese 54-80 
 
 Barium.... ....136-80 
 
 The sixteen elements here mentioned form about ninety- 
 nine parts of the earth's crust; the other elements constitute 
 only about a hundredth part, though they include gold, sil- 
 
 1 This number has within the last few years been increased by the alleged 
 discovery of no fewer than fourteen new metals Some of these bodies, how- 
 ever, have not yet been satisfactorily proved to be new. T. S. Humpidge, Nature, 
 xxii. p. 232.
 
 GEOGNOSY 113 
 
 ver, copper, tin, lead, and the other useful metals, iron ex- 
 cepted. By far the most abundant and important element 
 is Oxygen. It forms about 23 per cent by weight of air, 
 88-87 per cent of water, and about a half of all the rocks 
 which compose the visible portion or crust of the globe. 
 Another metalloid, Silicon, always united with oxygen, 
 ranks next in abundance as a constituent of the crust. Of 
 the remaining metalloids, Carbon and Sulphur sometimes 
 occur in the free state, but more usually in combination. 
 Chlorine (save perhaps at volcanic vents) does not occur 
 in a free state, but is abundant in combination with the 
 alkalies, especially with sodium. Fluorine is always found 
 in combination, and has only recently been isolated by arti- 
 ficial chemical processes. It is the only element which has 
 not been combined with oxygen. It chiefly occurs in union 
 with Calcium as the mineral fluor-spar, and constitutes more 
 than half of the mineral cryolite; but traces of its presence 
 have been detected in other minerals, in sea-water, and in 
 the bones, teeth, blood and milk of mammalia. Hydrogen 
 occurs chiefly in combination with oxygen as the oxide, 
 water, of which it forms 11-18 per cent by weight; also in 
 combination with carbon as the hydrocarbons (mineral oils 
 and gases), produced by the slow decomposition of organic 
 matter. Phosphorus occurs with oxygen principally in cal- 
 cic phosphate. Of the metals, a few are found in the native 
 state (gold, silver, copper, etc), but those of importance in 
 the framework of the earth's crust have entered into com- 
 bination with metalloids or with each other. Putting the 
 more important metals and metalloids together, we may 
 compute that oxygen, silicon, aluminium, magnesium, cal- 
 cium, potassium, sodium, iron and carbon, form together 
 more than 97 per cent of the whole known crust.
 
 114 TEXT-BOOK OF GEOLOGY 
 
 So far as accessible to observation, the outer portion of 
 our planet consists mainly of metalloids. Its metallic con- 
 stituents have already in great part entered into combination 
 with oxygen, so that the atmosphere contains the residue of 
 that gas which has not yet united itself to terrestrial com- 
 pounds. In a broad view of the arrangement of the chem- 
 ical elements in the external crust, the suggestive specula- 
 tion of Durocher deserves attention.* He regarded all rocks 
 as referable to two layers or magmas coexisting in the 
 earth's crust, the one beneath the other, according to their 
 specific gravities. The upper or outer shell, which he 
 termed the acid or siliceous magma, contains an excess of 
 silica, and has a mean density of 2-65. The lower or inner 
 shell, which he called the basic magma, has from six to 
 eight times more of the earthy bases and iron-oxides, with 
 a mean density of 2 -96. To the former he assigned the early 
 plutonic rocks, granite, felsite, etc., with the more recent 
 trachytes; to the latter he relegated all the heavy lavas, 
 basalts, diorites, etc. The ratio of silica is 7 in the acid 
 magma to 6 in the basic. Though the proportion of silicic 
 acid or of the earthy and metallic bases cannot be regarded 
 as any certain evidence of the geological date of rocks, nor 
 of their probable depth of origin, it is nevertheless a fact 
 that (with many important exceptions) the eruptive rocks of 
 the older geological periods are very generally super-sili- 
 cated and of lower specific gravity, while those of later time 
 are very frequently poor in silica, but rich in the earthy 
 bases and in iron and manganese, with a consequent higher 
 specific gravity. The latter, according to Durocher, have 
 been forced up from a lower zone through the lighter sili- 
 
 9 Ann. dee Mines, 1857. Translated by Haughton, "Manual of Geology," 
 1866, p. 16.
 
 GEOGNOSY 115 
 
 ceous crust. The sequence of volcanic rocks, as first an- 
 nounced by Richthofen, has an interesting connection with 
 this speculation. 3 
 
 The main mass of the earth's crust is composed of a few 
 predominant compounds. Of these in every respect the 
 most abundant and important is Silicon-dioxide or Silica 
 (Kieselerde) SiO s . As the fundamental ingredient of the 
 mineral kingdom, it forms more than one-half of the known 
 crust, which it seems to bind firmly together, entering as a 
 main ingredient into the composition of most crystalline and 
 fragmental rocks as well as into the veins that traverse them. 
 It occurs in the free state as the abundant rock-forming 
 mineral quartz, which strongly resists ordinary decay, and 
 is therefore a marked constituent of many of the more en- 
 during kinds of rock. As one of the acid-forming oxides 
 (H 4 SiO 4 , Silicic acid, Kieselsaure) it forms combinations 
 with alkaline, earthy, and metallic bases, which appear as 
 the prolific and universally diffused family of the silicates. 
 Moreover, it is present in solution in terrestrial and oceanic 
 waters, from which it is deposited in pores and fissures of 
 rocks. It is likewise secreted from these waters by abun- 
 dantly diffused species of plants and animals (diatoms, radio- 
 larians, etc.). It has been largely effective in replacing the 
 organic textures of former organisms, and thus preserving 
 them as fossils. 
 
 Alumina or aluminium-oxide (Thonerde), A1 8 O 3 , occurs 
 sparingly as corundum, which, however, according to F. A. 
 Genth, was the original condition of many now abundant 
 complex aluminous minerals and rocks. The most common 
 condition of aluminium is in union with silica. In this 
 
 3 Posted, Book in. Part I. Section i. 5,
 
 116 TEXT-BOOK OF GEOLOGY 
 
 form it constitutes the basis of the vast family of the alumi- 
 nous silicates, of which so large a portion of the crystalline 
 and fragmental rocks consists. Exposed to the atmosphere, 
 these silicates lose some of their more soluble ingredients, 
 and the remainder forms an earth or clay consisting chiefly 
 of silicate of aluminium. 
 
 Carbon is the fundamental element of organic life. In 
 combination with hydrogen, as well as with oxygen, nitrogen 
 and sulphur, it forms the various kinds of coal, and thus 
 takes rank as an important rock-forming element. As car- 
 bon-dioxide, CO 5 , it is present in the air, in rain, in the sea 
 and in ordinary terrestrial waters. This oxide is soluble in 
 water, 4 giving rise then to a dibasic acid termed Carbonic 
 Acid (Kohlensaure), CO(OH) 2 or H 2 CO , which forms carbo- 
 nates, its combination with calcium having been instrumen- 
 tal in the formation of vast masses of solid rock. Carbon- 
 dioxide constitutes a fifth part of the weight of ordinary 
 limestone. 
 
 Sulphur (Soufre, Schwefel) occurs uncombined in occa- 
 sional deposits like those of Sicily and Naples, to be after- 
 ward described, also in union with iron and other metals as 
 sulphides ; but its principal condition as a rock-builder is in 
 combination with oxygen as sulphuric acid (Schwefelsaure), 
 HsSO 4 , which forms sulphates of lime, magnesia, etc. 
 
 Calcium enters into the composition of many crystalline 
 rocks in combination with silica and with other silicates. 
 But its most abundant form is in union with carbon -dioxide, 
 when it appears as the mineral, calcite (CaCO 3 ), or the rock, 
 limestone. Calcium-carbonate, being soluble in water con- 
 taining carbonic acid, is one of the most universally diffused 
 
 4 One volume of water at C. dissolves 1-7967 volumes of carbon-dioxide; 
 at 15 C. the amount is reduced.to 1-0020 volumes.
 
 GEOGNOSY 117 
 
 mineral ingredients of natural waters. It supplies the varied 
 tribes of mollusks, corals, and many other invertebrates with 
 mineral substance for the secretion of their tests and skele- 
 tons. Such too has been its office from remote geological 
 periods, as is shown by the vast masses of organically- 
 formed limestone, which enter so conspicuously into the 
 structure of the continents. In combination with sulphuric 
 acid, calcium forms important beds of gypsum and anhy- 
 drite. 
 
 Magnesium, Potassium, and Sodium play a less conspicu- 
 ous but still essential part in the composition of the earth's 
 crust. Magnesium, in combination with silica, forms a class 
 of silicates of prime importance in the composition of vol- 
 canic and metamorphic rocks. As a carbonate, it unites 
 with calcium-carbonate to form the widely diffused rock, 
 dolomite. In union with chlorine, it takes a prominent 
 place among the salts of sea-water. Potassium or Sodium, 
 combined with silica, is present in small quantity in most 
 silicates. In union with chlorine, as common salt, sodium 
 is the most important mineral ingredient of sea-water, and 
 can be detected in minute quantities in air, rain, and in ter- 
 restrial waters. In the old chemical formulae hitherto em- 
 ployed in mineralogy the metals of the alkalies and alkaline 
 earths are represented as oxides. Thus lime (calcium-mon- 
 oxide), soda (sodium-monoxide), potash (potassium-monox- 
 ide), magnesia (magnesium-oxide), are denoted as in union 
 with carbonic acid, sulphuric acid, silica, etc., forming car- 
 bonates, sulphates, silicates of lime, soda, etc. 
 
 Iron and Manganese are the two most common heavy 
 metals, occurring both in the form of ores, and as constitu- 
 ents of rocks. Iron is the great pigment of nature. Its 
 peroxide or sesquioxide, now known as ferric oxide, forms
 
 118 TEXT-BOOK OF GEOLOGY 
 
 large mineral masses, and together with the protoxide or 
 ferrous oxide, occurs in smaller or larger proportions in the 
 great majority of crystalline rocks. Iron (as sulphate or in 
 combination with organic acids) is removed in solution in 
 the water of springs, and precipitated as a hydrous perox- 
 ide. Manganese is commonly associated with iron in mi- 
 nute proportions in igneous rocks, and being similarly 
 removed in solution in water, is thrown down as bog 
 manganese or wad. 
 
 Silicic Acid, Carbonic Acid, and Sulphuric Acid are 
 the three acids with which most of the bases that compose 
 the earth's crust have been combined. With these we may 
 connect the water which, besides merely percolating through 
 rocks, or existing inclosed in the vesicles of minerals, has 
 been chemically absorbed in the process of hydration, and 
 which thus constitutes more than 10 or even 20 per cent of 
 some rocks (gypsum). 
 
 Chemical analysis has revealed the numerous combina- 
 tions in which the elements are united to form minerals and 
 rocks. Considerable additional light has been thrown on 
 the subject by chemical synthesis, that is, by artificially 
 producing the minerals and rocks which are found in na- 
 ture. The experiments have been varied indefinitely so as 
 to imitate as far as possible the natural conditions of produc- 
 tion. Further reference to this subject will be found on 
 pp. 161, 505 et seq. 
 
 Although every mineral may be made to yield data of 
 more or less geological significance, only those minerals 
 need be referred to here which enter as chief ingredients 
 into the composition of rock-masses, or which are of fre- 
 quent occurrence as accessories, and special note may be 
 taken of those of their characters which are of main interest
 
 GEOGNOSY 119 
 
 from a geological point of view, such as their modes of oc- 
 currence in relation to the genesis of rocks, and their weath- 
 ering as indicative of the nature of rock-decomposition. 
 
 ii. Rock-forming Minerals 
 
 Minerals, as constituents of rocks, occur in four condi- 
 tions, according to the circumstances under which they 
 have been produced. 
 
 (1.) Crystalline, as (a) more or less regularly defined 
 crystals, which, exhibiting the outlines proper to the 
 mineral to which they belong, are said to be idiomorphic; 
 (6) amorphous granules, aggregations or crystalloids, having 
 an internal crystalline structure, in most cases easily recog- 
 nizable with polarized light, as in the quartz of granite, 
 and an external form which has been determined by con- 
 tact with the adjacent mineral particles; such crystalline 
 bodies which do not exhibit their proper crystalline out- 
 lines are said to be allotriomorphic; (c) ''crystallites" or 
 "microlites," incipient forms of crystallization, which are 
 described on p. 206. The crystalline condition may arise 
 from igneous fusion, aqueous solution, or sublimation. 6 
 
 (2.) Glassy or vitreous, as a natural glass, usually in- 
 cluding either crystals or crystallites, or both. Minerals 
 have assumed this condition from a state of fusion, also 
 from solution. The glass may consist of several minerals 
 fused into one homogeneous substance. Where it has as- 
 sumed a lithoid or stony structure, these component min- 
 erals crystallize out of the glassy magma, and may be 
 recognized in various stages of growth (postea, pp. 194- 
 214). 
 
 6 For the microscopic characters of minerals and rocks, see p. 192.
 
 120 TEXT-BOOK OF GEOLOGY 
 
 (3.) Colloid, as a jelly-like though stony substance, de- 
 posited from aqueous solution. The most abundant mineral 
 in nature which takes the colloid form is silica. Opal is 
 a hardened colloidal condition of this substance. Chalced- 
 ony, doubtless originally colloidal silica, now unites the 
 characters of quartz and opal, being only partially soluble 
 in caustic potash and partially converted into a finely 
 fibrous, doubly-refracting substance. 
 
 (4.) Amorphous, having no crystalline structure or form, 
 and occurring in indefinite masses, granules, streaks, tufts, 
 stainings, or other irregular modes of occurrence. 
 
 A mineral which has replaced another and has assumed 
 the external form of the mineral so replaced, is termed a 
 Pseudomorph. A mineral which incloses another has been 
 called a Perimorph] one inclosed within another, an 
 Endomorph. 
 
 Essential or accessory, original or secondary 'minerals. 
 A mineral is an essential ingredient when its absence would 
 so alter the character of a rock as to make it something 
 fundamentally different. The quartz of granite, for exam- 
 ple, is an essential constituent of that rock, the removal of 
 which would alter the petrographical species. A mineral 
 is said to be accessory when its absence would not change 
 the essential character of the rock. All essential minerals 
 are original constituents of a rock, but all the original con- 
 stituents are not essential. In granite, such minerals as 
 topaz, beryl, and sphene often occur under circumstances 
 which show that they crystallized out of the original magma 
 of the rock. But they form so trifling a proportion in the 
 total mass, and their absence would so little affect the gen- 
 eral character of that mass, that they are regarded as ac- 
 cessory, though undoubtedly original and often important
 
 GEOGNOSY 121 
 
 ingredients. 6 Again, in rocks of eruptive origin, the essen- 
 tial ingredients cannot be traced back further than the 
 eruption of the mass containing them. They are not only 
 original, as constituents of the lava, but are themselves 
 original and non-derivative minerals, produced directly 
 from the crystallization of molten minerals ejected from 
 beneath the earth's crust, though, as Michel-LeVy has 
 shown, the debris of older minerals may sometimes be 
 traced amid the later crystals of massive rocks. 7 In rocks 
 of aqueous origin, however, there are many, such as con- 
 glomerates and sandstones, where the component minerals, 
 though original ingredients of the rocks, are evidently of 
 derivative origin. The little quartz-granules of a sand- 
 stone have formed part of the rock ever since it was ac- 
 cumulated, and are its essential constituents. Yet each 
 of these once formed part of some older rock, the destruc- 
 tion of which yielded materials for the production of the 
 sandstone. The minute crystals of zircon, rutile, tourma- 
 line and other minerals so often found in sands, clays, 
 sandstones, shales and other sedimentary deposits, have 
 been derived from the degradation of older crystalline 
 rocks. 
 
 The same mineral may occur both as an original and 
 as a secondary constituent. Quartz, for example, appears 
 everywhere in both conditions; indeed, it may sometimes 
 be found in a twofold form even in the same rock, though 
 
 6 Some of the "accessory" minerals may be of great importance as indicative 
 of the conditions under which the rock was formed. 
 
 1 Bull. Soc. Geol. France, 3d ser. iii. 199. See also Fouque and Michel- 
 LeVy, "Mineralogie Micrographique, " p. 189. Some eruptive rocks abound in 
 corroded or somewhat, rounded or broken crystals which obviously have belonged 
 to some previous state of consolidation. Such crystals, which are obviously 
 moie undent than those forming the general mass of the rock, have been called 
 alloyenic, while those which belong to the time of formation of the rock, or to 
 some subsequent change within the rock, are known as authigenic. 
 GEOLOGY Vol. XXIX 6
 
 122 TEXT-BOOK OF GEOLOGY 
 
 there is then usually some difference between the orig- 
 inal and secondary quartz. A quartz-felsite, for instance, 
 abounds in original little kernels, or in double pyramids 
 of the mineral, often inclosing fluid cavities, while the 
 secondary or accidental forms usually occur in veins, retic- 
 ulations, or other irregular aggregates. 
 
 Accessory minerals frequently occur in cavities where 
 they have had some room to crystallize out from the general 
 mass. The "drusy" cavities, or open spaces lined with well- 
 developed crystals, found in some granites are good exam- 
 ples, for it is there that the non-essential minerals are 
 chiefly to be recognized. The veins of segregation found 
 in many crystalline rocks, particularly in those of the 
 granite series, are further illustrations of the original sepa- 
 ration of mineral ingredients from the general magma of 
 a rock (see Book IV. Part VII. 3). 
 
 In some cases minerals assume a concretionary shape, 
 which may be observed chiefly though not entirely in rocks 
 formed in water. Some minerals are particularly prone 
 to occur in concretions. Siderite (ferrous carbonate) is to 
 be found in abundant nodules, mixed with clay and or- 
 ganic matter among consolidated muddy deposits. Calcite 
 (calcium-carbonate) is likewise abundantly concretionary. 
 Silica in the forms of chert and flint appears in irregular 
 concretions, in calcareous formations, composed mainly of 
 the remains of marine organisms. 
 
 Secondary minerals have been developed as the result 
 of subsequent changes in rocks, and are almost invariably 
 due to the chemical action of percolating water, either from 
 above or from below. Occurring under circumstances in 
 which such water could act with effect, they are found 
 in cracks, joints, fissures and other divisional planes and
 
 GEOGNOSY 123 
 
 cavities of rocks, especially in the minute interspaces be- 
 tween the component grains or minerals. Subterranean 
 channels, frequently several feet or even yards wide, 
 have been gradually filled up by the deposit of mineral 
 matter on their sides (see the Section on Mineral Veins). 
 The cavities formed by expanding steam in ancient lavas 
 (amygdaloids) have offered abundant opportunities for de- 
 posits of this kind, and have accordingly been in large 
 measure occupied by secondary minerals (amygdales), as 
 calcite, chalcedony, quartz and zeolites. 
 
 In the subjoined list of the more important rock- forming 
 minerals, attention is drawn mainly to those features that 
 are of geological importance; the physical, chemical and 
 microscopic characters of these minerals will be found in 
 a text-book of mineralogy or petrography. Eeference is 
 therefore made here to features of more special signifi- 
 cance to the geologist, such as modes of occurrence, 
 whether original or secondary; modes of origin, whether 
 igneous, aqueous, or organic; pseudomorphs, that is, the 
 various minerals which any given mineral has replaced, 
 while retaining their external forms, and likewise those 
 which are found to have supplanted the mineral in ques- 
 tion while in the same way retaining its form a valuable 
 clew to the internal chemical changes which rocks undergo 
 from the action of percolating water (Book III. Part H. 
 Section ii. 1 and 2); and lastly, characteristics or peculi- 
 arities of weathering, where any such exist that deserve 
 special mention. 
 
 1. NATIVE ELEMENTS are comparatively of rare occur- 
 rence, and only two of them, Carbon and Sulphur, occa- 
 sionally play the part of noteworthy essential and accessory 
 constituents of rocks. A few of the native metals, more
 
 124 TEXT-BOOK OF GEOLOGY 
 
 especially copper and gold, now and then appear in suffi- 
 cient quantity to constitute commercially important ingredi- 
 ents of veins and rock-masses. 
 
 Graphite is found chiefly in ancient crystalline rocks, as 
 gneiss, mica-schist, granite, etc. ; some of the Laurentian 
 limestones of Canada being so full of the diffused mineral 
 as to be profitably worked for it; in rare instances coal has 
 been observed changed into it by intrusive basalt (Ayrshire). 
 In some cases graphite results from the alteration of im- 
 bedded organic matter, especially remains of plants; but its 
 presence, and that of diamond, among ancient crystalline 
 rocks and in meteorites can hardly be thus accounted for. 
 Occasionally it is observed as a pseudomorph after calcite 
 and pyrites, and sometimes inclosing sphene and other 
 minerals. 8 
 
 Sulphur occurs 1st, as a product of volcanic action in the 
 vents and fissures of active and dormant cones. Volcanic 
 sulphur is formed from the oxidation of the sulphuretted 
 hydrogen, so copiously emitted with the steam that issues 
 from volcanic vents, as at the Solfatara, near Naples. It 
 may also be produced by the mutual decomposition of the 
 same gas and anhydrous sulphuric acid. 2d, in beds and 
 layers, or diffused particles, resulting from the alteration 
 of previous minerals, particularly sulphates, or from deposit 
 in water through decomposition of sulphuretted hydrogen. 
 The frequent crystallization of sulphur shows that the 
 mineral must have been formed at ordinary temperatures, 
 for its natural crystals melt at 238-1 Fahr. Its formation 
 may be observed 'in progress at many sulphureous springs, 
 where it falls to the bottom as a pale mud through the oxi- 
 dation of the sulphuretted hydrogen in the water. It occurs 
 in Sicily, Spain and elsewhere, in beds of bituminous lime- 
 stone and gypsum. These strata, sometimes full of remains 
 of fresh-water shells and plants, are interlaminated with 
 sulphur, the very shells being not infrequently replaced 
 by this mineral. Here the presence of the sulphur may be 
 traced to the reduction of the calcium-sulphate to the state 
 of sulphide, through the action of the decomposing organic 
 matter, and the subsequent production and decomposition 
 of sulphuretted hydrogen, with consequent liberation of 
 sulphur." The sulphur deposits of Sicily furnish an excel- 
 
 8 Vom Rath. Sitzungsber. Wien. Akad. x. p. 67 ; Sullivan in Jukes' "Manual 
 of Geology," 3d edit. (1872), p. 56. 
 
 9 Braun, Bull. Soc. Geol. France, 1st ser. xil p. 171.
 
 GEOGNOSY 125 
 
 lent illustration of the alternate deposit of sulphur and 
 limestone. They consist mainly of a marly limestone, 
 through which the sulphur is partly disseminated and 
 partly interstratified in thin laminae and thicker layers, 
 some of which are occasionally 28 feet deep. Below these 
 deposits lie older Tertiary gypseous formations, the decom- 
 position of which has probably produced the deposits of 
 sulphur in the overlying more recent lake basins. 10 The 
 weathering of sulphur is exemplified on a considerable 
 scale at these Sicilian deposits. The mineral, in presence 
 of limestone, oxygen and moisture, becomes sulphuric acid, 
 which, combining with the limestone, forms gypsum, a cu- 
 rious return to what was probably the original substance 
 from the decomposition of which the sulphur was derived. 
 Hence the site of the outcrop of the sulphur beds is marked 
 at the surface by a white earthy rock, or borscale, which is 
 regarded by the miners in Sicily to be a sure indication 
 of sulphur underneath, as the gossan of Cornwall is indic- 
 ative of underlying metalliferous veins. 11 
 
 Iron, the most important of all the metals, is found only 
 sparingly in the native state, in blocks which have fallen as 
 meteorites, also in grains or dust inclosed in hailstones, in 
 snow of the Alps, Sweden and Siberia, in the mud of the 
 ocean-floor at remote distances from land, and in some erup- 
 tive rocks. There can be no doubt that a small but con- 
 stant supply of native iron (cosmic dust) is falling upon the 
 earth's surface from outside the terrestrial atmosphere. 1 * 
 This iron is alloyed with nickel, and contains small quan- 
 tities of cobalt, copper and other ingredients. Dr. An- 
 drews, however, showed in 1852 that native iron, in minute 
 spicules or granules, exists in some basalts 'and other vol- 
 canic rocks 13 and Mr. J. Y. Buchanan has detected it in ap- 
 
 >o Memorie del R. Comitato Geologico d'ltalia, i. (1871). 
 
 11 Journ. Soc. Arts, 1873, p. 170. B. Ledoux, Ann. dea Mines, 7me. ser. 
 vii. p. 1. The Sicilian sulphur beds belong to the Oeningen stage of the Upper 
 Tertiary deposits. They contain numerous plants and some insects. H. T. 
 Geyler, Palseontographica, xxiii. Lief. 9, p. 317. Von Lasaulx, Neues Jahrb. 
 1879, p. 490. 
 
 12 See Bhrenberg, Frorieps Notizen, Feb. 1846; Nordenskiold, Comptes 
 
 reudus, Ixxvii. p. 463, Ixxviii. p. 236. Tissandier, op. cit. Ixxviii. p. 821, Ixxx. 
 
 &58, Ixxxi. p. 576. See Ixxv. (1872) p. 683. Yung, Bull. Soc. Vaudoise Sci. 
 at. (1876), xiv. p. 493. Ranyard, Monthly Not. Roy. Astron. Soc. xxxix. 
 
 (1879) p. 161. S. L. Phipson, Comptes rend. Ixxxiii. p. 364. A Committee of 
 the British Association was appointed in 1880 to investigate the subject of cosmic 
 dust. See its reports for 1881-83. 
 
 13 Brit. Assoc. Rep. 1852, postea, p. 7(38.
 
 126 TEXT-BOOK OF GEOLOGY 
 
 preciable quantity in the gabbro of the west of Scotland. 
 It occurs also in the basalts of Bohemia and Greenland. 14 
 
 In the great majority of cases the OXIDES occur com- 
 bined with some acid. A few uncombined take a promi- 
 nent place as essential constituents or frequent ingredients 
 of rocks, especially the oxides of silicon and iron. 
 
 2. SILICA (SiO 9 ) is found in three chief forms, Quartz, 
 Tridymite, and Opal. 
 
 Quartz is abundant as (1) an essential constituent of rocks, 
 as in granite, gneiss, mica-schist, rhyolite (quartz-trachyte), 
 quartz-porphyry, sandstone; (2) a' secondary ingredient, 
 wholly or partially filling veins, joints, cracks, and cavi- 
 ties. It has been produced from (a) igneous action, as in 
 volcanic rocks; (&) aquo-igneous or plutonic action, as in 
 granites, gneisses, etc.; (c) solution in water, as where it 
 lines cavities or replaces other minerals. The last mode 
 of formation is that of the crystallized quartz and chalced- 
 ony found as secondary ingredients in rocks. 
 
 The study of the endomorphs and pseudomorphs of 
 quartz is of great importance in the investigation of the 
 history of rocks. No mineral is so conspicuous for the 
 variety of other minerals inclosed within it. In some sec- 
 ondary quartz-crystals, each prism forms a small mineralog- 
 ical cabinet inclosing a dozen or more distinct minerals, as 
 rutile, haematite, limonite, pyrites, chlorite, and many 
 others. 16 Quartz may be observed replacing calcite, arago- 
 nite, siderite, gypsum, rock-salt, haematite, etc. This facil- 
 ity of replacement makes silica one of the most valuable 
 
 14 Nordenskiold describes fifteen blocks of iron on the island of Disco, Green- 
 land, the weight of the two largest being 21,000 and 8000 kilogrammes (20 and 
 8 tons, respectively). He observed that at the same locality, the underlying 
 basalt contains lenticular and disk-shaped blocks of precisely similar iron, and 
 inferred that the whole of the blocks may belong to a meteoric shower which 
 fell during the time (Tertiary) when the basalt was poured out at .the surface. 
 He dismissed the suggestion that the iron could possibly be of telluric origin 
 (Geol. Mag. ix. (1872) p. 462). But the microscope reveals in this basalt the 
 presence of minute particles of native iron which, associated with viridite, are 
 molded round the crystals of labradorite and augite (Fouque" and Michel-Levy, 
 op. cit. p. 443). Steenstrup, Daubr6e, and others appear therefore to be justi- 
 fied in regarding this iron as derived from the inner metallic portions of the 
 globe, which lie at depths inaccessible to our observations, but from which the 
 vast Greenland basalt eruptions have brought up traces to the surface (K. J. T. 
 Steenstrup, Vid. Medd. Nat. Foren. Copenhagen (1875) Nos. 16-19, p. 284; 
 Zeitsch. Deutsch. Geol. Ges. xxviii. (1876) p. 225 ; Mineralog. Mag. July, 1884. 
 P. Wohler, Neues Jahrb. 1879, p. 832. Daubree, Discours Acad. Sci. 1 March, 
 1880, p. 17. W. Flight, Geol. Mag. ii. (2d ser.) p. 152. 
 
 16 See Sullivan, in Jukes' "Manual of Geology," 3d. edit. (1872), p. 61.
 
 GEOGNOSY 127 
 
 petrifying agents in nature. Organic bodies which have 
 been silicified retain, often with the utmost perfection, their 
 minutest and most delicate structures. 
 
 Quartz may usually be identified by its external charac- 
 ters, and especially oy its vitreous lustre and hardness. 
 When in the form of minute blebs or crystals, it may be 
 recognized in many rocks with a good lens. Under the 
 microscope, it presents a characteristic brilliant chromatic 
 polarization, and in convergent light gives a black cross. 
 Where it is an original and essential constituent of a rock, 
 quartz very commonly contains minute rounded or irregular 
 cavities or pores, partially filled with liquid. So minute are 
 these cavities that a thousand millions of them may, when 
 they are closely aggregated, lie within a cubic inch. The 
 liquid is chiefly water, not uncommonly containing sodium 
 chloride or other salt, sometimes liquid carbon-dioxide and 
 hydrocarbons. 1 ' Chalcedony exhibits under the microscope 
 a minute radial fibrous structure. 
 
 Bock-crystal and crystalline quartz resist atmospheric 
 weathering with great persistence. Hence the quartz-grains 
 may usually be easily discovered in the weathered crust of 
 a quartziferous igneous rock. But corroded quartz-crystals 
 have been observed in exposed mountainous situations, with 
 their edges rounded and eaten away. 17 The chalcedonic and 
 more or less soluble forms of silica are more easily affected. 
 Flint and many forms of colored chalcedony weather with a 
 white crust. 6ut it is chiefly from the weathering of sili- 
 cates (especially through the action of organic acids) that the 
 soluble silica of natural waters is derived. (Book III. Part 
 II. Section ii. 7.) 
 
 Tridymite has been met with chiefly among volcanic rocks 
 (trachytes, andesites, etc.), both as an abundant constituent 
 of those which have been poured out in the form of lava, 
 and also in ejected blocks (Vesuvius). 18 
 
 Opal, a hydrous condition of silica formed from solution 
 in water, is usually disseminated in veins and nests through 
 rocks. Semi-opal occasionally replaces the original sub- 
 
 ' See Brewster, Trans. Roy. Soc. Edin. x. p. 1. Sorby, Quart. Journ. Geol. 
 Soc. xiv. p. 463. Proc. Roy. Soc. xv. p. 153; xvii. p. 299. Zirkel, "Mikro- 
 skopische Beschaffenheit der Mineralien und Gesteine," p. 39. Rosenbusch, 
 "Mikroskopische Physiographic, " i. p. 30. Hartley, Journ. Chem. Soc. Feb- 
 ruary, 1876. The occurrence of fluid-cavities in the crystals of rocks is more 
 fully described in Part II. iv. of this Book. 
 
 17 Roth, Chern. Geol. i. p. 94. 
 
 18 Vom Rath, Z. Deutsch. Geol. Ges. xxv. p. 236, 1873.
 
 128 TEXT-BOOK OF GEOLOGY 
 
 stance of fossil wood (wood-opal). Several forms of opal 
 are deposited by geysers, and are known under the general 
 appellation of sinters. Closely allied to the opals are the 
 forms in which hydrous (soluble) silica appears in the or- 
 ganic world, where it constitutes the frustules of diatoms, 
 the skeletons of radiolaria, etc. Tripoli powder (Kiesel- 
 guhr), randanite, and other similar earths, are composed 
 mainly or wholly of the remains of diatoms, etc. 
 
 Corundum, aluminium-oxide, is found in crystalline rocks, 
 particularly in certain serpentines and schists, gneiss, gran- 
 ite, dolomite, and rocks of the metamorphic series. 
 
 3. IRON OXIDES. Four minerals, composed mainly of 
 iron oxides, occur abundantly as essential and accessory in- 
 gredients of r.ocks. Haematite, Limonite, Magnetite, and 
 Titanic iron. 
 
 Hxmatite (Fer oligiste, Eotheisen, Eisenglanz, Fe,O 3 = 
 Fe70, O30) in the crystallized form occurs in veins, as well 
 as lining cavities and fissures of rocks. The fibrous and 
 more common form (which often has portions of its mass 
 passing into the crystallized condition) lies likewise in 
 strings or veins; also in cavities, which, when of large 
 size, have given opportunity for the deposit of great masses 
 of haematite, as in cavernous limestones (Westmoreland). 
 It occurs with other ores and minerals as an abundant com- 
 ponent of mineral veins, likewise in beds interstratified with 
 sedimentary or schistose rocks. Scales and specks of opaque 
 or clear bright red haematite, of frequent occurrence in the 
 crystals of rocks, give them a reddisn color or peculiar lustre 
 (perthite, stilbite). Haematite appears abundantly as a prod- 
 uct of sublimation in clefts of volcanic cones and lava 
 streams. It is probably in most cases a deposition from 
 water, resulting from the alteration of some previous solu- 
 ble combination of the metal, such as the oxidation of the 
 sulphate, and occurs in veins and beds, and as the earthy 
 pigment that gives a red color to sandstones, clays and other 
 rocks. It is found pseudomorphous after ferrous carbonate, 
 and this has probably been the origin of beds of red ochre 
 occasionally intercalated among stratified rocks. It likewise 
 replaces calcite, dolomite, quartz, barytes, pyrites, magne- 
 tite, rock-salt, fluor-spar, etc. 
 
 Ljmonite (Brown iron -ore, 2Fe 2 O 8 +3H,O = FeA 85-56, 
 H 2 O 14-44) occurs in beds among stratified formations, and 
 may be seen in the course of deposit, through the action of 
 organic acids, on marsh- land (bog-iron-ore) and lake-bottoms. 
 (Book IV. Part II. Section iii.) In the form of yellow
 
 GEOGNOSY 129 
 
 ochre, it is precipitated from the waters of chalybeate 
 springs containing green vitriol derived from the oxidation 
 pi iron-sulphides." It is a common decomposition product 
 in rocks containing iron among their constituents. It is 
 thus always a secondary or derivative substance, resulting 
 from chemical alteration. It is the usual pigment which 
 gives tints of yellow, orange and brown to rocks. The 
 pseudomorphous forms of limonite show to what a large ex- 
 tent combinations of iron are carried in solution through 
 rocks. The mineral has been found replacing calcite, sid- 
 erite, dolomite, haematite, magnetite, pyrite, marcasite, ga- 
 lena, blende, gypsum, barytes, fluor-spar, pyroxene, quartz, 
 garnet, beryl, etc. 
 
 Magnetite (Fer oxydule*, Magneteisen, Fe 3 O<) occurs abun- 
 dantly in some schists, in scattered octohedral crystals; in 
 crystalline massive rocks like granite, in diffused grains or 
 minute crystals; among some schists and gneisses (Norway 
 and the Eastern States of North America), in massive beds; 
 in basalt and other volcanic rocks, as an essential constitu- 
 ent, in minute octohedral crystals, or in granules or crystal- 
 lites. It is likewise found as a pseudomorphous secondary 
 product, resulting from the alteration of some previous min- 
 eral, as olivine, hematite, pyrite, quartz, hornblende, au- 
 gite, garnet and sphene. It occurs with haematite, etc., as a 
 product of sublimation at volcanic foci, where chlorides of 
 the metals in presence of steam are resolved into hydro- 
 chloric acid and anhydrous oxides. It may thus result 
 from either aqueous or igneous operations. It is liable to 
 weather by the reducing effects of decomposing organic mat- 
 ter, whereby it becomes a carbonate, and then by exposure 
 passes into the hydrous or anhydrous peroxide. The mag- 
 netite grains of basalt-rocks are very generally oxidized at 
 the surface, and sometimes even for some depth inward. 
 
 Titanic Iron (Titaniferous Iron, Menaccanite, Ilmenite, Fer 
 titane, Titaneisen (FeTi) 9 O 3 ) occurs in scattered grains, plates 
 and crystals as an abundant constituent of many crystalline 
 rocks (basalt-rocks, diabase, gabbro and other igneous 
 masses); also in veins or beds in syenite, serpentine and 
 metamorphic rocks ; ' scarcely to be distinguished from 
 magnetite when seen in small particles under the micro- 
 scope, but possessing a brown semi-metallic lustre with re- 
 
 " Sullivan, Jukes' "Manual of Geology," p. 63. 
 
 20 Some of the Canadian masses of this mineral are 90 feet thick and manj 
 yards in length.
 
 130 TEXT-BOOK OF GEOLOGY 
 
 fleeted light; resists corrosion by acids when the powder of 
 a rock containing it is exposed to their action, while mag- 
 netite is attacked and dissolved. Titanic iron frequently 
 resists weathering, so that its black glossy granules project 
 from a weathered surface of rock. In other cases, it is de- 
 composed either by oxidation of its protoxide, when the 
 usual brown or yellowish color of the hydrous ferric oxide 
 appears, or by removal of the iron. The latter is believed 
 to oe the origin of a peculiar milky white opaque substance, 
 frequently to be observed under the microscope, surround- 
 ing and even replacing crystals of titanic iron, and named 
 Leucoxene by Grumbel. ai In other cases the decomposition 
 has resulted in the production of sphene. 
 
 Chromite (FeCr,O 4 ) occurs in black opaque grains and 
 crystals not infrequently in altered oHvine-rocks. 
 
 Spinels, a group of minerals, may be taken here. They 
 are closely related to each other, having cubic forms and 
 varying in composition from magnetite (see above) at the 
 one end to spinet (MgAl ? O 4 ) at the other. They are not in- 
 frequent as minute grains or crystals in some igneous and 
 metamorphic rocks. Between magnetite and spinel come 
 intermediate varieties, as chromite (see above), Picotite, Her- 
 cynite and Pleonaste. 
 
 4. MANGANESE OXIDES are frequently associated with 
 those of iron in ordinary rock-forming minerals, but in such 
 minute proportions as to have been generally neglected in 
 analyses. Their presence in the rocks of a district is some- 
 times shown by deposits of the hydrous oxide in the forms 
 of Psilomelane (H 2 MnO 4 +H,O) and Wad (MnO 8 -f-MnO+ 
 HO). These deposits sometimes take place as black or 
 dark brown branching, plant-like or dendritic impressions 
 between the divisional planes of close-grained rocKs (lime- 
 stone, felsite, etc.), sometimes as accumulations of a black 
 or brown earthy substance in hollows of rocks, occasionally 
 as deposits in marshy places, like those of bog-iron-ore, and 
 abundantly on someparts of the sea-floor. (See p. 769.) 
 
 5. SILICATES. These embrace by far the largest and 
 most important series of rock-forming minerals. Their 
 chief groups are the anhydrous aluminous and magnesian 
 silicates embracing the Felspars, Hornblendes, Augites, 
 
 S1 "Die Palaolitische Eruptivgesteine dee Fichtelgebirges," 1874, p. 29. See 
 Rosenbusch, Mik. Physiog. ii. p. 336. De la Vallee Pouesin and Renard, Mem. 
 Gouronnees Acad. Roy. de Belgique, 1876, xl. Plate vi. pp. 34 and 35. Fouque" 
 and Michel-Levy, "Mineralogie Micrograph," p. 426. See postea, p. 1040.
 
 GEOGNOSY 131 
 
 Micas, etc., and the hydrous silicates which include the 
 Zeolites, Clays, talc, chlorite, serpentine, etc. 
 
 The family of the Felspars forms one of the most im- 
 
 Krtant of all the constituents of rocks, seeing that its inem- 
 rs constitute by much the largest portion of the plutonic 
 and volcanic rocks, are abundantly present among many 
 crystalline schists, and by their decay nave supplied a great 
 part of the clay out of which argillaceous sedimentary for- 
 mations have been constructed. 
 
 The felspars are usually divided into two series. 1st, 
 The orthoclastic or monociinic felspars, consisting of two 
 species or varieties, Orthoclase and Sanidine; and, 2d, The 
 plagioclastic or triclinic felspars, among which, as constitu- 
 ents of rocks, may be mentioned the species albite, anorthite, 
 oligoclase, andesine, labradorite, and microcline. 
 
 Orthoclase (K,O 16-89, A1 2 O 3 1843, SiO, 64-68) occurs 
 abundantly as an original constituent of many crystalline 
 rocks (granite, syenite, felsite, gneiss, etc.), likewise in cavi- 
 ties and veinings in which it has segregated from the sur- 
 rounding mass (pegmatite); seldom found in unaltered 
 sedimentary rocks except in fragments derived from old 
 crystalline masses; generally associated with quartz, and 
 often with hornblende, while the felspars less rich in silica 
 more rarely accompany free quartz. It is an original con- 
 stituent of plutonic and old volcanic rocks (granite, felsite, 
 etc.), and of gneiss and various schists. A few examples 
 have been noticed where it has replaced other minerals 
 (prehnite, analcime, laumontite). Under the microscope it 
 is recognizable from quartz by its characteristic rectangular 
 forms, cleavage, twinning, angle of extinction, turbidity, 
 and frequent alteration." Orthoclase weathers on the whole 
 with comparative rapidity, though durable varieties are 
 known. The alkali and some of the silica are removed, 
 and the mineral passes into clay or kaolin (j). 140). 
 
 Sanidine, the clear glassy fissured variety of ortho- 
 clase so conspicuous in the more silicated Tertiary and 
 modern lavas, occurs in some trachytes in large flat tables 
 (hence the name "sanidine"); more commonly in fine clear 
 or gray crystals or crystalline granules; an eminently vol- 
 canic mineral. 
 
 Plagioclase (Triclinic) Felspars. While the different felspars 
 which crystallize in the triclinic system may be more or less 
 
 48 On microscopic determination of felspars, see Fouque and Michel-Levj, 
 op. cit. pp. 209, 227, and postea, pp. 108-172.
 
 132 TEXT-BOOK OF GEOLOGY 
 
 easily distinguished in large crystals or crystalline aggre- 
 gates, they are difficult to separate in the minute forms in 
 which they commonly occur as rock constituents. They 
 have been grouped oy petrographers under the general 
 name Plagioclase (with oblique cleavage), proposed by 
 Tschermak, who regards them as mixtures in various pro- 
 portions of two fundamental compounds albite or soda- 
 felspar, and anorthite or lime-felspar. 
 
 They occur mostly in well-developed crystals, partly in 
 irregular crystalline grains, crystallites or microlites. On 
 a fresh fracture, their crystals often appear as clear glassy 
 strips, on which may usually be detected a fine parallel 
 lineation or ruling, indicating a characteristic polysynthetio 
 twinning which never appears in orthoclase. A felspar 
 striated in this manner can thus be at once pronounced to 
 be a triclinic form, though the distinction is not invariably 
 present. Under the microscope, the fine parallel lamella- 
 tion or striping, best seen with polarized light, forms one 
 of the most distinctive features of this group of felspars. 
 The chief triclinio felspars are, Microcline (potash -felspar, 
 KjAl f Si 6 O lg ), which occurs in granites, particularly as the 
 common felspar of the graphic varieties; also in some 
 gneisses, etc.; Albite (soda-felspar, Na 2 O 11-82, A1 8 O 8 
 18-56, Si s O 68-62), found in some granites, and in several 
 volcanic rocks; Oligoclase (soda-lime and lime-soda felspars, 
 Na 2 8-2, CaO 4-8, AL,O 3 23-0, SiO, 62-8) occurs in many 
 
 ?ranites and other eruptive rocks; Andesine (Na s O 7"7, CaO 
 0, Al 2 O 8 25-6, SiO, 60-0), observed in some syenites, etc.; 
 Labradorite (Na,O 4-6, GaO 12-4, A1 8 F 8 30-2, SiO 2 52-9), an 
 essential constituent of many lavas, etc., abundant in masses 
 in the azoic rocks of Canada, etc. ; Anorthite (lime-felspar, 
 CaO 20-10, A1A 36-82, SiO 2 43-08) found in many volcanic 
 rocks, sometimes in granites and metamorphic rocks. 
 
 The triclinic felspars have been produced sometimes 
 directly from igneous fusion, as can be studied in many 
 lavas, where often one of the first minerals to appear in 
 the devitrification of the original molten glass has been the 
 labradorite or other plagioclase. In other cases, they have 
 resulted from the operation of the processes to which the 
 formation of the crystalline schists was due; large beds as 
 well as abundant diffused strings, veinings, and crystals of 
 triclinic felspar (labradorite) form a marked feature among 
 the ancient gneisses of Eastern Canada. The more highly 
 silicated species (albite, oligoclase) occur with orthoclase as
 
 GEOGNOSY 133 
 
 essential constituents of many granites and other plutonic 
 rocks. The more basic forms (labradorite, anorthite) are gen- 
 erally absent where free silica is present; bat occur in the 
 more basic igneous rocks (basalts, etc.). 
 
 Considerable differences are presented by the triclinic 
 felspars in regard to weathering. On an exposed face of 
 rock they lose their glassy lustre and become white and 
 opaque. This change, as in orthoclase, arises from loss of 
 bases and silica, and from hydration. Traces of carbonates 
 may often be observed in weathered crystals. The original 
 steam cavities of old volcanic rocks have generally been 
 filled with infiltrated minerals, which in many cases have 
 resulted from tbe weathering and decomposition of the tri- 
 clinic felspars. Calcite, prennite, and the family of zeolites 
 have been abundantly produced in this way. The student 
 will usually observe that where these minerals abound in 
 the cells and crevices of a rock, the rock itself is for the 
 most part proportionately decomposed, showing the relation 
 that subsists between infiltration-products and the decom- 
 position of the surrounding mass. Abundance of calcite in 
 veins and cavities of a felspathic rock affords good ground 
 for suspecting the presence in the latter of a lime-felspar." 
 (See under " Albitization, " postea, p. 1040.) 
 
 Saussurite, formerly described as a distinct mineral 
 species, is now found to be the result of the -decomposi- 
 tion of felspars, which have thus acquired a dull white 
 aspect and contain secondary crystallizations (zoisite) out 
 of the decomposed substance of the original felspar. Such 
 saussuritic felspars occur in varieties of gabbro and diorite. 
 Under the microscope they present a confused aggregate of 
 crystalline needles and granules imbedded in an amorphous 
 matrix. (See postea, p. 1040.) 
 
 Lcucite (K S O 21-58, A1 5 O 3 28-50, SiO 2 54-97) is a markedly 
 volcanic mineral, occurring as an abundant constituent of 
 many ancient and modern Italian lavas, and in some vari- 
 eties of basalt. Under the microscope, sections of this min- 
 eral are eight-sided or nearly circular, and very commonly 
 contain inclosures of magnetite, etc., conforming in arrange- 
 ment to the external form of the crystal or disposed radially. 
 
 Nepheline (Na,O 17-04, A1.O, 85-26, K S O 6-46, SiO, 41-24), 
 essentially a volcanic mineral, being an abundant constit- 
 
 88 A valuable essay on the stages of the weathering of triclinic felspar as 
 revealed by the microscope was published by Q-. Rose in 1867. Zeitsck, 
 Deutech. GeoL Ges. xix. p. 276.
 
 134 TEXT-BOOK OF GEOLOGY 
 
 uent of phonolite, of some Vesuvian lavas, and of some 
 forms of basalt, presents under the microscope various six- 
 sided and even four-sided forms, according to the angles 
 at which the prisms are cut. 84 Under the name of JElceolite 
 are comprised the greenish or reddish, dull, greasy-lustred, 
 compact or massive varieties of nepheline, which occur in 
 some syenites and other ancient crystalline rocks. 
 
 THE MICA FAMILY embraces a number of minerals, 
 distinguished especially by their very perfect basal cleav- 
 age, whereby they can be split into remarkably thin elastic 
 laminae, and by a predominant splendent pearly lustre. 
 They consist essentially of silicates of alumina, magnesia, 
 iron and alkalies, and may be conveniently divided into 
 two groups, the white micas, which are silicates of alumina 
 with alkalies, iron and magnesia, and the black micas, in 
 which the magnesia and iron play a more conspicuous part. 
 
 Muscovite (Potash-mica, white mica, Glimmer, K 2 O 3'07- 
 12-44, Na 2 0-4-10, FeO 0-1-16, Fe 2 O 3 0-46-8-80, MgO 
 0-37-3-08, A1A 28-05-38-41, SiO, 43-47-51-73, H 2 O 0-98- 
 6-22), abundant as an original constituent of many crystal- 
 line rocks (granite, etc.), and as one of the characteristic 
 minerals of the crystalline schists; also in many sandstones, 
 where its small parallel flakes, derived, like the surround- 
 ing quartz grains, from older crystalline masses, impart a 
 silvery or "micaceous" lustre and fissility to the stone.** 
 The persistence of muscovite under exposure to weather is 
 shown by the silvery plates of the mineral, which may be 
 detected on a crumbling surface of granite or schist where 
 most of the other minerals, save the quartz, have decayed; 
 also by the frequency of the micaceous lamination of 
 sandstones. 
 
 Biotite (Magnesia-mica, black mica, MgO 10-30 per cent) 
 occurs abundantly as an original constituent of many 
 granites, gneisses, and schists; also sometimes in basalt, 
 trachyte, and as ejected fragments and crystals in tuff. Its 
 small scales, when cut transverse to the dominant cleavage, 
 may usually be detected under the microscope by their re- 
 markably strong dichroism, their fine parallel lines of cleav- 
 age, and their frequently frayed appearance at the ends. 
 Under the action of the weather it assumes a pale, dull, soft 
 
 24 On the microscopic distinction between nepheline and apatite, see Fouque" 
 and Michel-LeVy, "Mineral. Micrograph." p. 276. 
 
 25 On the microscopic determination of the micas, see Fouque and Michel- 
 Levy, op. cit. p. 333.
 
 GEOGNOSY 135 
 
 crust, owing to removal of its bases. The mineral rubellan, 
 which occurs in hexagonal brown or red opaque inelastic 
 tables in some basalts and other igneous rooks, is regarded 
 as an altered form of biotite. 
 
 Phlogopite is another dark ferro-magnesian mica which 
 contains a little fluorine. Lepidolite (Lithia-mica) occurs in 
 some granites and crystalline schists, especially in veins. 
 Damourite, merely a variety of muscovite, occurs among 
 crystalline schists. Sericite, a talc-like variety of musco- 
 vite, occurs in soft inelastic scales in many schists, as a 
 result of the alteration of orthoclase felspar.*' Margarodite, 
 a silvery talc-like hydrous mica, is widely diffused as a 
 constituent of granite and other crystalline rocks. Para- 
 gonite, a scaly micaceous mineral, forms the main mass of 
 certain alpine schists. 
 
 Hornblende (Monoclinic Amphibole, CaO, 10-12, MgO 11- 
 24, FeAO-10, A1 8 O 3 5-18, SiO 4 40-^50 also usually with 
 some Na 8 O, K S and FeO). Divided into two groups. 1st. 
 Non-aluminous, including the white and pale green or gray 
 fibrous varieties (tremolite, actinolite, etc.). 2d. Aluminous, 
 embracing the more abundant dark green, brown, or black 
 varieties. Under the microscope, hornblende presents 
 cleavage-angles of 124 30', the definite cleavage-planes 
 intersecting each other in a well-marked lattice work, 
 sometimes with a finely fibrous character superadded. It 
 also shows a marked pleochroism with polarized light, 
 which, as Tschermak first pointed out, usually distin- 
 guishes it from augite." Hornblende has abundantly re- 
 sulted from the alteration (paramorphism) of augite (see 
 below, Uralite). In many rocks the ferro-magnesian sili- 
 cate which is now hornblende was originally augite; the 
 epidiorites, for instance, were probably once dolerites or 
 allied pyroxenic rocks. The pale non-aluminous horn- 
 blendes are found among gneisses, crystalline limestones, 
 and other metamorphic rocks. The dark varieties, though 
 also found in similar situations, sometimes even forming 
 entire masses of rock (amphibolite, hornblende-rock, horn- 
 blende-schist), are the common forms in granitic and vol- 
 canic rocks (syenite, diorite, hornblende-andesite, etc.). The 
 former group naturally gives rise by weathering to various 
 
 36 On the occurrence of this mineral in schists, see Lessen, Zeitsch. Deutsch. 
 Geol. Ges. 1867, pp. 546, 661. 
 
 * 7 Wien. Acad. May, 1869. See also Fouque and Michel-LeVy, op. cit. pp. 
 349, 365.
 
 136 TEXT- BOOK OF QEOLOQY 
 
 hydrous magnesian silicates, notably to serpentine and talc. 
 In the weathering of the aluminous varieties, silica, lime, 
 magnesia, and a portion of the alkalies are removed, with 
 conversion of part of the earths and the iron into carbon- 
 ates. The further oxidation of the ferrous carbonate is 
 shown by the yellow and brown crust so commonly to be 
 seen on the surface or penetrating cracks in the hornblende. 
 The change proceeds until a mere internal kernel of unal- 
 tered mineral remains, or until the whole has been converted 
 into a ferruginous clay. 
 
 Anthophyflite (Khombic Amphibple (MgFe)SiO 8 ) is a min- 
 eral which occurs in bladed, sometimes rather fibrous forms, 
 among the more basic parts of old gneisses; also in zones 
 of alteration round some of the ferro-magnesian minerals of 
 certain gabbros. 
 
 Soda-amphiboles resemble ordinary hornblende, but, as their 
 name denotes, they contain a more marked proportion of 
 soda. They include a blue variety called Glaucophane, 
 which is found abundantly in certain schists; Riebeckite, 
 which is also blue and occurs in some granites and micro- 
 granites; Arfvedsonite, a dark greenish or brown variety. 
 
 Uralite is the name given to a mineral which was origi- 
 nally pyroxene, but has now by a process of paramorphism 
 acquired the internal cleavage 'and structure of hornblende 
 (amphibole). Under the microscope a still unchanged ker- 
 nel of pyroxene may in some specimens be observed in 
 the centre of a crystal surrounded by strongly pleochroic 
 hornblende, with its characteristic cleavage and actinolitic 
 needles (postea, p. 1040). Smaragdite is a beautiful grass- 
 green variety also resulting from the alteration of a pyroxene. 
 
 Augite (Monoclinic Pyroxene, CaO 12-27-5, MgO 3-22 -5, 
 FeO 1-34, FeA 0-10, ALA 0-11; SiO, 40-57-4). Divided 
 like -hornblende into two groups. 1st. Non-aluminous, with 
 a prevalent green color (malacolite, coccolite, diopside, sah- 
 lite, etc.). 2d. Aluminous, including generally the dark 
 green or black varieties (common augite, fassaite). It would 
 appear that the substance of hornblende and augite is 
 dimorphous, for the experiments of Berthier, Mitscherlich 
 and Q-. Kose showed that hornblende, when melted and 
 allowed to cool, assumed the crystalline form of augite; 
 whence it has been inferred that hornblende is the result 
 of slow, and augite of comparatively rapid cooling. 118 Under 
 
 28 The same results have been obtained recently by Fouque and Michel-Levy, 
 "Synthese des Mine'raux et des Roches," 1882, p. 78.
 
 GEOGNOSY 137 
 
 the microscope, augite in thin slices is only very feebly 
 pleocbroic, and presents cleavage lines intersecting at an 
 angle of 87 5'. It is often remarkable for the amount of ex- 
 traneous materials inclosed within its crystals. Like some 
 felspars, augite may be found in basalt with merely an outer 
 casing of its own substance, the core being composed of 
 magnetite, of the ground-mass of the surrounding rock, 
 or of some other mineral (Fig. 7). The distribution of au- 
 gite resembles that of hornblende; the pale, non-aluminous 
 varieties are more specially found among gneisses, marbles, 
 and other crystalline, foliated, or metamorphic rocks; the 
 dark-green or black varieties enter as essential constituents 
 into many igneous rocks of all ages, from Paleozoic up to 
 recent times (diabase, basalt, andesite, etc.). Its weather- 
 ing also agrees with that of hornblende. The aluminous 
 varieties, containing usually some lime, give rise to calca- 
 reous and ferruginous carbonates, from which the fine inter- 
 stices and cavities of the surrounding rock are eventually 
 filled with threads and kernels of calcite and strings of 
 hydrous ferric oxide. In basalt and dolerite, for example, 
 the weathered surface often acquires a rich yellow color 
 from the oxidation and hyd ration of the ferrous oxide. 
 
 Omphacite, a granular variety of pyroxene, grass green 
 in color, and commonly associated with red garnet in the 
 rock known as eclogite. 
 
 Diallage, a variety of augite, characterized by its some- 
 what metallic lustre and foliated aspect, is especially a con- 
 stituent of gabbro. 
 
 Rhombic-Pyroxenes. There are three rhombic forms of 
 pyroxene, which occur as important constituents of some 
 rocks, Enstatite, Bronzite and Hypersthene. Enstatite oc- 
 curs in Iherzolite, serpentine, and other olivine rocks; also 
 in meteorites. Bronzite is found under similar conditions 
 to enstatite, from which it is with difficulty separable. It 
 occurs in some basalts and in serpentines; also in meteor- 
 ites. Bronzite and enstatite weather into dull green ser- 
 pentinous products. Bastite or Schiller-spar is a frequent 
 product of the alteration of Bronzite or Enstatite, and may 
 be observed with its characteristic pearly lustre in serpen- 
 tine. Hypersthene occurs in hypersthenite and hypersthene- 
 andesite; also associated with other magnesian minerals 
 among the crystalline schists. 
 
 Olivine (Peridot, MgO 32-4-50-5, FeO 6-29-7, SiO.81-6- 
 42-8) forms an essential ingredient of basalt, likewise the 
 main part of various so-called olivine-rocks or peridotitea
 
 138 TEXT-BOOK OF GEOLOGY 
 
 (as Iherzolite and pikrite), and occurs in many gabbros; 
 under the microscope with polarized light, gives, when 
 fresh, bright colors, specially red and green, but it is not 
 perceptibly pleochroic. Its orthorhombic outlines can some- 
 times be readily observed, but it often occurs in irregularly 
 shaped granules or in broken crystals, and is liable to be 
 traversed by fine fissures, which are particularly developed 
 transverse to the vertical axis. It is remarkably prone to 
 alteration. The change begins on the outer surface and 
 extends inward and specially along the fissures, until the 
 whole is converted either into a green granular or fibrous 
 substance, which is probably in most cases serpentine (Fig. 
 26), or into a reddish-yellow amorphous mass (limonite). 
 
 Hauyne (SiO 8 34-06, Al 27'64, Na ? O 11-79, K 2 O 4-96, CaO 
 10-60, SO 4 11-25) occurs abundantly in Italian lavas, in basalt 
 of the Bifel, and elsewhere. 
 
 Nosean (SiO, 83-79, Al 28-76, Na 2 O 26-20, SO 4 11-26), 
 under the microscope, is one of the most readily recognized 
 minerals, showing a hexagonal or quadrangular figure, with 
 a characteristic broad dark border corresponding to the ex- 
 ternal contour of the crystal, and where weathering has not 
 proceeded too far, inclosing a clear colorless centre. It oc- 
 curs in minute forms in most phonolites, also in large crys- 
 tals in some sanidine volcanic rocks. Both hauyne and 
 nosean are volcanic minerals associated with the lavas of 
 more recent geological periods. 
 
 Epidote (Pistacite, CaO 16-30, MgO 0-4-9, Fe 2 O 3 7-5-17-24, 
 A1 2 O 8 14-47-28-9, SiO 2 33-81-57-65) occurs in many crystal- 
 line rocks, as a result of the alteration of other silicates such 
 as felspars and hornblende (see postea, p. 1040); largely dis- 
 tributed in certain schists and quartzites, sometimes associ- 
 ated with beds of magnetite and hematite. 
 
 Zoisite is allied to epidote but contains no iron. It occurs 
 in altered basic igneous rocks and also (sometimes in large 
 aggregations) in metamorphic groups. 
 
 Vesuvianite (Idocrase, CaO 27-7-37-6, MgO 0-10-6, FeO 
 0-16, A1 2 O 8 10-5-26-1, SiO 2 35-39-7, H 2 O 0-2-73) occurs in 
 ejected blocks of altered limestone at Somma, also among 
 crystalline limestones and schists. 
 
 ' Andalusite (A1 9 O 3 50-96-62-2, Fe 2 O 3 0-5-7, SiO 8 35-3-40-17). 
 Found in crystalline schists. The variety Chiastolite, 
 abundant in some dark clay-slates, is distinguished by the 
 regular manner in which the dark substance of the surround- 
 ing matrix has been inclosed, giving a cross-like transverse 
 section. These crystals have been developed in the rock
 
 GEOGNOSY 139 
 
 after its formation, and are regarded as proofs of contact- 
 metamorphism. (Book IV. Part VIII.) Sillimanite or Fi~ 
 brolite is the name given to a fibrous variety which is not 
 infrequent among schistose rocks. 
 
 Dichroite (Cordierite, lolite, MgO 8-2-20-45, FeO 0-11-58, 
 A1 2 O, 28-72-33-11, Si0 2 48-1-50-4, H 8 0-2-66) occurs in 
 gneiss, sometimes in large amount (cordierite-gneiss); occa- 
 sionally as an accessory ingredient in some granites; also in 
 talc-schist. Undergoes numerous alterations, having been 
 found changed into pinite, chlorophyllite, mica, etc. 
 
 Scapolites, a series of minerals consisting of silicates of 
 alumina, lime and soda, with a little chlorine. They are 
 found among the cavities of lavas, but more frequently 
 among metamorphic rocks, where they appear in associa- 
 tion with altered felspars. Dipyre, Couseranite and Meionite 
 are varieties of the series. 
 
 Kyanite (Al 2 SiO 5 ) occurs in bladed aggregates of a beauti- 
 ful delicate blue color among schistose rocks; also in granu- 
 lar forms. 
 
 Garnet (CaO 0-5-78, MgO 0-10-2, Fe 2 O 3 0-6-7, FeO 24-82- 
 89-68, MnO 0-6-43, A1 8 O 3 15-2-21-49, SiO 2 35-75-52-11. 
 The common red and brown varieties occur as essential con- 
 stituents of eclogite, garnet- rock; and often as abundant 
 accessories in mica-schist, gneiss, granite, etc. Under the 
 microscope, garnet as a constituent of rocks, presents three- 
 sided, four-sided, six-sided, eight-sided (or even rounded) 
 figures according to the angle at which the individual crys- 
 tals are cut; it is usually clear, but full of flaws or of cavi- 
 ties; passive in polarized light. 
 
 Tourmaline (Schorl, CaO 0-2-2, MgO 0-14-89, Na 2 O 0-4-95, 
 K 2 O 0-3-59, FeO 0-12, Fe 2 O 3 0-13-08, A1 2 O 3 30-44-44-4, SiO, 
 35-2-41-16, B 3-63-11-78, F 1-49-2-58), with quartz, forms 
 tourmaline-rock; associated with some granites; occurs also 
 diffused through many gneisses, schists, crystalline lime- 
 stones, and dolomites, likewise in sands (see Zircon). Pleo- 
 chroism strongly marked. 
 
 Zircon (ZrO, 63-5-67-16, Fe 2 O 3 0-2, SiO 2 32-35-26) occurs 
 as a chief ingredient in the zircon-syenite of Southern Nor- 
 way; frequent in granites, diorites, gneisses, crystalline lime- 
 stones and schists; in eclogite; as clear red grains in some 
 basalts, and also in ejected volcanic blocks; of common oc- 
 currence in sands, clays, sandstones, shales and other sedi- 
 mentary rocks derived from crystalline masses such as 
 granite, etc. 
 
 Thamte (Sphene, CaO 2176-33, TiO 2 33-43-5, SiO 2 30-35),
 
 140 TEXT-BOOK OF GEOLOGY 
 
 dispersed in small characteristically lozenge-shaped crystals 
 in many syenites, also in granite, gneiss, and in some vol- 
 canic rocks (basalt, trachyte, phpnoiite). 
 
 Zeolites. Under this name is included a characteristic 
 family of minerals, which have resulted from the alteration, 
 and particularly from the hydra tion, of other minerals, es- 
 pecially of felspars. Secondary products, rather than origi- 
 nal constituents of rocks, they often occur in cavities both 
 as prominent amygdales and veins, and in minute interstices 
 only perceptible by the microscope. In these minute forms 
 they very commonly present a finely fibrous divergent struc- 
 ture. As already remarked, a relation may often be traced 
 between the containing rock and its inclosed zeolites. Thus 
 among the basalts of the Inner Hebrides, the dirty green 
 decomposed amygdaloidal sheets are the chief repositories 
 of zeolites, while the firm, compact, columnar beds are com- 
 paratively free from these alteration products." Among the 
 more common zeolites are Analcime, Natrolite, Prehnite and 
 Stilbite. 
 
 Kaolin (A1 2 3 38-6-40-7, CaO 0-3-5, K 2 O 0-1-9, SiO,45'5- 
 46-53, H 2 O 9-14-54) results from the alteration of potash- and 
 soda-felspars exposed to atmospheric influences. Under the 
 microscope the fine white powdery substance is found to in- 
 clude abundant minute six-sided colorless plates and scales 
 which have been formed by recrystallization of the decom- 
 posed substance of the felspar. The purest white kaolin is 
 called china-clay, from its extensive use in the manufacture 
 of porcelain. Ordinary clay is impure from admixture of 
 iron, lime*, and other ingredients, among which the debris of 
 the undecomposed constituents of the original rock may 
 form a marked proportion. 
 
 Talc (MgO 23-19-35-4, FeO 0-4-5, A1 2 O 3 0-5-67, SiO, 
 56-62-64-53, H 2 O 0-6-65) occurs as an essential constitu- 
 ent of talc-schist, and as an alteration product replacing 
 mica, hornblende, augite, olivine, diallage, and other min- 
 erals in crystalline rocks. 
 
 Chlorite (MgO 24-9-36, FeO 0-5-9, FeA -0-11-84 A1 2 3 
 10-5-19-9, SiO 2 30-33-5, H 8 O 11'5-1 6 )> including several 
 varieties or species, occurs in small green hexagonal tables 
 or scaly vermicular or earthy aggregates ; is an essential in- 
 gredient of chlorite-schist, and occurs abundantly as an 
 alteration product (of hornblende, etc.) in fine filaments, 
 incrustations, and layers in many crystalline rocks. (See 
 
 89 See Sullivan in Jukes' "Manual of Geology," p. 85.
 
 GEOGNOSY Ul 
 
 under "Chloritization," postea, p. 1040.) Among the min- 
 erals grouped under the general head of chlorites are Ghloro- 
 phceite, Clinochlore, Delessite, Pennine, Ripidolite, and others. 
 
 Ottrelite (Chloritoid, H 2 (FeMg) Al a SiO 7 ) occurs in small 
 lustrous iron-black or greenish-black lozenge-shaped or six- 
 side plates in certain schists. It resembles chlorite but is at 
 once distinguishable from that mineral by its much greater 
 hardness. 
 
 Serpentine (MgO 28^, FeO 1-10-8, A1,O 3 0-6-5, SiO, 37-5- 
 44-5, H 2 O 9-5-14*6) is a product of the alteration of pre-ex- 
 isting minerals, and especially of olivine. It occurs in nests, 
 grains, threads, and veins in rocks which once contained 
 olivine 30 (p. 138), also massive as a rock, in which it has re- 
 placed olivine, enstatite or some other magnesian bisilicate 
 (pp. 300, 1040). Under the microscope it presents, in very 
 thin slices, a pale leek-green or bluish-green base, showing 
 aggregate polarization. Through this base runs a network 
 of dark opaque threads and veinings. Sometimes among 
 these veinings, or through the network of green serpentinous 
 matter in the base, the forms of original olivine crystals may 
 be traced (Figs. 26, 27). 
 
 Glaucomte(CaO 0-4-9, MgO 0-5-9, K O 0-12-9, Na,O 0-2-5, 
 FeO 3-25-5, Fe a O 3 0-28-1, A1 2 O 3 1-5-13-3, SiO, 46-5-60-09, 
 H 2 O 0-14-7). Found in many stratified formations, particu- 
 larly among sandstones and limestones, where it envelops 
 grains of sand, or fills and coats foraminifera and other or- 
 ganisms, giving a general green tint to the rock. It is at 
 present being formed on the sea-floor off the coasts of Geor- 
 gia and South Carolina, where Pourtales found it filling the 
 chambers of recent polythalamia. 
 
 6. CARBONATES. This familv of minerals furnishes only 
 four which enter largely into the formation of rocks, viz. 
 Carbon-ate of Calcium in its two forms, Calcite and Arago- 
 nite, Carbonate of Magnesium (and Calcium) in Dolomite, 
 and Carbonate of Iron in Siderite. 
 
 Calcite (CaCOj) occurs as (1) an original constituent of 
 many aqueous rocks (limestone, calcareous shale, etc.), 
 either as a result of chemical deposition from water (calc- 
 sinter, stalactites, etc.), or as a secretion by plants or ani- 
 mals; 81 or (2) as a secondary product resulting from weath- 
 
 30 See Tschermak, Wien. Akad. Ivi. 1867. 
 
 81 Mr. Sorby has investigated the condition in which the calcareous matter 
 of the harder parts of invertebrates exists. He finds, that in forarainifera, 
 echinoderms, brachiopods, Crustacea, and some lamellibranchs and gasteropoda,
 
 142 TEXT-BOOK OF GEOLOGY 
 
 ering, when it is found filling or lining cavities, or diffused 
 through the capillary interstices of minerals and rocks. It 
 probably never occurs as an original ingredient in the mas- 
 sive crystalline rocks, such as granite, felsite, and lavas. 
 Under the microscope, calcite is readily distinguishable by 
 its intersecting cleavage lines, by a frequent twin lamella- 
 tion (sometimes giving interference colors), strong double 
 refraction, weak or inappreciable pleochroism, and charac- 
 teristic iridescent polarization tints of gray, rose and blue. 
 
 From the readiness with which water absorbs carbon- 
 dioxide, from the increased solvent power which it thereby 
 acquires, and from the abundance of calcium in various 
 forms among minerals and rocks, it is natural that calcite 
 should occur abundantly as a pseudomorph replacing other 
 minerals. Thus, it has been observed taking tne place of a 
 number of silicates, as orthoclase, oligoclase, garnet, augite 
 and several zeolites; of the sulphates, anhydrite, gypsum, 
 barytes, and celestine; of the carbonates, aragonite, dolo- 
 mite, cerussite; of the fluoride, fluor-spar; and of the sul- 
 phide, galena. Moreover, in many massive crystalline rocks 
 (diorite, dolerite, etc.), which have been long exposed to at- 
 mospheric influence, this mineral may be recognized by the 
 brisk effervescence produced by a drop of acid, and in 
 microscopic sections it appears filling the crevices, or send- 
 ing minute veins among the decayed mineral constituents. 
 Calcite is likewise the great petrifying medium: the vast 
 majority of the animal remains found in the rocky crust of 
 the globe have been replaced by calcite, sometimes with a 
 complete preservation of internal organic structure, some- 
 times with a total substitution of crystalline material for 
 that structure, the mere outer form of the organism alone 
 surviving." 
 
 Aragonite (CaCO 8 ), harder, heavier, and much less abun- 
 dant than calcite, which is the more stable form of calcium- 
 carbonate; occurs with beds of gypsum, also in mineral 
 veins, in strings running through basalt and other igneous 
 rocks, and in the shells of many mollusca. It is thus always 
 a deposit from water, sometimes from warm mineral springs, 
 sometimes as the result of the internal alteration of rocks, 
 
 it occurs as calcite; tbat in nautilus, sepia, most gasteropoda, many lamelli- 
 branchs, etc., it is aragonite; and that in not a few cases the two forms occur 
 together, or that the carbonate of lime is hardened by an admixture of phos- 
 phate. Quart. Journ. Geol. Soc. 1879. Address, p. 61. 
 32 See index sub voc. Calcite.
 
 GEOGNOSY 143 
 
 and sometimes through the action of living organisms. Be- 
 ing more easily soluble than calcite, it has no doubt in many 
 cases disappeared from limestones originally formed mainly 
 of aragonite shells, and has been replaced by the more dur- 
 able calcite, with a consequent destruction of the traces of 
 organic origin. Hence what are now thoroughly crystalline 
 limestones may have been formed by a slow alteration of 
 such shelly deposits (p. 811). 
 
 Dolomite (Bitter-spar (Ca; Mg)CO 3 , p. 264) occurs (1) as 
 an original deposit in massive beds (magnesian limestone), 
 belonging to many different geological formations; (2) as a 
 product of alteration, especially of ordinary limestone or of 
 aragonite (Dolomitization, p. 546). 
 
 Siderite (Brown Ironstone, Spathic Iron, Chalybite, Fer- 
 rous Carbonate, FeCO 3 ) occurs crystallized in association 
 with metallic ores, also in beds and veins of many crystal- 
 line rocks, particularly with limestones; the compact argil- 
 laceous varieties (clay-ironstone) are found in abundant 
 nodules and beds in the shales of Carboniferous and other 
 formations where they have been deposited from solution in 
 water in presence of decaying organic matter (see pp. 257, 
 267). 
 
 7. SULPHATES. Among the sulphates of the mineral 
 kingdom, only two deserve notice here as important com- 
 pounds in the constitution of rocks viz. calcium-sulphate 
 or sulphate of lime in its two forms, Anhydrite and Gyp- 
 sum; and barium-sulphate or sulphate of baryta in Barytes. 
 
 Anhydrite (CaSO 4 ) occurs more especially in association 
 with beds of gypsum and rock-salt (see p. 265). 
 
 Gypsom (Selenite, CaSO 4 +2H s O). Abundant as an origi- 
 nal aqueous deposit in many sedimentary formations (see 
 p. 265). 
 
 Barytes (Heavy Spar, BaSO 4 ). Frequent in veins and 
 especially associated with metallic ores as one of their 
 characteristic vein-stones. 
 
 8. PHOSPHATES. The phosphates which occur most con- 
 spicuously as constituents or accessory ingredients of rocks 
 are the tricalcic phosphate or Apatite, and triferrous phos- 
 phate or Vivianite. 
 
 Apatite (3Ca, (PO 4 )4-CaF,) occurs in many igneous rocks 
 (granites, basalts, etc.), in minute hexagonal non-pleochroic 
 needles, giving faint polarization tints; also in large crystals 
 and massive beds associated with metamorphic rocks. 
 
 Vivianite (Blue iron-earth, Fe s P s O 8 , 8H 8 0) occurs crystal- 
 lized in metalliferous veins; the earthy variety is not infre-
 
 144 TEXT-BOOK OF GEOLOGY 
 
 quent in peat-mosses where animal matter has decayed, and 
 is sometimes to be observed coating fossil fishes as a fine 
 layer like the bloom of a plum. 
 
 9. FLUORIDES. The element fluorine, though widely 
 diffused in nature, occurs as an important constituent of 
 comparatively few minerals. Its most abundant compound 
 is with Calcium as the common mineral Fluorite. It occurs 
 also with sodium and aluminium in the mineral Cryolite. 
 
 Fluorite (Fluor-spar, CaF a ) occurs generally in veins, espe- 
 cially in association with metallic ores. 
 
 10. CHLORIDES. There is only one chloride of impor- 
 tance as a constituent of rocks sodium-chloride or common 
 salt (NaCl), which, occurring chiefly in beds, is described 
 among the rocks at p. 259. Carnallite (KClMgCl,6H.,O), 
 a hydrated chloride of potassium and magnesium, occurs 
 in beds associated with rock-salt, gypsum, etc., in some 
 salt districts (p. 260). 
 
 11. SULPHIDES. Sulphur is found united with metals 
 in the form of sulphides, many of which form common 
 minerals. The sulphides of lead, silver, copper, zinc, anti- 
 mony, etc., are of great commercial importance. Iron- 
 disulphide, however, is the only one which merits con- 
 sideration here as a rock-forming substance. It is formed 
 at the present day by some thermal springs, and has been 
 developed in many rocks as a result of the action of infil- 
 trating water in presence of decomposing organic matter and 
 iron salts. It occurs in two forms, Pyrite and Marcasite. 
 
 Pyrite (Eisenkies, Schwefelkies, FeS a ) occurs disseminated 
 through almost all kinds of rocks, often in great abundance, 
 as among diabases and clay-slates; also frequent in veins or 
 in beds. In microscopic sections of rocks, pyrite appears 
 in small cubical, perfectly opaque crystals, which with re- 
 flected light show the characteristic brassy lustre of the 
 mineral, and cannot thus be mistaken for the isometric 
 magnetite, of which the square sections exhibit a charac- 
 teristic blue-black color. Pyrite when free from marcasite 
 yields but slowly to weathering. Hence its cubical crystals 
 may be seen projecting still fresh from slates which have 
 been exposed to the atmosphere for several generations. 88 
 
 Marcasite (Hepatic pyrites) occurs abundantly among sedi- 
 mentary formations, sometimes abundantly diffused in mi- 
 nute particles which impart a blue-gray tint, and speedily 
 
 33 For an elaborate paper on the decomposition of Pyrites, see A.. A. Julien, 
 Annals New York Acad. Sci. vols. iii. and iv.
 
 GEOGNOSY 145 
 
 weather yellow on exposure and oxidation; sometimes 
 segregated in layers, or replacing the substance of fossil 
 plants or animals; also in veins through crystalline rocks. 
 This form of the sulphide is especially characteristic of 
 stratified fossiliferous rocks, and more particularly of those 
 of Secondary and Tertiary date. It is extremely liable to 
 decomposition. Hence exposure for even a short time 
 to the air causes it to become brown; free sulphuric acid 
 is produced, which attacks the surrounding minerals, some- 
 times at once forming sulphates, at other times decomposing 
 aluminous silicates and dissolving them in considerable 
 quantity. Dr. Sullivan mentions that the water annually 
 pumped from one mine in Ireland carried up to the surface 
 more than a hundred tons of dissolved silicate of alumina. 34 
 Iron disulphide is thus an important agent in effecting the 
 internal decomposition of rocks. It also plays a large part 
 as a petrifying medium, replacing the organic matter of 
 plants and animals, and leaving casts of their forms, often 
 with bright metallic lustre. Such casts when exposed to 
 the air decompose. 
 
 Pyrrhotine (Magnetic pyrites, Fe,S 8 ) is much less abundant 
 than either of the forms of ordinary iron-pyrites, from which 
 it is distinguished by its inferior hardness and its magnetic 
 character. 
 
 It will be observed that great differences exist in the 
 relative importance of the minerals above enumerated as 
 constituents of rocks. Prof. Rosenbusch points out that 
 they may be naturally arranged in four groups 1st, ores 
 and accessory ingredients (magnetite, haematite, ilmenite, 
 apatite, zircon, spinel, titanite), 2d, magnesian and ferru- 
 ginous silicates (biotite, amphibole, pyroxene, olivine), 3d, 
 felspathic constituents (felspar proper, nepheline, leucite, 
 melilite, sodalite, hauyne), 4th, free silica. 86 
 
 iii. Determination of Rocks 
 
 Rocks considered as mineral substances are distinguished 
 from each other by certain external characters, such as the 
 
 34 Jukes' "Manual of Geology," p. 65. 35 Neues Jahrb. 1882 (ii.) p. 5. 
 
 GEOLOGY Vol. XXIX 7
 
 146 TEXT-BOOK OF GEOLOGY 
 
 size, form, and arrangement of their component particles. 
 These characters, readily perceptible to the naked eye, and 
 in the great majority of cases observable in hand specimens, 
 are termed megascopic or macroscopic (pp. 146-156), to dis- 
 tinguish them from the more minute features which, being 
 only visible or satisfactorily observable when greatly mag- 
 nified, are known as microscopic (pp. 161-172). The larger 
 (geotectonic) aspects of rock-structure, which can only be 
 properly examined in the field and belong to the general 
 architecture of the earth's crust, are treated of in Book IV. 3 " 
 
 In the discrimination of rocks, it is not enough to specify 
 their component minerals, for the same minerals may con- 
 stitute very distinct varieties of rock. For example, quartz 
 and mica form the massive crystalline rock, greisen, the 
 foliated crystalline rock, mica-schist, and the sedimentary 
 rock, micaceous sandstone. Chalk, encrinal limestone, sta- 
 lagmite, statuary marble are all composed of calcite. It is 
 needful to take note of the megascopic and microscopic 
 structure and texture, the state of aggregation, color, and 
 other characters of the several masses. 
 
 Four methods of procedure are available in the investi- 
 gation and determination of rocks: 1st, megascopic (macro- 
 scopic) examination, either by the rough and ready, but 
 often sufficient, appliances for use in the field, or by those 
 for more careful work indoors; 2d, chemical analysis; 3d, 
 chemical synthesis; 4th, microscopic investigation. 
 
 i. Megascopic (Macroscopic) Examination 
 Tests in the field. The instruments indispensable for the 
 
 36 The student who would pursue physical geology by original research in 
 the field and abroad may consult Bone, "Guide du Geologue Voyageur," 2 vols. 
 1835; 6lie de Beaumont, "Lecons de Geologic pratique," vol. i. 1845; Penning 
 and Jukes-Brown. "Field Geology," 2d edit. 1880; A. Geikie, "Outlines of Field 
 Geology, " 4th. edit. 1891. F. v. Richthoferi, "Fiihrer fiir Forschungsreisende," 
 1886; Grenville Cole, "Aids in Practical Geology," 1891.
 
 GEOGNOSY 14:7 
 
 investigation of rocks in the field are few in number, and 
 simple in character and application. The observer will be 
 sufficiently accoutred if he carries with him a hammer of 
 such form and weight as will enable him to break off clean, 
 sharp, un weathered chips from the edges of rock- masses, 
 a small lens, a pocket-knife of hard steel for determining 
 the hardness of rocks and minerals, a magnet or a magne- 
 tized knife-blade, and a small pocket-phial of dilute hydro- 
 chloric acid, or better still some citric acid in powder. 
 
 Should the object be to form a collection of rocks, a 
 hammer of at least three or four pounds in weight should 
 be carried: also one or two chisels and a small trimming 
 hammer, weighing about \ lb., for reducing the specimens 
 to shape. A convenient size of specimen is 4x3x1 inches. 
 They should be as nearly as possible uniform in size, so 
 as to be capable of orderly arrangement in the drawers 
 or shelves ol a case or cabinet. Attention should be paid 
 not only to obtain a thoroughly fresh fracture of a rock, but 
 also a weathered surface, wherever there is anything char- 
 acteristic in the weathering. Every specimen should have 
 affixed to it a label, indicating as exactly as possible the 
 locality from which it was taken. This information ought 
 always to be written down in the field at the time of col- 
 lecting, and should be affixed to or wrapped up with the 
 specimen, before it is consigned to the collecting bag. If, 
 however, the student does not purpose to form a collection, 
 but merely to obtain such chips as will enable him to judge 
 of the characters of rocks, a- hammer weighing from 1J to 2 
 Ibs., with a square face and tapering to a chisel- edge at the 
 opposite end, will be most useful. The advantage of this 
 form is that the hammer can be used not only for breaking 
 hard stones, but also for splitting open shales and other 
 fissile rocks, so that it unites the uses of hammer and chisel. 
 
 It is, of course, desirable that the learner should first 
 acquire some knowledge of the nomenclature of rocks, b ; y 
 carefully studying a collection of correctly named and judi- 
 ciously selected rock-specimens. Such collections may now 
 be purchased at small cost from mineral dealers, or may be 
 studied in the museums of most towns. Having accustomed 
 his eye to the ordinary external characters of rocks, and be- 
 come familiar with their names, the student may proceed 
 to determine them for himself in the field. 
 
 Finding himself face to face with a rock-mass, and after 
 noting its geotectonic characters (Book IV.), the observer 
 will proceed to examine the exposed or weathered surface.
 
 148 TEXT-BOOK OF GEOLOGY 
 
 The earliest lesson he has to learn, and that of which per- 
 haps he will in after life meet with the most varied illustra- 
 tions, is the extent to which weathering conceals the true 
 aspect of rocks. From what has been said in previous 
 pages, the nature of some of the alterations will be under- 
 stood, and further information regarding the chemical proc- 
 esses at work will be found in Book III. The practical 
 study of rocks in the field soon discloses the fact, that 
 while, in some cases, the weathered crust so completely 
 obscures the essential character of a rock that its true 
 nature might not be suspected, in other instances, it is the 
 weathered crust that best reveals the real structure of the 
 mass. Spheroidal crusts of a decomposing yellow ferrugi- 
 nous earthy substance, for example, would hardly be iden- 
 tified as a compact dark basalt, yet, on penetrating within 
 these crusts, a central core of still undecomposed basalt 
 may not infrequently be discovered. Again, a block of 
 limestone when broken open may present only a uniformly 
 crystalline structure, yet if the weathered surface be exam- 
 ined it may show many projecting fragments of shells, 
 polyzoa, corals, crinoids, or other organisms. The really 
 fossiliferous nature of an apparently unfossiliferous rock 
 may thus be revealed by weathering. Many limestones 
 also might, from their fresh fracture, be set down as toler- 
 ably pure carbonate of lime; but from the thick crust of 
 yellow ochre on their weathered faces are seen to be highly 
 ferruginous. Among crystalline rocks, the weathered sur- 
 face commonly throws light upon the mineral constitution 
 of the mass, for some minerals decompose more rapidly than 
 others, which are thus left isolated and more easily recog- 
 nizable. In this manner, the existence of quartz in many 
 felspathic rocks may be detected. Its minute blebs or crys- 
 tals, which to the naked eye or lens are lost among the 
 brilliant facets of the felspars, stand out amid the dull clay 
 into which these minerals are decomposed. 
 
 The depth to which weathering extends should be noted. 
 The student must not be too confident that he has reached 
 its limit, even when he comes to the solid, more or less 
 hard, splintery, and apparently fresh stone. Granite some- 
 times decomposes into Kaolin and sand to a depth of twenty 
 or thirty feet or more. Limestones, on the other hand, 
 have often a mere film of crust, because their substance is 
 almost entirely dissolved and removed by rain (Book III. 
 Part II. Section ii. 2). 
 
 With some practice, the inspection of a weathered sur-
 
 GEOGNOSY 149 
 
 face will frequently suffice to determine the true nature 
 and name of a rock. Should this preliminary examination, 
 and a comparison of weathered and unweathered surfaces, 
 fail to afford the information sought, we proceed to apply 
 some of the simple and useful tests available for field-work. 
 The lens will usually enable us to decide whether the rock 
 is compact and apparently structureless, or crystalline, or 
 fragmental. Having settled this point, we proceed to ascer- 
 tain the hardness and color of streak, by scratching a fresh 
 surface of the stone. A drop of acid placed upon the 
 scratched surface or on the powder of the streak may re- 
 veal the presence of some carbonate. By practice, consider- 
 able facility can be acquired in approximately estimating 
 the specific gravity of rocks merely by the hand. The fol- 
 lowing table may be of assistance, but it must be under- 
 stood at the outset that a knowledge of rocks can never 
 be gained from instructions given in books, but must be 
 acquired by actual handling and study of the rocks them- 
 selves. 
 
 i. A fresh fracture shows the rock to be close-grained, dull, with no 
 distinct structure. 37 
 
 . H. 0-5 or less up to 1. Soft, crumbling or easily 
 scratched with the knife, if not with the finger-nail; 
 emits an earthy smell when breathed upon, does not 
 effervesce with acid; is dark gray, brown, or blue, 
 perhaps red, yellow, or even white = probably some 
 clay rock, such as mudstone, massive shale, or fire- 
 clay (p. 234); or a decomposed felspar-rock, like a 
 close-grained felsite or orthoclase porphyry. If the 
 rock is hard and fissile it may be shale or clay-slate 
 (p. 235). 
 
 /9. H. 1-5-2. Occurs in beds or veins (perhaps fibrous), 
 white, yellow, or reddish. Sp. gr. 2-2-2*4. Does 
 not effervesce= probably gypsum (pp. 143, 265). 
 Y. Friable, crumbling, soils the fingers, white, or yellow- 
 ish, brisk effervescence= chalk, marl, or some pul- 
 verulent form of limestone (pp. 244, 260). 
 d. H. 3-4. Sp. gr. 2-5-2-7. Pale to dark green or red- 
 dish, or with blotched and clouded mixtures of these 
 colors. Streak white; feels soapy; no effervescence, 
 
 31 In this table, H. = hardness ; Sp. gr. = specific gravity. The scale of 
 hardness usually employed is 1, Talc; 2, Rock-salt or gypsum; 3, Calcite; 
 4, Fluorite; 5, Apatite; 6, Orthoclase; 7, Quartz; 8, Topaz; 9, Corundum; 
 10, Diamond.
 
 150 TEXT-BOOK OF GEOLOGY 
 
 splintery to subconchoidal fracture, edges subtrans- 
 lucent. See serpentine (p. 301). 
 
 e. H. averaging 3. Sp. gr. 2-6-2-8. White, but more 
 frequently bluish-gray, also yellow, brown and 
 black; streak white; gives brisk effervescence = 
 some form of limestone (pp. 244, 260). 
 
 C. H. 3-5^-5. Sp. gr. 2-8-2-95. Yellowish, white, or 
 pale brown. Powder slowly soluble in acid with 
 feeble effervescence, which becomes brisker when 
 the acid is heated with the powder of the stone. See 
 dolomite (pp. 143, 264). 
 
 7. H. 3^. Sp. gr. 3-3-9. Dark brown to dull black, 
 streak yellow to brown, feebly soluble in acid, which 
 becomes yellow; occurs in nodules or beds, usually 
 with shale; weathers with browii or blood-red crust 
 = brown iron-ore. See clay-ironstone (pp. 256, 267); 
 and limonite(pp. 128, 266); if the rock is reddish and 
 gives a cherry-red streak, see haematite (pp. 128, 266). 
 
 6. Sp. gr. 2-55. White, gray, yellowish, or bluish, rings 
 under the hammer, splits into thin plates, does not 
 effervesce, weathered crust white and distinct=per- 
 haps some compact variety of phonolite (p. 289. See 
 also felsite, p. 280, and porphyrite, p. 292). 
 
 e. Sp. gr. 2-9-3-2. Black or dark green, weathered crust 
 yellow or brown= probably some close-grained va- 
 riety of basalt (p. 296), andesite (p. 289), aphanite 
 (p. 288), or ampnibolite (p. 314). 
 
 *. H. 6-6-5, but less according to decomposition. Sp. 
 gr. 2-55-2-7. Can with difficulty be scratched with 
 the knife when fresh. White, bluish-gray, yellow, 
 lilac, brown, red; white streak; sometimes with well 
 denned white weathered crust, no effervesce nee = 
 probably a felsitic rock (p. 280). 
 
 H. H. 7. Sp. gr. 2-5-2-9. The knife leaves a metallic 
 streak of steel upon the resisting surface. The rock 
 is white, reddish, yellowish, to brown or black, very 
 finely granular or of a horny texture, gives no reac- 
 tion with acid = probably silica in the form of jasper, 
 hornstone, flint, chalcedony, halleflinta (pp. 127, 316), 
 adinole (p. 317). 
 
 ii. A fresh fracture shows the rock to be glassy. 
 
 Leaving out of account some glass-like but crystalline 
 minerals, such as quartz and rock-salt, the number of vitre- 
 ous rocks is comparatively small. The true nature of the 
 mass in question will probably not be difficult to determine.
 
 GEOGNOSY 151 
 
 It must be one of the Massive volcanic rocks (p. 269 et seq.}. 
 If it occurs in association with siliceous lavas (liparites, 
 trachytes) it will probably be obsidian (p. 282), or pitchstone 
 (p. 283); if it passes into one of the basalt-rocks, as so com- 
 monly happens along the edges of dikes and intrusive 
 sheets, it is a glassy form of basalt (p. 297). Each of the 
 three great series of eruptive rocks, Acid, Intermediate, and 
 Basic, has its glassy varieties (see pp. 282-284, 297). 
 iii. A fresh fracture shows the rock to be crystalline. 
 If the component crystals are sufficiently large for deter- 
 mination in the field, they may suggest the name of the 
 rock. Where, however, they are too minute for identifica- 
 tion even with a good lens, the observer may require to sub- 
 mit the rock to more precise investigation at home, before 
 its true character can be ascertained. For the purposes of 
 field-work, however, the following points should be noted, 
 a. The rock can be easily scratched with the knife. 
 
 (a) Effervesces briskly with acid = limestone. (5) Pow- 
 der of streak effervesces in hot acid. See dolo- 
 mite (p. 264). (c) No effervescence with acid: 
 may be granular crystalline gypsum (alabaster) 
 or anhydrite (pp. 143, 265). 
 
 /J. The rock is not easily scratched. It is almost cer- 
 tainly a silicate. Its character should be sought 
 among the massive crystalline rocks (p. 268). If it 
 be heavy, appear to be composed of only one min- 
 eral, and have a marked greenish tint, it may be 
 some kind of amphibolite (p. 314); if it consist of 
 some white mineral (felspar) and a green mineral 
 which gives it a distinct green color, while the 
 weathered crust shows more or less distinct effer- 
 vescence, it may be a fine-grained diorite (p. 286), 
 or diabase (p. 296); if it be gray and granular, with 
 striated felspars and dark crystals (augite and mag- 
 netite), with a yellowish or brownish weathered 
 crust, it is probably a dolerite (p. 294) or andesite 
 (p. 289); if it be compact, finely-crystalline, scratched 
 with difficulty, showing crystals of orthoclase, and 
 with a bleached argillaceous weathered crust, it is 
 probably an orthoclase-porphyry (p. 285), or quartz- 
 porphyry (p. 278). The occurrence of distinct blebs 
 or crystals of quartz in the fresh fracture or weath- 
 ered face will suggest a place for the rock in the 
 quartziferous crystalline series (granites, quartz-por- 
 phyries, rhyolites), or among the gneisses and schists.
 
 152 TEXT-BOOK OF GEOLOGY 
 
 iv. A fresh fracture shows the rock to have a foliated structure. 
 
 The foliated rocks are for the most part easily recogniz- 
 able by the prominence of their component minerals (p. 303). 
 Where the minerals are so intimately mingled as not to be 
 separable by the use of the leas, the following hints may 
 be of service: 
 
 a. The rock has an unctuous feel, and is easily scratched. 
 It may be talc-schist (p. 315), chlorite-schist (p. 315), 
 sericitic mica-schist (p. 319), or foliated serpentine 
 
 (p. 316). 
 ^he 
 
 /9. The rock: emits an earthy smell when breathed on, is 
 harder than those included in a, is fine-grained, dark- 
 gray in color, splits with a slaty fracture and con- 
 tains perhaps scattered crystals of iron-pyrites or 
 some other mineral. It is some argillaceous-schist 
 or clay-slate, the varieties of which are named from 
 the predominant inclosed mineral, as chiastolite- 
 slate, andalusite-schist, ottrelite-schist, etc. (p. 309); 
 if it has a silky lustre it may be phyllite. 
 f. The rock is composed of a mass of ray-like or fibrous 
 crystals matted together. If the fibres are exceed- 
 ingly fine, silky, and easily separable, it is probably 
 asbestos; if tney are coarser, greenish to white, 
 glassy, and hard, it is probably an actinolite-schist 
 (p. 314). Many serpentines are seamed with veins 
 of the fine silky fibrous variety termed chrysotile, 
 which is easily scratched. 
 
 8. The rock has a hardness of nearly 7, and splits with 
 some difficulty along micaceous folia. It is proba- 
 bly a quartzose variety of mica-schist, quartz-schist, 
 or gneiss (pp. 309, 317-319). 
 
 e. The rock shows on its weathered surface small parti- 
 cles of quartz and folia of mica in a fine decompos- 
 ing base. It is probably a fine-grained variety of 
 mica-schist or gneiss. 
 
 v. A fresh fracture shows the rock to have a fragmental (clastic) 
 structure. 
 
 Where the component fragments are large enough to be 
 seen by the naked eye or with a lens, there is usually little 
 difficulty in determining the true nature and proper name 
 of the rock. Two characters require to be specially consid- 
 ered the component fragments and the cementing paste. 
 
 1. The Fragments. According to the shape, size, and 
 composition of the fragments, different names are assigned 
 to clastic rocks.
 
 GEOGNOSY 153 
 
 a. Shape. If the fragments are chiefly rounded, the 
 rock may be sought in the sand and gravel series (p. 224), 
 while if they are large and angular, it may be classed as a 
 breccia (p. 230). Some mineral substances, however, do not 
 acquire rounded outlines, even after long-continued attri- 
 tion. Mica, for example, splits up into thin Iamina3, which 
 may be broken into small flakes or spangles, but never be- 
 come rounded granules. Other minerals, also, which have 
 a ready cleavage, are apt to break up along their cleavage- 
 
 E lanes, and thus to retain angular contours. Gale-spar is a 
 imiliar example of this tendency. Organic remains com- 
 posed of this mineral (such as crinoids and echinoids) may 
 often be noticed in a very fragmentary condition, having 
 evidently been subjected to long-continued comminution. 
 Yet angular outlines and fresh or little worn cleavage-sur- 
 faces may be found among them. Many limestones consist 
 largely of sub-angular organic debris. ' Angular inorganic 
 detritus is characteristic of volcanic breccias and tuffs 
 (p. 238}. 
 
 /?. Size. Where the fragments are hard, rounded, or 
 sub-angular quartzose grains, the size of a pin's head or less, 
 the rock is probably some form of sandstone (p. 231). 
 Where they range up to the size of a pea, it may be a 
 pebbly sandstone, fine conglomerate or grit; where they 
 vary from the size of a pea to that of a walnut, it is an 
 ordinary gravel or conglomerate; where they range up to 
 the size of a man's head or larger, it is a coarse shingle or 
 conglomerate. A considerable admixture of sub-angular 
 stones makes it a brecciated conglomerate or breccia; but 
 where the materials are loosely aggregated, the deposit may 
 be some kind of glacial drift, such as moraine-stuft or bowl- 
 der-clay (p. 235). Large angular and irregular blocks are 
 characteristic of coarse volcanic agglomerates (p. 240). 
 
 Y. Composition. In the majority of cases, the frag- 
 ments are of o.uartz, or at least of some siliceous and endur- 
 ing mineral. Sandstones consist chiefly of rounded quartz- 
 grains (p. 231). Where these are unmixed with other 
 ingredients, the rock is sometimes distinguished as a quart- 
 zose sandstone. Such a. rock when indurated becomes 
 quartzite (p. 311). Among the quartz-grains, minute frag- 
 ments of other minerals may be observed. When any one 
 of these is prominent, it may give a name to the variety of 
 sandstone, as felspathic, micaceous (p. 186). Volcanic tuffs 
 and breccias are characterized by the occurrence of lapilli 
 (very commonlv cellular) of the lavas from the explosion of
 
 154 TEXT-BOOK OF GEOLOGY 
 
 which they have been formed. Among inter-bedded vol- 
 canic rocks, the student will meet with some which he may 
 be at a loss whether to class as volcanic, or as formed of 
 ordinary sediment. They consist of an intermixture of vol- 
 canic detritus with sand or mud, and pass on the one side 
 into true tuffs, on the other into sandstones, shales, lime- 
 stones, etc. If the component fragments of a non-crystalline 
 rock give a brisk effervescence with acid, they are calcare- 
 ous, and the rock (most likely a limestone, or at least of cal- 
 careous composition) may be searched for traces of fossils. 
 
 2. The Paste. It sometimes happens that the component 
 fragments of a clastic rock cohere merely from pressure and 
 without any discoverable matrix. This is occasionally the 
 case with sandstone. Most commonly, however, there is 
 some cementing paste. If a drop of weak acid produces 
 effervescence from between the component non-calcareous 
 grains of a rock, the paste is calcareous. If the grains are 
 coated with a red crust which, on being bruised between 
 white paper, gives a cherry-red powder, the cementing ma- 
 terial is the anhydrous peroxide of iron. A dark brown or 
 black matrix which can be dissipated by heating is bitumi- 
 nous. Where the component grains are so firmly cemented 
 in an exceedingly hard matrix that they break across rather 
 than separate from each other when the stone is fractured, 
 the paste is probably siliceous. 
 
 Determination of Specific Gravity. The student will find this 
 character of considerable advantage in enabling him to dis- 
 criminate between rocks. He may acquire some dexterity 
 in estimating, even with the hand, the probable specific 
 gravity of substances; but he should begin by determin- 
 ing it with a balance. Jolly's spring balance is a simple 
 and serviceable instrument for this purpose. It consists of 
 an upright stem having a graduated strip of mirror let into 
 it, in front of which hangs a long spiral wire, with rests at 
 the bottom for weighing a substance in air and in water. 
 For most purposes it is sufficiently accurate, and a determi- 
 nation can be made with it in the course of a few minutes. 38 
 Another and more convenient instrument has been invented 
 by W. N. "Walker, consisting -of a lever graduated into 
 inches and tenths, and resting on a knife-edge stand, on one 
 side of whjch is placed a movable weight, while on the long 
 
 88 Jolly's spring balance can be obtained through any optician or mineral 
 dealer from Berberich, of Munich, for nine florins or 27s. In the United States 
 it is manufactured by Geo. Wade & Co., at the Hoboken Institute.
 
 GEOGNOSY 155 
 
 graduated side the substance to be weighed is suspended. 
 This instrument has the advantage of not being so liable to 
 get out of order as other contrivances. 8 ' 
 
 Mechanical Analysis. Much may be learned regarding the 
 composition of a rock by reducing it to powder. In the case 
 of many sandstones and clays this reduction may easily be 
 effected by drying the stone and crumbling it between the 
 fingers. But where the material is too compact for such 
 treatment some fragments of it placed within folds of paper 
 upon a surface of steel may be reduced to powder by a few 
 smart blows of a hammer. The powder can be sifted 
 through sieves of varying degrees of fineness and the sepa- 
 rate fragments may be picked out with a fine brush and ex- 
 amined with a lens. If they are dark in color they may be 
 placed on white paper, if light-colored they are more readily 
 observed upon a black paper. Portions of this powder may 
 be carefully washed and mounted with Canada balsam on 
 glass, as in the way described below for microscopic slices. 
 In this way the constituent minerals of many crystalline 
 rocks may be isolated and studied with great facility. For 
 purposes of comparison specimens of the rock-forming min- 
 erals should be procured and treated in a similar way. A 
 series of typical preparations of the powder or minute frag- 
 ments of such minerals affords to the student an admirable 
 basis from which to start in his study of the crystallographic 
 and optical characters of the minerals which he will require 
 to identify among the constituents of rocks. 
 
 Another method of isolating the several components of 
 certain rocks is by washing the triturated materials in water 
 and allowing the sediment to subside. The finer and lighter 
 particles may be drawn off, while the coarser and heavier 
 grains will sink according to their respective specific gravi- 
 ties, and may then be separated and collected. This may 
 be done by means of a wide tube with a stop-cock at the 
 bottom, or by gently washing the powder with water on 1 an 
 inclined surface, when, as in the analogous treatment of 
 veinstones and ores in mining, the particles arrange them- 
 selves according to their respective gravities, the lightest 
 being swept away by the current. 
 
 Magnetic particles may be extracted with a magnet, the 
 end of which is preserved from contact with the powder by 
 
 39 See Geol. Mag. 1883, p. 109, for a description and drawing of this instru- 
 ment, and the manner of using it. It may be obtained of Lowden, optician, 
 Dundee, and How & Co., Farringdon Street, London. Its price is 31s. 6d.
 
 156 TEXT-BOOK OF GEOLOGY 
 
 being covered with fine tissue-paper. An electro-magnet 
 will at once withdraw the particles of minerals which con- 
 tain far too little iron to be ordinarily recognized as mag- 
 netic; in this way the particles of a ferruginous magnesian 
 mica may in a few seconds be gathered out of the powder 
 of a granite. 40 
 
 Where the difference between the specific gravity of the 
 component minerals of a rock is slight, they may "be sepa- 
 rated by means of a solution of given density. Mr. E. Son- 
 stadt proposed the use of a saturated solution of iodide of 
 mercury in iodide of potassium, which has a maximum den- 
 sity of nearly 3*2. 41 Kohrbach's solution, consisting of 
 iodide of mercury and iodide of barium, has a density of 
 as much as 3'588. 4S More serviceable is the solution of 
 borotungstate of cadium, with a density of 8 '28, proposed 
 by D. Klein. 43 The powder of a rock being introduced into 
 one of these liquids, those particles whose specific gravity- 
 exceeds that of the liquid will sink to the bottom, while 
 those which are lighter will float. This process allows of 
 the separation of the felspars from each other, and at once 
 eliminates the heavy minerals such as hornblende, augite, 
 and black mica. By the addition of water or other liquid, 
 as the case may be, the specific gravity may be reduced, and 
 different solutions of given density may be employed for de- 
 termining and isolating rock-constituents. This method of 
 analysis is important in affording a ready means of separat- 
 ing the quartz and felspar of a rock. 44 
 
 Hydrofluoric acid may be used in separating the mineral 
 constituents of rocks. The rock to be studied is reduced 
 to powder and introduced gently into a platinum capsule 
 containing the concentrated acid. During the consequent 
 effervescence, the mixture is cautiously stirred with a plati- 
 num spatula. Some minerals are converted into fluorides, 
 others into fluosilicates, while some, particularly the iron- 
 
 40 Mem. Acad. des Sci. xxxii. No. 11; Fouque and Michel-Levy, "Mineral- 
 ogie Micrographique, " p. 115. 
 
 41 Chem. News, xxix. (1874), p. 128. 44 Neues Jahrb. 1883, p. 186. 
 
 43 Compt. rend, xciii. (1881), p. 318. More recently R. Brauus has intro- 
 duced methylene iodide, which gives a density of 3 -33 and is diluted with ben- 
 zole. Neues Jahrb. 1886, ii. p. 72. See also J. W. Retgers, op. cit. 1889, 
 ii. p. 185. 
 
 44 Fouque and Michel-Levy, "Mineralogie Micrographique," p. 171. Thou- 
 let, Bull. Soc. Miii. France, ii. (1879), p. 17. A cheap form of instrument for 
 isolating minerals by means of heavy solutions is described by Mr. J. W. Evans, 
 Geol. Mag. 1891, p. 67.
 
 GEOGNOSY 157 
 
 magnesia species, remain undissolved. The thick jelly of 
 silica and alumina is removed with water, and the crystal- 
 line minerals lying at the bottom can then be dried and 
 examined. By arresting the solution at different stages the 
 different minerals may be isolated. This process is admi- 
 rably adapted for collecting the pyroxene of pyroxenic 
 rocks." 
 
 ii. Chemical Analysis 
 
 The determination of the chemical composition of rocks 
 by detailed analysis in the wet way, demands an acquaint- 
 ance with practical chemistry which comparatively few 
 geologists possess, and is consequently for the most part 
 left in the hands of chemists, who are not geologists. But 
 as some theoretical questions in geology involve a consider- 
 able knowledge of chemical processes, so a satisfactory 
 analysis of rocks is best performed by one who under- 
 stands the nature of the geological problems on which 
 such an analysis may be expected to throw light. As a 
 rule, detailed chemical analysis lies out of the sphere of 
 a geologist's work: yet the wider his knowledge of chemical 
 laws and methods the better. He should at least be able 
 to employ with accuracy the simpler processes of chemical 
 research. 
 
 Treatment with Acid. The geologist's accoutrements for 
 the field should include a small bottle of powdered citric 
 acid, or one with a mineral acid, and provided with a glass 
 stopper prolonged downward into a point. Dilute hydro- 
 chloric acid has been commonly employed; but H. C. 
 Bolton proposed in 1877 the use of organic acids in place 
 of the usual mineral acids. Citric acid is particularly ser- 
 viceable for the purpose, and has the advantage over the 
 mineral acids that it can be carried in powder, and a strong 
 solution of it in water can be made in such quantity and 
 at such time as may be required. A little of the powder 
 placed with the point of a Knife on a surface of limestone 
 and moistened with a drop of water will give the proper 
 reaction. 46 
 
 When a drop of acid gives effervescence upon a surface 
 of rock, the reaction is caused by the liberation of bubbles 
 
 45 Fouque and Michel-Levy, op. cit. p. 116. 
 
 46 Ann. New York Acad. Sci. i. (1879) p. 1. Chem. News, xxxvi. xxxvii., 
 xxxviii., xliii.
 
 158 TEXT-BOOK OF GEOLOGY 
 
 of carbon dioxide, as this oxide is replaced by the more 
 powerful acid. Hence effervescence is an indication of the 
 presence of carbonates, and when brisk is specially char- 
 acteristic of calcium-carbonate. Limestone and markedly 
 calcareous rocks may thus at once be detected. By the 
 same means, the decomposition of such rocks as dolerite 
 may be traced to a considerable distance inward from the 
 surface, the original lime-bearing silicate of the rock having 
 been decomposed by infiltrating rain-water, and partially 
 converted into carbonate of lime. This carbonate being far 
 more sensitive to the acid-test than the other carbonates 
 usually to be met with among rocks, a drop of weak cold 
 acid suffices to produce abundant effervescence even from 
 a crystalline face. But the effervescence becomes much 
 more marked if we apply the acid to the powder of the 
 stone. For this purpose, a scratch may be made and then 
 touched with acid, when a more or less copious discharge 
 of carbonic acid may be obtained, where otherwise it might 
 appear so feebly as perhaps even to escape observation. 
 Some carbonates, dolomite for example, are hardly affected 
 by acid until it is heated. This is done by placing some 
 fragments of the substance at the bottom of a test-tube, 
 covering them with acid and applying a flame. 
 
 It is a convenient method of roughly estimating the 
 purity of a limestone, to place a fragment of the rock in 
 acid. If there is much impurity (clay, sand, oxide of iron, 
 etc.), this will remain behind as an insoluble residue, and 
 may then be further tested chemically, or examined with 
 the microscope. In this way many limestones among the 
 crystalline schists may be dissolved, in acetic acid, leaving 
 a residue of pyroxenes, amphiboles, micas or other silicates. 
 Of course the acid, especially if strong mineral acid is em- 
 ployed, may attack some of the non-calcareous constituents, 
 so that it cannot be concluded that the residue absolutely 
 represents everything present in the rock except the car- 
 bonate of lime; but the proportion of non-calcareous matter 
 so dissolved by the acid will usually be small. 
 
 Further chemical processes. A thorough chemical anal- 
 ysis of a rock or mineral is indispensable for the elucida- 
 tion of its composition. But there are several processes by 
 which, until that complete analysis has been made, the 
 geologist may add to his knowledge of the chemical nature 
 of the objects of his study. It is commonly the case that 
 minerals about which he may be doubtful are precisely
 
 GEOGNOSY 159 
 
 those which, from their small size, are most difficult of 
 separation from the rest of the rock preparatory to analyt- 
 ical processes. The mineral apatite, for example, occurs 
 in minute hexagonal prisms, which on cross-fracture might 
 be mistaken for nepheline, or even sometimes for quartz. 
 If, however, a drop of nitric acid solution of molybdate 
 of ammonia be placed upon one of these crystals, a yellow 
 precipitate will appear if it be apatite. Nepheline, which is 
 another hexagonal mineral likewise abundant in some rocks, 
 gives no yellow precipitate with the ammonia solution, while 
 if a drop of hydrochloric acid be put over it, crystals of 
 chloride of sodium or common salt will be obtained. These 
 reactions can be observed even with minute crystals or frag- 
 ments, by placing them on a glass slide under the micro- 
 scope and using an exceedingly attenuated pipette for drop- 
 ping the liquid on the slide.** 
 
 Two ingenious applications of chemical processes to the 
 determination of minute fragments of minerals are now in 
 use. In one of these, devised by Boricky, 48 hydrofluosilicic 
 acid of extreme purity is employed. This acid decomposes 
 most silicates, and forms from their bases hydrofluosilicates. 
 A particle about the size of a pin's head of the mineral to 
 be examined is fixed by its base upon a thin layer of Canada 
 balsam spread upon a slip of glass, and a drop of the acid is 
 placed upon it. The preparation is then set in moist air 
 near a saucer of water under a bell-glass for twenty-four 
 hours, after which it is inclosed in dry air, with chloride of 
 calcium. In a few hours the hydrofluosilicates crystallize 
 out upon the balsam and can be examined with the micro- 
 scope. Those of potassium take the form of cubes, of 
 sodium hexagonal prisms, etc. 
 
 The second process, devised by Szabo, consists in utiliz- 
 ing the colorations given to the 'flame of a Bunseu-burner 
 by sodium and potassium. An elongated splinter of the 
 mineral to be examined is first placed in the outer or oxi- 
 dizing part of the flame near the base, and then in the re- 
 
 47 An excellent treatise on the chemical examination of minerals under the 
 microscope is that by MM. Klement and Renard, "Reactions microchemiques A 
 cristaux et leur application en analyse qualitative," Brussels, 1886. See also 
 H. Behrens, Ann. ficole Polytechnique de Delft, i. 1885, p. 176; Neues Jahrb. 
 vii. Beilage Band. p. 435; Zeitsch. f. Analyt. Chemie, xxx. ii. p. 126-174 (1891). 
 
 48 Archiv Naturwiss. Landesdurchforschung von Bohmen, iii. fasc. 3, 1876. 
 "Elemente einer neuen chemisch-mikroskopischen Mineral- und Gesteinsana- 
 lyae." Prag. 1877.
 
 TEXT-BOOK OF GEOLOGY 
 
 ducing part further up and nearer the centre. The amount 
 of sodium present in the mineral is indicated by the extent 
 to which the flame is colored yellow. The potassium is 
 similarly estimated, but the flame is then looked at with 
 cobalt glass, so as to eliminate the influence of the sodium. 4 ' 
 Blow-pipe Tests. The chemical tests with the blow-pipe 
 are simple, easily applied, and require only patience and 
 practice to give great assistance in the determination of 
 minerals, if unacquainted with blow-pipe analysis, the 
 student must refer to one or other of the numerous text- 
 books on the subject, some of which are mentioned below. 80 
 For early practice the following apparatus will be found 
 sufficient: 
 
 1. Blow-pipe. 
 
 2. Thick-wicked candle, or a tin box filled with the 
 material of Child's night-lights, and furnished with a piece 
 of Freyberg wick in a metallic support. 
 
 3. "Platinum-tipped forceps. 
 
 4. A few pieces of platinum wire in lengths of three or 
 four inches. 
 
 5. A few pieces of platinum foil. 
 
 6. Some pieces of cnarcoal. 
 
 7. A number of closed and open tubes of hard glass. 
 
 8. Three small stoppered bottles containing sodium- 
 carbonate, borax, and microcosmic salt. 
 
 9. Magnet. 
 
 This list can be increased as experience is gained. The 
 whole apparatus may easily be packed into a box which will 
 go into the corner of a portmanteau. 
 
 iii. Chemical Synthesis 
 
 As already remarked (p. 118), much interesting light has 
 been thrown on the natural conditions in which minerals 
 
 49 Szabo, "Deber eine neue Methods die Felspathe auch in Gesteinen zu 
 bestimmen." Buda-Pesth, 1876. 
 
 50 The great work on the blow-pipe is Plattner's, of which an English trans- 
 lation has been published. Elderhorst's "Manual of Qualitative Blow-pipe 
 Analysis and Determinative Mineralogy," by H. B. Nason and C. F. Chandler 
 (Philadelphia: N. S. Porter and Coates), is a smaller but useful volume; while 
 still less pretending is Scheerer's "Introduction to the Use of the Mouth Blow- 
 pipe," of which a third edition by H. F. Blandford was published in 1875 by 
 F. Norgate. An admirable work of reference will be found in Prof. Brush's 
 "Manual of Determinative Mineralogy" (New York: J. Wiley and Son). F. v. 
 KobelPs "Tafeln zur Bestimmung der Mineralien" (Munich) are useful. A valu- 
 able summary will be found in Prof. Cole's "Aids in Practical Geology," 1891.
 
 GEOGNOSY 161 
 
 and rocks have been formed, by actual experiments in 
 which these bodies are reproduced artificially. Since the 
 classic experiments of Hall much progress has been made 
 in this subject, notably from the prolonged and admirable 
 researches carried on in Paris by Prof. Dauhre'e and by 
 Messrs. Fouque" and Michel-LeVy. To some of the results 
 obtained by these observers reference will be made in Book 
 III. Part I. Sect. iv. The processes of investigation have 
 been grouped in three classes. 1st. Those by the "dry 
 way" as in fusion and sublimation, sometimes simply, 
 sometimes with the' intervention of a mineralizing agent 
 such as borax, borates, fluorides, chlorides, etc. 2d. Those 
 by the "wet way" where water or steam are used as dis- 
 solvents either by themselves or with the aid of some 
 mineralizing agent; and 3d, Those where some combina- 
 tion of the two foregoing methods is employed, that is, 
 where water or steam is made to act at a high temperature 
 and under great pressure." 
 
 iv. Microscopic Investigation" 
 
 The value of the microscope as an aid in geological re- 
 search is now everywhere acknowledged. Some informa- 
 tion may here be given as to the methods of procedure in 
 microscopical inquiry. 
 
 1. Preparation of microscopic slides of rocks and minerals. The 
 observer ought to be able to prepare his own slices, and in 
 many cases will find it of advantage to do so, or at least 
 personally to superintend their preparation by others. It 
 is desirable that he should know at the outset that no costly 
 or unwieldy set of apparatus is needful for his purpose. If 
 he is resident in one place and can accommodate a cutting 
 machine such as a lapidary's lathe, he will find the process 
 
 51 See on this subject Daubree's great work "Geologie Experimentale, " 
 1879; Fouque et Michel-Levy, "Synthese des Mine>aux et des Roches," 1882; 
 Stanislas Meunier, "Les M&hodes de Synthese en MineValogie, " 1891; also 
 postea, p. 513 et seq. 
 
 5i On the microscopic investigation of rocks consult Fouque and Michel-Levy, 
 "MineralogieMicrographique," 2 vols. Paris, 1879; Michel -Levy, "Les Mineraux 
 des Roches, ' ' Paris, 1888 ; Michel-Levy and Lacroix, "Tableaux des Mineraux des 
 Roches," 1889; Rosenbusch, "Mikroskopische Physiographic der Mineralien 
 und Gesteine," 2 vols., one of which has been translated into English by 
 Iddinga and published by Macmillan & Co. ; also his "Hulfstabellen zur Mikro- 
 skopischen Mineralbestimmung," 1888, translated into English by F. H. Hatch 
 aud published by Swan Som;enschein & Co. ; F. Rutley, "Rock-forming Min- 
 erals," London, 1888, and Prof. Cole's volume above cited.
 
 162 TEXT-BOOK OF GEOLOGY 
 
 of preparing rock-slices greatly facilitated." The thickness 
 of each slice must be mainly regulated by the nature of the 
 rock, the rule being to make the slice as thin as can con- 
 veniently be cut, so as to save labor in grinding down 
 afterward. Perhaps the thickness of a shilling may be 
 taken as a fair average. The operator, however, may still 
 further reduce this thickness oy cutting and polishing 
 a face of the specimen, cementing that on glass in the way 
 to be immediately described, and then cutting as close as 
 possible to the cemented surface. The thin slice thus left on 
 the glass can then be ground down with comparative ease. 
 
 Excellent rock-sections, however, may be prepared with- 
 out any machine, provided the operator possesses ordinary 
 neatness of hand and patience. He must procure as thin 
 chips as possible. Should the rocks be accessible to him 
 in the field, he should select the freshest portions of them, 
 and by a dexterous use of the hammer, break off from 
 a sharp edge a number of thin splinters or chips, out of 
 which he can choose one or more for rock-slices. These 
 chips may be about an inch square. It is well to take sev- 
 eral of them, as the first specimen may chance to be spoiled 
 in the preparation. The geologist ought also always to carry 
 off a piece of the same block from which his chip is taken, 
 that he may have a specimen of the rock for future reference 
 and comparison. Every such hand-specimen, as well as the 
 chips belonging to it, ought to be wrapped up in paper on 
 the spot where it is obtained, and with it should be placed 
 a label containing the name of the locality and any notes 
 that may be thought necessary. It can hardly be too fre- 
 quently reiterated that all such field-notes ought as far as 
 possible to be written down on the ground, when the actual 
 facts are before the eye for examination. 
 
 53 A. machine well adapted for both cutting and polishing was devised some 
 years ago by Mr. J. B. Jordan, and may be had of Messrs. Cotton and John- 
 son, Gerrard" Street, Soho, London, for 10 10s. Another slicing and polishing 
 machine, invented by Mr. F. G. Cuttell, costs 6 10s. These machines are too 
 unwieldy to be carried about the country by a field-geologist. Fuess o f Berliu 
 supplies two small and convenient hand-instruments, one for slicing, the other 
 for grinding and polishing. The slicing-machine is not quite so satisfactory 
 for hard rocks as one of the larger, more solid forms of apparatus worked by 
 a treadle. But the grinding-machine is useful, and might be added to a geolo- 
 gist's outfit without material inconvenience. If a lapidary is within reach, 
 much of the more irksome part of the work may be saved by getting him to 
 cut off the thin slices in directions marked for him upon the specimens. Many 
 lapidaries now undertake the whole labor of cutting and mounting microscopic 
 slides.
 
 GEOGNOSY 168 
 
 Having obtained his thin slices, either by having them 
 slit with a machine or by detaching with a hammer as thin 
 splinters as possible, the operator may proceed to the prepa- 
 ration of them for the microscope. For this purpose the 
 following simple apparatus is all that is absolutely needful, 
 though if a grinding-machine be added it will save time 
 and Tabor. 
 
 List of Apparatus required in the Preparation of Thin Slices 
 of Rocks and Minerals for Microscopical Examination 
 
 1. A cast-iron plate \ inch thick and 9 inches square. 
 
 2. Two pieces of plate-glass, 9 inches square. 
 
 3. A Water of Ayr stone, 6 inches long by 2* inches 
 broad. 
 
 4. Coarse emery (1 Ib. or so at a time). 
 
 5. Fine or flour-emery (ditto). 
 
 6. Putty powder (1 oz.). 
 
 7. Canada balsam. (There is an excellent kind prepared 
 by Rimmington, Bradford, specially for microscopic prepa- 
 rations, and sold in shilling bottles.) 
 
 8. A small forceps, and a common sewing-needle with 
 its head fixed in a cork. 
 
 9. Some oblong pieces of common flat window-glass; 
 2x1 inches is a convenient size. 
 
 10. Glasses with ground edges for mounting the slices 
 upon. They may be had at any chemical instrument mak- 
 er's in different sizes, the commonest in this country being 
 3x1 inches, though this size is rather too long for conven- 
 ient handling on a rotating stage. 
 
 11. Thin covering-glasses, square or round. These are 
 sold by the ounce; J oz. will be sufficient to begin with. 
 
 12. A small bottle of spirits of wine. 
 
 The first part of the process consists in rubbing down 
 and polishing one side of the chip or slice, if this has not 
 already been done in cutting off a slice affixed to glass, as 
 above mentioned. We place the chip upon the wheel of the 
 grinding-machine, or, failing that, upon the iron plate, with 
 a little coarse emery and water. If the chip is so shaped 
 that it can be conveniently pressed by the finger against the 
 plate and kept there in regular horizontal movement, we 
 may proceed at once to rub it down. If, however, we find 
 a difficulty, from its small size or otherwise, in holding the 
 chip, one side of it may be fastened to the end of a bobbin
 
 164 TEXT-BOOK OF GEOLOGY 
 
 or other convenient bit of wood by means of a cement 
 formed of three parts of resin and one of beeswax, which 
 is easily softened by heating. A little practice will show 
 that a slow, equable motion with a certain steady pressure is 
 most effectual in producing the desired flatness of surface. 
 When all the roughnesses have been removed, which can be 
 told after the chip has been dipped in water so as to remove 
 the mud and emery, we place the specimen upon the square 
 of plate-glass, and with flour-emery and water continue to 
 rub it down until all the scratches caused by the coarse 
 emery have been removed and a smooth polished surface 
 has been produced. 54 Care should be taken to wash the 
 chip entirely free of any grains of coarse emery before the 
 polishing on glass is begun. It is desirable also to reserve 
 the glass for polishing only. The emery gets finer and finer 
 the longer it is used, so that by remaining on the plate it 
 may be used many times in succession. Of course the glass 
 itself is worn down, but by using alternately every portion 
 of its surface and on both sides, one plate may be made to 
 last a considerable time. If after drying and examining it 
 carefully, we find the surface of the chip to be polished and 
 free from scratches, we may advance to the next part of the 
 process. But it will often happen that the surface is still 
 finely scratched. In this case we may place the chip upon 
 the Water of Ayr stone and with a little water gently rub it 
 to and fro. It should be held quite flat. The Water of 
 Ayr stone, too, should not be allowed to get worn into a 
 hollow, but should also be kept quite flat, otherwise we 
 shall lose part of the chip. Some soft rocks, however, will 
 not take an unscratched surface even with the Water of Ayr 
 stone. These may be finished with putty powder, applied 
 with a bit of woollen rag. 
 
 The desired flatness and polish having been secured, and 
 all trace of scratches and airt having been completely re- 
 moved, we proceed to a further stage," which consists in 
 grinding down the opposite side and reducing the chip to 
 the requisite degree of thinness. The first step is now to 
 cement the polished surface of the chip to one of the pieces 
 of common glass. A thin piece of iron (a common shovel 
 
 some 
 pour- 
 
 M Exceedingly impalpable emery powder may be obtained by stirring 
 of the finest emery in water, and after the coarse particles have subsided, 
 ing off the liquid and allowing the fine suspended dust gradually to subsida 
 Filtered and dried, the residue can be kept for the more delicate parts of the 
 polishing.
 
 GEOGNOSY 165 
 
 does quite well) is heated over a fire, or is placed between 
 two supports over a gas-flame." On this plate must be laid 
 the piece of glass to which the slice is to be affixed, together 
 with the slice itself. A little Canada balsam is dropped on 
 the centre of the glass and allowed to remain until it has ac- 
 quired the necessary consistency. To test this condition, 
 the point of a knife should be inserted into the balsam, and 
 on being removed should be rapidly cooled by being pressed 
 against some cold surface. If it soon becomes hard enough 
 to resist the pressure of the finger nail, it has been suffi- 
 ciently heated. Care, however, must be observed not to let 
 it remain too long on the hot plate; for it will then become 
 brittle and start from the glass at some future stage, or at 
 least will break away from the edges of the chip and leave 
 them exposed to the risk of being frayed off. The heat 
 should be kept as moderate as possible, for if it becomes too 
 great it may injure some portions of the rock. Chlorite, for 
 example, is rendered quite opaque if the heat is so great as 
 to drive off its water. 
 
 When the balsam is found to be ready, the chip, which 
 has been warmed on the same plate, is lifted with the for- 
 ceps, and laid gently down upon the balsam. It is well to 
 let one end touch the balsam first, and then gradually to 
 lower the other, as in this way the air is driven out. With 
 the point of a needle or a knife the chip should be moved 
 about a little, so as to expel any bubbles of air and promote 
 a firm cohesion between the glass and the stone. The glass 
 is now removed with the forceps from the plate and put 
 upon the table, and a lead weight or other small heavy ob- 
 ject is placed upon the chip, so as to keep it pressed down 
 until the balsam has cooled and hardened. If the operation 
 has been successful, the slide ought to be ready for further 
 treatment as soon as the balsam has become cold. If, how- 
 ever, the balsam is still soft, the glass must be again placed 
 on the plate and gently heated, until on cooling, the balsata. 
 fulfils the condition of resisting the pressure of the finger- 
 nail. 
 
 Having now produced a firm union of the chip and the 
 glass, we proceed to rub down the remaining side of the 
 stone with coarse emery on the iron plate as before. If 
 the glass cannot be held in the hand or moved by the sim- 
 
 " A piece of wire-gauze placed over the flame, with an interval of an inch 
 or more between it and the overlying thin iron plate, tends to diffuse the heat 
 -nd prevail: th? ^wlsam fi -om being unequally heated.
 
 166 TEXT-BOOK OF OEOLOGY 
 
 pie pressure of i,be fingers, which usually suffices, it may be 
 fastened to the end of the bobbin with the cement as before. 
 When the chip has been reduced until it is tolerably thin; 
 until, for example, light appears through it when held be- 
 tween the eye and the window, we may, as before, wash it 
 clear of the coarse emery and continue the reduction of it on 
 the glass plate with fine emery. Crystalline rocks, such 
 as granite, gneiss, diorite, dolerite, and modern lavas, can 
 be thus reduced to the required thinness on the glass plate. 
 Softer rocks may require gentle treatment with the Water 
 of Ayr stone. 
 
 The last parts of the process are the most delicate of all. 
 We desire to make the section as thin as possible, and for 
 that purpose continue rubbing until after one final attempt 
 we may perhaps find to our dismay that great part of the 
 slice has disappeared. The utmost caution should be used. 
 The slide should be kept as flat as possible, and looked at 
 frequently, that the first indications of disruption may be 
 detected. The thinness desirable or attainable depends in 
 great measure upon the nature of the rock. Transparent 
 minerals need not be so much reduced as more opaque ones. 
 Some minerals, indeed, remain absolutely opaque to the 
 last, like pyrite, magnetite, and ilmenite. 
 
 The slide is now ready for the microscope. It ought al- 
 ways to be examined with that instrument at this stage. 
 We can thus see whether it is thin enough, and if any 
 chemical tests are required they can readily be applied to 
 the exposed surface of the slice. If the rock has proved 
 to be very brittle, and we have only succeeded in procuring 
 a thin slice after much labor and several failures, nothing 
 further should be done with the preparation, unless to cover 
 it with glass, as will be immediately explained, which not 
 only protects it, but adds to its transparency. But where 
 the slice is not so fragile, and will bear removal from its 
 original rough scratched piece of glass, it should be trans- 
 ferred to one of the glass-slides (No. 10). For this purpose, 
 the preparation is once more placed on the warm iron plate, 
 and close alongside of it is put one of the pieces of glass 
 which has been carefully cleaned, and on the middle of 
 which a little Canada balsam has been dropped. The heat 
 gradually loosens the cohesion of the slice, which is then 
 very gently pushed with the needle or knife along to the 
 contiguous clean slip of glass. Considerable practice is 
 needed in this part of the work, as the slice, being so thin, 
 is apt to go to pieces in being transferred. A gentle incli-
 
 GEOGNOSY 167 
 
 nation of the warm plate, so that a tendency may be given 
 to the slice to slip downward of itself on to the clean glass, 
 may be advantageously given. We must never attempt to 
 lift the slice. All shifting of its position should be per- 
 formed with the point of the needle or other sharp instru- 
 ment. If it goes to pieces we may yet be able to pilot the 
 fragments to their resting-place on the balsam of the new 
 glass, and the resulting slide may be sufficient for the re- 
 quired purpose. 
 
 When the slice has been safely conducted to the centre 
 of the glass slip, we put a little Canada balsam over it, and 
 warm it as before. Then taking one of the thin cover- 
 glasses with the forceps, we allow it gradually to rest upon 
 the slice by letting down first one side, and then by degrees 
 the whole. A few gentle circular movements of the cover- 
 glass with the point of the needle or forceps may be needed 
 to insure the total disappearance of air-bubbles. When 
 these do not appear, and when, as before, we find that the 
 balsam has acquired the proper degree of consistence, 
 the slide containing the slice is removed, and placed on 
 the table with a small lead weight above it in the same 
 way as already described. On becoming quite cold and 
 hard the superabundant balsam round the edge of the cover- 
 glass may be scraped off with a knife, and any which still 
 adheres to the glass may be removed with a little spirits of 
 wine. Small labels should be kept ready for affixing to the 
 slides to mark localities and reference numbers. Thus 
 labelled, the slide may be put away for future study and 
 comparison. 
 
 The whole process seems perhaps a little tedious. But 
 in reality much of it is so mechanical, that after the mode 
 of manipulation has been learned by a little experience, the 
 rubbing-down may be done while the operator is reading. 
 Thus in the evening, when enjoying a pleasant book after 
 his day in the field, he may at the same time, after som'e 
 practice, rub down his rock-chips, and thus get over the 
 drudgery of the operation almost unconsciously. 
 
 Boxes, with grooved sides or with flat trays for carrying 
 microscopic slides, are sold in different sizes. Such boxes 
 are most convenient for a travelling equipage, as they go 
 into small space, and with the help of a little cotton-wool 
 they hold the glass slides firmly without the risk of break- 
 age. For a final resting-place, a case with shallow trays or 
 drawers in which the slides can lie flat is most convenient. 
 
 2. The Microscope. Unless the observer proposes to enter
 
 168 TEXT-BOOK OF GEOLOGY 
 
 into great detail in the investigation of the minuter parts of 
 rock structure, he does not require a large and expensive in- 
 strument. For most geological purposes, objectives of 2, 1, 
 and J inch focal length are sufficient. But it is desirable 
 also for special work, such as the investigation of crystal- 
 lites and inclusions of minerals, to have an objective capable 
 of magnifying up to 200 or 300 diameters. An instrument 
 with fairly good glasses of these powers, according to the 
 arrangement of object-glasses and eye-pieces, may be had of 
 some London makers for 5. But for some of the most im- 
 portant parts of the microscopical study of rocks a rotating 
 stage is requisite, the presence of which necessarily adds to 
 the cost or the instrument. One of the best microscopes 
 specially adapted for petrographical research is that devised 
 by Mr. A. Dick, and manufactured by Swift & Son, of 81 
 Tottenham Court Road, London, price 18 without objec- 
 tives. 
 
 Among the indispensable adjuncts are two Nicol-prisms, 
 one (polarizer) to be fitted below the stage, the other (ana- 
 lyzer) most advantageously placed over the eye-piece. A 
 quartz-wedge is useful in examination with polarized light. 
 A nose-piece for two objectives, screwed to the foot of the 
 tube, saves time and trouble by enabling the observer at 
 once to pass from a low to a high power. The numerous 
 pieces of apparatus necessary for physiological work are not 
 needed in the examination of rocks and minerals. 
 
 8. Methods of Examination. A few hints may be here given 
 for the guidance of the student in making his own micro- 
 scopic observations, but he must consult some of the special 
 treatises, mentioned on p. 161, for full details. 
 
 Reflected Light. It is not infrequently desirable to ob- 
 serve with the microscope the characters of a rock as an 
 opaque object. This cannot usually be done with a broken 
 fragment of the stone, except of course with very low pow- 
 ers. Hence one of the most useful preliminary examinations 
 of a prepared slice is to place it in the field, and, throwing 
 the mirror out of gear, to converge as strong a light upon it 
 as can be had, short of bright direct sunlight. The observer 
 can then see some way into the rock and observe the rela- 
 tive^ thicknesses and forms of its constituents. The advan- 
 tage' of this method is particularly noticeable in the case of 
 opaque minerals. The sulphides and iron-oxides so abun- 
 dant in rocks appear as densely black objects with trans- 
 mitted light, and show only their external form. But by 
 throwing a strong light upon their surface, we may often
 
 GEOGNOSY 169 
 
 discover not only their distinctive colors, but their charac- 
 teristic internal structure. Titaniferous iron is an admirable 
 example of the advantage of this method. Seen with trans- 
 mitted light, that mineral appears in black, structureless 
 grains or opaque patches, though frequently bounded by 
 definite lines and angles. But with reflected light, the 
 cleavage and lines of growth of the mineral can then often 
 be clearly seen, and what seemed to be uniform black 
 patches are found in many cases to inclose bright brassy 
 kernels of pyrite. Magnetite also presents a characteristic 
 blue-black color, which distinguishes it from the other 
 iron-oxides. 
 
 Transmitted Light. It is, of course, with the light 
 allowed to pass through prepared slices that most of the 
 microscopic examination of minerals and rocks is per- 
 formed. A little experience will show the learner that, 
 in viewing objects in this way, he may obtain somewhat 
 different results from two slices of the same rock according 
 to their relative thinness. In the thicker one, a certain 
 mineral or rock, obsidian for example, will appear perhaps 
 brown or almost black, while in the other what is evidently 
 the same substance may be pale yellow, green, brown, or 
 almost colorless. Triclinic felspars seen in polarized light 
 give only a pale milky light when extremely thin, but 
 present bright chromatic bands when somewhat thicker. 
 
 Polarized Light. By means of polarized light, an ex- 
 ceedingly delicate method of investigation is made avail- 
 able. We use both the Nicol-prisms. If the object be 
 singly-refracting, such as a piece of glass, or an amorphous 
 body, or a crystal belonging to some substance which crys- 
 tallizes in the isometric or cubic system (or if it be a tetrag- 
 onal, hexagonal or rhombohedral crystal, cut perpendicular 
 to its principal axis), the light will reach our eye apparently 
 unaffected by the intervention of the object. The field will 
 remain dark when the axes of the two prisms are at right 
 angles (crossed Nicols), in the same way as if no interven- 
 ing object were there. Such bodies are isotropic. In all 
 other cases, the substance is doubly-refracting and modifies 
 the polarized beam of light. On rotating one of the prisms, 
 we perceive bands or flashes of color, and numerous lines 
 appear which before were invisible. The fie.d no longer re- 
 mains dark when the two Nicol-prisms are crossed. Such 
 a substance is anisotropic. 
 
 M But the effect of pressure may give weak color-tints in glasses and in 
 cubic crystals. 
 
 GEOLOGY Vol. XXIX 8
 
 170 TEXT-BOOK OF GEOLOGY 
 
 It is evident, therefore, that we may readily tell by this 
 means whether or not a rock contains any glassy constit- 
 uent. If it does, then that portion of its mass will become 
 dark when the prisms are crossed, while the crystalline 
 parts which, in the vast majority of cases, do not belong 
 to the cubic system, will remain conspicuous by their 
 brightness. A thin plate of quartz makes this separation 
 of the glassy and crystalline parts of a rock even more 
 satisfactory. It is placed between the Nicol-prisms, which 
 may be so adjusted with reference to it that the field of the 
 microscope appears uniformly violet. The glassy portion 
 of any rock, being singly-refracting or isotropic, placed on 
 the stage will allow the violet light to pass through un- 
 changed, but the crystalline portions, being doubly-refract- 
 ing or anisotropic, will alter trie violet light into other pris- 
 matic colors. The object should be rotated in the field, 
 and the eye should be kept steadily fixed upon one portion 
 of the slide at a time, so that any change may be observed. 
 This is an extremely delicate test for the presence of glassy 
 and crystalline constituents. 
 
 In searching for the crystallographic system to which 
 a mineral in a microscopic slide should be referred, atten- 
 tion is given to the directions in which the mineral placed 
 between crossed Nicols appears dark, or to what are called 
 the directions of its extinction. It is extinguished (that is, 
 the normal darkness of the field between the crossed Nicols 
 is ^restored) when two of its axes of elasticity for vibrations 
 of light coincide with the principal sections of the two 
 prisms. During a complete rotation of the slide in the 
 field of the microscope the mineral becomes dark in four 
 positions 90 apart, each of which marks that coincidence. 
 W hen, on the other hand, the prisms are placed parallel to 
 each other, the coincidence of their principal sections with 
 the axes of elasticity in the mineral allows the maximum 
 of light to pass through, which likewise occurs four times 
 in a complete rotation of the mineral. The different crys- 
 tallographic systems are distinguishable by the relation be- 
 tween their crystallographic axes and their axes of elasticity. 
 By noting this relation in the case of any given mineral (and 
 there are usually sections enough of each mineral in the 
 same rock-slice to furnish the required data) its crystalline 
 system may be fixed. But in many cases it has-been found 
 possible to establish characteristic distinctions for individual 
 mineral species, by noting the angle between the direction 
 of their extinction and certain principal faces.
 
 GEOGNOSY 171 
 
 The determination of whether the component grains of 
 a rock belong to uniaxial or biaxial doubly-refracting min- 
 erals is a point of much importance, which is effected by 
 means of an achromatic condenser inserted in the aperture 
 of the stage below the slide and suitably adjusted so as to 
 converge the rays of light within the grain or crystal. The 
 Nicols having been crossed, the eye-piece is removed, and 
 the eye when held a little distance from the open end of the 
 tube will perceive a dark bar, ring, or cross move across 
 the field as the stage is rotated, if the mineral examined 
 has been cut at a Favorable angle. By the form and be- 
 havior of these indications the uniaxial or biaxial character 
 is made evident. 
 
 Pleochroism (Dichroism). Some minerals show a change 
 of color when a Nicol-pnsm is rotated below them; horn- 
 blende, for example, exhibiting a gradation from deep 
 brown to dark yellow. A mineral presenting this change 
 is said to be pleochroic (polychroic, dichroic, trichroic). 
 To ascertain the pleochroism of any mineral we may re- 
 move the upper polarizing prism (analyzer) and leave only 
 the lower (polarizer). If as we rotate the latter, no change 
 of tint can be observed, there is no pleochroic mineral 
 present, or at least none which shows pleochroism at the 
 angle at which it has been bisected in the slice. But in 
 a slice of any crystalline rock, crystals may usually be 
 observed which offer a change of hue as the prism goes 
 round. These are examples of pleochroism. This behavior 
 may be used to detect the mineral constituents of rocks. 
 Thus the two minerals hornblende and augite, which in so 
 many respects resemble each other, cannot always be distin- 
 guished by cleavage angles, in microscopic slices. But as 
 Tschermak pointed out, augite remains passive or nearly 
 so as the lower prism is rotated: it is not pleochroic, or 
 only very feebly so; while hornblende, on the other hand, 
 especially in its darker varieties, is usually strongly pleo- v 
 chroic. It is to be observed, however, that the same mineral 
 is not always equally pleochroic, and that the absence of 
 this property is therefore less reliable as a negative test, 
 than its presence is as a positive test. 
 
 It would be beyond the scope of this volume to enter 
 into the complicated details of the microscopic structure 
 of minerals and rocks. This information must be sought 
 in some of the works specially devoted to it, a few of which 
 are cited on p. 161. 
 
 In his examination of rocks with the microscope, the
 
 172 TEXT-BOOK OF GEOLOGY 
 
 student may find an advantage in propounding to himself 
 the following questions, and referring to the pages here 
 cited. 
 
 1st, Is the rock entirely crystalline (pp. 174, 258, 268), 
 consisting solely of crystals of different minerals interlaced; 
 and if so, what are these minerals ? 2d, Is there any trace 
 of a glassy ground-mass or base (pp. 178, 204)? Should 
 this be detected, the rock is certainly of volcanic origin 
 (pp. 282, 297). 3d, Can any evidence be found of the de- 
 vitrification of what may have been at one time the glassy 
 basis of the whole rock? This devitrification might be 
 shown by the appearance of numerous microscopic hairs, 
 rods, bundles of feather-like irregular or granular aggrega- 
 tions (p. 204). 4th, In what order did the minerals crystal- 
 lize? This may often be made out with a microscope, as, 
 for instance, where one mineral is inclosed within another 
 (p. 204)." 5th, "What is the nature of any alteration which 
 the rock may have undergone ? In a vast number of cases 
 the slices show abundant evidence of such alteration: fel- 
 spar passing into granular kaolin, augite changing into 
 viridite, olivine into serpentine, while secondary calcite, 
 epidote, quartz, and zeolites run in minute veins or fill up 
 interstices of the rock (p. 587). 6th, Is the rock a frag- 
 mental one; and if so, what is the nature of its component 
 grains (pp. 224-225)? Is any trace of organic remains to 
 be detected? 
 
 iv. General outward or Megascopic (Macroscopic) Characters 
 of Rocks 68 
 
 1. Structure. 69 The different kinds of rock-structures dis- 
 
 51 It is possible, however, that a crystal inclosed within another may some- 
 times have crystallized there out of a portion of the surrounding magma of the 
 rock whicli has been inclosed within the larger crystal (postea p. 514). 
 
 68 The following general text-books on rocks may be referred to: Maccul- 
 loch, "A Geological Classification of Rocks," etc., London, 1821. B. von 
 Cotta, "Rocks Classified and Described," translated by Lawrence, London, 
 1866. Zirkel, "Lehrbuch der Petrographie, " two vols. Bonn, 1866. Senft, 
 "Classification der Felsarten," Breslau, 1857; "Die Krystallinischen Felsge- 
 mengtheile," Berlin, 1868. Ken ngott, "Elemente der Petrographie," Leipz., 
 1868. A. von Lasaulx, "Elemente der Petrographie," Bonn, 1876. Bischof, 
 "Chemical Geology," translated for Cavendish Society, 1854-59, and supple- 
 ment, Bonn, 1871. Roth, "Allgemeine und Chemische Geologic," Berlin, 
 1879. Other works in which the microscopical characters are more specially 
 treated of, are enumerated on p. 193. 
 
 59 In the 3d edition of Jukes' "Student's Manual of Geology" (1871), p. 93, 
 it was proposed to reserve the term "Structure" for large features, such as char- 
 acterize rock-blocks, and to use the term "Texture" for the minuter characters,
 
 GEOGNOSY 173 
 
 tinguishable by the unaided eye are denoted either by ordi- 
 nary descriptive adjectives, or by terms derived from rocks 
 in which the special structures are characteristically de- 
 veloped, such as granitoid, brecciated, shaly. It must be 
 borne in mind, bowever, that the external character of 
 a rock does not always supply us with its true internal 
 structure, which may be gained only by microscopic ex- 
 amination. This is of course more especially true of the 
 close-grained kinds, where to the naked eye no definite 
 structure is discernible. Some of the definitions originally 
 founded on external appearance have been considerably 
 modified by microscopic investigation. Many compact 
 rocks, for instance, have been proved to be wholly crys- 
 talline. 
 
 The same rock- mass may show very different structures 
 and textures in different parts of its extent. This is true 
 alike of sedimentary and igneous materials. It may be 
 observed even in the several portions of one continuous 
 mass of erupted rock variations in the rate of cooling, in 
 temperature, and other circumstances have combined to 
 produce sometimes the most extraordinary textural and 
 even structural, as well as chemical and mineralogical 
 contrasts in a boss or sheet of igneous rock." Hence the 
 student must be on his guard against concluding that two 
 portions of rock strikingly unlike each other in outward 
 appearance cannot be portions of one original continuous 
 mass. 
 
 such as can be judged of in hand specimens. M. De Lapparent makes a similar 
 distinction (Traite", p. 602, note). But the practice of using the word structure 
 as it is employed above in the text, has received such a support from the petrog- 
 raphers of Germany that though I still think it would be preferable to distin- 
 guish between texture and structure, I have adopted what has now the sanction 
 of common usage. 
 
 60 See Book IV. Part VTI. ; G. P. Becker, Amer. Journ. Sci. xxxiii. (1887), 
 p. 50. J. H. L. Vogt, GeoL Foren. Forhand. Stockholm, xiii. (1891).
 
 174 TEXT-BOOK OF GEOLOGY 
 
 Crystalline (P h a n er oc ry sta 1 1 i n e), consisting 
 wholly or chiefly of crystalline particles or crystals. 61 
 Where the individual elements of the rocks are of large 
 size, the structure is coarse-crystalline (granitic), as in many 
 granites. When the particles are readily visible to the 
 naked eye, and are tolerably uniform in size, as in marble, 
 many granites and dolomites, the rock is said to be granular- 
 crystalline. Successive stages in the diminution of the size 
 of the particles may be traced until these are no longer rec- 
 ognizable with the naked eye, and the structure must then 
 be resolved with the microscope (fine-crystalline, micro-crys- 
 talline, crypto-crystalline). Fine-grained rocks may also be 
 called compact, though this term is likewise applicable to 
 the more close-grained varieties of the fragmental series. 
 The microscopic characters of such rocks should always 
 be ascertained where possible." 
 
 Many crystalline rocks consist not only of crystals, but 
 of a magma or paste, in which the crystalline particles are 
 seen by the naked eye to be imbedded. It is of course im- 
 possible, except from analogy, to determine macroscopically 
 what may be. the nature of this magma. It may be entirely 
 composed of minute crystals, or may consist of various 
 crystallitic products of devitrification. Its intimate struc- 
 ture can only be ascertained with the microscope. But its 
 existence is often strikingly manifest even to the unassisted 
 eye, for in what are termed "porphyries" it forms a large 
 part of their mass. The term "ground-mass" is employed 
 to denote this megascopic matrix. Microscopic examination 
 shows that a ground-mass may consist of minute crystals, 
 
 61 Prof. Rosenbusch proposed the term "holocrystalline" for rocks in which 
 there is no morphous material among the crystalline constituents. 
 
 61 On the crystallization of igneous rocks, see J. P. Iddings, Bull. Phil. Soc. 
 Washington, xi. (1889), p. 71.
 
 GEOGNOSY 175 
 
 or crystallites, or granules and filaments, or glass, or combi- 
 nations of these in various proportions. (See pp. 194, 207.) 
 
 Lithoid, compact and stony in aspect, with no very 
 distinct crystalline structure. The term is especially ap- 
 plied to the devitrified condition of once glassy rocks, 
 such as obsidians, which have assumed the character of 
 perlites or f el sites. 
 
 Granitic (Granitoid), thoroughly crystalline, and 
 consisting of crystals approximately uniform in size, as in 
 granite. This structure is characteristic of many eruptive 
 rocks. Though usually distinctly recognizable by the 
 naked eye ("macromerite" of Vogelsang 83 ), it sometimes 
 becomes very fine ("micromerite"), and may be only 
 recognizable with the microscope as thoroughly crystal- 
 line (microgranitic); at other times it passes into a porphy- 
 ritic or porphyroid character by the appearance of large 
 crystals dispersed through a 'general ground-mass. 
 
 Pegmatitic (Pegmatoid, Graphic), exhibiting 
 the peculiar arrangement of crystalline constituents seen in 
 pegmatite or graphic granite (p. 
 275), where the quartz and felspar 
 have crystallized simultaneously 
 so as to be inclosed within each 
 other. This structure may be 
 seen on a large scale in many 
 massive veins of pegmatite; where 
 it takes an exceedingly minute 
 form it is known as micropeg- 
 
 . . Fig. 5. Micropesrmatitic Structure, 
 
 m a 1 1 1 1 C (r Ig. 5). Such micro- wanophyre, Mull. (Magnified.) 
 
 scopic intergrowth of quartz and felspar is characteristic of 
 large masses of eruptive rock (micropegmatite, granophyre). 
 
 68 Z. Deutsch. Geol. Ges. xxiv. p. 534.
 
 176 TEXT-BOOK OF GEOLOGY 
 
 Aphanitic, a name given to the very close texture 
 exhibited by some igneous rocks (diabases, diorites) where 
 the component ingredients cannot be determined except with 
 the microscope. 
 
 Porphyritic (Porphyroid), composed of a com- 
 pact or finely crystalline ground-mass, through which larger 
 crystals of earlier consolidation, 64 often of felspar, are dis- 
 persed (Fig. 6). This and the granitic structure are the two 
 
 Fig. 6. Porphyritic Structure. (Nat. size.) 
 
 great structure- types of the eruptive rocks. By far the 
 larger number of these rocks belong to the porphyritic type. 
 Microscopic research has thrown much light on the nature 
 of the ground-mass of porphyritic rocks. Vogelsang pro- 
 posed to classify these rocks in three divisions: " 1st, Grano- 
 phyre, where the ground-mass is a microscopic crystalline 
 mixture of the component minerals with absence or sparing 
 development of an imperfectly individualized magma (see 
 p. 209); 2d, Felsophyre, having usually an imperfectly indi- 
 vidualized or felsitic magma for the ground-mass (pp. 208, 
 
 64 Phenocrysts, Iddings, Bull. Phil. Soc. Washington, ii. (1889), p. 13. 
 
 65 Vogelsang, loc. cit. Compare the classification into granitoid and trachy- 
 toid, p. 271. 

 
 GEOGNOSY 177 
 
 211); 3d, Vitrophyre, where the ground-mass is a glassy 
 magma (pp. 204, 212). The second subdivision embraces 
 most of the porphyries, and a very large number of eruptive 
 rocks of all ages. 66 
 
 Segregated. In granite and other crystalline mas- 
 sive rocks, vein-like portions, coarser (or finer) in texture 
 than the rest of the mass, may be observed. . These belong 
 to the last phase of consolidation, when segregations from 
 the original molten or viscous magma took place along cer- 
 tain lines or round particular centres, where the individual 
 minerals crystallized out from the general mass. They have 
 been sometimes termed "segregation," or "exudation" 
 veins. They are to be distinguished from the veins, 
 usually of finer and more acid material, which ramify 
 through a mass of igneous rock and probably represent 
 portions of the original molten magma which remained 
 still liquid and were injected into rents of the already con- 
 solidated parts. These are the true "contemporaneous 
 veins" (Book IV. Part VII.) 
 
 Gr r a n u 1 a r a somewhat vague term applied to rocks 
 composed" of approximately equal grains, which are some- 
 times worn fragments, as in sandstone, sometimes crystalline 
 particles, as in granite and marble. This texture may be- 
 come so fine as to pass insensibly into compact.' 7 The 
 peculiar granular structure found so abundantly among 
 metamorphic rocks wkich have been intensely crushed and 
 in which there seems to have been a process of recrystalli- 
 
 66 According to Rosenbusch the porphyritic massive rocks are those in 
 which, during the different stages of their produciion, the same minerals 
 have been formed more than once. Neuea Jahrb. 1882 (ii.), p. 14. 
 
 61 As applied to massive (eruptive) rocks, Rosenbusch would restrict the 
 term granular to those in which each individual constituent separated out during 
 but one definite stage of the process of rock-building. Loc. cit.
 
 178 TEXT-BOOK OF GEOLOGY 
 
 zation among the powdered particles, has been termed 
 gra null tic (p. 210). This word, however, is liable to 
 the objection that in Germany it is applied to rocks bearing 
 that structure while in France it is used for a holocrystalline 
 granite. 68 
 
 Vitreous or glassy, having a structure like that of 
 artificial glass ; as in obsidian. Among the crystalline rocks 
 there is often present a variable amount of an amorphous 
 ground-mass, which may increase until it forms the main 
 part of the substance. The nature of this amorphous por- 
 tion is described at pp. 203, 212. Its most obvious mega- 
 scopic condition is that of a volcanic glass. Most vitreous 
 rocks present, even to the naked eye, dispersed grains, crys- 
 tals, or other inclosures. Under the microscope, they are 
 found to be often crowded with minute crystals and imper- 
 fect or incipient crystalline forms (pp. 194, 205). Resin- 
 ous is the term applied to vitreous rocks having the lustre 
 of pitchstone, and to others which are still less vitreous. 
 Devitrification is the conversion of the vitreous into a crys- 
 talline or lithoid structure (pp. 206, 214). 
 
 Streaked, arranged in streaky inconstant lines (Germ. 
 Schliereri), either parallel or convergent, and often undulat- 
 ing. This structure, conspicuously shown by the lines of 
 flow in vitreous rocks (flow-structure, fluxion-structure, 
 fluidal-structure) is less marked where the materials have 
 assumed definite crystalline forms. It can be seen on a 
 minute scale, however, in many crystalline masses when 
 examined with the microscope (p. 211). 
 
 Banded, arranged in parallel bands, distinguished from 
 each other by color, texture, structure or composition ; char- 
 
 68 Michel-Levy, Ann. des Mines, viii. (1875), p. 387; "Structure et Classifi- 
 cation des Roches Eruptives," 1889, p. 14.
 
 GEOGNOSY 179 
 
 acteristic of many gneisses, and of jaspers, flints, halleflintas 
 and other flinty rocks. This term may frequently be ap- 
 plied to the flow-structure of igneous rocks referred to in 
 the previous paragraph, likewise to the segregation veins of 
 eruptive bosses and sheets, and to the parallel arrangement 
 of materials produced in rocks which have under intense 
 mechanical pressure been crushed and sheared. With the 
 naked eye it is often hardly possible to distinguish between 
 the banded structure of devitrified igneous rocks and that 
 resulting from intense mechanical deformation. 
 
 Mylonitic, a term introduced to denote the peculiar 
 granular structure of rocks which have undergone intense 
 crushing. The materials have been reduced to minute grains 
 which have not recrystallized as they have done in the gran- 
 ulitic structure. Many remarkable examples of this struc- 
 ture have been observed among the schists of the Scottish 
 Highlands. 
 
 Spherulitic, composed of, or containing small glob- 
 ules or spherules which may 
 be colloid and isotropic, or 
 more or less distinctly crystal- 
 line, particularly with an internal 
 fibrous divergent structure (Figs. 
 7, 17). This structure occurs in 
 vitreous rocks, where it is one of 
 the stages of devitrification in 
 
 Obsidian, pitchstone, etc.' 9 (p. 214). Fig. T.-SpherulRU^Structure. 
 
 69 On the constitution and origin of spherulite in acid eruptive rocks, see 
 "Whitman. Cross, Phil. Soc. "Washington, xi. p. 411 (1891), and J. P. Iddings, 
 op. cit. p. 445. Quartz assumes in some rocks (e.g. banded eurites) a finely 
 globular structure which was developed before the cessation of the motion that 
 produced flow -structure, and which, according to M. Michel-Levy, may be re- 
 garded as connecting the colloid and crystallized conditions of silica. Bull. Soc. 
 Geol. France (3), v. p. 140.
 
 180 TEXT-BOOK OF GEOLOGY 
 
 The term lithophyse has been applied by F. von 
 Richthofen to large bladder-like sphemlites wherein inter- 
 spaces lined with crystals occur between the successive con- 
 centric internal layers. 70 Many ancient rhyolites present an 
 aggregate of nodular bodies (Pyromeride) due originally to 
 devitrification and subsequently more or less altered espe- 
 cially by the deposition of silica within them (postea, p. 260). 
 
 Orbicular structure is one in which the component 
 minerals of a rock have crystallized in such a way as to 
 
 Fig. 8. Orbicular Structure. Napoleonite, Corsica. (Nat. Size.) 
 
 form spheroidal aggregations sometimes with an internal 
 radial or concentric grouping. It is typically seen in the 
 napoleonite or ball-diorite (kugeldiorite, orbicular diorite, 
 p. 287) of Corsica (Fig. 8), but occurs in other rocks, some- 
 times even in granite. 
 
 P e r 1 i t i c (Figs. 9 and 20), having the structure of the 
 rock formerly termed perlite, wherein between minute rec- 
 
 70 Jahrb. K. K. Geol. Reichsanst, 1860, p. 180. See Iddings, 7th Ann. Rep. 
 U. S. Geol. Surv. (1885-86), p. 249. Amer. Journ. Sci. xxxiii. (1887), p. 36.
 
 GEOGNOSY 
 
 181 
 
 Pig. 9 Perlitic Structure. 
 
 (Magnified.) 
 
 tilinear fissures the substance of the mass has assumed, 
 during the contraction resulting from cooling, a finely 
 globular character, not unlike the 
 spheroidal structure seen in 
 weathered basalt which is also a 
 phenomenon of contraction dur- 
 ing the cooling and consolidation 
 of an igneous rock. 
 
 Horny, flinty, having a 
 compact, homogeneous, dull text- 
 ure, like that of horn or flint, as 
 in chalcedony, jasper, flint, and 
 many halleflintas and felsites. 
 
 Cavernous (porous), containing irregular cavities 
 due, in most cases, to the abstraction of some of the min- 
 erals; but occasionally, as in some limestones (sinters), 
 dolomites and lavas, forming part of the original struc- 
 ture of the rock. 
 
 C e 1 1 u 1 a r. Many lavas, ancient and modern, have 
 been saturated with steam at the time of their eruption, 
 and in consequence of the segregation and expansion of 
 this imprisoned vapor, have had spherical cavities devel- 
 oped in their mass. When this cellular structure is mark'ed 
 by comparatively few and small holes, it may be called 
 vesicular; where the rock consists partly of a roughly 
 cellular, and partly of a more compact substance inter- 
 mingled, as in the slag of an iron furnace, it is said to be 
 slaggy; portions where the cells occupy about as much 
 space as the solid part, and vary much in size and shape, 
 are called scoriaceous, this being the character of the 
 rough clinker-like scoriae of recent lava-streams; when the 
 cells are so much more numerous than the solid part, that
 
 182 TEXT-BOOK OF GEOLOGY 
 
 the stone would almost or quite float on water, the structure 
 is called pumiceous, the term pumice being applied to 
 the froth-like part of obsidian. As the cellular structure 
 can only be developed while the rock is still liquid, or at 
 least viscid, and as, while in this condition, the mass is 
 often still moving away from its point of emission, the 
 cells are not infrequently elongated in the direction of 
 movement. Subsequently water, infiltrating through the 
 
 Fig. 10. Amygdaloidal Structures; Porphyrite, Old Red Sandstone, Ayrshire. 
 (Nat. size.) 
 
 rock, deposits various mineral substances (calcite, quartz, 
 chalcedony, zeolites, etc.) from solution, so that the flat- 
 tened and elongated almond-shaped cells are eventually 
 filled up. A cellular rock which has undergone this change 
 is said to be an amygdaloid, or arnygdaloidal, and 
 the almond-like kernels are known as amygdales (Fig. 
 10). Where the cells or cavernous spaces of a rock are 
 lined with crystals and empty inside they are said to be 
 druses or drusy cavities. 
 
 Cleaved, having a fissile structure superinduced by 
 pressure and known as cleavage (see pp. 531, 532). The 
 planes of cleavage are independent of those of bedding,
 
 GEOGNOSY 183 
 
 though they may coincide with them. A cleaved structure 
 is best seen in fine-grained material, and is typically devel- 
 oped in roofing-slate, but it may occur in any compact 
 igneous rock. 
 
 Foliated, consisting of minerals that have crystallized 
 in approximately parallel, lenticular, and usually wavy 
 layers or folia. Rocks of this kind commonly contain 
 layers of mica, or of some equivalent readily cleavable 
 mineral, the cleavage-planes of which coincide generally 
 with the planes of foliation. Gneiss, mica-schist and talc- 
 schist are characteristic examples. So distinctive, indeed, 
 is this structure in schists, that it is often spoken of as 
 schistose. In gneiss, it attains its most massive form; 
 in chlorite-schist and some other schists, it becomes so 
 fine as to pass into a kind of minutely scaly texture, often 
 only perceptible with the microscope, the rock having on 
 the whole a massive structure. 
 
 Fibrous, consisting of one or more minerals composed 
 of distinct fibres. Sometimes the fibres are remarkably 
 regular and parallel, as in fibrous gypsum, and veins of 
 chrysotile, fibrous aragonite or calcite (satin-spar); in other 
 instances, they are more tufted and irregular, as in asbestos 
 and actinolite-schist. 
 
 Clastic, fragmental, composed of detritus (p. 214). 
 Kocks possessing this character have, in the great majority 
 of cases, been formed in water, and their component frag- 
 ments are usually more or less rounded or water-worn. 
 Different names are applied, according to the form or 
 size of the fragments. Brecciated, composed, like a 
 breccia, of angular fragments, which may be of any degree 
 of coarseness. Agglomerated, consisting of large, 
 roughly rounded and tumultuously grouped blocks, as
 
 184 TEXT-BOOK OF GEOLOGY 
 
 in the agglomerate filling old volcanic funnels. Con- 
 glomerated (Conglomeratic), made up of well- 
 rounded blocks or pebbles; rocks having this character 
 have been formed by and deposited in water. Pebbly, 
 containing dispersed water- worn pebbles, as in many coarse 
 sandstones, which thus by degrees pass into conglomerates. 
 P s a m m i t i c, or sandstone-like, composed of rounded 
 grains, as in ordinary sandstone: when the grains are 
 larger (often sharp and somewhat angular) the rock is 
 gritty, or a grit. M uddy (peli tic), having a tex- 
 ture like that of dried mud. Cryptoclastic or com- 
 pact, where the grains are too minute to reveal to the 
 naked eye the truly fragmental character of the rock, as 
 in fine mudstones and other argillaceous deposits. 
 
 Concretionary, containing, or consisting of mineral 
 matter, which has been collected,. either from the surround- 
 ing rock or from without, round some centre, so as to form 
 a nodule or irregularly shaped lump. This aggregation of 
 material is of frequent occurrence among water-formed 
 rocks, where it may be often observed to have taken place 
 round some organic centre, such as a leaf, cone, shell, fish- 
 bone, or other relic of plant or animal. (Book IV. Part I.) 
 Among the most frequent minerals found in concretionary 
 forms as constituents of rocks, are calcite, siderite, pyrite, 
 marcasite, and various forms of silica. In a true concre- 
 tion, the material at the centre has been deposited first, 
 and has increased by additions from without, either during 
 the formation of the inclosing rock, or by subsequent con- 
 centration and aggregation. Where, on the other hand, 
 cavities and fissures have been filled up by the deposition 
 of materials on their walls, and gradual growth inward, 
 the result is known as a secretion. Amygdales and
 
 GEOGNOSY 185 
 
 the successive coatings of mineral veins are examples of 
 the latter process. 
 
 Septaria n a structure often exhibited by concretions 
 of limestone and clay-ironstone which in consolidating have 
 shrunk and cracked internally. These shrinkage-cracks ra- 
 diate in an irregular way from the middle toward the cir- 
 cumference, but die out before reaching the latter (Fig. 26). 
 Usually they have been filled with some subsequently in- 
 filtrated mineral, notably calcite. 
 
 Oolitic, a structure like fish-roe, formed of spherical 
 grains, each of which has an internal radiating and concen- 
 tric structure, and often possesses a central nucleus of some 
 foreign body. This structure is specially found among lime- 
 stones (see p. 262). When the grains are as large as peas, 
 the structure is termed pisolitic. 
 
 Various structures which affect large masses of rock 
 rather than hand-specimens will be found described in 
 Book IV. But a few of the more important may be 
 included here. 
 
 Massive, unstratified, having no arrangement in 
 definite layers or strata. Lava, granite, and generally all 
 crystalline rocks which have been erupted to the surface, 
 or have solidified below from a state of fusion are massive 
 rocks. 
 
 Stratified, bedded, composed of layers or beds ly- 
 ing parallel to each other, as in shale, sandstone, limestone, 
 and other rocks which have been deposited in water. Suc- 
 cessive streams of lava, poured one upon another, have also 
 a bedded arrangement. Laminated, consisting of fine, 
 leaf-like strata or laminse; this structure being characteris- 
 tically exhibited in shales, is sometimes also called s h a 1 y.
 
 186 TEXT-BOOK OF GEOLOGY 
 
 Jointed, traversed by the divisional planes termed 
 Joints which are fully treated of in Book IV. Part II. 
 
 Columnar, divided into prismatic joints or columns. 
 This structure is typically represented among the basalts 
 and other basic lavas (p. 883 and Figs. 230-232), but it may 
 also be observed as an effect of contact-metamorphism 
 among stratified rocks which have been invaded by intru- 
 sive masses (Book^IV. Part VIII.) 
 
 2. Composition* Before having recourse to chemical or 
 microscopic analysis, the geologist can often pronounce as 
 to the general chemical or mineralogical nature of a rock. 
 Most of the terms which he employs to express his opinion 
 are derived from the names of minerals, and in almost all 
 cases are self-explanatory. The following examples may 
 suffice. Calcareous, consisting of or containing car- 
 bonate of lime. Argillaceous, consisting of or con- 
 taining clay. Felspathic, having some form of felspar 
 as a main constituent. Siliceous, formed of or contain- 
 ing silica; usually applied to the chalcedonic forms of this 
 cementing oxide. Quartzose, containing or consisting 
 entirely of some form of quartz. Carbonaceous, con- 
 taining coaly matter, and hence usually associated with 
 a dark color. P y r i t o u s, containing diffused disulphide 
 of iron. Grypseous, containing layers, nodules, strings 
 or crystals of calcium-sulphate. Saliferous, containing 
 beds of, or impregnated with rock-salt. Micaceous, full 
 of layers of mica-flakes. 
 
 As rocks are not definite chemical compounds, but mix- 
 tures of different minerals in varying proportions, they ex- 
 hibit many intermediate varieties. Transitions of this kind 
 are denoted by such phrases as "granitic gneiss," that is, 
 a gneiss in which the normal foliated structure is nearly
 
 GEOGNOSY 187 
 
 merged into the massive structure of granite; "argillaceous 
 limestone" a rock in which the limestone is mixed with 
 clay; "calcareous shale" a fissile rock, consisting of clay 
 with a proportion of lime. It is evident that such rocks 
 may graduate so insensibly into each other, that no sharp 
 line can be drawn between them either in the field or in 
 their terminology. 
 
 As already alluded to, and as will be more fully ex- 
 plained in later pages, the progress of research goes to show 
 that even in the same mass of eruptive rock considerable 
 differences of chemical composition may be found. These 
 differences seem to point to some separation of the constit- 
 uents, by gravity or otherwise, before consolidation. Thus 
 the picrite of Bathgate shades upward into a rock in which 
 the heavy magnesian silicates are replaced in large measure 
 by felspars/ 1 Mr. Iddings has recently called attention to 
 some remarkable gradations of composition among the 
 volcanic rocks of the Tewar Mountains, New Mexico, 
 where be believes a series of intermediate varieties to be 
 traceable from obsidian at the one end to basalt at the 
 other." A remarkable instance of a similar kind is de- 
 scribed by Mr. Teall and Mr. Dakyns from the Scottish 
 Highlands. 
 
 3. State of Aggregation. The hardness or softness of a 
 rock, in other words, its induration, friability, or the degree 
 of aggregation of its particles, may be either original or ac- 
 quired. Some rocks (sinters, for example) are soft at first 
 and harden by degrees; the general effect of exposure, how- 
 
 11 Trans. Roy. Soc. Edin. vol. xxix. (1879), p. 504. 
 
 78 Bull. D. S. Geol. Surv. No. 66 (1890), Bull. Phil. Soc. Washington, xi. 
 (1890), pp. 65, 191, and postea, pp. 457, 458. Teall and Dakyns, Quart. Journ. 
 Geol. Soc. 1892.
 
 188 TEXT-BOOK OF GEOLOGY 
 
 ever, is to loosen the cohesion of the particles of rocks. A 
 rock which can easily be scratched with the nail is almost 
 always much decomposed, though some chloritic and talcose 
 schists are soft enough to be thus affected. Compact rocks 
 which can easily be scratched with the knife, and are ap- 
 parently not decomposed, may be fine-grained limestones, 
 dolomites, ironstones, mudstones, or some other simple 
 rocks. Crystalline rocks, except limestone, cannot, as a 
 rule, be scratched with the knife unless considerable force 
 be used. They are chiefly composed of hard silicates, so 
 that when an instance occurs where a fresh specimen can be 
 easily scratched, it will usually be found to be a limestone 
 (pp. 148, 139, 149). The ease with which a rock may be 
 broken is the measure of its frangibility. Most rocks break 
 most easily in one direction; attention to this point will 
 sometimes throw light upon their internal structure. 
 
 Fracture is the surface produced when a rock is split 
 or broken, and depends for its character upon the texture of 
 the mass. Finely granular, compact rocks are apt to break 
 with a splintery fracture where wedge-shaped plates ad- 
 here by their thicker ends to, and lie parallel with the gen- 
 eral surface. When the rock breaks off into concave and 
 convex rounded shell-like surfaces, the fracture is said to 
 be conchoidal, as may be seen in obsidian and other 
 vitreous rocks and in exceedingly compact limestones. The 
 fracture may also be f o 1 i a t e d, s 1 a t y, or s h a 1 y, accord- 
 ing to the structure of the rock. Many opaque, compact 
 rocks are translucent on the thin edges of fracture, and 
 afford there, with the aid of a lens, a glimpse of their inter- 
 nal composition. A rock is said to be flinty, when it is 
 hard, close-grained, and breaks with a smooth or conchoidal 
 fracture like flint; friable, when it crumbles down like
 
 GEOGNOSY 189 
 
 dry clay or chalk; plastic, when, like moist clay, it can 
 be worked into shapes; pulverulent, when it falls read- 
 ily to powder; earthy, when it is decomposed into loam 
 or earth; incoherent or loose, when its particles are 
 quite separate, as in dry blown sand. 
 
 4. Color and Lustre* These characters vary so much, 
 even in the same rock, according to the freshness of the sur- 
 face examined, that they possess but a subordinate value. 
 Nevertheless, when cautiously used, color may be made 
 to afford valuable indications as to the probable nature and 
 composition of rocks. It is, in this respect, always desir- 
 able to compare a freshly -broken with a weathered piece 
 of the rock. 7 * 
 
 White indicates usually the absence or a comparatively 
 small amount of the heavy metallic oxides, especially iron. 
 It may either be the original color, as in chalk and calc-sin- 
 ter, or may be developed by weathering, as in the white crust 
 on flints and on many porphyries. Gray is a frequent color 
 of rocks which, if quite pure, would be white, but which 
 acquire a grayish tint by admixture of dark silicates, organic 
 matter, diffused pyrites, etc. Blue or bluish-gray is a char- 
 acteristic tint of rocks through which iron-disulphide is dif- 
 fused in extremely minute subdivision. But as a rule it 
 rapidly disappears from such rocks on exposure, especially 
 where they contain organic matter also. The .stiff blue clay 
 of the sea-bottom which is colored by iron-disulphide be- 
 comes reddish-brown when dried, and then shows no trace 
 of sulphide. 74 Black may be due either to the presence of 
 carbon (when weathering will not change it much), or to 
 
 13 Alterations of the colors of minerals and rocks are effected by heat and 
 even by sunlight. See Janettaz, Bull. Soc. Geol. xxix. (1872), p. 300. 
 
 14 J. Y. Buchanan, Brit. Assoc. 1881, p. 584.
 
 190 TEXT- BOOK OF GEOLOGY 
 
 some iron-oxide (magnetite chiefly), or some silicate rich in 
 iron (as hornblende and augite). Many rocks (basalts and 
 melaphyres particularly) which look quite black on a fresh 
 surface, become red, brown or yellow on exposure, black 
 being comparatively seldom a weathered color. Yellow (or 
 Orange), as a dull earthy coloring matter, almost always in- 
 dicates the presence of hydrated peroxide of iron. In mod- 
 ern volcanic districts it may be due to iron-chloride, sulphur, 
 etc. Bright, metallic, gold-like yellow is usually that of 
 iron-disulphide. Brown is the normal color of some carbo- 
 naceous rocks (lignite), and ferruginous deposits (bog-iron- 
 ore, clay-ironstone, etc.). It very generally, on weathered 
 surfaces, points to the oxidation and hydration of minerals 
 containing iron. Red, in the vast majority of cases, is due 
 to the presence of anhydrous peroxide of iron. This min- 
 eral gives dark blood-red to pale flesh-red tints. As it is 
 liable, however, to hydration, these hues are often mixed 
 with the brown, orange and yellow colors of limonite. 76 
 Green, as the prevailing tint of rocks, occurs among schists, 
 when its presence is usually due to some of the hydrous 
 magnesian silicates (chlorite, talc, serpentine). It appears 
 also among massive rocks, especially those of older geologi- 
 cal formations, where hornblende, olivine, or other silicates 
 have been altered. Among the sedimentary rocks, it is 
 principally due to ferrous silicate (as in glauconite). Car- 
 bonate of copper colors some rocks emerald- or verdigris- 
 green. The mottled character so common among many 
 stratified rocks is frequently traceable to unequal weather- 
 ing, some portions of the iron being more oxidized than 
 others; while some, on the other hand, become deoxidized 
 
 u See I. C. Russell, Bull. U. S. Geol. Surv. No. 52 (1889).
 
 - GEOGNOSY 191 
 
 from the reducing action of decaying organic matter, as in 
 the circular green spots so often found among red strata. 
 
 Lustre, as an external character of rocks, does not pos- 
 sess the value which it has among minerals. In most rocks, 
 the granular texture prevents the appearance of any distinct 
 lustre. A completely vitreous lustre without a granular tex- 
 ture, is characteristic of volcanic glass. A splendent semi- 
 metallic lustre may often be observed upon the foliation 
 planes of schistose rocks and upon the laminae of micaceous 
 sandstones. As this silvery lustre is almost invariably due 
 to the presence of mica, it is commonly called distinctively 
 micaceous. A metallic lustre is met with sometimes in beds 
 of anthracite; more usually its occurrence among rocks in- 
 dicates the presence of metallic oxides or sulphides. A 
 resinous lustre is characteristic of many pitchstones. Lustre- 
 mottling is a term applied to the interrupted sheen on the 
 cleavage faces of minerals which have inclosed much smaller 
 crystals or grains of other minerals. It is well seen on the 
 surfaces of some of the constituents of serpentine rocks. 
 
 5. Feel and SmelL These minor characters are occasion- 
 ally useful. By the feel of a mineral or rock is meant the 
 sensation experienced when the fingers are passed across its 
 surface. Thus hydrous magnesian silicates have often a 
 marked soapy or greasy feel. Some sericitic mica-schists 
 show the same character. Trachyte received its name from 
 its characteristic rough or harsh feel. Some rocks adhere 
 to the tongue, a quality indicative of their tendency to ab- 
 sorb water. 
 
 Smell. Many rocks, when freshly broken, emit dis- 
 tinctive odors. Those containing volatile hydrocarbons give 
 sometimes an appreciable bituminous odor, as is the case 
 with certain eruptive rocks, which in central Scotland have
 
 192 TEXT-BOOK OF GEOLOGY 
 
 been intruded through coal-seams and carbonaceous shales. 
 Limestones have often a fetid odor; rocks full of decompos- 
 ing sulphides are apt to give a sulphurous odor; those which 
 are highly siliceous yield, on being struck, an empyreumatic 
 odor. It is characteristic of argillaceous rocks to emit a 
 strong earthy smell when breathed upon. 
 
 6. Specific Gravity* This is an important character 
 among rocks as well as among minerals. It varies from 
 0'6 among the hydrocarbon compounds to 3 - l among the 
 basalts. As already stated, the average specific gravity of 
 the rocks of the earth's crust may be taken to be about 2-5, 
 or from that to 3 - 0. Instruments for taking the specific 
 gravity of rocks have been already (p. 154) referred to. 
 
 7. Magnetism is so strongly exhibited by some crystal- 
 line rocks as powerfully to affect the magnetic needle, and 
 to vitiate observations with this instrument. It is due to 
 the presence of magnetic iron, the existence of which may 
 be shown by pulverizing the rock in an agate mortar, wash- 
 ing carefully the triturated powder, and drying the heavy 
 residue, from which grains of magnetite or of titaniferous 
 magnetic iron may be extracted with a magnet. This may 
 be done with any basalt (p. 156). A freely swinging mag- 
 netic needle is of service, as by its attraction or repulsion it 
 affords a delicate test for the presence of even a small quan- 
 tity of magnetic iron. 
 
 v. Microscopic Characters of Rocks 
 
 No department of Geology has been more advanced in 
 recent years than Lithology, and this has been mainly due 
 to the introduction of the microscope as an instrument for 
 investigating minute internal structure. As far back as the 
 year 1827, a method of making thin transparent sections of
 
 GEOGNOSY 193 
 
 fossil wood, and mounting them on glass with Canada bal- 
 sam, had been devised by William Nicol of Edinburgh, and 
 was employed by Henry Witham in his "History of Fossil 
 Vegetables." 76 
 
 It was not, however, until 1856 that Mr. H. C. Sorby, 
 applying this method to the investigation of minerals and 
 rocks, showed how many and important were the geological 
 questions on which it was calculated to shed light." Refer- 
 ence will be made in subsequent pages to the remarkable 
 results then announced by him. To the publication of his 
 memoir the subsequent rapid development of the micro- 
 scopic study of rocks may be distinctly traced. The micro- 
 scopic method of analysis is now in use in every country 
 where attention is paid to the history of rocks.' 8 
 
 In iii. p. 161 information has been given regarding the 
 
 16 Small 4to, Edinburgh, 1831. This work, though dedicated to Nicol, does 
 not distinctly recognize him as the actual inventor of the process of slicing min- 
 eral substances for microscopic investigation. All that was original in Witham'a 
 researches he owed either directly or indirectly to Nicol. 
 
 11 Brit. Assoc. 1856, Sect. p. 78. Quart. Jourii. Geol. Soc. xiv. 1858. Micr. 
 Journ. xvii. (1877), p. 113. 
 
 18 Among the best text-books on this subject the following may be mentioned : 
 "Mikroskopische Beschaffenheit der Miner-alien und Gesleine," F. Zirke), 
 
 1 vol. 1873. "Mikroskopische Physiographic der Mineralien und Gesteiue," 
 H. Rosenbusch, 2 vols. 2d Edit. 1885-87, and the English translation of the 
 first volume quoted on p. 161; likewise the Tables translated by F. H. Hatch 
 quoted on p. 161. "Elemente der Petrographie," A. von Lasaulx, 1875. "Min- 
 eralogie microgaphique: roches eruptives francaises," Fouque and Michel-Levy, 
 
 2 vols. 4to, Paris, 1879. "Microscopical Petrography," Zirkel, being vol. vi. of 
 ihe Geol. Explor. of 40th Parallel, Washington, 1876. "British Petrography," 
 J. J. H. Teall, London, 1888. "Les Mineraux des Roches," Michel-Levy and 
 Lacroix, Paris, 1888. The volumes for the last fifteen or twenty years of the 
 Quarterly Journal of the Geological Society, Geological Magazine, Neues Jahr- 
 buch fiir Mineralogie, etc., Zeitschrift der Deutschen Geologischen Gesellschaft, 
 Bulletin de la Societe geologique de France,, Jahrbuch der K. K. Geologischen 
 Reichsanstalt (Vienna), contain numerous papers on the microscopic structure 
 of rocks. Rutley's "Study of Rocks," 1879, and his "Rock-forming Minerals," 
 1888; Cole's "Aids in Practical Geology," 1891; and Hatch's "Petrology 
 Igneous Rocks," 1891, are useful handbooks. The manual of Rosenbusch and 
 the work of Fouque aud Michel-Levy, contain a tolerably ample bibliography of 
 the subject, to which the student is referred. The titles of some of the more 
 important memoirs which have recently appeared will be given in footnotes. 
 
 GEOLOGY Vol. XXIX 9
 
 194 TEXT-BOOK OF GEOLOGY 
 
 preparation of sections of rocks for microscopical examina- 
 tion, the methods of procedure in the practice of this part 
 of geological research, and some of the terms employed in 
 the following pages. 
 
 1. Microscopic Elements of Rocks 
 
 Rocks when examined in thin sections with the micro- 
 scope are found to be composed of or to contain various ele- 
 ments, of which the more important are, 1st, crystals, or 
 crystalline substances; 2d, glass; 3d, crystallites; 4th, de- 
 tritus. 
 
 A. CRYSTALS OR CRYSTALLINE SUBSTANCES. Rock- 
 forming minerals, when not amorphous, may be either 
 crystallized in their proper crystallographic forms (idio- 
 morphic), or while possessing a crystalline internal struc- 
 ture, may present no definite external geometrical form 
 (allotriomorphic, p. 209). The latter condition is more 
 prevalent, seeing that minerals have usually been devel- 
 oped round and against each other, thus mutually hinder- 
 ing the assumption of determinate crystallographic contours. 
 Other causes of imperfection are fracture by movement in 
 the original magma of the rock, and partial solution in that 
 magma (Fig. 12), as in the corroded quartz of quartz-por- 
 phyries and rhyolites, and the hornblende crystals of basalts. 
 The ferro-magnesian minerals of earlier consolidation among 
 basalts and andesites, are sometimes surrounded with a dark 
 shell called the corrosion-zone. In some rocks, such as 
 granite, the thoroughly crystalline character of the com- 
 ponent ingredients is well marked, yet they less frequently 
 present the definite isolated crystals so often to be observed 
 in porphyries and in many old and modern volcanic rocks. 
 Among thoroughly crystalline rocks, good crystals of the
 
 GEOGNOSY 195 
 
 component minerals may be obtained from fissures and cavi- 
 ties in which there has been room for their formation. It is 
 in the "drusy" cavities of granite, for example, that the 
 well-defined prisms of felspar, quartz, mica, topaz, beryl 
 and other minerals are found. Successive stages in order 
 of appearance or development can readily be observed 
 among the crystals of rocks. Some appear as large, but 
 frequently broken, or corroded forms. These have evi- 
 dently been formed first. Others are smaller but abundant, 
 usually unbroken, and often disposed in lines. Others have 
 been developed by subsequent alteration within the rock. 7 ' 
 
 A study of the internal structure of crystals throws light 
 not merely on their own genesis, but on that of the rocks of 
 which they form part, and is therefore well worthy of the 
 attention of the geologist. That many apparently simple 
 crystals are in reality compound, may not infrequently be 
 detected by the different condition of weathering in the two 
 opposite parts of a twin on an exposed face of rock. The 
 internal structure of a crystal modifies the action of solvents 
 on its exterior (e.g. weathered surfaces of calcite, aragonite 
 and felspars). Crystals may occasionally be observed built 
 up of rudimentary "microlites," as if these were the simplest 
 forms in which the molecules of a mineral begin to appear 
 (p. 205). 
 
 A microscopic examination of some rocks shows that a 
 subsequent or secondary growth of different minerals has 
 taken place after their original crystalline form was com- 
 plete. These later additions are in optical continuity with 
 the original crystal, and sometimes have taken place even 
 upon worn or imperfect forms. They may be occasionally 
 detected among the silicates of igneous rocks, and also even 
 
 19 Fouque and Michel-Levy, "Min. Micrograph." p. 161.
 
 196 TEXT-BOOK OF GEOLOGY 
 
 among the sandgrains of sandstones which have thus had 
 their rounded forms converted into crystallographic faces. 80 
 
 Crystalline minerals are seldom free from extraneous in- 
 clusions. These are occasionally large enough to be readily 
 seen by the naked eye. But the microscope reveals them 
 in many minerals in almost incredible quantity. They 
 are, a, vesicles containing gas; /?, vesicles containing liquid; 
 f, globules of glass or of some lithoid substance; <J, crys- 
 tals; e, filaments, or other indefinitely-shaped pieces, patches, 
 or streaks of mineral matter. 
 
 a. Gr as-filled cavities are most frequently globu- 
 lar or elliptical, and appear to be due to the presence of gas 
 or steam in the crystal at the time of consolidation. Zirkel 
 estimates them at 360,000,000 in a cubic millimetre of the 
 hauyne from Melfi. 81 In some instances the cavity has a 
 geometric form belonging to the crystalline system of the 
 inclosing mineral. Such a space defined by crystallographic 
 contours is a negative crystal. A cavity filled with gas con- 
 tains no bubble, and its margin is marked by a broad dark 
 band. The usual gas is nitrogen, with traces of oxygen 
 and carbon-dioxide; sometimes it is entirely carbon-dioxide 
 or hydrogen and hydrocarbons. 
 
 y3. Vesicles containing liquid (and gas). As 
 far back as the year 1823, Brewster studied the nature of 
 certain fluid-bearing cavities in different minerals. 62 The 
 
 80 H. C. Sorby, Presidential Address, Geol. Soc. 1880, p. 62. R. D. Irving 
 and C. R. Van Hise "On secondary enlargements of Mineral Fragments in cer- 
 tain rocks." Bull. U. S. Geol. Surv. No. 8 (1884). J. W. Judd, Quart. Journ. 
 Geol. Soc. xlv. (1889), p. 175. 
 
 si "Mik. Beschaff." p. 86. 
 
 * 2 Edin. Phil. Journ. ix. p. 94. Trans. Roy. Soc. Edin. x. p. 1. See also 
 W. Nicol, Edin. New Phil. Journ. (1828), v. p. 94; De la Vallee Poussin and 
 Renard, Acad. Roy. Belg. 1876, p. 41 ; Hartley, Journ. Chem. Soc. ser. 2, xiv. 
 137; ser. 3, ii. p. 241; Microscop. Journ. xv. p. 170; Brit. Assoc. 1877, Sect. 
 p. 232.
 
 GEOGNOSY 197 
 
 first observer who showed their important bearing on geo- 
 logical researches into the origin of crystalline rocks was 
 Mr. Sorby, in whose paper, already cited, they occupy a 
 prominent place. Vesicles entirely filled with liquid are 
 distinguished by their sharply-defined and narrow black 
 borders. Vesicular spaces containing fluid may be noticed 
 in many artificial crystals formed from aqueous solutions 
 (crystals of common salt show them well) and in many min- 
 erals of crystalline rocks. They are exceedingly various in 
 form, being branching, curved, oval, or spherical, and some- 
 times assuming as negative crystals a geometric form, like 
 that characteristic of the mineral in which they occur, as 
 cubic in rock-salt and hexagonal in quartz. They also vary 
 greatly in size. Occasionally in quartz, sapphire, and other 
 minerals, large cavities are readily observable with the naked 
 eye. But they may be traced with high magnifying powers 
 down to less than ]0 J 00 of an inch in diameter. Their propor- 
 tion in any one crystal ranges within such wide limits, that 
 whereas in some crystals of quartz few may be observed, in 
 others they are so minute and abundant that many millions 
 must be contained in a cubic inch. The fluid present is 
 usually water, frequently with saline solutions, particularly 
 chloride of sodium or of potash, or sulphates of potash, 
 soda, or lime. Carbon-dioxide may be present in the water; 
 sometimes the cavities are partially occupied with it in 
 liquid form, and the two fluids, as originally observed by 
 Brewster, may be seen in the same cavity unmingled, the 
 carbon-dioxide remaining as a freely moving globule within 
 the carbonated water. Cubic crystals of chloride of sodium 
 may be occasionally observed in the fluid, which must in 
 such cases be a saturated solution of this salt (Fig. 11, low- 
 est figure in Column A). Usually each cavity contains a
 
 198 TEXT-BOOK OF GEOLOGY 
 
 small globule or bubble, sometimes stationary, sometimes 
 movable from one side or end of the cavity to the other, as 
 the specimen is turned. With a high magnifying power, 
 the minuter bubbles may be observed to be in motion, some- 
 ^^ times slowly pulsating from 
 &JS (2^2 side to side, or rapidly vi- 
 *?& ^/ l 8 ^!^ brating like a living organ- 
 
 ^^ ^^ ^^ ism. The cause of this 
 
 *^ H*^ r trepidation, which resem- 
 
 j 4P bles the so-called "Brown- 
 ian movements," has been 
 Fig. ii.-cavities in Crystals, highly mag- plausibly explained by the 
 
 nified; A, Liquid Inclusions; B, Glass . , ... 
 
 Inclusions; c, Cavities showing the de- inCCSSant interchange OI the 
 vitrification of the original glass by the 
 
 in the molecules from the 
 
 to the vaporous condition 
 along the surface where vapors and liquid meet an inter- 
 change which, though not visible on the large bubbles, makes 
 itself apparent in the minute examples, of which the dimen- 
 sions are comparable to those of the intermolecular spaces. 83 
 The bubble may be made to disappear by the application of 
 heat. 
 
 With regard to the origin of the bubble, Sorby pointed 
 out that it can be imitated in artificial crystals, in which 
 he explained its existence by diminution of volume of the 
 liquid owing to a lowering of temperature after its inclosure. 
 By a series of experiments he ascertained the rate of expan- 
 sion of water and saline solutions up to a temperature of 
 200 C. (392 Fahr.), and calculated from them the tempera- 
 ture at which the liquid in crystals would entirely fill its 
 
 83 Charbonelle and Thirion, Rev. Quest. Scientif. vii. (1880)43. On the criti- 
 cal point of water, etc., in these cavities see Hartley, Journ. Chem. Soc. ser. 3, 
 vol. ii. p. 241. Pop. Sci. Rev. new ser. i. p. 119.
 
 GEOGNOSY 199 
 
 inclosing cavities. Thus, in the nepheline of the ejected 
 blocks of Monte Somma, he found that the relative size of 
 the vacuities was about -28 of the fluid, and assuming the 
 pressure under which the crystals were formed to have been 
 not much greater than sufficient to counteract the elastic 
 force of the vapor, he concluded that the nepheline may 
 have been formed at a, temperature of about 340 C. (644 
 Fahr.), or a very dull red heat, only just visible in the 
 dark. He estimated also from the fluid cavities in the 
 quartz of granite that this rock has probably consolidated 
 at somewhat similar temperatures, under a pressure some- 
 times equal to that of 76,000 feet of rock. 84 Zirkel, however, 
 has pointed out that even in contiguous cavities, where there 
 is no evidence of leakage through fine fissures, the relative 
 size of the vacuole varies within very wide limits, and in 
 such a manner as to indicate no relation whatever to the 
 dimensions of the inclosing cavities. Had the vacuole been 
 due merely to the contraction of the liquid on cooling, it 
 ought to have always been proportionate to the size of the 
 cavity. 86 
 
 MM. De la Valle'e Poussin and Renard, attacking the 
 question from another side, measured the relative dimen- 
 sions of the vesicle and of its inclosed water and cube of 
 rock-salt, as contained in the quartziferous diorite of 
 Quenast in Belgium. The temperature at which the asper- 
 tained volume of water in the cavity would dissolve its 
 salt was found by calculation to be 307 C. (520 Fahr.). 
 But as the law of the solubility of common salt has not 
 been experimentally determined for high temperatures, this 
 figure can only be accepted provisionally, though other 
 
 84 Sorby, Q. J. Geol. Soc. xiv. pp. 480, 493. <* "Mik. Beschaff." p. 46.
 
 200 TEXTBOOK OF GEOLOGY 
 
 considerations go to indicate that it is probably not far 
 from the truth. Assuming then that this was the tempera- 
 ture at which the vesicle was formed, these authors proceed 
 to determine the pressure necessary to prevent the complete 
 vaporization of the water at that temperature, and obtain, as 
 the result, a pressure of 87 atmospheres, equal to 84 tons 
 per square foot of surface. 8 * That many rocks were formed 
 under great pressure is well shown by the liquid carbon- 
 dioxide in the pores of their crystals. 
 
 Although, in almost all cases, the liquid inclusions are 
 to be referred to the conditions under which the mineral 
 crystallized out of the original magma, they may be excep- 
 tionally developed long subsequently, either in one of the 
 original minerals during decomposition, or in a mineral of 
 secondary origin, such as quartz of subsequent introduc- 
 tion. 87 
 
 Liquid inclusions may be dispersed at random through 
 a crystal, or as in the quartz of granite, gathered in inter- 
 secting planes (which look like fine fissures and which may 
 sometimes have become real fissures, owing to the line of 
 weakness caused by fche crowding of the cavities), or dis- 
 posed regularly in reference to the contour of the crystal. 
 In the last case they are sometimes confined to the centre, 
 sometimes arranged in zones along the lines of growth of 
 the crystal. 68 They are specially conspicuous in the quartz 
 
 88 "Memoire snr lea Roches dites Plutoniennea de la Belgique," De la Vallee 
 Poussin and A. Renard. Acad. Roy. Belg. 1876, p. 41. See also Ward, Q. J. 
 Geol. Soc. xxxi. p. 568, who believed that the granites of Cumberland consoli- 
 dated at a maximum depth of 22,000 to 30,000 feet. 
 
 81 See Whitman Cross on the development of liquid inclusions in plagioclase 
 during the decomposition of the gneiss of Brittany. Tschermak's Min. MittheiL 
 1880, p. 369; also G. F. Becker, "Geology of Comstock Lode." U. S. Geol. 
 Surv. 1882, p. 371. 
 
 88 The way in which vesicles, inclosed crystals, etc., are grouped along the 
 zones of growth of crystals is illustrated in Fig. 12.
 
 GEOGNOSY 201 
 
 of granite and other massive rocks, as well as of gneiss and 
 mica-schist; also in felspars, topaz, beryl, augite, nepheline, 
 olivine, leucite and other minerals. 
 
 f. Inclusions of glass or of some lithoid 
 substance. In many rocks which have consolidated 
 from fusion, the component crystals contain globules or 
 irregularly shaped inclosures of a vitreous nature (Fig. 11, 
 Column B). These inclosures are analogous to the fluid- 
 inclusions just described. They are portions of the original 
 glassy magma out of which the minerals of the rock crystal- 
 lized, as portions of the mother-liquor are inclosed in artifi- 
 cially formed crystals of common salt. That magma is in 
 reality a liquid at high temperatures, though at ordinary 
 temperatures it becomes a solid. At first, these glass- 
 vesicles may be confounded with the true liquid-cavities, 
 which in some respects they closely resemble. But they 
 may be distinguished by the immobility of their bubbles, 
 of which several are sometimes present in the same cavity; 
 by the absence of any diminution of the bubbles when heat 
 is applied; by the elongated shape of many of the bubbles; 
 by the occasional extrusion of a bubble almost beyond the 
 walls of the vesicle; by the usual pale greenish or brownish 
 tint of the substance filling the vesicle, and its identity 
 with that forming the surrounding base or ground-mass 
 in which the crystals are imbedded; and by the com- 
 plete passivity of the substance in polarized light (see 
 p. 169). 
 
 Glass inclusions occur abundantly in some minerals, 
 aggregated in the centre of a crystal or ranged along its 
 zones of growth with singular regularity. They appear in 
 felspars, quartz, leucite, and other crystalline ingredients 
 of volcanic rocks, and of course prove that in such posi-
 
 202 TEXT-BOOK OF QEOLOGY 
 
 tions these minerals, even the refractory quartz, have 
 undoubtedly crystallized out of molten solutions. 
 
 In inclusions of a truly vitreous nature, traces of de- 
 vitrification may not infrequently be seen. In particular, 
 microscopic crystallites (p. 205) make their appearance, like 
 those in the ground-mass of the rock. Sometimes the in- 
 clusions, like the general ground-mass, have an entirely 
 stony character (Fig. 11, C). This may be well observed 
 in those which have not been entirely separated from the 
 surrounding ground-mass, but are connected with it by a 
 narrow neck at the periphery of the inclosing crystal, la 
 some granites and in elvans, the quartz by irregular contrac- 
 tion, while still .in a plastic state, appears to have drawn 
 into its substance portions of the surrounding already 
 lithoid base; 89 but this appearance may sometimes be due 
 to irregular corrosion of the crystals by the magma. 90 
 
 d. Crystals and crystalline bodies. Many com- 
 
 Fig. 12. Section of a fractured and corroded Augite crystal 
 from a dike, Crawfordjohn, Lanarkshire (magnified), showing 
 lines of growth with vesicles and magnetite crystals. 
 
 ponent minerals of rocks contain other minerals (Fig. 12). 
 These occur sometimes as perfect crystals, more usually 
 
 89 J. A. Phillips, Q. J. Geol. Soc. xxxi. p. 338. 
 
 90 Fouque and Michel-Levy, "Min. Micrograph."
 
 GEOGNOSY 203 
 
 as what ar.e termed microlites (p. 205). Like the glass- 
 inclusions, they tend to range themselves in lines along 
 the successive zones of growth, in the inclosing mineral. 
 Microlites are of frequent occurrence in leucite, garnet, 
 augite, hornblende, calcite, fluorite, etc. From the fact 
 that microlites of the easily fusible augite are, in the 
 Vesuvian lavas, inclosed within the extremely refractory 
 leucite, it was supposed that the relative order of fusibility 
 is not always followed in the microlites and enveloping 
 crystals. But this has been satisfactorily explained by 
 Fouque" and Michel-Levy, who have shown experimentally 
 that leucite, when crystallizing from fusion, tends to catch 
 up inclusions of the surrounding glass, which, should the 
 glass be pyroxenic, may assume the form of augite. 91 
 
 e. Filaments, streaks, patches, discolorations. 
 Besides the inclosures already enumerated, crystals like- 
 wise frequently inclose irregular portions of mineral matter, 
 due to alteration of the original substance of the minerals 
 or rocks. Thus tufts and vermicular aggregates of certain 
 green ferruginous silicates are of common occurrence among 
 the crystals and cavities of old pyroxenic volcanic rocks. 
 Orthoclase crystals are often mottled with patches of a gran- 
 ular nature, due to partial conversion of the mineral into 
 kaolin. The magnetite, so frequently inclosed within min- 
 erals, is abundantly oxidized, and has given rise to brown 
 and yellow patches and discolorations. Care must be taken 
 not to confound these results of infiltrating water with the 
 original characters of a rock. Practice will give the student 
 confidence in distinguishing them, if he familiarizes his eye 
 with decomposition products by studying slices of weathered 
 minerals and of the weathered parts of rocks. 
 
 91 "Synthese des Mineraux," 1882, p. 156.
 
 204 TEXT-BOOK OF GEOLOGY 
 
 B. GLASS. Even to the unassisted eye, many volcanic 
 rocks consist obviously in whole or in great measure of 
 glass. 92 This substance in mass is usually black or dark 
 green, but when examined in thin sections under the micro- 
 scope, it presents for the most part a pale brown tint, or is 
 nearly colorless. In its purest condition, it is quite struc- 
 tureless, that is, it contains no crystals, crystallites, or other 
 distinguishable individualized bodies. But even in this 
 state it may sometimes be observed to be marked by clot- 
 like patches or streaks of darker and lighter tint, arranged 
 in lines or eddy-like curves, indicative of the flow of the 
 original fluid mass. Rotated in the dark field of crossed 
 Nicol-prisms, such a natural glass remains dark, as, unless 
 where it has undergone internal stresses, it is perfectly inert 
 in polarized light. Being thus isotropic, it may readily be 
 distinguished from any inclosed crystals which, acting on 
 the light, are anisotropic (p. 169). Perfectly homogeneous 
 structureless glass, without inclosures of any kind, occurs 
 for the most part only in limited patches, even in the most 
 thoroughly vitreous rocks. Originally the structure of all 
 glassy rocks, at the time of most complete fusion, may have 
 been that of perfectly unindividualized glass. But as these 
 masses tended toward a solid form, devitrification of their 
 glass set in. Many forms of incipient or imperfect crystal- 
 lization, as well as perfect crystals, were developed in the 
 still fluid and moving mass, and, together with crystals of 
 earlier growth, were arranged in the direction of motion. 
 Devitrification has in frequent examples proceeded so far 
 that no trace remains of any actual glass. 93 
 
 w See B. Cohen on glassy Rocks. Neues .Tahrb. 1880 (ii.), p. 23. 
 
 93 Consult a paper on the microscopic character of devitrified glass and some 
 analogous rock-structures, by D. Herman and F. Rutley. Proc. Roy. Soc. 1885, 
 p. 87.
 
 GEOGNOSY 205 
 
 C. CRYSTALLITES AND MICROLITES.'* Under these 
 names may be included minute inorganic bodies possess- 
 ing a more or less definite form, but generally without the 
 geometrical characters of crystals. They occur most com- 
 monly in rocks which have been formed from igneous fusion, 
 but are found also in others which have resulted from, or 
 have been altered by, aqueous solutions. They seem to be 
 early or peculiar forms of crystallization. They are abun- 
 dantly developed in artificial slags, and appear in many 
 modern and ancient vitreous rocks, but the conditions under 
 which they are produced are not yet well understood. 96 
 
 Crystallites are disti nguished by remaining isotropic 
 in polarized light. The simplest are extremely minute drop- 
 like bodies or globulites, sometimes crowded confusedly 
 through the glass, giving it a dull or somewhat granular 
 character, while in other cases they are arranged in lines or 
 groups. Gradations can be traced from spherical or sphe- 
 roidal globulites into other forms more elliptical in shape, 
 but still having a rounded outline and sometimes sharp ends 
 (longulites). There does not appear to be any essential dis- 
 tinction, save in degree of development, between these forms 
 and the long rod-like or needle-shaped bodies which have 
 been termed belonites. Existing sometimes as mere simple 
 needles or rods, these more elongated crystallites may be 
 traced into more complex forms, curved or coiled, at one 
 
 94 The word crystallite was first used by Sir James Hall to denote the lithoid 
 substance obtained by him after fusing and then slowly cooling various "whin- 
 stones" (diabases, etc.). Since its revival in lithology it has been applied to 
 the minuter bodies above described. The student should consult Vogelsang's 
 "Philosophic der Geologic," p. 139; "Krystalliten, " Bonn, 8vo, 1875; also 
 his descriptions in Archives Neerlandaises, v. 1870, vi. 1871. Sorby, Brit. 
 Assoc. 1880. 
 
 95 They are well exhibited also in ordinary blow-pipe beads. See Sorby, 
 Brit. Assoc. 1880, or Geol. Mag. 1880, p. 468. They have been produced ex- 
 perimentally in the artificial rocks fused by Messrs. Fouque and Michel-Le"vy.
 
 206 
 
 TEXT-BOOK OF GEOLOGY 
 
 time solitary, at another in groups. In most cases, crystal- 
 lites are transparent and colorless, or slightly tinted, but 
 sometimes they are black and opaque, from a coating of fer- 
 ruginous oxide, or only appear so as an optical delusion 
 from their position. Black, seemingly opaque, hair- like, 
 twisted and curved forms, termed trichites, occur abundantly 
 in obsidian. 
 
 Mi.crolites are other incipient forms of crystallization 
 which differ from crystallites in that they react on polarized 
 light. They assume rod -like or needle-shaped forms, some- 
 times occurring singly, sometimes in aggregates, and even 
 occasionally grouped into skeleton-crystals. They can fofr 
 the most part be identified as rudimentary forms of definite 
 minerals such as augite, hornblende, felspar, olivine, and 
 magnetite. 
 
 Good illustrations of the general character and grouping 
 
 
 Fig-. 13. Augite Crystal surrounded 
 by Crystallites and Microlites. 
 from the vitreous Andesite of 
 Eskdalemuir, magnified 800 Di- 
 ameters. 
 
 Fig. 14. Microlites of the Pitch- 
 stone of Arran, magnified 70 
 Diameters. (See p. 283 ) 
 
 of crystallites and rnicrolites are shown in some vitreous 
 basalts. In Fig. 13, for example, the outer portion of the 
 field displays crowded globulites and longulites as well as 
 here and there a few belonites and some curved and coiled 
 trichites. Bound the rude augite crystal, these various
 
 GEOGNOSY 207 
 
 bodies have been drawn together out of the surrounding 
 glass. Numerous rod-like microlites diverge from the crys- 
 tal, and these are more or less thickly crusted with the sim- 
 pler and smaller forms." In Fig. 14, the remarkably beau- 
 tiful structure of an Arran pitchstone is shown; the glassy 
 base being crowded with minute microlites of hornblende 
 which are grouped in a fine feathery or brush-like arrange- 
 ment round tapering rods. In this case, also, we see that 
 the glassy base has been clarified round the larger individ- 
 uals by the abstraction of the crowded smaller microlites. 
 By the progressive development of crystallites, microlites, 
 or crystals during the cooling and consolidation of a molten 
 rock, a glass loses its vitreous character and becomes lithoid; 
 in other words, undergoes devitrification. 
 
 The characteristic amorphous or indefinitely granular 
 and fibrous or scaly matter, constituting the microscopic 
 base in which the definite crystals of felsites and porphyries 
 are imbedded (pp. 278-281), has been the subject of much 
 discussion. Between crossed Nicol-prisms it sometimes be- 
 haves isotropically, like a glass, but in other cases allows a 
 mottled glimmering light to pass through. It is now well 
 understood to be a product of the devitrification of once 
 glassy rocks wherein the crystallitic and microlitic forms 
 can still be recognized or have been more or less effaced by 
 subsequent alteration by infiltrating water.' 7 ,, 
 
 Every gradation in the relative abundance of crystallites 
 may be traced. In some obsidians and other vitreous rocks, 
 portions of the glass can be obtained with comparatively 
 few of them; but in the same rocks we may not infrequently 
 
 96 Proc. Eoy. Phys. Soc. Edin. v. p. 246, Fig. 5. J. J. H. Teall, Q. J. GeoL 
 Soc. xl. p. 221, Plate xii. Fig. 2a. 
 
 91 See Zirkel, "Mik. BeschafE." p. 280. Rosenbusch, voL ii. p. 60.
 
 208 TEXT-BOOK OF GEOLOGY 
 
 observe adjacent parts where they have been so largely de- 
 veloped as to usurp the place of the original glass, and give 
 the rock in consequence a lithoid aspect (Fig. 11, C and 
 pp. 278-283). 
 
 D. DETRITUS. Many rocks are composed of the detritus 
 of pre-existing materials. In the great majority of cases 
 this can be readily detected, even with the naked eye. But 
 where the texture of such detrital or fragmental (clastic) 
 rocks becomes exceedingly fine, their true nature may re- 
 quire elucidation with the microscope (Figs. 21, 22). An 
 obvious distinction can be drawn between a mass of com- 
 pact detritus and a crystalline or vitreous rock. The de- 
 trital materials are found to consist of various and irregu- 
 larly shaped grains, with more or less of an amorphous and 
 generally granular paste. In some cases, the grains are 
 broken and angular, in others they are rounded or water- 
 worn (pp. 227-228). They may consist of minerals (quartz, 
 chert, felspars, mica, etc.), or of rocks (slate, limestone, 
 basalt, etc.), or of the remains of plants or animals (spores 
 of lycopods, fragments of shells, crinoids, etc.). It is evi- 
 dent therefore that though some of them may be crystalline, 
 the rock of which they now form part is a non-crystalline 
 compound. Water, with carbonate of lime or other min- 
 eral matter in solution, permeating a detrital rock, has some- 
 times allowed its dissolved materials to crystallize among 
 the interstices of the detritus, thus producing a more or less 
 distinctly crystalline structure. But the fundamentally sec- 
 ondary or derivative nature of the mass is not always thereby 
 effaced. 
 
 2. Microscopic Structures of Rocks 
 
 We have next to consider the manner in which the fore- 
 going microscopic elements are associated in rocks. This
 
 GEOGNOSY 209 
 
 inquiry brings before us the minute structure or texture of 
 rocks, and throws great light upqn their origin and history.* 8 
 
 Four types of rock-structure are revealed by the micro- 
 scope. A, holocrystalline; B, hemi-crystalline; C, glassy; 
 D, clastic. 
 
 A. HOLOCRYSTALLINE, consisting entirely of crystals or 
 crystalline individuals, whether visible to the naked eye, 
 or requiring the aid of a microscope, imbedded in each 
 other without any intervening amorphous substance. Rocks 
 of this type are exemplified by granite (Figs. 15 and 29) and 
 by other igneous rocks. But they occur also among the 
 crystalline limestones and schists, as in statuary marble, 
 which consists entirely of crystalline granules of calcite 
 (Fig. 28). 
 
 According to the classification proposed by Prof. Rosen- 
 busch the holocrystalline structure is idiomorphic or panidio- 
 morphic when each of the component crystals has assumed 
 its own crystallographic form, and allotriomorphic when it 
 has its outlines determined by those of its neighbors. When 
 interspaces have been left between the crystals or crystal- 
 line grains the structure is miarolitic or saccharoid. 
 
 The holocrystalline eruptive rocks (p. 269) are typically 
 represented by granite, hence the term granitoid has been 
 adopted to express their microscopic structure. Varieties 
 of this structure are designated according to the relations ,of 
 the component minerals. Where no one mineral greatly 
 preponderates, but where they are all confusedly and toler- 
 ably equally distributed in individuals readily observable 
 by the naked eye, as ordinary granite, the structure is 
 
 98 The first broad classification of the microscopic structure of rocks was that 
 proposed by Zirkel, which, with slight modification, is here adopted. "Mik. 
 Beschaff." p. 265. "Basaltgesteine, " p. 88. See also Rosenbusch's suggestive 
 paper already cited, Neues. Jahrb. 1882 (ii.), p. 1.
 
 210 
 
 TEXT-BOOK OF GEOLOGY 
 
 granitic (see granular, p. 177). Where a similar struc- 
 ture is so fine that it can only be recognized with the 
 microscope, it has been called microgranitic or euritic. 
 Where the minerals are grouped in small, isolated, grain- 
 like individuals, each having its own independent crystal- 
 line structure, so that under the microscope in polar- 
 ized light, the rock presents the appearance of a brilliant 
 
 Fig. 15 -Holocrvstalline Structure. Pig. 16.-Hemi-crystallme Structure. 
 
 Granite (20 Diameters). The white Do erite, consisting ol a tnclimc 
 
 portions are Quartz/the striped Felspar, Augite and Magnetite 
 
 parts Felspar, the long, dark, finely IP. a devitrihed Ground-mass (20 
 
 striked stripes are Mica. (See Wa^).^ = ero^ 
 
 the broader monoclinic forms, 
 slightly shaded in the drawing-, 
 are Augite; the black specks are 
 Magnetite; the needle-shaped 
 forms are Apatite. (See p. 294.) 
 
 mosaic, the structure has been named granulitic or micro- 
 granulitic (panidiomorphic granular or porphyric of Rosen - 
 busch). Where the quartz and felspar of a granitic rock 
 have crystallized together, one within the other, the struc- 
 ture is pegmatitic (Fig. 31) where visible to the naked eye, 
 and micropegmatitic (granophyric of Rosenbusch) where the 
 help of a microscope is needed (Fig. 5). 9 ' 
 
 99 Fouque and Michel-Levy, "Min. Micrograph." The micropegmatite of 
 Michel-Levy is the same as the structure subsequently named granophyre by 
 Rosenbusch. Michel-Levy, "Roches Eruptives," p. 19.
 
 GEOGNOSY 211 
 
 B. HEMI-CRYSTALLINE.'"" This division probably com- 
 prehends the majority of the massive eruptive or igneous 
 rocks. It is distinguished by the occurrence of what ap- 
 pears to the naked eye as a compact or finely granular 
 ground-mass, through which more or less recognizable crys- 
 tals are scattered. Examined with the microscope, this 
 ground-mass is found to present considerable diversity 
 (Figs. 16. 18, 32). It may be (1) wholly a glass, as in some 
 basalts, trachytes, and other volcanic products; (2) partly 
 devitrified through separation of peculiar little granules 
 and needles (crystallites and microlites) which appear in 
 a vitreous base; (3) still further devitrified, until it becomes 
 an aggregation of such little granules, needles, and hairs, 
 between which little or no glass-base appears (micro-crystal- 
 litic); or (4) "microfelsitic" (petrosiliceous), closely related 
 to the two previous groups, and consisting of a nearly struc- 
 tureless mass, marked usually with indefinite or half -effaced 
 granules and filaments, but behaving like a singly-refracting, 
 amorphous body (p. 204). 
 
 In rocks belonging to this type, a spherulitic structure 
 has sometimes been produced by the appearance of globular 
 bodies composed of a crystalline internally radiating sub- 
 stance, sometimes with concentric shells of amorphous 
 material. In many cases, spherulites are only recognizable 
 with the microscope, when they each present a black crgss 
 between crossed Nicol-prisms, and thereby characteristically 
 reveal the microspherulitic structure (Figs. 7 and 17). 101 
 
 100 p or t n j a structure the term "mixed" has been proposed, as being a mix- 
 ture of the crystalline and amorphous (glassy) structures. It has been desig- 
 nated by Fouqu^ and Michel-Levy "trachytoid, " as being typically developed 
 among the trachytes (postea, p. 288). It is called "hypocrystalline" by Rosen- 
 busch. 
 
 101 Fouque and Michel-Levy, "Min. Micrograph." Some remarkably beau- 
 tiful examples of microspherulitic structure occur in the quartz- porphyries that 
 traverse the lower Cambrian tuffs at St. David's. Q. J. Geol. Soc. xxxix. p. 313.
 
 212 TEXT-BOOK OF GEOLOGY 
 
 The term op hi tic is applied to a structure in which 
 one mineral after crystallizing has been inclosed within an- 
 other- during the consolidation of an igneous rock (Fig. 18). 
 It is abundant in many dolerites and diabases where some 
 bisilicate such as augite serves as a matrix in which the 
 felspars and other crystals are inclosed. The name is de- 
 rived from the so-called "ophites" of the Pyrenees. 102 
 
 Fig. 17. Spherulitic Structure. Pitch- Fig. 18. Ophitic Structure. Dolerite, 
 
 stone, Raasay (magnified). Skye (magnified). 
 
 C. GLASSY. Composed of a volcanic glass such as has 
 already been described. It seldom happens, however, that 
 rocks which seem to the eye to be tolerably homogeneous 
 glass do not contain abundant crystallites and minute crys- 
 tals. Hence truly vitreous rocks tend to graduate into the 
 second or hemi-crystalline type. This gradation and the 
 abundant traces of a devitrified base or magma between 
 the crystals of a vast number of eruptive rocks, lead to the 
 belief that the glassy type was the original condition of most 
 if not all of these rocks. Erupted as molten masses, their 
 mobility would depend upon the fluidity of the glass. Yet 
 even while still deep within the earth's crust, some of their 
 constituent minerals (felspars, leucite, magnetite, etc.) were 
 
 102 These rocks (diabases) have been critically studied by J. Kuhn, Zeitsch. 
 Deutsch. Geol. Ges. xxxiii. (1881) 372.
 
 GEOGNOSY 213 
 
 often already crystallized, and suffered fracture and corrosion 
 by subsequent action of the inclosing magma. This is well 
 shown by what is termed the flow-structure or fluxion- 
 structure. Crystals and crystallites are ranged in current- 
 like lines, with their long axes in the direction of these 
 lines. Where a large older crystal occurs, the train of 
 minuter individuals is found to sweep round it and to 
 reunite on the further side, or to be diverted in an eddy- 
 like course (Fig. 19). So thoroughly is this arrangement 
 characteristic of the motion of a somewhat viscid liquid, 
 that there cannot be any doubt that such was the condition 
 
 Fig. 19. Flow-structure in Obsidian. ilg. 30. Perlitic Structure. Felsitic 
 
 (20 Diameters). glass. Mull (magnified). 
 
 of these masses before their consolidation. The flow- 
 structure may be detected in many eruptive rocks, from 
 thoroughly vitreous compounds like* obsidian, on the one 
 hand, to completely crystalline masses like some dolecites, 
 on the other. It occurs not only in what are usually re- 
 garded as volcanic rocks, but also in plutonic or deep-seated 
 masses which, there is reason to believe, consolidated be- 
 neath the surface, as for instance^in the Bode vein of the 
 Harz, among quartz-porphyries associated with granites in 
 Aberdeenshire, and in felsite dikes and bosses in the 
 Shetlands, Skye, central Scotland, and County Waterford.
 
 214 TEXT-BOOK OF GEOLOGY 
 
 The structure, therefore, cannot be regarded as certainly 
 indicating that the rock in which it is found ever flowed 
 out at the surface as lava. 
 
 Some glassy rocks, in cooling and consolidating, have 
 had spherulites developed in them (Fig. 17); also by con- 
 traction the system of reticulated and spiral cracks known 
 as perlitic structure (p. 180 and Figs. 9 and 20). 
 
 The final stiffening of a vitreous mass into solid stone 
 has resulted (1st) from mere solidification of the glass: this 
 is well seen at the edge of dikes and intrusive sheets of 
 different basalt-rocks, where the igneous mass, having been 
 suddenly congealed along its line of contact with the sur- 
 rounding rocks, remains there in the condition of glass, 
 though only an inch further inward from the edge the 
 vitreous magma has disappeared, as represented in Fig. 
 287; (2d) from the devitrification of the glass by the abun- 
 dant development of microfelsitic granules and filaments, 
 as in quartz-porphyry, or of crystallites, microlites and 
 crystals, as in such glassy rocks as obsidian and- tachy lite; 
 or (3d) from the complete crystallization of the whole of the 
 original glassy base, as may be observed in some dolerites. 
 
 D. CLASTIC. Composed of detrital materials, such as 
 have been already described (p. 183 and Fig. 21). Where 
 these materials consist of grains of quartz-sand, they with- 
 stand almost any subsequent change, and hence can be 
 recognized even among a highly metamorphosed series 
 of rocks. Quartzite from such a series can sometimes be 
 scarcely distinguished under the microscope from unaltered 
 quartzose sandstone. Where the detritus has resulted from 
 the destruction of aluminous or magnesian silicates, it is 
 more susceptible of alteration. Hence it can be traced in 
 regions of local metamorphism, becoming more and more
 
 GEOGNOSY 
 
 215 
 
 crystalline, until the rocks formed of or containing it pass 
 into true crystalline schists. 
 
 Detritus derived from the comminution or decay of 
 organic remains presents very different and characteristic 
 structures 103 (Fig. 22). Sometimes it is of a siliceous nature, 
 as where it has been derived from diatoms and radiolarians. 
 But most of the organically-derived detrital rocks are cal- 
 careous, formed from the remains of foraminifera, corals, 
 echinoderms, polyzoa, cirripeds, annelids, mollusks, crusta- 
 cea and other invertebrates, with occasional traces of fishes 
 
 Fig. 21. Clastic Structure, of Inor- 
 game origin Section of a Piece 
 of Greywacke. (10 Diameters. 
 See p. 232.) 
 
 Pig 22. Clastic Structure, of Organic 
 Origin Structure of Chalk (Sorby). 
 Magnified 100 Diameters. (See p. 
 2460 
 
 or even of higher vertebrates. Distinct differences of micro- 
 scopic structure can be detected in the hard parts of some of 
 the living representatives of these forms, and similar differ- 
 ences have been detected in beds of limestone of all ages. 
 Mr. Sorby, in the paper cited below, has shown how char- 
 acteristic and persistent are some of these distinctions, and 
 how they may be made to indicate the origin of the rock 
 in which they occur. There is an important difference be- 
 tween the two forms in which carbonate of lime is made 
 
 103 ^6 gtudent who would further investigate this subject, will find a sug- 
 gestive and luminous essay upon it by Mr. Sorby in his Presidential Address to 
 the Geological Society, Quart. Journ. Geol. Soc. 1879.
 
 216 TEXT-BOOK OF GEOLOGY 
 
 use of by invertebrate animals; aragonite being much less 
 durable than calcite (pp. 141, 244). Hence while shells of 
 gasteropods, many lamellibranchs, corals and other organ- 
 isms, formed largely or wholly of aragonite, crumble down 
 into mere amorphous mud, pass into crystalline calcite, or 
 disappear, the fragments of those consisting of calcite may 
 remain quite recognizable. 
 
 It is evident, therefore, that the absence of all trace of 
 organic structure in a limestone need not invalidate an in- 
 ference from other evidence that the rock has been formed 
 from the remains of organisms. The calcareous organic 
 debris of a sea-bottom may be disintegrated, and reduced 
 to amorphous detritus, by the mechanical action of waves 
 and currents, by the solvent chemical action of the water, 
 by the decay of the binding material, such as the organic 
 matter of shells, or by being swallowed and digested by 
 other animals (postea, p. 243). 104 
 
 Moreover, in clastic calcareous rocks, owing to their 
 liability to alteration by infiltrating water, there is a ten- 
 dency to acquire an internal crystalline texture (p. 621). 
 At the time of formation, little empty spaces lie between 
 the component granules and fragments, and according to 
 Mr. Sorby these interspaces may amount to about a quarter 
 of the whole mass of the rock. They have very commonly 
 been filled up by calcite introduced in solution. This infil- 
 trated calcite acquires a crystalline structure, like that of 
 ordinary mineral- veins. But the original component organic 
 granules also themselves become crystalline, and, save in 
 so far as their external contour may reveal their original 
 
 JM Sorby, Presidential Address, Q. J. Geol. Soc. 1879. G. Rose. AbhandL 
 Acad. Berlin, 1858; Giimbel, Zeitsch. Dentach. Geol. Gesellsch. 1884, p. 38& 
 Cornish and Kendall, Geol. Mag. 1888, p. 66.
 
 GEOGNOSY 217 
 
 organic source, they cannot be distinguished from mere 
 mineral-grains. In this way, a cycle of geological change 
 is completed. The calcium-carbonate originally dissolved 
 out of rocks by infiltrating water, and carried into the sea, 
 is secreted from the oceanic waters by corals, foraminifera, 
 echinoderms, mollusks and other invertebrates. The re- 
 mains of these creatures collected on the sea-bottom slowly 
 accumulate into beds of detritus, which in after times are 
 upheaved into land. Water once more percolating through 
 the calcareous mass, gradually imparts to it a crystalline 
 structure, and eventually all trace of organic forms may 
 be effaced. But at the same time, the rock, once exposed 
 to meteoric influences, is attacked by carbonated water, its 
 molecules are carried in solution into the sea, where they 
 will again be built up into the framework of marine or- 
 ganisms. 
 
 E. ALTERATION OF ROCKS BY METEORIC WATER. An 
 important revelation of the microscope is the extent to which 
 rocks suffer from the influence of infiltrating water. The 
 nature of some of these changes is described in subsequent 
 pages. (Book III. Part. II. Sect. ii. 2.) It may be suffi- 
 cient to note here a few of the more obvious proofs of altera- 
 tion. Threads and kernels of calcite running through an 
 eruptive rock, such as diabase, dolerite, or andesite, are a 
 good index of internal decomposition. They usually pojnt 
 to the decay of some lime-bearing mineral in the rock. 
 Some other minerals are likewise frequent signs of altera- 
 tion, such as serpentine (often resulting from the alteration 
 of olivine, Figs. 33, 34), chlorite, epidote, limonite, chalced- 
 ony, etc. In many cases, however, the decomposition prod- 
 ucts are so indefinite in form and so minute in quantity as 
 not to permit of their being satisfactorily referred to any 
 
 GEOLOGY Vol. XXIX 10
 
 218 TEXT-BOOK OF GEOLOGY 
 
 known species of mineral. For these indeterminate, but 
 frequently abundant substances, the following short names 
 were proposed by Vogelsang to save periphrasis, until the 
 true nature of the substance is ascertained. Viridite green 
 transparent or translucent patches, often in scaly or fibrous 
 aggregations, of common occurrence in more or less de- 
 composed rocks containing hornblende, augite, or olivine: 
 probably in many cases serpentine, in others chlorite 'or 
 delessite. Ferrite yellowish, reddish, or brownish amor- 
 phous substances, probably consisting of peroxide of iron, 
 either hydrous or anhydrous, but not certainly referable to 
 any mineral, though sometimes pseudomorphous after ferru- 
 ginous minerals. Opacite black, opaque grains and scales 
 of amorphous earthy matter, which may in different cases be 
 magnetite, or some other metallic oxide, earthy silicates, 
 graphite, etc. 105 
 
 vi. Classification of Rocks 
 
 It is evident that Lithology may be approached from two 
 very different sides. We may, on the one hand, regard 
 rocks chiefly as so many masses of mineral matter, present- 
 ing great variety of chemical composition and marvellous 
 diversity of microscopic structure. Or, on the other hand, 
 passing from the details of their chemical and mineralogical 
 characters, we may look at them rather as the records of 
 ancient terrestrial changes. In the former aspect, they pre- 
 sent for consideration problems of the highest interest in in- 
 organic chemistry and mineralogy; in the latter view, they 
 invite attention to the great geological revolutions through 
 which the planet has passed. It is evident, therefore, that 
 
 106 Vogelsang, Z. Deutsch. Geol. Ges. xxiv. (1872), p. 529. Zirkel, Geol. 
 Expl. 40th Parallel, vol. vi. p. 12.
 
 GEOGNOSY 219 
 
 two distinct systems of classification may be followed, the 
 one based on chemical and mineralogical, the other on 
 geological considerations. 
 
 From a chemical point of view, rocks may be grouped 
 according to their composition; as Oxides, exemplified by 
 formations of quartz, hsematite, or magnetite; Carbonates, 
 including the limestones and clay-ironstones; Silicates, em- 
 bracing the vast majority of rocks, whether composed of a 
 single mineral, or of more than one; Phosphates, such as 
 guano and the older bone- beds and coprolitic deposits. A 
 classification of this kind, however, pays no regard to the 
 mode of origin or conditions of occurrence of the rocks, and 
 is not well suited for the purposes of the geologist. 106 
 
 From the mineralogical side, rocks may be classified with 
 reference to their prevailing mineral constituent. Thus such 
 subdivisions as Calcareous rocks, Quartzose rocks, Ortho- 
 clase rocks, Plagioclase rocks, Pyroxenic rocks, Horn- 
 blendic rocks, etc., may be adopted; but these terms are 
 hardly less objectionable to the geologist, and are in fact 
 suited rather for the arrangement of hand-specimens in a 
 museum, than for the investigation of rocks in situ. 
 
 From the standpoint of geological inquiry, rocks have 
 been classified according to their mode of origin. In one 
 system they are arranged under three great divisions: 1st, 
 Igneous, embracing all which have been erupted from the 
 heated interior of the earth; 2d, Aqueous or Sedimentary, 
 including all which have been laid down as mechanical or 
 chemical deposits from water or air, and all which have 
 resulted from the growth and decay of plants or animals; 
 3d, MetamorphiCj those which have undergone subsequent 
 
 106 The eruptive rocks are susceptible of a convenient, though not strictly 
 accurate, chemical classification into acid, intermediate and basic (see p. 273).
 
 220 TEXT-BOOK OF GEOLOGY 
 
 change within the crust of the earth, whereby their original 
 character has been so modified as to be sometimes quite 
 indeterminable. Another geological arrangement is based 
 upon the general structure of the rocks, and consists of two 
 divisions: 1st, Stratified, embracing all the aqueous and sedi- 
 mentary, with part of the less altered metamorphic rocks; 
 2d, Unstratified, nearly conterminous with the term igne- 
 ous, since it includes all the eruptive rocks. Further sub- 
 divisions of this series have been proposed according to dif- 
 ferences of structure or texture, as porphyritic, granitic, etc. 
 These geological subdivisions, however, ignore the chemical 
 and mineralogical characters of the rocks, and are based on 
 deductions which may not always be sound. Thus, rocks 
 may be included in the igneous series which further re- 
 search may show not to be of igneous origin; others may be 
 classed as metamorphic, regarding the true origin of which 
 there may be considerable uncertainty. 
 
 A further system of classification, based upon relative 
 age, has been applied to the arrangement of the eruptive 
 rocks, those masses which were erupted prior to Secondary 
 time being classed as "older," and those of Tertiary and 
 later date as "younger. " This system has been elaborated 
 in great detail by Michel-Levy, who maintains that the same 
 types have been reproduced nearly in the same order in the 
 two series, though basic rocks, often with vitreous charac- 
 ters, rather predominate in the later. 107 It must, indeed, be 
 
 1OT See on this subject, J. D. Dana, Amer. J. Sci. xvi. 1878, p. 336. Michel- 
 Levy, Bull. Soc. Geol. France, 3d ser. iii. (1874), p. 199; vi. p. 173. Ann. des 
 Mines, viii. (1875) "Roches Eruptives," 1889. Fouque and Michel-Levy, "Min- 
 eralogie Microgr." p, 150. Rosenbusch, "Mik. Physiog." ii. Reyer, "Physik 
 der Eruptioaen," 1877, part iii. opposes the adoption of relative age as a basis 
 of classification. On the classification of compound silicated rocks, see Vogel- 
 sang, Z. Deutsch. Geol. Ges. xxiv. p. 507 ; and for an incisive criticism of a too 
 merely mineralogical classification. Lessen, op. cit. xxiv. p. 782. Consult also 
 0. Lang, "Ueber die Individual! tat der Gesteine" in Tschermak's Min. Miuheil. 
 vol. xi. part 6 (1890), p. 467.
 
 GEOGNOSY 221 
 
 admitted that certain broad distinctions between the older 
 and the later eruptive rocks have been well ascertained, and 
 appear to hold generally over the world. Among these dis- 
 tinctions may be mentioned as more characteristic of the 
 Palaeozoic rocks the presence of microcline, turbid ortho- 
 clase in Carlsbad twins, muscovite, enstatite, bronzite, dial- 
 lage, tourmaline, anatase, rutile, cordierite, and in the 
 younger rocks the presence of sanidine, tridymite, leucite, 
 nosean, hauyne, and zeolites. Even where the same min- 
 eral occurs in both the older and newer series, it often pre- 
 sents a somewhat different aspect in each, as in the case of 
 the plagioclase and augite, which in the younger series are 
 distinguished by the occurrence in them of vitreous and 
 gaseous inclusions which are rare or absent in those of the 
 older series. 108 Throughout the younger eruptive rocks, the 
 vitreous condition is much more frequent and perfectly de- 
 veloped than in the older group, where, on the other hand, 
 the granitic structure is characteristically displayed. Still, 
 to these rules so many exceptions occur that it may be 
 doubted whether enough of positively ascertained data have 
 been collected regarding the relative ages of eruptive rocks 
 to warrant the adoption of any classification upon a chrono- 
 logical basis. There can be no doubt that, making due 
 allowance for the alterations arising from permeation by 
 meteoric water, there is no essential difference between some 
 types of volcanic rock in Paleozoic and in recent times. 
 The Carboniferous basalts and trachytes of Scotland, for 
 example, present the closest resemblance to those of Ter- 
 tiary age. 109 
 
 Though no classification which can at present be pro- 
 
 108 See J. Murray and A. Renard, Proc. Roy. Soc. Edin. xi. p. 669. 
 
 109 See Nature, iii. (1871), p. 303.
 
 222 TEXT-BOOK OF GEOLOGY 
 
 posed is wholly satisfactory, one which shall do least vio- 
 lence, at once to geological and mineralogical relationships, 
 is to be preferred. The arrangement which has met with 
 the most general acceptance is threefold. 1st, Sedimen- 
 tary Bocks, including first the rocks which have resulted 
 from the accumulation of detritus, either inorganic or or- 
 ganic, under water or on land, and secondly those which 
 have been deposited from aqueous solution. The former 
 are mechanical, the latter chemical accumulations; but they 
 have often been deposited together. Certain rocks of me- 
 chanical origin, such as detrital limestones, may by subse- 
 quent alteration be converted into materials that cannot be 
 distinguished from others of true chemical origin. Hence 
 the whole series is intimately linked together. 2d, Mas- 
 sive, Eruptive, or Intrusive Rocks, embracing all 
 those which have solidified from fusion within the earth's 
 crust, or have been erupted as lava to the surface. 3d, 
 Schistose Rocks, and their accompaniments, including 
 the so-called Metamorphic rocks which have reached their 
 present condition as a consequence of the alteration some- 
 times of sedimentary, sometimes of igneous rocks. This 
 group graduates into the two others, but it contains some 
 distinctive masses, the origin of which is still involved in 
 doubt. 
 
 It must be kept in view that in this proposed system 
 of classification, and in the following detailed description of 
 rocks, many questions regarding the origin and decomposi- 
 tion of these mineral masses must necessarily be alluded to. 
 The student, however, will find these questions discussed in 
 later pages, and will probably recognize a distinct advantage 
 in this unavoidable preliminary reference to them in connec- 
 tion with the rocks by which they are suggested.
 
 GEOGNOSY M6 
 
 vii. A Description of .the more Important Rocks of the Earth's Crust 
 
 Full details regarding the composition, microscopic struc- 
 ture, and other characters of rocks must be sought in such 
 general treatises and special memoirs as those already cited 
 (pp. 160, 172, 193). The purposes of the present text-book 
 will be served by a succinct account of the more common or 
 important rocks which enter into the composition of the 
 crust of the earth. 
 
 I. SEDIMENTARY 
 
 A. FRAGMENTAL (CLASTIC) 
 
 This great series embraces all rocks of a secondary or 
 derivative origin ; in other words, all formed of fragmentary 
 materials which have previously existed on or beneath the 
 surface of the earth in another form, and the accumulation 
 and consolidation of which gives rise to new compounds. 
 Some of these materials have been produced by the me- 
 chanical action of wind, as in the sand-hills of sea-coasts 
 and inland deserts (^Eolian rocks); others by the operation 
 of moving water, as the gravel, sand and mud of shores and 
 river-beds (Aqueous sedimentary rocks); others by the ac- 
 cumulation of the entire or fragmentary remains of once liv- 
 ing plants and animals (Organically -formed rocks); while 
 yet another series has arisen from the gathering together of 
 the loose debris thrown out by volcanoes (Volcanic tuffs). 
 It is evident that in dealing with these various detrital for- 
 mations, the degree of consolidation is of secondary impor- 
 tance. The soft sand and mud of a modern lake-bottom 
 differ in no essential respect from ancient lacustrine strata, 
 and may tell their geological story equally well. No line is 
 to be drawn between what is popularly termed rock and the
 
 224 TEXT-BOOK OF GEOLOGY 
 
 loose, as yet uncompacted, debris out of which solid rocks 
 may eventually be formed. Hence in the following arrange- 
 ment, the modern and the ancient, being one in structure 
 and mode of formation, are classed together. 
 
 It will be observed that, in several directions, we are led 
 by the fragmental rocks to crystalline stratified deposits, 
 some of which have been deposited from chemical solution, 
 while others have resulted from the gradual conversion of 
 a detrital into a crystalline structure. Both series of de- 
 posits are accumulated simultaneously and are often inter- 
 stratified. Calcareous rocks formed of organic remains 
 (p. 243) exhibit very clearly this gradual internal change, 
 which more or less effaces their detrital origin, and gives 
 them such a crystalline character as to entitle them to be 
 ranked among the crystalline limestones. 
 
 1. Gravel and Sand Rocks (Psammites) 
 
 As the deposits included in this subdivision are pro- 
 duced by the disintegration and removal of rocks by the 
 action of the atmosphere, rain, rivers, frost, the sea, and 
 other superficial agencies, they are mere mechanical accu- 
 mulations, and necessarily vary indefinitely in composition, 
 according to the nature of the sources from which they are 
 derived. As a rule, they consist of the detritus of siliceous 
 rocks, these being among the most durable materials. 
 Quartz, in particular, enters largely into the composition of 
 sandy and gravelly detritus. Fragmentary materials tend 
 to group themselves according to their size and relative 
 density. Hence they are apt to occur in layers, and to 
 show the characteristic stratified arrangement of sedimen- 
 tary rocks. They may inclose the remains of any plants or 
 animals entombed on the same sea-floor, river-bed, or lake- 
 bottom. 
 
 In the majority of these rocks, their general mineral 
 composition is obvious to the naked eye. But the appli- 
 cation of the microscope to their investigation has thrown 
 considerable light upon their composition, formation, and 
 subsequent mutations. Their component materials are thus
 
 GEOGNOSY 225 
 
 ascertained to be divisible into 1st, derived fragments, of 
 which the most abundant are quartz, after which come fel- 
 spar, mica, iron-ores, zircon, rutile, apatite, tourmaline, 
 garnet, sphene, augite, hornblende, fragments of various 
 rocks, and clastic dust; 2d, constituents which have been 
 deposited between the particles, and which in many cases 
 serve as the cementing material of the rock. Among the 
 more important of these are silicic acid in the form of 
 quartz, chalcedony and opal; carbonates of lime, iron or 
 magnesia; haematite, limonite; pyrite and glauconite. 110 
 
 Cliff-Debris, Moraine StdF angular rubbish disengaged by 
 frost and ordinary atmospheric waste from cliffs, crags, and 
 steep slopes. It slides down the declivities of hilly regions, 
 and accumulates at their base, until washed away by rain 
 or by brooks. It forms talus-slopes of as much as 40, 
 though for short distances, if the blocks are large, the 
 general angle of slope may be much steeper. It naturally 
 depends for its composition upon the nature of the solid 
 rocks from which it is derived. Where cliff-debris falls 
 upon and is borne along by glaciers it is called "Moraine- 
 stuff," which may be deposited near its source, or may be 
 transported for many miles on the surface of the ice (p. 714). 
 
 Perched Blocks, Erratic Blocks large masses of rock, often as 
 big as a house, which have been transported by glacier-ice, 
 and have been lodged in a prominent position in glacier 
 valleys or have been scattered over hills and plains. An 
 examination of their mineralogical character leads to the 
 identification of their source, and, consequently, to the 
 
 Stth taken by the transporting ice. (See liook III. Part 
 . Section ii. 5.) 
 
 Rain-wash a loam or earth which accumulates on the 
 lower parts of slopes or at their base, and is due to 
 the gradual descent of the finer particles of disintegrated 
 rocks by the transporting action of rain. Brick-earth 
 is the name given in the southeast of England to thick 
 masses of such loam, which is extensively used for making 
 bricks. 
 
 Soil the product of the subaerial decomposition of rocks 
 and of the decay of plants and animals. Primarily the 
 character of the soil is determined by that of the subsoil, 
 of which indeed it is merely a further disintegration. Ac- 
 
 110 G. Klemm, Zeitsch. Deutsch. Geol. Ges. xxxiv. (1882), p. 771. H. C. 
 Sorby, Quart. Journ. Geol. Soc. xxxvi. (1880). J. A. Phillips, op. cit. xxxvii. 
 (1881), p. 6.
 
 226 TEXT-BOOK OF GEOLOGY 
 
 cording to the nature of the rock underneath, a soil may 
 vary from a stiff clay, through various clayey and sandy 
 loams, to mere sand. The formation of soil is treated of 
 in Book III. Part II. Section ii. 1. 
 
 Subsoil the broken- up part of the rocks immediately 
 under the soil. Its character, of course, is determined by 
 that of the rock out of which it is formed by subaerial dis- 
 integration. (Book III. Part II. Section ii. 1.) 
 
 Blown Sand loose sand usually arranged in lines of 
 dunes, fronting a sandy beach or in the arid interior of 
 a continent. It is piled up by the driving action of wind. 
 (Book III. Part II. Section i.) It varies in composition, 
 being sometimes entirely siliceous, as upon shores where 
 siliceous rocks are exposed; sometimes calcareous, where 
 derived from triturated shells, nullipores, or other calca- 
 reous organisms. The minute grains from long-continued 
 mutual friction assume remarkably rounded and polished 
 forms. Layers of finer and coarser particles often alternate, 
 as in water-formed sandstone. On many coast-lines in 
 Europe, grasses and other plants bind the surface of the 
 shifting sand. These layers of vegetation are apt to be 
 covered by fresh encroachments of the loose material, and 
 then by their decay to give rise to dark peaty seams in the 
 sand. Calcareous blown sand is compacted into hard stone 
 by the action of rain-water, which alternately dissolves a 
 little of the lime, and re-deposits it on evaporation as a thin 
 crust cementing the grains of sand together. In the Baha- 
 mas and Bermudas, extensive masses of calcareous blown 
 sand have been cemented in this way into solid stone, which 
 weathers into picturesque crags and caves like a limestone 
 of older geological date. 111 At Newquay, Cornwall, blown 
 sand has been by the decay of abundant land-shells solidi- 
 fied into a material capable of being used as a building- 
 stone. 
 
 River-sand, Sea-sand. When the rounded water-worn de- 
 tritus is finer than that to which the term gravel would be 
 applied, it is called sand, though there is obviously no line 
 to pe drawn between the two kinds of deposit, which neces- 
 sarily graduate into each other. The particles of sand range 
 down to such minute forms as can only be distinctly dis- 
 
 111 For interesting accounts of the ./Eolian deposits of the Bahama and Ber- 
 mudas, see Nelson, Q. J. Geol. Soc. ix. p. 200, Sir Wyville Thomson's "Atlan- 
 tic," vol. i. ; also J. J. Rein, Senckenb. Nat. Gesellsch. Bericht. 1869-70, p. 140, 
 1872-73, p. 131. On the Red Sands of the Arabian Desert, see J. A. Phillips, 
 Q. J. Geol. Soc. xxxviii. (1882), p. 110, also op. cit. xxzvii. (1881), p. 12.
 
 GEOGNOSY 227 
 
 cerned with a microscope. The smaller forms are generally 
 less well rounded than those of greater dimensions, no doubt 
 because their diminutive size allows them to remain sus- 
 pended in agitated water, and thus to escape the mutual 
 attrition to which the larger and heavier grains are exposed 
 upon the bottom. (Book III. Part II. Section ii.J So far 
 as experience has yet gone, there is no method by which 
 inorganic sea-sand can be distinguished from that of rivers 
 or lakes. As a rule, sand consists largely (often wholly) 
 of quartz-grains. The presence of fragments of marine 
 shells will of course betray its salt-water origin; but in 
 the trituration to which sand is exposed on a coast-line, 
 the shell-fragments are in great measure ground into cal- 
 careous mud and removed. 
 
 Mr. Sorby has shown that, by microscopic investigation, 
 much information may be obtained regarding the history 
 and source of sedimentary materials. He has studied the 
 minute structure of modern sand, and finds that sand-grains 
 present the following five distinct types, which, however, 
 graduate into each other. 
 
 1. Normal, angular, fresh -formed sand, such as has been 
 derived almost directly from the breaking up of granitic or 
 schistose rocks. 
 
 2. Well-worn sand in rounded grains, the original angles 
 being completely lost, and the surfaces looking like fine 
 ground glass. 
 
 3. Sand mechanically broken into sharp angular chips, 
 showing a glassy fracture. 
 
 4. Sand having the grains chemically corroded, so as 
 to produce a peculiar texture of the surface, differing from 
 that of worn grains or crystals. 
 
 5. Sand in which the grains have a perfectly crystalline 
 outline, in some cases undoubtedly due to the deposition 
 of quartz upon rounded or angular nuclei of ordinary non- 
 crystalline sand. 11 " 
 
 The same acute observer points out that, as in the 
 familiar case of conglomerate pebbles, which have some- 
 times been used over again in conglomerates of very differ- 
 ent ages, so with the much more minute grains of sand, we 
 must distinguish between the age of the grains and the age 
 of the deposit formed of them. An ancient sandstone may 
 consist of grains that had hardly been worn before they were 
 
 u ' 2 Address, Q. J. Geol. Soc. xxxvi. (1880), p. 58, and Monthly Microscop. 
 Journ. Anniv. Address, 1877.
 
 228 TEXT-BOOK OF GEOLOGY 
 
 finally brought to rest, while the sand of a modern beach 
 may have been ground down by the vaves of many suc- 
 cessive geological periods. 
 
 Sand taken by Mr. Sorby from the old gravel terraces 
 of the River Tay was found to be almost wholly angular, 
 indicating how little wear and tear there may be among 
 particles of quartz TTO of an inch in diameter, even though 
 exposed to the drifting action of a rapid river. 113 Sand from 
 the bowlder clay at Scarborough was likewise ascertained to 
 be almost entirely fresh and angular. On the other hand, 
 in geological formations which can be traced in a given 
 direction for several hundred miles, a progressively large 
 proportion of rounded particles may be detected in the 
 sandy beds, as Mr. Sorby has found in following the Green- 
 sand from Devonshire to Kent. In wind-blown sand ex- 
 posed for a long period to drift to and fro along the sur- 
 lace the larger particles and pebbles acquire a remarkably 
 smoothed and polished surface. 
 
 The occurrence of various other minerals besides quartz 
 in ordinary sand has long been recognized, but we owe to 
 the recent observations of Mr. A. B. Dick the discovery 
 that among these minerals some of the most plentiful and 
 most perfectly preserved belong to species that were not 
 supposed to be so widely diffused, such as zircon, rutile, 
 and tourmaline. He has found that these heavy minerals 
 constitute sometimes as much as 4 per cent of the Bagshot 
 sand of the older Tertiary series of the London basin. 114 
 Felspars, micas, hornblendes, pyroxenes, magnetite, glau- 
 conite and other minerals may likewise be recognized. The 
 remarkable perfection of some of the crystallographic forms 
 of the minuter mineral constituents of certain sands has been 
 well shown by Mr. Dick. 
 
 Varieties of river- or sea-sand may be distinguished by 
 names referring to some remarkable 'constituent, e.g. mag- 
 netic sand, iron-sand, gold-sand, auriferous sand, etc. 
 
 Gravel, Shingle names applied to the coarser kinds of 
 rounded water-worn detritus. In Gravel, the average size 
 of the component pebbles ranges from that of a small pea up 
 to about that of a walnut, though of course many included 
 fragments will be observed which exceed these limits. In 
 Shingle, the stones are coarser, ranging up to blocks as big 
 . _ 
 
 113 See Book III. Part II. Section ii. 3. 
 
 114 Nature, xxxvi. (1887), p. 91, Mem. Geol. Surv. "Geology of London," 
 vol. i. (1889), p. 523. Teall, "Microscopic Petrography," Plate xliv.
 
 GEOGNOSY 229 
 
 as a man's head or larger. German geologists distinguish 
 as "schotter," a shingle containing dispersed bowlders, and 
 "schotter-conglomerate," a rock wherein these materials 
 have become consolidated. 115 All these names are applied 
 quite irrespective of the composition of the fragments, which 
 varies greatly from point to point. As a rule, the stones 
 consist of hard rocks, since these are best fitted to with- 
 stand the powerful grinding action to which they are 
 
 Conglomerate (PuddingstoneV a rock formed of consoli- 
 dated gravel or shingle. The component pebbles are 
 rounded and water-worn. They may consist of any kind 
 of rock, though usually of some hard and durable sort, 
 such as quartz or quartzite. A special name may be given 
 according to the nature of the pebbles, as quartz-conglom- 
 erate, limestone-conglomerate, granite-conglomerate, etc., or 
 according to that of the paste or cementing matrix, which 
 may consist of a hardened sand or clay, and may be sili- 
 ceous, calcareous, argillaceous, or ferruginous. In the coarser 
 conglomerates, where the blocks may exceed six feet in 
 length, there is often very little indication of stratification. 
 Except where the flatter stones show by their general paral- 
 lelism the rude lines of deposit, it may be only when the 
 mass of conglomerate is taken as a whole, in its relation to 
 the rocks below and above it, that its claim to be considered 
 a bedded rock will be conceded. The occurrence of occa- 
 sional bands of conglomerate in a series of arenaceous strata 
 is analogous probably to that of a shingle-bank or gravel- 
 beach on a modern coast-line. But it is not easy to under- 
 stand the circumstances under which some ancient conglom- 
 erates accumulated, such as that of the Old Red Sandstone 
 of Central Scotland, which attains a thickness of many thou- 
 sand feet, and consists of well-rounded and smoothed blocks 
 often several feet in diameter. 
 
 In many old conglomerates (and even in those of Miocene 
 age in Switzerland) the component pebbles may be observed 
 to have indented each other. In such cases also they may 
 be found elongated, distorted or split and recementect; 
 sometimes the same pebble has been crushed into a num- 
 ber of pieces, which are held together by a retaining cement. 
 These phenomena point to great pressure, and some internal 
 
 115 See, for example, an account of the schotter-conglomerates of Northern 
 Persia by E. Tietze, Jahrb. Geol. ReichsansU Vienna, 188], p. 68.
 
 230 - TEXT-BOOK OF GEOLOGY 
 
 relative movement in the rocks. (Book III. Part I. Section 
 iv. 3.) 
 
 Breccia a rock composed of angular, instead of rounded, 
 fragments. It commonly presents less trace of stratification 
 than conglomerate. Intermediate stages between these two 
 rocks, where the stones are partly angular and partly sub- 
 angular and rounded, are known as brecciated conglomerate. 
 Considered as a detrital deposit formed by superficial waste, 
 breccia points to the disintegration of rocks by the atmos- 
 phere, and the accumulation of their fragments with little or 
 no intervention of running water. Thus it may be formed 
 at the base of a cliff, either subaerially, or where the ddbris 
 of the cliff falls at once into a lake or into deep sea-water. 
 
 The term Breccia has, however, been applied to rocks 
 formed in a totally different manner. -Angular blocks of 
 all sizes and shapes have been discharged from volcanic 
 orifices, and, falling back, have consolidated there into 
 masses of brecciated material (volcanic breccia). Intrusive 
 igneous eruptions have sometimes torn off fragments of 
 the rocks through which they have ascended, and these 
 angular fragments have been inclosed in the liquid or pasty 
 mass. Or the intrusive rock has cooled and solidified ex- 
 ternally while still mobile within, and in its ascent has 
 caught up and involved some of these consolidated parts 
 of its own substance. Again, where solid masses of rock 
 within the crust of the earth have ground against each 
 other, as in dislocations, angular fragmentary rubbish has 
 been produced, which has subsequently been consolidated 
 by some infiltrating cement (Fault-rock). It is evident, 
 however, that breccia formed in one or other of these 
 hypogene ways will not, as a rule, be apt to be mistaken 
 for the true breccias, arising from superficial disintegration. 
 
 Sandstone (Gres) 116 a rock composed of consolidated sand. 
 As in ordinary modern sand, the integral grains of sandstone 
 are chiefly quartz, which must here be regarded as the resi- 
 due left after all the less durable minerals of the original 
 rocks have been carried away in solution or in suspension as 
 fine mud. The colors of sandstones arise, not so much from 
 that of the quartz, which is commonly white or gray, as 
 from the film or crust which often coats the grains and holds 
 
 116 See J. A. Phillipa on the constitution and history of grits and sandstones. 
 Quart. Journ. Geol. Soc. xxxvii. (1881), p. 6. For analyses of some British 
 sandstones used as building stones, see Wallace, Proc. Phil. Soc. Glasgow, xiv. 
 (1883), p. 22.
 
 GEOGNOSY 231 
 
 them together as a cement. Iron, the great coloring ingre- 
 dient of rocks, gives rise to red, brown, yellow, and green 
 hues, according to its degree of oxidation and hydration. 
 
 Like conglomerates, sandstones differ in the nature of 
 their component grains, and in that of the cementing matrix. 
 Though consisting for the most part of siliceous grains, they 
 include others of clay, felspar, mica, zircon, rutile, tourma- 
 line, or other minerals such as occur in sand (p. 227), and 
 these may increase in number so as to give a special charac- 
 ter to the rock. Thus, sandstones may be argillaceous, fel- 
 spathic, micaceous, calcareous, etc. By an increase in the 
 argillaceous constituents, a sandstone may pass into one of 
 the clay-rocks, just as modern sand on the sea-floor shades 
 imperceptibly into mud. On the other hand, by an augmen- 
 tation in the size and sharpness of the grains, a sandstone 
 may become a g r i t, and by an increase in the size and num- 
 ber of pebbles may pass into a pebbly or conglomeratic sand- 
 stone, and thence into a fine conglomerate. A piece of fine- 
 grained sandstone, seen under the microscope, looks like a 
 coarse conglomerate, so that the difference between the two 
 rocks is little more than one of relative size of particles. 
 
 The cementing material of sandstones may be ferrugi- 
 nous, as in most ordinary red and yellow sandstones, where 
 the anhydrous or hydrous iron-oxide is mixed with clay or 
 other impurity in red sandstones the grains are held to- 
 gether by a haematitic, in yellow sandstones by a limonitic 
 cement; argillaceous, where the grains are united by a base 
 of clay, recognizable by the earthy smell when breathed 
 upon; calcareous, where carbonate of lime occurs either as 
 an amorphous paste or as a crystalline cement between the 
 grains; siliceous, where the component particles are bound 
 together by silica, as in the exposed blocKs of Eocene sand- 
 stone known as "gray weathers" in Wiltshire, and which 
 occur also over the north of France toward the Ardennes. 
 
 Among the varieties of sandstone the following may heje 
 be mentioned. F 1 a g s t o n e a thin- bedded sandstone, 
 capable of being split along the lines of stratification into 
 thin beds or flags; Micaceous sandstone (mica-psam- 
 mite) a rock so full of mica-flakes that it splits readily into 
 thin laminae, each of which has a lustrous surface from the 
 quantity of silvery mica. This rock is called "fakes" in 
 Scotland. Freeston e a sandstone (the term being ap- 
 plied sometimes also to limestone) which can be cut into 
 blocks in any direction, without a marked tendency to split 
 in any one plane more than in another. Though this rock
 
 232 TEXT-BOOK OF GEOLOGY 
 
 occurs in beds, each bed is not divided into laminae, and it 
 is the absence of this minor stratification which makes the 
 stone so useful for architectural purposes (Craigleith and 
 other sandstones at Edinburgh, some of which contain 98 per 
 cent of silica). Glauconitic sandstone (green-sand) 
 a sandstone containing kernels and dusty grains of glau- 
 conite, which imparts a general greenish hue to the rock. 
 The glauconite has probably been deposited in association 
 with decaying organic matter, as where it fills echinus- 
 spines, foraminifera, shells and corals on the floor of the 
 present ocean. 117 Buhrstone a highly siliceous, ex- 
 ceedingly compact, though cellular rock (with Chara seeds, 
 etc.), found alternating with unaltered Tertiary strata in the 
 Paris basin, and forming from its hardness and roughness an 
 excellent material for the grindstones of flour-mills, may be 
 mentioned here, though it probably has been formed by the 
 precipitation of silica through the action of organisms. Ar- 
 k o s e (granitic sandstone) a rock composed of disintegrated 
 granite, and found in geological formations of different ages, 
 which have been derived from granitic rocks. Crystal- 
 lized sandstone an arenaceous rock in which a de- 
 posit of crystalline quartz has taken place upon the indi- 
 vidual grains, each of which becomes the nucleus of a more 
 or less perfect quartz crystal. Mr. Sorby has observed such 
 crystallized sand in deposits of various ages from the Oolites 
 down to the Old Red Sandstone. 118 
 
 Graywacke a compact aggregate of rounded or subangular 
 grains of quartz, felspar, slate, or other minerals or rocks, 
 cemented by a paste which is usually siliceous, but may be 
 argillaceous, felspathic, calcareous, or anthracitic (Fig. 21). 
 Gray, as its name denotes, is the prevailing color: but it 
 passes into brown, brownish-purple, and sometimes, where 
 anthracite predominates, into black. The rock is distin- 
 guished from ordinary sandstone by its darker hue, its hard- 
 ness, the variety of it's component grains, and, above all, by 
 the compact cement in which the grains are imbedded. In 
 many varieties, so pervaded is the rock by the siliceous 
 paste, that it possesses great toughness, and its grains seem 
 to graduate into each other as well as into the surrounding 
 
 m Ante, p. 141 ; Sollas, Geol. Mag. iii. 2d ser. p. 539. 
 
 118 Q. J. Geol. Soc. xxxvi. p. 63. See Daubree, Ann. des Mines, 2d ser. i. 
 p. 206. A. A. Young, Amer. Journ. Sci. 3d ser. xxiii. 257 ; xxiv. 47, and es- 
 pecially the work of Irving and Van Hise (quoted on p. 196), which gives some 
 excellent figures of enlarged quartz-graius.
 
 GEOGNOSY 233 
 
 matrix. Such rocks, when fine-grained, can hardly, at first 
 sight or with the unaided eye, be distinguished from some 
 compact igneous rocks, though a microscopic examination 
 at once reveals their fragmental character. In other cases, 
 where the graywacke has been formed mainly out of the 
 debris of granite, quartz- porphyry, or other felspathic 
 masses, the grains consist so largely of felspar, and the 
 
 Easte also is so felspathic, that the rock might be mistaken 
 }r some close-grained granular porphyry. Graywacke oc- 
 curs extensively among the Palaeozoic formations, in beds 
 alternating with shales and conglomerates. It represents 
 the muddy sand of some of the Palaeozoic sea-floors, retain- 
 ing often its ripple-marks and sun-cracks. The metamor- 
 phism it has undergone has generally not been great, and 
 lor the most part is limited to induration, partly by pres- 
 sure and partly by permeation of a siliceous cement. But 
 where felspathic ingredients prevail, the rock has offered 
 facilities for alteration, and has been here and there changed 
 into highly crystalline mica-schists full of garnets and other 
 secondary minerals (contact-metamorphism at the granite of 
 New Galloway, Scotland, postea, Book IV. Part Viil.V 
 
 The more fissile fine-grained varieties of this rock have 
 been termed graywacke-slate (p. 238). In these, as well as 
 in graywacke, organic remains occur among the Silurian and 
 Devonian formations. Sometimes in the Lower Silurian 
 rocks of Scotland, these strata become black with carbona- 
 ceous matter, among which vast numbers of graptolites may 
 be observed. Gradations into sandstone are termed Gray- 
 wac ke-s a n d s ton e. In Norway the reddish felspathic 
 graywacke or sandstone of the Primordial rocks is called 
 Sparagmite; similar material forms much of the Torri- 
 don sandstone of Scotland. 
 
 Quartzite. An altered siliceous sandstone (see p. 311). 
 
 2. Clay Rocks (Pelites) 
 
 These are composed of fine argillaceous sediment or mud, 
 derived from the waste of rocks. Perfectly pure clay or 
 kaolin, bydrated silicate of alumina, may be obtained where 
 granites and other felspar-bearing rocks decompose. But, 
 as a rule, the argillaceous materials are mixed with various 
 impurities. 
 
 Clay, Mud. The decomposition of felspars and allied 
 minerals gives rise to the formation of hydrous aluminous 
 silicates, which, occurring usually in a state of fine subdivi-
 
 234 TEXT-BOOK OF GEOLOGY 
 
 siou, are capable of being held in suspension in water, and 
 of being transported to great distances. These substances, 
 differing much in composition, are embraced under the gen- 
 eral term Clay, which may be defined as a white, gray, 
 brown, red, or bluish substance, which when dry is soft arid 
 friable, adheres to the tongue, and shaken in water makes it 
 mechanically turbid; when moist is plastic, when mixed 
 with much water becomes mud. It is evident that a wide 
 range is possible for varieties of this substance. The fol- 
 lowing are the more important. 
 
 Kaolin (Porcelain-clay, China-clay) has been already no- 
 ticed (p. 140). 
 
 Pipe-clay white, nearly pure, and free from iron. 
 
 Fire-clay largely found in connection with coal-seams, 
 contains little iron, and is nearly free from lime and alka- 
 lies. Some of the most typical fire-clays are those long used 
 as Stourbridge, Worcestershire, for the manufacture of pot- 
 tery. The best glass-house pot-clay, that is, the most re- 
 fractory, and therefore used for the construction of pots 
 which have to stand the intense heat of a glass-house, has 
 the following composition: silica, 73 '82; alumina, 15*88; 
 protoxide of iron, 2 -95; lime, trace; magnesia, trace; alka- 
 lies, -90; sulphuric acid, trace; chlorine, trace; water, 6-45; 
 specific gravity, 2-51. 
 
 Gannister a very siliceous close-grained variety, found in 
 the Lower Coal measures of the North of England, and now 
 largely ground down as a material for the hearths of iron 
 furnaces. 
 
 Brick-clay properly rather an industrial than a geological 
 term, since it is applied to any clay, loam, or earth, from 
 which bricks or coarse pottery are made. It is an impure 
 clay, containing a good deal of iron, with other ingredients. 
 An analysis gave the following composition of a brick-clay: 
 silica, 49 - 44; alumina, 34-26; sesquioxide of iron, 7'74; 
 lime, 1-48; magnesia, 5 '14; water, 1-94. 
 
 Fuller's Earth (Terre a foulon, Walkerde) a greenish or 
 brownish, earthy, soft, somewhat unctuous substance, with 
 a shining streak, which does not become plastic with water, 
 but crumbles down into mud. It is a hydrous aluminous 
 silicate with some magnesia, iron-oxide and soda. The yel- 
 low fuller's earth of Reigate contains silica 44, alumina 11, 
 oxide of iron 10, magnesia 2, lime 5, soda 5.' 18 In England 
 
 119 tire's Diet. Arts, etc. ii. p. 142.
 
 GEOGNOSY 235 
 
 fuller's earth occurs in beds among the Jurassic and Creta- 
 ceous formations. In Saxony it is found as a result of the 
 decomposition of diabase and gabbro. 
 
 Wacke a dirty-green to brownish -black, earthy or com- 
 pact, but tender and apparently homogeneous clay, which 
 arises as the ultimate stage of the decomposition of basalt- 
 rocks in situ. 
 
 Loam an earthy mixture of clay and sand with more or 
 less organic matter. The black soils of Russia, India, etc. 
 (Tchernosem, Regur), are dark deposits of loam rich in or- 
 ganic matter, and sometimes upward of twenty feet deep. 
 
 Loess a pale, somewhat calcareous clay, probably of wind- 
 drift origin, found in some river-valleys (Rhine, Danube, 
 Mississippi, etc.), and over wide regions in China and else- 
 where. It is described in Book III. Part. II. Sect. i. 1. 
 
 Laterite a cellular, reddish, ferruginous clay, found in 
 some tropical countries as the result of the subaerial decom- 
 position of rocks; it acquires great hardness after being 
 quarried out and dried. 
 
 Till, Bowlder-clay a stiff sandy and stony clay, varying in 
 color and composition, according to the character of the 
 rocks of the district in which it lies. It is full of worn 
 stones of all sizes, up to blocks weighing several tons, and 
 often well-smoothed and striated. It is a glacial deposit, 
 and will be described among the formations of the Glacial 
 Period. 
 
 Mudstone a fine, usually more or less sandy, argillaceous 
 rock, having no fissile character, and of somewhat greater 
 hardness than any form of clay. The term C 1 a-y-r o c k has 
 been applied by some writers to an indurated clay requiring 
 to be ground and mixed with water before it acquires plas- 
 ticity. 
 
 Shale (Schiste, Schieferthon) a general term to describe 
 clay that has assumed a thinly stratified or fissile structure. 
 Under this term are included laminated and somewhat hard- 
 ened argillaceous rocks, which are capable of being split 
 along the lines of deposit into thin leaves. They present 
 almost endless varieties of texture and composition, passing, 
 on the one hand, into clays, or, where much indurated, into 
 slates and argillaceous schists, on the other, into flagstones 
 and sandstones, or again, through calcareous gradations into 
 limestone, or through ferruginous varieties into clay-iron- 
 stone, and through bituminous kinds into coal. 
 
 Clay-slate (Schiste ardoise, ThonsehieferY Under this 
 name are included certain hard fissile argillaceous masses,
 
 236 TEXT-BOOK OF GEOLOGY 
 
 composed primarily of compact clay, sometimes with mega- 
 scopic and microscopic scales of one or more micaceous min- 
 erals, granules of quartz and cubes or concretions of pyrites, 
 as well as veins of quartz and calcite. The fissile structure 
 is specially characteristic. In some cases this structure 
 coincides with that, of original deposit, as is proved by the 
 alteration of fissile beds with bands of hardened sandstone, 
 conglomerate or fossiliferous limestone. But for the most 
 part as the rocks have been much compressed, the fissile 
 structure of the argillaceous bands is independent of stratifi- 
 cation, and can be seen traversing it. Sorby has shown that 
 this superinduced fissility or " cleavage' ' has resulted from 
 an internal rearrangement of the particles in planes perpen- 
 dicular to the direction in which' the rocks have been com- 
 pressed (see Book III. Part I. Section iv. 3). In England 
 the term "slate" or "clay-slate" is given to argillaceous, not 
 obviously crystalline rocks possessing this cleavage -struc- 
 ture, w here the micaceous lustre of the finely disseminated 
 superinduced mica is prominent, the rocks are phyllites. 
 
 Microscopic examination shows that while some argilla- 
 ceous rocks consist mainly of granular debris, many cleaved 
 clay-slates contain a large proportion of a micaceous mineral 
 in extremely minute flakes, which in the best Welsh slates 
 have an average size of 2 <Jx, of an inch in breadth, and ^ of 
 an inch in thickness, together with very fine black hairs 
 which may be magnetite. 1110 Moreover, many clay-slates, 
 though to outward appearance thoroughly noncrystalline, 
 and evidently of f ragmen tal composition and sedimentary 
 origin, yet contain, sometimes in remarkable abundance, 
 microscopic microlites and crystals of different minerals 
 placed with their long axes parallel with the planes of fis- 
 sility. These minute bodies include yellowish- brown nee- 
 dles of rutile, greenish or yellowish flakes of mica, scales of 
 calcite, and probably other minerals. 121 Small granules of 
 quartz containing fluid-cavities, show on their surfaces a 
 distinct blending with the substance of the surrounding 
 
 120 Sorby, Q. J. Geol. Soc. xxvi. p. 68. 
 
 121 These "clay-slate needles" were probably not crystallized contemporane- 
 ously with the deposit of the original rock. In some cases they may have been 
 deposited with the rest of the sediment as part of the debris of pre-existing 
 crystalline rocks (see p. 228); but in general they appear to have been devel- 
 oped where they now occur by subsequent actions (see postea, pp. 531, 632). 
 For their character see Zirkel, '"'Mik. Beschaff." p. 490. Kalkowsky, N. Jahrb. 
 1879, p. 382; A. Cathrein, op. cit. 1882 (i.) p. 169. F. Penck, Sitzb. Bayer. 
 Akad. Math. Phys. 1880, p. 461. A. Wichmann, Q. J. Geol. Soc. xxxv. p. 156.
 
 GEOGNOSY 237 
 
 rock. M. Renard has found that the Belgian whet-slate is 
 full of minute crystals of garnet. 1 " Some of the more crys- 
 talline varieties (phyllite) are almost wholly composed of 
 minute crystalline particles of mica, quartz, felspar, chlorite, 
 and rutile, and form an intermediate stage between ordinary 
 clay-slate and mica-schist. 
 
 'A distinction has been drawn by some petrographers be- 
 tween certain rocks (phyllite, Urthonschiefer) which occur 
 in Archaean regions or in groups probably of high antiq- 
 uity, and others (ardoise, Thonschiefer) which are found in 
 Palaeozoic and later formations. But there does not appear 
 to be adequate justification for this grouping, which has 
 probably been suggested rather by theoretical exigencies 
 than by any essential differences between the rocks them- 
 selves. That the whole of the series of argillaceous rocks, 
 beginning with clay and passing through shale into slate 
 and pbyllite, is of sedimentary origin is indicated by the 
 organic remains, false bedding, ripple-mark, etc., found in 
 those at one end of the series, and by the insensible grada- 
 tion of the mineralogical characters through increasing stages 
 of metamorphism to the other end. Some microscopic crys- 
 tals may possibly have been originally formed among the 
 muddy sediment on the sea-floor (see p. 770). Others may 
 have formed part of the original mechanical detritus that 
 went to make the slate. But, for the most part, they have 
 been subsequently developed within the rock, and represent 
 early stages of the process which has culminated in the pro- 
 duction of crystalline schists. The development of crystals 
 of chiastolite and other minerals in clay-slate is frequently 
 to be observed round bosses of granite, as one of the phases 
 of contact-metamorphism. 
 
 A number of varieties of Clay-slate are recognized. 
 Roofing slate (Dachschiefer) includes the finest, most 
 compact, homogeneous and durable kinds, suitable for roof- 
 ing houses or the manufacture of tables, chimney-pieces, 
 writing-slates, etc. ; it occurs in the Silurian and Devonian 
 formations of Central and Western Europe. Anthra- 
 ci tic-si ate (anthracite-phyllite, alum-slate), dark carbo- 
 naceous slate with, much iron-disulphide. Bands of this 
 nature sometimes run through a clay-slate region. The 
 
 128 Acad. Roy. Belgique, xli. (1877). See also his paper on the composition 
 and structure of the phyllades of the Ardennes, Bull. Mus. Roy. Belg. iii. (1884), 
 p. 281.
 
 238 TEXT- BOOK OF GEOLOGY 
 
 carbonaceous material arises from the alteration of the re- 
 mains of plants (fucoids) or animals (frequently graptolites). 
 The marcasite so abundantly associated with these organ- 
 isms decomposes on exposure, and the sulphuric acid pro- 
 duced, uniting with the alumina, potash, and other bases of 
 the surrounding rocks, gives rise to an efflorescence of alum, 
 or the decomposition produces sulphurous springs like those 
 of Moffat. The name Graywacke-slate has been ap- 
 plied to extremely fine-grained, hard, shaly, more or less 
 micaceous and sandy bands, associated with graywacke 
 among the older Palaeozoic rocks. Whet-slate, Novae- 
 ulite, Hone-stone, is an exceedingly hard fine-grained 
 siliceous rock, some varieties of which derive their economic 
 value from the presence of microscopic crystals of garnet. 
 The various forms of altered clay-slate are described at p. 309 
 among the metamorphic rocks. 
 
 Porcellamte (Ar^illite) or baked shale a name applied to 
 the exceedingly indurated sometimes partially fused condi- 
 tion which shales are apt to assume in contact with dikes 
 and intrusive sheets or bosses. For an account of this form 
 of contact-metamorphism see Book IV. Part VIII. 
 
 3. Volcanic Fragmental Rocks Toffs 
 
 This section comprises all deposits which have resulted 
 from the comminution of volcanic rocks. They thus in- 
 clude (1) those which consist of the fragmentary materials 
 ejected from volcanic foci, or the true ashes and tuffs ; and 
 (2) some rocks derived from the superficial disintegration of 
 already erupted and consolidated volcanic masses. Obvi- 
 ously the second series ought properly to be classed with 
 the sandy or clayey rocks above described, since they have 
 been formed in the same way. In practice, however, these 
 detrital reconstructed rocks cannot always be certainly dis- 
 tinguished from those which have been formed by the con- 
 solidation of true volcanic dust and sand. Their chemical 
 and lithological characters, both megascopic and micro- 
 scopic, are occasionally so similar, that their respective 
 modes of origin have to be decided by other considera- 
 tions, such as the occurrence of lapilli, bombs, or slags in 
 the truly volcanic series, and of well water-worn pebbles 
 of volcanic rocks in the other. Attention to these features, 
 however, usually enables the geologist to make the distinc- 
 tion, and to perceive that the number of instances where 
 he may be in doubt is less than might be supposed. Only
 
 GEOGNOSY 239 
 
 a comparatively small number of the rocks classed here are 
 not true volcanic ejections. 123 
 
 Referring to the account of volcanic action in Book III. 
 Part I. Sect, i., we may here merely define the use of the 
 names by which the different kinds of ejected volcanic 
 materials are known. 
 
 Volcanic Blocks angular, sub-angular, round, or irregu- 
 larly-shaped masses of lava, several feet in diameter, some- 
 times of uniform texture throughout, as if they were large 
 fragments dislodged by explosion from a previously consoli- 
 dated rock, sometimes' compact in the interior and cellular 
 or slaggy outside. 
 
 Bombs round, elliptical, or discoidal pieces of lava from 
 a few inches up to one or more feet in diameter. They are 
 frequently cellular internally, while the outer parts are fine- 
 grained. Occasionally they consist of a mere shell of lava 
 with a hollow interior like a bombshell, or of a casing of 
 lava inclosing a fragment of rock. Their mode of origin 
 is explained in Book III. Part I. Sect. i. 1. 
 
 Lapilli (rapilli) ejected fragments of lava, round, angular, 
 or indefinite in shape, varying in size from a pea to a walnut. 
 Their mineralogical composition depends upon that of the 
 lava from which they have been thrown up. Usually they 
 are porous or finely vesicular in texture. 
 
 Volcanic Sand, Volcanic Ash the finer detritu's erupted from 
 volcanic orifices, consisting partly of rounded and angular 
 fragments up to about the size of a pea derived from the 
 explosion or lava within eruptive vents, partly of vast 
 quantities of microlites and crystals of some of the min- 
 erals of the lava. The finest dust is in a state of extremely 
 minute subdivision. When examined under the microscope, 
 it is sometimes found to consist not only of minute crystals 
 and microlites, but of volcanic glass, which may be observed 
 adhering to the microlites or crystals round which it flowed 
 when still part of the fluid lava. The presence of minutely 
 cellular fragments is characteristic of most volcanic f rag- 
 mental rocks, and this structure may commonly be observed 
 in the microscopic fragments and filaments of glass. 
 
 When these various materials are allowed to accumulate, 
 they become consolidated and receive distinctive names. 
 In cases where they fall into the sea or into lakes, they are 
 liable at the outer margin of their area to be mingled with, 
 
 143 For a classification of tuffs and tuffaceous deposits see E. Reyer, Jakrb. 
 K. K. Geol. Reichsanst. xxxi. (1881), p. 51.
 
 2-iO TEXT-BOOK OF GEOLOGY 
 
 and insensibly to pass into ordinary non-volcanic sediment. 
 Hence we may expect to find transitional varieties between 
 rocks formed directly from the results of volcanic explosion 
 and ordinary sedimentary deposits. 
 
 Volcanic Conglomerate a rock composed mainly or entirely 
 of rounded or sub-angular fragments, chiefly or wholly of 
 volcanic rocks, in a paste of the same materials, usually 
 exhibiting a stratified arrangement, and often found inter- 
 calated between successive sheets of lava. Conglomerates 
 of this kind may have been formed by the accumulation of 
 rounded materials ejected from volcanic vents; or as the 
 result of the aqueous erosion of previously solidified lavas, 
 or by a combination of both these processes. Well-rounded 
 and smoothed stones almost certainty indicate long-continued 
 water-action, rather than trituration in a volcanic vent. In 
 the Western Territories of the United States vast tracts of 
 country are covered with masses of such conglomerate, 
 sometimes 2000 feet thick. Captain Button has shown that 
 similar deposits are in course of formation there now, merely 
 by the influence of disintegration upon exposed lavas.' 34 
 
 Volcanic conglomerates receive different names accord- 
 ing to the nature of the component fragments; thus we have 
 basalt-conglomerates, where these fragments are wholly or 
 mainly of basalt, trachyte -conglomerates, porphy rite -conglom- 
 erates, phonolite- conglomerates, etc. 
 
 Volcanic Breccia resembles Volcanic Conglomerate, except 
 that the stones are angular. This angularity indicates an 
 absence of aqueous erosion, and, under the circumstances 
 in which it is found, usually points to immediately adjacent 
 volcanic explosions. There is a great variety of breccias, 
 as basalt-breccia, diabase-breccia, etc. 
 
 Volcanic Agglomerate a tumultuous assemblage of blocks of 
 all sizes up to masses several yards in diameter, met with 
 in the "necks" or pipes of old volcanic orifices. The stones 
 and paste are commonly of one or more volcanic rocks, 
 such as felsite, porphyrite or basalt, but they include also 
 fragments of the surrounding rocks, whatever these may 
 be, through which the volcanic orifice has been drilled. 
 As a rule, agglomerate is devoid of stratification; but some- 
 times it includes portions which have a more or less distinct 
 arrangement into beds of coarser and finer detritus, often 
 placed on end, or inclined in different directions at high 
 angles, as described in Book IV. Part VII. Sect. i. 4. 
 
 184 "High Plateaus of Utah," p. 77.
 
 GEOGNOSY 241 
 
 Volcanic Tuff. This general term may be made to include 
 all the finer kinds of volcanic detritus, ranging, on the one 
 hand, through coarse gravelly deposits into conglomerates, 
 and on the other, into exceedingly compact fine-grained 
 rocks, formed of the finest and most impalpable kind of 
 volcanic dust. Some modern tuffs are full of microlites, 
 derived from the lava which was blown into dust. Others 
 are formed of small rounded or 
 angular grains of different lavas, 
 with fragments of various rocks 
 through which the volcanic fun- 
 nels have been drilled. The tuffs 
 of earlier geological periods have 
 often been so much altered, that 
 it is difficult to state what may 
 have been their original condi- 
 tion. The absence of microlites 
 and glass in them is no proof 
 that they are not true tuffs; for 
 the presence of these bodies de- plg w ._ Mtewsoplo structure * 
 
 pends Upon the nature of the Carboniferous Palagonite Tuff 
 
 lavas. If the latter were not 
 
 vitreous and microlitic, neither would be the tuffs derived 
 from them. In the Carboniferous volcanic area of Central 
 Scotland, the tuffs are made up of debris and blocks of 
 the basaltic lavas, and, like these, are not microlitic, though 
 in some places they abound in fragments of the basic glass 
 called palagonite. (Fig. 23, and infra, p. 242.) 
 
 Tuffs have consolidated sometimes under water, some- 
 times on dry land. As a rule, they are distinctly stratified. 
 Near the original vents of eruption they commonly present 
 rapid alternations of finer and coarser detritus, indicative of 
 successive phases of volcanic activity. They necessarily 
 shade off into the sedimentary formations with which they 
 were contemporaneous. Thus, we have tuffs passing grad- 
 ually into shale, limestone, sandstone, etc. The interme- 
 diate varieties have been called ashy shale, tuffaceous shale, 
 or shaly tuff, etc. From the circumstances of their forma- 
 tion, tuffs frequently preserve the remains of plants and 
 animals, both terrestrial and aquatic. Those of Monte 
 Somma contain fragments of land-plants and shells. Some 
 of those of Carboniferous age in Central Scotland have 
 
 E "elded crinoids, brachiopods, and other marine organisms. 
 ike the other fragmentary volcanic rocks, the tuffs may 
 be subdivided according to the nature of the lava from the 
 GEOLOGY Vol. XXIX 11
 
 F, 
 
 242 TEXT-BOOK OF GEOLOGY 
 
 disintegration of which they have been formed. Thus we 
 have felsite-tuffs, trachyte-tuffs, basalt-tuffs, pumice-tuffs, por- 
 phyrite-tuffs, etc. A few varieties with special characteris- 
 tics may be mentioned here. 1 " 
 
 Trass a pale yellow or gray rock, rough to the feel, com- 
 osed of an earthy or compact pumiceous dust, in which 
 fragments of pumice, trachyte, graywacke, basalt, carbon- 
 ized wood, etc., are imbedded. It has filled up some of the 
 valleys of the Eifel, where it is largely quarried as a hy- 
 draulic mortar. 
 
 Peperino a dark-brown, earthy or granular tuff, found in 
 considerable quantity among the Alban Hills near Borne, 
 and containing abundant crystals of augite, mica, leucite, 
 magnetite, and fragments of crystalline limestone, basalt, 
 and leucite-lava. 
 
 Palagonite-Tuff a bedded aggregate of dust and fragments 
 of basaltic lava, among which are conspicuous angular 
 pieces and minute granules of the pale yellow, green, red, 
 or brown basic glass called palagonite. This vitreous sub- 
 stance is intimately related to the basalts (p. 298). It ap- 
 pears to have gathered within volcanic vents and to have 
 been emptied thence, not in streams, but by successive 
 aeriform explosions, and to have been subsequently more 
 or less altered. The percentage composition of a specimen 
 from -the typical locality, Palagonia, in the Val d.i Noto, 
 Sicily, was estimated by Sartorius von Waltershausen to 
 be: silica, 41-26; alumina, 8-60; ferric oxide, 25-32; lime, 
 5-59; magnesia, 4-84; potash, 0-54; soda, 1-06; water, 12-79. 
 This rock is largely developed among the products of the 
 Icelandic and Sicilian volcanoes; it occurs also in the Eifel 
 and in Nassau. It has been found to be one of the charac- 
 teristic features of tuffs of Carboniferous age in Central 
 Scotland 188 (Fig. 23). 
 
 Schalstem. Under this name, German petrographers have 
 placed a variety of green, gray, red, or mottled fissile rocks, 
 impregnated with carbonate of lime. They are inter-strati- 
 fied with the Devonian formations of Nassau, the Harz and 
 
 196 On the occurrence and structure of tuffs, see J. C: Ward, Q. J. Geol. Soc. 
 xxxi. p. 388; Reyer, Jahrb. Geol. Reichsanst. 1881, p. 57; Geikie, Trans. Roy. 
 Soc. Edin. xxix. ; Vogelsang, Z. Deutsch. Geol. Ges. xxiv. p. 643 ; Penck, op. 
 cit. xxxi. p. 504. On the basalt-tuffs of Scania, F. Eichstadt, Sveriges Geol. 
 Undersokn, ser. c. No. 58 (1883). On the metamorphism of tuffs into lava-like 
 rocks, see Button's "High Plateaux of Utah" (U. S. Geograph. and Geol. Sur- 
 vey of Rocky Mounts.), 1880, p. 79. 
 
 156 Trans. Roy. Soc. Edin. xxix. p. 514.
 
 GEOGNOSY 243 
 
 Devonshire, and with the Silurian rocks of Bohemia. They 
 sometimes contain fragments of clay-slate, and are occasion- 
 ally fossiliferous. They present amygdaloidal and porphy- 
 ritic, as well as perfectly laminated structures. Probably 
 they are in most cases true diabase-tuffs, but sometimes 
 they may be forms of diabase-lavas, which, like the strati- 
 fied formations in which they lie, have undergone altera- 
 tion, and in particular have acquired a more or less dis- 
 tinctly fissile structure, as the result of lateral pressure and 
 internal crushing. 1ST 
 
 4. Fragmental Rocks of Organic Origin 
 
 This series includes deposits formed either by the growth 
 and decay of organisms in situ, or by the transport and sub- 
 sequent accumulation of their remains. These may be con- 
 veniently grouped, according to their predominant chemical 
 ingredient, into Calcareous, Siliceous, Phosphatic, Carbona- 
 ceous, and Ferruginous. 
 
 1. CALCAREOUS. Besides the calcareous formations 
 which occur among the stratified crystalline rocks as re- 
 sults of the deposition of chemical precipitates (p. 260), 
 a more important series is derived from the remains of 
 living organisms, either by growth on the spot or by trans- 
 port and accumulation as mechanical sediment. To by far 
 the larger part of the limestones intercalated in the rocky 
 framework of our continents, an organic origin may with 
 probability be assigned. It is true, as has been above men- 
 tioned (p. 216), that limestone, formed of the remains of 
 animals or plants, is liable to an internal crystalline re- 
 arrangement, the effect of which is to obliterate the organic 
 structure. Hence in many of the older limestones no trace 
 of any fossils can be detected, and yet these rocks were 
 almost certainly formed of organic remains. An attentive 
 microscopic study of organic calcareous structures, and , of 
 the mode of their replacement by crystalline calcite, some- 
 times detects indications of former organisms, even in the 
 midst of thoroughly crystalline materials." 8 
 
 IOT 0. Koch, Jahrb. Ver. Nat. Nassau, xiii. (1858), 216, 238. J. A. Phillips, 
 Q. J. Geol. Soc. xxxii. p. 155, xxxiv. p. 471. 
 
 128 Sorby, Address to Geol. Society, February, 1879, and the paper of Messrs. 
 Cornish and Kendall, cited ante, p. 216. Gumbel has suggested that the different 
 durability of the calcite and aragonite organic forms may be due rather to struc- 
 ture than mineral composition.
 
 244 TEXT-BOOK OF GEOLOGY 
 
 Limestone, composed of the remains of calcareous organ- 
 isms, is found in layers which range from mere thin larninaa 
 up to massive beds several feet or even yards in thickness. 
 In some instances, such as that of the Carboniferous or 
 Mountain limestone of Britain and Belgium, and that of the 
 Coal-measures in Wyoming and Utah, it occurs in continu- 
 ous superposed beds to a united thickness of several thou- 
 sand feet, and extends for hundreds of square miles, forming 
 a rock out of which picturesque gorges, hills, and table-lands 
 have been excavated. 
 
 Limestones of organic origin present every gradation of 
 texture and structure, from mere soft calcareous mud or 
 earth, evidently composed of entire or crumbled organisms, 
 up to solid compact crystalline rock, in which indications 
 of an organic source can hardly be perceived. Mr. Sorby, 
 in the address already cited, called renewed attention to the 
 importance of the form in which carbonate of lime is built 
 up into animal structures. Quoting the opinion of Rose ex- 
 pressed in 1858, that the diversity in the state of preserva- 
 tion of different shells might be due to the fact that some of 
 them had their lime as calcite, others as aragonite, he showed 
 that this opinion is amply supported by microscopic exami- 
 nation. Even in the shells of a recent raised beach, he ob- 
 served that the inner aragonite layer of the common mussel 
 had been completely removed, though the outer layer of cal- 
 cite was well preserved. In some shelly limestones contain- 
 ing casts, the aragonite shells have alone disappeared, and 
 where these still remain represented by a calcareous layer, 
 this has no longer the original structure, but is more or less 
 coarsely crystalline, being in fact a pseudomorph of calcite 
 after aragonite, and quite unlike contiguous calcite shells, 
 which retain their original microscopical and optical char- 
 acters. 18 * 
 
 The following list comprises some of the more distinctive 
 and important forms of organically-derived limestones. 
 
 S h e 1 1-M a r 1 a soft, white, earthy, or crumbling de- 
 posit, formed in lakes and ponds by the accumulation of the 
 remains.of shells and Entomostraca on the bottom. When 
 such calcareous deposits become solid compact stone they 
 are known as fresh-water (lacustrine) limestones. These are 
 generally of a smooth texture, and either dull white, pale 
 
 129 The student will find the address from which these citations are made 
 lull of suggestive matter in regard to the origin and subsequent history of lime- 
 stones.
 
 GEOGNOSY 
 
 245 
 
 gray, or cream-colored, their fracture slightly conchoidal, 
 rarely splintery. 
 
 Lumachell e a compact, dark gray or brown lime- 
 stone, charged with ammonites or other fossil shells, which 
 are sometimes iridescent, giving bright green, blue, orange, 
 and dark red tints (fire-marble). 
 
 Calcareous (Foraminiferal) Ooz e a white or 
 gray calcareous mud, of organic origin, found covering vast 
 areas of the floor of the Atlantic and other oceans, and 
 formed mostly of the remains of Foraminif&ra, particularly 
 
 Pig. 34. Foraminiferal (Globigerina) Ooze, dredged by the "Challenger" 
 
 Expedition in Lat. 50 1' S., Long. 123 V E., from a depth of 1800 
 
 fathoms (magnified 50 Diameters). 
 
 of forms of the genus Globigerina (Fig. 24). Further ac- 
 count of this and other organic deep-sea deposits is given in 
 Book III. Part II. Section iii. 
 
 S h e 1 1-S a n d a deposit composed in great measure or 
 wholly of comminuted shells, found commonly on a low 
 shelving coast exposed to prevalent on-shore winds. When 
 thrown above the reach of the waves and often wetted by 
 rain, or by trickling runnels of water, it is apt to become 
 consolidated into a mass, owing to the solution and rede- 
 posit of lime round the grains of shell (p. 216). 
 
 Coral-rock a limestone formed by the continuous
 
 246 TEXT-BOOK OF GEOLOGY 
 
 growth of coral-building polyps. This substance affords 
 an excellent illustration 01 the way in which organic struc- 
 ture may be effaced from a limestone entirely formed of the 
 remains of once living animals. Though the skeletons of 
 the reef -building corals remain distinct on the upper sur- 
 face, those of their predecessors beneath them are gradually 
 obliterated by the passage through them of percolating water, 
 dissolving and redepositing calcium carbonate. We can 
 thus understand how a mass of crystalline limestone may 
 have been produced from one formed out of organic re- 
 mains, without the action of any subterranean heat, but 
 merely by the permeation of water from the surface. 180 
 
 C halk a white soft rock, meagre to the touch, soiling 
 the fingers, formed of a fine calcareous flour derived from 
 the remains of Foraminifera, echinoderms, mollusks, and 
 other marine organisms. By making thin slices of the rock 
 and examining them under the microscope, Sorby has found 
 that Foraminifera, particularly Globigerina, and single de- 
 tached cells of comparatively shallow-water forms, probably 
 constitute less than half of the rock by bulk (Fig. 22), the 
 remainder consisting of detached prisms of the outer calcare- 
 ous layer of Inoceramus, fragments of Ostrea, Pecten, echino- 
 derms, spicules of sponges, etc. It is not quite like any known 
 modern deep-sea deposit. A microscopic investigation of 
 chalk from the neighborhood of Lille showed that, besides 
 the usual organic constituents, the rock contains minute 
 grains and crystals of quartz, tourmaline, zircon, rutile, 
 garnet and felspars, 131 these minerals being among the most 
 widely diffused and persistent ingredients in the finer sedi- 
 ments that are derived from the denudation of crystalline 
 rocks (see p. 228). 
 
 Crinoidal (Encrinite) Limeston e a rock com- 
 posed in great part of crystalline joints of encrinites, with 
 Foraminifera, corals, and mollusks. It varies in color from 
 white or pale gray, through shades of bluish-gray (sometimes 
 yellow or brown, less commonly red) to a dark gray or even 
 black color. It is abundant among Palaeozoic formations, 
 being in Western Europe especially characteristic of the 
 lower part of the Carboniferous system. 
 
 130 See Dana's "Coral and Coral Islands," p. 354; also the account of the 
 Devonian and Carboniferous limestones in the present volume. Dupont has 
 shown that many of the massive limestones of Belgium have been formed by 
 reef -like masses of Stromatopora or allied organisms. 
 
 181 L. Cayeux, Ann. Soc. Geol. Nord. xvii. (1890), p. 283.
 
 GEOGNOSY 247 
 
 2. SILICEOUS. Silica is directly eliminated from both 
 fresh and salt water by the vital growth of plants and ani- 
 mals. (Book TIL Part II. Section iii.) 
 
 Diatom-earth, Tn polite (Infusorial earth, Kieselguhr) a sili- 
 ceous deposit formed chiefly of the frustules of diatoms, laid 
 down both in salt and in fresh water. Wide areas of it are 
 now being deposited on the bed of the South Pacific (Dia- 
 tom-ooze, Fig. 181). In Virginia, United States, an exten- 
 sive tract occurs covered with diatom-earth to a depth of 40 
 feet. It likewise underlies peat-mosses, probably as an 
 original lake-deposit. It is used as Tripoli powder for pol- 
 ishing purposes (see p. 807). 
 
 Radiolanan ooze a pale chalk-like abysmal marine deposit 
 consisting mainly of the remains of siliceous radiolarians and 
 diatoms. It is further referred to in Book III. Part II. 
 Section iii. 
 
 Flint (Silex, Feuerstein) a .gray or black, excessively 
 compact rock, with the hardness of quartz and a perfect 
 conchoidal fracture, its splinters being translucent on the 
 edges. Consists of an intimate mixture of crystalline in- 
 soluble silica and of amorphous silica soluble in caustic 
 potass. Its dark color, which can be destroyed by heat, 
 arises chiefly from the presence of carbonaceous matter. 
 Flint occurs principally as nodules, dispersed in layers 
 through the Upper Chalk of England and the northwest of 
 Europe. It frequently incloses organisms such as sponges, 
 echini and brachiopods. It has been deposited from sea- 
 water, at first through organic agency, and subsequently by 
 direct chemical precipitation round' the already deposited 
 silica. (Book III. Part II. Section iii.) Chert (phtanite) 
 is a name applied to impure calcareous varieties of flint, in 
 layers and nodules which are found among the Palaeozoic 
 and later formations, especially but not exclusively in lime- 
 stones. 132 In some cases, as in the spicules of sponges, the 
 silica has had a directly organic origin, having been secreted 
 from sea- water by the living organisms; in other cases, 
 where, for example, we find a calcareous shell, or echinus, 
 or coral converted into silica, it would seem that the substi- 
 tution of silica for calcium-carbonate has been effected by a 
 
 139 Consult Hull and Hardman, Trans. Roy. Dublin Soc. i. (1878), p. 71. 
 Renard, Bull. Acad. Roy. Belgique, 2d ser. vol. xlvi. p. 471; Sollas, Ann. 
 Mag. Nat. Hist. vii. (1881), p. 141 ; Scientific Proc. Roy. Dublin Soc. vi. (1887), 
 part. i. G. J. Hinde, Geol. Mag. 1887, p. 43fi. Bands of radiolarian chert oc- 
 cupy persistent horizons among the Lower Silurian rocks of southern Scotland.
 
 248 TEXT-BOOK OF GEOLOGY 
 
 process of chemical pseudomorphism, either after or during 
 the formation of the limestone. The vertical ramifying 
 masses of flint in Chalk show that the calcareous ooze had 
 to some extent accumulated before the segregation of these 
 masses. 188 
 
 3. PHOSPHATIC. A few invertebrata contain phosphate 
 of lime. Among these may be mentioned the brachiopoda 
 Lingula and Orbicula, 19 * also Conularia, Serpulites, and some 
 recent and fossil Crustacea. The shell of the recent Lingula 
 ovalis was found by Hunt to contain, after calcination, 61 
 per cent of fixed residue, which consisted of 85-70 per cent 
 of phosphate of lime; 11-75 carbonate of lime, and 2-80 mag- 
 nesia. The bones of vertebrate animals likewise contain 
 about 60 per cent of phosphate of lime, while their excre- 
 ment sometimes abounds in the same substance. Hence de- 
 posits rich in phosphate of lime have resulted from the ac- 
 cumulation of animal remains from Silurian times up to the 
 present day. Associated with the Bala limestone, in the 
 Lower Silurian series of North Wales, is a band composed 
 of concretions cemented in a black, graphitic, slightly phos- 
 phatic matrix, and containing usually 64 per cent of phos- 
 phate of lime (phosphorite). 136 The tests of the trilobites and 
 other organisms among the Cambrian rocks of Wales also 
 contain phosphate of lime, sometimes to the extent of 20 per 
 cent. 138 Phosphatic, though certainly far inferior in extent 
 and importance to calcareous, and even to siliceous, forma- 
 tions, are often" of singular geological interest. The follow- 
 ing examples may serve as illustrations. 137 
 
 Cuano a deposit consisting mainly of the droppings of 
 sea-fowl, formed on islands in rainless tracts off the western 
 coasts of South America and of Africa. It is a brown, light, 
 powdery substance with a peculiar ammoniacal odor, and 
 occurs in deposits sometimes more than 100 feet thick. 
 Analyses of American guano give combustible organic 
 matter and acids, 11-3; ammonia (carbonate, urate, etc.), 
 31-7; fixed alkaline salts, sulphates, phosphates, chlorides, 
 etc., 8-1; phosphates of lime and magnesia, 22-5; oxalate of 
 lime, 2-6; sand and earthy matter, 1-6; water, 22-2. This 
 
 133 On formation of chalk- flints, see Book III. Part II. Section iii. 3. 
 
 134 Sterry Hunt, Amer. Journ. Soc. xvii. (1854), p. 236. Logan's ''Geology 
 of Canada," 1863, p. 461. 
 
 136 D. C. Davies, Q. J. Geol. Soc. xxxi. p. 357. 13 Hicks, op. cit. p. 368. 
 
 131 For an exhaustive account of deposits of phosphate of lime, see R. A. P. 
 Penrose, Jr. ; Bull. U. S. Geol. Surv. No. 46, 1888, also postea, Book III. Part 
 II. Sect. iii. 3.
 
 GEOGNOSY 249 
 
 remarkable substance is highly valuable as a source of arti- 
 ficial manures. (Book III. Part II. Section iii.) 
 
 Bone-Breccia a deposit consisting largely of fragmentary 
 bones of living or extinct species of mammalia, found some- 
 times under stalagmite on the floors of limestone caverns, 
 more or less mixed with earth, sand, or lime. In some older 
 geological formations, bone-beds occur, formed largely of 
 the remains of reptiles or fishes, as the "Lias bone-bed," 
 and the "Ludlow bone-bed." 
 
 Coprolitic nodules and beds 138 are formed of the accumu- 
 lated excrement (coprolites) of vertebrated animals. Among 
 the Carboniferous shales 01 the basin of the Firth of Forth, 
 coprolitic nodules are abundant, together with the bones and 
 scales of the larger ganoid fishes which voided them: abun- 
 dance of broken scales and bones of the smaller ganoids can 
 usually be observed in the coprolites. Among the Lower 
 Silurian rocks of Canada, numerous phosphatic nodules, 
 supposed to be of coprolitic origin, occur. 13 * The phos- 
 phatic beds of the Cambridgeshire Cretaceous rocks are now 
 largely worked as a source of artificial manure. In popular 
 and especially commercial usage, the word "coprolitic" is 
 applied to nodular deposits which can be worked for phos- 
 phate of lime, thougn they may contain few or no true 
 coprolites. 
 
 Phosphatic Chalk. In the Chalk of France and Belgium, 
 more sparingly in that of England, certain lavers occur 
 where the original calcareous matter has been replaced to 
 a considerable extent by phosphate of lime. Such bands 
 have frequently a brownisn tint, which on examination is 
 found to result from the abundance of minute brown grains 
 composed mainly of phosphate. The foraminifera and other 
 minuter or fragmentary fossils have been changed into this 
 brown substance. The proportion of phosphate of lime 
 ranges up to 45 per cent or more. 140 
 
 4. CARBONACEOUS. The formations here included^have 
 almost always resulted from the decay and entombment of 
 vegetation on the spot where it grew, sometimes by the 
 drifting of the plants to a distance and their consolidation 
 
 188 On the origin of phosphatic nodules and beds, see Gruner, Bull. Soc. 
 Geol. France, xxviii. (2d ser.), p. 62. Martin, op. cit. iii. (3d ser.), p. 273. 
 
 139 Logan's "Geology of Canada," p. 461. 
 
 140 See A. F. Renard and J. Cornet, Bull. Acad. Roy. Belgique, xxi. (1891), 
 p. 126. A. Strahan, Quart. Journ. Geol. Soc, xlvii. (1891).
 
 250 TEXT-BOOK OF GEOLOGY 
 
 there. (See Book 111. Part II. Section iii. 3.) In the 
 latter case, they may be mingled with inorganic sediment, 
 so as to pass into carbonaceous shale. 
 
 Peat vegetable matter, more or less decomposed and 
 chemically altered, found throughout temperate climates 
 in boggy places where marshy plants grow and decay. It 
 varies from a pale yellow or brown fibrous substance, like 
 turf or compressed hay, in which the plant-remains are 
 abundant and conspicuous, to a compact dark brown or 
 black material, resembling black clay when wet, and some 
 varieties of lignite when dried. The nature and proportions 
 of the constituent elements of peat, after being dried at 
 100 C., are illustrated by the analysis of an Irish example 
 which gave carbon, 60-48; hydrogen, 6'10; oxygen, 32*55; 
 nitrogen, 0'88; while the ash was 3 '30. There is always a 
 large proportion of water which cannot be driven off even 
 by drying the peat. In the manufacture of compressed peat 
 for fuel this constituent, which of course lessens the value 
 of the peat as compared with an equal weight of coal, is 
 driven off to a great extent by chopping the peat into fine 
 pieces, and thereby exposing a large surface to evaporation. 
 The ash varies in amount from less than 1-00 to more than 
 65 per cent, and consists of sand, clay, ferric oxide, sul- 
 phuric acid, and minute proportions of lime, soda, potash 
 and magnesia. 141 Under a pressure of 6000 atmospheres 
 peat is converted into a hard, black, brilliant substance 
 having the physical aspect of coal, and showing no trace 
 of organic structure. 142 
 
 Lignite (Brown Coal) compact or earthy, compressed and 
 chemically altered vegetable matter, often retaining a lamel- 
 lar or ligneous texture, with stems showing woody fibre 
 crossing each other in all directions. It varies from pale 
 brown or yellow to deep brown or black. Some shade of 
 brown is the usual color, whence the name Brown Coal, 
 by which it is often known. It contains from 55 to 75 per 
 cent of carbon, has a specific gravity of 0*5 to 1 P 5, burns 
 easily to a light ash with a sooty flame and a strong burned 
 smell. It occurs in beds chiefly among the Tertiary strata, 
 under conditions similar to those in which coal is found in 
 
 141 See Senft's "Humus-, Marsch-, Torf- und Limonit-bildungen," Leipzig, 
 1862. J. J. Friih, "Ueber Torf und Dopplerit, Zurich," 1883, and the various 
 memoirs quoted postea, p. 802. 
 
 14i Spring, Bull. Acad. Roy. Bruxelles, xJix. (1880), p. 367.
 
 GEOGNOSY 251 
 
 older formations. It may be regarded as a stage in the 
 alteration and mineralization of vegetable matter, inter- 
 mediate between peat and true coal. 
 
 Coal a compact, usually brittle, velvet-black to pitch- 
 black, iron-black, or dull, sometimes brownish rock, with 
 a grayish- black or brown streak, and in some varieties a 
 distinctly cubical cleavage, in others a conchoidal fracture. 
 It contains from 75 to 90 per cent of carbon, and a small 
 percentage of sulphur, generally in the form of iron-disul- 
 phide. It has a specific gravity of 1-2-1 '35, and burns with 
 comparative readiness, giving a clear flame, a strong aro- 
 matic or bituminous smell, some varieties fusing and caking 
 into cinder, others burning away to a mere white or red 
 
 Ptg. S6. Microscopic Structure of Dalkeith Coal, 
 
 showing Lycopodiaceous Sporangia 
 
 (magnified 300 Diameters). 
 
 ash. Though it consists of compressed vegetation, no trace 
 of organic structure is usually apparent. 143 An attentive 
 examination, however, will often disclose portions of stems, 
 leaves, etc., or at least of carbonized woody fibre. Some 
 kinds are almost wholly made up of the spore-cases of 
 lycopodiaceous plants (Fig. 25). There is reason to believe 
 that different varieties of coal may have arisen from original 
 diversities in the nature of the vegetation out of which they 
 were formed. The accompanying table shows the chemical 
 gradation between unaltered vegetation and the more highly 
 mineralized forms of coal. 
 
 143 On the influence of pressure on the formation of coal, see Fre'my, Compt. 
 rend. 20th May 1879. Spring, Bull. Acad. Roy. Bruxelles, 1880, p. 367.
 
 TEXT-BOOK OF GEOLOGY 
 
 TABLE SHOWING THE GRADUAL CHANGE IN COMPOSITION FBOM WOOD TO 
 CHARCOAL'" 
 
 
 a 
 
 i 
 
 
 
 UP 
 
 Substance 
 
 I 
 
 
 1 
 
 !'f 
 
 
 
 
 I 
 
 O 
 
 fill! 
 
 1. Wood (mean of several analyses) 
 2. Peat ( " " " ) 
 
 100 
 100 
 
 12-18 
 9-85 
 
 83-07 
 55-67 
 
 1-80 
 2-89 
 
 3 Lignite (mean of 15 varieties) 
 
 100 
 
 8-37 
 
 42-42 
 
 3-07 
 
 4. Tan-yard coal of S. Staffordshire basin 
 
 100 
 
 6-12 
 
 21-23 
 
 3-47 
 
 5 Steam coal from the Tyne .... 
 
 100 
 
 5-91 
 
 18-32 
 
 3-62 
 
 6 Peutrefelin coal of S Wales 
 
 100 
 
 4-75 
 
 5-28 
 
 4-09 
 
 7. Anthracite from Pennsylvania, TJ. S... 
 
 100 
 
 2-84 
 
 1-74 
 
 2-63 
 
 Coal occurs in seams or beds intercalated between strata 
 of sandstone, shale, fireclay, etc., in geological formations 
 of Palaeozoic, Secondary, and Tertiary age. It should be 
 remembered that the word coal is rather a popular than 
 a scientific term, being indiscriminately applied to any 
 dense, black mineral substance capable of being used as 
 fuel. Strictly employed, it ought only to be used with 
 reference to Beds of fossilized vegetation, the result either 
 of the growth of plants on the spot or of the drifting of 
 them thither. 
 
 The following analyses show the chemical composition 
 of peat, lignite, and some of the principal varieties or coal: 14 * 
 
 
 Peat 
 
 Devon- 
 shire 
 
 Lignite 
 
 Bovey, 
 Tracey, 
 Devon 
 
 Caking 
 Coal 
 North- 
 umber- 
 land 
 
 Non-Cak- 
 ing Coal 
 
 8. Staf- 
 fordshire 
 
 Cannel 
 Coal 
 
 Wigan 
 
 Anthra- 
 cite 
 
 S. Wales 
 
 Carbon 
 
 54-02 
 
 66-31 
 
 78-69 
 
 78-57 
 
 80-07 
 
 90-39 
 
 Hydrogen... 
 
 5-21 
 
 5-63 
 
 6-00 
 
 5-29 
 
 5-53 
 
 3-28 
 
 Oxygen 
 
 28-18 
 
 22-86 
 
 10-07 
 
 12-88 
 
 8-08 
 
 2-98 
 
 Nitrogen ... 
 
 2-30 
 
 0-57 
 
 2-37 
 
 1-84 
 
 2-12 
 
 0-83 
 
 Sulphur ... 
 
 0-56 
 
 2-36 
 
 1-51 
 
 0'39 
 
 1-50 
 
 0-91 
 
 Ash 
 
 9-73 
 
 2-27 
 
 1-36 
 
 1-03 
 
 2-70 
 
 1-61 
 
 
 
 
 
 
 
 
 Specific gravity 
 
 0-850 
 
 1-129 
 
 1-259 
 
 1-278 
 
 1-276 
 
 1-392 
 
 144 Percy's "Metallurgy," vol. i. p. 268. 
 146 From Percy's "Metallurgy," vol. i.
 
 GEOGNOSY 253 
 
 These analyses are exclusive of water, which in the peat 
 amounted to 2&-56, and in the lignite to 34-66 per cent. 
 
 Arthracite the most highly mineralized form of vegetation 
 is tn iron-black to velvet-black substance, with a strong 
 metahoidal to vitreous lustre, hard and brittle, containing 
 over 90 per cent of carbon, with a specific gravity of 
 1-35-1-7. It kindles with difficulty, and in a strong draught 
 burns without fusing, smoking, or smelling, but giving out 
 a great heat. It is a coal from which the bituminous parts 
 have been eliminated. It occurs in beds like ordinary coal, 
 but in positions where probably it has been subjected to 
 some change whereby its volatile constituents have been 
 expelled. It is found largely in South Wales, and sparingly 
 in the Scottish coal-fields where the ordinary coal-seams 
 have been approached by intrusive masses of igneous rock. 
 It is largely developed in the great coal-field of Pennsylvania. 
 Some Lower Silurian shales are black from diffused anthracite, 
 and have in consequence led to fruitless searches for coal. 
 
 Oil-shale (Brandschiefer} shale containing such a propor- 
 tion of hydrocarbons as to be capable of yielding mineral 
 oil on slow distillation. This substance occurs as ordinary 
 shales do, in layers or beds, interstratified with other aqueous 
 deposits, as in the Scottish coal-fields. It is in a geological 
 sense true shale, and owes its peculiarity to the quantity of 
 vegetable (or animal) matter which has been preserved 
 among its inorganic constituents. It consists of fissile argil- 
 laceous layers, highly impregnated with bituminous matter, 
 passing on one side into common shale, on the other into 
 cannel or parrot coal. The richer varieties yield from 30 
 to 40 gallons of crude oil to the ton of shale. They may be 
 distinguished from non-bituminous or feebly bituminous 
 shales (throughout the shale districts of Scotland), by the 
 peculiarity that a thin paring curls up in front of the knife, 
 and shows a brown lustrous streak. Some of the oil-shales 
 in the Lothians are crowded with the valves of ostracod 
 crustaceans, besides scales, coprolites, etc., of ganoid fishes. 
 It is possible that the bituminous matter may in some cases 
 have resulted from animal organisms, though the abun- 
 dance of plant remains indicates that it is probably in most 
 cases of vegetable origin. Under the name "pyroschists" 
 Sterry Hunt classed the clays or shales (of ail geological 
 ages) which are hydrocarbonaceous, and yield by distilla- 
 tion volatile hydrocarbons, infammable gas, etc. 
 
 Petroleum a general term, under which is included a series 
 of natural mineral oils. These are fluid hydrocarbon com-
 
 254 TEXT-BOOK OF GEOLOGY 
 
 pounds, varying from a thin, colorless, watery liquidity to 
 a black, opaque, tar-like viscidity, and in specific gravity 
 from 0-8 to 1-1. The paler, more limpid varieties are gen- 
 erally called naphtha, the darker, more viscid kinds 
 mineral tar, while the name petroleum, or rock-oil, 
 has been more generally applied to the intermediate kinds. 
 Petroleum occurs sparingly in Europe. A few localities for 
 it are known in Britain. It is found in large quantity along 
 the country stretching from the Carpathians, through Gal- 
 licia and Moldavia, also at Baku on the Caspian. 146 The 
 most remarkable and abundant display of the substance, 
 however, is in the so-called oil-regions' of North America, 
 particularly in Western Canada and Northern Pennsylvania, 
 where vast quantities of it have been obtained in recent 
 years. In Pennsylvania it is found especially in certain 
 porous beds of sandstone or "sand-rocks," which occur as 
 low down as the Old Red Sandstone, or even as the top 
 of the Silurian system. In Canada it is largely present in 
 still lower strata. Its origin in these ancient formations, 
 where it cannot be satisfactorily connected with any de- 
 structive distillation of coal, is still an unsolved problem. 
 Asphalt a smooth, brittle, pitch-like, black or brownish- 
 black mineral, having a resinous lustre and conchoidal 
 fracture, streak paler than surface of fracture, and specific 
 gravitv of 1-0 to 1-68. It melts at about the temperature 
 of boiling water, and can be easily kindled, burning with 
 a bituminous odor and a bright but smoky flame. It is 
 composed chiefly of hydrocarbons, with a variable admix- 
 ture of oxygen and nitrogen. It occurs sometimes in asso- 
 ciation with petroleum, of which it may be considered a 
 hardened oxidized form, sometimes as an impregnation 
 filling the pores or chinks of rocks, sometimes in inde- 
 pendent beds. In Britain it appears as a product of the 
 destructive distillation of coals and carbonaceous shales by 
 intrusive igneous rocks, as at Binny Quarry, Linlithgow- 
 shire, but also in a number of places where its origin is 
 not evident, as in the Cornish and Derbyshire mining dis- 
 tricts, and among the dark flagstones of Caithness and 
 Orkney, which are laden with fossil fishes. At Seyssel 
 (Departement de 1'Ain) it forms a deposit 2500 feet long 
 and 800 feet broad, which yields 1500 tons annually. It 
 exudes, in a liquid form from the ground round the borders 
 
 146 Abich, Jahrb. Geol. Reichsanst, xxix. (1879), p. 165. Trautschold, 
 Zeitsch. Deutsch. Geol. Ges. xxvi. (1874), p. 257. See postea, Book III. 
 Part I. Sect. i. 2, where other authorities are cited.
 
 GEOGNOSY 255 
 
 of the Dead Sea. In Trinidad it forms a lake li miles in 
 circumference, which is cool and solid near the shore, but 
 increases in temperature and softness toward the centre. 
 
 Graphite. This mineral occurs in masses of sufficient size 
 and importance to deserve a place in the enumeration of 
 carbonaceous rocks. Its mineralogical characters have al- 
 ready (p. 124) been given. It occurs in distinct lenticular 
 beds, and also diffused in minute scales, through slates, 
 schists, and limestones of the older geological formations, as 
 in Cumberland, Scotland, Canada, and Bohemia. It is likewise 
 found occasionally as the result of the alteration of a coal 
 seam by intrusive basalt, as at New Cumnock in Ayrshire. 
 
 5. FERRUGINOUS. The decomposition of vegetable mat- 
 ter in marshy places and shallow lakes gives rise to certain 
 organic acids, which, together with the carbonic acid x so 
 generally also present, decompose the ferruginous minerals 
 of rocks and carry away soluble salts of iron. Exposure 
 to the air leads to the rapid decomposition and oxidation 
 of those solutions, which consequently give rise to precipi- 
 tates, consisting partly of insoluble basic salts and partly 
 of the hydrated ferric oxide. These precipitates, mingled 
 with clay, sand, or other mechanical impurity, and also 
 with dead and decaying organisms, form deposits of iron- 
 ore. Operations of this kind appear to have been in prog- 
 ress from a remote geological antiquity. Hence ironstones 
 with traces of associated organic remains belong to many 
 different geological formations, and are being formed still. 1 * 1 
 
 Bog Iron-Ore (Lake-ore, minerai des marais, Sumpferz) 
 a dark-brown to black, earthy, but sometimes compact mix- 
 ture of hydrated peroxide of iron, phosphate of iron, and 
 hydrated oxide 01 manganese, frequently with clay, sand, 
 and organic matter. An ordinary specimen yielded, per- 
 oxide of iron, 62-59; oxide of manganese, 8-52: sand, 11-37; 
 phosphoric acid, 1-50; sulphuric acid, traces; water and 
 organic matter, 16-02=100-00. Bog iron-ore may either be 
 formed in situ from still water, or may be laid down by cur- 
 rents in lakes. Of the former mode of formation, a familiar 
 illustration is furnished by the "moor-band pan" or hard 
 ferruginous crust, which in boggy places and on some ill- 
 drained land, forms at the bottom of the soil, on the top 
 of a stiff and tolerably impervious subsoil. Abundant 
 bog-iron or lake-ore is obtained from the bottoms of some 
 lakes in Norway and Sweden. It forms everywhere on the 
 
 147 See Senft's work already (p. 260) cited, p. 168; also postea, Book in. 
 Part II. Sect. iii.
 
 256 TEXT-BOOK OF GEOLOGY 
 
 shallower slopes near banks of reeds, where there is no strong 
 current of water, occurring in granular concretions (Bohnerz) 
 that vary from the size of grains of coarse gunpowder up to 
 nodules 6 inches in diameter, and forming layers 10 to 200 
 yards long, 5 to 15 yards broad, and 8 to 30 inches thick. 
 These deposits are worked during winter by inserting per- 
 forated iron shovels through holes cut in the ice; and so 
 rapidly do they accumulate, that instances are known 
 where, after having been completely removed, the ore at 
 the end of twenty-six years was found to have gathered 
 again to a thickness of several inches. A layer of loose 
 earthy ochre 10 feet thick is believed to have formed in 
 600 years on the floor of the Lake Tisken near the old 
 copper mine of Falun in Sweden. 148 According to Ehren- 
 berg, the formation of bog-ore is due, not merely to the 
 chemical actions arising from the decay of organic matter, 
 but to a power possessed by diatoms of separating iron from 
 water and depositing it as hydrous peroxide within their 
 siliceous framework. 
 
 Aluminous Yellow Iron-Ore is closely related to tho fore- 
 going. It is a mixture of yellow or pale brown, Irydrated 
 peroxide of iron, with clay and sand, sometimes wi h sili- 
 cate of iron, hydrated oxide of manganese, and carbonate of 
 lime, and occurs in dull, usually pulverulent grains and 
 nodules. Occasionally these nodules may be observed to 
 consist of a shell of harder material, within which the yel- 
 low oxide becomes progressively softer toward the centre, 
 which is sometimes quite empty. Such concretions are known 
 as aetites or eagle-stones. This ore occurs in the Coal-measures 
 of Saxony and Silesia, also in the Harz, Baden, Bavaria, etc., 
 and among the Jurassic rocks in England. 
 
 Clay-Ironstone (Sphaerosiderite) has been already (p. 143) 
 referred to. It occurs abundantly in nodules and beds in 
 the Carboniferous system in most parts 
 of Europe. The nodules are generally 
 oval and flattened in form, varying in 
 size from a small bean up to concretions 
 a foot or more in diameter, and with an 
 
 internal system of radiating cracks, often 
 
 Pig. 26-septarian NO- filled with calcite (Fig. 26). In many 
 
 dule of Clay-u-onstone. ^^ ^ COQtaiQ V ir f the centre some 
 
 organic substance, such as a coprolite, fern, cone, shell, or 
 fish, that has served as a surface round which the iron in 
 
 148 A. F. Thoreld, Geol. Foren. Forhand. Stockholm, iii. p. 2.0, postea, pp. 
 407, 483.
 
 GEOGNOSY 257 
 
 the water and the surrounding mud could be precipitated. 
 Seams of clay-ironstone vary in thickness from mere paper- 
 like partings up to beds several feet deep. The Cleveland 
 seam in the middle Lias of Yorkshire is about 20 feet thick. 
 In the Carboniferous system of Scotland certain seams 
 known as Black-band contain from 10 to 52 per cent of coaly 
 matter, and admit of being calcined with the addition of 
 little or no fuel. They are sometimes crowded with organic 
 remains, especially lamellibranchs (Anthracosia, Anthra- 
 comya, etc.) and fishes (Rhizodus, Megalichthys, etc.). 
 
 A microscopic examination of some black-bana iron- 
 stones reveals a very perfect oolitic structure, showing that 
 the iron has either replaced an original calcareous oolite or 
 has been precipitated in water having such a gentle move- 
 ment as to keep the granules quietly rolling along, while / 
 their successive concentric layers of carbonate were being 
 deposited. Mr. Sorby has observed in the Cleveland iron- 
 stones an abnormal form of oolitic structure, and remarks 
 that one specimen bore evidence that the iron, mostly in the 
 form of small crystals of the carbonate, had been introduced 
 subsequently to the formation of the rock, as it had replaced 
 some of the aragonite of the inclosed shells. 149 
 
 The subjoined analyses show the composition of some 
 varieties of clay-ironstones. 160 
 
 Clay iron-ore Black-band Cleveland ore 
 (Coal measures), (Carboniferous), (Lias), 
 
 Peroxide of iron 
 
 Yorkshire 
 1-45 
 
 Scotland 
 2-72 
 
 Yorkshire 
 2-86 
 
 Protoxide of iron 
 
 36-14 
 
 40-77 
 
 43-02 
 
 Protoxide of manganese. 
 Alumina 
 
 1-38 
 6'74 
 
 
 '0-40 
 6-87 
 
 Lime 
 
 2-70 
 
 0*90 
 
 6-14 (zinc) 
 
 Magnesia.. . ,. 
 
 2-11 
 
 0-72 
 
 5-21 
 
 Potash 
 
 0*65 
 
 
 
 Silica 
 
 ...17-37 
 
 10-10 
 
 7-17 
 
 Carbonic acid 
 
 .. 26-57 
 
 26-41 
 
 25-50 
 
 Phosphoric acid 
 Sulphuric acid 
 
 0-34 
 trace 
 
 
 1-81 
 
 Iron pyrites 
 
 0-10 
 
 
 
 "Water 
 
 1'77 
 
 1-0 
 
 3-48 
 
 Organic matter 
 
 2-40 
 
 17-38 
 
 0-15 
 
 Percentage of iron... 
 
 99-78 
 29-12 
 
 100-00 
 34-60 
 
 100-61 
 35-46 
 
 M Address to Geol. Soc. February, 1879. 
 
 140 See Percy's "Metallurgy," voL ii. Bischof, "Chem. und Phys. GeoL' 
 aupp. (1871), p. 65.
 
 258 TEXT- BOOK OF GEOLOGY 
 
 B. CRYSTALLINE, INCLUDING ROCKS FORMED FROM CHEMICAL PRECIPITATION 
 
 This division consists mainly of chemical deposits, but 
 includes also some which, originally formed of organic cal- 
 careous de'bris, have acquired a crystalline structure. The 
 rocks included in it occur as laminae and beds, usually inter- 
 calated among clastic formations, such as sandstone and 
 shale. Sometimes they attain a thickness of many thou- 
 sand feet, with hardly any interstratification of mechani- 
 cally derived sediment. They are being formed abundantly 
 at the present time by mineral springs and on the floor of 
 inland seas; while on the bottom of lakes and of the main 
 ocean, calcareous organic accumulations are in progress, 
 which will doubtless eventually acquire a thoroughly crys- 
 talline structure like that of many limestones. 
 
 Ice. So large an area of the earth's surface is covered 
 with ice, that this substance deserves notice among geologi- 
 cal formations. Ice is commonly and conveniently classified 
 in two divisions, snow-ice and water-ice, according as 
 it results from the compression and alternate melting and 
 freezing of fallen snow, or from the freezing of the surface 
 or bottom of sheets of water. 
 
 S n o'w-i c e (see Book III. Part II. Sect. ii. 5) is of 
 two kinds. 1st, Fallen snow on mountain slopes above the 
 snow-line gradually assumes a granular structure. The lit- 
 tle crystalline needles and stars of ice are melted and frozen 
 into rounded granules which form a more or less compact 
 mass known in Switzerland as Neve or Firn. 2d, When the 
 granular neve slowly slides down into the valleys, it ac- 
 quires a more compact crystalline structure and becomes 
 glacier-ice. According to the researches of F. Klocke, gla- 
 cier-ice is, throughout its mass, an irregular aggregate of 
 distinct crystalline grains, the boundaries of which form 
 the minute capillary fissures so often described. 1 " Its struc- 
 ture thus closely corresponds to that of marble (p. 263). 
 
 151 Neues Jahrb. 1881 (i.), p. 23. Grad and Dupre (Ann. Club. Alp. Franc. 
 (1874) show how the characteristic structure of glacier-ice may be revealed by 
 allowing colored solutions to permeate it.
 
 GEOGNOSY 259 
 
 Glacier-ice in small fragments is white or colorless, and 
 often shows innumerable fine bubbles of air, sometimes also 
 fine particles of mud. In larger masses, it has a blue or 
 green-blue tint, and displays a veined structure, consisting 
 of parallel vertical veinings of white ice full of air-bubbles, 
 and of blue clear ice without air-bubbles. Snow-ice is 
 formed above the snow-line, but may descend in glaciers 
 far below it. It covers large areas of the more lofty moun- 
 tains of the globe, even in tropical regions. Toward the 
 poles it descends to the sea, where large pieces break oif 
 and float away as icebergs. 
 
 Water-i'ce (see Book III. Part II. Sect. ii. 5) is 
 formed, 1st, by the freezing of the surface of fresh water 
 (river-ice, lake-ice), or of the sea (ice-foot, floe-ice, pack- 
 ice); this is a compact, clear, white or greenish ice. 2d, by 
 the freezing of the layer of water lying on the bottom of 
 rivers, or the sea (bottom-ice, ground-ice, anchor-ice); this 
 variety is more spongy, and often incloses mud, sand and 
 stones. 1 " 
 
 Rock-Salt (Sel gemme, Steinsalz, p. 144) occurs in layers 
 or beds from less than an inch to many hundred feet in 
 thickness. The salt deposits at Stassfurt", for example, are 
 1197 feet thick, of which the lowest beds comprise 685 feet 
 of pure rock-salt, with thin layers of anhydrite J-inch thick 
 dividing the salt at intervals of from one to eight inches. 
 Still more massive are the accumulations of Sperenberg near 
 Berlin, which have been bored through to a depth of 4200 
 feet, and those of Wieliczka in Grallicia which are here and 
 there more than 4600 feet thick. 
 
 The more insoluble salts (notably gypsum or anhydrite) 
 are apt to appear in the lower parts of a saliferous series. 
 Wlien purest, rock-salt is clear and colorless, but usually is 
 colored red (peroxide of iron), sometimes green, or blue 
 (chloride or silicate of copper). It varies in structure, being 
 sometimes beautifully crystalline and giving a cubical cleav- 
 age; laminated, granular r or less frequently fibrous/ It 
 usually contains some admixture of clay, sand, anhydrite, 
 bitumen, etc., and is often mixed with chlorides of magne- 
 sium, calcium, etc. In some places it is full of vesicles (not 
 infrequently of cubic form) containing saline water; or it 
 abounds with minute cavities filled with hydrogen, nitro- 
 
 181 On the properties of ice with some interesting geological bearings, see 
 O. Pettersson, "Vega-Expeditionens Vetenskapliga lakttagelser, " vol. ii. p. 249, 
 Stockholm, 1883.
 
 260 TEXT-BOOK OF GEOLOGY 
 
 gen, carbon-dioxide, or with some hydrocarbon gas. Oc- 
 casionally remains of minute forms of vegetable and animal 
 life, bituminous wood, corals, shells, crustaceans, and fish 
 teeth are met with in it. Owing to its ready solubility, it is 
 not found at the surface in moist climates. It has been 
 formed by the evaporation of very saline water in inclosed 
 basins a process going on now in many salt-lakes (Great 
 Salt Lake of Utah, Dead Sea), and on the surface of some 
 deserts (Kirgis Steppe). In different parts of the world, 
 deposits of salt have probably always been in progress from 
 very early geological times. Saliferous formations of Ter- 
 tiary and Secondary age are abundant in Europe, while in 
 America they occur even in rocks as old as the Upper Si- 
 lurian period, and among the Punjab Hills in still more 
 ancient strata. 163 
 
 Carnallite a chloride of potassium and magnesium (p. 144). 
 It occurs in a bed 20 to 30 metres thick which overlies the 
 rock-salt in the saliferous series of Stassfurt, and has been 
 found in other old salt deposits, as well as among the "salt- 
 erns" or "salines" along the Mediterranean coast where the 
 water of that inland sea is evaporated in the manufacture of 
 salt. It so closely resembles rock-salt that it was formerly 
 included with it, but it is much less frequently met with. 
 It is a valuable source for the manufacture of potash-salts. 
 
 Limestone (Calcaire, Kalkstein) essentially a mass of cal- 
 cium-carbonate, sometimes nearly pure, and entirely or al- 
 most entirely soluble in hydrochloric acid, sometimes loaded 
 with sand, clay, or other intermixture. Few rocks vary 
 more in texture and composition. It may be a hard, close- 
 grained mass, breaking with a splintery or conchoidal frac- 
 ture; or a crystalline rock built up of fine crystalline grains 
 of calcite, and resembling loaf-sugar in color and texture; 
 or a dull earthy friable chalk-like deposit; or a compact, 
 massive, finely-granular rock resembling a close-grained 
 sandstone or freestone. As its hardness is about 3, it can 
 easily be scratched with a knife and the white powder gives 
 a copious effervescence with acid. The specific gravity 
 naturally varies according to the impurity of the rock, 
 ranging from 2 -5 to 2-8. The colors, too, vary extensively, 
 the most common being shades of blue-gray and cream-color 
 passing into white. Some limestones are highly siliceous, 
 the calcareous matter having been accompanied with silica 
 
 163 On salt deposits of various ages, see A. C. Ramsay, Brit. Assoc. Rep. 
 1880, p. 10; also Index, sub voc. "Salt Deposits."
 
 GEOGNOSY 261 
 
 in the act of deposition; others are argillaceous, sandy, fer- 
 ruginous, dolomitic, or bituminous. By far the larger num- 
 ber of limestones are of organic origin; though owing to 
 internal rearrangement, their original clastic character has 
 frequently been changed into a crystalline one. Under the 
 present subdivision are placed all those limestones which 
 have had a distinctly chemical origin, and also those which 
 though doubtless, in many cases, originally formed of or- 
 ganic debris, have lost their fragmental, and have assumed 
 instead a crystalline structure. (For the organic limestones 
 see p. 244.)" 
 
 Compact, common limestone a fine-grained 
 crystalline-granular aggregate, occurring in beds or lamina 
 interstratified with other aqueous deposits. When purest it 
 is readily soluble in acid with effervescence, leaving little 
 or no residue. Many varieties occur, to some of which 
 separate names are given. Hydraulic limestone contains 10 
 per cent or more of silica (and usually alumina) and, when 
 burned and subsequently mixed with water, forms a cement 
 or mortar, which has the property of "setting" or harden- 
 ing under water. Limestones containing perhaps as much 
 as 25 per cent of silica, alumina, iron, etc., that in them- 
 selves would be unsuitable for many of the ordinary pur- 
 poses for which limestones are used, can be employed for 
 making hydraulic mortar. These limestones occur in beds 
 like those in the Lias of Lyme Regis, or in nodules like 
 those of Sheppey, from which Roman cement is made. 
 Gementstone is the name given to many pale dull ferrugi- 
 nous limestones, which contain an admixture of clay, and 
 some of which can be profitably used for making hydraulic 
 mortar or cement. Fetid limestone (stinkstein, swinestone) 
 gives off a fetid smell (sulphuretted hydrogen gas), when 
 struck with a hammer. In some cases, the rock seems to 
 have been deposited by volcanic springs containing decom- 
 posable sulphides as well as lime. In other instances., the 
 odor may be connected with the decomposition of imbedded 
 organic 'matter. In some quarries in the Carboniferous 
 Limestone of Ireland, as mentioned by Jukes, the freshly- 
 broken rock may be smelled at a distance of a hundred yards 
 when the men are at work, and occasionally the stench be- 
 comes so strong that the workmen are sickened by it, and 
 require to leave off work for a time. Cornstone is an arena- 
 ceous or siliceous limestone particularly characteristic of 
 some of the Palaeozoic red sandstone formations. Rotten- 
 stone is a decomposed siliceous limestone from which most
 
 262 TEXT-BOOK OF GEOLOGY 
 
 or all of the lime has been removed, leaving a siliceous 
 skeleton of the rock. A similar decomposition takes place 
 in some ferruginous limestones, with the result of leaving a 
 yellow skeleton of ochre. Common limestone, having been 
 deposited in water usually containing other substances in 
 suspension or solution, is almost always mixed with impuri- 
 ties, and where the mixture is sufficiently distinct it receives 
 a special name, such as siliceous limestone, sandy limestone, 
 argillaceous limestone, bituminous limestone, dolomitic lime- 
 stone. 
 
 Travertine (calcareous tufa, calc-sinter) is the porous 
 material deposited by calcareous springs, usually white or 
 yellowish, varying in texture from a soft chalk-like or marly 
 substance to a compact building-stone. (See Book III. Part 
 II. Sect. iii. 3, 6.) Stalactite is 'the name given to the 
 calcareous pendent deposit formed on the roofs of limestone- 
 caverns, vaults, bridges, etc. ; while the water, from which 
 the hanging lime-icicles are derived, drips to the floor, and 
 on further evaporation there, gives rise to the crust-like 
 deposit known as stalagmite. Mr. Sorby has shown that in 
 the calcareous deposits from fresh water there is a constant 
 tendency toward the production of calcite crystals with the 
 principal axis perpendicular to the surface of deposit. 
 Where that surface is curved, there is a radiation or con- 
 vergence of the fibre-like crystals, well seen in sections of 
 stalactites and of some calcareous tufas (Fig. 108). 
 
 O o 1 i t e a limestone formed wholly or in part of more 
 or less perfectly spherical grains, and having somewhat the 
 aspect of fish-roe. Each grain consists of successive con- 
 centric shells of carbonate of lime, frequently with an in- 
 ternal radiating fibrous structure, which gives a black cross 
 between crossed Nicols (Fig. 27). The calcareous material 
 was deposited round some minute particle of sand or other 
 foreign body which was kept in motion, so that all sides 
 could in turn become incrusted Oolitic grains of this char- 
 acter are now forming in the springs of Carlsbad (Sprudel- 
 stein); but they may no doubt also be produced where 
 gentle currents in lakes, or in partially inclosed areas of 
 the sea, keep grains of sand or fragments of shells drifting 
 along in water, which is so charged with lime as to be ready 
 to deposit it upon any suitable surface. An oolitic lime- 
 stone may contain much impurity. Where the calcareous 
 granules are cemented in a somewhat argillaceous matrix 
 the rock is known in Germany as Rogenstein. Where the 
 individual grains of an oolitic limestone are as large as
 
 GEOGNOSY 263 
 
 peas, the rock is called a p i s o 1 i t e (pea-grit). The gran- 
 ules sometimes consist of aragonite. Oolitic structure is 
 found in limestones of all ages from Palaeozoic down to 
 recent times. 154 Mr. E. Wethered has recently pointed out 
 that many oolitic grains show curious vermiform twistings 
 in their outer concentric coats, which he regards as of 
 organic origin, either plant or animal (Q-irvanelld). 1 In 
 some instances oolites nave had their calcareous matter re- 
 placed by carbonate or oxide of iron, so as to become oolitic 
 ironstones. 
 
 Marble (granular limestone) a crystalline-granular ag- 
 gregate composed of crystalline calcite-granules of remark- 
 ably uniform size, each' of which has its own independent 
 
 Big. 27. Microscopic Structure of Oolitic Pig. 28. Microscopic Structure of white 
 Limestone, after Sorby. (Magnified Statuary Marble. (Magnified 50 Di- 
 
 30 Diameters.) ameters.) 
 
 twin lamella ("often giving interference colors) and cleavage 
 lines. This characteristic structure is well displayed when 
 a thin slice of ordinary statuary marble is placed under the 
 microscope (Fig. 28). Typical marble is white, but the rock 
 is also yellow, gray, blue, green, red, black, or streaked and 
 mottled, as may be seen in the numerous kinds used for 
 ornamental purposes. Its granular structure gives it a re- 
 semblance to loaf-sugar, whence the term "saccliaroid" 
 applied to it. Fine silvery scales of mica or talc may often 
 
 154 Oolitic structure is found even among the limestones of the Dalradian 
 metamorphic series of Scotland (Islay) which may possibly be pre- Palaeozoic. 
 
 165 Geol. Mag. 1889, p. 196; Quart. Journ. Geol. Soc. xlvi. (1890), p. 270. 
 Mr. 0. Reid has suggested that these tubular bodies may be due to the deposit 
 of lime round organic filaments (Algae) like the calcareous incrustation formed 
 round fibres of hemp in kettles and boilers.
 
 264 TEXT-BOOK OF GEOLOGY 
 
 be noticed even in the purest marble (Oipolino). Some 
 crystalline limestones associated with gneiss and schist 
 are peculiarly rich in minerals mica, garnet, tremolite, 
 actinolite, anthophyllite, zoisite, vesuvianite, pyroxenes, 
 and many other species occurring there often in great 
 abundance. These inclusions can be isolated by dissolv- 
 ing the surrounding rock in acid (ante, p. 157). 
 
 Marble is regarded by most geologists as a metamorphio 
 rock, that is, one in which the calcium-carbonate, whether 
 derived from an organic or inorganic source, has been en- 
 tirely recrystallized in situ. In the course of this change 
 the original clay, sand or other impurities of the rock have 
 been also crystallized, and now appear as the crystalline 
 silicates just referred to. Marble occurs in beds and large 
 lenticular masses associated with crystalline schists on many 
 different geological horizons. In Canada it occurs of Lau- 
 rentian; in Scotland of Cambrian; in Utah of Upper Car- 
 boniferous; in Southern Europe of Triassic, Jurassic and 
 Cretaceous age. 
 
 Dolomite (Magnesian Limestone) consists typically of a 
 yellow or white, crystalline, massive aggregate of the 
 mineral dolomite; but the relative proportions of the cal- 
 cium and magnesium-carbonates vary indefinitely, so that 
 every gradation can be found, from pure limestone without 
 magnesium -carbonate up to pure dolomite containing 45'65 
 per cent of that carbonate. Ferrous carbonate is also of 
 common occurrence in this rock. The texture of dolomite 
 is usually distinctly crystalline, the individual crystals 
 being occasionally so loosely held together that the rock 
 readily crumbles into a crystalline sand. A fissured cavern- 
 ous structure apparently due to a process of contraction 
 during the process of dolomitization," is of common oc- 
 currence: even in compact varieties, cellular spaces occur, 
 lined with crystallized dolomite (Rauchwacke), the crystals 
 of which are often hollow and sometimes inclose a kernel of 
 calcite. Other varieties are built up of spherical, botryoidal 
 and irregularly-shaped concretionary masses. Dolomite, in 
 its more typical forms, is distinguishable from limestone 
 by its greater hardness (3-5-4*5), higher specific gravity 
 (2*8-2'95), and much less easy solubility in acid. It oc- 
 curs sometimes in beds of original deposit, associated with 
 gypsum, rock-salt and other results of the evaporation of 
 saturated saline waters; it is also found replacing what wag 
 once ordinary limestone. The process by which carbonate 
 of lime is replaced by carbonate of magnesia, is referred to
 
 GEOGNOSY 265 
 
 in Book III. Part I. Sect. iv. 2. IM Dolomite sometimes 
 forms picturesque mountain masses, as in the Dolomite 
 Mountains of the Eastern Alps. 
 
 Gypsum a fine granular to compact, sometimes fibrous 
 or sparry aggregate of the mineral gypsum, having a hard- 
 ness of only 1-5-2 (therefore scratched with the nail), and 
 a specific gravity of about 2-32, and being unaffected by 
 acids; hence readily distinguishable from limestone, which 
 it occasionally resembles. It is normally white, but may be 
 colored gray or brown by an admixture of clay or bitumen, 
 or yellow and red by being stained with iron-oxide. It oc- 
 curs in beds, lenticular intercalations and strings, usually 
 associated with beds of red clay, rock-salt, or anhydrite, in 
 formations of many various geological periods from Silurian 
 (New York) down to recent times. The Triassic gypsum 
 deposits of Thuringia, Hanover and the Harz have long 
 been famous. One of them runs along the south flank of 
 the Harz Mountains as a great band six miles long and 
 reaching a height of sometimes 430 feet. 
 
 Gypsum furnishes a good illustration of the many dif- 
 ferent ways in which some mineral substances can originate. 
 Thus it may be produced," 1st, as a chemical precipitate 
 from solution in water, as when sea- water is evaporated; 
 2d, through the decomposition of sulphides and the action 
 of the resultant sulphuric acid upon limestone; 3d, through 
 the mutual decomposition of carbonate of lime and sulphates 
 of iron, copper, magnesia, etc. ; 4th, through the hydration 
 of anhydrite; 5th, through the action of the sulphurous 
 vapors and solutions of volcanic orifices upon limestone 
 and calcareous rocks. 1 " It is in the first of these ways that 
 the thick beds of gypsum associated with rock-salt in many 
 geological formations have been formed. The first mineral 
 to appear in the evaporation of sea-water being gypsum, it 
 has been precipitated on the floors of inland seas and saline 
 lakes before the more soluble salts. 
 
 Anhydrite the anhydrous variety of calcium-sulphate, 
 occurs as a compact or granular, white, gray, bluish or 
 reddish aggregate in saliferous deposits. It is less frequent 
 than gypsum, from which it is distinguished by its much 
 greater hardness (3-3 5) and into which it readily passes by 
 
 156 On the mineralogical nature of dolomite see 0. Meyer, Z. Deutsch, GeoL 
 Ges. xxxi. p. 445, Loretz, op. cit. xxx. p. 387, xxxi. p. 756. Renard, Bull. Acad. 
 Roy. Belg. xlvii. (1879), No. 5. 
 
 151 Roth. Chem. Geol. i. p. 553. 
 GEOLOGY Vol. XXIX 12
 
 266 TEXT-BOOK OF GEOLOGY 
 
 taking up 0-2625 of its weight of water. 158 It often occurs 
 in thin seams or partings in rock-salt; but it also forms 
 large hill-like masses, of which the external parts have 
 been converted into gypsum. 
 
 Ironstone. Under this general term are included various 
 iron-ores in which the peroxide, protoxide, carbonate, etc., 
 are mingled with clay and other impurities. They have 
 generally been deposited as chemical precipitates on the 
 bottoms of lakes, under marshy ground, or within fissures 
 and cavities of rocks. Some iron-ores are associated with 
 schistose and massive rocks; others are found with sand- 
 stones, shales, limestones and coals; while some occur in 
 the form of mineral veins. Those which have resulted 
 from the co-operation of organic agencies are described at 
 p. 254. 
 
 Haematite (red iron-oreX a compact, fine-grained, 
 earthy, or fibrous rock of a blood-red to brown-red color, 
 but where most crystalline, steel-gray and splendent, with 
 a distinct cherry-red streak. Consists of anhydrous ferric 
 oxide, but usually is mixed with clay, sand, or other in- 
 gredient, in such varying proportions as to pass, by insen- 
 sible gradations, into ferruginous clays, sands, quartz, or 
 jasper. Occurs as beds, huge concretionary masses, and 
 veins traversing crystalline rocks; sometimes, as in West- 
 moreland, filling up cavernous spaces in limestone. Is 
 found occasionally in beds of an oolitic structure among 
 stratified formations. Some at least of the oolitic or piso- 
 litic ironstones have resulted from the conversion of original 
 grains of calcite in ordinary oolites into carbonate of iron 
 which on oxidation has become magnetite, haematite, or 
 limonite. 
 
 Limonit^e (brown iron-ore), an earthy or ochreous, 
 compact, fine-grained or fibrous rock, of an ochre-yellow 
 to a dark brown color, distinguishable from haematite by 
 being hydrous and giving a yellow streak. Occurs in beds 
 and veins, sometimes as the result of the oxidation of fer- 
 rous carbonate; abundant on the floors of some lakes; com 
 monly found under marshy soil where it forms a hard 
 brown crust upon the impervious subsoil (bog -iron-ore). 
 Found likewise in oolitic concretions sometimes as large as 
 walnuts, consisting of concentric layers of impure limonite 
 
 158 See G. Rose on formation of this rock in presence of a solution of chlo- 
 ride of sodium. Neues. Jahrb. Min. 1871, p. 932. Also Bischof, "Chem. und 
 Pliya. Geol." Suppl. (1871), p. 188.
 
 GEOGNOSY 267 
 
 with sand and clay (Bohnerz). (See p. 255 and Book III. 
 Part II. Sect. iii. 3.) 
 
 Spathic Iro n-o r e, a coarse or fine crystalline or dull 
 compact aggregate of the mineral siderite or ferrous car- 
 bonate, usually with carbonates of calcium, manganese and 
 magnesium; has a prevalent yellowish or brownish color, 
 and when fresh, its rhombohedral cleavage-faces show a 
 
 early lustre, which soon disappears as the surface is oxi- 
 ized into limonite or hsematite. Occurs in beds and veins, 
 especially among older geological formations. The colossal 
 Erzberg at Eisenerz in Styria, which rises more than 2700 
 feet above the valley, consists almost wholly of siderite. 1 * 9 
 
 Clay-ironstone (Sphserosiderite), a. dull brown or 
 black, compact form of siderite, with a variable mixture 
 of clay, and usually also of organic matter. Occurs in the 
 Carboniferous and other formations, in the form either of 
 nodules, where it has usually been deposited round some 
 organic centre, or of beds interstratified with shales and 
 coals. It is more properly described at p. 256, with the 
 organically derived rocks. 
 
 Magnet ic iro n-o r e, a granular to compact aggregate 
 of magnetite, of a black color and streak, more or less per- 
 fect metallic lustre, and strong magnetism. Commonly con- 
 tains admixtures of other minerals, notably of hematite, 
 chrome-iron, titanic-iron, pyrites, chlorite, quartz, horn- 
 blende, garnet, epidote, felspar. Occurs in beds and enor- 
 mous lenticular masses (Stocke) among crystalline schists, 
 likewise in segregation-veins of gabbros and other eruptive 
 rocks; also occasionally in an oolitic form (probably as 
 a pseudomorph after an original calcareous oolite) among 
 Palaeozoic rocks, as in the so-called "pisolitic iron-ore" 01 
 North Wales. Among the Scandinavian gneisses lies the 
 iron mountain of Gellivara in Lulea-Lappmark, 17,000 feet 
 long, 8500 feet broad, and 525 feet high. 
 
 Siliceous Sinter (Greyserite, Kieselsinter), the siliceous de- 
 posit made by hot springs, including varieties that are 
 crumbling and earthy, compact and flinty, finely laminated 
 and shaly, sometimes dull and opaque, sometimes trans- 
 lucent, with pearly or waxy lustre, and with chalcedonic 
 alterations in the older parts. The deposit may occur as 
 an incrustation round the orifices of eruption, rising into 
 dome-shaped, botryoidal, coralloid, or columnar elevations, 
 or investing leaves and stems of plants, shells, insects, etc., 
 
 169 Zirkel, Lehrb. i. p. 345.
 
 268 TEXT-BOOK OF GEOLOGY 
 
 or hanging in pendent stalactites from cavernous spaces 
 which are from time to time reached by the hot water. 
 Wben purest, it is of snowy whiteness, but is often tinted 
 yellow or flesh color. It consists of silica 84 to 91 per cent, 
 with small proportions of alumina, ferric oxide, lime, mag- 
 nesia, and alkali, and from 5 to 8 per cent of water. (See 
 Book III. Part II. Sect. iii. 3, par. 6.) 
 
 Flint and Chert have been already described among 
 the rocks of organic origin (ante, p. 247). Hornstone, an ex- 
 cessively compact siliceous rock, usually of some dull dark 
 tint, occurs in nodular masses or irregular bands and veins. 
 The name has sometimes been applied to fine flinty forms 
 of felsite. Vein-Quartz may be alluded to here as a substance 
 which sometimes occurs in large masses. It is a massive 
 form of quartz found filling veins (sometimes many yards 
 broad) in crystalline and clastic rocks; more especially in 
 metamorphic areas. (See Quartz Bocks, p. 310.) 
 
 Some of the other varieties of silica occurring in large 
 masses may be classed as rocks. Such are Jasper, and 
 Ferruginous Quartz. These, as well as common 
 vein-quartz, occur as veins traversing both stratified and 
 unstratified rocks; also as beds associated with the crystal 
 line schists. With them may be grouped Lydian-Stone 
 (Lydite, Kieselschiefer\ a black or dark-colored, excessively 
 compact, hard, infusible rock with splintery fracture, occur- 
 ing in thin, sharply defined bands, split by cross joints into 
 polygonal fragments, which are sometimes cemented by fine 
 layers of quartz. It consists of an intimate mixture of silica 
 with alumina, carbonaceous materials, and oxide of iron, 
 and under the microscope shows minute quartz-granules 
 with dark amorphous matter. It occurs in thin layers or 
 bands in the Silurian and later Palaeozoic formations inter- 
 stratified with ordinary sandy and argillaceous strata. As 
 these rocks have not oeen materially altered, the bands of 
 Lydian-stone may be of original formation, though the ex- 
 tent to which they are often veined with quartz shows that 
 they have, in many cases, been permeated by siliceous water 
 since their deposit. The siliceous rocks due to the opera- 
 tions of plant and animal life are described on p. 247, also in 
 Book III. Part II. Sect. iii. 3. 
 
 Some originally clastic siliceous rocks have acquired a 
 more or less crystalline structure from the action of thermal 
 water or otherwise. One of the most marked varieties has 
 been termed Crystallized Sandstone (see p. 232). Another 
 variety, known as Quartzite, is a granular and compact ag-
 
 GEOGNOSY 269 
 
 gregate of quartz, which will be described in connection with 
 the schistose rocks among which it generally occurs (p. 311). 
 
 II. MASSIVE ERUPTIVE IGNEOUS 
 
 Almost all the members of this important subdivision 
 have been produced from within the crust of the earth, in a 
 molten condition. Nearly all consist of two or more min- 
 erals. Considered from a chemical point of view, they may 
 be described as mixtures, in different proportions, of sili- 
 cates of alumina, magnesia, lime, potash, and soda, usually 
 with magnetic iron and phosphate of lime. In one series, 
 the silicic acid has not been more than enough to combine 
 with the different bases; in another, it occurs in excess as 
 free quartz. Taking this feature as a basis of arrangement, 
 some petrographers have proposed to divide the rocks into 
 an acid group, including such rocks as granite, quartz-por- 
 phyry and rhyolite, where the percentage of silica ranges 
 from 60 to 75 or more, a basic group, typified by such rocks 
 as basalt, where the proportion of silica is only about 50 per 
 cent or less, and an intermediate group represented by the 
 andesites with a proportion of silica ranging between that of 
 the other two groups. Ib 
 
 In the vast majority of igneous rocks, the chief silicate 
 is a felspar the number of rocks where the felspar is repre- 
 sented by another silicate (as leucite or nepheline) being 
 comparatively few and unimportant. As the felspars group 
 themselves into two divisions, the monoclinic or orthoclase, 
 and the triclinic or plagioclase, the former with, on the 
 whole, a preponderance of silica; and as these minerals oc- 
 
 160 See a paper on the chemical relations of the eruptive rocks by Prof. 
 Rosenbusch, Tschermak's Min. Mittheil. xi. (1889), p. 144, also the paper 
 quoted in footnote ( 163 ) on p. 272, and a Memoir OH "the origin of Igneous 
 Rocks," by J. P. Iddings, Phil. Soc. Washington, 1892, p. 90.
 
 270 TEXT-BOOK OF GEOLOGY 
 
 cur under tolerably distinct and definite conditions, other 
 petrographers divide the felspar-bearing Massive rocks into 
 two series: (1) the Orthoclase rocks, having orthoclase as 
 their chief silicate, and often with free silica in excess, and 
 (2) the Plagioclase rocks, where the chief silicate is some 
 species of triclinic felspar. The former series corresponds 
 generally to the acid group above mentioned, while the 
 plagioclase rocks are intermediate and basic. It has been 
 objected to this arrangement that the so-called plagioclase 
 felspars are in reality very distinct minerals, with propor- 
 tions of silica, ranging from 43 to 69 per cent; soda from 
 to 12; and lime from to 20."" In addition to the felspar- 
 rocks, there must be noted those in which felspar is either 
 wholly absent or sparingly present, and where the chief part 
 in rock-making has been taken by nepheline, leucite, oli- 
 vine, or serpentine. 
 
 From the point of view of internal structure, a classifica- 
 tion based upon microscopic research has been adopted by 
 other writers, who recognize three leading types of micro- 
 structure Granular, Porphyritic and Glassy, or Holocrys- 
 lalline, Hemi-crystalline and Vitreous. MM. Fouque and 
 Michel-Levy, pointing out that most eruptive rocks are the 
 result of successive stages of crystallization, each recogniz- 
 able by its own characters, show that two phases of consoli- 
 dation are specially to be observed, the first (porphyritic) 
 marked by the formation of large crystals (phenocrysts), 
 which were often broken and corroded by mechanical and 
 chemical action within the still unsolidified magma; the 
 second by the formation of smaller crystals, crystallites, 
 etc., which are molded round the older series. In some 
 
 141 Dana, Amer. Journ. Sci. 1878, p. 432. The modern methods of separat- 
 ing the felspars remove some of the difficulty above referred to.
 
 GEOGNOSY 271 
 
 rocks the former, in others the latter of these two phases 
 is alone present. Two leading types of structure are rec- 
 ognized by these authors among the eruptive rocks. 1. 
 Granitoid, where the constituents are mainly those of 
 the second epoch of consolidation, 'but where neither amor- 
 phous magma nor crystallites are to be seen. This struc- 
 ture includes three varieties, (a) the granitoid proper, having 
 crystals of approximately equal size; (b) pegmatoid, where 
 there has been a simultaneous crystallization and regular 
 arrangement of two constituents; (c) ophitic, in which the 
 felspars are ranged parallel to one of their crystalline faces, 
 forming a kind of transition into microlitic rocks. 2. 
 Trachytoid, distinguished by a more marked contrast 
 between the crystals of the first and second consolidation, 
 the usual presence of an amorphous magma, and the fluxion 
 structure. Three varieties are named: (a) petrosiliceous, with 
 trains and spherulites of a finely clouded substance charac- 
 teristic of the more acid rocks; (b} microlitic, characterized 
 by the abundance of microlites of felspars and other min- 
 erals; (c) vitreous, derived from the two foregoing varieties 
 by the predominance of the amorphous paste. 192 
 
 It is common to introduce a chronological element into 
 the classification of the massive rocks and to divide them 
 into an ancient (Palaeozoic and Mesozoic) and modern (Ter- 
 tiary and recent) series. Certain broad distinctions can 
 doubtless be made between many ancient and modern 
 eruptive rocks; but, for reasons already stated, it seems 
 inexpedient, in the present state of our knowledge, to em- 
 ploy relative antiquity (which must be determined by a 
 totally distinct branch of geological inquiry, and may be 
 
 1M "Mineralogie Micrographique, " p. 150.
 
 272 TEXT-BOOK OF GEOLOGY 
 
 erroneously determined) as a basis of petrographical ar- 
 rangement. 19 * 
 
 In the following arrangement the threefold division first 
 mentioned above is adopted, according to the relative abun- 
 dance of silica: 1st, Acid; 2d, Intermediate; 8d, Basic. In 
 each of these series there is a range of structure from com- 
 pletely crystalline to completely glassy. The holocrystal- 
 line rocks are as a rule the deep-seated representatives of 
 each series, while the vitreous and semi-vitreous are those 
 which have either been erupted to the surface or have been 
 connected with volcanic rather than plutonic action. No 
 system of classification yet proposed can avoid incongruities, 
 and it must be remembered that the hard and fast lines of 
 our nomenclature do not represent any really abrupt demar- 
 cations in nature. As one rock graduates into another, our 
 terminology should be elastic, so as to include such transi- 
 tional forms. 
 
 i. Acid Series 
 
 In this family the silicic acid has been in such excess as 
 often to separate out in the form of free quartz. Sometimes, 
 as in granite, it has not assumed a definitely crystallized form, 
 but is molded round the other crystals as a later stage of 
 consolidation. In other rocks (quartz- porphyry, etc.) it oc- 
 curs as a product of earlier consolidation, and often assumes 
 perfect crystallographic contours, occurring even in double 
 pyramids.' The texture of the rocks is (1> holocrystalline 
 or crystalline-granular (granitoid), as typically developed in 
 granite; (2) hemi-crystalline (porphyritic, trachytoid), as in 
 quartz-porphyry or felsite; (3) vitreous, as in obsidian. 
 
 Granite." 4 A thoroughly crystalline-granular admixture of 
 
 1M For a tabular arrangement of the massive (eruptive) rocks and critical 
 remarks on their classification, see Roaenbusch, Neues .Tahrb. 1882, ii. p. 1. 
 
 184 Oil the structure of granite, see the manuals of Zirkel and Rosenbusch 
 and the memoirs there cited; also Zirkel's "Microscop. Petrography," 1876, 
 p. 39; Phillips, Q. J. Geol. Soc. xxxi. p. 330; xxxvi. p. 1. J. C. Ward, op. cit. 
 p. 569; and xxxii. p. 1. King's "Systematic Geology" (vol. i. of Explor. 40th
 
 GEOGNOSY 278 
 
 quartz, felspar, and mica, in particles of tolerably uniform 
 size (Figs. 15 and 29). The felspar is chiefly white or pink 
 orthoclase, but triclinic felspars (oligpclase and albite) may 
 often be observed in smaller quantity, frequently distin- 
 guishable by their fine striation 
 and more waxy lustre. Micro- 
 cline is not infrequent, as well 
 as the intercrystallization of or- 
 thoclase and plagioclase (Per- 
 thite). The mica may be the 
 potash (muscovite) variety, usu- 
 ally of a white silvery 'aspect, 
 but more commonly biotite or 
 other dark brown or black vari- 
 ety. The quartz may be observed . 
 to'form a kind of paste or magma 
 
 Wrapping round the Other in- pjg. 29. Holocrystalline Structure 
 
 gradients. Only in cavities of of Granite (magnified), 
 
 the granite do the component minerals occur in independent 
 well-formed crystals, and there too the accessory minerals 
 (beryl, topaz, tourmaline, garnet, etc.) are chiefly found. 
 
 From a microscopic examination of granite, it was for- 
 merly inferred that the rock has a thoroughly crystalline 
 structure, with no megascopic ground -mass, nor microscopic 
 base of any kind between the crystals or crystalline individ- 
 uals. More recent and exhaustive study of the subject, 
 however, has led to the conclusion that though nothing like 
 a vitreous, or even porphyritic, ground-mass can be de- 
 tected, there is yet sometimes discernible an analogous 
 kind of entirely crystalline magma, in which the crystals 
 or crystalline debris of the rock are imbedded, and in which 
 they are partially dissolved. Having regard to the relations 
 between this magma and its inclosed minerals, M. Michel- 
 Le'vy has observed that microscopic examination points to 
 a distinction between granites in which the quartz is more 
 recent than the other constituents and has consolidated at 
 once, and those in which there are remains of earlier bi- 
 pyramidal quartz. He distinguishes these two series as 
 (A) Ancient granites, composed of black mica, hornblende, 
 oligoclase, and orthoclase, forming a crystalline debris im- 
 
 Parallel), p. Ill et seq. Michel-Levy, Bull. Soc. G-eol. Prance, 3d ser. iii. p. 199. 
 Rosenbusch, Zeitsch. Deutsch. Geol. Gesell. xxviii. (1876), p. 369. H. Mohl, 
 Nyt. Mag. Nat. xxiii. p. 1 et seq. J. Lehmann, "Unterauchungen iiber die Ent- 
 stehung der Altkry stall inischen Schiefergesteine," 1884, p. 3. W. J. Sollas, 
 Trans. Roy. Irish Acad. xxix. Part xiv. (1891).
 
 274 TEXT-BOOK OF GEOLOGY 
 
 bedded in a more recent crystalline magma of orthoclase and 
 quartz. (B) Porphyroid granites, generally finer in grain 
 than the preceding, and further distinguished by the oc- 
 currence of bi-pyramidal crystals of quartz (which made 
 their appearance' between the old felspar and the recent 
 orthoclase), and of a notable quantity of white mica (rare 
 among the ancient granites) posterior in advent even to the 
 more recent quartz. 1 " 
 
 Among the component minerals of granite, the quartz 
 presents special interest under the microscope. It is often 
 found to oe full of cavities containing liquid, sometimes 
 in such numbers as to amount to a thousand millions in 
 a cubic inch and to give a milky turbid aspect to the min- 
 eral. The liquid in these cavities appears usually to be 
 water containing sodium and potassium chlorides, with 
 sulphates of these metals and of calcium (p. 196). 
 
 The mean of eleven analyses of granites made by Dr. 
 Haughton gave the following average composition: silica, 
 72-07; alumina, 14-81; peroxide of iron, 2-22; potash, 6-11; 
 soda, 2-79; lime, 1-63; magnesia, 0-33; loss by ignition, 
 1-09; total, 100-05, with a mean specific gravity of 2-66. 
 
 Most large masses of granite present differences of tex- 
 ture in different parts of their area. Some of these varia- 
 tions depend on the relation of the mass to the surrounding 
 rocks (Bk. IV. Pt. VII.). Others may occur in any portion 
 of a granite boss, and have been produced by the circum- 
 stances in which the mass consolidated. Some granites are 
 marked by the occurrence of the cavities above referred to 
 where the individual minerals have had room to assume 
 sharply defined crystalline forms. Many granites are apt 
 to be traversed by veins, sometimes due to a segregation 
 of the surrounding minerals in rents of the original pasty 
 magma, sometimes to a protrusion of a less coarsely crys- 
 talline (micro-granitic, felsitic) material into the main rock 
 (Fig. 30). Some of the more important of these varieties 
 are distinguished by special names. Thus, where the com- 
 ponent quartz and felspar have crystallized together so as 
 to inclose each other and assume a tendency to an orienta- 
 tion of their longer axes in one general direction, as thej 
 are specially apt to do in segregation -veins, the rock is 
 termed Pegmatite. 186 One of the most interesting struc- 
 
 166 Bull. Soc. Geol. Prance, 3d ser. iii. (1875), p. 199. 
 
 166 For an admirable and exhaustive account of the Pegmatite veins, and 
 their associated minerals in Southern Norway, see the great monograph by Prof. 
 W. C. Brogger in Groth's Zeitsch. Krystallographie, xvi. (1890).
 
 GEOGNOSY 275 
 
 tural varieties is the form of pegmatite termed Graphic 
 Granite, in which the orientation of the quartz and fel- 
 spar is singularly well developed (Fig. 31). The quartz has 
 assumed the shape of long imperfect columnar shells, placed 
 parallel to each other and inclosed within the orthoclase, so 
 that a transverse section bears some resemblance to Hebrew 
 writing. The two minerals have crystallized together, with 
 their principal axes parallel. This intergrowth seems to 
 show that there could have been little or no internal move- 
 
 Fig. 30. Vein of finer grain (aplite) traversing a coarsely 
 crystalline Granite. 
 
 ment of the veins, in which it so frequently occurs, when 
 the component minerals assumed their crystalline forms. 
 Where the intergrowth is on a minute scale it is known as 
 micropegmatite, and it forms the base of the rock 
 to which the name of Granop hy re has been given (Fig. 
 6). Here and there c an example may be found of a eranite 
 becoming fine-grained, but containing large scattered felspar 
 crystals. Such a rock may be termed a porphyritic granite. 
 Some granites abound in inclosed crystalline concretions or 
 fragments. These are sometimes mere segregations of the 
 materials of the granite, when they are usually ovoid in
 
 276 TEXT-BOOK OF GEOLOGY 
 
 form and porphyritic in structure; in other cases, they are 
 fragments of other rocks, and are then commonly schistose 
 in structure and irregular in form. 167 In rare examples the 
 component minerals of granite have crystallized with a 
 radial concentric arrangement into rounded ball-like aggre- 
 gates (spheroidal, orbicular granite). J68 In the centre, as 
 well as round the edges of large bosses of granite, the min- 
 erals occasionally assume a more or less perfectly schistose 
 
 Fig. 81. Graphic Granite (nat. size). 
 
 arrangement. When this takes place, the rock is called 
 gneissose or gneiss granite. (See Book IV. Part VII.) 
 
 Differences in the proportions or nature of the component 
 minerals have likewise suggested distinctive names. Oi these 
 the following are the more important: Gr r an i ti t e (biotite 
 granite) a mixture of pink orthoclase and abundant oligo- 
 clase, with a little quartz, some blackish green magnesia- 
 mica, and occasionally with hornblende or augite. Horn- 
 blend e-gra nit e a rock with hornblende added to the 
 other normal constituents of granite, and usually poorer in 
 quartz than normal granite. A well-known variety occurs 
 at Syene in Upper Egypt, whence it was obtained anciently 
 in large blocks for obelisks and other architectural works. 
 The well-known Egyptian monoliths are made of it. It was 
 called by Pliny "Syenite" a name adopted by Werner as 
 a general designation for horneblendic granites without 
 quartz. The rock of Syene is really a hornblende-biotite- 
 
 ' J. A. Phillips, Q. J. Geol. Soc. xxxvi. (1880), p. 1. 
 
 188 W. C. Brogger and H. Backstrom, Geol. Stockholm Forhandl. ix. (1887^ 
 p. 37. Hatch, Quart.. Journ. Geol. Soc. xliv. (1888), p. 648, and authorities 
 there cited.
 
 GEOGNOSY 277 
 
 granite. A u g i t e-g r a n i t e a variety in which augite oc- 
 curs with black mica. Tourmaline granite agranitite 
 with disseminated tourmaline. Greisen a rare granitic 
 rock from which the felspar has disappeared, found in some 
 granite districts, especially in those wherein mineral-veins 
 occur. A p 1 i t e a fine-grained mixture of quartz and fel 
 spar, which have not infrequently intergrown (micropeg- 
 matite); found especially in veins in granite. "Elvan is 
 a Cornish term for a crystalline-granular mixture of quartz 
 and ortboclase, forming veins which proceed from granite, 
 or occur only in its neighborhood, and are evidently asso- 
 ciated with it. 109 Under the name Granulite M. Michel- 
 LeVy includes certain fine-grained granites with white mica, 
 'which to the naked eye appear to be composed entirely of 
 felspar and quartz, or of felspar alone, though both mica 
 and quartz appear in abundance when the rocks are micro- 
 scopically examined. He includes in this category most of 
 the rocks of the Alps described as "protogine." 
 
 Surrounding large masses of granite there are usually 
 numerous veins, which consist of granite, quartz- porphyry, 
 felsite, or sometimes even spherulitic material (Mull). There 
 can be no doubt that these finer-grained protrusions really 
 proceed from the crystalline granite mass. Lessen has shown 
 that the Bode vein in the Harz has a granitoid centre, with 
 compact porphyry sides, in which he found with the micro- 
 scope a true glassy base. 170 Sometimes the rocks associated 
 in this wav with granite differ in composition from the main 
 granite. 'Tourmaline is one of the characteristic minerals 
 of granite-veins, though less observable in the main body of 
 the rock; with quartz, it forms Schorl-rock. 
 
 Granite weathers chiefly by the decay of its felspars. 
 These are converted into kaolin, the mica becomes yellow 
 and soft, while the quartz stands out scarcely affected. The 
 granite of the southwest of England has weathered to a 
 depth of 50 feet and upward, so that it can be dug out with 
 a spade, and is largely used as a source of porcelain-clay. <. 
 
 Granite occurs (1) as an eruptive rock, forming huge 
 bosses, which rise through other formations both stratified 
 and unstratified, and sending out veins into the surrounding 
 and overlying rocks, which usually show evidence of much 
 alteration as they approach the granite ; (2) connected with 
 
 169 J. A. Phillips, Q. J. Geol. Soc. xxxi. p. 334. Michel-IxSvy, Bull. Soc. 
 Gteol. France, iii. 3d ser. p. 201. 
 
 110 Zeitsch. Deutsch. Geol. Gea. xxvi. (1874), p. 866.
 
 278 TEXT-BOOK OF GEOLOGY 
 
 true volcanic rocks (as in the Tertiary granophyres of Mull 
 and Skye), and forming, perhaps, the lower portions of 
 masses which flowed out at the surface as lavas. Granite 
 is thus a decidedly plutonic rock; that is, it has consolidated 
 at some depth beneath the surface, and in this respect differs 
 from the superficial volcanic rocks, such as lavas, which 
 have flowed out above ground from volcanic orifices. 
 
 Quartz- Porphyry (Microgranite, Eurite)." 1 A fine-grained 
 microgranitic ground-mass, composed mainly of felspar and 
 quartz, through which are usually scattered conspicuous 
 porphyritic crystals of one or other or both of the same 
 minerals. 
 
 To the naked eye the ground-mass varies from an ex- 
 ceedingly compact texture to one where abundant minute 
 c^stals can be detected. Of the porphyritic constituents 
 the quartz occasionally occurs in bi-pyramidal crystals; the 
 felspar is usually orthoclase, while black mica occasionally 
 appears. Under the microscope the structure of the rock is 
 found to be microgranitic, with frequently a micropegma- 
 titic arrangement of the quartz and felspar (granophyre). 
 
 The flesh-red quartz- porphyry of Dobritz, near Meissen, 
 in Saxony, was found by Kentzsch to have the following 
 chemical composition: Silica, 76-92: alumina, 12-89; pot- 
 ash, 4-27; soda, 0-68; lime, 0-68; magnesia, 0*68; oxide of 
 iron, 1-15; water, 1-97; total, 99-54 specific gravity, 2-49. 
 
 The colors of the rock depend chiefly upon those of 
 the felspar pale flesh-red, reddish-brown, purple, yellow, 
 bluish or slate-gray, passing into white, being in different 
 places characteristic. It will be observed in this, as in other 
 rocks containing much felspar, that the color, besides de- 
 pending on the nue of that mineral, is greatly regulated by 
 the nature and stage of decomposition. A rock, weathering 
 externally with a pale yellow or white crust, may be found 
 to be dark in the central undecayed portion. When the 
 base is very compact, and the felspar-crystals well-defined 
 and of a different color from the base, the rock, as it takes 
 a good polish, may be used with effect as an ornamental 
 stone. In popular language, such a stone is classed with 
 the "marbles, under the name of "porphyry." 
 
 The Quartz-porphyries occur (1) with plutonic rocks, 
 as eruptive bosses or veins, often associated with granite, 
 from which, indeed, they may be seen to proceed directly; 
 
 171 Zirkel, "Microscop. Petrog." p. 71. See particularly Rosenbusch, "Mik. 
 Phys." ii. p. 60.
 
 GEOGNOSY 279 
 
 of frequent occurrence also as veins and irregularly intruded 
 masses among, highly convoluted rocks, especially when 
 these have been more or less metamorphosed; (2) in the 
 chimneys of old volcanic orifices, forming there the "neck" 
 or plug by which a vent is filled up; and (3) as bosses some- 
 times of large size which have been protruded in connection 
 with volcanic action. Between the granophyres which are 
 characterized by a micropegmatitic structure and the fel- 
 sites or ancient rhyolites there is a close relation. Quartz- 
 porphyries are abundant in Britain among formations of 
 Lower Silurian, Old Red Sandstone and Lower Carbonifer- 
 ous age. In the Inner Hebrides they occur in large bosses 
 or domes (granophyre) rising through the older Tertiary 
 basaltic plateau. 
 
 Many of the rocks called "quartz-porphyry" are not 
 microgranitic but have the "felsitic" structure arising from 
 the devitrification of ancient forms of rhyolite (see p. 280). 
 
 Rhyolite 172 (Liparite, Quartz -trachyte) a rock having a 
 compact pale-gray, yellowish, greenish or reddish ground- 
 mass, sometimes with glassy patches and layers, often 
 showing perfect flow-structure, not infrequently also with 
 spherulitic and perlitic structures, and with crystals of 
 orthoclase (sanidine), granules of quartz and minute crystals 
 of black mica, augite, more rarely hornblende. Consider- 
 able diversity exists in the texture of the rock. Frequently 
 it is finely cavernous, the cavities being lined with chal- 
 cedony, quartz, amethyst, jasper, etc. Some varieties are 
 coarse and granitoid in character. Intermediate varieties 
 may be obtained like the quartz- porphyries, and these pass 
 by degrees into more or less distinctly vitreous rocks. 
 Throughout these gradations, however, which doubtless 
 represent different stages in the crystallization of an orig- 
 inal molten glass, a characteristic ground-mass can be seen 
 under the microscope having a glassy, enamel-like, porce- 
 laneous, microlitic character, with characteristic spherulitic 
 and fluxion structures. In the quartz, glass-inclusiofis, 
 having a dihexahedral form, may often be detected; but 
 liquid inclusions are absent. An analysis by Vom Rath 
 of a rhyolite from the Euganean Hills 'gave silica, 76-03; 
 'alumina, 13*32; soda, 5-29; potash, 3*83; protoxide of iron, 
 1-74; magnesia, 0-30; lime, 0-85; loss, 0-32; total, 101-68 
 specific gravity, 2 '553. 
 
 1W On rhyolite see Richthofen, Jahrb. K. K. Geol. Reichsanst. xi. 156. 
 Zirkel, "Micro. Petrog." p. 163. King, "Explor. 40th Parallel," vol. i. p. 606.
 
 280 TEXT-BOOK OF GEOLOGY 
 
 The perlitic structure is so characteristic of this rock 
 that the varieties which specially exhibit it were formerly 
 regarded as a distinct rock-species under the name of Perlite 
 or Pearlstone. As the name indicates, the structure presents 
 enamel-like or vitreous globules which, occasionally assum- 
 ing polygonal forms by mutual pressure, sometimes consti- 
 tute the entire rock, their outer portions shading off into 
 each other, so as to form a compact mass; in other cases, 
 separated by and cemented in a compact glass or enamel. 
 They consist of successive very thin shells, which, in a 
 transverse section, are seen as coiled or spiral rings, usually 
 full of the same kind of hair-like crystallites and crystals as 
 in the more glassy parts of the rhyolite (Fig. 9). As these 
 bodies both singly and in fluxion-streams traverse the glob- 
 ules, the latter may be regarded as a structure developed by 
 contraction in the rock, during its consolidation, analogous 
 to the concentric spheroidal structure seen in weathered 
 basalt (Fig. 94). Among these concentrically laminated 
 globules true spherulites occur, distinguished by their in- 
 ternal radiating fibrous structure (Figs. 7, 17). 
 
 Rhyolite is an acid rock of volcanic origin. It forms 
 enormous masses in the heart of extinct volcanic districts in 
 Europe (Hungary, Euganean Hills, Iceland, Lipari), and 
 in North America (Wyoming, Utah, Idaho, Oregon, Cali- 
 fornia). 
 
 N e v a d i t e a variety of rhyolite named by Richthofen 
 from its development in Nevada, and characterized by its 
 resemblance to granite, owing to the abundance of its por- 
 phyritic crystals, and the relatively small amount of ground- 
 mass in which they are imbedded. The granitoid aspect is 
 external only, as the ground-mass is distinct, and varies 
 from a holocrystalline character to one with abundant glass, 
 and the texture ranges from dense to porous. ITS 
 
 F e 1 s i t e (Felstone). Under this name a large series of 
 rocks has been grouped which appear for the most part to 
 have been originally vitreous lavas like the rhyolites, but 
 which have undergone complete devitrification, though fre- 
 quently retaining the perlitic, spherulitic, and flow-struc- 
 
 113 Hague and Iddings, Amer. Journ. Sci. xxvii. (1884), p. 461. These au- 
 thors distinguish between Nevadite and Liparite, the latter being characterized 
 by the small number of porphyritic crystals imbedded in a relatively large 
 amount of ground-mass, which, as in Nevadite, may be holocrystalline or 
 glassy. They also distinguish Lithoidal Rhyolite and Hyaline RJiyolite as 
 additional varieties.
 
 GEOGNOSY 281 
 
 tures. They vary in color from nearly white through shades 
 of gray, blue and red or brown to nearly black, often weath- 
 ering with a white crust. They are close-grained in texture, 
 often breaking with a sub-conchoidal fracture and showing 
 translucent edges. Porphyritic felspars (both orthoclase and 
 plagioclase) and blebs of quartz are of frequent occurrence. 
 The flow-structure is occasionally strongly marked by bands 
 of different color and texture, sometimes curiously bent and 
 curled over, indicating the direction of movement of the 
 still unconsolidated rock. The spherulitic structure also 
 may be found so strongly marked that the individual spher- 
 ules measure an inch or more in diameter, so that the rock 
 seems composed of an aggregate of balls, and was formerly 
 mistaken for a conglomerate (Pyromeri.de). 1 ' 1 * Under the 
 microscope many of the typical structures of rhyolite can 
 be detected in felsites. The ground-mass of these rocks has 
 given rise to much discussion, but it is now generally recog- 
 nized as a more or less altered condition of the devitrifi- 
 cation of an original vitreous mass (p. 207). Secondary 
 changes have in large measure destroyed the original micro- 
 litic structure, but traces of it can often be found, while the 
 spherulitic and perlitic forms frequently remain almost as 
 fresh as in a recent rock. Felsites with a large propor- 
 tion of alkalies, especially soda, have been called Kerato- 
 phyres. 1 ''* 
 
 Felsites have been found abundantly as interbedded 
 lavas with tuffs and agglomerates associated with Silurian 
 and older rocks in Wales and Shropshire. 17 " Soda-felsites 
 or keratophyres have been found to play a considerable part 
 among the materials erupted by the Lower Silurian vol- 
 canoes of the southeast of Ireland. 177 
 
 The vitreous acid rocks form an interesting group in which 
 we may still detect what was probably the original condition 
 
 114 On nodular felsites see G. Cole, Quart. Journ. Geol. Soc. xli. (18Sb), 
 p. 162; xlii. p. 183; Miss Raisin, op. cit. xlv. (1889), p. 247. Harker "Bala 
 Volcanic Bocks," 1889, p. 28. 
 
 115 Gumbel, "Palaeolit. Eruptivgest. Fichtelgebirg." (1874), p. 43. Rosen- 
 busch, "Mikrosop. Physiog." ii. 434. 
 
 116 Mr. Allport described some ancient forms of perlitic structure fronl Shrop- 
 shire, in what were probably once ordinary rhyolites, Q. J. Geol. Soc. xxxiii. 
 p. 449 ; and Mr. Rutley showed the presence of the same structure among the 
 Lower Silurian lavas of North Wales. Op. cit. xxxv. p. 508. 
 
 111 F. H. Hatch, Mem. Geol. Surv. Ireland, Explanation of Sheet 130; Geol. 
 Mag. 1889, p. 70.
 
 282 TEXT-BOOK OF GEOLOGY 
 
 of at least the rhyolites and felsites. Every gradation can 
 be traced from a perfect glass into a thoroughly devitrified 
 and even crystalline rock. As already remarked, the origi- 
 nal vitreous condition of rhyolite can still be seen even with 
 the naked eye in the clots and streaks of glass that occa- 
 sionally run 'through it in the direction of its flow-structure. 
 Various names have been given to the glassy rocks, of which 
 the chief are obsidian, pitchstone and pumice. These, how- 
 ever, are not to be regarded as distinct rock-species but 
 rather as the glassy condition of different lavas. 
 
 Obsidian (rhyolite-glass) the most perfect form of 
 volcanic glass, externally resembling bottle glass, having a 
 perfect conchoidal fracture, and breaking into sharp splinters, 
 transparent at the edges. Its colors are black, brown, or 
 grayish-green, rarely yellow, blue, or red, but not infre- 
 quently streaked or banded with paler and darker hues. A 
 thin slice of obsidian prepared for the microscope is found 
 to be very pale yellow, brown, gray, or nearly colorless, and 
 on being magnified shows that the usual dark colors are al- 
 most always produced by the presence of minute opaque crys- 
 tallites, which present themselves as black opaque trichytes, 
 sometimes beautifully arranged in eddy-like lines showing 
 the original fluid movement of the rock (Fig. 19); also as 
 rod-like transparent microlites. They occasionally so in- 
 crease in abundance as to make the rock lose the aspect of 
 a glass and assume that of a dull flint-like or enamel-like 
 stone. This devitrification can only be properly studied 
 with the microscope. Again, dull gray enamel-like spheru- 
 lites appear in some parts of the rock in great abundance, 
 drawn out intd layers so as to give the rock a fissile struc- 
 ture, while steam- or gas-cavities likewise occur, sometimes 
 so large and abundant as to impart a cellular aspect. The 
 occurrence of abundant sanidine crystals gives rise to Por- 
 phyritic Obsidian. Many obsidians, from the increase in the 
 number of their steam-vesicles, pass into pumice. Now and 
 then, the steam-pores are found in enormous numbers, of ex- 
 tremely minute size, as in an obsidian from Iceland, a plane 
 of which, about one square millimetre in size, has been esti- 
 mated to include 800,000 pores. The average chemical com- 
 position of obsidian is silica, 71-0; alumina, 13-8; potash, 
 4'0; soda, 5 - 2; lime, !!; magnesia, 0'6; oxides of iron and 
 manganese, 3-7; loss, 0-6 (little or no water). Mean specific 
 gravity, 2*40. Obsidian occurs as a product of the volca- 
 noes of late geological periods. It is found in Lipari, Ice- 
 land, and Teneriffe; in North America, it has been erupted
 
 GEOGNOSY 283 
 
 from many points among the Western Territories; 178 it is 
 met with also in New Zealand. 
 
 Pitchstone is a name given to the less perfectly 
 glassy acid rocks, which are distinguished by a resinous 
 or pitch-like lustre, and internally by a more advanced de- 
 velopment of microlites than in obsidian. They thus rep- 
 resent a further stage of devitrification. These rocks are 
 easily frangible, breaking with a somewhat splintery frac- 
 ture, translucent on thin edges, with usually a black or dark 
 green color, that ranges through shades of green, brown, and 
 yellow to nearly white. Examined microscopically, they 
 are found to consist of glass in which are diffused hair-like, 
 feathery and rod-shaped microlites, or more definitely 
 formed crystals of orthoclase, plagioclase, quartz, horn- 
 blende, augite, magnetite, etc. The pitchstone of Corrie- 
 gills, in the island of Arran, presents abundant green, 
 leathery, and dendritic microlites of hornblende (Fig. 14). l " 
 Occasionally, as in Arran, pitchstone assumes a spherulitic 
 or perlitic structure. Sometimes it becomes porphyritic, by 
 the development of abundant sanidine crystals (Isle of Eigg). 
 
 Pitchstone is found as (1) intrusive dikes, veins, or 
 bosses, probably in close connection with former volcanic 
 activity, as in the case of the dikes, which in Arran traverse 
 Lower Carboniferous rocks, but are probably of Miocene 
 age, and those which in Meissen send veins through and 
 overspread the younger Palaeozoic felsite-porphyries; (2) 
 sheets which have flowed at the surface, as in the remark- 
 able mass forming the Scuir of Eigg, which has filled up a 
 river-channel of older Tertiary age. 180 * 
 
 Pumice (Ponce, Bimstein) a general term for the 
 loose, spongy, cellular, filamentous or froth-like parts of 
 lavas. So distinctive is this structure, that the term pumi- 
 ceous has come into general use to describe it. There can be 
 no doubt that this froth-like rock owes its peculiarity to the 
 abundant escape of steam or gas through its mass while still 
 in a state of fusion. The most perfect forms of pumice are 
 found among the acid lavas, but this type of rock may be 
 met with in the other groups. Microscopic examination of 
 a rhyolitic pumice reveals a glass crowded with enormous 
 
 118 For an account of the obsidian of the Yellowstone Park see J. P. Iddmgs, 
 tth Kept. U. S. Geol. Surv. (1885-86), p. 255; consult also Zirkel, "Microscop. 
 Petrog." 
 
 119 See F. A. Gooch, Min. Mittheil. 1876, p. 185. Allport, Geol. Mag. 1881, 
 p. 438. 
 
 110 Quart. Journ. Geol. Soc. (1871), p. 303.
 
 284 TEXT-BOOK OF GEOLOGY 
 
 numbers of minute gas- or vapor-cavities, usually drawn out 
 in one direction, also abundant crystallites like those of ob- 
 sidian. Owing to its porous nature, pumice possesses great 
 buoyancy and readily floats on water, drifting on the ocean 
 to distances of many hundreds of miles from land, until the 
 cells are gradually filled with water, when the floating masses 
 sink to the bottom. 1 " 1 Abundant rounded blocks of purnice 
 were dredged up by the "Challenger" from the floor of the 
 Atlantic and Pacific Oceans. 
 
 ii. Intermediate Series 
 
 In this series, the average percentage of silica is consid- 
 erably less than in the acid series (56-66 per cent). Free 
 quartz is not found as a marked constituent, although occa- 
 sionally it occurs in some quantity, as microscopic examina- 
 tion has shown in the case even of some rocks where the 
 mineral was formerly believed to be absent. A range of 
 structure is displayed similar to that in the acid series. The 
 thoroughly crystalline varieties are typified by syenite (and 
 diorite), representing the granites of the acid rocks, those 
 
 among ' the trachytes and andesites by dark glasses of the 
 obsidian and pitchstone types. 
 
 Syenite. This name, formerly given in England to a 
 granite with hornblende replacing mica, is now restricted to 
 a rock consisting essentially of a holocrystalline mixture of 
 orthoclase and hornblende, to which -plagioclase, biotite, 
 augite, magnetite, or quartz may be added. As already 
 mentioned, the word, first used by Pliny in reference to the 
 rock of Syene, was introduced by Werner as a scientific 
 designation. It was applied by him to the rock of the 
 Plauenscher-Grund, Dresden; he afterward, however, made 
 that rock a greenstone. The base of all syenites, like that 
 of granites, is thoroughly crystalline, without an amorphous 
 ground-mass. The typical syenite of the Plauenscher- 
 Grund, formerly described as a coarse-grained mixture of 
 flesh-colored orthoclase and black hornblende, containing 
 no quartz, and with no indication of plagioclase, was re- 
 garded as a normal orthoclase- hornblende rock. Micro- 
 
 181 On porosity, hydration, and flotation of pumice, see Bischof, "Chem. und 
 Pbys. Geol." suppl. (1871), p. 177.
 
 GEOGNOSY 280 
 
 scopical research has, however, shown that well-striated 
 tri clinic felspars, as well as quartz, occur in it. Its com- 
 position is: silica, 59 '3; alumina, 16'85; protoxide of iron, 
 7-01; lime, 4-43; magnesia, 2-61; potash, 6-57; soda, 2;44; 
 water, etc., 1'29; total, 101 -03. Average specific gravity, 
 2-75 to 2-90. 
 
 Syenite is of much less frequent occurrence than granite. 
 While always thoroughly granitic in structure, it varies in 
 texture from coarse granular, where the individual minerals 
 can readily be distinguished by the naked eye, to compact. 
 Among its accessory minerals of common occurrence may 
 be mentioned titanite (sphene), quartz, apatite, epidote, 
 orthite, magnetite, pyrite, zircon. The predominance of 
 one or more of the ingredients has given rise to the sepa- 
 ration of a few varieties under distinctive names. In the 
 typical syenite, the dark silicate is almost wholly horn- 
 blende; sometimes there are to be found traces of augite 
 within the hornblende, indicating that the former mineral 
 was the original constituent and has been changed by para- 
 morphism. Where the ferro-magnesian silicate is mainly 
 augite, as in the well-known rock of Monzoni, the rock is 
 termed Augite-syenite or Monzonite; where brown 
 mica predominates it gives rise to Mica-syneite or Mi- 
 nette. 
 
 Elaeolite-syenite (Nepheline-syenite) is a granitoid 
 rock, characterized by the association of the variety of 
 nepheline known as elaeolite with orthoclase, and with 
 minor proportions of plagioclase, microcline, hornblende, 
 augite, biotite, sodalite, zircon, and sphene. It is distin- 
 guished by the rare minerals, upward of fifty in number, 
 which it contains, and in which some of the rarer elements 
 are combined, such as thorium, yttrium, cerium, lanthanum, 
 tantalum, niobium, zirconium, etc. It is typically devel- 
 oped in Southern Norway (Brevig, Laurvig). Where zir- 
 con enters as an abundant constituent the rock is known as 
 Zircon-syenite. Foyaite is the name given t<s> a 
 hornblendic variety found at Mount Foya, Portugal; Mias- 
 cite is a variety with abundant mica, found at Miask; 
 D i troi te, containing eodalite, spinel, etc., occurs at Ditro 
 in Transylvania. 
 
 Orthoclase-Porphyry (Micro-syenite, Quartzless-porphyry, Or- 
 thophyre) stands to the syenites in the same relation that 
 quartz-porphyry or micro-granite does to the granites. It 
 is composed of a compact micro-granitic ground-mass, with 
 little or no free quartz, but through which are usually scat-
 
 286 TEXT-BOOK OF GEOLOGY 
 
 tered numerous crystals of orthoclase, sometimes also a tri- 
 clinic felspar, black hornblende and glancing scales of dark 
 biotite. It contains from 55 to 65 per cent of silica, thus 
 differing from quartz-porphyry and felsite in its smaller pro- 
 portion of this acid. It is also rather more easily scratched 
 with the knife, but except by chemical or microscopical ana- 
 lysis, it is often impossible to draw a distinction between this 
 rock and its equivalents in the acid series. 
 
 Orthoclase-porphyry occurs in veins, dikes, and intru- 
 sive sheets. Probably many so-called "felstones," whether 
 occurring as lavas or as intrusive masses among the older 
 Palaeozoic formations, are really orthoclase-porphyries. 
 Some highly micaceous varieties have been called Mica- 
 trap a vague term under which have also been in- 
 cluded Minettes, Micaceous Quartz-porphyries, etc. The 
 name Lamprophyre, originally given by Giimbel to 
 some rnica-traps from the Fichtelgebirge, has been pro- 
 posed by Rosenbusch as a general term for the Mica-traps, 
 divisible into two groups the Orthoclastic, or syenitic, 
 where the felspar is orthoclase (Minettes), and the Plagio- 
 clastic or dioritic, where the felspar is a plagioclase variety 
 (Kersantites). 182 The lamprophyres occur abundantly as 
 dikes or veins of a fine-grained texture, and dull reddish 
 to brownish color, among the older Palaeozoic rocks of 
 Britain. 183 
 
 The orthoclase-porphyry of Pieve in the Vicentin was 
 found by Von Lasaulx to have the following composition: 
 silica, 61 '07; alumina, 18 '56; peroxides of iron and man- 
 ganese, 2-60; potash, 6-83; soda, 3-18; lime, 2-86; mag- 
 nesia, 1'18; carbonic acid, 1-36; loss, 2 p 13 specific gravity, 
 2-59. m 
 
 Dioritc. 1 " Under this name is comprehended a group of 
 
 IM The typical locality for these rocks is Kersanton in Brittany, where they 
 are dark-green and remarkably durable. A singular vein of kersantite, 3 to Q\ 
 feet broad, has been traced for nearly five miles in the Harz. Lessen, Zeitsch. 
 Deutsch. Geol. Ges. xxxii. (1880) p. 445. Jahrb. Preuss. Geol. Landesanst. 
 1880. A. von Groddeck, op. cit. 1882. M. Koch, op. cit. 1886. Barrois, 
 Assoc. Francaise (1880), p. 661 ; Ann. Soc. Geol. Nord, xiv. (1886), p. 31. 
 
 188 For an account of the Lamprophyres of the classical district of the Plauen- 
 scher-Grund, see B. Doss, Tschermak's Mineral Mittheil. xi. (1889). 
 
 184 Zeitsch. Deutsch. GeoL Ges. xxv. p. 320. 
 
 185 On diorite, its structure and geological relations, consult the memoir on 
 Belgian plutonic rocks by De la Vallee Poussin and A. Renard, Mem. Acad. 
 Royale Belg. 1876; Behrens, Neues Jalirb. Min. 1871, p. 460; Zirkel, "Micro- 
 Bcopical Petrog." p. 83. J. A. Phillips, Q. J. Geol. Soc. xxxii. p. 165, and xxxiv. 
 p. 471 two valuable papera in which the constitution of some of the "green-
 
 GEOGNOSY 287 
 
 rocks, which, possessing a granitic structure, differ from the 
 granites in their much smaller percentage of silica, and from 
 the syenites in containing plagioclase instead of orthoclase 
 as their chief constituent. They are sometimes divided into 
 two sections, the quartz-diorites and the normal diorites. 
 Many of these rocks were formerly included in the general 
 division of "Greenstones. " 
 
 Q u a r t z-d i o r i t e a holocrystalline mixture of plagio- 
 clase (oligoclase, less frequently labradorite) and quartz with 
 some hornblende, augite, or mica. It outwardly resembles 
 
 fray granite, and, indeed, includes many so-called granites, 
 ts silica ranges up to 67 per cent. In normal Diorite, 
 quartz is almost entirely absent; hornblende and black mica 
 occur together in some varieties, while pyroxene character- 
 izes others. Under the microscope a thoroughly crystalline 
 structure is seen, and among the pyroxene-diorites the fel- 
 spar and pyroxene are sometimes intergrown in ophitic ag- 
 gregates. The average chemical composition of quartzless 
 diorite is: silica, 54; alumina, 16-18; potash, 1-5-2-5; soda, 
 2-3; lime, 6-7 '5; magnesia, 6-0; oxides of iron and man- 
 ganese, 10-14; mean specific gravity, about 2'95. 
 
 Among the varieties of diorite, the following may be 
 mentioned. C o r s i t e (from Corsica) a granitoid mixture 
 of grayish-white plagioclase, blackish-green hornblende, and 
 some quartz, which have grouped themselves into globular 
 aggregations with an internal radial and concentric structure 
 (Orbicular diorite, Kugeldiorit, Napoleonite 
 Fig. 8). T o n a 1 i t e (from Monte Tonale, Tyrol) a vari- 
 ety containing quartz, hornblende, and biotite in strongly 
 contrasted colors. E p i d i o r i t e a name given to ancient 
 rocks which have originally been pyroxenic eruptive masses, 
 but, by metamorphism, have acquired a crystalline re- 
 arrangement of their constituents, the pyroxene being 
 changed into hornblende, often fibrous or actinolitic, the 
 felspar becoming granular, and the whole rock having ac- 
 quired a more or less distinct schistose structure. The dark 
 intrusive sheets associated with the crystalline schists of the 
 Scottish Highlands and the north of Ireland are largely 
 epi diorites. Some of these rocks are quartziferous, but 
 many of them belong to the basic series (see p. 1052). 
 As the granites pass into fine-grained quartz-porphyries, 
 
 stones" of the older geologists is clearly worked out. Many of these ancient 
 rocks are there shown to be forms of doleritic lava and the change of their 
 original augite into hornblende is traced.
 
 288 TEXT-BOOK OF GEOLOGY 
 
 and the syenites into compact orthoclase-porphyries, so the 
 diorites have their close-textured varieties, which are com- 
 prised under the general term Aphanite, divisible into 
 Quartz-aphanite and Normal-aphanite. The general charac- 
 teristic of these rocks is that the constituent minerals be- 
 come so minute as to disappear from the naked eye. They 
 are dark heavy close-grained masses. They merge into the 
 basic diabases (p. 170). 
 
 Trachyte 186 a term originally applied to modern volcanic 
 rocks possessing a characteristic roughness (17^1/9) under the 
 finger, is now restricted to a compact, usually pale, porphjr- 
 ritic, frequently cellular, rock, consisting essentially of sani- 
 dine, with more or less triclinic felspar, augite, hornblende, 
 and biotite, sometimes with apatite, and tridymite. It is dis- 
 tinguished from rhyolite, or quartz-trachyte, by the absence 
 of free quartz, and by the smaller proportion of vitreous or 
 microlitic (rnicro-felsitic) ground-mass. The sanidine crys- 
 tals present abundant steam-pores and glass-inclusions, as 
 well as hornblende-microlites and magnetite. In some 
 varieties, the ground-mass appears to be entirely com- 
 posed of microlites; in others, minor degrees of devitrifi- 
 cation can be traced, until the ground -mass passes into a 
 glass (trachyte-glass, obsidian). The trachytes of Hungary 
 have been grouped as Augite-trachyte, Amphibole-trachyte 
 and Biotite-trachyte. Average composition of Trachyte 
 silica, 60 '0-64 -0; alumina, 17*0; protoxide and peroxide of 
 iron, 6-0-8-0; magnesia, 1/0; lime, 3 -5; soda, 4-0; potash, 
 2-0-2-5. Average specific gravity, 2-65. 
 
 Trachyte is an abundantly diffused lava of Tertiary and 
 Post-tertiary date. It occurs in most of the volcanic dis- 
 tricts of Europe (Siebengebirge, Nassau, Transylvania, Bay 
 of Naples, Euganean Hills); in the Western Territories of 
 the United States; 187 in New Zealand. It also occurs among 
 the Carboniferous lavas of Scotland. 
 
 188 On trachyte, see Zirkel, "Micro. Petrog." p. 143. King in vol. i. of 
 "Explor. 40th Parallel," p. 578. On the relative age and classification of Hun- 
 garian trachytes, Szabo, Zeitsch. Deutsch. Geol. Ges. xxix. p. 635, and "Compte 
 rend. Congres Internationale de Geologic" (1878), Paris, 1880. For the Scottish 
 Carboniferous trachytes see Presidential Address to the Geological Society 1892, 
 and P. H. Hatch, Trans. Roy. Soc. Edin. 1892. 
 
 187 It would appear that much of what has been regarded as trachyte in 
 Western America is andesite, consisting essentially of plagioelase, and not of 
 sanidine. The normal trachytes are now described as hornblende-mica-audesites, 
 and the augite-trachytes are hyperethene-augite-andesites, most of the rest being 
 dacites, and some of them rhyolites. Hague and Iddings, Amer. Journ. 8d. 
 xxvii. (1884), p. 456.
 
 GEOGNOSY 289 
 
 D o m i t e (so named from the Puy-de-D6me) is a porous 
 loosely aggregated trachyte, having a microhtic ground- 
 mass, through which are dispersed tridymite, sanidine, 
 much plagioclase, hornblende, magnetite, biotite, and spec- 
 ular iron. Soda-trachyte (Pantellerite) is a variety rich 
 in oligoclase, found in Pantelleria. 
 
 Phonolite (Nepheline-trachyte, Clinkstone) 188 a term sug- 
 gested by the metallic ringing sound emitted by the fresh 
 compact varieties when struck, is applied to a compact, gray 
 or brown, quartzless mixture of sanidine and nepheline, 
 with nosean, hauyne, leucite, pyroxene, hornblende, or 
 mica. The rock is rather subject to decomposition, hence 
 its fissures and cavities are frequently filled with zeolites. 
 An average specimen gave on analysis silica, 57 - 7; alu- 
 mina, 2O6; potash, 6 - 0; soda, 7 g O; lime, 1'5; magnesia, 0'5; 
 oxides of iron and manganese, 3'5; loss by ignition, 3*2 per 
 cent. The specific gravity may be taken as about 2-58. 
 Phonolite is sometimes found splitting into thin slabs 
 which can be used for roofing purposes. Occasionally it 
 assumes a porphyritic texture from the presence of large 
 crystals of sanidine or of hornblende. When the rock is 
 partly decomposed and takes a somewhat porous texture, 
 it resembles normal trachyte. 
 
 It is a thoroughly volcanic rock, and generally of Ter- 
 tiary date. It occurs sometimes filling the pipes or volcanic 
 orifices, sometimes as sheets which have been poured out in 
 the form of lava-streams, and sometimes in dikes and veins, 
 as in Bohemia and Auvergne. Some of the great bosses 
 or eruptive vents connected with the trachyte lavas of the 
 Carleton Hills, Haddingtonshire, have recently been deter- 
 mined by Dr. Hatch to be true phonolites. 
 
 With the pbonolites may be classed Leucite- 
 trachyte, or Le uci te-p h o noli te, where the fel- 
 spathoid is leucite instead of nepheline, and Nosean- 
 trachyte (Nosean-phonolite), or Hauyne-trachyte 
 (Hauyne-phonolite), with nosean or hauyne taking he 
 place of the felspar of ordinary phonolite. 
 
 Andesite a name originally given by Von Buch to some 
 lavas found in the Andes, is now applied to a large series 
 of rocks distinguished from the trachytes in that their fel- 
 spar is plagioclase, and passing by the addition of olivine 
 
 188 Boricky, "Petrograph. Stud. Phonolitgestein. Bohmens." Archiv. Lan- 
 desdurchforchung Bohmen, 1874. G. P. Fohr, "Die Phonolite des Hegau's," 
 Verb. Phys. Med. Ges. Wiirzburg, xviii. (1883). P. H. Hatch, Trans. Roy. 
 Soc. Edin. 1892. 
 
 GEOLOGY Vol. XXIX 13
 
 290 TEXT-BOOK OF GEOLOGY 
 
 into dolerite and basalt. In fresh examples they are dark 
 gray, or even black rocks with a compact ground-mass, 
 through which striated felspar prisms may generally be ob- 
 served. They often assume cellular and porphyritic struc- 
 tures. At the one end of the series stand rocks containing 
 free silica (Dacite), while at the other are basalt-like masses 
 of much more basic composition (Aguite-andesite). Under 
 the microscope the ground-mass presents more or less of 
 a pale brownish glass with abundant felspar microlites. 
 
 Dacite (Quartz-andesite) composed mainly of plagio- 
 clase, quartz, and mica, with a varying amount of sanidine 
 as an accessory constituent, and, by addition of hornblende 
 and pyroxene, graduating into hornblende-andesite. The 
 ground-mass has a felsitic, sometimes spherulitic, glassy, 
 or finely granular base. Composition: silica, 69-36; alu- 
 mina, 16-23; iron oxides, 2-41; lime, 3-17; magnesia, 1*34; 
 alkalies, 7 -08; water, 0-45. Mean specific gravity, 2 -60. 
 This rock is extensively developed in the Great Basin and 
 other tracts of western North America among Tertiary 
 and recent volcanic outbursts. 
 
 Hornblende-andesite 189 consists of a triclinic fel- 
 spar (usually oligoclase), with hornblende, augite, or mica. 
 The ground-mass resembles that of trachyte, presenting 
 sometimes remains of a pale glass. The porphyritic min- 
 erals frequently show evidence of having been much 
 corroded before consolidation. Composition: silica, 61 '12; 
 alumina, 11-61; oxides of iron, 11 -64; lime, 4-33; magnesia, 
 0-61; potash, 3-52; soda, 3*85; ignition, 4-35. Hornblende- 
 andesite is a volcanic rock of Tertiary and post-Tertiary 
 date found in Hungary, Transylvania, Siebengebirge, and 
 in some of the Western Territories of the United States. 
 According to researches by Messrs. Hague and Iddings, 
 gradations from this rock into basalt and hypersthene- 
 andesite can be traced in California, Oregon, and Washing- 
 ton. These rocks, therefore, cannot be said to have sharply 
 denned and distinct forms. 190 Under the name of Horn- 
 blende-mica-andesite American petrographers have described 
 a frequent variety of rock throughout the Great Basin, char- 
 acterized by the vitreous appearance of its felspar, its rough 
 porous trachyte-like ground-mass, and the presence of mica 
 
 189 See Zirkel, "Microscop. Petrog." p. 122. King, in vol. i. of "Explor. 
 40th Parallel," p. 562. Hague and Iddings, Amer. Journ. Sci. xxvi. (1883), 
 p. 230. 
 
 190 Amer. Jouru. Sci. Sept. 1883, p. 233.
 
 GEOGNOSY 291 
 
 as an essential constituent. This term will include a large 
 proportion of the rocks hitherto classed as trachytes, but 
 in which the felspar proves to be plagioclase and not 
 sanidine. 191 
 
 P y r o x e n e-a n d e s i t e consisting of labradorite or 
 oligoclase, with augite (less frequently a rhombic pyrox- 
 ene) and abundant magnetite, sometimes with hornblende 
 or mica, forming a dark heavy basalt-like compound, with 
 a compact sometimes more or less distinctly vitreous ground- 
 mass. Composition: silica, 57*15; alumina, 16'10; protox- 
 ide of iron, 13 - 0; lime, 5 '75; magnesia, 2*21; potash, 1*81; 
 soda, 3'88. Mean specific gravity, 2'75-2-85. 
 
 It was formerly supposed that the pyroxene of the ande- 
 sites was always augite. But rhombic forms of the mineral 
 have now been frequently detected. Under the name of 
 Hypersthene-andesite, certain Tertiary or recent rocks, 
 stretching over vast areas in Western America, have been 
 described as associated with other andesites and basalts. 
 They are black to gray, or reddish-gray, in color, and vary 
 in texture from dense, thoroughly crystalline forms, to 
 others approaching white glassy pumice, the base under 
 the microscope ranging from a brown glass to a holocrvstal- 
 line structure. The magnesian silicate is pyroxene, cniefly 
 in the orthorhombic form as hypersthene, but partly also 
 augite. An analysis of the pumiceous form of the rock 
 gave 62 per cent of silica, while the percentage of the same 
 constituent in the glass of the base was found to rise to 
 69.94."" 
 
 Pyroxene-andesite occurs in dikes, lava-streams, plateaus, 
 sheets, and neck-like bosses in regions of extinct and active 
 volcanoes, as in the Inner Hebrides, Antrim, Transylvania, 
 Hungary, Santorin, Iceland, Teneriffe, the Western Terri- 
 tories of North America, the Andes, New Zealand, etc. 
 Many of the rocks of these regions now classed under this 
 name have long been known and described as dolerites and 
 basalts. Indeed, there is the closest relation between them 
 and the true olivine-bearing dolerites and basalts. The 
 latter occur among the Tertiary volcanic plateaus of Britain, 
 interstratified with rocks which, not containing olivine, have 
 been placed among the andesites. Neither in their mode of 
 occurrence nor to the eye in hand specimens is there any 
 
 191 Hague and Iddings, Amer. Journ. Sci. xxvii. (1884), p. 460. 
 92 Whitman Cross, Bull. U. S. Geol. Survey, 1883, No. 1. Hague and 
 Iddinga, Amer. Journ. Sci. xxvi. (1883), p. 226; xxvii. (1884), p. 457.
 
 292 TEXT-BOOK OF GEOLOGY 
 
 good distinction to be drawn between them. Under the 
 name of T hoi ei te some interesting augite-andesites have 
 been described, in which the felspar prisms form a network 
 filled in with granular augite and interstitial matter (inter- 
 sertal structure). In other varieties of andesite the felspar- 
 mesh has been filled with large crystalline patches of augite, 
 which thus incloses the felspar (ophitic structure). 
 
 Tephrite (Nepheline-andesite, Leucite-andesite, No- 
 Bean- or Hauyne-andesite) a group of andesites, in which 
 the felspar is partly replaced by one of the felspathoids, 
 nepheline, leucite, nosean, or hauyne. 
 
 Porphyrit e a name for old forms of andesite which 
 have generally undergone considerable alteration, and conse- 
 quently appear as dull, sometimes earthy, generally reddish 
 or brownish rocks. When fresh they- are dark grav or 
 black. They are commonly porphyritic, and show abun- 
 dant scattered crystals of plagioclase, less commonly of 
 mica. Their texture varies from coarse crystalline to ex- 
 ceedingly close-grained, passing occasionally into vitreous 
 varieties (Yetholm, Cheviot Hills). Rocks of this type have 
 been abundantly poured forth as lavas during Palaeozoic 
 time, and they occur as interstratified lava-beds, eruptive 
 sheets, dikes, veins, and irregular bosses. In Scotland they 
 form masses, several thousand feet thick, erupted in the 
 time of the Lower Old Red Sandstone, and others of wide 
 extent and several hundred feet in depth belonging to the 
 Lower Carboniferous period. . In Germany porphyrites ap- 
 pear also at numerous points among formations of later 
 Palaeozoic age. 
 
 Propylite a name given by Richthofen to certain 
 Tertiary volcanic rocks of Hungary, Transylvania, and the 
 Western Territories of the United States, consisting of a 
 triclinic felspar and hornblende in a fine-grained non-vitre- 
 ous ground-mass, and closely related to the Hornblende- 
 andesites. Their distinguishing feature is the great altera- 
 tion which they have undergone, whereby their ferro-mag- 
 nesian constituents have been converted into chlorite, and 
 their felspars into epidote. Some quartziferous propy likes 
 have been described by Zirkel from Nevada, wherein the 
 quartz abounds in liquid inclusions containing briskly-mov- 
 ing bubbles, and sometimes double inclosures with an inte- 
 rior of liquid carbon-dioxide. 193 A specimen from Storm 
 
 193 Zirkel's "Microscopical Petrography," p. 110. King, "Exploration of 
 40th Parallel," vol. i. p. 54=5. C. E. Button's "High Plateaus oi Utah" (U. S.
 
 GEOGNOSY 293 
 
 Canon, Fish Creek Mountains, contained silica, 60-58; 
 alumina, 17-52; ferric oxide, 2-77; ferrous oxide, 2-53; 
 manganese, a trace; lime, 3 -78; magnesia, 2-76; soda, 
 3-30; potash, 4-46; carbonic acid, a trace. Loss by igni- 
 tion, 2-25; specific gravity, 2-6-2-7. The geologists of the 
 Geological Survey of the United States believe that the 
 rocks included under the term "propylite" in the western 
 parts of America represent various stages of the decomposi- 
 tion of granular diorite, porphyritic diorite, diabase, quartz- 
 porphyry, hornblende-andesite, and augite-andesite. 194 The 
 name has been more recently applied by Rosenbusch to 
 rocks which have undergone alteration by solfataric action. 
 
 iii. Basic Series 
 
 This third series of eruptive rocks is distinguished by its 
 low silica percentage, and the relative abundance of its basic 
 constituents. A similar range of structure can be traced in 
 it as in the other two series. At the one extreme come 
 rocks with a holocrystalline structure like the gabbros. 
 These pass into others of a hemi-crystalline character, where, 
 amid abundant crystals, crystallites, and microlites, there 
 are still traces of the original glass. At the other end lie 
 true basic volcanic glasses, which externally might be mis- 
 taken for the pitchstones and obsidians of the acid rocks. 
 
 Gabbro 195 (Euphotide) a group of coarsely crystalline 
 rocks composed of plagioclase (labradorite) or anorthite, 
 magnetite or titaniferous iron, and some lerro-magnesian 
 mineral, which in the normal gabbros is augite or diallage, 
 but may be a rhombic pyroxene, hornblende, olivine, or 
 mica. These minerals occur in allotriomorphic forms, as 
 in granite; but they sometimes assume ophitic relations 
 which lead into the rock termed dolerite. The felspar has 
 often lost its vitreous lustre and passed into the dull opaque 
 condition known as saussurite. The augite is usually in the 
 form of diallage, distinguished by its schiller-spar lustre.' 
 
 Geographical and Geological Survey of the Rocky Mountains), chaps, iii. and 
 iv. Hague and Iddings, Amer. Journ. Sci. 1883. 
 
 194 G. F. Becker on the Comstock Lode. Reports of U. S. Geological Survey 
 1880-81, aud his full memoir in vol. iii. of the Monographs of U. S. Geol. Sur- 
 vey (1882). Hague and Iddings, Amer. Journ. Sci. xxvii. (1884), p. 454. 
 
 195 On Gabbro see Lossen, Z. Deutoch. GeoL Ges. xix. p. 651. Lang, op. 
 cit. xxxi. p. 484. Zirkel on Gabbros of Scotland, op. cit. xxiii. 1871. Judd, 
 Quart. Journ. Geol. Soc. xlii. (1886), p. 49. G. H. Williams, Bull. U. S. GeoL 
 Surv. No. 28 (1886). P. D. Chester, op. cit. No. 59 (1890). M. E. Wadaworth, 
 GeoL Surv. Minnesota, BulL 2, 1SS7.
 
 294 TEXT-BOOK OF GEOLOGY 
 
 Gabbro occurs as an eruptive rock among the older for- 
 mations, likewise in large bosses and dikes in volcanic cores 
 of Tertiary age (Mull, Skye). Average composition: silica, 
 49; alumina, 15; lime, 9 -5; magnesia, 9-7; oxides of iron 
 and manganese, 11-5; potash, 0-3; soda, 2 - 5. Loss by igni- 
 tion, 2*5; specific gravity, 2-85-3-10. 
 
 T he following varieties may be noticed : O 1 i v i n e-g a b- 
 bro a granitoid or ophitic compound of plagioclase, au- 
 gite, olivine, and magnetic or titaniferous iron; good exam- 
 ples are found among the deep-seated parts of some of the 
 Tertiary volcanic vents of the Inner Hebrides. Hypers- 
 thene-gabbro or Norite (Hypersthenite, Hyperite, 
 Schillerfels) with a rhombic pyroxene in addition to or 
 in place of the augite. Troctolite (Forellenstein) a 
 mixture of white anorthite with dark-green olivine, re- 
 ceives its name from the supposed resemblance of its 
 speckled appearance to that of the side of a trout. P y- 
 r oxe n e-g r a n ul i t e (granular diorite, trap-granulite) 
 consisting of plagioclase, pyroxene (monoclimc and rhom- 
 bic), hornblende, and garnet, distinguished by the granular 
 condition of these minerals, and found among gneisses and 
 other schistose rocks; this is probably an altered condition 
 of some original pyroxenic eruptive rock. 
 
 Dolente an important group of basic rocks, which con- 
 nect the gabbros with the basalts and include many of the 
 rocks once termed "Greenstones." They are composed of 
 labradorite (or anorthite), with some ferro-magnesian min- 
 eral (augite, enstatite, olivine, or mica) and magnetic or 
 titaniferous iron. As a rule, they are holocrystalline, the 
 constituent felspar and pyroxene or olivine being character- 
 istically grouped in ophitic structure, but a little residual 
 glass may occasionally be detected. They occur in bosses, 
 intrusive sheets, and dikes, especially as the subterranean 
 accompaniments of the volcanic action which has thrown 
 out augite-andesites and basalts to the surface. 
 
 Normal or ordinary dolerite consists of plagioclase and 
 augite, with magnetite or titanic iron and frequently olivine. 
 Average composition: silica, 45-55; alumina, 12-16; lime, 
 7-13; magnesia, 3-9: oxides of iron and manganese, 9-18; 
 potash, 0-1; soda, 2-5. Loss by ignition (water, etc.), 0-5-3; 
 specific gravity, 2-75-2-96. 
 
 Different names have been proposed for the chief varie- 
 ties. The most important of these are 1 i v i n e-d o 1 e r i t e 
 a dark, heavy, close-grained finely-crystalline rock, with 
 scattered olivine, apt to weather with a brown curst. 1 i-
 
 GEOGNOSY 290 
 
 vine-free dole rite a similar rock but containing no 
 olivine. Enstatit e-d o I e r i t e contains enstatite in ad- 
 dition to the other ingredients. N e p h e 1 i n e-d o 1 e r i t e, 
 has the felspar largely or entirely replaced by nepheliue (see 
 Nephelinite, p. 299). 
 
 As varieties of dolerite depending for their peculiarities 
 mainly upon their antiquity and the consequent alteration 
 they have undergone, we may include the rocks compre- 
 hended under the term Diabase. 198 This name was given 
 to certain dark green or black eruptive rocks found in older 
 geological formations, and consisting essentially of triclinic 
 felspar, augite, magnetite or titaniferous iron, apatite, some- 
 times olivine, usually with more or less of diffused greenish 
 chloritic substances (viridite) which have resulted from the 
 alteration of the augite or olivine. The average composi- 
 tion of typical diabase may be taken to be: silica, 48-50; 
 alumina, 16-0; protoxide of iron, 12-15; lime, 5-11; mag- 
 nesia, 4-6; potash, 0-8-l*5; soda, 3-4'5; water, l - 5-2. Spe- 
 cific gravity about 2'9. There is generally carbonic acid 
 present, united with some of the lime as a decomposition 
 
 Eroduct. As in ordinary dolerite, gradations may be traced 
 om coarsely crystalline diabase 197 into exceedingly fine- 
 grained and compact varieties (Diabase-aphanite), which 
 sometimes assume a fissile character (Diabase-schiefer) where 
 they have been subjected to crushing or cleavage. Some 
 kinds present a porphyritic structure, and show dispersed 
 crystals of the component minerals (Diabase-porphyry, Lab- 
 rador-porphyry, Augite-porphyry); or, as in some varieties 
 of diorite, a concretionary arrangement is produced by the 
 appearance of abundant pea-like bodies of a compact felsitic 
 material, imbedded in a compact or finely crystalline ground- 
 mass (Variolite). When the green compact ground-mass 
 contains small kernels of carbonate of lime, sometimes in 
 great numbers, it is called Calcareous aphanite or Calcaph- 
 anite. Sometimes the rock is abundantly amygdaloidal. 
 Though, as a rule, free silica does not occur in it, some 
 varieties found to contain this mineral, possibly a second- 
 ary product, have been distinguished as Quartz-diabase. 
 The presence of olivine has suggested the name Olivine- 
 
 194 The student will find in the Zeitschrift. Deutsch. G-eol. Ges. 1874, p. 1, 
 an important memoir by Dathe on the composition and structure of diabase. 
 See also Zirkel's "Microscop. Petrog." p. 97. 
 
 191 Michel-LeVy. Bull. Soc. Geol. France, 3d ser. xi. p. 282. Geikie, Trans. 
 Roy. Soc. Edin. xxix. p. 487.
 
 296 
 
 TEXT-BOOK OF GEOLOGY 
 
 diabase as distinguished from the normal kinds in which 
 this mineral is absent. A variety containing hornblende is 
 termed Proterobase. Ophite, a variety occurring in the Pyre- 
 nees, contains diallage and epidote (see p. 212). 
 
 Diabase occurs both in contemporaneous beds and in in- 
 trusive dikes and sheets. 
 
 Basalt 198 a black, extremely compact, apparently homo- 
 geneous rock, which breaks with a splintery or conchoidal 
 fracture, and in which the component minerals can only 
 be observed with the microscope, unless where they are 
 scattered porphyritically through the mass (Fig. 32). The 
 minerals consist of plagioclase 
 (labradorite or anorthite), pyrox- 
 ene (usually augite, but occasion- 
 ally a rhombic form), olivine, 
 magnetite or titaniferous iron. 
 Many years ago, Andrews de- 
 tected native iron in the basalt 
 of Antrim, and more recently 
 Nordenskiold found this sub- 
 stance abundantly diffused in the 
 basalt of Disco Island, occurring 
 even in large blocks like meteor- 
 ites (ante, p. 125). The ground- 
 mass of basalt presents under the 
 microscope traces of glass in 
 which are imbedded minute gran- 
 ules, hairs, needles, and mierolites 
 
 gated into a large compound of felspar and augite. The pro- 
 crystal. The black specks are portion or this base varies within 
 wide limits, insomuch that while 
 in some parts of a basalt it so preponderates that the indi- 
 vidual crystals are scattered widely through it, or are drawn 
 out into beautiful streaks and eddies of fluxion structure, in 
 others it almost disappears, and the rock then appears as a 
 nearly crystalline mass, which thus graduates into dolerite 
 and basic andesite. The component minerals frequently 
 appear porphyritically dispersed, especially the olivine, the 
 pale yellow grains of which are characteristic. 
 
 198 On basalt rocks see Zirkel's "Basaltgesteine," 1870. Boricky's "Petro- 
 graphische Studien en den Basaltgesteinen Bohmens," in Archiv fur Naturwisa. 
 Landesdurchforschung von Bohmen, ii. 1873. Allport, Q. J. Geol. Soc. xxx. 
 p. 529. Geikie, Trans. Roy. Soc. Edin. xxix. Mohl, Nov. Act. Acad. Leop. 
 Carol, xxxvi. (1873), p. 74; Neues Jahrb. 1873, pp. 449, 824. F. Eichstadt on 
 Basalts of Scania, Sveriges Geol. Undersok, ser. c. No. 51, 1882. E. Svedmark, 
 op. cit. No. 60, 1883. 
 
 shaded crystals are Olivine con- 
 siderably serpentinized : the nu- 
 merous small white prisms are 
 Plagioclase. A few Augite prisms 
 occur which, to the right of the 
 centre of the drawing, are aggre- 
 gated into a large compound
 
 GEOGNOSY 297 
 
 Two types of basalt have been recognized in the great 
 basaltic outbursts of Western America: (1) the porphyritic, 
 consisting of a glassy and microlitic or micro-crystalline 
 ground-mass, bearing relatively large crystals of olivine, 
 felspar, and occasionally augite, a structure showing close 
 relations to that of many andesites; (2) the granular (in the 
 sense in which that term is used by Eosenbusch, ante, p. 177) 
 an aggregate of quite uniform "grains, composed of well- 
 developed plagioclase and olivine crystals, with ill-defined 
 patches of augite, and frequently with a considerable amount 
 of glass-base. By diminution of olivine and augmentation 
 of silica, and the appearance of hypersthene, gradations can 
 be traced from true olivine-basalts into normal andesites. 
 Basalts with free quartz are not infrequent in Western 
 America."* 
 
 Basalt occurs in amorphous and columnar sheets, which 
 may alternate with each other or with associated tuffs. It 
 also forms abundant dikes, veins, and intrusive bosses. It 
 frequently assumes a cellular structure, which becomes 
 amygdaloidal by the deposit of calcite, zeolites, or other 
 minerals in the vesicles. A relation may be traced between 
 the development of amygdales and the state of the rock; the 
 more amygdaloidal the rock, the more is it decomposed, 
 showing that the amygdales have probably in large meas- 
 ure been derived by infiltrating water from the basalt itself. 
 
 Yitreous Basalt (Basalt-glass, Tachylyte, Hyalo- 
 melan). 200 Basalt passes into a condition which, even to 
 the naked eye, is recognizable as that of a true glass. This 
 more especially takes place along the edges of dikes and. 
 intrusive sheets. Where an external skin of the original 
 molten rock has rapidly cooled and consolidated, in contact 
 with the rocks through which the eruption took place, a 
 transition can be traced within the space of less than a quar- 
 ter of an inch from a crystalline dolerite, anamesite, basalt, 
 or andesite into a black glass, which under the microsqope 
 assumes a pale brown or yellowish color, and is isotropic, 
 but generally contains abundant microlites, sometimes with 
 a globular or spherulitic concretionary structure. In such 
 cases it seems indisputable that this glass represents what 
 
 199 Hague and Iddings, Amer. Journ. Sci. xxvii. (1884), p. 456. Iddings, 
 op. cit. xxxvi. (1888), p. 208, Bull. U. S. Geol. Surv. Noa. 66 and 79 (J. S. Diller). 
 
 400 See Judd & Cole, Q. J. Geol. Soc. xxxix. (1883), p. 444. Cole, op. cit. 
 xliv. (1888), p. 300. Cohen, Neues Jahrb. 1876, p. 744; 1880 (vol. ii.), p. 23 
 (Sandwich Islands).
 
 298 TEXT-BOOK OF GEOLOGY 
 
 was the general condition of the whole molten mass at the 
 time of eruption, and that the present crystalline structure 
 of the rock was developed during cooling and consolidation. 
 The glassy forms of basalt undergo alteration into a yellow- 
 ish substance called Palagonite (p. 242). It is worthy 
 of remark that in the analyses of vitreous basalts, the per- 
 centage of silica rises usually above, while their specific 
 gravity falls below, that of ordinary crystalline basalt. 
 
 The average composition of basalt is silica, 45-55; 
 alumina, 10-18; lime, 7-14; magnesia, 3-10; oxides of 
 iron and manganese, 9-16; potash, 0-5-3; soda, 2-5. Loss 
 by ignition (water, etc.), l-x>; specific gravity, 2'85-3'10. 
 
 The basalt-rocks are thoroughly volcanic in origin, ap- 
 pearing in lava-streams, plateaus, sills, necks, dikes, and 
 veins. The columnar structure is so common among the 
 finer-grained varieties that the term "basaltic" has been 
 popularly used to denote it. As already stated, it has been 
 assumed by some writers that basalt did not begin to be 
 erupted until the Tertiary period. But true basalt occurs 
 abundantly in Scotland as a product of Lower Carboniferous 
 volcanoes, and exhibits there a variety of types of minute 
 structure." 01 
 
 Basic Pumice. Though the acid lavas furnish most 
 of the pumice with which we are familiar, some of the basic 
 kinds also assume a similar structure. Thus at Hawaii, the 
 basic pyroxenic or olivine lavas give rise to a pumiceous 
 froth. 
 
 Melaphyre a name originally proposed by Brong- 
 niart and subsequently applied in various senses by different 
 writers to include rocks which range in structure 'and corn- 
 position from the more basic andesites to true olivine- basalts. 
 The melaphyres for the most part belong to pre-Tertiary 
 eruptions (though some Tertiary lavas have been described 
 as melaphyre) and have undergone more or less alteration. 
 If the word is to be retained as a definite rock- name it should 
 be restricted to an altered type, as is now generally agreed, 
 and preferentially to the older altered basalts. The mela- 
 phyres will then bear somewhat the same relation to the 
 basalts that the diabases do to the dolerites and the porphy- 
 rites to the andesites. But it must necessarily happen that 
 difficulty will be experienced in deciding which of the three 
 
 201 See Trans. Boy. Soc. Edin. xxix. <1879), p. 437, and Presidential Address, 
 Quart. Journ. Geol. Soc. (1892), p. 129, where the types of microscopic structure 
 observed by Dr. Hatch are enumerated. 
 
 \
 
 GEOGNOSY 299 
 
 names would be best applied to some of the eruptive rocks 
 of the older geological formations. The melaphyres, as thus 
 defined, are somewhat dull, dark brown, reddish, or green 
 rocks, often amygdaloidal and showing their porphyritic 
 minerals in an altered condition, the olivines especially 
 being changed into serpentine or replaced by magnetite or 
 even by haematite. 2011 
 
 Nepheline-basalt (Nepheline-Basanite). Zirkel 
 proved that certain black heavy rocks, having externally 
 the aspect of ordinary basalt, contain little or no felspar, 
 the part of that mineral being taken in some by nephelme, 
 in others by leucite.* 08 They are volcanic masses of late 
 Tertiary age, but occur much more sparingly than the true 
 basalts. They are found in the Odenwald, Thuringer Wuld, 
 Brzgebirge, Baden, etc. Mean composition silica, 45'52; 
 alumina, 16*50; ferric and ferrous oxides, 11*20; lime, 
 10-62; magnesia, 4*35; potash, 1*95; soda, 5*40; water, 
 2*68. Mean specific gravity, 2-9-3-1. Nephelinite is 
 a form of basalt with no felspar or olivine. 
 
 L e u c i t e-b a s a 1 1 (Leucite-Basanite) contains little or 
 no felspar, but has leucite in place of it. Externally it 
 resembles ordinary basalt. This rock occurs among the 
 extinct volcanoes of the Bifel and of Central Italy, and 
 forms the lavas of Vesuvius. Leucitite contains no fel- 
 spar and no olivine. 
 
 M el i lite-basalt. In continuation of Zirkel's re- 
 search, A. Stelzner has shown that in some basults the 
 part of felspar and nepheline is played by melilite. * 04 In 
 outer appearance the rocks possessing this composition, and 
 to which the name of Melilite-basalt has been given, cannot 
 be distinguished from ordinary basalt. Under the micro- 
 scope, the ground-mass appears to be mainly composed of 
 transparent sections of melilite, either disposed without 
 order, or ranged in fluxion lines round the large olivine 
 and augite crystals; but it also contains chromite (?), niicro- 
 litic augite, brown mica, abundant magnetite, with perow- 
 skite, apatite, and probably nepheline. (Swabian Alb, 
 Bohemia, Saxon Switzerland, etc.) 
 
 * J For some account of the use of the word melaphyre see Brongniart, 
 "Classification et Caracteres mineralotriques des Roches homogenes et hetero- 
 genes," 1827, p. 106. Naumann, "Lehrbuch der Geognosie," i. p. 587. Zir- 
 kel, "Petrographie, " ii. p. 39. Rosenbusch, "Mikroskop. Physiogr." ii. p. 484. 
 
 903 "Basaltgesteine," 1870. 
 
 404 Neues Jahrb. (Beilageband), 1883, p. 369-439.
 
 300 TEXT-BOOK OF GEOLOGY 
 
 Under the awkward name of "ultra-basic," the follow- 
 ing group of rocks is included in which the proportion of 
 silica sinks to a still smaller amount than in the basalts. 
 
 Ltmburgtte (Magma-basalt) a fine-grained to vitreous rock 
 composed of augite, olivine, magnetite or titaniferous iron, 
 and apatite. The base is generally glassy and the propor- 
 tion of silica in the rock is only about 42 per cent. The 
 typical locality is Limburg, near the Kaiserstuhl in Baden. 
 
 Peridotite Group. The rocks here embraced, stand at the 
 extreme end of the basic igneous rocks as the rhyolites and 
 granites stand at the opposite end of the acid series. They 
 contain no felspar, or at least an insignificant proportion of 
 it, and consist of olivine, with augite, hornblende or mica, 
 magnetic or titaniferous iron, chromite and other allied 
 minerals of the spinel type. They contain silica, 39-i5; 
 alumina, 0-6; ferrous oxide, 8-10; lime, 0-2; magnesia, 
 35-48; and have a mean specific gravity between 3 - and 
 3 - 3. When quite fresh these rocks have a holocrystalline 
 structure, but they are generally more or less altered, and 
 in their extreme condition of alteration form rocks known 
 as serpentines. They occur for the most part as intrusive 
 masses belonging to the deeper-seated portions of volcanic 
 eruptions. The following varieties may be noticed: 
 
 Pikrite 206 (Palaeopikrite, Pikrite-porphyry) a rock 
 rich in olivine, usually more or less serpentinized, with 
 augite, magnetite, or ilmenite, brown biotite, hornblende, 
 or apatite; occurs as an eruptive rock among Palaeozoio 
 formations; is closely related to the diabases into which 
 by the addition of plagioclase it naturally passes. When 
 hornblende predominates over pyroxene the rock has been 
 called hornblende-pikrite. 
 
 Lh erzo li te aoe so named from L'herz in the Ariege, 
 is a holocrystalline rock composed of olivine, diallage, and 
 a rhombic" pyroxene, with a lesser proportion of a spinel- 
 loid sometimes brown (chromite, picotite), sometimes green 
 (pleonast), and iron ores. 
 
 D u n i t e, named by F. von Hochstetter from the Dun 
 Mountain, New Zealand, consists of a granitoid mixture of 
 olivine with chromite or other spinelloid. Such a rock 
 passes naturally by alteration into a serpentine. 
 
 905 So named from mtpfc, bitter, in allusion to the large proportion of bitter- 
 earth (Magnesia) a character shared by all the peridotites. GiimbeJ, "Die 
 Palaeolithischen Eruptivgesteine des Fichtelgebirges" ; Munich, 1874. 
 
 206 On the eruptive nature of Lherzolite, see A. Lacrois, Compt. rend. civ. 
 (1892), pp. 974 and 976.
 
 GEOGNOSY 301 
 
 Serpentine. 9 " Under this name are included rocks which, 
 whatever may have been their original character and com- 
 position, now consist mainly or wholly of serpentine. As 
 already stated, olivine readily passes into the condition of 
 serpentine, while the other minerals may remain nearly un- 
 affected, as is admirably seen in some pikrites. Most ser- 
 pentine-rocks originally consisted principally of olivine 
 (see Fig. 33). Diorite, gabbro, and other rocks, consisting 
 largely of magnesian silicates, likewise pass into serpen- 
 tine. " If varieties due to different phases of alteration were 
 judged worthy of separate designation, each member of the 
 peridotites might of course have a conceivable or actual 
 representative among the serpentines. But without attempt- 
 
 Fig. 83. -Stages in the alteration of Olivine. A, the nearly fresh crystal; B, the 
 alteration half completed; c, the crystal wholly serpentinized. 
 
 ing this minuteness of classification, we may with advantage 
 treat by itself, as deserving special notice, the massive form 
 of the mineral serpentine from whatsoever rock it may have 
 originated. 
 
 Massive serpentine is a compact or finely granular, faintly 
 glimmering, or dull rock, easily cut or scratched, having a 
 prevailing dirty-green color, sometimes variously streaked 
 or flecked with brown, yellow, or red. It frequently con- 
 tains other minerals besides serpentine. One of its com- 
 monest accompaniments is chrysotile or fibrous serpentine, 
 which in veinings of a silky lustre often ramifies through 
 the rock in all directions. Other common inclosures are 
 
 201 See Tschermak, Sitz. Akad. Wien, Ivi. July, 1867; it was this author 
 who first showed the derivation of serpentine from original olivine rocks ; Bon- 
 ney, Q. J. Geol. Soc. xxxiii. p. 884, xxxiv. p. 769; Geol. Mag. (2) vi. p. 362; 
 (3) i. p. 406; Michel-LeVy, Bull. Soc. Ge"ol. France, vi. 3d ser. p. 156; Sterry 
 Hunt, Trans. Roy. Soc. Canada, i. (1883); Dathe, Neues Jahrb. 1876, pp. 236, 
 337, where Garnet-serpentine and Bronzite-serpentine are described from the 
 Saxon granulite region. J. S. Diller, Bull. U. S. Geol. Burv. No. 38 (1887); 
 M. E. Wadsworth, "Lithological Studies" (1884), p. 118.
 
 302 TEXT-BOOK OF GEOLOGY 
 
 bronzite, enstatite, magnetite, and chrome-spinels, besides 
 traces of the original olivine, pyroxene, amphibole, mica, 
 or felspar in the rocks, which have been altered into 
 serpentine. 
 
 Serpentine occurs in two distinct forms; 1st, in beds or 
 bands intercalated among schistose rocks, and associated 
 especially with crystalline limestones; 2dly, in dikes, veins, 
 or bosses traversing other rocks. 
 
 As to its mode of origin, there can be no doubt that in 
 most cases it was originally an eruptive rock, as is clearly 
 shown by its occurrence in dikes and irregular bosses. The 
 frequent occurrence of recognizable olivine crystals, or of 
 their still remaining contours, in 
 the midst of the serpentine-matrix, 
 affords good grounds for assigning 
 an eruptive origin to many serpen- 
 tines which have no distinctly erup- 
 tive external form (Fig. 34). The 
 rock cannot, of course, have been 
 ejected as the hydrous magnesian 
 silicate serpentine; we must regard 
 it as having been originally an 
 eruptive olivine rock, or a highly 
 hornblendic or micaceous diorite, 
 F\g. 34.-Microscopic structure of or olivine-gabbro. But, on the 
 
 Serpentine (20 D.ameters). ^ interca l ation Q f 
 
 beds of serpentine among schistose rocks, and particularly 
 the frequent occurrence of serpentine in connection with 
 more or less altered limestones (West of Ireland, High- 
 lands of Scotland) suggests another mode of origin in these 
 cases. Some writers have contended that such serpentines 
 are products of the alteration of dolomite, the magnesia 
 having been taken up by silica, leaving the carbonate of 
 lime behind as beds of limestone. Others have supposed 
 the original rocks, from which the serpentines were derived, 
 to have been a deposit from oceanic water, as has been sug- 
 gested by Sterry Hunt in the case of those associated with 
 the crystalline schists. 208 Beds of serpentine intercalated with 
 limestone might conceivably have been due to the elimina- 
 tion of magnesian silicates from sea- water by organic agency, 
 like the glauconite now found filling the chambers of Jo- 
 raminifera, the cavities of corals, the canals in shells, sea- 
 urchin spines and other organisms on the floor of the present 
 
 * 8 "Chemical Essays," p. 123.
 
 GEOGNOSY 303 
 
 sea." 09 Among the limestone and crystalline schists of 
 Banffshire (p. 316), serpentine occurs in thick lenticular 
 beds which possess a schistose crumpled structure and 
 agree in dip with the surrounding rocs. They may have 
 been deposits of contemporaneous origin with the lime- 
 stones and schists among which they occur, and in asso- 
 ciation with which they have undergone the characteristic 
 schistose puckering and crumpling. Sometimes they sug- 
 gest a source from the alteration of highly basic volcanic 
 tuffs. In other cases they may have been erupted peridotites 
 which have acquired a schistose character from the same 
 process of mechanical deformation that has played so large 
 a part in producing the foliation of the crystalline schists. 
 
 III. SCHISTOSE (METAMORPHIC) 
 
 In this section is comprised a series of rocks which 
 present a remarkable system of divisional planes that are 
 not original but have been superinduced upon them. At 
 the one end stand rocks which are unmistakably of sedi- 
 mentary origin, for their original bedding can often be dis- 
 tinctly seen, and they also contain organic remains similar 
 to those found in ordinary unaltered sedimentary strata. 
 At the other end come coarsely crystalline masses, which 
 in many respects resemble granite, and the original char- 
 acter of which is not obvious. An apparently unbroken 
 gradation can be traced between these extremes, and the 
 whole series has been termed "metamorphic" from the 
 changed form in which its members are believed now to 
 appear. In the earlier stages the change has taken the form 
 of cleavage as in ordinary slate. Even in slate, however, 
 as already remarked (p. 236), a beginning may be detected 
 in the development of crystalline particles, and the crystal- 
 line re-arrangement may be traced in constantly advancing 
 
 209 According to Berthier, one of the glauconitic deposits in a Tertiary lime- 
 stone is a true serpentine. See Sterry Hunt, "Chem. Essays," p. 303.
 
 304 TEXT-BOOK OF GEOLOGY 
 
 progression until the whole mass has become crystalline, 
 and forms what is known as a schist. 
 
 The Crystalline Schists, properly so called, constitute a 
 well-defined series of rocks. They are mainly composed 
 of silicates. Their structure is crystalline, but is distin- 
 guished from that of the Massive or Eruptive rocks by its 
 more or less closely parallel layers or folia, consisting of 
 
 Fig. 35. Profile of a piece of Gneiss, showing the lenticular character of 
 its folia, natural size. (B. N. Peach.) 
 
 materials which have assumed a crystalline character along 
 these layers. The folia may be composed of only one min- 
 eral, but usually consist of two or more, which occur either 
 in distinct, often alternate laminae, or intermingled in the 
 same layer. This structure resembles that of the stratified 
 rocks, but it is differentiated (1) by a prevalent striking 
 want of continuity in the folia, which, as a rule, are con- 
 spicuously lenticular, thickening out and then dying away,
 
 GEOGNOSY 805 
 
 and reappearing after an interval on the same or a different 
 plane (Fig. 35); (2) by a peculiar and very characteristic 
 welding of the folia into each other, the crystalline par- 
 ticles of one layer being so intermingled with those of the 
 layers above and below it that the whole coheres as a tough, 
 
 Pig. 86. View of a hand-specimen of contorted mica-schtot, two-thirds natural 
 size. (B. N. Peach.) 
 
 not easily fissile mass; (3) by a frequent remarkable and 
 eminently distinctive puckering or crumpling (with frequent 
 minute faulting) of the folia, which becomes sometimes so 
 fine as to be discernible only under the microscope 118 (Fig. 
 
 210 On the microscopic structure of the crystalline schists see Zirkel, "Micro- 
 scopical Petrography" (vol. vi. of King's Exploration of 40th Parallel), 1876, 
 L14. Allport, Q. J. Geol. Soc. xxxii. p. 407. Sorby, op. cit. xxxvi. p. 81, 
 limann's "Untersuchungen uber d. Entstehung. Altkryst. Schiefer," Bonn, 
 1884; and other memoirs ciled in subsequent pages.
 
 306 TEXT-BOOK OF GEOLOGY 
 
 87), but is often present conspicuously in hand-specimens 
 (Fig. 36), and can be traced in increasing dimensions, till 
 it connects itself with gigantic curvatures of the strata, 
 which embrace whole mountains. These characters are 
 sufficient to indicate a great difference between schistose 
 rocks and ordinary stratified formations, in which the strata 
 lie in continuous flat, parallel, and more or less easily 
 separable layers. 
 
 In some instances, the folia can be seen to coincide with 
 original bedding, as where a band of quartzite or of con- 
 glomerate is intercalated between sheets of phyllite or mica- 
 schist. In such cases, there cannot be any doubt that the 
 rock, though now more or less reconstructed and crystal- 
 line, was originally mere accumulated mechanical sediment. 
 Many clay slates, phyllites, and mica-schists are obviously 
 only altered marine clays, and some of them still retain their 
 recognizable fossils. From such rocks, gradations can be 
 followed into chiastolite-schist, mica-schist, and fine gneiss. 
 Quartzites and quartz-schists often still retain the false-bed- 
 ding of the original sandy sediment of which they are com- 
 posed. The pebbly and conglomeratic bands associated with 
 some schists afford convincing proof of their original clastic 
 nature. Thus, at the one end of the schistose series we find 
 rocks in which an original sedimentary character remains 
 unmistakable. At the other end, after many intermediate 
 stages, we encounter thoroughly amorphous crystalline 
 masses, that bear the closest resemblance to eruptive rocks 
 into which they insensibly pass. In such instances, there 
 can be little doubt that the amorphous structure is the origi- 
 nal one, which has become schistose by subsequent defor- 
 mation (Book IV. Part. VIII.). The banded arrangement 
 of many coarse gneisses, however, may be an original segre-
 
 GEOGNOSY 307 
 
 gation-stnicture, like that observable in sills and bosses of 
 eruptive rocks (p. 1035). 
 
 In the more thoroughly reconstructed and recry stall ized 
 schists all trace of the original structures has been lost. 
 The foliation is not coincident with bedding, nor with any 
 structure of eruptive rocks, but has been determined by 
 planes of cleavage or of shearing, or by the alignment as- 
 sumed by minerals crystallizing under the influence of in- 
 tense pressure. Along these surfaces the constituents have 
 rearranged themselves, and new chemical and mineralogical 
 combinations have been effected during the progress of the 
 ' ' metamorphism. ' ' 
 
 A rock possessing a crystalline arrangement into sepa- 
 rate folia is in English termed a Schist. 811 This word, 
 though employed as a general designation to describe the 
 structure of all truly foliated rocks, is also made use of as 
 a suffix to the names of the minerals of which some of the 
 foliated rocks largely consist. Thus we have "mica-schist," 
 "chlorite-schist, " "hornblende-schist." If the mass loses 
 its fissile tendency, owing to the felting together of the com- 
 ponent mineral into a tough coherent whole, the word rock 
 is usually substituted for schist, as in "hornblende-rock," 
 "actinolite-rock," and so on. The student must bear in 
 mind that while the possession of a foliated structure is the 
 distinctive character of the crystalline schists, it is not al- 
 ways present in every individual bed or mass associated 
 with these rocks. Yet the non-schistose portions are so ob- 
 viously integral parts of the schistose series that they can- 
 not, without great violation of natural affinities, be separated 
 
 211 In French this term has no such definite signification, being applied both 
 to schists and to shales. In German also the corresponding word "schiefer" 
 designates schists, but is also employed for non-crystalline shaly rocks; thon- 
 schiefer = clay-slate : schieferthon = shale.
 
 308 TEXT-BOOK OF GEOLOGY 
 
 from them. Hence in the following enumeration they are 
 included as common accompaniments of the schists. Quart- 
 zite also may be placed in this subdivision, though in its 
 typical condition it shows no schistose structure. 
 
 The origin of the crystalline schists has been the subject 
 of. long discussion among geologists. Werner held that, 
 like other rocks of high antiquity, they were chemical pre- 
 cipitates from a universal ocean. Hutton and his followers 
 maintained that they were mechanical aqueous sediments 
 altered by subterranean heat. These two doctrines in vari- 
 ous modifications are still maintained by opposite schools. 
 In recent years much light has been thrown upon the origin 
 of the schistose structure, which has been shown to be in 
 many cases due to the mechanical crushing and chemical 
 readjustment and recrystallization of the materials of both 
 sedimentary and igneous rocks. This subject is discussed 
 in a later part of this work. (See Book IV. Part VIII.) 
 
 It is obvious that a wide series of rocks embracing vari- 
 ously altered forms of both sedimentary and igneous mate- 
 rials hardly admits of any simple system of classification. 
 Regarding them from the point of view of the nature of the 
 metamorphism they have undergone, geologists have some- 
 times grouped these rocks as resulting either from contact 
 metamorphism, that is, from the effects of the protrusion of 
 igneous matter from within the earth's interior, or from re- 
 gional metamorphism where the changes have been brought 
 about by some widespread terrestrial disturbance (Book IV. 
 Part VIII.). But this arrangement, though of value in dis- 
 cussing questions of metamorphism, has the disadvantage of 
 introducing theoretical considerations, and of placing in dif- 
 ferent groups rocks which undoubtedly present the same 
 general petrographical characters. Avoiding all disputed
 
 GEOGNOSY 309 
 
 questions as to modes of origin, I shall group the schists 
 according to their mineral characters, beginning with those 
 which are obviously only a further stage of the alteration of 
 clay-slates, and ending with the gneisses, which bear a close 
 affinity to granites. 
 
 1. ARGILLITES, ARGILLACEOUS SCHISTS, PHYLLITES. 
 The rocks included in this group may often be traced into 
 the clay-slates described on p. 235. They mark a further 
 stage of metamorphism, wherein besides mechanical defor- 
 mation there has been a more or less decided recrystalliza- 
 tion of the materials, which is demonstrated by the abundant 
 secondary mica and by the appearance of such minerals as 
 chiastolite, andalusite, staurolite, garnet, etc. When a clay- 
 slate becomes lustrous by the development of mica, it is 
 known as P h y 1 1 i t e a term which may be regarded as 
 embracing the intermediate group of rocks between normal 
 clay-slates and true mica-schists. 
 
 'Chiastolite-slate (schiste made), a clay-slate i n 
 which crystals of chiastolite have been developed, even 
 sometimes side by side with still distinctly preserved grap- 
 tolites or other organic remains 8 " 1 (Skiddaw, Aberdeenshire, 
 Brittany, the Pyrenees, Saxony, Norway, Massachusetts, 
 etc.). Staurolite-slate, a micaceous clay-slate with 
 crystals of staurolite (Banffshire, Pyrenees). Ottrelite- 
 slate, a clay-slate marked by minute, six-sided, grayish 
 or blackish green lamellae of ottrelite (Ardennes, where it is 
 said to contain remains of trilobites, Bavaria, New England). 
 Di pyre-slate is full of small crystals of dipyre. Seri- 
 ci t e-p hy Hi te is a name proposed by Lossen for those 
 compact, greenish, reddish, or violet sericite-schists in which 
 the naked eye can no longer distinguish the component min- 
 erals. M i c a-p h y 1 1 i t e (phyllade gris feuiltete of Du- 
 mont), a silky, usually very fissile slate, with minute scales 
 of mica. German petrographers have distinguished by name 
 some other varieties found in metamorphic areas and char- 
 acterized by different kinds of concretions, but to which no 
 
 212 A good illustration of this association is figured by Kjerulf in his "Geo- 
 logie des Siidlichen und Mittleren Norwegen," Plate xiv. fig. 246. See also 
 Brogger's memoir on Upper Silurian fossils among the crystalline rocks of Ber- 
 gen. Christiania, 1882. A similar association occurs in the graptolite-shalea 
 next the granite of Galloway, Scotland.
 
 310 TEXT-BOOK OF GEOLOGY 
 
 special designations have been given in English. K n o- 
 tense hiefer (Knotted schist) contains little knots or con- 
 cretions of a dark-green or brown, fine-granular, faintly 
 glimmering substance, of a talcose or micaceous nature, im- 
 bedded in a finely-laminated matrix of a talc-like or mica- 
 like mineral." 18 These aggregations appear to be in many 
 cases incipient stages in the formation ot definite crystals of 
 such minerals as andalusite. In Fruchtschiefer the 
 concretions are like grains of corn ; in Garbenschiefer, 
 like caraway seeds ; in Fleckschiefer, like flecks or 
 spots. Some of these rocks might be included with the 
 mica-schists, into varieties of which they seem to pass. 
 Eound some of the eruptive diabase of the Harz, the clay- 
 slates have been altered into various crystalline masses to 
 which names have been attached. Tbus Spilosite is a 
 greenish, schistose rock, composed of finely granular or com- 
 pact felspathic material, with small chlorite concretions or 
 scales. Desmosite is a schistose mass in which similar 
 materials are disposed in more distinct alternations. 814 
 
 2. QUARTZ KocKS." 16 Quartz-schist (schistose quartzite), 
 an aggregate of granular (or granulitic) quartz with a suffi- 
 cient development of fine folia of mica to impart a more or 
 less definitely schistose structure to the rock. The disap- 
 pearance of the mica gives quartzite, and the greater promi- 
 nence of this mineral affords gradations into mica-schist. 
 Such gradations are quite analogous to those among recent 
 sedimentary materials from pure sand, through muddy sand, 
 and sandy mud, into mud or clay, and between sandstones 
 and shales. The Highlands of Scotland, for instance, em- 
 brace large tracts of quartz-schists-rocks which are not prop- 
 erly either mica-schist or ordinary quartzite. They consist 
 of granular (granulitized) quartz, with fine parallel laminae 
 of mica, and are capable of being split into thick or thin 
 flagstones. Interstratified pebbly varieties occur. 
 
 Itacolumite a schistose quartzite, in which the 
 quartz-granules are separated by fine scales of mica, talc, 
 
 418 A, von Lasaulx, Neues Jahrb. 1872, p. 840. K. A. Lossen, Z. Deutsch, 
 Geol. Ges. 1867, p. 585 (where a detailed description of the Taunus phyllitea 
 will be found), 1872, p. 757. 
 
 814 Other names are Bandschiefer, Contactschiefer, etc. See K. A. Lessen, 
 Zeitach. Deutsch. Geo. Gs. xix. (1867), p. 509, xxi. p. 291, xxiv. p. 701. 
 Kayser, op. cit xxii. p. 103. 
 
 " J. Maculloch, Trans. Geol. Soc. 1st ser. ii. (1814), p. 450, iv. {1817), p . 
 264; 2d ser. i. (1819), p. 53. Lossen, Zeitsch. Deutsch. Geol. Ges. lix. (1867), 
 pp. 615-634.
 
 GEOGNOSY 311 
 
 chlorite, and sericite. Occasionally these pliable scales are 
 so arranged as to give a certain' flexibility to the stone 
 (flexible sandstone). This rock occurs in the southeastern 
 States of North America; also in Brazil, as the matrix in 
 which diamonds are found. 
 
 Siliceous schist (Lydian stone, Lydite, Kiesel- 
 schiefer) has already been described (p. 268) among the 
 stratified rocks; but ilralso occurs among the crystalline 
 schists, sometimes as the result of the pulverization of 
 quartzose rocks (mylonite). 
 
 Quartzite (Quartz-rock), though not properly a schistose 
 rock, may be most conveniently considered here, as it is so 
 constant an accompaniment of the schists, and, like them, 
 can often be directly traced to the alteration of former sedi- 
 mentary formations. It is a granular to compact mass of 
 quartz, generally white, sometimes yellow or red with a 
 characteristic lustrous fracture. It occurs in thin and thick 
 beds in association with schists, sometimes in continuous 
 masses several thousand feet thick. In Scotland it forms 
 
 Fig. 37. Contorted Micaceous-schist, Pig. 38. Microscopic Structure of 
 
 as seen under the microscope with Quartzite. (Magnified 30 diame- 
 
 a magnifying power of 50 diameters. ters.) 
 
 ranges of mountains, and is there frequently accompanied 
 by beds of limestone, which in Sutherlandshire contain 
 Cambrian fossils. 816 
 
 Even to the naked eye, the finely granular or arenaceous 
 structure of qiiartzite is distinctly visible. Microscopic ex- 
 amination shows this structure still more clearly, and leaves 
 no doubt that the rock originally consisted of a tolerably 
 pure quartz-sand (Fig. 38). More or less distinct evidence 
 
 818 See the chapters on the Pre-Cambrian and Cambrian systems postea. 
 On the metarnorphic quartzoae rocks of Morbihan, France, see Barrels, Ann. 
 Soc. Geol. Nord, xi. (1884).
 
 312 TEXT-BOOK OF GEOLOGY 
 
 of crashing and deformation of the grains may often be ob- 
 served, likewise proof of the transfusion of a siliceous cement 
 among the particles. This cement was probably produced 
 by the solvent action of heated water upon the quartz grains, 
 which seem to shade off into each other, or into the interven- 
 ing silica. It is owing, no doubt, to the purely siliceous 
 character of the grains that the blending of these with the 
 surrounding cement is so intimate as often to give the rock 
 an almost flinty homogeneous texture. That quartzite, as 
 here described, is an original sedimentary rock, and not a 
 chemical deposit, is shown not only by its granular texture, 
 but by the exact resemblance of all its leading features to 
 ordinary sandstone false-bedding, alternation of coarser and 
 finer layers, worm-burrows, and fucoid-casts. The lustrous 
 fracture that distinguishes this rock from sandstone is due 
 to the exceedingly firm cohesion of the component grains, 
 which break across rather than separate, and to the conse- 
 quent production of innumerable minute clear vitreous sur- 
 faces of quartz. A sandstone, on the other hand, has its 
 grains so loosely coherent that when the rock is broken the 
 fracture passes between them, and the new surface obtained 
 presents innumerable dull rounded grains. 
 
 Besides occurring in alternation with schists, quartzite 
 is also met with locally as an altered form of sandstone, 
 which, when traversed by igneous dikes, is indurated for a 
 distance of a few inches or feet from the intrusive mass. 
 These local productions of quartzite show the characteristic 
 lustrous fracture, and have not yet been distinguished by 
 the microscope from the quartz-rock of wide metamorphic 
 regions. There is yet another condition under which this 
 rock, or one of analogous structure, may be seen. Highly 
 silicated bands, having a lustrous aspect, fine grain, and 
 great hardness, occur among the unaltered shales and other 
 strata of the Carboniferous system. In such cases the sup- 
 position of any general metamorphism being inadmissible, 
 we may infer either that these quartzose bands have been 
 indurated, for example, by the passage through them of 
 thermal silicated water, or that they are an original for- 
 mation. 
 
 Schistose Conglomerate Rocks. In some regions of schists, 
 not only bands of quartzite occur, representing former sand- 
 stones, but also pebbly or conglomeratic bands, in which 
 pebbles of quartz and other materials from less than an inch 
 to more than a foot in diameter are imbedded in a foliated 
 matrix, which may be phyllite, mica-schist, gneiss, quart-
 
 GEOGNOSY 313 
 
 zite, etc. 317 Examples of this kind are found in the pass 
 of the Tete Noire between Martigny and Chamouni, in the 
 Saxon granulite region, in the Bergen region of Norway, in 
 the northwest of France, in northwest Ireland, in the islands 
 of Islay and Garvelloch, and in Perthshire and other parts 
 of the central Highlands of Scotland. The pebbles are not 
 to be distinguished from the water- worn WOCKS of ordinary 
 conglomerates; but the original matrix which incloses them 
 has been so altered as to acquire a micaceous foliated struc- 
 ture, and to wrap the pebbles round as with a kind of glaze. 
 These facts, like those already referred to in the structure 
 of quartzite and argillaceous and quartz-schist, are of con- 
 siderable value in regard to the theory of the origin of some 
 crystalline schists. 
 
 3. PYROXENE-ROCKS. Augite-schist a fine-grained schis- 
 tose aggregate of pale or dark-green augite, with sometimes 
 quartz, plagioclase, magnetite, or chlorite; found rarely 
 among the crystalline schists. Among the schistose rocks 
 of the Taunus, Lessen has described some interesting varie- 
 ties under the name of Augite-schist (Augitschiefer). They 
 are green, compact, sometimes soft and yielding to the 
 finger-nail, usually distinctly schistose, and interbedded with 
 the gneisses and schists. They are composed of a fine dull 
 diabase-like ground-mass, through which are dispersed crys- 
 tals of augite, 1 to 2 mm. in length, which in the typical 
 varieties are the only components distinctly recognizable by 
 the naked eye." 18 Aogite-rock a granular aggregate of augite 
 (with tourmaline, sphene, scapolite, etc.), found in beds in 
 the Laurentian limestone of Canada. Malacolite-rock 
 is a pale granular to compact, or even fibrous aggregt^te of 
 malacolite found in beds in crystalline limestone (Riesenge- 
 birge). Schistose Cabbro a granular to schistose aggregate of 
 plagioclase and diallage, occurs in lenticular bands among 
 the amphibolites and granulites of the crystalline schists. 
 The diailage may occur in conspicuous crystals, and is some- 
 times associated with abundant olivine, as in ordinary gab- 
 bro (p. 268). 2 " 
 
 817 Prof. "Wichmann describes some curious examples of serpentine conglom- 
 erates. See his paper in "Beitrage zur Geologie Ost-Asiens und Australiens, " 
 ii. pp. 35, 111. On the conglomerate- schists of Saxony, see A. Sauer, "GeoL 
 Specialkarte Sachsen," Sect. "Elterlein," also Lehmann'a "Altkryst. Schiefer- 
 gesteine," p. 124. Reusch, "Silurfossiler og Pressede Konglomerater, " Chris- 
 tiania, 1882. Barrois, Ann. Soc. Geol. Nord. xi. 1884. 
 
 818 Lessen, Zeitsch. Deutscu. Geol. Ges. xix. (1867), p. 598. 
 
 219 Rocks of this character occur in the Saxon "Granulitgebirge" and also 
 GEOLOGY Yol. XXIX 14
 
 314 TEXT-BOOK OF GEOLOGY 
 
 These pyrpxenic intercalations among the schists, like 
 the hornblendic and olivine bands mentioned below, seem to 
 represent bands of igneous material (lavas or tuffs) either 
 erupted contemporaneously with the deposition of the origi- 
 nal material of the schists, or subsequently intruded into it, 
 and thereafter exposed to the metamorphism which produced 
 the foliation of the schists. 
 
 4. HORNBLENDE-ROCKS. Amphibolites a name applied to 
 a group of rocks, composed mainly of hornblende, some- 
 times schistose, sometimes thick-bedded. Besides the horn- 
 blende, numerous other minerals, such as are common among 
 the schists, likewise occur orthoclase, plagioclase, quartz, 
 augite and varieties, garnet, zoisite, mica, rutile, etc. 
 Where the rock is schistose, it becomes an amphibolite-schist 
 or hornblende-schist: or if the hornblende takes the form of ac- 
 tinolite, Actinolite-schist. Glaucophane-schist 
 a bluish-gray or black rock, in which the hornblende oc- 
 curs in the form of glaucophane, forms large masses in the 
 Southern Alps, and occurs locally in Anglesey. Where an 
 amphibolite is not schistose, it used to be termed hornblende- 
 rock. Nephrite (Jade) is a compact, extremely finely 
 fibrous variety. The presence of other minerals in notice- 
 able quantity may furnish names for other varieties. Thus, 
 where plagioclase (and some orthoclase) occurs, the rock be- 
 comes a F elsp ar-am p h i bo 1 i t e, Dioritic amphib- 
 olite, or Di o rite-schist. 820 Amphibolites occur as 
 bands associated with gneiss and other schistose formations. 
 It was suggested by Jukes that they may possibly represent 
 former beds of hornblendic or augitic lava and tuff, which 
 have been metamorphosed together with the strata among 
 which they were intercalated. This suggestion has received 
 confirmation from the researches of the Geological Survey 
 in the north of Scotland and in Ireland, where what were 
 doubtless originally pyroxenic masses erupted prior to the 
 metamorphism of the region, have had their augite changed 
 by paramorphism into hornblende, and have partially as- 
 sumed a foliated structure, passing into Epidiorite, Epi- 
 d i o r i t e-s c h i s t, amphibolite-schists, and even serpentine. 
 
 in Lower Austria. F. Becke, Tschermak's Min. Mitth. IV. p. 352. J. Leh- 
 mann's "Untersuchungen iiber die Entstehung der Altkrystallinischen Schiefer- 
 gesteine," Bonn, 1884, p. 190. On the diabase-schists of the Taunus, see L. 
 Milch, Zeitsch. Deutsch. Geol. Ges. xli. (1889), p. 394. 
 
 ** See F. Becke, Tschermak's Min. Mitth. IV. p. 233. The author likewise 
 distinguishes diallage-amphibolite, garaet-amphibolite, salite-amphibolite, zoisite- 
 amphibolite.
 
 GEOGNOSY 315 
 
 The connection of some schists with original masses of dio- 
 rite, gabbro, and diabase has been pointed out by Lehmann 
 and subsequently by many other observers." 1 
 
 5. G-ARNET-Rpcks. Eclogite, one of the most beautiful 
 members of the crystalline-schist series, is a granular aggre- 
 gate of grass-green omphacite (pyroxene) and red garnet, 
 through which are frequently dispersed bluish kyanite and 
 white mica. It occurs in bands in the Archa3an gneiss and 
 mica-schist. To those varieties where the kyanite becomes 
 predominant, the name of K y a n i t e-r o c k has been given. 
 Garnet-rock is a crystalline-granular rock composed 
 mainly of garnet, with hornblende and magnetite; by the 
 diminution of the garnet it passes into an amphibolite. 
 Kinzigite a crystalline schistose rock, composed of 
 plagioclase, garnet, and black mica, found in the Black 
 Forest (Kinsig) and the Odenwald. 
 
 6. EPIDOTE-ROCKS. Epidosite (Pistacite-rock) an aggre- 
 gate of bright green epidote with some quartz, occurs with 
 chlorite-schist (Canada), with granite and serpentine (Elba), 
 and with syenite. E p i d o t e-s c h i s t, a schistose greenish 
 rock, with silvery lustre on the foliation surfaces, composed 
 of epidote, sericite, magnetite, quartz, calcite, plagioclase, 
 and specular iron. 4 " 
 
 7. CHLORITE-ROCKS. Chlorite-schist a scaly schistose ag- 
 gregate of greenish chlorite, usually with quartz and often 
 with felspar, talc, mica, or magnetite, the last-named min- 
 eral frequently appearing in beautifully perfect disseminated 
 octohedra. Occurs with gneiss and other schists in evenly 
 bedded masses. 
 
 8. TALC-ROCKS. Talc-schist a schistose aggregate of scaly 
 talc, often with quartz, felspar, and other minerals; having 
 an unctuous feel, and white or greenish color. Occurs some- 
 what rarely in beds associated with mica-schist and clay- 
 slate, and frequently contains magnetite, chlorite, mica, 
 kyanite, and other minerals, including carbonates. A 
 massive variety, composed of a finely felted aggregate of 
 
 221 "Untersuchungen iiber die Entstehung der Altkrystall Schief. " See also 
 Giimbel, "Die Palaolitischen Eruptivgesteine des Fichtelgebirges, " Munich, 
 1874, p. 9; Teall, Quart. Journ. Geol. Soc. xli. (1883), p. 133; "British Petro- 
 graphy," p. 198. Hatch, Mem. Geol. Survey, Explanation of Sheets, 138, 139, 
 Ireland, p. 49. Hyland, Mem. Geol. Survey, Explanations of Northwest Done- 
 gal, and of Southwest Donegal, Petrographical appendices, also postea, Book 
 IV. pt. viii. G. H. Williams, Bull. U. S. Geol. Surv. No. 62, 1890. 
 
 422 See Wichmann on Rocks of Timor, "Beitrage zur Geologie Ost-Asiens 
 und Australiens," II. part 2, p. 97, Leyden, 1884.
 
 316 TEXT-BOOK OF GEOLOGY 
 
 scales of talc, with chlorite and serpentine, is called 
 Potstone (Topfstein). Many rocks with a soapy or 
 unctuous feel have been classed as talc-schist, which. con- 
 tain no talc, but a variety of mica (sericite-schist, etc.). 
 Talc-schist, though not specially abundant, occurs in con- 
 siderable mass in the Alps (Mont Blanc, Monte Rosa, 
 Carinthia, etc.), and is found also among the Apennine 
 and Ural mountains. 
 
 9. OLIVINE-ROCKS, or PERIDOTITES of the Crystalline 
 Schists."" Eocks of which olivine forms a main constit- 
 uent, occur as subordinate bands or irregular masses asso- 
 ciated with gneisses and other schistose rocks. They were 
 probably eruptive masses, contemporaneous with or subse- 
 quent to the surrounding gneisses and schists (p. 314). The 
 olivine is commonly associated with some pyroxenic min- 
 eral, hornblende, garnet, etc. Some of the rocks mentioned 
 on p. 300 may also be included here. Dunite, for example, 
 which occurs in apparently eruptive form at Dun Mountain, 
 near Nelson, New Zealand, is found in North Carolina in 
 beds with laminated structure intercalated in hornblende- 
 gneiss. Many of these rocks have undergone much crush- 
 ing and deformation, and pass into foliated forms of 
 Serpentine, which must thus be reckoned as one of the 
 schistose as well as one of the eruptive series. Some re- 
 markable schistose serpentines occur interbedded among 
 phyllites, mica-schists, and limestones in Banffshire. 
 
 10. FELSITOID-EOCKS. These are distinguished by an 
 exceedingly compact felsite-like matrix. They occur in 
 beds or bed-like masses, sometimes in districts of contact 
 metamorphism, sometimes associated with vast masses of 
 schists. 
 
 Halleflinta an exceedingly compact, hornstone-like, fel- 
 sitic, gray, yellowish, greenish, reddish, brownish, or black, 
 rock, composed of an intimate mixture of microscopic par- 
 ticles of felspar and quartz, with fine scales of mica and 
 chlorite. It breaks with a splintery or conchoidal fracture, 
 presents under the microscope a finely-crystalline structure, 
 occasionally with nests of quartz, and is only fusible in fine 
 splinters before the blow-pipe. Some of the rocks to which 
 this name has been applied are probably felsitic lavas; 
 others, though externally presenting a resemblance to fel- 
 
 823 See Tschermak, Sitzb. Akad. Wissen., Vienna, Ivi. (1867). F. Becke, 
 Tschermak's Min. Mitth. IV. (1882), p. 322. E. Dathe, Neues Jahrb. 1876, 
 pp. 265-337.
 
 GEOGNOSY 317 
 
 site, occur in beds intimately associated with foliated rocks 
 (Norway), and may be metamorphic products (perhaps al- 
 tered fine sediments) due to the same series of changes that 
 gave rise to the crystalline schists among which they lie. 2 "* 
 
 Adinole (Adinole-schist) a rock externally resembling 
 the last, but distinguished from it by its greater fusibility. 
 It is an intimate mixture of quartz and albite, containing 
 about ten per cent of soda. It is a product of alteration, 
 being found among the altered Carboniferous shales around 
 the eruptive diabases of the Harz, in the altered Devonian 
 rocks of the Taunus, and in the altered Cambrian rocks of 
 South Wales." 6 
 
 Porphytoid a name bestowed upon certain rocks composed 
 of a felsite-like ground-mass which has assumed a more or 
 less schistose structure from the development of micaceous 
 scales, and which contains porphyritically scattered crystals 
 of felspar and quartz. The felspar is either orthoclase or al- 
 bite, and may be obtained in tolerably perfect crystals. The 
 quartz occasionally presents doubly terminated pyramids. 
 The micaceous mineral may be paragonite or sericite. Por- 
 phyroid occurs in circumstances which suggest considerable 
 mechanical deformation, as among the schistose rocks of 
 Saxon v," 8 in the Paleozoic area of the Ardennes," 7 as- well 
 as in Westphalia and other parts of Europe.** 8 Some por- 
 pbyroids are probably sheared forms of quartz-porphyry, 
 felsite, or some similar rock; others may be more of the 
 nature of tuffs. 
 
 11. QUARTZ- AND TOUBMALINE-ROCKS. Tourmaline-schist 
 (Schorl -schist, schorl-rock), a blackish, finely granular, 
 quartzose rock with abundant granules and needles of 
 black tourmaline (schorl), which occurs as one of the 
 products of contact-metamorphism in the neighborhood of 
 some granites (Cornwall). 
 
 12. QUARTZ- AND MICA-BOCKS. Mica-schist (Mica-slate, 
 Glimmerschiefer), a schistose aggregate of quartz and mica, 
 
 224 For analyses see H. Santesson, "Kemiska Bergsartanalyser," 8vo, Stock- 
 holm, 1877. 
 
 226 Lessen, Zeitsch. Deutsch. Geol. Gesel. xix. (1867), p. 573. See also 
 Quart. Journ. Geol. Soc. xxxix. (1883), pp, 302, 320. Rosenbusch, "Mikro- 
 skopische Physiographic, " ii. p. 235. F. Posepny, Tschermak's Mineral. Mitth. 
 x. 175. 
 
 288 Rothpletz, Geol. Survey Saxony, Explanation of Section Rochlitz. 
 
 221 De la Vallee Poussin and Renard, Mem. Couronnees Acad. Roy. Belg. 
 1876, p. 85. 
 
 228 Lessen, Site. Gesellsch. Naturf. Freunde, 1883, No. 9.
 
 318 TEXT-BOOK OF GEOLOGY 
 
 the relative proportions of the two minerals varying widely 
 even in the same mass of rock. Each is arranged in lentic- 
 ular wavy laminae. The quartz shows great inconstancy 
 in the n amber and thickness of its folia. It often presents 
 a granular character, like that of quartz-rock, or passing into 
 granulite. The mica lies in thin plates, sometimes so dove- 
 tailed into each other as to form long continuous irregular 
 crumpled folia, separating the quartz layers, and often in 
 the form of thin spangles and membranes running in the 
 C[uartz. (Figs. 36 and 37.) As the rock splits open along 
 its micaceous folia, the quartz is not readily seen save in- 
 a cross fracture. 
 
 The mica in typical mica-schist is generally a white 
 variety; but it is sometimes replaced by a dark species. 
 In many lustrous, unctuous schists which are now found 
 to have a wide extent, the silvery foliated mineral is ascer- 
 tained to be a mica (margarodite, damourite, etc.), and not 
 talc, as was once supposed. These were named by Dana 
 hydro-mica-schists. Among the accessory minerals, garnet 
 (specially characteristic), schorl, felspar, hornblende, kya- 
 nite, staurolite, chlorite, and talc may be mentioned. Mica- 
 schist readily passes into other members of the schistose 
 family. By addition of felspar, it merges into gneiss. By 
 loss of quartz and increase 01 chlorite, it passes into chlorite- 
 schist, and by loss of mica, into quartz-schist and quartzite. 
 By failure 01 quartz and diminution of mica, with an in- 
 creasing admixture of calcite, it may shade into calc-mica- 
 schist (see below), and even into marble. Mica-schist varies 
 in color mainly according to the hue of its mica. 
 
 Mr. Sorby has stated that thin slices of some mica-schists, 
 when examined under the microscope, show traces of orig- 
 inal grains of quartz-sand and other sedimentary particles 
 of which the rock at first consisted. He has ajso found 
 indications of what he supposes to have been current- 
 bedding or ripple-drift, like that seen in many fine sedi- 
 mentary deposits, and he concludes that mica-schist is a 
 crystalline metamorphosed sedimentary rock. 229 In many, 
 if not in most cases, however, the foKation does not corre- 
 spond with original bedding, but with structural planes 
 (cleavage, faulting) superinduced by pressure, tension, or 
 
 229 Q. J. Geol. Soc. (1863), p. 401, and his address in vol. xxxvi. (1880), p. 85. 
 The apparent current-bedding of many granulitic and other metarnorphic rocks 
 is certainly deceptive, and must be due to planes of shearing or slipping in the 
 mechanical movements which produced the metamorphism.
 
 GEOGNOSY 319 
 
 otherwise, upon rocks which may not always have been 
 of sedimentary origin. 
 
 Among the varieties of mica-schist may be mentioned 
 S e r i c i t e-s c h i s t (which may be also included among the 
 phyllites), composed of an aggregate of fine folia of the 
 silky variety of mica called sericite, in a compact honestone- 
 like quartz; Paragonite-schist, where the mica is the 
 hydrous soda variety, paragonite; Gr n eiss-mica-sc hi st, 
 containing dispersed kernels of orthoclase. Some of these 
 rocks contain little or no quartz, the place of which is 
 taken by felspar. Gal c-m i c a-s c h i s t, a schistose calca- 
 reous rock, which in many, if not in all cases, was origi- 
 nally a limestone with more or less muddy impurity. The 
 carbonate of lime has assumed a granular-crystalline form, 
 while the aluminous silicates have recrystallized as fine 
 scales of white mica. * Tremolite, zoisite, and other min- 
 erals are not infrequent in this rock. 
 
 Normal mica-schist, together with other schistose rocks, 
 forms extensive regions in Norway, Scotland, the Alps, 
 and other parts of Europe, and vast tracts of the " Archa3an" 
 regions of North America. Some of its varieties are also 
 found encircling granite masses (Scotland. Ireland, etc.) as 
 a zone or aureole of contact-metamorphism from a few yards 
 to a mile or so broad, which shades away into unaltered 
 graywacke or slate outside. In these cases, mica-schist is 
 unquestionably a metamorphosed condition of ordinary 
 sedimentary strata, the change being connected with the 
 extravasation of granite. (Book IV. Part VIII.) 
 
 Though the possession of a fissile structure, showing 
 abundant divisional surfaces covered with glistening mica, 
 is characteristic of mica-schist, we must distinguish between 
 this structure and that of many micaceous sandstones which 
 can be split into thin seams, each splendent with the sheen 
 of its mica-flakes. A little examination will show that in 
 the latter case the mica has not crystallized in situ, but 
 exists merely in the form of detached worn scales, which, 
 though lying on the same general plane, are not welded 
 into each other as in a schist; also that the quartz does not 
 exist in folia but in rounded separate grains. 
 
 13. QUARTZ- AND FELSPAR-KOCKS. The replacement of 
 the mica of a mica-schist by felspar, or the disappearance 
 of the mica from a gneiss, gives rise to an aggregate of fel- 
 spar and quartz. Such a rock may be observed in thin 
 bands or courses, alternating with the surrounding mass. 
 In mineral composition, it may be compared to the quartz-
 
 320 TEXT-BOOK OF GEOLOGY 
 
 porphyries or granite-porphyries of the massive rocks, but 
 it is usually distinguishable by a more or less foliated struc- 
 ture, and by the absence of felsitic ground-mass. 
 
 14. QUARTZ-, FELSPAR-, AND MICA-ROCKS. Gneiss. 
 This name, formerly restricted to a schistose aggregate of 
 orthoclase (sometimes micro line or a plagioclastic felspar, 
 either separate or crystallized together), quartz, and mica, 
 is now commonly employed in a wider sense to denote the 
 coarser schists which so often present granitoid characters."* 
 Many gneisses, indeed, differ from granite chiefly in the 
 foliated arrangement of the minerals. The quartz some- 
 times contains abundant liquid inclusions, in which liquid 
 carbon-dioxide has been detected. The relative proportions 
 of the minerals, and the manner in which they are grouped 
 with each other, present great variations. As a rule, the 
 folia are coarser, and the schistose character less perfect 
 than in mica-schist. Sometimes the quartz lies in tolerably 
 pure bands, a foot or even more in thickness, with plates 
 of mica scattered through it. These quartz layers may be 
 replaced by a crystalline mixture of quartz and felspar, 
 or the felspar will take the form of independent lenticular 
 folia, while the laminae of mica which lie so abundantly in 
 the rock give it its fissile structure. The felspar of many 
 gneisses presents under the microscope a remarkable fibrous 
 structure, due to the crystallization of fine lamella of some 
 plagioclase (albite or oligoclase) in the main mass of ortho- 
 clase or microcline. a81 Among the accessory minerals, gar- 
 net, tourmaline or schorl, hornblende, apatite, graphite, py- 
 rites, and magnetite may be enumerated. 
 
 There can be no doubt that many gneisses owe their 
 characteristic schistose structure to the crushing and shear- 
 ing of some original eruptive rock such as granite. In- 
 stances, however, occur where the materials are segregated 
 in bands which so closely resemble those of true flow-struc- 
 ture or segregation in igneous bosses and sheets as to suggest 
 that they may possibly have resulted from the movement 
 of a still unconsolidated eruptive mass (pp. 306, 1022). 
 Analogies to such structures may be observed among 
 ancient and modern lavas. 
 
 830 See Kalkowsky's "Gneissformation des Eulengebirges, " Leipzig, 1878; 
 Lehmann's "Altkrystallinische Schief ergesteine, " 1884; F. Becke, Tschermak'g 
 Min. Mitth. 1882, p. 194; E. "Weber, op. cit. 1884, p. 1, and postea Book IV. 
 Part VIII. ii. and Book VI. Pre-Cambrian. 
 
 P. Becke (Tscharmak's Min. Mitth. 1882, (iv.) p. 18) described thto 
 structure and named it micropertMte.
 
 GEOGNOSY 321 
 
 Many varieties of gneiss occur. Some are distinguished 
 by peculiarities of structure or composition, as Granite- 
 gneiss, where the schistose arrangement is so coarse as 
 to be unrecognizable, save in a large mass of the rock; 
 Diorite-gneiss, gabbr o-g n e i s s, composed of the 
 materials of a diorite or gabbro but with a coarsely schis- 
 tose structure; Porphyritic gneiss or Augen- 
 gneiss, in which large eye-like kernels of orthoclase or 
 quartz are dispersed through a finer matrix and represent 
 larger crystals or crystalline aggregates which have been 
 broken down and dragged along by shearing movements in 
 the rock. Other varieties are named from the occurrence 
 in them of one or more distinguishing minerals, as Horn- 
 blende-gneiss (syenitic gneiss), in which hornblende 
 occurs instead of or in addition to mica; Protogine- 
 gneiss, where the ordinary mica is altered into chlorite or 
 a talc-like substance; Serici te-g ne iss, a schistose ag- 
 gregate of sericite, albite, (juartz, with less frequently white 
 and black mica and a chloritic mineral ; 23a A u g 1 1 e-g n e i s s, 
 containing an augitic mineral (not of the diallage group) and 
 potash-felspar or potash-soda-felspar or scapolite, with horn- 
 blende (which has often crystallized parallel with the augite), 
 brown mica, more or less quartz, and also frequently with 
 garnet, calcite, titanite, etc.;" 3 P 1 ag i oclas e-g n ei ss, 
 with plagioclase more abundant than orthoclase, sometimes 
 containing hornblende, sometimes augite; Cordierite- 
 gneiss, with the bluish vitreous mineral cordierite. 
 
 The most typical gneisses occur among the so-called 
 "Archaean rocks," of which they form the leading type, 
 and where they probably represent original eruptive rocks. 
 (See Book VI.' Part L) They cover considerable areas in 
 Scandinavia, N. W. Scotland, Bohemia, Bavaria, Erzge- 
 birge, Moravia, Central Alps, Canada, etc. But rocks to 
 which the name of gneiss cannot be refused appear also 
 among the products of the metamorpliism of various strati- 
 fied formations. Such are the gneisses associated with iriany 
 other crystalline schists among the altered Cambrian and 
 Silurian rocks of Scotland, Norway, and New England, 
 
 m K. A. Lessen, Zeitsch. Deutsch. Geol. Ges. xix. (1867), p. 565. 
 
 838 The occurrence of augite as an abundant constituent of some gneisses 
 has been made known by microscopic research. Rocks of this naiure occur in 
 Sweden (A. Stelzner, N. .Tahrb. 1880 (ii.), p. 103), and have been fully de- 
 scribed from Lower Austria (P. Becke, Tschermak's Min. Mitth. 1882 (iv.), pp. 
 219-365). They are likewise well developed among the oldest gneisses of the 
 northwest of Sutherland in Scotland.
 
 322 TEXT-BOOK OF GEOLOGY 
 
 the altered Devonian rocks of the Taunus, and other re- 
 gions, which will be described in Book IV. Part VIII. 
 Some of these may also be eruptive granites, diorites t 
 etc., which have undergone shearing and have acquired 
 a schistose character. 
 
 15. QUARTZ-, FELSPAR-, AND GARNET-ROCKS. Cranolite"* 
 (Eurite-schistoide, Leptynite of French authors, Weiss-stein) 
 a fine-grained granular aggregate of pale reddish, yellow- 
 ish, or white felspar with quartz and small red garnets, oc- 
 casionally with kyanite, biotite, and microscopic rutile and 
 tourmaline. The felspar, which is the predominant constit- 
 uent, presents the peculiar fibrous structure referred to in 
 the foregoing description of gneiss (microperthite, micro- 
 cline), and appears seldom to be true orthoclase. The 
 quartz is conspicuous in thin partings between thicker more 
 lelspathic bands, giving a distinctly fissile bedded character 
 to the mass. A dark variety, interstratified with the normal 
 rock, is distinguished by the presence of microscopic augite 
 or diallage (Augitgranulite of Saxony). Granulite occurs in 
 bands among the gneiss and other members of the crystal- 
 line schist series in Saxony, Bohemia, Lower Austria, the 
 Vosges, and Central France. The term "granulite" is also 
 employed in a structural sense to denote a rock which has 
 been crushed down by dynamic metamorphism, and has ac- 
 quired this characteristic fine granular structure. (See pp. 
 177, 211.) 
 
 16. FELSPAR- AND MICA-ROCKS. Rocks composed essen- 
 tially of a schistose aggregate of minutely scaly mica with 
 some felspar, quartz, andalusite, or other mineral, occur in 
 regions of metamorphism. Cornubianite was a name 
 proposed bv Boase for a rock composed of a felspar base, 
 witn abundant mica. 8 " It is found around the granite of 
 Cornwall, of which it is a metamorphic product. By some 
 writers this rock has been associated with the gneisses, but 
 it is distinguished by the scarcity or absence of quartz. 
 
 884 Michel-LeVy has proposed to reserve the names "Leptynite" for schistose 
 and "Granulite" for eruptive rocks. Bull. Soc. Geol. France, 3d ser. ii. pp. 
 177, 189, iii. p. 287, iv. p. 730, vii. p. 760; Lory, op. cit. viii. p. 14. Scheerer, 
 Neues Jahrb. 1873, p. 673. Dathe, N. Jahrb. 1876, p. 225; Z. Deutsch. GeoL 
 Ges. 1877, p. 274. Details regarding the great development of the granulite of 
 Saxony (Granulitgebirge) will be found in the explanatory pamphlets published 
 with the sheets of the Geological Survey of Saxony, especially those of sections 
 Rochlitz, Geringswalde and Waldheim. The history of the origin of granulite 
 is discussed by J. Lehmann, "Untersuchungen iiber die Entstehung der Altkry- 
 Btall. Schiefergesteine." 
 
 236 "Geology of Cornwall" (1832), pp. 226, 230.
 
 GEOGNOSY 
 
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 TEXT-BOOK OF QEOLOGY 
 
 BOOK HI 
 
 DYNAMICAL GEOLOGY 
 
 DYNAMICAL GEOLOGY investigates the processes 
 of change at present in progress upon the earth, 
 whereby modifications are made on the structure 
 and composition of the crust, on the relations between the 
 interior and the surface, as shown by volcanoes, earth- 
 quakes, and other terrestrial disturbances, on the distri- 
 bution of land and sea, on the outlines of the land, on the 
 form and depth of the sea -bottom, on marine currents, and 
 on climate. Bringing before us, in short, the whole range 
 of geological activities, it leads us to precise notions regard- 
 ing their relations to each other, and the results which they 
 achieve. A knowledge of this branch of the subject is thus 
 the essential groundwork of a true and fruitful acquaintance 
 with the principles of geology. The study of the present 
 order of nature provides a key for the interpretation of the 
 past. 
 
 The operations considered by Dynamical Geology may 
 be regarded as a vast cycle of change, into the investigation 
 of which the student may break at any point, and round 
 which he may travel, only to find himself brought back to 
 his starting-point. It is a matter of comparatively small mo- 
 ment at what part of the cycle the inquiry is begun. The 
 changes seen in action will always be found to have resulted 
 from some that preceded, and to give place to others that 
 follow them.
 
 DYNAMICAL GEOLOGY 325 
 
 At an early time in the earth's history, anterior to any 
 of the periods of which a record remains in the visible rocks, 
 the chief sources of geological energy probably lay within 
 the earth itself. The planet still retained much of its initial 
 heat, and in all likelihood was the theatre of great chemical 
 changes. As it cooled, and as the superficial disturbances 
 due to internal heat and chemical action became less marked, 
 the influence of the sun, which must always have operated, 
 and which in early geological times may have been more 
 effective than it afterward became, would then stand out 
 more clearly, giving rise to that wide circle of surface 
 changes wherein variations of temperature and the circula- 
 tion of air and water ovej* the surface of the earth come into 
 play. 
 
 In the pursuit of his inquiries into the past history and 
 into the present economy of the earth, the student must 
 needs keep his mind ever open to the reception of evidence 
 for kinds, and especially for degrees, of action which he had 
 not before encountered. Human experience has been too 
 short to allow him to assume that all the causes and modes 
 of geological change have been definitely ascertained. Be- 
 sides the fact that both terrestrial and solar energy were 
 once probably more intense than now, there may remain for 
 future discovery evidence of former operations by heat, 
 magnetism, chemical change, or other agency, that may 
 explain phenomena with which geology has to deal. "Of 
 the influences, so many and profound, which the sun exerts 
 upon our planet, we can as yet only perceive a little. Nor 
 can we tell what other cosmical influences may have lent 
 their aid in the revolutions of geology. 
 
 In the present state of knowledge, all the geological 
 energy upon and within the earth must ultimately be traced
 
 326 TEXT-BOOK OF GEOLOGY 
 
 back to the primeval energy of the parent nebula, or suii. 
 There is, however, a certain propriety and convenience in 
 distinguishing between that part of it which is due to the 
 survival of some of the original energy of the planet, and 
 that part which arises from the present supply of energy re- 
 ceived day by day from the sun. In the former case, the 
 geologist has to deal with the interior of the earth and its 
 reaction upon the surface; in the latter, he is called upon to 
 study the surface of the earth, and to some extent its re- 
 action on the interior. This distinction allows of a broad 
 treatment of the subject under two divisions: 
 
 I. Hypogene or Plutonic Action the changes 
 within the earth, caused by original internal heat and by 
 chemical action. 
 
 II. Epigene or Surface Action .the changes 
 produced on the superficial parts of the earth, chiefly by 
 the circulation of air and water set in motion by the sun's 
 heat. 
 
 PART I. HYPOGENE ACTION 
 
 An Inquiry into the Geological Changes in Progress 
 beneath the Surface of the Earth 
 
 In the discussion of this branch of the subject, it is use- 
 ful to carry in the mind the conception of a globe still in- 
 tensely hot within, radiating heat into space, and conse- 
 quently contracting in bulk. Portions of molten rocks from 
 inside are from time to time poured out at the surface. 
 Sudden shocks are generated, by which earthquakes are 
 propagated to and along the surface. Wide geographical 
 areas are upraised or depressed. In the midst of these move- 
 ments, the rocks of the crust are fractured, squeezed, 
 sheared, crumpled, rendered crystalline, and even fused.
 
 DYNAMICAL GEOLOGY 327 
 
 Section i. Volcanoes and Volcanic Action 1 
 1. Volcanic Products 
 
 The term volcanic action (volcanism or volcanicity) em- 
 braces all the phenomena connected with the expulsion of 
 heated materials from the interior of the earth to the surface. 
 Among these phenomena, some possess an evanescent char- 
 acter, while others leave permanent proofs of their existence. 
 It is naturally to the latter that the geologist gives chief at- 
 tention, for it is by their means that he can trace former 
 phases of volcanic activity in regions where, for many ages, 
 there have been no volcanic eruptions. In the operations 
 of existing volcanoes, he can observe only superficial mani- 
 festations of volcanic action. But examining the rocks of 
 the earth's crust, he discovers that amid the many terrestrial 
 revolutions which geology reveals, the very roots of former 
 volcanoes have been laid bare, displaying subterranean 
 
 1 The student is referred to the following general works on the phenomena 
 of volcanoes. Scrope, "Considerations on Volcanoes," London, 1825; "Vol- 
 canoes," London, 2d edit. 1872; "Extinct Volcanoes of Central France," Lon- 
 don, 1858; "On Volcanic Cones and Craters," Quart. Jouru. Geol. Soc. 1859. 
 Daubeny, "A Description of Active and Extinct Volcanoes," 2d edit., London, 
 1858. Darwin, "Geological Observations on Volcanic Islands," 2d edit., Lon- 
 don, 1876. A. von. Humboldt, "Deber den Bau und die Wirkung der Vul- 
 kane," Berlin, 1824. L. von Buch, "Deber die Natur der vuikanischen Er- 
 scheinungen auf den Canarischen Inseln," Poggend. Annalen (1827), ix. x. ; 
 "Ueber Erhebungskratere und Vulkane," Poggend. Annalen (1836), xxxvii. 
 E. A. von Hoff, "Geschichte der durch Ueberlieferung nachgewiesenen natur- 
 lichen Veranderungen der Erdoberflache" (part ii., "Vulkane und Erdbebeh"), 
 Gotha, 1824. C. W. C. Fuchs, "Die vuikanischen Erscheinungen der Erde," 
 Leipzig, 1865. R. Mallet, "On Volcanic Energy," Phil. Trans. 1873. J. 
 Schmidt, "Vulkanstudien," Leipzig, 1874. Sartorius von Waltershausen and 
 A. von Lasaulx, "Der Aetna," 4to, Leipzig, 1880. E. Reyer, "Beitrag zur 
 Physik der Eruptionen," Vienna, 1877; "Die Euganeen; Bau und Geschichte 
 eines Vulkanes. " Vienna, 1877. Fouque, "Santorin et ses eruptions," Paris, 
 1879. Judd, "Volcanoes," 1881. G. Mercalli, "Vulcani e Fenomeui vulcanici 
 in Italia," Milan, 1883. Ch. Velain, "Les Volcans," Paris, 1884. J. D. Dana, 
 "Characteristics of Volcanoes," 1890. "Volcanoes Past and Present," E. Hull, 
 1892. "The South Italian Volcanoes," H. J. Johnston -La vis, Naples, 1891. 
 References will be found in succeeding pages to other and more special memoirs.
 
 328 TEXT-BOOK OF GEOLOGY 
 
 phases of volcanism which could not be studied in any 
 modern volcano. Hence an acquaintance only with active 
 volcanoes will not afford a complete knowledge of volcanic 
 action. It must be supplemented and enlarged by an inves- 
 tigation of the traces of ancient volcanoes preserved in the 
 crust of the earth. (Book IV. Part VII.) 
 
 The word "volcano" is applied to a conical hill or moun- 
 tain (composed mainly or wholly of erupted materials), from 
 the summit and often also from the sides of which, hot 
 vapors issue, and ashes and streams of molten rock are in- 
 termittently expelled. The term "volcanic" designates all 
 the phenomena essentially connected with one of these chan- 
 nels of communication between the surface and the heated 
 interior of the globe. Yet there is good reason to believe 
 that the active volcanoes of the present day do not afford by 
 any means a complete type of volcanic action. The first 
 effort in the formation of a new volcano is to establish a 
 fissure in the earth's crust. A volcano is only one vent or 
 group of vents established along the line of such a fissure. 
 But in many parts of the earth, alike in the Old World and 
 the New, there have been periods in the earth's history when 
 the crust was rent into innumerable fissures over areas thou- 
 sands of square miles in extent, and when the molten rock, 
 instead of issuing, as it does at a modern volcano, in nar- 
 row streams from a central elevated cone, welled out from 
 numerous small vents along the rents, and flooded enormous 
 tracts of country without forming any mountain or conspicu- 
 ous volcanic cone in the usual sense of these terms. Of 
 these "fissure-eruptions," apart from central volcanic cones, 
 no examples appear to have occurred within the times of 
 human history, except in Iceland, where vast lava-floods 
 issued from a fissure in 1783 (pp. 378, 434). They can best
 
 DYNAMICAL GEOLOGY 329 
 
 be studied from the remains of former convulsions. Their 
 importance, however, has not yet been generally recognized 
 in Europe, though acknowledged in America, where they 
 have been largely developed. Much still remains to be 
 done before their mechanism is as well understood as that 
 of the lesser type to which all present volcanic action be- 
 longs. In the succeeding narrative an account is first pre- 
 sented of the ordinary and familiar volcano and its products; 
 and in 3, ii., some details are given of the general aspect 
 and character of fissure-eruptions. 
 
 The openings by which heated materials from the in- 
 terior now reach the surface include volcanoes (with their 
 various associated orifices) and hot-springs. 
 
 The prevailing conical form of a volcano is that which 
 the ejected materials naturally assume round the vent of 
 eruption. The summit of the cone is truncated (Figs. 39, 
 45), and presents a cup-shaped or caldron-like cavity, termed 
 the crater, at the bottom of which is the top of the main 
 funnel or pipe of communication with the heated interior. 
 A volcano, when of small size, may consist merely of one 
 cone; when of the largest dimensions, it forms a huge moun- 
 tain, with many subsidiary cones and many lateral fissures 
 or pipes, from which the heated volcanic products are given 
 out. Mount Etna (Fig. 39), rising from the sea to a height 
 of 10,840 feet, and supporting, as it does, some 200 minor 
 cones, many of which are in themselves considerable hills, 
 is a magnificent example of a colossal volcano. 4 
 
 2 The structure and history of Etna are fully described in the great work of 
 Sartorius von Waltershausen and A. von Lasaulx cited on p. 327 a treasure- 
 house of facts in volcanic geology. See also G. F. Rodwell, "Etna, a history of 
 the mountain and its eruptions," London, 1878; 0. Silvestri, "Un Viaggio all' 
 Etna," 1879. Notices of recent eruptions of the mountain will be found in 
 Nature, vols. xix., xx., xxi., xxii., xxv. (observatory on Etna, p. 394), xxvii., 
 xlvi. ; Compt. rend. Ixvi. The work of Mercalli, cited on p. 327, gives descrip- 
 tions of this and the other Italian volcanic centres.
 
 330 
 
 TEXT-BOOK OF GEOLOGY 
 
 The materials erupted from volcanic vents may be classed 
 as (1) gases and vapors, (2) water, (3) lava, (4) fragmentary 
 substances. A brief summary under each of these heads 
 
 may be given here; the share taken by the several products 
 in the phenomena of an active volcano is described in 2. 
 1. Gases and Vapors exist dissolved in the molten magma
 
 DYNAMICAL GEOLOGY 331 
 
 within the earth's crust. They play an important part in 
 volcanic activity, showing themselves in the earliest stages 
 of a volcano's history, and continuing to appear for centu- 
 ries after all other subterranean action has ceased. By 
 much the most abundant of them all is water-gas, which, 
 ultimately escaping as steam, has been estimated to form 
 Aftnrths of the whole cloud that hangs over an active volcano 
 (Fig. 40). In great eruptions, steam rises in prodigious 
 
 Fig. 40. View of Vesuvius as seen from Naples during the eruption of 1872, 
 showing the dense clouds of condensed aqueous vapor. 
 
 quantities, and is rapidly condensed into a heavy rainfall. 
 M. Fouque calculated that, during 100 days, one of th'e 
 parasitic cones on Etna had ejected vapor enough to form, 
 if condensed, 2,100,000 cubic metres (462,000,000 gallons) 
 of water. But even from volcanoes which, like the Sol- 
 fatara of Naples, have been dormant for centuries, steam 
 sometimes still rises without intermission and in consider- 
 able volume. Jets of vapor rush out from clefts in the
 
 332 TEXT-BOOK OF GEOLOGY 
 
 sides and bottom of a crater with a noise like that made 
 by the steam blown off by a locomotive. The number of 
 these funnels or "fumaroles" is often so large, and the 
 a/mount of vapor so abundant, that only now and then, 
 when the wind blows the dense cloud aside, can a momen- 
 tary glimpse be had of a part of the bottom of the crater; 
 while at the same time the rush and roar of the escaping 
 steam remind one of the din of some vast factory. Aqueous 
 vapor rises likewise from rents on the outside of the vol- 
 canic cone. It issues so copiously from some flowing lavas 
 that the stream of rock may be almost concealed from view 
 by the cloud; and it continues to escape from fissures of 
 the lava, far below the point of exit, for a long time after 
 the rock r lias solidified and come to rest. So saturated are 
 many molten lavas with water-vapor that Mr. Scrope 
 thought that they owed their mobility to this cause.' In 
 the deep volcanic magma the water-substance must be far 
 above its critical temperature, which is about 773 Fahr. 
 
 Probably in no case is the steam mere pure vapor of 
 water, though when it condenses into copious rain, it is 
 fresh and not salt water. It is associated with other vapors 
 and gases disengaged from the potent chemical laboratory 
 underneath. There seems to be always a definite order in 
 the appearance of these vapors, though it may vary for 
 different volcanoes. The hottest and most active "fuma- 
 roles," or vapor-vents, may contain all the gases and vapors 
 of a volcano, but as the heat diminishes, the series of gas- 
 eous emanations is reduced. Thus in the Vesuvian erup- 
 tion of 1855-56, the lava, as it cooled and hardened, gave 
 out successively vapors of hydrochloric acid, chlorides, and 
 
 8 "Considerations on Volcanoes" (1825), p. 110.
 
 DYNAMICAL GEOLOGY 333 
 
 sulphurous acid; then steam; and, finally, carbon-dioxide 
 and combustible gases. 4 More recent observations tend to 
 corroborate the deductions of C. Sainte-Claire Deville that 
 the nature of the vapors evolved depends on the tempera- 
 ture or degree of activity of the volcanic orifice, chlorine 
 (and fluorine) emanations indicating the most energetic 
 phase of eruptivity, sulphurous gases a diminishing condi- 
 tion, and carbonic acid (with hydrocarbons) the dying out 
 of the activity. 6 A "solfatara," or vent emitting only gase- 
 ous discharges, is believed to pass through these successive 
 stages. Wolf observed that on Cotopaxi while hydrochloric 
 acid and even free chlorine escaped from the summit of the 
 cone, sulphuretted hydrogen and sulphurous acid issued 
 from the middle and lower slopes.' Fouque^s studies at 
 Santorin have shown also that from submarine vents a 
 similar order of appearance obtains among the volcanic 
 vapors, hydrochloric and sulphurous acids being only found 
 at points of emission having a temperature above 100 C., 
 
 4 C. Sainte-Claire Deville and Leblanc, Ann. Chim. et Phys., 1858, lii. p. 19 
 et seq. For accounts of Vesuvius and its eruptions, besides the general works 
 already cited on p. 327, consult J. Phillips' "Vesuvius," 1869; "Mount Vesu- 
 vius," J. L. Lobley, 1889; J. Schmidt, "Die Eruption des Vesuv. 1855," Vienna, 
 1856; Mercalli's "Vulcani, etc."; H. J. Johnston-Lavis, Q. J. Geol. Soc. xl. 35; 
 Geol. Mag. 1888, p. 445. A diary of the volcano's behavior for six months is 
 given in Nature, xxvi. ; one for four years (1882-86) by Dr. Johnston-Lavis 
 "Spettatore del Vesuvio," Naples, 1887; a valuable series of reports on the 
 mountain by the same author will be found in recent volumes of the Reports of 
 the British Association (1885-91) and a large detailed map of the volcano, alpo 
 by him, published by Philip, London, 1891. 
 
 5 He distinguished volcanic emanations according to their order of appear- 
 ance as regards time, nearness to the vent, and temperature: viz. 1. Dry fuma- 
 roles (without steam), where anhydrous chlorides are almost the only discharge, 
 and where the temperature is very high (above that of melted zinc). 2. Acid 
 fumaroles, with sulphurous and hydrochloric acids and steam. 3. Alkaline 
 (ammoniacal) fumaroles; temperature about 100 0. ; abundant steam with chlo- 
 ride of ammouium. 4. Cold fumaroles; temperature below 100 C., with nearly 
 pure steam accompanied by a little carbon-dioxide, and sometimes sulphuretted 
 hydrogen. 5. Mofettes ; emanations of carbon-dioxide with nitrogen and oxygen, 
 marking the last phase of volcanic activity. 
 
 6 Neues Jahrb. 1878, p. 164.
 
 334 TEXT-BOOK OF GEOLOGY 
 
 while carbon- dioxide, sulphuretted hydrogen, and nitrogen 
 occur at all the fumaroles, even where the temperature is 
 not higher than that of the atmosphere.* 
 
 The following are the chief gases and acids evolved at 
 volcanic fumaroles. Hydrochloric acid is abundant 
 at Vesuvius, and probably at many other vents whence it 
 has not been recorded. It is recognizable by its pungent, 
 suffocating fumes, which make approach difficult to the 
 clefts from which it issues. Sulphuretted hydrogen 
 and sulphurous acid are distinguishable by their 
 odors. The liability of the former gas to decomposition 
 leads to the deposition of a yellow crust of sulphur; 
 occasionally, also, the production of sulphuric acid is 
 observed at active vents. From observations made at Vesu- 
 vius in May, 1878, Mr. Siemens concluded that vast quan- 
 tities of free hydrogen or of combustible com- 
 pounds of this gas exist dissolved in the magma of the 
 earth's interior, and that these, rising and exploding in the 
 funnels of volcanoes, give rise to the detonations and clouds 
 of steam. 8 At the eruption of Santorin in 1866, the same 
 gases were also distinctly recognized by Fouque, who for 
 the first time established the existence of true volcanic 
 flames. These were again studied spectroscopically in the 
 following year by Janssen, who found them to arise essen- 
 tially from the combustion of free hydrogen, but with traces 
 of chlorine, soda, and copper. Fouque" determined by analy- 
 sis that, immediately over the focus of eruption, free hydro- 
 gen formed thirty per cent of the gases emitted, but that 
 the proportion of this gas rapidly diminished with distance 
 from the active vents and hotter lavas, while at the same 
 time the proportion of marsh-gas and carbon-dioxide rapidly 
 increased. The gaseous emanations collected by him were 
 found to contain abundant free o^ y g e n as well as hy- 
 drogen. One analysis gave the following results: carbon- 
 dioxide 0-22, oxygen 21-11, nitrogen 21-90, hydrogen 56-70, 
 marsh-gas 0'07=100'00. This gaseous mixture, on coming 
 in contact with a burning body, at once ignites with a sharp 
 explosion. Foucpe' infers that the water- vapor of volcanic 
 vents may exist in a state of dissociation within the mol- 
 ten magma whence lavas rise. 8 C a r b o n-d i o x i d e rises 
 
 ' "Santorin et ses Eruptions," Paris, 1879. 
 
 8 Monatsb. K. Preuss. Akad. 1878, p. 588. 
 
 9 Fouque, "Santorin et ses eruptions," p. 225.
 
 DYNAMICAL GEOLOGY 335 
 
 chiefly (a) after an eruption has ceased and the volcano re- 
 lapses into quiescence ; or (b) after volcanic action has other- 
 wise become extinct. Of the former phase, instances are on 
 record at Vesuvius where an eruption has been followed 
 by the emission of this gas so copiously from the ground as 
 to suffocate hundreds of hares, pheasants, and partridges. 
 Of the second phase, good examples are supplied by the 
 ancient volcanic regions of the Eiiel and Auvergne, where 
 the gas still rises in prodigious quantities. Bischof esti- 
 mated that the volume of carbonic acid evolved in the 
 Brohl Thai amounts to 5,000,000 cubic feet, or 300 tons of 
 gas in one day. Nitrogen, derived perhaps from the 
 decomposition of atmospheric air dissolved in the water 
 which penetrates into the volcanic foci, has been frequently 
 detected among the gaseous emanations. At Santorin it 
 was found to form from 4 to 88 per cent of - the gas ob- 
 tained from different fumaroles. 10 Fluorine and iodine 
 have likewise been noticed. 
 
 With these gases and vapors are associated many sub- 
 stances which, sublimed by the volcanic heat or resulting 
 from reactions among the escaping vapors, appear as Sub- 
 limates along crevices wherein they reach the air and are 
 cooled. Besides sulphur, there are several chlorides 
 (particularly that of sodium, and less abundantly those of 
 potassium, iron, copper, and lead); also free sulphuric 
 acid, sal-ammoniac, specular iron, cxide of 
 copper, boracic acid, alum, sulphate of lime, 
 fe Is pars, pyroxene, and other substances. Carbonate 
 of soda occurs in large quantities among the fumaroles of 
 Etna. Sodium-chloride sometimes appears so abundantly 
 that wide spaces of a volcanic cone, as well as of the newly 
 erupted lava, are crusted with salt, which can even be profit- 
 ably removed by the inhabitants of the district. Consid- 
 erable quantities of chlorides, etc., may thus be buried 
 between successive sheets of lava, and in long subsequent 
 times may give rise to mineral springs, as has been sug- 
 gested witn reference to the saline waters which issue from 
 volcanic rocks of Old Red Sandstone and Carboniferous age 
 in Scotland." The iron-chloride forms a bright yellow and 
 reddish crust on the crater walls, as well as on loose stones 
 on the slopes of the cone. Specular iron, from the decom- 
 position of iron-chloride, forms abundantly as thin lamellae 
 
 10 Fouqu6, loc. cit " Proc. Ro7- Soc. Edin. ix. p. 367.
 
 336 TEXT-BOOK OF GEOLOGY 
 
 in the fissures of Vesuvian lavas. In the spring of 1873 the 
 author observed delicate brown filaments of tenorite (copper- 
 oxide, CuO) forming in clefts of the crater of Vesuvius. 
 They were upheld by the upstreaming current of vapor until 
 blown off by the wind. Fouque" has described tubular vents 
 in the lavas of Santorin with crystals of anorthite, spheae, 
 and pyroxene, formed by sublimation. In the lava stalac- 
 tites of Hawaii needle-like fibres of breislakite abound. 
 
 2. Water* Abundant discharges of water accompany 
 some volcanic explosions. Three sources of this water may 
 be assigned : (1) from the melting of snow by a rapid acces- 
 sion of temperature previous to or during an eruption; this 
 takes place from time to time on Etna, in Iceland, and 
 among the snowy ranges of the Andes, where the cone of 
 Cotopaxi is said to have been entirely divested of its snow 
 in a single night by the heating of the mountain; (2) from 
 the condensation of the vast clouds of steam which are dis- 
 charged during an eruption; this undoubtedly is the chief 
 source of the destructive torrents so frequently observed to 
 form part of the phenomena of a great volcanic explosion; 
 and (3) from the disruption of reservoirs of water filling sub- 
 terranean cavities, or of lakes occupying crater-basins; this 
 has several times been observed among the South American 
 volcanoes, where immense quantities of dead fish, which in- 
 habited the water, have been swept down with the escaping 
 torrents. The volcano of Agua in Guatemala received its 
 name from the disruption of a crater-lake at its summit by 
 an earthquake in 1540, whereby a vast and destructive de- 
 bacle of water was discharged down the slopes of the moun- 
 tain." In the beginning of the year 1817, an eruption took 
 place at the large crater of Idjen, one of the volcanoes of 
 
 19 For an account of this mountain see K. v. Seebach, Abh. Gesell. Wiai. 
 Gottingen, xxxviii. (1892) p. 216.
 
 DYNAMICAL GEOLOGY 337 
 
 Java, whereby a steaming lake of hot acid water was dis- 
 charged with frightful destruction down the slopes of the 
 mountain. After the explosion, the basin filled again with 
 water, but its temperature was no longer high. 18 
 
 In many cases, the water rapidly collects volcanic dust 
 as it rushes down, and soon becomes a pasty mud; or it 
 issues at first in this condition from the volcanic reservoirs 
 after violent detonations. Hence arise what are termed 
 mud-lavas, or aqueous lavas, which in many respects 
 behave like true lavas. This volcanic mud eventually con- 
 solidates into one of the numerous forms of tuff, a rock 
 which, as has been already stated (p. 238), varies greatly in 
 the amount of its coherence, in its composition, and in its 
 internal arrangement. Obviously, unless where subsequently 
 altered, it cannot possess a crystalline structure like that of 
 true lava. As a rule, it betrays its aqueous origin by more 
 or less distinct evidence of stratification, by the multifarious 
 pebbles, stones, blocks of rock, tree-trunks, branches, shells, 
 bones, skeletons, etc., which it has swept along in its Bourse 
 and preserved within its mass. Sections of this compacted 
 tuff may be seen at Herculaneum. 14 The trass of the Brohl 
 Thai and other valleys in the Eifel district, referred to on 
 p. 242, is another example of an ancient volcanic mud. 
 
 13 See Junghuhn's "Java." For an account of the volcanoes of the Sunda 
 Island and Moluccas, see F. Scheider, Jahrb. Geol. Reichsanst. Vienna, xxxv. 
 (1885), p. 1. Consult also for the Javanese volcanoes the works on Krakfctoa 
 quoted on p. 362. 
 
 14 Mallet thought that the so-called "mud-lavas" of Herculaneum and Pom- 
 peii were not aqueous deposits (Journ. Roy. Geol. Soc. Ireland, IV. (1876), p. 
 144). But theie seems no reason to doubt that while an enormous amount of 
 ashes fell during the eruption of A.D. 79, there were likewise, especially in the 
 later phases of eruption, copious tori cuts of water that mingled with the line ash 
 and became "mud-lavas." The sharpness of outline and the absence of any 
 trace of abdominal distension in the molds of the human bodies found at Pom- 
 peii, probably show that, these victims of the catastrophe were rapidly enveloped 
 in a firm coherent matrix which could hardly have been mere loose dual. See 
 H. J. Johnstou-Lavis, Q. J. Geol. Soc. xl. p. 89. 
 
 GEOLOGY Vol. XXIX 15
 
 338 TEXT-BOOK OF GEOLOGY 
 
 3. Lava. The term lava is applied generally to all the 
 molten rocks of volcanoes. 1 * The use of the word in this 
 broad sense is of great convenience in geological descrip- 
 tions, by directing attention to the leading character of the 
 rocks as molten products of volcanic action, and obviating 
 the confusion and errors which are apt to arise from an ill- 
 defined or incorrect lithological terminology. Precise defi- 
 nitions of the rocks, such as those given above in Book II., 
 can be added when required. A few remarks regarding 
 some of the general lithological characters of lavas may be 
 of service here; the behavior of the rocks in their emission 
 from volcanic orifices will be described in 2. 
 
 While still flowing or not yet cooled, lavas differ from 
 each other in the extent to which they are impregnated with 
 gases and vapors. Some appear to be saturated, others con- 
 tain a much smaller gaseous impregnation; and hence arise 
 important distinctions in their behavior (pp. 370-395). After 
 solidification, lavas present some noticeable characters, then 
 easily ascertainable. (1) Their average specific gravity may 
 be takn as ranging between 2-37 and 3-22. (2) The heavier 
 varieties contain much magnetic or titaniferous iron, with 
 augite and olivine, their composition being basic, and their 
 proportion of silica averaging about 45 to 55 per cent. In 
 this group come the basalts, nepheline-lavas, and leucite- 
 lavas. The lighter varieties contain commonly a minor pro- 
 portion of metallic bases, but are rich in silica, their per- 
 centage of that acid ranging between 70 and 75. They are 
 thus not basic but acid rocks. Among their more important 
 varieties are the rhyolites and obsidians. Some intermedi- 
 ate varieties (trachytes, phonolites, and andesites) connect 
 the acid and basic series. (3) Lavas differ much in structure 
 and texture, (a) Some are entirely crystalline, consisting 
 of an interlaced mass of crystals and crystalline particles, as 
 in some dolerites, and granitoid rhyolites. Even quartz, 
 which used to be considered a non-volcanic mineral, charac- 
 
 15 "All es ist Lava was im Vulkane fliesst und durch seine Flussigkeit neue 
 Lagerstatter einnimmt" is Leopold von Buch's comprehensive definition.
 
 DYNAMICAL GEOLOGY 339 
 
 teristic of the older and chiefly of the plutonic eruptive 
 rocks, has been observed in large crystals in modern lava 
 (liparite and quartz-andesite"). (6) Some show more or less 
 of a half-glassy or stony (devitrified) matrix, in which the 
 constituent crystals are imbedded; this is the most common 
 arrangement, (c) Others are entirely vitreous, such crystals 
 or crystalline particles as occur in them being c[uite subor- 
 dinate, and, so to speak, accidental inclosures in the main 
 glassy mass. Obsidian or volcanic glass is the type of this 
 group, (d) They further differ in the extent to which mi- 
 nute pores or larger cellular spaces have been developed in 
 them. According to Biscbof, the porosity of lavas depends 
 on their degree of liquidity, a porous lava or slag, when re- 
 duced in his fusion-experiments to a thin-flowing consist- 
 ency, hardening into a mass as compact as the densest lava 
 or basalt. 17 The presence of interstitial steam in lavas, by 
 expanding the still molten stone, produces an open cellular 
 texture, somewhat like that of sponge or of bread. Such a 
 vesicular arrangement very commonly appears on the upper 
 surface of a lava current, which assumes a slaggy or cindery 
 aspect. In some forms of purnice the proportion of air 
 cavities is 8 or 9 times that of the inclosing glass. (4) Lavas 
 vary greatly in color and general external aspect. The 
 heavy basic kinds are usually dark gray, or almost black, 
 though, on exposure to the weather, they acquire a brown 
 tint from the oxidation and hydration of their iron. Their 
 surface is commonly rough and ragged, until it has been 
 sufficiently decomposed by the atmosphere to crumble into 
 soil which, under favorable circumstances, supports a luxu- 
 riant vegetation. The less dense lavas, such as phonolites 
 and trachytes, are frequently paler in color, sometimes yel- 
 low or buff, and decompose into light soils; but the obsid- 
 ians present rugged black sheets of rock, roughened with 
 ridges and heaps of gray froth-like pumice. Some of the 
 most brilliant surfaces of color in any rock-scenery on the 
 globe are to be found among volcanic rocks. The walls of 
 active craters glow with endless hues of red and yellow. 
 The Grand Oaflon of the Yellowstone Eiver has been dug 
 out of the most marvellously tinted lavas and tuffs. 
 
 16 Wolf, Neues Jahrb. 1874, p. 377. 
 
 11 "Chem. und Phys. Geol." supp. (1871), p. 144. On the production of the 
 vesicular structure consult Dana, "Characteristics of Volcanoes," p. 161. Com- 
 pare also Judd, Geol. Mag. 1888, p. 7.
 
 S40 TEXT-BOOK OF GEOLOGY 
 
 4. Fragmentary Materials. Under this title may be in- 
 cluded all the substances which, driven up into the air by 
 volcanic explosions, fall in solid form to the ground the 
 dust, ashes, sand, cinders, and blocks of every kind which 
 are projected from a volcanic orifice. These materials differ 
 in composition, texture, and appearance, even during a sin- 
 gle eruption, and still more in successive explosions of the 
 same volcano. For the sake of convenience, separate names 
 are applied to some of the more distinct varieties, of which 
 the following may be enumerated. 
 
 (1) Ashes and san d. In many eruptions, vast quan- 
 tities of an exceedingly fine light gray powder are ejected. 
 As this substance greatly resembles what is left after a piece 
 of wood or coal is burned in an open fire, it has been popu- 
 larly termed ash, and this name has been adopted by geolo- 
 gists. If, however, by the word ash, the result of combus- 
 tion is implied, its employment to denote any product of 
 volcanic action must be regretted, as apt to convey ;i wrong 
 impression. The fine ash-like dust ejected by a volcano is 
 merely lava in an extremely fine state of comminution. So 
 minute are the particles that they find their way readily 
 through the finest chinks of a closed room, and settle down 
 upon floor and furniture, as ordinary dust does when a house 
 is shut up. From this finest form of material, gradations 
 may be traced, through what is termed volcanic sand, into 
 the coarser varieties of ejected matter. In composition, the 
 ash and sand vary necessarily with the nature of the lava 
 from which they are derived. Their microscopic structure, 
 and especially their abundant microlites, crystals, and vol- 
 canic glass, have been already referred to (pp. 239-241). 
 
 (2) L a p i 1 1 i or r a p i 1 1 1 (p. 239) are Dejected fragments 
 ranging from the size of a pea to that of* a walnut; round, 
 subangular, or angular in shape, and having the same indefi- 
 nite range of composition as the finer dust. As a rule, the 
 larger pieces fall nearest the focus of eruption. Sometimes 
 they are solid fragments of lava, but more usually they have 
 a cellular texture, while sometimes they are so light and 
 porous as to float readily on water, and, when ejected near 
 the sea, to cover its surface. Well formed crystals occur in 
 the lapilli of many volcanoes, and are also ejected separately.
 
 DYNAMICAL GEOLOGY 341 
 
 It has been observed indeed that the fragmentary materials 
 not infrequently contain finer crystals than the accompany- 
 ing lava. 18 
 
 (3) Volcanic Blocks (p. 239) are larger pieces of 
 stone, often angular in shape. In some cases they appear 
 to be fragments loosened from already solidified rocks in the 
 chimney of the volcano. Hence we find among them pieces 
 of non-volcanic rocks, as well as of older tuffs and lavas rec- 
 ognizably belonging to early eruptions. In many cases, 
 they are ejected in enormous quantities during the earlier 
 phases of violent eruption. The great explosion from the 
 side of Ararat in 1840 was accompanied by the discharge of 
 a vast quantity of fragments over a space of many square 
 miles around tne mountain. Whitney has described the oc- 
 currence in California of beds of such fragmentary volcanic 
 breccia, hundreds of feet thick and covering many square 
 miles of surface. Junghuhn, in his account of the eruption 
 in Java in 1772, mentions that a valley ten miles long was 
 filled to an average depth of fifty feet with angular volcanic 
 de'bris." 
 
 Among the earlier eruptions of a volcano, fragments of 
 the rocks through which the vent has been drilled may fre- 
 quently be observed. These are in many cases not volcanic. 
 Blocks of schist and granitoid rocks occur in the cinder-beds 
 at the base of the volcanic series of Santorin. In the older 
 tuffs of Somma, pieces of altered limestone (sometimes 
 measuring 200 cubic feet or more and weighing upward 
 of 15 tons) are abundant and often contain cavities lined 
 with the characteristic "Yesuvian minerals." 80 Blocks of 
 a coarsely crystalline granitoid (but really trachytic) lava 
 have been particularly observed both on Etna" and Vesu- 
 vius. In the year 1870 a mass of that kind, weighing several 
 tons, was to oe seen lying at the foot of the upper cone of 
 Vesuvius, within the entrance to the Atrio del Cavallo. 
 Similar blocks occur among the Carboniferous volcanic 
 pipes of central Scotland, together sometimes with frag- 
 ments of sandstone, shale, or limestone, not infrequently 
 full of Carboniferous fossils." Enormous masses of various 
 
 18 Sartorius von Waltershausen, "Sicilien und Island," 1853, p. 328. 
 
 19 But see the remarks already made on volcanic conglomerate, ante. p. 283. 
 
 20 See H. J. Johnston -Lavis, Q. J. Geol. Soc. xl. p. 76. 
 
 21 For the erupted blocks (Answurflinge) of Etna see "Der Aetna," ii. pp. 
 216, 330, 461. 
 
 82 Trans. Roy. Soc. Edin. xxix. p. 469. See postea, Book IT. Sect. vii. 1, 4.
 
 342 TEXT-BOOK OF GEOLOGY 
 
 schists have been carried up by the lavas of the Tertiary 
 volcanic plateau of the Inner Hebrides. 28 
 
 (4) Volcanic Bombs and slags. These have orig- 
 inally formed portions of the column of lava ascending the 
 pipe of a volcano, and have been detached and hurled into 
 the air by successive explosions of steam. A bomb (Fig. 
 41) is a round, elliptical, or pear-shaped, often discoidal 
 mass of lava, from a few inches to several feet in diameter; 
 sometimes tolerably solid throughout, more usually coarsely 
 cellular inside. Not infrequently its interior is hollow, and 
 the bomb then consists of a shell which is most close-grained 
 
 Fig. 41. Section of Volcanic Bomb, one-third natural size. 
 
 toward the outside, or the centre is a block of stone with an 
 external coating of lava. There can be no doubt that, when 
 torn by eructations of steam from the surface of the boiling 
 lava, the material of these bombs is in as thoroughly molten 
 a condition as the rest of the mass. From the rotatory mo- 
 tion imparted by its ejection, it takes a circular form, and 
 in proportion to its rapidity of rotation and fluidity is the 
 amount of its "flattening at the poles." The centrifugal 
 force within allows the expansion of the interstitial vapor, 
 while the outer surface rapidly cools and solidifies; hence 
 the solid crust, and the porous or cavernous interior. Such 
 bombs, varying from the size of an apple to that of a man's 
 body, were found by Darwin abundantly strewn over the 
 ground in the Island of Ascension; they were also ejected 
 
 23 Trans. Roy. Soc. Ediu. xxxv. (1888), p. 82.
 
 DYNAMICAL GEOLOGY 343 
 
 in vast quantities during the eruption of Santorin in 1866." 
 Among the tuffs of the Eifel region, small bombs, consist- 
 ing mostly of granular olivine, are of common occurrence, 
 as also pieces of sanidine or other less fusible minerals 
 which have segregated out of the magma before ejection. 
 In like manner, among the tuffs filling volcanic necks, 
 probably of Permian age, which pierce the Carboniferous 
 rocks of Fife, large worn crystals of orthoclase, biotite, etc., 
 are found. When the ejected fragment of lava has a rough 
 irregular form and a porous structure, like the clinker of 
 an iron furnace, it is known as a slag." 
 
 The fragmentary materials erupted by a volcano and de- 
 posited around it acquire by degrees more or less consolida- 
 tion, partly from the mere pressure of the higher upon the 
 lower strata, partly from the influence of infiltrating water. 
 It has been already stated (p. 240) that different names are 
 applied to the rocks thus formed. The coarse, tumultuous, 
 unstratified accumulation of volcanic de'bris within a crater 
 or funnel is called Agglomerate. When the debris, 
 though still coarse, is more rounded, and is arranged in a 
 stratified form on the slopes of the cone or on the country 
 beyond, it becomes aYolcanic Conglomerate. The 
 finer-grained varieties, formed of dust and lapilli, are in- 
 cluded in the general designation of Tuffs. These are 
 usually pale yellowish, grayish, or brownish, sometimes 
 black rocks, granular, porous, and often incoherent in tex- 
 ture. They occur interstratified with and pass into ordinary 
 non-volcanic sediment. 
 
 Organic remains sometimes occur in tuff. Where vol- 
 canic debris has accumulated over the floor of a lake, or of 
 the sea, the entombing and preserving of shells and other 
 organic objects must continually take place. Examples of 
 this kind are cited in later pages of this work from older 
 geological formations. Professor Guiscardi of Naples found 
 about 100 species of marine shells of living species in the 
 old tuffs of Vesuvius. Marine shells have been picked up 
 within the crater of Monte Niiovo, and have been frequently 
 observed in the old or marine tuff of that district. Showers 
 of ash, or sheets of volcanic mud, often preserve land-shells, 
 insects, and vegetation living on the area at the time. The 
 
 24 Darwin, "Geological Observations on Volcanic Islands," 2d edit. p. 42. 
 Fouque, "Santoriu," p. 79. 
 
 25 On the ratio between the pores and volume of the rock in slags and lavas, 
 see determinations by Biscliof, "Chem. imd Phys. Geol." supp. (1871), p. 158.
 
 344 TEXT-BOOK OF GEOLOGY 
 
 older tuffs of Vesuvius have yielded many remains of the 
 shrubs and trees which at successive periods have clothed 
 the flanks of the mountain. Fragments of coniferous wood, 
 which once grew on the tuff-cones of Carboniferous age in 
 central Scotland, are abundant in the "necks" of that re- 
 gion, while the minute structure of some of the lepidoden- 
 droid plants has also been admirably preserved there in 
 tuff." 
 
 2. Volcanic Action 
 
 Volcanic action many be either constant or periodic. 
 Stromboli, in the Mediterranean, so far as we know, has 
 been uninterruptedly emitting hot stones and steam, from 
 a basin of molten lava, since the earliest period of history." 
 Among the Moluccas, the volcano Sioa, and in the Friendly 
 Islands, that of Tofua, have never ceased to be in eruption 
 since their first discovery. The lofty cone of Sangay, among 
 the Andes of Quito, is always giving off hot vapors; Goto- 
 paxi, too, is ever constantly active. 88 But, though exam- 
 ples of unceasing action may thus be cited from widely 
 different quarters of the globe, they are nevertheless excep- 
 tional. The general rule is that a' volcano breaks out from 
 time to time with varying vigor, and after longer or shorter 
 intervals of quiescence. 
 
 Active, Dormant, and Extinct Phases, It is usual to class 
 volcanoes as active, dormant, and extinct. This arrangement, 
 however, often presents considerable difficulty in its appli- 
 cation. An active volcano cannot of course be mistaken, 
 for even when not in eruption, it shows by its discharge of 
 
 86 Trans. Roy. Soc. Edin. xxix. p. 470; postea, Book IV. Part VII. Sect, 
 ii. 2. 
 
 91 For accounts of Stromboli see Spallanzani's "Voyages dans les deux 
 Siciles." Scrope's "Volcanoes." Judd, Geol. Mag. 1875. Mercalli's "Vul- 
 cani," etc. p. 135; and his papers in Atti Soc. Ital. Sci. Nat. xxii., xxiv., xxvii., 
 xxix., xxxi. L. "W. Pulcher, Geol. Mag. 1890, p. 347. 
 
 28 For descriptions of Cotopaxi, see Wolf, Neues Jahrb. 1878; Whymper, 
 Nature, xxiii. p. 323; "Travels amongst the Great Andes," chap. vi.
 
 DYNAMICAL GEOLOGY 345 
 
 steam and hot vapors that it might break out into activity 
 at any moment. But in many cases, it is impossible to de- 
 cide whether a volcano should be called extinct or only 
 dormant. The volcanoes of Silurian age in Wales, of Car- 
 boniferous age in Ireland, of Permian age in the Harz, of 
 Miocene age in the Hebrides, of younger Tertiary age in 
 the Western States and Territories of North America, are 
 certainly all extinct. Bat the older Tertiary volcanoes 
 of Iceland are still represented there by Skaptar-Jokull, 
 Hecla, and their neighbors." Somma, in the first century 
 of the Christian era, would have been naturally regarded 
 as an extinct volcano. Its fires had never been known to 
 have been kindled; its vast crater was a wilderness of wild 
 vines and brushwood, haunted, no doubt, by wolf and 
 wild boar. Yet in a few days, during the autumn of the 
 year 79, the half of the crater walls was blown out by a 
 terrific series of explosions, the present Vesuvius was then 
 formed within the limits of the earlier crater, and since that 
 time volcanic action has been intermittently exhibited up 
 to the present day. Some of the intervals of quietude, 
 however, have been so considerable that the mountain 
 might then again have been claimed as an extinct vol- 
 cano. Thus, in the 131 years between 1500 and 1631, so 
 completely had eruptions ceased that the crater had once 
 
 89 On the volcanic phenomena of Iceland consult G. Mackenzie's "Travels in 
 the Island of Iceland during the Summer of 1810." E. Henderson's "Iceland." 
 Zirkel, "De geognostica Islandse constitutione observationes, " Bonn, 1861. Tho- 
 roddsen, "Oversigt over de islandske Vulkaners Historie," translated in resume 
 by G. H. Boehmer, Smithsonian Inst. Rep. 1885, part i. p. 495; also Bihang t. 
 Svensk. Yet. Akad. Handl. 14, ii. (1888), 17, ii. (1891); Geol. Mag. 1880, p. 
 458; Nature, .Oct. 1884. Mitth. K. K. Geogr. Ges. Vienna, xxiv. (1891), p. 
 117. Keilhack, Zeitsch. Deutsch. Geol. Gesel. xzxviii. (1886), p. 376; Schmidt, 
 op. cit. xxxvii. (1885), p. 737; A. Holland, "Lakis Kratere og Lava-strome, " 
 Universitets Programme, Christiania, 1885 ; Breon, "Geologic de 1'Islande, et des 
 UesFoeroe," Paris, 1884; T. Anderson, Journ. Soc. Arts, vol. xl. (1892), p. 397.
 
 346 TEXT-BOOK OF GEOLOGY 
 
 more become choked with copsewood. A few pools and 
 springs of very salt and hot water remained as memorials 
 of the former condition of the mountain. But this period 
 of quiescence closed with the eruption of 1631 the most 
 powerful of all the known explosions of Vesuvius, except 
 the great one of 79. In the island of Ischia, Mont' Epomeo 
 was last in eruption in the year 1302, its previous outburst 
 having taken place, it is believed, about seventeen centuries 
 before that date. From the craters of the Eifel, Auvergne, 
 the Yivarais, and central Italy, though many of them look 
 as if they had only recently been formed, no eruption has 
 been known to come during the times of human history or 
 tradition. In the west of North America, from Arizona to 
 Oregon, numerous stupendous volcanic cones occur, but 
 even from the most perfect and fresh of them nothing but 
 steam and hot vapors has yet been known to proceed. 30 
 But the presence there of hot springs and geysers proves 
 the continued existence of one phase of volcanic action. 
 
 In short, no essential distinction can be drawn between 
 dormant and extinct volcanoes. Volcanic action, as will be 
 afterward pointed out, is apt to show itself again and again, 
 even at vast intervals, within the same regions and over the 
 same sites. The dormant or waning condition of a volcano, 
 when only steam and various gases and sublimates are given 
 off, is sometimes called the Solfatara phase, from the well- 
 known dormant crater of that name near Naples. 
 
 Sites of Volcanic Action, Volcanoes may break through 
 any geological formation. In Auvergne, in the Miocene 
 period, they burst through the granitic and gneissose 
 plateau of central France. In Lower Old Bed* Sandstone 
 
 80 Eruptions occurred perhaps less than 100 years ago. Diller, Bull. U. S. 
 Geol. Surv., No. 79.
 
 DYNAMICAL GEOLOGY 347 
 
 times, they pierced contorted Silurian rocks in central 
 Scotland. In late Tertiary and post-Tertiary ages, they 
 found their way through recent soft marine strata, and 
 formed the huge piles of Etna, Somma, and Vesuvius; 
 while in North America, during the same cycle of geolog- 
 ical time, they flooded with lava and tuff many of the river- 
 courses, valleys, and lakes of Nevada, Utah, Wyoming, 
 Idaho, and adjacent territories. On the banks of the Rhine, 
 at Bonn and elsewhere, they have penetrated some of the 
 older alluvia of that river. In many instances, also, newer 
 volcanoes have appeared on the sites of older ones. In 
 Scotland, the Carboniferous volcanoes have risen on the 
 ruins of those of the Old Red Sandstone, those of the 
 Permian period have broken out among the earlier Car- 
 boniferous eruptions, while the older Tertiary dikes have 
 been injected into all these older volcanic masses. The 
 newer puys of Auvergne were sometimes erupted through 
 much older and already greatly denuded basalt-streams. 
 Somma and Vesuvius have risen out of the great Neapoli- 
 tan plain of older marine tuff, while in central Italy newer 
 cones have been thrown up upon the wide Roman plain of 
 more ancient volcanic de'bris. 31 The vast Snake River lava- 
 fields of Idaho overlie denuded masses of earlier trachytic 
 lavas, and similar proofs of a long succession of intermit- 
 tent and widely-separated volcanic outbursts can be traced 
 northward into the Yellowstone Valley. 
 
 When a volcanic vent is opened, it might be supposed 
 always to find its way to the surface along some line of fis- 
 
 31 According to Prof. Gr. Pozzi, the principal volcanic outbursts of Italy are 
 of the Glacial Period. Atti Lincei, 3d ser. vol. ii. (1878), p. 35. Stefani re- 
 gards those of Tuscany as partly Miocene, partly Pliocene and post- Pliocene. 
 (Proc. Tosc. Soc. Nat. Pisa, 1. p. xxi.)
 
 348 TEXT-BOOK OF GEOLOGY 
 
 sure, valley, or deep depression. No doubt many, if not 
 most, modern as well as ancient vents, especially those of 
 large size, have done so. It is a curious fact, however, that 
 in innumerable instances minor vents have appeared where 
 there was no visible line of dislocation in the rocks at the 
 surface to aid them. This has been well shown by a study 
 of the ancient volcanic rocks of the Old Red Sandstone, Car- 
 boniferous, and Permian formations of Scotland. 88 It has 
 likewise been most impressively demonstrated by the way in 
 which the minor basalt cones and craters of Utah have 
 broken out near the edges or even from the face of cliffs, 
 rather than at the bottom. Captain Button remarks that 
 among the high plateaus of Utah, where there are hundreds 
 of basaltic craters, the least common place for them is at the 
 base of a cliff, and that, though they occur near faults, it is 
 almost always on the lifted, rarely upon the depressed side.' 8 
 On a small scale, a similar avoidance of the valley bottom is 
 shown on the Rhine and Moselle, where eruptions have 
 taken place close to the edge of the plateau through which 
 these rivers wind. Why outbreaks should have occurred in 
 this way is a question not easily answered. It suggests that 
 the existing depressions and heights of the earth's surface 
 may sometimes be insignificant features, compared with the 
 depth of the sources of volcanoes and the force employed in 
 volcanic eruption. On the other hand, it is remarkable that 
 in Scotland the Palaeozoic eruptions took place on the low 
 ground and valleys, and continued to show themselves there 
 during a long succession of volcanic periods. Especially 
 noteworthy is the way in which the Permian vents were 
 
 88 Trans. Roy. Soc. Edin. xxix. p. 437. 
 
 33 "High Plateaus of Utah," Geol. and Geog. Survey of Territories, 1880, 
 p. 62.
 
 DYNAMICAL GEOLOGY 349 
 
 opened in lines and groups along the bottom of long narrow 
 valleys in the Silurian uplands. 34 
 
 Ordinary phase of an active Volcano. The interval be- 
 tween two eruptions of an active volcano shows a gradual 
 augmentation of energy. The crater, emptied by the last 
 discharge, has its floor slowly upraised by the expansive 
 force of the lava-column underneath. Yapors rise in con- 
 stant outflow, accompanied sometimes by discharges of ^dust 
 or stones. Through rents in the crater-floor red-hot lava 
 may be seen only a few feet down. Where the lava is main- 
 tained at or above its fusion-point and possesses great liquid- 
 ity, it may form boiling lakes, as in the great crater of 
 Kilauea, where acres of seething lava may be watched throw- 
 ing up fountains of molten rock, surging against the walls 
 and re-fusing large masses that fall into the burning flood. 
 The lava-column inside the pipe of a volcano is all this time 
 gradually rising, until some weak part of the wall allows it 
 to escape, or until the pressure of the accumulated vapors 
 becomes great enough to burst through the hardened crust 
 of the crater-floor and give rise to the phenomena of an 
 eruption. 
 
 Conditions of Eruption* Leaving for the present the gen- 
 eral question of the cause of volcanic action, it may be here 
 remarked that the conditions determining any particular 
 eruption are still unknown. The explosions of a volcano 
 may be to some extent regulated by the conditions of atmos- 
 pheric pressure over the area at the time. In the case of a 
 volcanic funnel like Stromboli, where, as Scrope pointed 
 out, the expansive subterranean force within, and the re- 
 pressive effect of atmospheric pressure without, just balance 
 
 34 Quart. Journ. Geol. Soc. vol. xlviii. (1892). Presidential Address, p. 156.
 
 350 TEXT-BOOK OF GEOLOGY 
 
 each other, any serious disturbance of that pressure might 
 be expected to make itself evident by a change in the con- 
 dition of the volcano. Accordingly, it has long been re- 
 marked by the fishermen of the Lipari Islands that in stormy 
 weather there is at Stromboli a more copious discharge of 
 steam and stones than in fine weather. They make use of 
 the cone as a weather-glass, the increase of its activity indi- 
 cating a falling, and the diminution a rising barometer. In 
 like manner, Etna, according to Sartorius von Waltershau- 
 sen, is most active in the winter months. Mr. Coan has in- 
 dicated a relation between the eruptions of Kilauea and the 
 rainy seasons of Hawaii, most of the discharges of that 
 crater taking place within the four months from March to 
 June. 35 
 
 When we remember the connection, now indubitably 
 established, between a more copious discharge of fire-damp 
 in mines and a lowering of atmospheric pressure, we may be 
 prepared to find a similar influence affecting the escape of 
 vapors from the upper surface of the lava-column of a vol- 
 cano; for it is not so much to the lava itself as to the expan- 
 sive vapors impregnating it that the manifestations of vol- 
 canic activity are due. Among the Vesuvian eruptions since 
 
 35 Dana, "Characteristics of Volcanoes," p. 125. For accounts of the vol- 
 canic phenomena of Hawaii, see W. Ellis, "Polynesian Researches. " Wilkes' 
 TT. S. Exploring Expedition, 1838-42, "Geology," by J. D. Dana. The Rev. 
 T. Coan, a missionary resident in Hawaii, observed the operations of the vol- 
 canoes for upward of forty years, and published from time to time short notices 
 of them in the American Journal of Science, vols. xiii. (1852) xiv., xv., xviii., 
 xxi., xxii., xxiii., xxv., xxvii., xxxvii., xl., xliii., xlvii., xlix; 3d ser. ii. (1871) 
 iv., vii., viii., xiv., xviii., xx., xxi., xxii. (1881). Prof. Dana has recently re- 
 visited these volcanoes and fully discussed their phenomena in the Amer. Jouru. 
 Sci. vols. xxxiii. -xxxvii. (1887-89), and in his "Characteristics of Volcanoes." 
 See also C. E. Dutton, Amer. Journ. Sci. xxv. (1883), p. 219; Report U. S. 
 Geological Survey, 1882-83. L. Green, "Vestiges of the Molten Globe," 1887. 
 For an account of the remarkable glassy lavas of Hawaii, see E. Cohen, Neues 
 Jahrb. 1880 (ii.), p. 23; and a general account of the petrography of the islands, 
 by E. S. Dana, Amer. Journ. Sci. xxxvii. (1889), p. 441.
 
 DYNAMICAL GEOLOGY 851 
 
 the middle of the seventeenth century, the number which 
 took place in winter and spring has been to that of those 
 which broke out in summer and autumn as 7 to 4. In 
 Japan, also, the greater number of recorded eruptions have 
 taken place during the cold months of the year, February 
 to April. 36 
 
 There may be other causes besides atmospheric pressure 
 concerned in these differences; the preponderance of rain 
 during the winter and spring may be one of these. Accord- 
 ing to Mr. Coan, previous to the great Hawaiian eruption of 
 1868 there had been unusually wet weather, and to this fact 
 he attributes the exceptional severity of the earthquakes and 
 volcanic explosions. The greater frequency of Japanese 
 volcanic eruptions and earthquakes in winter has been re- 
 ferred in explanation to the fact that the average barometric 
 gradient across Japan is steeper in winter than in summer, 
 while the piling up of snow in the northern regions gives 
 rise to long-continued stresses, in consequence of which cer- 
 tain lines of weakness in the earth's crust are more prepared 
 to give way during the winter months than they are in sum- 
 mer." The effects of varying atmospheric pressure, how- 
 ever, can probably only slightly and locally modify volcanic 
 activity. Eruptions, like the great one of Cotopaxi in 1877, 
 have in innumerable instances taken place without, so far as 
 can be ascertained, any reference to atmospheric conditions. 
 
 Kluge has sought to trace a connection between the years 
 of maximum and minimum sun-spots and those of greatest 
 and feeblest volcanic activity, and has constructed lists to 
 show that years which have been specially characterized by 
 terrestrial eruptions have coincided with those marked by 
 
 36 J. Milne, Seismol. Soc. Japan, IX. Part ii. p. 174. 
 *> J. Milne, loc. cit
 
 352 TEXT-BOOK OF GEOLOGY 
 
 few sun-spots and diminished magnetic disturbance. 88 Such 
 a connection cannot be regarded as having yet been satisfac- 
 torily established. Again, the same author has called atten- 
 tion to the frequency and vigor of volcanic explosions at or 
 near the time of the August meteoric shower. But in this 
 case, likewise, the cited examples can hardly yet be looked 
 upon as more than coincidences. 
 
 Periodicity of Eruptions. At many volcanic vents the 
 eruptive energy manifests itself with more or less regularity. 
 At Stromboli, which is constantly in an active state, the ex- 
 plosions occur at intervals varying from three or four to ten 
 minutes and upward. A similar rhythmical movement has 
 been often observed during the eruptions at other vents 
 which are not constantly active. Volcano, for example, 
 during its eruption of September, 1873, displayed a succes- 
 sion of explosions which followed each other at intervals of 
 from twenty to thirty minutes. At Etna and Vesuvius a simi- 
 lar rhythmical series of convulsive efforts has often been ob- 
 served during the course of an eruption." Among the vol- 
 canoes of the Andes a periodic discharge of steam has been 
 observed: Mr. Whymper noticed outrushes of steam to pro- 
 ceed at intervals of from twenty to thirty minutes from the 
 summit of Sangai, while during his inspection of the great 
 crater of Cotopaxi, this volcano was seen to blow off steam 
 at intervals of about half an hour. 40 At the eruption of the 
 Japanese volcano, Oshima, in 1877, Mr. Milne observed that 
 the explosions occurred nearly every two seconds, with occa- 
 
 1 "Ueber Synchronismus und Antagonismus," 8vx>, Leipzig, 1863, p. 72. 
 A. Poey (Comptes Rend. Ixxviii. (1874), p. 51) believes that among the 786 erup- 
 tions recorded, by Kluge, between 1749 and 1861, the maxima correspond to 
 periods of minimum in solar spots. See, however, postea, p. 477 
 
 39 G. Mercalli, Atti. Soc. Ital. Sci. Nat. xxiv. (1881). 
 
 40 "Travels amongst the Great Andes of the Equator," 1892, pp. 74, 163.
 
 DYNAMICAL GEOLOGY 353 
 
 sional pauses of 15 or 20 seconds. 41 Kilauea, in Hawaii, 
 seems to show a regular system of grand eruptive periods. 
 Dana has pointed out that outbreaks of lava have taken 
 place from that volcano at intervals of from eight to nine 
 years, this being the time required to fill the crater up to the 
 point of outbreak, or to a depth of 400 or 500 feet." 
 
 Some volcanoes have exhibited a remarkable paroxysmal 
 phase of activity, when after comparative or complete quies- 
 cence a sudden gigantic explosion has taken place, followed 
 by renewed and prolonged repose. Vesuvius supplies the 
 most familiar illustration of this character of volcanic en- 
 ergy. The great eruption of A.D. 79, which truncated the 
 upper part of the old cone of Somma, was a true paroxysmal 
 explosion, unlike anything that had preceded it within his- 
 toric times, and far more violent than any subsequent mani- 
 festation of the same volcano. The crater-basin of Santorin, 
 of which the islands Thera and Therasia represent portions 
 of the rim, seems to have been blown out by some stupen- 
 dous paroxysm in prehistoric times. The vast explosion of 
 Krakatoa in 1883 was another memorable example. In 
 these instances there was an earlier period of ordinary vol- 
 canic activity, during which a large cone was gradually built 
 up. In the case of Somma and Krakatoa the energy died 
 down for a time, and the paroxysm came with hardly any 
 premonitory warning. It has been succeeded by a time of 
 comparatively feeble activity. At Vesuvius the great ex- 
 plosion of 1631, which terminated nearly 1500 years of 
 quiescence, may be regarded as a minor paroxysm, since 
 which the mountain has remained more continuously active. 
 
 41 Trans. Seism. Soc. Japan, ix. part ii. p. 82. 
 
 4-2 "Characteristics of Volcanoes," p. 124. On periodicity of eruptions, see 
 Kluge, Neues Jahrb. 1862, p. 582.
 
 854 TEXT-BOOK OF GEOLOGY 
 
 General sequence of events in an Eruption. The ap- 
 proach of an eruption is not always indicated by any pre- 
 monitory symptoms, for many tremendous explosions are 
 recorded to have taken place in different parts of the world 
 without perceptible warning. Much in this respect would 
 appear to depend upon the condition of liquidity of the lava, 
 and the amount of resistance offered by it to the passage 
 of the escaping vapors through its mass. In Hawaii, where 
 the lavas are remarkably liquid, vast outpourings of them 
 have taken place quietly without earthquakes during the 
 present century. But even there, the great eruption of 
 1868 was accompanied by violent earthquakes. 
 
 The eruptions of Vesuvius are often preceded by failure 
 or diminution of wells and springs. But more frequent in- 
 dications of an approaching outburst are conveyed by sym- 
 pathetic movements of the ground. Subterranean rumblings 
 and groanings are heard; slight tremors succeed, increasing 
 in frequency and violence till they become distinct earth- 
 quake shocks. The vapors from the crater grow more abun- 
 dant, as the lava-column in the pipe or funnel of the volcano 
 ascends, forced upward and kept in perpetual agitation by 
 the passage of elastic vapors through its mass. After a 
 long previous interval of quiescence, there may be much 
 solidified lava toward the top of the funnel, which will re- 
 strain the ascent of the still molten portion underneath. A 
 vast pressure is thus exercised on the sides of the cone, 
 which, if too weak to resist, will open in one or more rents, 
 and the liquid lava will issue from the outer slope of the 
 mountain; or the energies of the volcano will be directed 
 toward clearing the obstruction in the chief throat, until with 
 tremendous explosions, and the rise of a vast cloud of dust 
 and fragments, the bottom and sides of the crater are finally
 
 DYNAMICAL GEOLOGY 355 
 
 blown out, and the top of the cone disappears. The lava 
 ma}' now escape from the lowest part of the lip of the crater, 
 while, at the same time, immense numbers of red-hot bombs, 
 scoriae, and stones are shot up into the air. The lava at first 
 rushes down like one or more rivers of melted iron, but, as 
 it cools, its rate of motion lessens. Clouds of steam rise 
 from its surface, as well as from the central crater. Indeed, 
 every successive paroxysmal convulsion of the mountain is 
 marked, even at a distance, by the rise of huge ball-like 
 wreaths or clouds of steam, mixed with dust and stones, 
 forming a column which towers sometimes a couple of miles 
 or more above the summit of the cone. By degrees these 
 eructations diminish in frequency and intensity. The lava 
 ceases to issue, the showers of stones and dust decrease, and 
 after a time, which may vary from hours to days or months, 
 even in the regime of the same mountain, the volcano be- 
 comes once more tranquil. 4 * 
 
 In the investigation of the subject, the student will nat- 
 urally devote attention specially to those aspects of volcanic 
 action which have more particular geological interest from 
 the permanent changes with which they are connected, or 
 from the way in which they enable us to detect and realize 
 conditions of volcanic energy in former periods. 
 
 Fissures. The convulsions which culminate in the forma- 
 tion of a volcano usually split open the terrestrial crust by 
 a more or less nearly rectilinear fissure, or by a system Of 
 fissures. In the subsequent progress of the mountain, the 
 ground at and around the focus of action is liable to be again 
 and again rent open by other fissures. These tend to di- 
 
 43 See Schmidt's narrative of the eruption of Vesuvius in May, 1855 (ante, 
 p. 333). An account of the great eruption of Cotopaxi in June, 1877, by Dr. 
 Th. Wolf, will be found in Neues Jahrb. 1878, p. 113.
 
 356 TEXT-BOOK OF GEOLOGY 
 
 verge from the focus; but around the vent where the rocks 
 have been most exposed to concussion, the fissures some- 
 times intersect each other in all directions. In the great 
 eruption of Etna, in the year 1669, a series of six parallel 
 fissures opened on the side of the mountain. One of these, 
 with a width of two yards, ran for a distance of 12 miles, in 
 a somewhat winding course, to within a mile of the top of 
 the cone. 44 Similar fissures, but on a smaller scale, have 
 often been observed on Vesuvius and other volcanoes.** A 
 fissure sometimes reopens for a subsequent eruption. 
 
 Two obvious causes may be assigned for the pushing 
 upward of a crater-floor and the fissuring of a volcanic 
 cone (1) the enormous pressure of the dissolved vapors 
 or gases acting upon the walls and roof of the funnel and 
 convulsing the cone by successive explosions; and (2) the 
 hydrostatic pressure of the lava-column in the funnel, which 
 may be taken to be about 120 Ibs. per square inch, or nearly 
 8 tons on the square foot, for each 100 feet of depth. Both 
 of these causes may act simultaneously, and their united 
 effect has been to uplift enormous superincumbent masses 
 of solid rock and to produce a widespread series of long 
 and continuous fissures reaching from unknown depths to 
 various distances from the surface and even opening up 
 sometimes on the surface. These results of the expansive 
 energy of volcanic action are of special interest to the 
 geologist, for he encounters evidence of similar operations 
 in former times preserved in the crust of the earth (see 
 Book IV. Part VII. Sect. i.). 
 
 Into rents thus formed, the water-substance or vapor 
 
 44 For fissures on Etna, see Silvestri, Boll. R. Geol. Com. Ital. 1874. 
 
 45 For a description of those of Iceland (which run chiefly N.E. to S.W., 
 and N. to S.) see T. Kjerulf, Nyt. Mag. xxi. 147.
 
 DYNAMICAL GEOLOGY 
 
 357 
 
 rises with great expansive force, followed by the lava 
 which solidifies there like iron in a mold. Where fissures 
 are vertical or highly inclined the igneous rock takes the 
 form of dikes or veins] where the intruded material has 
 forced its way more or less in a horizontal direction be- 
 tween strata of tuff, beds of non-volcanic sediments, or 
 
 Fjg. 42. View of Lava-dikes, Valle del Bove, Etna (Abich). 
 
 flows of lava, it takes the form of sheets (sills) or beds. The 
 cliffs of many an old crater show how marvellously they 
 have been injected by such veins, dikes, or sheets of lava. 
 Those of Somma, and the Valle del Bove on Etna (Fig. 42), 
 which have long been known, project now from the softer 
 tuffs like walls of masonry. 48 The crater cliffs of Santorin 
 also present an abundant series of dikes. The permanent 
 separation of the walls of fissures by the consolidation of 
 the lava that rises in them as dikes must widen the dimen- 
 
 46 S. von Waltershausen "Der Aetna," ii. p. 341.
 
 358 
 
 TEXT-BOOK OF GEOLOGY 
 
 sions of a cone, for the fissures are not due to shrinkage, 
 although doubtless the loosely piled fragmentary materials, 
 in the course of their consolidation, develop lines of joint. 
 Sometimes the lava has evidently risen in a state of extreme 
 fluidity, and has at once filled the rents prepared for it, 
 
 Fig. 43. Dike contorting beds of tuff. Crater of Vesuvius (Abich). 
 
 cooling rapidly on the outside as a true volcanic glass, but 
 assuming a distinctly crystalline structure inside (ante, p. 
 296). Dikes of this kind, with a vitreous crust on their 
 sides, may be seen on the crater-wall of Somma, and not 
 uncommonly among basalt dikes in Iceland and Scotland. 
 In other cases, the lava had probably already acquired 
 a more viscous or even lithoid char- 
 acter ere it rose in the fissure, and 
 in this condition was able to push 
 aside and even contort the strata of 
 tuff through which it made its way 
 
 Fig. 44. Section of Dikes of ,_.. . _,. .. , ,. , , , 
 
 Lava traversing the bedded (Fig. 43). There can be little doubt 
 
 tuffs of a volcanic cone. 
 
 that in the architecture of a volcano, 
 
 dikes must act the part of huge beams and girders (Fig. 
 44), binding the loose tuffs and intercalated lavas together, 
 and strengthening the cone against the effects of subse- 
 quent convulsions.
 
 DYNAMICAL GEOLOGY B59 
 
 From this point of view, an explanation suggests itself 
 of the observed alternations in the character of a volcano's 
 eruptions. These alternations may depend in great measure 
 upon the relation between the height of the cone, on the 
 one hand, and the strength of its sides, on the other. When 
 the sides have been well braced together by interlacing 
 dikes, and further thickened by the spread of volcanic 
 materials all over their slopes, they may resist the effects 
 of explosion and of the pressure of the ascending lava- 
 column. In this case, the volcano may find relief only 
 from its summit, and if the lava flows forth, it will do so 
 from the top of the cone. As the cone increases in eleva- 
 tion, however, the pressure from within upon its sides aug- 
 ments. Eventually egress is once more established on the 
 flanks by means of fissures, and a new series of lava-streams 
 is poured out over the lower slopes (see Fig. 62). 
 
 In the deeper portions of a volcanic vent the convulsive 
 efforts of the lava-column to force its way upward must 
 often produce lateral as well as vertical rifts, aud into 
 these the molten material will rush, exerting as it goes an 
 enormous upward pressure on the mass of rock overlying 
 it. At a modern volcano these subterranean manifestations 
 cannot be seen, but among the volcanoes of Tertiary and 
 older time they have been revealed by the progress of 
 denudation. Some of these older examples teach us the 
 prodigious upheaving power of the sheets of molten rock 
 intruded between volcanic or other strata. An account of 
 this structure (sills, laccolites), with reference to some 
 examples of it, will be found in Book IV. Part VII.* T 
 
 Though lava very commonly issues from the lateral 
 
 See particularly the description of intrusive sheets or laccolites.
 
 360 TEXT-BOOK OF GEOLOGY 
 
 fissures on a volcanic cone, it may sometimes approach the 
 surface in thejn without actually flowing out. The great 
 fissure on E'tna in 1669, for example, was visible even from 
 a distance, by the long line of vivid light which rose from 
 the incandescent lava within. Again, it frequently happens 
 that minor volcanic cones are thrown up on the line of a 
 fissure, either from the congelation of the lava round the 
 point of emission, or from the accumulation of ejected 
 scoriae round the fissure-vent. One of the most remark- 
 able examples of this kind is that of the Laki fissure in 
 Iceland, the whole length of which (12 miles) bristles with 
 small cones and craters almost touching each other." 
 
 Explosions. Apart from the appearance of visible fis- 
 sures, volcanic energy may be, as it were, concentrated 
 on a given point, which will usually be the weakest in the 
 structure of that part of the terrestrial crust, and from 
 which the solid rock, shattered ,into pieces, is hurled into 
 the air by the enormous expansive energy of the volcanic 
 vapors." This operation has often been observed in vol- 
 canoes already formed, and has even been witnessed on 
 ground previously unoccupied by a volcanic vent. The 
 history of the cone of Vesuvius brings before us a long 
 series of such explosions, beginning with that of A.D. 79, 
 and coming down to the present day (Fig. 45). Even now, 
 in spite of all the lava and ashes poured out during the last 
 eighteen centuries, it is easy to see how stupendous must 
 have been that earliest explosion, by which the southern 
 half of the ancient crater was blown out. At every suc- 
 
 48 A. Eelland, "Lakis Kratere og Lava-strome, " cited on p. 345. On this 
 straight fissure some 600 craters rise, varying from 6 to 450 feet high. 
 
 49 See Daubree's experiments on the mechanical effects of gas at high pres- 
 sures, Comptes Rend, cxi., cxii. cxiii. and Bull. Soc. Geol. France, xix. (1891), 
 p. 313.
 
 DYNAMICAL GEOLOGY 361 
 
 cessive important eruption, a similar but minor operation 
 takes place within the present cone. The hardened cake 
 of lava forming the floor is burst open, and with it there 
 usually disappears much of the upper part of the cone, and 
 sometimes, as in 1872, a large segment of the crater-wall. 
 The Valle del Bove on the eastern flank of Etna is a chasm 
 probably due mainly to some gigantic prehistoric explo- 
 sion. 80 The islands of Santorin (Figs. 65 and 66) bring 
 before us evidence of a prehistoric catastrophe of a similar 
 
 ?. 45. View of Vesuvius from the south, 
 Showing the remaining part of the old crater-wall of Somma behind. 
 
 nature, by which a large volcanic cone was blown up. The 
 existing outer islands are a chain of fragments of the pe- 
 riphery of the cone, the centre of which is now occupied by 
 the sea. In the year 1538 a new volcano, Monte Nuovo, 
 was formed in 24 hours on the margin of the Bay of Naples. 
 An opening was drilled by successive explosions, and such 
 quantities of stones, scori*, and ashes were thrown out from 
 it as to form a hill that rose 440 English feet above the sea- 
 level, and was more than a mile and a half in circumfer- 
 ence. Most of the fragments c^w to be seen on the slopes 
 of this cone and inside its beautifully perfect crater are of 
 
 60 "Der Aetna," p. 400. 
 GEOLOGY Vol. XXIX 16
 
 362 TEXT-BOOK OF GEOLOGY 
 
 various volcanic rocks, many of them being black scoriae; 
 but pieces of Roman pottery, together with fragments of 
 the older underlying tuff, and some marine shells, have 
 been obtained doubtless part of the soil and subsoil dis- 
 located and ejected during the explosions. 
 
 One of the most stupendous volcanic explosions on 
 record was that of Krakatoa in the Sunda Strait on the 
 26th and 27th of August, 1883. 61 After a series of convul- 
 sions, the greater portion of the island was blown out with 
 a succession of terrific detonations which were heard more 
 than 150 miles away. A mass of matter, estimated at about 
 1-g cubic miles in bulk, was hurled into the air in the form 
 of lapilli, ashes, and the finest volcanic dust. The effects 
 of this volcanic outburst were marked both upon the atmos- 
 phere and the ocean. A series of barometrical disturbances 
 passed round the globe in opposite directions from the vol- 
 cano at the rate of about 700 miles an hour. The air-wave, 
 travelling from east to west, is supposed to have passed three 
 and a quarter times round the earth (or 82,200 miles) before 
 it ceased to be perceptible." The sea in the neighborhood 
 was thrown into waves, one of which was computed to have 
 risen more than 100 feet above tide-level, destroying towns, 
 villages, and 36,380 people. Oscillations of the water were 
 perceptible even at Aden, 1000 miles distant, at Port Eliza- 
 beth in South Africa, 5450 miles, and among the islands of 
 the Pacific Ocean, and they are computed to have travelled 
 with a maximum velocity of 467 statute miles in the hour." 
 
 51 See "The Eruption of Krakatoa," by a Committee of the Royal Society, 
 1888. "Krakatau," R. D. M. Verbeck, Batavia, 1886. 
 
 58 Scott and Strachey, Proc. Roy. Soc. xxxvi. (1883). Royal Society's 
 Report, p. 57. 
 
 63 Wharton, Royal Society's Report, p. 89. For a great Japanese explosion, 
 see Nature, 13th Sept. 1888.
 
 DYNAMICAL GEOLOGY 363 
 
 It is not necessary, and it does not always happen, that 
 any actual solid or liquid volcanic rock is erupted by explo- 
 sions that shatter the rocks through which the funnel passes. 
 Thus, among the cones of the extinct volcanic tract of the 
 Eifel, some occur which consist entirely, or nearly so, of 
 comminuted debris of the surrounding Devonian graywacke 
 and slate through which the various volcanic vents have 
 been opened (see pp. 341, 417, 970). Evidently, in such 
 cases, only elastic vapors forced their way to the surface; 
 and we see what probably often takes place in the early 
 stages of a volcano's history, though the fragments of the 
 underlying disrupted rocks are in most instances buried and 
 lost under the far more abundant subsequent volcanic mate- 
 rials. Sections of small ancient volcanic "necks" or pipes 
 sometimes afford an excellent opportunity of observing that 
 these orifices were originally opened by the blowing out of 
 the solid crust and not by the formation of fissures. Exam- 
 ples will be cited in later pages from Scottish volcanic areas 
 of Old Red Sandstone, Carboniferous, and Permian age. 
 The orifices are there filled with fragmentary materials, 
 wherein portions of the surrounding and underlying rocks 
 form a noticeable proportion.* 4 (See Figs. 296-301.) 
 
 Showers of Dost and Stones. A communication having 
 been opened, either by fissuring or explosion, between the 
 heated interior and the surface, fragmentary materials are 
 commonly ejected from it, consisting at first mainly of 'the 
 rocks through which the orifice has been opened, afterward 
 of volcanic substances. In a great eruption, vast numbers 
 of red-hot stones are shot up into the air, and fall back 
 partly into the crater and partly on the outer slopes of the 
 
 54 Trans. Roy. Soc. Edin. xxix. p. 458; Quart. Journ. Geol. Soc. (1892), 
 President's Address, pp. 86, 118, 135, 143, 153.
 
 864 TEXT-BOOK OF GEOLOGY 
 
 cone. According to Sir W. Hamilton, cinders were thrown 
 by Vesuvius, during the eruption of 1779, to a height of 
 10,000 feet. Instances are known where large stones, ejected 
 obliquely, have described huge parabolic curves in the air, 
 and fallen at a great distance. Stones 8 Ibs. in weight occur 
 among the ashes which buried Pompeii. The volcano of 
 Antuco in Chile is said to send stones flying to a distance 
 of 36 (?) miles, Cotopaxi is reported to have hurled a 200- 
 ton block 9 miles, 66 and the Japanese volcano, Asama, is 
 said to have ejected many blocks of stone, measuring from 
 40 to more than 100 feet in diameter. 66 
 
 But in many great eruptions, besides a constant shower 
 of stones and scoriae, a vast column of exceedingly fine dust 
 rises out of the crater, sometimes to a height of several 
 miles, and then spreads outward like a sheet of cloud. The 
 remarkable fineness of this dust may be understood from 
 the fact that during great volcanic explosions no boxes, 
 watches, or close-fitting joints have been found to be able 
 to exclude it. Mr. Whymper collected some dust that fell 
 65 miles away from Cotopaxi, and which was so fine that 
 from 4,000 to 25,000 particles were required to weigh a 
 grain." So dense is the dust-cloud as to obscure the sun, 
 and for days together the darkness of night may reign for 
 miles around the volcano. In 1822, at Vesuvius, the ashes 
 not only fell thickly on the villages round the base of the 
 mountain, but travelled as far as Ascoli, which is 56 Italian 
 miles distant from the volcano on one side, and as Casano, 
 105 miles on the other. The eruption of Cotopaxi, on 26th 
 
 55 D. Forbes, Geol. Mag. vii. p. 320. 
 
 56 J. Milne, Seism. Soc. Japan, ix. p. 179, where an excellent account of the 
 volcanoes of Japan is given. See also "The Volcanoes of Japan," by J. Milne 
 and W. K. Burton. 
 
 M Royal Society Report on Krakatoa, p. 183.
 
 DYNAMICAL GEOLOGY 365 
 
 June, 1877, began by an explosion that sent up a column of 
 fine ashes to a prodigious height into the air, where it rapidly 
 spread out and formed so dense a canopy as to throw the 
 region below it into total darkness. 68 So quickly did it 
 diffuse itself, that in an hour and a half, a previously bright 
 morning became at Quito, 33 miles distant, a dim twilight, 
 which in the afternoon passed into such darkness that the 
 hand placed before the eye could not be seen. At Guaya- 
 quil, on the coast, 150 miles distant, the shower of ashes 
 continued till the 1st of July. Dr. Wolf collected the ashes 
 daily, and estimated that at that place there fell 315 kilo- 
 grammes on every square kilometre during the first thirty 
 hours, and on the 30th of June, 209 kilogrammes in twelve 
 hours. 68 During a much less important eruption of the same 
 mountain on 3d July, 1880, the amount of volcanic dust 
 ejected, according to Mr. Whymper, could not have been 
 less, and was probably vastly more, than two millions of 
 tons,* equivalent to a mass of lava containing more than 
 150,000 cubic feet. 
 
 The explosion of Krakatoa in August, 1883, was accom- 
 panied by the discharge of enormous quantities of volcanic 
 dust, some of which was carried to vast distances. It was 
 estimated that the clouds of fine dust were hurled from that 
 volcano to a height of 17 miles, and the darkness which they 
 caused extended for 150 miles from the focus of eruption. 
 The diffusion and continued suspension of the finer particles 
 
 58 During the comparatively insignificant eruption of the volcano in 1880 
 Mr. Whymper noticed that a column of inky blackness, formed doubtless of 
 volcanic dust, went straight up into the air with such velocity that in less than a 
 minute it had risen 20,000 feet above the rim of the crater, or 40,000 feet above 
 the sea. "Travels amongst the Great Andes," p. 322. 
 
 59 Neues Jahrb. 1878, p. 141. An account of this eruption is given by Mr. 
 Whymper in his "Travels amongst the Great Andes," chap. vi. 
 
 eo "Travels amongst the Great Andes," p. 328.
 
 366 TEXT-BOOK OF GEOLOGY 
 
 of this dust in the upper air has been regarded as the proba- 
 ble cause of the remarkably brilliant sunsets of the follow- 
 ing winter and spring over a large part of the earth's sur- 
 face. 61 One of the most stupendous outpourings of volcanic 
 ashes on record took place, after a quiescence of 26 years, 
 from the volcano Coseguina, in Nicaragua, during the early 
 part of the year 1835. On that occasion, utter darkness pre- 
 vailed over a circle of 35 miles radius, the ashes falling so 
 thickly that, even 8 leagues from the mountain, they cov- 
 ered the ground to a depth of about 10 feet. It was esti- 
 mated that the rain of dust and sand fell over an area at 
 least 270 geographical miles in diameter. Some of the finer 
 materials, thrown so high as to come within the influence of 
 an upper air-current, were borne away eastward, and fell, 
 four days afterward, at Kingston, in Jamaica a distance of 
 700 miles. During the great eruption of Sumbawa, in 1815, 
 the dust and stones fell over an area of nearly one million 
 square miles, and were estimated by Zollinger to amount to 
 fully fifty cubic miles of material, and by Junghuhn to be 
 equal to one hundred and eighty-five mountains like Vesu- 
 vius. Toward the end of the 18th century, during a time 
 of great disturbance among the Japanese volcanoes, one of 
 them, Sakurajima, threw out so much pumiceous material 
 that it was possible to walk a distance of 23 miles upon the 
 floating debris in the sea. 
 
 An inquiry into the origin of these showers of fragmen- 
 tary materials brings vividly before us some of the essential 
 features of volcanic action. We find that bombs, slags, and 
 lapilli may be thrown up in comparatively tranquil states of 
 a volcano, but that the showers of fine dust are discharged 
 
 Royal Society Report, pp. 151-463.
 
 DYNAMICAL GEOLOGY 367 
 
 with violence, and only appear when the volcano becomes 
 more energetic. Thus, at the constantly, but quietly, active 
 volcano of Stromboli, the column of lava in the pipe may be 
 watched rising and falling with a slow rhythmical move- 
 ment. At every rise, the surface of the lava swells up into 
 blisters several feet in diameter, which by and by burst with 
 a sharp explosion that makes the walls of the crater vibrate. 
 A cloud of steam rushes out, carrying with it hundreds of 
 fragments of the glowing lava, sometimes to a height of 1200 
 feet. It is by the ascent of steam through its mass, that a 
 column of lava is kept boiling at the bottom of the crater, 
 and by the explosion of successive larger bubbles of steam, 
 that the various bombs, slags, and fragments of lava are torn 
 off and tossed into the air. It has often been noticed at 
 Vesuvius that each great concussion is accompanied by a 
 huge ball-like cloud of steam which rushes up from the 
 crater. Doubtless it is the sudden escape of that steam 
 which causes the explosion. 
 
 The varying degree of liquidity or viscosity of the lava 
 probably modifies the force of explosions, owing to the dif- 
 ferent amounts of resistance offered to the upward passage 
 of the absorbed gases and vapors. Thus explosions and ac- 
 companying scoriae are abundant at Vesuvius, where the 
 lavas are comparatively viscid; they are almost unknown at 
 Kilauea, where the lava is remarkably liquid. 
 
 In tranquil' conditions of a volcano, the steam, whether 
 collecting into larger or smaller vesicles, works its way up- 
 ward through the substance of the molten lava, and as the 
 elasticity of this compressed vapor overcomes the pressure 
 of the overlying lava, it escapes at the surface, and there the 
 lava is thus kept in ebullition. But this comparatively 
 quiet operation, which may be watched within the craters
 
 368 TEXT-BOOK OF GEOLOGY 
 
 of many active volcanoes, does not produce clouds of fine 
 dust. The collision or friction of millions of stones ascend- 
 ing and descending in the dark column above the crater 
 must doubtless cause much dust and sand. But the explo- 
 sive action of steam is probably also an immediate cause of 
 much trituration. The aqueous vapor or water-gas which 
 is so largely dissolved in many lavas must exist within the 
 lava-column, under an enormous pressure, at a temperature 
 far above its critical point (p. 332), even at a white heat, and 
 therefore possibly in a state of dissociation. The sudden 
 ascent of lava so constituted relieves the pressure rapidly 
 without sensibly affecting the temperature of the mass. 
 Consequently, the white-hot gases or vapors at length ex- 
 plode, and reduce the molten mass to the finest powder, like 
 water shot out of a gun." 
 
 Evidently no part of the operations of a volcano has 
 greater geological significance than the ejection of such 
 enormous quantities of fragmentary matter. In the first 
 place, the fall of these loose materials round the orifice of 
 discharge is one main cause of the growth of the volcanic 
 cone. The heavier fragments gather around the vent, and 
 there too the thickest accumulation of dust and sand takes 
 place. Hence, though successive explosions may blow out 
 the upper part of the crater- walls and prevent the mountain 
 from growing so rapidly in height, every eruption must in- 
 crease the diameter of the cone. In the second place, as 
 every shower of dust and sand adds to the height of the 
 ground on which it falls, thick volcanic accumulations may 
 
 68 Messrs. Murray and Eenard (Proc. Roy. Soc. Bdin. xii. (1884), p. 480) 
 concluded that the fragmentary condition and the fresh fractures of the dust 
 particles of the Krakatoa eruption were due to a tension phenomenon, which 
 affects these vitreous matters in a manner analogous to what is observed in 
 "Rupert's drops."
 
 DYNAMICAL GEOLOGY Sbi) 
 
 be formed far beyond the base of the mountain. The vol- 
 cano of Sangay, in Ecuador, for instance, has buried the 
 country around it to a depth of 4000 feet under its ashes." 
 In such loose deposits are entombed trees and other kinds of 
 vegetation, together with the bodies of animals, as well as 
 the works of man. In some cases, where the layer of vol- 
 canic dust is thin, it may merely add to the height of the 
 soil, without sensibly interfering with the vegetation. But 
 it has been observed at Santorin that though this is true in 
 dry weather, the fall of rain with the dust at once acts detri- 
 mentally. On the 3d of June, 1866, the vines were there 
 withered up, as if they had been burned, along the track of 
 the smoke cloud.* 4 By the gradual accumulation of vol- 
 canic ashes, new geological formations arise which, in their 
 component materials, not only bear witness to the volcanic 
 eruptions that produced them, but preserve a record of the 
 land-surfaces over which they spread. In the third place, 
 besides the distance to which the fragments may be hurled 
 by volcanic explosions, or to which they may be diffused by 
 the ordinary aerial movements, we have to take into account 
 the vast spaces across which the liner dust is sometimes 
 borne by upper air-currents. In the instance already cited, 
 ashes from Coseguina fell 700 miles away, having been car- 
 ried all that long distance by a high counter-current of air, 
 moving apparently at the rate of about seven miles an hour 
 in an opposite direction to that of the wind which blew at the 
 surface. By the Sumbawa eruption, also referred to above, 
 the sea west of Sumatra was covered with a layer of ashes 
 two feet thick. On several occasions ashes from the Ice- 
 landic volcanoes have fallen so thickly between the Orkney 
 
 63 D. Forbes, Geol. Mag. vii. 320. ** Fouque, "Santorin," p. 81.
 
 370 TEXT-BOOK OF GEOLOGY 
 
 and Shetland Islands, that vessels passing there have had 
 the unwonted deposit shovelled off their decks in the morn- 
 ing. In the year 1783, during the memorable eruption of 
 Skaptar-Jdkull, so vast an amount of fine dust was ejected 
 that the atmosphere over Iceland continued loaded with it 
 for months afterward. It fell in such quantities over parts 
 of Caithness a distance of 600 miles as to destroy the 
 crops; that year is still spoken of by the inhabitants as the 
 year of "the ashie." Traces of the same deposit have been 
 observed in Norway, and even as far as Holland.' 6 Hence 
 it is evident that volcanic accumulations may take place in 
 regions many hundreds of miles distant from any active vol- 
 cano. A single thin layer of volcanic detritus in a group of 
 sedimentary strata would not thus of itself prove the exist- 
 ence of contemporaneous volcanic action in its neighbor- 
 hood. Failing other proof of adjacent volcanic activity, it 
 might have been wind-borne from a volcano in a distant 
 region. 
 
 Lava-streams. At its exit from the side of a volcano, 
 lava glows with a white heat, and flows with a motion which 
 has been compared to that of honey or of melted iron. It 
 soon becomes red, and like a coal fallen from a hot fireplace, 
 rapidly grows dull as it moves along, until it assumes a 
 black, cindery aspect. At the same time the surface con- 
 geals, and soon becomes solid enough to support a heavy 
 block of stone. The aspect of the stream varies with the 
 composition and fluidity of the lava, form of the ground, 
 angle of slope, and rapidity of flow. Viscous lavas, like 
 those of Vesuvius, break up along the surface into rough 
 
 66 Nordenskiold, Geol. Mag. 2d dec. iii. p. 292. G. vom Rath, Mouatsber. 
 K. Preuss. Akad. Wiss. 1876, p. 282. Neues Jahrb. 1876, p. 52, and postea, 
 p. 675.
 
 DYNAMICAL GEOLOGY 
 
 371 
 
 brown or black cinder-like slags and irregular ragged cakes, 
 bristling with jagged points ("aa" 86 ), which, in their onward 
 motion, grind and grate against each other with a harsh me- 
 tallic sound, sometimes rising into rugged mounds or be- 
 coming seamed with rents and gashes, at the bottom of 
 which the red-hot glowing lava may be seen (Fig. 46). In 
 lavas possessing somewhat greater fluidity, the surface pre- 
 sents froth-like, curving lines, as in the scum of a slowly 
 
 Figr. 46. View of portion of a Lava-stream on Vesuvius (Abich). 
 
 flowing river, or is arranged in curious ropy folds, as the 
 layers nave successively flowed over each other and con- 
 gealed ("pahoehoe" ""). These, and many other fantastic 
 coiled shapes were exhibited by the Yesuvian lava of 1858, 
 and are admirably displayed by the peculiarly liquid glassy 
 lavas of Kilauea. 86 Basalts possessing extreme liquidity 
 
 66 For descriptions of Vesuvian lava-streams, see the various memoirs and 
 works cited, anle, p. 333. For those of Etna, Sartorius von Waltershausen and 
 A. von Lasaulx, "Der Aetna," ii. p. 390. The rugged scoriaceous lava-surfaces 
 are known in Hawaii as aa, the smooth coiled and ropy surfaces are there called
 
 TEXT-BOOK OF GEOLOGY 
 
 have flowed for great distances with singularly smooth sur- 
 faces. A large area which has been flooded with lava is 
 perhaps the most hideous and appalling scene of desolation 
 anywhere to be found on the surface of the globe. 
 
 A lava-stream usually spreads out as it descends from its 
 point of escape, and moves more slowly. Its sides look like 
 huge embankments, or like some of the long mounds of 
 
 Pig. 47. View of houses surrounded and partly demolished by the Lava 
 of Vesuvius, 1872. 
 
 "clinkers" in a great manufacturing district. The advancing 
 end is often much steeper, creeping onward like a great wall 
 or rampart, down the face of which the rough blocks of 
 hardened lava are ever rattling (Fig. 47). 
 
 Outflow of Lava. This appears to be immediately 
 due to the expansion of the absorbed vapors and gases in 
 the molten rock. Though these vapors may reach the sur- 
 
 paJwehoe. Dana, "Characteristics of Volcanoes," p. 9. The same stream of 
 lava may exhibit both these aspects in different parts of its course. Ibid. p. 209 
 and Mr. Johnston-Lavis' papers on Vesuvius, already cited p. 333.
 
 DYNAMICAL GEOLOGY 373 
 
 face, and even produce tremendous explosions, without an 
 actual outcome of lava, yet so intimately are vapors and 
 lava commingled in the subterranean reservoirs, that they 
 commonly rise together, and the explosions of the one lead 
 to the outflow of the other. The first point at which the 
 lava makes its appearance at the surface will largely depend 
 upon the structure of the ground. Two causes have been 
 assigned on a foregoing page (p. 356) for the fissuring of a 
 volcanic cone. As the molten mass rises within the chim- 
 ney of the volcano, continued explosions of vapor take 
 place from its upper surface. The violence of these may 
 be inferred from the vast clouds of steam, ashes, and stones 
 hurled to so great a height into the air, and from the con- 
 cussions of the ground, which may be felt at distances of 
 more than 100 miles from the volcano. It need not be 
 a matter of surprise, therefore, that the sides of a great 
 vent, exposed to shocks of such intensity, should at last 
 give way, and that large divergent fissures should be 
 opened down the cone. Again, the hydrostatic pressure 
 of the column of lava must, at a depth of 1000 feet below 
 the top of the column, exert a pressure of between 70 and 
 80 tons on each square foot of the surrounding walls (p. 356). 
 We may well believe that such a force, acting upon the 
 walls of a funnel already shattered by a succession of ter- 
 rific explosions, may prove too great for their resistance. 
 When this happens, the lava pours forth from the outside 
 of the cone. On a much-fissured cone, lava may issue 
 freely from many points, so that a volcano so affected has 
 been graphically described as "sweating fire." 
 
 In a lofty volcano, lava occasionally rises to the lip of 
 the crater and flows out there; but more frequently it 
 escapes from some fissure or orifice in a weak part of the
 
 874 TEXT-BOOK OF GEOLOGY 
 
 cone. In minor volcanoes, on the other hand, where the 
 explosions are less violent, and where the thickness of 
 the cone in proportion to the diameter of the funnel is often 
 greater, the lava very commonly rises into the crater. 
 Should the crater-walls be too weak to resist the pressure 
 of the molten mass, they give way, and the lava rushes 
 out from the breach. This is seen to have happened in 
 several of the puys of Auvergne, so well figured and de- 
 scribed by Scrope (Fig. 48). " But if the crater be massive 
 
 Fig. 48. View of one of the Tuff-cones of Auvergne, broken aown on one side 
 by the escape of a stream of Lava. (After Scrope.) 
 
 enough to withstand the pressure, the lava may at last flow 
 out from the lowest part of the rim. 
 
 In a tall column of molten lava, there may be a varia- 
 tion in the density of its different parts, the heaviest 
 naturally gravitating to the bottom. It has been observed 
 by Ch. Velain that at the Isle of Bourbon (Reunion), the 
 lavas escaping from the base of the volcanic cone are denser 
 
 61 For descriptions of this region, see Scrope's "Geology and Extinct Volca- 
 noes of Central France, " 2d edit. 1858. H. Lecoq's "Epoques ge"ologiques de 
 1'Auvergne," 1867. Michel-Levy, Bull. Soc. Geol. France, xviii. (1890), p. 688. 
 The succession of volcanic rocks in Velay is described by M. Boule, Bull. Soc. 
 Geol. France, xviii. (1889), p. 174, and in Bull. Carte Geol. de la France, No. 28 
 (1892); see also op. cit. No. 13 for a memoir by P. Termier.
 
 DYNAMICAL GEOLOGY 375 
 
 and more basic than those which flow out from the lip of 
 the crater. 88 
 
 As soon as the molten rock reaches the surface, the 
 superheated water- vapor or gas, dissolved within its mass, 
 escapes copiously, and hangs as a dense white cloud over 
 the moving current. The lava-streams of Vesuvius some- 
 times appear with as dense a steam-cloud at their lower 
 ends as that which escapes at the same time from the main 
 crater. Even after the molten mass has flowed several 
 miles, steam continues to rise abundantly both from its 
 end and from numerous points along its surface, and con- 
 tinues to do so for many weeks, months, or it may be for 
 several years. 
 
 Should the point of escape of a lava-stream lie well 
 down on the cone, far below the summit of the lava-column 
 in the funnel, the molten rock, on its first escape, driven 
 by hydrostatic pressure, will sometimes spout up high into 
 the air a fountain of molten rock. This was observed in 
 1794 on Vesuvius, and in 1832 on Etna. In the eruption 
 of 1852 at Mauna Loa, an unbroken fountain of lava, from 
 200 to 700 feet in height and 1000 feet broad, burst out at 
 the base of the cone. Similar "geysers" of molten rock 
 have subsequently been noticed in the same region. Thus 
 in March and April, 1868, four fiery fountains, throwing 
 lava to heights varying from 500 to 1000 feet, continued 
 to play for several weeks. According to Mr. Coan, such 
 outbursts take place from the bottom of a column of lava 
 3000 feet high. The volcano of Mauna Loa strikingly illus- 
 trates another feature of volcanic dynamics in the position 
 and outflow of lava. It bears upon its flanks at a distance 
 
 68 l 'Les Volcans," p. 36. For references relating to this island, see p. 416.
 
 376 TEXT-BOOK OF GEOLOGY 
 
 of 20 miles, but 10,000 feet lower, the huge crater Kilauea. 
 As Dana has pointed out, these orifices form part of one 
 mountain, yet the column of lava stands 10,000 feet higher 
 in one conduit than in the other. On a far smaller scale 
 the same independence occurs among the several pipes of 
 some of the geysers in the Yellowstone region of North 
 America. 
 
 From the wide extent of basalt-dikes, such as those of 
 Tertiary age in Britain, which rise to the surface at a dis- 
 tance of 200 miles from the main remnants of the volcanic 
 outbursts of their time, and are found over an area of per- 
 haps 100,000 square miles, it is evident that molten lava 
 may sometimes occupy a far greater space within the crust 
 than might be inferred from the dimensions and outpour- 
 ings even of the largest volcanic cone. There can be no 
 doubt that vast reservoirs of melted rock, impregnated with 
 superheated vapors, must formerly have existed, if they do 
 not exist still, beneath extensive tracts of countiy (p. 967). 
 Yet even in these more stupendous manifestations of vol- 
 canism, the lava should be regarded rather as the sign than 
 as the cause of volcanic action. The cause of the ascent of 
 the lava in volcanic pipes is still obscure: it may possibly 
 be due to the compression arising from the secular contrac- 
 tion of the earth. But it is doubtless the pressure of the 
 imprisoned vapor, and its struggles to get free, which pro- 
 duce the subterranean earthquakes and the explosions from 
 the vents. As soon as the vapor finds relief, the terrestrial 
 commotion calms down again, until another accumulation 
 of vapor demands a repetition of the same phenomena. 
 
 Bate of flow of Lava. The rate of movement is 
 regulated by the fluidity of the lava, by its volume, and 
 by the form and inclination of the ground. Hence, as a
 
 DYNAMICAL GEOLOGY 377 
 
 rule, a lava-stream moves faster at first than afterward, be- 
 cause it has not had time to stiffen, and its slope of descent 
 is usually steeper than further down the mountain. One 
 of the most fluid and swiftly flowing lava-streams ever 
 observed on Vesuvius was that erupted on 12th August, 
 1805. It is said to have rushed down a space of 3 Italian 
 (3f English) miles in the first four minutes, but to have 
 widened out and moved more slowly as it descended, yet 
 finally to have reached Torre del Greco in three hours. A 
 lava erupted by Maun a Loa in 1852 went as fast as an ordi- 
 nary stage-coach, or fifteen miles in two hours; but some of 
 the lavas from that mountain have in parts of their course 
 moved with double that rapidity. Long after a current 
 has been deeply crusted over with slags and rough slabs 
 of lava, it may continue to creep slowly forward for weeks 
 or even months. 
 
 It happens sometimes that, as the lava moves along, the 
 still molten mass inside bursts through the outer hardened 
 and deeply seamed crust, and rushes out with, at first, a 
 motion much more rapid than that of the main stream. 
 Any sudden change in the form or slope of the ground 
 affects the flow of the lava. Thus, reaching the edge of 
 a steep defile or cliff, the molten rock pours over in a cata- 
 ract of glowing, molten rock, with clouds of steam, showers 
 of fragments, and a noise utterly indescribable. Or, on the 
 other hand, encountering a ridge or hill across its path, it 
 accumulates until it either finds egress round the side or 
 actually overrides and entombs the obstacle. The hardened 
 crust or shell, within which the still fluid lava moves, serves 
 to keep the mass from spreading. Here and there, inside 
 this crust, the lava subsides, leaving cavernous spaces and 
 tunnels into which, when the whole is cold, one may creep,
 
 378 TEXT-BOOK OF GEOLOGY 
 
 and which are sometimes festooned with stalactites of lava 
 (p. 387). 
 
 Size of Lava-streams. In some cases, lava escap- 
 ing from craters or fissures comes to rest before reaching 
 the base of the slopes, like the obsidian current which has 
 congealed on the side of the little volcanic island of Vol- 
 cano. 8 ' In other instances, the molten rock not only reaches 
 the plains but flows for many miles away from the point of 
 eruption. Sartorius von Waltershausen computed the lava 
 emitted by Etna in 1865 at 92 millions of cubic metres, that 
 of 1852 at 420 millions, that of 1669 at 980 millions, and 
 that of a prehistoric lava-stream near Randazzo at more 
 than 1000 millions. 70 The most stupendous outpouring of 
 lava on record was that which took place in Iceland in the 
 year 1783. Successive streams issued from a fissure about 
 12 miles long, filling up river-gorges which were sometimes 
 600 feet deep and 200 feet broad, and advancing into the 
 alluvial plains in lakes of molten rock 12 to 15 miles wide 
 and 100 feet deep. Two currents of lava which, filling up 
 the valley of the Skapta, escaped in nearly opposite direc- 
 tions, extended for 45 and 50 miles respectively, their usual 
 thickness being 100 feet. Bischof estimated that the total 
 amount of lava poured forth during this single eruption 
 "surpassed in magnitude the bulk of Mont Blanc." 71 
 
 Varying liquidity of Lava. All lava, at the 
 time of its expulsion, is in a molten condition. It usually 
 
 69 Recent eruptions in this island have consisted entirely of ashes. A. Balt- 
 zer, Zeitsch. Deutsch. Geol. Ges. xxvi. (1875), p. 36. G. Mercalli, "Le Eruzi- 
 oni dell' Isola Vulcano, " Rassegna Nazionale, 1889; also a paper by same 
 author in Atti. Soc. Ital. Sci. Nat., vol. xxxi. 
 
 10 "Der Aetna," ii. 303. 
 
 11 Lyell, "Principles," ii. p. 49. Holland, "Lakis Kratere," cited ante, 
 p. 345.
 
 DYNAMICAL GEOLOGY 379 
 
 consists of a glassy magma in which, by reason of the high 
 temperature, most or even all of the mineral constituents 
 exist dissolved. Considerable differences, however, have 
 been observed in the degree of liquidity. Humboldt and 
 Scrope long ago called attention to the thick, short, lumpy 
 form* presented by masses of solidified trachytic rocks, 
 which are lighter and more siliceous, and to the thin, 
 widely extended sheets assumed by basalts, which are 
 heavy and contain much iron and basic silicates. It may 
 be inferred that, as a rule, the basalts or basic lavas have 
 been more liquid than the trachytes or siliceous lavas. 
 The cause of this difference has been variously explained. 
 It may depend partly upon chemical composition, the sili- 
 ceous being naturally less fusible than the basic rocks. 
 But as great differences of fluidity are observable even 
 among lavas having nearly the same composition, there 
 would seem to be some further cause for the diversity. 
 Eeyer has ingeniously maintained that we must look to 
 original differences in the extent to which the subterra- 
 nean igneous magma that supplied the lava has been 
 saturated with vapors and gases. Molten rock highly im- 
 pregnated gives rise, he holds, to fragmentary discharges, 
 while when feebly impregnated it flows out tranquilly." 
 On the other hand, Captain C. E. Dutton, who has studied 
 the volcanic phenomena of Western America and Hawaii, 
 suggests that the different degrees of liquidity may depend 
 not only on chemical differences, but on variations of tem- 
 perature. He supposes that the basaltic lavas which have 
 spread so far in thin sheets, and which must have had a 
 comparatively great liquidity, flowed at temperatures far 
 
 TO Scrope, "Considerations on Volcanoes" (1825), p. 93. 
 13 "Beitrag zur Physik der Eruptionen," p. 77.
 
 380 TEXT-BOOK OF GEOLOGY 
 
 above that of their melting-point, and were, to use his 
 phrase, "superfused." 74 
 
 The varying degrees of liquidity are manifested in a 
 characteristic way on the surface of lava. Thus, in the 
 great lava-pools of Hawaii, the rock exhibits a remarkable 
 liquidity, throwing up fountains of molten rock to a height 
 of 300 feet or more. During its ebullition in the crater- 
 pools, jets and dribblets, a quarter 
 of an inch in diameter, are tossed 
 up, and falling back on one another, 
 make "a column of hardened tears 
 of lava," one of which (Fig. 49) 
 was found to have attained a height 
 
 * &j>'f., ~' ; 'iv^vX of 40 feet, while in other places the 
 
 K^-Z$~ !''' : H; >v01&v^ 
 
 JwSsbhiL^L-^^JSc^sSPI jets thrown up and blown aside by 
 
 6f Kilauea (Dana)/' glagg which He thickly togeth er like 
 
 mown grass, and are known by the natives under the name 
 of "Pele's Hair," after one of their divinities." Yet al- 
 though the ebullition is caused by the uprise and escape of 
 highly heated vapors, there is no cloud over the boiling lake 
 itself, heavy white vapor only escaping at different points 
 along the edge. 
 
 On the other hand, the lavas of Vesuvius and of most 
 modern volcanoes, which issue so saturated with vapor as to 
 be nearly concealed from view in a cloud of steam, are ac- 
 companied by abundant explosions of fragmentary materials. 
 Slags and clinkers, torn by explosions of steam from the 
 
 14 "High Plateaus of Utah," Geog. and Geol. Sur. Territories. Washing- 
 ton, 1880, chap. v. 
 
 16 Dana, Geol. U. S. Explor. Exped., "Geology," p. 179; "Characteristics 
 of Volcanoes," p. 160.
 
 DYNAMICAL GEOLOGY 
 
 881 
 
 molten rock, are strewn abundantly over the cone, while the 
 surface of the lava is likewise rugged with similar clinkers, 
 which may now and then be observed piled up round some 
 more energetic steam-spiracle. Sometimes the vapor forces 
 up the lava round such a spiracle or furnarole and gradually 
 piles up a rugged column several feet or yards in height, as 
 
 Fig. 50. Lava-column (eight feet high), Vesuvius (Abich). 
 
 has been observed on Vesuvius 78 (Figs. 46, 49, 50). So vast 
 an amount of steam rushes out from one of these orifices, 
 and with such boiling and explosion, that the cone of bombs, 
 slags, and irregular lumps of lava forms a miniature or para- 
 sitic volcano, which will remain as a marked cone on its 
 
 16 Some good examples were observed on this mountain in the summer of 
 1891 by Mr. Johnston -La vis, Brit. Assoc. 1891, sect. C.
 
 382 TEXT-BOOK OF GEOLOGY 
 
 parent mountain long after the eruption which gave it birth 
 has ceased. The lava of the eruption at Santorin in 1866-67 
 at first welled out tranquilly, but after a few days its out- 
 flow was accompanied by explosions and discharges of in- 
 candescent fragments, which increased until they had cov- 
 ered the lava dome with ejected scoria?, and had opened a 
 number of crateriform mouths on its summit." 
 
 There can be no doubt, as above remarked, that the con- 
 dition of liquidity of the lava has in some measure deter- 
 mined the form of the eruptions. In one case, there are 
 quiet outwellings of the more liquid lavas, as at Hawaii; in 
 another, there are explosive discharges and cinder-cones, 
 accompanying the more viscid lavas, as at most modern vol- 
 canoes. The former has been the condition favorable to the 
 most colossal outpourings of molten rock, as we see in the 
 basalt-plateaus of Britain, Faroe, Greenland, Idaho, and 
 Oregon, the Ghauts, Abyssinia, etc. This subject is again 
 referred to at p. 433. 
 
 Crystallization of Lava. Pouring forth with a 
 liquidity like that of molten iron, lava speedily assumes a 
 more viscous condition and a slower motion. Obsidian and 
 other vitreous rocks have consolidated as glass: yet that 
 they are not always extremely fluid is indicated by the arrest 
 of the obsidian stream half-way down the steep northern 
 slope of Volcano. Even in such perfect natural glass as ob- 
 sidian, microscopic crystallites and crystals are usually pres- 
 ent, and in prodigious numbers (pp. 205, 282). In most 
 lavas, devitrification has proceeded so far before the final 
 stiffening, that the original glassy magma has passed into a 
 more or less completely lithoid or crystalline mass. 
 
 17 Fouque, "Santorin," p. xv.
 
 DYNAMICAL GEOLOGY 383 
 
 That lava may possess an appreciably crystalline struc- 
 ture while still in motion, has often been proved at Vesu- 
 vius, where well-defined crystals of the infusible leucite may 
 be observed in a molten magma of the other minerals, por- 
 tions of the white-hot rock in this condition being ladled 
 out, impressed with a stamp and suddenly congealed. The 
 fluxion-structure above described (pp. 178, 213) furnishes 
 interesting evidence of this fact in many ancient as well as 
 modern lavas. 
 
 There is reason to believe that in the molten magma be- 
 neath a volcano considerable progress may be made in the 
 development of some crystalline minerals out of the sur- 
 rounding glass, and that this crystalline portion may be to 
 some extent separated from the vitreous residue. Hence 
 where this has taken place, subsequent eruptions may give 
 rise to a more crystalline and probably more basic lava from 
 one point of emission and a more glassy and probably more 
 acid lava from another vent. Or we may conceive that the 
 two portions of the magma may be subsequently mingled 
 again in various proportions before eruption. 78 If the proc- 
 ess of differentiation should continue, as seems natural, dur- 
 ing the lapse of a whole cycle of a volcano's history, the 
 earlier lavas would be more basic than the later. 
 
 The crystalline structure of lava has been supposed to be 
 in some measure determined by the presence of the volcanic 
 vapors and gases with which the molten rock is impregnated, 
 the rapid escape of these vapors preventing the formation of 
 the crystalline structure, and leaving the lava in the condi- 
 tion of a more or less perfect glass. But the experiments of 
 
 18 Compare the observation of Oh. Velain cited ante, p. 219, and the remarks 
 postea, pp. 444, 457, 936. Consult on this subject a paper by Prof. Judd, Geol. 
 Mag. 1888, p. 1.
 
 384 TEXT-BOOK OF GEOLOGY 
 
 MM. Fouque* and Michel-LeVy (postea, p. 613) have shown 
 that rocks, having in every essential particular the charac- 
 ters of volcanic lavas, may be artificially produced under 
 ordinary atmospheric pressure by simple dry fusion. There 
 appears to be no doubt that the presence of water lowers the 
 fusion-point of silicates, though what precise influence the 
 dissolved vapors exert upon the ultimate consolidation of 
 molten lava has yet to be ascertained. Difference in the rate 
 of cooling has doubtless been an important, if not the main, 
 factor in determining the various conditions of texture of 
 lava-streams. The crystalline structure may be expected to 
 be most perfect where, as within thick masses of rock, the 
 cooling has been prolonged, and where, consequently, the 
 crystals have had ample time and opportunity for their for- 
 mation. On the other hand, the glassy structure will natu- 
 rally be most perfectly shown where the cooling has been 
 most rapid, as in the vitreous crust on the walls of dikes al- 
 ready referred to (pp. 297, 358). Rocks crystallizing in the 
 deeper parts of a volcano usually possess a more coarsely 
 crystalline structure than those which crystallize at or near 
 to the surface (p. 936). 
 
 Temperature of Lav a. It would be of the highest 
 interest and importance to know accurately the temperature 
 at which a lava-stream first issues. Measurements not alto- 
 gether satisfactory have been taken at various distances 
 below the point of emission, where the moving lava could 
 be safely approached. Experiments made at Vesuvius by 
 Scacchi and Sainte-Claire Deville in 1855, by thrusting thin 
 wires of silver, iron, and copper into the lava, indicated a 
 temperature of scarcely 700 C. (1228 Fahr.). Observations 
 of a similar kind, made in 1819, when a silver wire 3 oth inch 
 in diameter at once melted in the Vesuvian lava of that year,
 
 DYNAMICAL GEOLOGY 385 
 
 gave a greatly higher temperature, the melting-point of sil- 
 ver being about 1800 Fahr. But copper wire has also been 
 melted, the point of fusion of this metal being about 2204 
 Fahr. Evidence of the high temperature of lava has like- 
 wise been adduced from the alteration it has effected upon 
 refractory substances in its progress, as where, at Torre del 
 Greco, it overflowed the houses, and was afterward found to 
 have fused the fine edges of flints, to have decomposed brass 
 into its component metals, the copper actually crystallizing, 
 and to have melted silver, and even sublimed it into small 
 octahedral crystals (p. 393). The lava of Santorin has caught 
 up pieces of limestone, and has formed out of them nodules 
 containing crystallized anorthite, augite, sphene, black gar- 
 net, and particularly wollastonite. " The initial temperature 
 of lava, as it first issues from the Yesuvian funnel, is proba- 
 bly considerably more than 2000 Fahr. Obviously the dis- 
 solved water (or water-substance, for, as already remarked, 
 the temperature is far above the critical point of water, and 
 its component gases may exist dissociated) must possess as 
 high a temperature as that of the white-hot lava in which 
 it is contained. The existence of the elements of water at a 
 white beat, even in rocks which have reached the surface, is 
 a fact of no little significance in the theoretical consideration 
 of hypogene action. 
 
 Inclination and thickness of lava-flows. 
 It was at one time supposed that lava could not consoli- 
 date in beds on such steep slopes as those of most volca- 
 noes. Hence arose the "elevation -crater theory" (described 
 at p. 412), in which the inclined position of lavas round 
 a volcanic vent was explained by upheaval after their 
 
 19 Fouque, "Santorin," p. 206. 
 GEOLOGY Vol. XXIX 17
 
 386 TEXT-BOOK OF GEOLOGY 
 
 emission. Observations all over the world, however, have 
 now demonstrated that lava, with all its characteristic fea- 
 tures, can consolidate on slopes of even 35 and 40. 80 The 
 lava in the Hawaii Islands has cooled rapidly on slopes of 
 25, that from Vesuvius, in 1855, is here and there as steep 
 as 30, while the older lavas in Monte Somma are sometimes 
 inclined at 45. On the east side of Etna, a cascade of lava, 
 which in 1689 poured into the vast hollow of the Cava 
 Grande, has an inclination varying from 18 to 48, with 
 an average thickness of 16 feet. On Mauna Loa some lava- 
 flows are said to have congealed on slopes of 49, 60, and 
 even 90, 81 though in these cases it could only be a layer 
 of rock, stiffening and adhering to the surface of the de- 
 clivity. On the other hand, lava-streams have travelled 
 considerable distances over ground that to the eye looks 
 quite level. Among the Hawaiian Islands a declivity of 
 1 or less has been quite sufficient for the flow of the ex- 
 tremely liquid and mobile lavas of that region. In the 
 great lava-fields of the Snake River region of the Western 
 Territories of the United States the basalts, which must 
 also have been extremely liquid, have flowed over slopes 
 of much less than I . 82 The breadth and length of a lava- 
 stream, as well as the form of its surface, depend mainly 
 upon the liquidity of the molten material at the time of 
 outflow. Even when it consolidates on a steep slope, a 
 stream of lava forms a sheet with parallel upper and under 
 surfaces, a general uniformity of thickness, and often greater 
 evenness of surface, than where the angle of descent is low. 
 The thickness varies indefinitely; many basalts which have 
 
 80 Lyell on the consolidation of lava on steep slopes, Phil. Trans. 1858. 
 "' J. D. Dana, Amer. Jour. Sci. xxxv. (1888), p. 32. 
 82 J. D. Dana, "Characteristics of Volcanoes," p. 12.
 
 DYNAMICAL GEOLOGY 387 
 
 been poured out in a remarkably liquid condition have 
 solidified in beds not more than 10 or 12 feet thick. On 
 the other hand, more pasty lavas, and lavas which have 
 flowed into narrow valleys, may be piled up in solid masses 
 to a thickness of several hundred feet (pp. 378, 391). 
 
 Structure of a lava-stream. Lava-streams are 
 sometimes nearly homogeneous throughout. In general, 
 however, they each show three component layers. At the 
 bottom lies a rough, slaggy mass, produced by the rapid 
 cooling of the lava, and the breaking up and continued 
 onward motion of the scoriform layer. The central and 
 main portion of the stream consists of solid lava, often, 
 however, with a more or 
 
 less carious and vesicular e -^^^^^^si: J^ fSS 
 texture. The upper part, 
 as we have seen, may be 
 a mass of rough broken- 
 
 UD slabs, SCOriae, Or clink- Fig . 51.-Elongation of vesicles in direction 
 
 of flow of lava. 
 
 ers. The proportions borne 
 
 by these respective layers to each other vary continu- 
 ally. Some of the more fluid ropy lavas of Vesuvius 
 have an inconstant and thin slaggy crust; others may be 
 said to consist of little else than scoriae from top to bottom. 
 Throughout the whole mass of a lava-current, but more 
 especially along its upper surface, the absorbed or dis- 
 solved water-vapor expands with diminution of pressur-e, 
 and pushing the molten rock aside, segregates into small 
 bubbles or irregular cavities. Hence, when the lava solidi- 
 fies, these steam-holes are seen to be sometimes so abundant 
 that a detached portion of the rock containing them will 
 float in water (pumice). They are often elongated in the 
 direction of the motion of the lava-stream (Fig. 51). Some-
 
 388 TEXT-BOOK OF GEOLOGY 
 
 times, indeed, where the cells are numerous, their elonga- 
 tion in one direction gives a fissile structure to the rock. 
 
 A singular feature in many lava-streams are the tunnels 
 and caverns already referred to (p. 377) as observable in 
 them. These cavities have doubtless arisen during the 
 flow of the mass when the upper and under portions had 
 solidified and were creeping sluggishly onward, while the 
 still molten interior was able to move faster and thus to 
 leave empty spaces behind it. Such tunnels may frequently 
 be vseen among the Vesuvian lava-streams. Some remarkable 
 examples are described from the highly glassy lavas of 
 Hawaii, where they are sometimes from 2 to 10 feet in height 
 and 30 feet broad, but with large lateral expansions. The 
 walls of these Hawaiian lava-chambers are smooth and even 
 glassy, and from their roofs hang slender stalactites of lava 
 20 to 30 inches long, while on the floor below little mounds 
 of lava-stalagmite have formed. The precise mode of origin 
 of these curious appendages is not yet understood. 83 
 
 In passing from a fluid to a solid condition, and thus 
 contracting, lava acquires different structures. Lines of 
 divisional planes or joints traverse it, especially perpen- 
 dicular to the upper and under surfaces of the sheet. 
 These sometimes assume prismatic forms, dividing the 
 rock into columns, as is so frequently to be observed in 
 basalt. They are described in Book IV. Part II., together 
 with other forms of joints. 
 
 Vapors and sublimations of a lava-stream. 
 Besides steam, many other vapors, absorbed in the orig- 
 inal subterranean molten magma, escape from the fissures 
 of a lava-stream. Such vapors are copiously disengaged at 
 
 83 See Dana's "Characteristics of Volcanoes," pp. 209, 332.
 
 DYNAMICAL GEOLOGY 389 
 
 fumaroles (pp. 332, 334). Among the exhalations, chlorides 
 abound, particularly chloride of sodium, which appears, 
 not only in fissures, but even over the cooled crust of the 
 lava, in small crystals, in tufts, or as a granular and even 
 glassy incrustation. Chloride of iron is deposited as a 
 yellow coating at fumaroles, where also bright emerald- 
 green films and scales of chloride of copper may be more 
 rarely observed. Many chemical changes take place in the 
 escape of these vapors. Thus specular- iron, either the re- 
 sult of the mutual decomposition of steam and iron- 
 chloride, or of the oxidation of magnetite, forms abundant 
 scales, plates, and small crystals in the fumaroles and vesi- 
 cles of some lavas. Sal-ammoniac also appears in large 
 quantity on many lavas, not merely in the fissures, but 
 also on the upper surface. In these cases, it is not directly 
 a volcanic product, but results from some decomposition, 
 possibly from the gases evolved by the sudden destruction 
 of vegetation. It has, however, been observed also in the 
 crater of Etna, where the co-operation of organic substance 
 is hardly conceivable, and where perhaps it may arise from 
 the decomposition of aqueous vapor, whereby a combination 
 is formed with atmospheric nitrogen. Sulphur, breislakite, 
 szaboite, tenorite, alum, sulphates of iron, soda and potash, 
 and other minerals are also found. 
 
 Slow cooling of lava. The hardened crust of a 
 lava-stream is a bad conductor of heat. Consequently, the 
 surface of the stream may have become cool enough to be 
 walked upon, though the red-hot mass may be observed 
 through the rents to lie only a few inches below. Many 
 years, therefore, may elapse before the temperature of the 
 whole mass has fallen to that of the surrounding soil. 
 Eleven months after an eruption of Etna, Spallanzani
 
 390 TEXT-BOOK OF GEOLOGY 
 
 could see that the lava was still red-hot at the bottom of 
 the fissures, and a stick thrust into one of them instantly 
 took fire. The Vesuvian lava of 1785 was found by Breislak, 
 seven years afterward, to be still hot and steaming inter- 
 nally, though lichens had already taken root on its surface. 
 The ropy lava erupted by Vesuvius in 1858 was observed 
 by the author in 1870 to be still so hot, even near its termi- 
 nation, that steam issued abundantly from its rents, many 
 of which were too warm to allow the hand to be held in 
 them, and three years later it was still steaming abun- 
 dantly. Hoffmann records that from the lava which flowed 
 from Etna in 1787, steam was still issuing in 1830. Yet 
 more remarkable is the case of Jorullo, in Mexico, which 
 sent out lava in 1759. Twenty-one years later a cigar 
 could be lighted at its fissures; after 44 years it was still 
 visibly steaming; and even in 1846, that is, after 87 years 
 of cooling, two vapor-columns were still rising from it. 84 
 
 This extremely slow rate of cooling has justly been re- 
 garded as a point of high geological significance, in regard 
 to the secular cooling and probable internal temperature of 
 our globe. Some geologists have argued, indeed, that if so 
 comparatively small a portion of molten matter as a lava- 
 stream can maintain a high temperature under a thin, cold 
 crust for so many years, we may, from analogy, feel little 
 hesitation in believing that the enormously vaster mass of 
 the globe may, beneath a relatively thin crust, still continue 
 in a molten condition within. More legitimate deductions, 
 however, might be drawn from more accurate and precise 
 measurements of the rate of loss of heat, and of its varia- 
 tions in different lava-streams. Lord Kelvin, for instance, 
 
 84 E. Schleiden, quoted by Naumann, "Geognosie," i. p. 160.
 
 DYNAMICAL GEOLOGY 391 
 
 has suggested that, by measuring the temperature of in- 
 trusive masses of igneous rock in coal- workings and else- 
 where, and comparing it with that of other non-volcanic 
 rocks in the same regions, we might obtain data for calcu- 
 lating the time which has elapsed since these igneous sheets 
 were erupted (ante, p. 94). 
 
 Effects of lava-streams on superficial 
 waters and topography. In its descent, a stream 
 of lava may reach a water-course, and, by throwing itself 
 as an embankment across the stream, may pond back the 
 water and form a lake. Such is the origin of the pictur- 
 esque Lake Aidat in Auvergne. Or the molten current 
 may usurp the channel of the stream, and completely bury 
 the whole valley, as has happened again and again among 
 the vast lava-fields of Iceland. Few changes in physiog- 
 raphy are so rapid and so enduring as this. The channel 
 which has required, doubtless, many thousands of years 
 for the water laboriously to excavate, is sealed up in a few 
 hours under 100 feet or more of stone, and another vastly 
 protracted interval may elapse before this newer pile is 
 similarly eroded. 86 
 
 By suddenly overflowing a brook or pool of water, 
 molten lava sometimes has its outer crust shattered to 
 fragments by a sharp explosion of the generated steam, 
 while the fluid mass within rushes out on all sides. 8 * The 
 lava emitted by Mauna Loa, Hawaii, in the spring of 188 
 flowed out to sea, and added half a mile to the extent of 
 
 85 For an example of the conversion of a lava-buried river-bed into a hill-top 
 by long-continued denudation, see Quart. Journ. Geol. Soc. 1871, p. 303. 
 
 86 Explosions of this nature have been observed on Etna, where the lava has 
 suddenly come in contact with water or snow, considerable loss of life being 
 sometimes the result. Sartorius von Waltershauaen and A. von Lasaulx, "Der 
 Aetna," i. pp. 295, 300.
 
 392 TEXT-BOOK OF GEOLOGY 
 
 the island at that point. At the end of the stream three 
 cinder-cones formed from the contact of the lava with the 
 water, and Captain Button calls special attention to the fact 
 that not only in this instance, but in other examples among 
 the Hawaiian lavas which have reached the sea, there is 
 clear evidence of the formation of ordinary volcanic craters 
 by the accidental contact of lava with water. 87 The lavas 
 of Etna and Vesuvius have also protruded into the sea, but, 
 owing probably to their more viscous and lithoid condition 
 and lower temperature, they do not seem to have given rise 
 to explosive action at their seaward ends. Thus a current 
 from the latter mountain entered the Mediterranean at Torre 
 del Grreco in 1794, and pushed its way for 360 feet outward, 
 with a breadth of 1100 and a height of 15 feet. So quietly 
 did it advance, that Breislak could sail round it in a boat 
 and observe its progress. 
 
 By the outpouring of lava, two important kinds of geo- 
 logical change are produced. (1) Stream-courses, lakes, 
 ravines, valleys, in short, all the minor features of a land- 
 scape, may be completely overwhelmed under a thick sheet 
 of lava. The drainage of the district being thus effectually 
 altered, the numerous changes which flow from the opera- 
 tions of running water over the land are arrested and made 
 to begin again in new channels. (2) Considerable altera- 
 tions may likewise be caused by the effects of the heat and 
 vapors of the lava upon the subjacent or contiguous ground. 
 Instances have been observed in which the lava has actually 
 melted down opposing rocks, or masses of slags on its 
 own surface. Interesting observations, already referred to 
 (p. 386), have been made at Torre del Greco under the lava- 
 
 81 U. S. Geol. Report for 1882-83, p. 181.
 
 DYNAMICAL GEOLOGY 393 
 
 stream which overflowed part of that town in 1794. It was 
 found that the window-panes of the houses had been devitri- 
 fied into a white, translucent, stony substance; that pieces 
 of limestone had acquired an open, sandy, granular texture, 
 without loss of carbon-dioxide, and that iron, brass, lead, 
 copper, and silver objects bad been greatly altered, some of 
 the metals being actually sublimed. We can understand, 
 therefore, that, retaining its heat for so long a time, a mass 
 of lava may induce many crystalline structures, rearrange- 
 ments, or decompositions in the rocks over which it comes 
 to rest, and proceeds slowly to cool. This is a question of 
 considerable importance in relation to the behavior of an- 
 cient lavas which, after having been intruded among rocks 
 beneath the surface, have subsequently been exposed by 
 denudation (Book IV. Part VII.). 
 
 But, on the other hand, the exceedingly trifling change 
 produced, even by a massive sheet of lava, has often been 
 remarked with astonishment. On the flank of Vesuvius, 
 vines and trees may be seen still flourishing on little islets 
 of the older land-surface, completely surrounded by a flood 
 of lava. Dana has given an instructive account of the de- 
 scent of a lava-stream from Kilauea in June, 1840. Islet- 
 like spaces of forest were left in the midst of the lava, many 
 of the trees being still alive. Where the lava flowed round 
 the trees, the stumps were usually consumed, and cylindrical 
 holes or casts remained in the lava, either empty or filled 
 with charcoal. In many cases, the fallen crown of the tree 
 lay near, and so little damaged that the epiphytic plants on 
 it began to grow again. Yet so fluid was the lava that it 
 hung in pendent stalactites from the branches, which never- 
 theless, though clasped round by the molten rock, had barely 
 their bark scorched. Again, for nearly 100 years there has
 
 394 TEXT-BOOK OF GEOLOGY 
 
 lain on the flank of Etna a large sheet of ice, which, origi- 
 nally in the form of a thick mass of snow, was overflowed by 
 lava, and has thereby been protected from the evaporation 
 and thaw which would certainly have dissipated it long ago, 
 had it been exposed to the air. The heat of the lava has 
 not sufficed to melt it. Extensive tracts of snow were like- 
 wise overspread by lava from the same mountain in 1879. 
 In other cases, snow and ice have been melted in large quan- 
 tities by overflowing lava. The great floods of water which 
 rushed down the flank of Etna, after an eruption of the 
 mountain in the spring of 1755, and similar deluges at Coto- 
 paxi, are thus explained. 
 
 One further aspect of a lava-stream may be noticed here 
 the effect of time upon its surface. While all kinds of 
 lava must, in the end, crumble down under the influence 
 of atmospheric waste and, where other conditions permit, 
 become coated with soil, and support some kind of vegeta- 
 tion, yet extraordinary differences may be observed in the 
 facility with which different lava-streams yield to this 
 change, even on the flank of the same mountain. Every 
 one who ascends the slopes of Vesuvius remarks this fact. 
 After a little practice, it is not difficult there to trace the 
 limits of certain lavas even from a distance, in some cases 
 by their verdure, in others by their barrenness. Five hun- 
 dred years have not sufficed to clothe with green the still 
 naked surface of the Catanian lava of 1381; while some of 
 the lavas of the present century have long given footing to 
 bushes of furze." Some of the younger lavas of Auvergne, 
 which certainly flowed in times anterior to those of history, 
 are still singularly bare and rugged. Yet, on the whole, 
 
 88 On the weathering of the Etna lavas, see "Der Aetna," ii. p. 397.
 
 DYNAMICAL GEOLOGY 395 
 
 where lava is directly exposed to the atmosphere, without 
 receiving protection from occasional showers of volcanic ash, 
 or where liable to be washed bare by heavy torrents of rain, 
 its surface decays in a few years sufficiently to afford soil for 
 stray plants in the crevices. When these have taken root 
 they help to increase the disintegration; at last, as the rock 
 is overspread, the traces of its volcanic origin fade away 
 from its surface. Some of the Vesuvian lavas of the pres- 
 ent century already support vineyards. 
 
 Elevation and Subsidence* Proofs of elevation are fre- 
 quent among volcanic vents which, lying near the sea and 
 containing marine sediments among their older erupted ma- 
 terials, supply, in the inclosed marine organisms, evidence 
 of the movement. In this way, it is known that Etna, Vesu- 
 vius, and other Mediterranean volcanoes, began their his- 
 tory as submarine vents, and that they owe their present 
 dimensions not only to the accumulation of ejected mate- 
 rials, but also, to some extent, to an elevation of the sea- 
 bottom. 
 
 Proof of subsidence is less easily traced, but indications 
 have been observed of a sinking of the ground beneath a 
 volcanic vent. During the eruption of Santorin in 1866-67, 
 very decided but extremely local subsidence took place 
 near the vent in the centre of the old crater. The discharge 
 of such prodigious quantities of material may tend to pro- 
 duce cavernous spaces in the terrestrial crust, and the weight 
 of the ejected lavas and tuffs may still further contribute to 
 a general settlement of the ground around a volcanic focus. 
 
 If we consider the records of volcanic action in past geo- 
 logical time we meet with many proofs that it took place in 
 areas where the predominant terrestrial movement was one 
 of subsidence. Thus among the Palaeozoic systems of Brit-
 
 896 TEXT-BOOK OF GEOLOGY 
 
 ain, the Cambrian, Silurian, Devonian, Carboniferous, and 
 Permian volcanoes successively appeared, and their lavas 
 and tuffs were carried down and buried under thousands of 
 feet of sedimentary deposits. 89 
 
 Torrents of "Water and Mud. We have seen that large 
 quantities of water accompany many volcanic eruptions. In 
 some cases, where ancient crater-lakes or internal reservoirs, 
 shaken by repeated detonations, have been finally disrupted, 
 the mud which has thereby been liberated has issued from 
 the mountain. Such "mud-lava" {lava d'acqua), on account 
 of its liquidity and swiftness of motion, is more dreaded for 
 destructiveness than even the true melted lavas. On the 
 other hand, rain or melted snow or ice, rushing down the 
 cone and taking up loose volcanic dust, is converted into a 
 kind of mud that grows more and more pasty as it descends. 
 The mere sudden rush of such large bodies of water down 
 the steep declivity of a volcanic cone cannot fail to effect 
 much geological change. Deep trenches are cut out of the 
 loose volcanic slopes, and sometimes large areas of woodland 
 are swept away, the de*bris being strewn over the plains 
 below. 
 
 One of these mud-lavas invaded Herculaneum during 
 the great eruption of 79, and by quickly enveloping the 
 houses and their contents, has preserved for us so many 
 precious and perishable monuments of antiquity. In the 
 same district, during the eruption of 1622, a torrent of this 
 kind poured down upon the villages of Ottajano and Massa, 
 overthrowing walls, filling up streets, and even burying 
 houses with their inhabitants. During the great eruption 
 of Cotopaxi, in June, 1877, enormous torrents of water and 
 
 Presidential Addresses, Quart. Jouru. Geol. Soc. xlvii. (1891), xlviii. (1892).
 
 DYNAMICAL GEOLOGY 397 
 
 mud, produced by the melting of the snow and ice of the 
 cone, rushed down from the mountain. Huge portions of 
 the glaciers of the mountain were detached by the heat of 
 the rocks below them and rushed down bodily, breaking up 
 into blocks. The villages all round the mountain to a dis- 
 tance of sometimes more than ten geographical miles were 
 left deeply buried under a deposit of mud mixed with blocks 
 of lava, ashes, pieces of wood, lumps of ice, etc." 8 Many of 
 the volcanoes of Central and South America discharge large 
 quantities of mud directly from their craters. Thus, in the" 
 year 1691, Imbaburu, one of the Andes of Quito, emitted 
 floods of mud so largely charged with dead fish that pesti- 
 lential fevers arose from the subsequent effluvia. Seven 
 years later (1698), during an explosion of another of the 
 same range of lofty mountains, Carguairazo (14,706 feet), 
 the summit of the cone is said to have fallen in, while tor- 
 rents of mud containing immense numbers of the fish Pyme- 
 lodus Oyclopum, poured forth and covered the ground over 
 a space of four square leagues. The carbonaceous mud 
 (locally called moya) emitted by the Quito volcanoes some- 
 times escapes from lateral fissures, sometimes from the cra- 
 ters. Its organic contents, and notably its siluroid fish, 
 which are the same as those found living in the streams 
 above ground, prove that the water is derived from the sur- 
 face, and accumulates in craters or underground cavities 
 until discharged by volcanic action. Similar but even moce 
 stupendous and destructive outpourings have taken place 
 from the volcanoes of Java, where wide tracts of luxuriant 
 vegetation have at different times been buried under masses 
 of dark gray mud, sometimes 100 feet thick, with a rough 
 
 Wolf, Neues Jahrb. 1878, p. 133.
 
 398 TEXT-BOOK OF GEOLOGY 
 
 hiliocky surface from which the top of a submerged palm- 
 tree would here and there protrude. 
 
 Between the destructive effects of mere water-torrents 
 and that of these mud-floods there is, of course, the notable 
 difference that, whereas in the former case a portion of the 
 surface is swept away, in the latter, while sometimes consid- 
 erable demolition of the surface takes place at first, the main 
 result is the burying of the ground under a new tumultuous 
 deposit by which the topography is greatly changed, not 
 only as regards its temporary aspect, but in its more perma- 
 nent features, such as the position and form of its water- 
 courses. 
 
 Effects of the dosing of a Volcanic Chimney Sills and 
 Dikes* A study of the volcanic phenomena of former geo- 
 logical periods, where the structure of the interior of vol- 
 canoes and their funnels has been laid bare by denudation, 
 shows that in many cases a vent becomes plugged up by the 
 ascent and consolidation of solid material in it, while yet 
 the eruptive energy of the volcano, though lessened, has 
 not ceased. A time is reached when the ascending magma, 
 impelled by pressure from below, can no longer overcome 
 the resistance of the column of solid lava or compacted ag- 
 glomerate which has sealed up the orifice of discharge, or at 
 least when it can more easily force a passage for itself be- 
 tween the sedimentary strata on which the whole volcanic 
 pile may rest, or between the lava sheets at the base of the 
 pile, or into fissures in either or both of these groups. 
 Hence arise intrusive sheets or sills and dikes or veins (see 
 pp. 952, 958). That these later manifestations of volcanic 
 energy have sometimes taken place on a great scale is shown 
 by the number and size of the sills which are found at the 
 base of the Paleozoic volcanic groups of Britain. This fea-
 
 DYNAMICAL GEOLOGY 399 
 
 tare is a remarkably striking feature of the rocks that under- 
 lie the great Lower Silurian volcanic outflows of Arenig and 
 Cader Idris in North Wales. It recurs so frequently, not 
 only among Palaeozoic volcanic phenomena but quite as 
 markedly among those of Tertiary age in the British Isles, 
 that it must be regarded as marking an ordinary phase of 
 volcanic action. Bat it remains of course invisible until in 
 the progress of denudation a volcanic cone is cut down to 
 the roots. 
 
 Exhalations of Vapors and Gases. A volcano, as its ac- 
 tivity wanes, may pass into the Solfatara stage, when only 
 volatile emanations are discharged. The well-known Sol- 
 fatara near Naples, since its last eruption in 1198, has con- 
 stantly discharged steam and sulphurous vapors. The island 
 of Volcano has now passed also into this phase, though giv- 
 ing vent to occasional explosions. Numerous other exam- 
 ples occur among the old volcanic tracts of Italy, where they 
 have been termed soffioni. Steam, escaping in conspicuous 
 jets, sulphuretted hydrogen, hydrochloric acid, and carbonic 
 acid are particularly noticeable at these orifices. The vapors 
 in rising condense. The sulphuretted hydrogen partially 
 oxidizes into sulphuric acid, which powerfully corrodes the 
 surrounding rocks. The lava or tuff through which the hot 
 vapors rise is bleached into a white or yellowish crumbling 
 clay, in which, however, the less easily corroded crystals 
 may still be recognized in situ. At the same time, subli- 
 mates of sulphur or of chlorides may be formed, or the sul- 
 phuric acid attacking the lime of the silicates gives rise to 
 gypsum, which spreads in a network of threads and veins 
 through the hot, steaming, and decomposed mass. In this 
 way, at the island of Volcano, obsidian is converted into a 
 snow-white, dull, clay-stone-like substance, with crystals of
 
 400 TEXT-BOOK OF GEOLOGY 
 
 sulphur and gypsum in its crevices. Silica is likewise de- 
 posited from solution at many orifices, and coats the altered 
 rock with a crust of chalcedony, hyalite, or some form of 
 siliceous sinter. As the result of this action, masses of 
 rock are decomposed below the surface, and new deposits 
 of alum, sulphur, sulphides of iron and copper, etc., are 
 formed above them. Examples have been described from 
 Iceland, Lipari, Hungary, Terceira, Teneriffe, St. Helena, 
 and many other localities." The lagoons of Tuscany are 
 basins into which the waters from suffioni are discharged, 
 and where a precipitation of their dissolved salts takes place. 
 Among the substances thus deposited are gypsum, sulphur, 
 silica, and various alkaline salts; but the most important is 
 boracic acid, the extraction of which constitutes a thriving 
 industry. In Chile many solfataras occur among extinct 
 volcanoes.' 11 
 
 Another class of gaseous emanations betokens a condi- 
 tion of volcanic activity further advanced toward final ex- 
 tinction. In these, the gas is carbon-dioxide, either issuing 
 directly from the rock or bubbling up with water which is 
 often quite cold. The old volcanic districts of Europe fur- 
 nish many examples. Thus on the shores of the Laacher 
 See an ancient crater-lake of the Eifel the gas issues from 
 numerous openings called moffette, round which dead insects, 
 and occasionally mice and birds, may be found. In the 
 same region occur hundreds of springs more or less charged 
 with this gas. The famous Valley of Deatli in Java cou- 
 
 91 Von Buch, "Ganar. Inseln," p. 232. Hoffmann. Pogg. Ann. 1832, pp. 
 38, 40, 60. Bunsen, Ann. Chem. Pharm. 1847 (Ixii.), p. 10. Darwin, "Vol- 
 canic Islands," p. 29. The name Propylite, as already mentioned (ante, p. 293) 
 lias been proposed by Rosenbusch to be restricted to certain andesitesand allied 
 rocks altered by solfataric action. 
 
 94 Domeyko, Ann. Mines, ix. (7e. aer.). Large numbers of solfataras occur 
 also in Iceland.
 
 DYNAMICAL GEOLOGY 401 
 
 tains one of the most remarkable gas-springs in the world. 
 It is a deep, boskj hollow, from one small space on the bot- 
 tom of which carbon-dioxide issues so copiously as to form 
 the lower stratum of the atmosphere. Tigers, deer, and 
 wild-boar, enticed by the shelter of the spot, descend and 
 are speedily suffocated. Many skeletons, including those of 
 man himself, have been observed. 
 
 As a distinct class of gas-springs, we may group and de- 
 scribe here the emanations of volatile hydrocarbons, which, 
 when they take fire, are known as Fire- wells. These are 
 not of volcanic origin, but arise from changes within the 
 solid rocks underneath. They occur in many of the districts 
 where mud-volcanoes appear, as in northern Italy, on the 
 Caspian, in Mesopotamia, in southern Kurdistan, and in 
 many parts of the United States. It has been observed 
 that they frequently rise in regions where beds of rock-salt 
 lie underneath, and as that rock has been ascertained often 
 to contain compressed gaseous hydrocarbons, the solution of 
 the rock by subterranean water, and the consequent libera- 
 tion of the gas, has been offered as an explanation of these 
 fire-wells. 
 
 In the oil regions of Pennsylvania, certain sandy strata 
 occur at various geological horizons whence large quantities 
 of petroleum and gas are obtained (p. 254). In making the 
 borings for oil-wells, reservoirs of gas as well as subterra- 
 nean courses or springs of water are met with. When the 
 supply of oil is limited but that of gas is large, a contest 
 for possession of the bore-hole sometimes takes place be- 
 tween the gas and water. When the machinery is removed 
 and the boring is abandoned, the contest is allowed to pro- 
 ceed unimpeded and results in the intermittent discharge of 
 columns of water and gas to heights of 130 feet or more.
 
 402 TEXT-BOOK OF GEOLOGY 
 
 At night, when the gas has been lighted, the spectacle of 
 one of these "fire-geysers" is inconceivably grand." 
 
 Geysers. Eruptive fountains of hot water and steam, to 
 which the general name of Geysers (i.e. gushers) is given, 
 from the examples in Iceland, which were the first to be 
 seen and described, mark a declining phase of volcanic 
 activity. The Great and Little Geysers, the Strokkr and 
 other minor springs of hot water in Iceland, have long been 
 celebrated examples. More recently another series has been 
 discovered in New Zealand. But probably the most re- 
 markable and numerous assemblage is that which has been 
 brought to light in the northwest part of the territory of 
 Wyoming, and which has been included within the "Yel- 
 lowstone National Park" a region set apart by the Con- 
 gress of the United States to be forever exempt from settle- 
 ment, and to be retained for the instruction of the people. 
 In this singular region the ground in certain tracts is honey- 
 combed with passages which communicate with the surface 
 by hundreds of openings, whence boiling water and steam 
 are emitted. In most cases, the water remains clear, tran- 
 quil, and of a deep green-blue tint, though many of the 
 otherwise quiet pools are marked by patches of rapid ebul- 
 lition. These pools lie on mounds or sheets of sinter, and 
 are usually edged round with a raised rim of the same sub- 
 stance, often beautifully fretted and streaked with brilliant 
 colors. The eruptive openings usually appear on small, 
 
 93 Ashburner, Proc. Amer. Phil. Soc. xvii. (1877), p. 127. Stowell's Petro- 
 leum Reporter, 15th Sept. 1879. Second Geol. Survey of Pennsylvania, con- 
 taining Reports by J. Carll, 1877, 1880. J. S. Newberry, "The First Oil Well." 
 Harper's Magazine, Oct. 1890. On the naphtha districts of the Caspian Sea, 
 Abich, Jahrb. Geol. Reichs. xxix. (1879), p. 165. H. Sjogren, op. cit. xxxvii. 
 (1887), p. 47. C. Marvin, "Region of Eternal Fire," London, 1884. See also 
 for phenomena in Gallicia, Jahrb. Geol. Reichs. xv. pp. 199, 351; xvii. p. 291; 
 xviii. p. 311; xxxi. (1881), p. 131. Proc. Inst. Civ. Engineers, xlii. (1875), 
 p. 343.
 
 DYNAMICAL GEOLOGY 
 
 403 
 
 low, conical elevations of sinter, from each of which one 
 or more tubular projections rise. It is from these irregular 
 tube-like excrescences that the eruptions take place. 
 
 The term geyser is restricted to active openings whence 
 columns of hot water and steam are from time to time 
 ejected; the non-eruptive pools are only hot springs. A 
 true geyser should thus possess an underground pipe or 
 passage, terminating at the surface in an opening built 
 
 Fig. 52. View of OKI Faithful Geyser, and others in the distance, Fire 
 Hole River, Yellowstone Park. 
 
 round with deposits of sinter. At more or less regular 
 intervals, rumblings and sharp detonations in the pipe are 
 followed by an agitation of the water in the basin, and then 
 by the violent expulsion of a column of water and steam to 
 a considerable height in the air. In the Upper Fire Hole 
 basin of the Yellowstone Park, one of the geysers, named 
 "Old Faithful" (Fig. 52), has ever since the discovery of 
 the region sent out a column of mingled water and steam 
 every sixty-three minutes or thereabout. The column 
 rushes up with a loud roar to a height of more than 100
 
 404 TEXT-BOOK OF GEOLOGY 
 
 feet, the whole eruption not occupying more than about 
 live or six minutes. The other geysers of the same district 
 are more capricious in their movements, and some of them 
 more stupendous in the volume of their discharge. The 
 eruptions of the Castle, Giant, and Beehive vents are mar- 
 vellously impressive. 94 
 
 In examining the Yellowstone Geyser region in 1879, 
 the author was specially struck by the evident indepen- 
 dence of the vents. This was shown by- their very differ- 
 ent levels, as well as by their capricious and unsympathetic 
 eruptions. On the same hill-slope, dozens of quiet pools, 
 as well as some true geysers, were noticed at different 
 levels, from the edge of the Fire Hole River up to a 
 height of at least 80 feet above it. Yet the lower pools, 
 from which, of course, had there been underground con- 
 nection between the different vents, the drainage should 
 have principally discharged itself, were often found to be 
 quiet steaming pools without outlet, while those at higher 
 points were occasionally in active eruption. It seemed also 
 to make no difference in the height or tranquillity of one 
 of the quietly boiling caldrons, when an active projection 
 of steam and water was going on from a neighboring vent 
 on the same gentle slope. 
 
 Bunsen and Deseloiseaux spent some days experimenting 
 at the Icelandic geysers, and ascertained that in the Great 
 Geyser, while the surface temperature is about 212 Fahr., 
 that of lower portions of the tube is much higher a ther- 
 mometer giving as high a reading as 266 Fahr. The water 
 
 94 See Hayden's Reports for 1870 arid for 1878, in the latter of which will be 
 found a voluminous monograph on the Hot Springs by A. C. Peale ; Comstock's 
 Report in Jones's Reconnoissance of N. W. Wyoming, etc., 1874. The de- 
 posits of hot springs are further referred to on pp. 267, 809.
 
 DYNAMICAL GEOLOGY 405 
 
 at a little depth must consequently be 54 above the normal 
 boiling-point, but it is kept in the fluid state by the pressure 
 of the overlying column. At the basin, however, the water 
 cools quickly. After an explosion it accumulates there, and 
 eventually begins to boil. The pressure on the column 
 below being thus relieved, a portion of the superheated 
 water flashes into steam, and as the change passes down 
 the pipe, the whole column of water and steam rushes out 
 with great violence. The water thereafter gradually col- 
 lects again in the pipe, and after an interval of some hours 
 the operation is renewed. The experiments made by Bun- 
 sen proved the source of the eruptive action to lie in the 
 hot part of the pipe. He hung stones by strings to different 
 depths in the funnel of the geyser, and found that only 
 those in the higher part were cast out by the rush of water, 
 sometimes to a height of 100 feet, while, at the same time, 
 the water at the bottom was hardly disturbed at all. These 
 observations give much interest and importance to the phe- 
 nomena of geysers in relation to volcanic action. They 
 show that the eruptive force in geysers is steam; that the 
 water column, even at a comparatively small depth, may 
 have a temperature considerably above 212 ; that this high 
 temperature is local; and that the eruptions of steam and 
 water take place periodically, and with such vigor as to 
 eject large stones to a height of 100 feet. 95 
 
 The hot water comes up with a considerable percentage 
 of mineral matter in solution. According to the analysis 
 of Sandberger, water from the Great Geyser of Iceland 
 
 9S Comptes Rendus, xxiii. (1846), p. 934; Pogg. Annal. Ixxii. (1847), p. 159; 
 Ixxxiii. (1851), p. 197. Ann. Ghimie, xxxviii. (1853), pp. 215, 385. The expla- 
 nation proposed for the phenomena observed at the Great Geyser is probably not 
 applicable in those cases where the mere local accumulation of steam in suitable 
 reservoirs may be sufficient.
 
 406 TEXT-BOOK OF GEOLOGY 
 
 contains in 10,000 parts the following proportions of ingre- 
 dients: silica, 5-097; sodium-carbonate, 1-939; ammonium- 
 carbonate, 0-083; sodium-sulphate, 1-07; potassium-sulphate, 
 0-475; magnesium-sulphate, 0-042; sodium-chloride, 2-521; 
 sodium-sulphide, 0-088; carbonic acid, 0-557 = ll-872. 96 
 
 When the water has reached the surface, it deposits the 
 silica as a sinter on the surfaces over which it flows or on 
 which it rests. 97 The deposit, which is not due to mere 
 cooling and evaporation, is curiously aided by the presence 
 of living algae (postea, p. 809). It naturally takes place 
 fastest along the margins of the pools. Hence the curiously 
 fretted rims by which these sheets of water are surrounded, 
 and the tubular or cylindrical protuberances which rise 
 from the growing domes. Where numerous hot springs 
 have issued along a slope, a succession of basins gives a 
 curiously picturesque terraced aspect to the ground, as at 
 the Mammoth Springs of the Yellowstone Park and at the 
 now destroyed terraces of Rotamahana in New Zealand. 
 
 In course of time, the network of underground passages 
 undergoes alteration. Orifices that were once active cease 
 to erupt, and even the water fails to overflow them. Sinter 
 is no longer formed round them, and their surfaces, exposed 
 to the weather, crack into fine shaly rubbish like com- 
 minuted oyster-shells. Or the cylinder of sinter grows 
 upward until, by the continued deposit of sinter and the 
 failing force of the geyser, the tube is finally filled up, and 
 then a dry and crumbling white pillar is left to mark the 
 site of the extinct geyser. 
 
 " Annal. Chem. und Pharm. 1847, p. 49. A series of detailed analyses of 
 the hot springs of the Yellowstone National Park will be found in No. 47 of the 
 Bull. U. S. Geol. Surv. 1888. 
 
 91 For an account of the geyserite of the Yellowstone district, see papers by 
 W. H. Weed, Anier. Journ. Sci. xxxvii. (1889), and 9lh Ann. Rep. U. S. GeoL 
 Surv. 1890.
 
 DYNAMICAL GEOLOGY 407 
 
 Mud- Volcanoes* These are of two kinds: 1st, where 
 the chief source of movement is the escape of gaseous 
 discharges; 2d, where the active agent is steam. 
 
 (1) Although not volcanic in the proper sense of the 
 term, certain remarkable orifices of eruption may be no- 
 ticed here, to which the names of mud-volcanoes, salses, air- 
 volcanoes, and maccahtbas have been applied (Sicily, the 
 Apennines, Caucasus, Kertch, Tamar). These are conical 
 hills formed by the accumulation of fine and usually saline 
 mud, which, with various gases, is continuously or intermit- 
 tently given out from the orifice or crater in the centre. 
 They occur in groups, each hillock being sometimes less 
 than a yard in height, but ranging up to elevations of 100 
 feet or more. Like true volcanoes, they have their periods 
 of repose, when either no discharge takes place at all, or 
 mud oozes out tranquilly from the crater, and their epochs 
 of activity, when large volumes of gas, and sometimes col- 
 umns of flame, rush out with considerable violence and ex- 
 plosion, and throw up mud and stones to a height of several 
 hundred feet. The gases play much the same part, there- 
 fore, in these phenomena that steam does in those of true 
 volcanoes. They consist of marsh*gas and other hydrocar- 
 bons, carbon-dioxide, sulphuretted hydrogen, and nitrogen, 
 with petroleum vapors. The mud is usually cold. In the 
 water occur various saline ingredients, among which com- 
 mon salt generally appears; hence the name, Salses. Naph- 
 tha is likewise frequently present. Large pieces of stone, 
 differing from those in the neighborhood, have been ob- 
 served among the ejections, indicative doubtless of a some- 
 what deeper source than in ordinary cases. Heavy rains 
 may wash down the minor mud-cones and spread out the 
 material over the ground; but gas- bubbles again appear
 
 408 TEXT-BOOK OF GEOLOGY 
 
 through the sheet of mud, and by degrees a new series 
 of mounds is once more thrown up. 
 
 There can be little doubt that this type of mud- volcano 
 is to be traced to chemical changes in progress underneath. 
 Dr. Daubeny explained them in Sicily by the slow combus- 
 tion of beds of sulphur. The frequent occurrence of naph- 
 tha and of inflammable gas points, in other cases, to the 
 disengagement of hydrocarbons from subterranean strata." 
 
 (2) The second class of mud-volcano presents itself in 
 true volcanic regions, and is due to the escape of hot water 
 and steam through beds of tuff or some other friable kind 
 of rock. The mud is kept in ebullition by the rise of steam 
 through it. As it becomes more pasty and the steam meets 
 with greater resistance, large bubbles are formed which 
 burst, and the more liquid mud from below oozes out from 
 the vent. In this way, small cones are built up, many of 
 which have perfect craters atop. In the Geyser tracts of the 
 Yellowstone region, there are instructive examples of such 
 active and extinct mud-vents. Some of the extinct cones 
 there are not more than a foot high, and might be carefully 
 removed as museum specimens. 
 
 Mud-volcanoes occur*in Iceland, Sicily (Maccaluba), in 
 many districts of northern Italy, at Tamar and Kertch, at 
 Baku on the Caspian, near the mouth of the Indus, and 
 in other parts of the globe." 
 
 98 The "burning hills" of Turkestan are referred to the subterranean com- 
 bustion of beds of Jurassic Coal. J. Muschketoff, Neues Jahrb. 1876, p. 516. 
 
 99 On mud-volcanoes, see Bunsen, Liebig's Annual, Ixiii. (1847), p. 1; Abich, 
 Mem. Acad. St. Petersburg, 7e. ser. t. vi. No. 5, ix. No. 4; Daubeny's "Volca- 
 noes," pp. 264, 539; Buist, Trans. Bombay Geograph. Soc. x. p. 154; Roberts, 
 Journ. Roy. Asiatic Soc. 1850; De Verneuil, Mem. Soc. Geol. France, iii. (1838), 
 p. 4; Stiffe, Q. J. Geol. Soc. xxx. p. 60; Von LasauLx, Z. Deutsch. Geol. Gea. 
 xxxi. p. 457 ; Gumbel, Sitzb. Akad. Munch. 1879; F. R. Mallet, Rec. Geol. Surv. 
 India, xi. p. 188. H. Sjogren, Jahrb. Geol. Reichsansc. xxxvii. (1887), p. 233.
 
 DYNAMICAL GEOLOGY 409 
 
 3. Structure of Volcanoes 
 
 We have now to consider the manner in which the vari- 
 ous solid materials ejected by volcanic action are built up 
 at the surface. This inquiry will be restricted here to the 
 phenomena of modern volcanoes, including the active and 
 dormant, or recently extinct, phases. Obviously, however, 
 in a modern volcano we can study only the upper and ex- 
 ternal portions, the deeper and fundamental parts being still 
 concealed from view. But the interior structure has been, 
 in many cases, laid open among the volcanic products of 
 ancient vents. As these belong to the architecture of the 
 terrestrial crust, they are described in Book IV. The stu- 
 dent is therefore requested to take the descriptions there 
 given, in connection with the foregoing and present sec- 
 tions, as related chapters of the study of volcanism. 
 
 Confining attention at present to modern volcanic action, 
 we find that the solid materials emitted from the earth's 
 interior are arranged in two distinct types of structure, 
 according as the eruptions proceed from large central cones 
 or from less prominent vents connected with fissures. In 
 the former case, volcanic cones are produced; in the lat- 
 ter, volcanic plateaus or plains. The type of the volcanic 
 cone, or ordinary volcano, is now the most abundant and 
 best known. 
 
 i. Volcanic Cones 
 
 From some weaker point of a fissure, or from a vent 
 opened directly by explosion, volcanic discharges of gas 
 and vapors with their liquid and solid accompaniments 
 make their way to the surface and gradually build up 
 
 a volcanic hill or mountain. Occasionally, eruptions have 
 OBOLOGY Vol. XXIX 18
 
 410 TEXT-BOOK OF GEOLOGY 
 
 proceeded no further than the first stage of gaseous ex- 
 plosion. A caldron-like cavity has been torn open in the 
 ground, and ejected fragments of the solid rocks, through 
 which the explosion has emerged, have fallen back into 
 and round the vent. Subsequently, after possible subsi- 
 dence of the fragmentary materials in the vent, and even 
 of the sides of the orifice, water supplied by rain and filter- 
 ing from the neighboring ground may partially, or wholly, 
 fill up the cavity, so as to produce a lake either with 
 or without a superficial outlet. Under favorable circum- 
 stances, vegetation creeping over bare earth and stone may 
 so conceal all evidence of the original volcanic action as to 
 make the quiet sheet of water look as if it had always been 
 an essential part of the landscape. Explosion-lakes (Crater- 
 lakes) of this kind occur in districts of extinct volcanoes, 
 as in the Eifel (maare), central Italy, and Auvergne. The 
 crateriform hollow called the Gour de Tazenat, in Velay, 
 has a diameter of half a mile and lies in the granite, while 
 another cavity near Confolens, on the left bank of the 
 Loire, has also been blown out of the granite and has 
 given passage to no volcanic materials, but only to broken- 
 up granite. 100 Other illustrations in central France are to 
 be found in the Lakes of Pavin, Mont Sineire, Chauvet, 
 Beurdouse, Champedaze and La Grodival. 101 A remarkable 
 example is supplied by the Lonar Lake in the Indian penin- 
 sula, half-way between Bombay and Nagpur. It lies in the 
 midst of the volcanic plateau of the Deccan traps, which 
 extend around it for hundreds of miles in nearly flat beds 
 that slightly dip away from the lake. An almost circular 
 
 100 Tournaire, Bull. Soc. Geol. France, xxvi. (1869), p. 1166; Daubree, 
 Comptes rend. 1890, p. 859. 
 
 101 Scrope, "Volcanoes of Central France," pp. 81, 143, 144.
 
 DYNAMICAL GEOLOGY 411 
 
 depression, rather more than a mile in diameter, and from 
 300 to 400 feet deep, contains at the bottom a shallow lake 
 of bitter saline water, depositing crystals of trona (native 
 carbonate of soda), the nitrum of the ancients. Except to 
 the north and northeast, it is encircled with a raised rim 
 of irregularly piled blocks of basalt, identical with that of 
 the beds through which the cavity has been opened. The 
 rim never exceeds 100 feet, and is often not more than 40 
 or 60 feet in height, and cannot contain a thousandth part 
 of the material which once filled the crater. No other evi- 
 dence of volcanic discharge from this vent is to be seen. 
 Some of the contents of the cavity may have been ejected 
 in fine particles, which have subsequently been removed by 
 denudation; but it seems more probable that the existence 
 of the cavity is mainly due to subsidence after the original 
 explosion. 1 " 
 
 In most cases, explosions are accompanied by the ex- 
 pulsion of so much solid material that a cone gathers round 
 the point of emission. As the cone increases in height, by 
 successive additions of ashes or lava to its surface, these 
 volcanic sheets are laid down upon progressively steeper 
 slopes. The inclination of beds of lava, which must have 
 originally issued in a more or less liquid condition, offered 
 formerly a difficulty to observers, and suggested the fa- 
 mous theory of Elevation-craters (Erhebungskratere) of L. 
 von Buch, 108 Elie de Beaumont, 1 * 4 and other geologists. 
 
 104 This cavity may possibly mark one of the vents from which the basalt 
 Hoods issued. On explosion-craters and lakes, see Scrope's "Volcanoes." 
 Lecoq, "Epoquea geologiques de 1'Auvergne," tome iv. ; compare also Vogel- 
 sang, "Vulcane der Eifel," and in Neues Jahrb. 1870, pp. 199, 326, 460. On 
 Lonar Lake, see Malcolmson, Trans. Geol. Soc. 2d ser. v. p. 562. Medlicott and 
 Stanford's "Geology of India," p. 379. 
 
 103 Pogg. Ann. ix., x., xivii. p. 169. 
 
 104 Bull. Soc. Geol. France, iv. p. 357. Ann. dea Mines, ix. and x.
 
 412 TEXT-BOOK OF GEOLOGY 
 
 According to this theory, the conical shape of a volcanic 
 cone arises mainly from an upheaval or swelling of the 
 ground, round the vent from which the materials are finally 
 expelled. A portion of the earth's crust (represented in 
 Fig. 53 as composed of stratified deposits, a b g h] was 
 believed to have been pushed up like a huge blister, by 
 forces acting from below (at c) until the summit of the dome 
 gave way and volcanic materials were emitted. At first 
 these might only partially fill the cavity (as at/), but sub- 
 sequent eruptions, if sufficiently copious, would cover over 
 
 Fig. 58. Section illustrative of the Elevation-crater Theory. 
 
 the truncated edges of the pre-volcanic rocks (as at g ^), and 
 would be liable to further upheaval by a renewal of the 
 original upward swelling of the site. 
 
 It was a matter of prime importance in the interpretation 
 of volcanic action to have this question settled. To Poulett 
 Scrope, Constant Provost, and Lyell, belong the merit of 
 disproving the Elevation-crater theory. Scrope showed con- 
 clusively that the steep slope of the lava-beds of a volcanic 
 cone was original. 1 " Constant Provost pointed out that 
 there was no more reason why lava should not consolidate 
 
 106 "Considerations oa Volcanoes," 1825. Quart. Journ. Geol. Soc. xH. 
 p. 326.
 
 DYNAMICAL GEOLOGY 
 
 413 
 
 on steep slopes than that tears or drops of wax should not 
 do so. 10 " Lyell, in successive editions of his works, and 
 subsequently by an examination of the Canary Islands 
 with Hartung, brought forward cogent arguments against 
 the Elevation-crater theory. lif A comparison of Fig. 53 
 with Fig. 54 will show at a glance the difference between 
 
 Pig. 54. Diagram-sectioii of a normal Volcano. 
 
 * x, Pre-volcanic platform, supposed here to consist of upraised stratified rocks, 
 broken through by the funnel /, from which the cone of volcanic materials c c 
 has been erupted. Inside the crater u, previously cleared by some great explo- 
 sion, a minor cone may be formed during feebler phases of volcanic action, and 
 this inner cone may increase in size until the original cone is built up again, as 
 shown by the dotted lines. 
 
 this theory and the views of volcanic structure now uni- 
 versally accepted. The steep declivities on which lava 
 can actually consolidate have been referred to on p. 386. 
 The conical form of a volcano is that naturally assumed 
 by a self-supporting mass of coherent material. It varies 
 slightly according to the nature of the materials of the cone, 
 the progress of atmospheric denudation, the position of the 
 crater, the direction in which materials are ejected, the force 
 and direction of the wind during an eruption, the growth 
 
 m Comptes Rendus, i. (1835), 460; xli. (1855), p. 919. Geol. Soc. Prance: 
 Memoires, ii. p. 105, and Bull. xiv. 217. Societe Philom. Paris, Proc. Verb. 
 
 1843, p. 13. 
 
 IM Phil. Trans, 1858, p. 703. 
 4DO-422. 
 
 See the remarks of Fouque, "Santorin," pp.
 
 414 TEXT-BOOK OF GEOLOGY 
 
 of parasitic cones, and the collapse due to the dying out of 
 volcanic energy. 188 
 
 The cone grows by additions made to its surface during 
 successive eruptions, and though liable to great local varia- 
 tion of contour and topography, preserves its general form 
 with singular persistence. Many exaggerated pictures have 
 been drawn of the steepness of slope in volcanic cones, but 
 it is obvious that the angle cannot as a whole exceed the 
 maximum inclination of repose of the detrital matter ejected 
 from the central chimney. 108 A series of profiles of volcanic 
 cones taken from photographs shows how nearly they ap- 
 proach to a common average type. 110 One of the most 
 potent and constant agencies in modifying the outer forms 
 of these cones is undoubtedly to be found in rain and tor- 
 rents, which sweep down the loose detritus and excavate 
 ravines on the declivities till a cone may be so deeply 
 trenched as to resemble a half -opened umbrella. 111 
 
 The crater doubtless owes its generally circular form to 
 the equal expansion in all directions of the explosive vapors 
 from below. In some of the mud-cones already noticed, the 
 crater is not more than a few inches in diameter and depth. 
 From this minimum, every gradation of size may be met 
 with, up to huge precipitous depressions, a mile or more 
 in diameter, and several thousand feet in depth. In the 
 crater of an active volcano, emitting lava and scoriae, like 
 Vesuvius, the walls are steep, rugged cliffs of scorched and 
 
 108 J. Milne, Geol. Mag. 1878, p. 339; 1879, p. 506. Seismolog. Soc. Japan, 
 ix. p. 179. G. P. Becker, Amer. Journ. Sci. xxx. 1885, p. 283. 
 
 109 Cotopaxi is a notable example of such exaggerated representation. Mr. 
 "Whymper found that the general angles of the northern and southern slopes of 
 the cone were rather less than 30* ("Travels amongst the Great Andes," p. 123). 
 Humboldt depicted the angle as one of 60 1 
 
 110 See Milne, Seism. Soc. Japan, ix., and Geol. Mag. 1878, plate ix. 
 
 111 On the denudation of volcanic cones, see H. J. Johnston-Lavis, Q. J. Geol. 
 Soc. xl. p. 103.
 
 DYNAMICAL GEOLOGY 415 
 
 blasted rock red, yellow, and black. Where the material 
 erupted is only loose dust and lapilli, the sides of the crater 
 are slopes, somewhat steeper than those of the outside of 
 the cone. 
 
 The crater-bottom of an active volcano of the first class, 
 when quiescent, forms a rough plain dotted over with hil- 
 locks or cones, from many of which steam and hot vapors 
 are ever rising. At night, the glowing lava may be seen 
 lying in these vents, or in fissures, at a depth of only a few 
 feet from the surface. Occasional intermittent eruptions 
 take place and miniature cones of slag and scoriae are 
 thrown up. In some instances, as in the vast crater of 
 Gurung Tengger, in Java, the crater-bottom stretches out 
 into a wide level waste of volcanic sand, driven by the wind 
 into dunes like those of the African deserts. 
 
 A volcano commonly possesses one chief crater, oftem 
 also many minor ones, of varying or of nearly equal size. 
 The volcano of the Isle of Bourbon 
 (or Reunion) has three craters. 114 Not 
 infrequently craters appear successive- 
 ly, owing to the blocking up of the 
 pipe below. Thus in the accompany- 
 ing plan of the volcanic cone of the 
 island of Volcanello (Fig. 55), one of Fig. 55. Plan of voicaneiio, 
 
 ' . showing three successive 
 
 the Lipari group, the volcanic fun- craters. 
 
 nel has shifted its position twice, so that three craters 
 have successively appeared upon the cone, and partially 
 overlap each other. It may be from this cause that some 
 
 114 For receut information regarding this volcanic island, see B. von Drasche, 
 in Verhandl. Geol. Reichsanst. 1875, p. 266, and in Tschermak'a Min. Mittheil. 
 1875 (3), p. 217 (4), p. 39, and his work "Die Insel Reunion (Bourbon)," 4to, 
 Vienna, 1878. C. Velain. "Description geologique de la Presqu'ile d'Adeu, de 
 1'Ile de la Reunion, etc.," Paris, 4to, 1878; and his work, "Lea Volcans, " 1884.
 
 416 TEXT-BOOK OF GEOLOGY 
 
 volcanic mountains are now destitute of craters, or in other 
 cases, because the lava has welled up in dome form covered 
 perhaps with masses of scoriae, but without the production 
 of a definite crater. Mount Ararat, for example, is said to 
 have no crater; but so late as the year 1840 a fissure opened 
 on its side whence a considerable eruption took place. The 
 trachytic puys of Auvergne are dome-shaped hills without 
 craters. 
 
 Though the interior of modern volcanic cones can be at 
 the best but very partially examined, the study of the sites 
 of long-extinct cones, laid bare after denudation, shows that 
 subsidence of the ground has commonly taken place at and 
 round a vent. Evidence of subsidence has also been ob- 
 served at some modern volcanoes (ante, p. 395). Theoreti- 
 cally two causes may be assigned for this structure. In the 
 first place, the mere piling up of a huge mass of material 
 round a given centre tends to press down the rock under- 
 neath, as some railway embankments may be observed to 
 have done. This pressure must often amount to several 
 hundred tons on the square foot. In the second place, the 
 expulsion of volcanic material to the surface may leave 
 cavities underneath, into which the overlying crust will 
 naturally gravitate. These two causes combined, as sug- 
 gested by Mr. Mallet, afford a probable explanation of the 
 saucer-shaped depressions in which many ancient and some 
 modern vents appear to lie. 118 
 
 The following are the more important types of volcanic 
 cones: 114 
 
 113 Mallet, Q. J. Geol. Soc. xxxiii. p. 740. See also the account of "Volcanic 
 Necks," in Book IV. Part VII. 
 
 114 Von Seebach (Z. Deutsch. Geol. Ges. xviii. 644) distinguished two vol- 
 canic types. 1st, Bedded Volcanoes (Strato-Vulkane), composed of successive 
 sheets of lava and tuffs, and embracing the great majority of volcanoes. 
 2d, Dome Volcanoes, formiug hills composed of homogeneous protrusions of
 
 DYNAMICAL GEOLOGY 417 
 
 1. Cones of Non-volcanic Materials. These are due to the 
 discharge of steam or other aeriform product through the 
 solid crust without the emission of any true ashes or lava. 
 The materials ejected from the cavity are wholly, or almost 
 wholly, parts of the surrounding rocks through which the 
 volcanic pipe has been drilled. Some of the cones surround- 
 
 _ ^sJsSBKSte*-.- 
 
 Fig. 56 View of the Tuff-cones of Auvergne, taken from the top of the cone and 
 crater of Puy Pariou. 
 
 ing the crater lakes (maare) of the Eifel consist chiefly of 
 fragments of the underlying Devonian slates (pp. 341, 363). 
 2. Tuff-Cones, Cinder-Cones. Successive eruptions of fine 
 dust and stones, often rendered pasty by mixture with the 
 water so copiously condensed during an eruption, form a 
 cone in which the materials are solidified by pressure into 
 tuff. Cones made up only of loose cinders, like Monte 
 Nuovo in the Bay of Baiae, often arise on the flanks or 
 round the roots of a great volcano, as happens to a small 
 extent on Vesuvius, and on a larger scale upon Etna. They 
 
 lava, with little or no accompanying fragmentary discharges, without cratera 
 or chimneys, or at least with only minor examples of these volcanic features. 
 He believed that the same volcano might at different periods in its history be- 
 long to one or other of these types the determining cause being the nature of 
 the erupted lava, which, in the case of the dome volcanoes, is less fusible and 
 more viscid than in that of the bedded volcanoes. (See below, under "Lava- 
 cones.")
 
 418 
 
 TEXT-BOOK OF GEOLOGY 
 
 likewise occur by themselves apart from any lava-producing 
 volcano, though usually they afford indications that columns 
 of lava have risen in their 'funnels, and even now and then 
 that this lava has reached the surface. 
 
 The cones of the Eifel district have long been celebrated 
 for their wonderful perfection. Though small in size, they 
 exhibit with singular clearness many of the leading features 
 of volcanic structure. Those of Auvergne are likewise ex- 
 ceedingly instructive. 116 The high plateaus of Utah are 
 dotted with hundreds of small volcanic cinder-cones, the 
 singular positions of which, close to the edge of profound 
 river-gorges and on the upthrow side of faults, have already 
 (p. 348) been noticed. Among the Carboniferous volcanic 
 rocks of central Scotland the stumps of ancient tuff-cones, 
 frequently with a central core of basalt, or with dikes and 
 veins of that rock, are of common occurrence. 1 " 
 
 The materials of a tuff-cone are arranged in more or less 
 regularly stratified beds. On the outer side, they dip down 
 the slopes of the cone at the average angle of repose, which 
 
 Fig. 57. Section of the crater-rim of the Island of Volcano. 
 a, Older tuff; 6 6, younger ashes; the crater lies to the right. 
 
 may range between 30 and 40. From the summit of the 
 crater-lip they likewise dip inward toward the crater-bottom 
 at similar angles of inclination (Pig. 57). 
 
 3. Mud-Cones resemble tuff -cones in form, but are usually 
 smaller in size and less steep. They are produced by the 
 hardening of successive outpourings of mud from the ori- 
 fices already described (p. 407). In the region of the Lower 
 Indus, where they are abundantly distributed over an area 
 
 116 For Auvergne, see works cited on p. 374. For the Eifel, consult Hib- 
 bert, "History of the Extinct Volcanoes of the Basin of Neuwied on the Lower 
 Ehine," Edin. 1832. Von Dechen, "Geognostischer Fuhrer zu dem Laacher 
 See," Bonn, 1864. "Geognostischer Fuhrer in das Siebengebirge am Rhein," 
 Bonn, 1861. 
 
 " Trans. Roy. Soc. Edin. xxix. p. 455. See postea, Book IV. Part VII.
 
 DYNAMICAL GEOLOGY 419 
 
 of 1000 square miles, some of them attain a height 
 of 400 feet, with craters 30 yards across. 1 " 
 
 4. Lava-cones. Yolcanic cones composed en- 
 tirely of lava are comparatively rare, out occur 
 in some younger Tertiary and modern volcanoes. 
 Fouque describes the lava of 1866 at Santorin 
 as having formed a dome-shaped elevation, flow- 
 ing out quietly and rapidly without explosions. 
 After several days, however, its emission was ac- 
 companied with copious discharges of fragmentary 
 materials and the formation of several crateriform 
 mouths on the top of the dome. Where lava pos- 
 sesses extreme liquidity, and gives rise to little 
 or no fragmentary matter, it may build up a flat 
 cone as in the remarkable examples described by 
 
 Fig. 59. Plan of Lava-caldron, Kilauea, Hawaii (Dana, 1841 u). 
 
 Dana from the Hawaii Islands. 118 On the summit 
 of Mauua Loa (Fig. 58), a flat lava-cone 13,760 
 feet above the sea, lies a crater, which in its deep- 
 est part is about 8,000 feet broad, with vertical 
 walls of stratified lava rising on one side to a 
 height of 784 feet above the black lava- plain of 
 the crater-bottom. From the edges of this ele- 
 vated caldron, the mountain slopes outward at an 
 angle of not more than 6, until, at a level of 
 about 10,000 feet lower, its surface is indented 
 
 m Lyell, "Principles," ii. p. 77. 
 
 118 In Wilkes's Report of U. S. Exploring Expedition, 1838-42, and Dana's 
 "Characteristics of Volcanoes." See the works cited on p. 350. 
 
 119 For more recent maps showing the variations of this crater, see Dana's 
 "Characteristics. ' '
 
 420 TEXT-BOOK OF GEOLOGY 
 
 by the vast pit-crater, Kilauea, about two miles long, and 
 nearly a mile broad. So low are the surrounding slopes 
 that these vast craters have been compared to open quarries 
 on a hill or moor. The bottom of Kilauea is a lava-plain, 
 dotted with lakes of extremely fluid lava in constant ebul- 
 lition. The level of the lava has varied, for the walls sur- 
 rounding the fiery flood consist of beds of similar lava, and 
 are marked by ledges or platforms (Fig. 59) indicative of 
 former successive heights of lava, as lake terraces show 
 former levels of water. In the accompanying section (Fig. 
 60) the walls rising above the lower pit (p p') were found to 
 be 342 feet high, those bounding the higher terrace (o n n' o') 
 were 660 feet high, all being composed of innumerable beds 
 of lava, as in cliffs of stratified rocks. Much of the bottom 
 of the lower lava-plain has been crusted over by the solidifi- 
 cation of the molten rock. But large areas, which shift 
 
 Fig. 60. Section of Lava-terraces in Kilauea (Dana). 
 
 their position from time to time, remain in perpetual rapid 
 ebullition. The glowing flood, as it boils up with a fluidity 
 more like that of water than what is commonly shown by 
 molten rock, surges against the surrounding terrace walls. 
 Large segments of the cliffs, undermined by the fusion of 
 their base, fall at intervals into the fiery waves and are soon 
 melted. Recent observations by Captain Button point to a 
 diminution of the activity of this lava-crater. In Iceland, 
 and in the Western Territories of North America, low domes 
 of lava appear to mark the vents from which extensive 
 basalt floods have issued. 
 
 Where the lava assumes a more viscid character, as in 
 trachyte and liparite, dome-shaped eminences may be pro- 
 truded. As the mass increases in size by the advent of 
 fresh material injected from below, the outer layer will be 
 pushed outward, and successive shells will in like manner 
 be enlarged as the eruption advances. On the cessation of 
 discharges, we may conceive that a volcanic hill formed in 
 this way will present an onion-like arrangement of its com- 
 ponent sheets of rock. More or less perfect examples of this 
 structure have been observed in Bohemia, Auvergne, and
 
 DYNAMICAL GEOLOGY 
 
 421 
 
 the Eifel. 180 The trachytic domes of Auvergne form a con- 
 spicuous feature among the cinder cones of that region. 
 Huge conical protuberances of granophyre occur among the 
 Tertiary volcanic rocks of the Inner Hebrides, and similar 
 hills of liparite rise through the basalts of Iceland. 
 
 5. Cones of Tuff and Lava. This is by far the most abun- 
 dant type of volcanic structure, and includes the great vol- 
 canoes of the globe. Beginning, perhaps, as mere tuff-cones, 
 these eminences have gradually been built up by successive 
 
 Fig. 61. Plan of the Peak of Teneriffe, showing the large crater and minor cones. 
 
 outpourings of lava from different sides, and by showers of 
 dust and scoriae. At first, the lava, if the sides of the cone 
 are strong enough to resist its pressure, may rise until it 
 overflows from the crater. Subsequently, as the funnel be- 
 comes choked up, and the cone is shattered by repeated 
 explosions, the lava finds egress from different fissures and 
 openings on the cone. As the mountain increases in height, 
 the number of lava-currents from its summit will usually 
 
 120 B. Reyer (Jahrb. Geol. Reichs. 1879, p. 463) lias experimentally imitated 
 the process of extrusion by forcing up plaster of Paris through a hole in a board. 
 For drawings of the Puy de Sarcouy and other dome-shaped hills which pre- 
 sumably have had this mode of origin, see Scrope's "Geology and Extinct Vol- 
 canoes of Central France." Refer also to the remarks already made on the 
 liquidity of lava (ante, pp. 378-384), and the account of "Vulkanische Kup- 
 pen, " postea, p. 433.
 
 422 
 
 TEXT-BOOK OF GEOLOGY 
 
 decrease. Indeed, the taller a volcanic cone grows, the less 
 frequently as a rule does it erupt. The lofty volcanoes of 
 the Andes have each seldom been more than once in erup- 
 tion during a century. The Peak of Teneriffe (Fig. 61) was 
 three times active during 370 years prior to 1798. 181 The 
 
 Pig. 62. Map of Etna, after Sartorius von Waltershausen. 
 
 1, Lava of 1879; 2, Lavas of 1865 and 1852; 3, Lava of 1669; 4, Recent Lavas; 6, Lavas 
 
 of the Middle Ages; 6, Ancient Lavas of unknown date; 7, (.'ones and Craters; 
 
 8, Non-volcanic Bocks. 
 
 earlier efforts of a volcano tend to increase its height, as well 
 as its breadth; the later eruptions chiefly augment the 
 breadth, and are often apt to diminish the height by blow- 
 ing away the upper part of the cone. The formation of 
 fissures and the consequent intrusion of a network of lava- 
 
 181 For a recent account of Teneriffe, see A. Rothpletz, Pctermann's Mittlieil. 
 xxxv. (1889), p. 237.
 
 DYNAMICAL GEOLOGY 
 
 dikes, tend to bind the framework of the volcano and 
 strengthen it against subsequent explo- 
 sions. In this way, a kind of oscillation 
 is established in the form of the cone, pe- 
 riods of crater-eruptions being succeeded 
 by others when the emissions take place 
 only laterally (ante, p. 369). 
 
 One consequence of lateral eruption is 
 the formation of minor parasitic cones on 
 the flanks of the parent volcano (p. 329). 
 Those on Etna, more than 200 in number, 
 are really miniature volcanoes, some of 
 them reaching a height of 700 feet (Fig. 
 62). As the lateral vents successively be- 
 come extinct, the cones are buried under 
 sheets of lava and showers of debris thrown 
 out from younger openings or from the 
 parent cone. It sometimes happens that 
 the original funnel is disused, and. that the 
 eruptions of the volcano take place from a 
 newer main vent. Vesuvius, for example <Sl 
 (as shown in Figs. 63 and 45), stands on 
 the site of a portion of the rim of the more 
 ancient and much larger vent of Monte 
 Somma. The present crater of Etna lies 
 to the northwest of the former vaster crater. 
 The pretty little example of this shifting 
 furnished by Volcanello has been already 
 noticed (p. 415). 
 
 While, therefore, a volcano, and more 
 particularly one of great size, throwing 
 put both lava and fragmentary materials, 
 is liable to continual modification of its 
 external form, as the result of successive 
 eruptions, its contour is likewise usually 
 exposed to extensive alteration by the 
 effects of ordinary atmospheric erosion, 
 as well as from the condensation of the 
 volcanic vapors. Heavy and sudden floods, 
 produced by the rapid rainfall consequent 
 upon a copious discharge of steam, rush 
 down the slopes with such volume and force 
 as to cut deep gullies in the loose or only 
 partially consolidated tuffs and scoriaB. Bain continues the 
 erosion until the outer slopes, unless occasionally renewed
 
 424: TEXT-BOOK OF GEOLOGY 
 
 by fresh showers of detritus, assume a curiously furrowed 
 aspect, like a half-opened umbrella, the ridges being sepa- 
 rated by furrows that narrow upward toward the summit of 
 the cone. The outer declivities of Monte Soinma afford an 
 excellent illustration of this form of surface, the numerous 
 ravines on that side of the mountain presenting instructive 
 sections of the prehistoric lavas and tuns of the earlier and 
 more important period in the history of this volcano. 1 " 
 Similar trenches have been eroded on the southern or Vesu- 
 vian side of the original cone, but these have in great meas- 
 ure been filled up by the lavas of the younger mountain. 
 The ravines, in fact, form natural channels for the lava, as 
 may unfortunately be seen round the Yesuvian observatory. 
 This building is placed on one of the ridges between two 
 deep ravines; but the lava-streams of recent years have 
 poured into these ravines on either side, and are rapidly 
 filling them up. 
 
 Submarine Volcanoes. It is not only on the surface of 
 the land that volcanic action shows itself. It takes place 
 likewise under the sea, and as the geological records of the 
 earth's past history are chiefly marine formations, the char- 
 acteristics of submarine volcanic action have no small inter- 
 est for the geologist. In a few instances, the actual out- 
 break of a submarine eruption has been witnessed. Thus, 
 in the early summer of 1783, a volcanic eruption took place 
 about thirty miles from Cape Reykjanaes on the west coast 
 of Iceland. An island was built up, from which fire and 
 smoke continued to issue, but in less than a year the waves 
 have washed the loose pumice away, leaving a submerged 
 reef from five to thirty fathoms below sea-level. About a 
 month after this eruption, the frightful outbreak of Skaptar 
 Jokull, already referred to (p. 378), began, the distance of 
 this mountain from the submarine vent being nearly 200 
 miles. 1 ** A century afterward, viz. in July, 1884, another 
 
 1M See H. J. Johnstoii-Lbivis, Q. J. Geol. Soc. xl. p. 103. 
 183 Ljrell, "Principles," ii. p. 49.
 
 DYNAMICAL GEOLOGY 425 
 
 volcanic island is said to have been thrown up near the 
 same spot, having at first the form of a flattened cone, but 
 soon yielding to the power of the breakers. Many sub- 
 marine eruptions have taken place within historic times 
 in the Mediterranean. The most noted of these occurred 
 in the year 1831, when a new volcanic island (Graham's 
 Island, He Julia) was thrown up, with abundant discharge 
 of steam and showers of scoriae, between Sicily and the 
 
 Fig. 64. Sketch of submarine volcanic eruption (Sabrina Island) off St. Michael's, 
 June, 1811. 
 
 coast of Africa. It reached an extreme height of 200 feet 
 or more above the sea-level (800 feet above sea-bottom) 
 with a circumference of 3 miles, but on the cessation of. 
 the eruptions was attacked by the waves and soon demol- 
 ished, leaving only a shoal to mark its site. 184 In the year 
 1811, another island was formed by submarine eruption of 
 
 124 Phil Trans. 1832. Constant Prevost, Ann. des Sci. Nat. xxiv. Mem. 
 Soc. Geol. France, ii. p. 91. Mercalli's "Vulcani," etc., p. 117. For a recent 
 submarine eruption in the Mediterranean, see Bicco, Compt. Rend. Nov. 23, 
 1891.'
 
 426 TEXT-BOOK OF GEOLOGY 
 
 the coast off St. Michael's in the Azores (Fig. 64). Con- 
 sisting, like the Mediterranean example, of loose cinders, 
 it rose to a height of about 300 feet, with a circumference 
 of about a mile, but subsequently disappeared. 1 " In the 
 year 1796 the island of Johanna Bogoslawa, in Alaska, ap- 
 peared above the water, and in four years had grown into 
 a large volcanic cone, the summit of which was 3,000 feet 
 above sea-level. 1 " 
 
 Unfortunately, the phenomena of recent volcanic erup- 
 tions under the sea are for the most part inaccessible. Here 
 and there, as in the Bay of Naples, at Etna, among the 
 islands of the Greek Archipelago, and at Tahiti, elevation 
 of the sea-bed has taken place, and brought to the surface 
 beds of tuff or of lava, which have consolidated under water. 
 Both Vesuvius and Etna began their career as submarine 
 volcanoes. 187 It will be seen from the accompanying chart 
 (Fig. 65), that the Islands of Santorin and Therasia form 
 the unsubmerged portions of a great crater-rim rising round 
 a crater which descends 1278 feet below sea-level. The 
 materials of these islands consist of a nucleus of marbles 
 and schists nearly buried under a pile of tuft's (trass), 
 scoria?, and sheets of lava, the bedded character of which 
 is well shown in the accompanying sketch by Admiral 
 Spratt (Fig. 66), who, with the late Prof. Edward Forbes, 
 examined the geology of this interesting district in 1841. 
 They found some of the tuffs to contain marine shells, and 
 thus to bear witness to an elevation of the sea-floor since 
 volcanic action began. More recently the islands have been 
 carefully studied by various observers. K. von Fritsch has 
 
 l * De la Becbe, "Geological Observer," p. 70. 
 
 m D. Forbes, Geol. Mag. vii. p. 323. 
 m See, aa regards Etna, "Der Aetna," ii. p. 327.
 
 DYNAMICAL GEOLOGY 
 
 427 
 
 found recent marine shells in 
 many places up to heights of 
 nearly 600 feet above the sea. 
 The strata containing these 
 remains he estimates to be at 
 least 100 to 120 metres thick, 
 and he remarks that in every 
 case he found them to consist 
 essentially of volcanic debris 
 
 Fig. 85. Map of partially-submerged Tol 
 
 cano of Santorin. 
 
 o, Thera, or Santorin; b, Therasia; c, Mi- 
 kro Kaimeni; d, Neo Kaimeni. The fig- 
 ures denote soundings in fathoms, the 
 dotted line marks the 100 fathoms line. 
 
 and to rest upon volcanic 
 rocks. It is evident, there- 
 fore, that these shell-bearing 
 tuffs were originally de- 
 posited on the sea-floor 
 after volcanic action had 
 begun here, and that dur- 
 ing later times they were
 
 ^28 TEXT-BOOK OF GEOLOGY 
 
 upraised, together with the submarine lavas associated 
 with them. 1 * 8 Fouque' concludes that the volcano formed 
 at one time a large island with wooded slopes and a 
 somewhat civilized human population, cultivating a fer- 
 tile valley in the southwestern district, and that in prehis- 
 toric times the tremendous explosion occurred whereby the 
 centre of the island was blown out. 
 
 The similarity of the structure of Santorin to that of 
 Somma and Etna is obvious. Volcanic action still contin- 
 ues there, though on a diminished scale. In 1866-67 an 
 eruption took place on Neo Kaimeni, one of the later- 
 formed islets in the centre of the old crater, and greatly 
 added to its area and height. The recent eruptions of San- 
 torin, which have been studied in great detail, are specially 
 interesting from the additional information they have sup- 
 plied as to the nature of volcanic vapors and gases. Among 
 these, as already stated (p. 334), free hydrogen plays an 
 important part, constituting, at the focus of discharge, thirty 
 per cent of the whole. By their eruption under water, the 
 mingling of these gases with atmospheric air and the com- 
 bustion of the inflammable compounds is there prevented, 
 so that the gaseous discharges can be collected and ana- 
 lyzed. Probably were operations of this kind more prac- 
 ticable at terrestrial volcanoes, free hydrogen and its 
 compounds would be more abundantly detected than has 
 hitherto been possible. 
 
 The numerous volcanoes which dot the Pacific Ocean, 
 
 "8 See Fritsch, Z. Deutsch. Geol. Ges. xxiii. (1871), pp. 125-213. The most 
 complete and elaborate work is Fouque"'s monograph (already cited), "Santorin 
 et sea Eruptions, " Paris, 4to, 1880, where copious analyses of rocks, minerals 
 and gaseous emanations, with maps and numerous admirable views and sections, 
 are given. In this volume a bibliography of the locality will be found. Corn- 
 pare C. Doelter on Ihe Ponza Islands, Denksch. Akad. Wissensch. Vienna, xxxvi. 
 p. 141. Sitz. Akad. "Wissensch. Vienna, Ixxi. (1875), p. 49.
 
 DYNAMICAL GEOLOGY 429 
 
 probably in most cases began their career as submarine 
 vents, their eventual appearance as subaerial cones being 
 mainly due to the accumulation 
 of erupted material, but also par- 
 tially, as in the case of Santorin, 
 to actual upheaval of the sea- 
 bottom. The lonely island of St. 
 Paul (Figs. 67 and 69), lying in 
 the Indian Ocean more than 2000 
 miles from the nearest land is a 
 notable example of the summit 
 
 Fig. 67. Volcanic crater of St. 
 
 of a volcanic mountain rising to paul Island ' Indian ocean, 
 the sea-level in mid-ocean. Its circular crater, broken 
 down on the northeast side, is filled with water having 
 a depth of 30 fathoms. 11 * 
 
 Observations by K. von Drasche have shown that at 
 Bourbon (Reunion), during the early submarine eruptions 
 of that volcano, coarsely crystalline rocks (gabbro) were 
 emitted, that these were succeeded by andesitic and tra- 
 chytic lavas: but that when the vent rose above the sea, 
 basalts were poured out. 130 Fouque" observes that at San- 
 torin some of the early submarine lavas are identical with 
 those of later subaerial origin, but that the greater part of 
 
 129 For a general account of the volcanic islands of the ocean, see Darwin's 
 "Volcanic Islands, " 2d edit. 1876. For the Philippine volcanoes, see R. von 
 Drasche, Tschermak's Mirieralogische Mittheil. 1876; Semper's "Die Philip- 
 pinen und ihre Bewohner," Wurzburg, 1869. For the Kurile Islands, J. Milne, 
 G-eol. Mag. 1879, 1880, 1881 ; Volcanoes of Bay of Bengal (Barren Island, etc.), 
 V. Ball, Geol. Mag. 1879, p. 16; 1888, p. 404; F. R. Mallet, Mem. G-eol. Surv. 
 India, xxi. part iv. St. Paul (Indian Ocean), C. Velain, Assoc. Fran. 1875, p. 
 581; "Mission a i'ile St. Paul." 1879; "Description geologique de la Presqu'ile 
 d'Aden," etc., 4to, Paris, 1878; and "Les Voleans," 1884. For Isle of Bour- 
 bon, see authorities cited on p. 415, and for Hawaii, the references on p. 350. 
 
 130 Tschermak's Mineralogische Mittheil. 1876, pp. 42, 157. A similar struc- 
 ture occurs at Palma (Cohen, Neues Jahrb. 1879, p. 482) and in St. Paul (Velain 
 as above cited).
 
 430 TEXT-BOOK OF GEOLOGY 
 
 them belong to an entirely different series, being acid 
 rocks, belonging to the group of hornblende-andesites, 
 while the subaerial rocks are augite-andesites. The acidity 
 of these lavas has been largely increased by the infusion 
 into them of much silica, chiefly in the form of opal. They 
 differ much in aspect, being sometimes compact, scoriaceous, 
 hard, like millstone, with perlitic and spherulitic structures, 
 while they frequently present the characters of trass im- 
 pregnated with opal and zeolites. Among the fragmental 
 ejections there occur blocks of schist and granitoid rocks, 
 probably representing the materials below the sea-floor 
 through which the first explosion took place (pp. 341, 363, 
 417). During the eruption of 1866 some islets of lava rose 
 above the sea in the middle of the bay, near the active 
 vent. The rock in these cases was compact, vitreous, and 
 much cracked. 131 
 
 Among submarine volcanic formations, the tuffs differ 
 from those laid down on land chiefly in their organic con- 
 tents; but partly also in their more distinct and originally 
 less inclined bedding, and in their tendency to the ad- 
 mixture of non-volcanic or ordinary mechanical sediment 
 with the volcanic dust and stones. No appreciable dif- 
 ference either in external aspect or in internal structure 
 seems yet to have been established between subaerial and 
 submarine lavas. Some undoubtedly submarine lavas are 
 highly scoriaceous. There is no reason, indeed, why slaggy 
 lava and loose, non-buoyant scoriae should not accumulate 
 under the pressure of a deep column of the ocean. At 
 the Hawaii Islands, on 25th February, 1877, masses of 
 pumice, during a submarine volcanic explosion, were 
 
 131 Fouque, "Santorin."
 
 DYNAMICAL GEOLOGY 
 
 431 
 
 ejected to the surface, one of which struck the bottom 
 of a boat with considerable violence and then floated. 
 When we reflect, indeed, to what a considerable extent 
 the bottom of the great ocean-basins is dotted over with 
 volcanic cones, rising often solitary from profound depths', 
 we can believe that a large proportion of the actual erup- 
 tions in oceanic areas may take place under the sea. The 
 immense abundance and wide diffusion of volcanic detritus 
 (including blocks of pumice) over the bottom of the Pacific 
 
 Fig. 68. -View of the Peak of Teneriffe and its coast-erosion. 
 
 and Atlantic oceans, even at distances remote from land, as 
 made known by the voyage of the "Challenger," doubtless 
 indicate the prevalence and persistence of submarine vol- 
 canic action, even though, at the same time, an extensive 
 diffusion of volcanic debris from the islands is admitted to 
 be effected by winds and ocean -currents. 
 
 Volcanic islands, unless continually augmented by re- 
 newed eruptions, are attacked by the waves and cut down. 
 Graham's Island and the other examples above cited show
 
 432 TEXT-BOOK OF GEOLOGY 
 
 how rapid this disappearance may be. The island of Vol- 
 cano has the base of its slopes truncated by a line of cliff 
 due to marine erosion. The island of Teneriffe shows, in 
 the same way, that the sea is cutting back the land toward 
 the great cone (Fig. 68). The island of St. Paul (Figs. 67, 
 69) brings before us in a more impressive way the tendency 
 
 Fig. 69. View of St. Paul Island, Indian Ocean, from the east (Capt. Blackwood 
 in Admiralty Chart). 
 
 a, Nine-pin Rock, a stack of harder rock left by the sea; 6, entrance to crater lagoon 
 (see Fig. 67); c, d, e, cliffs composed of bedded volcanic materials dipping toward 
 the south, and much eroded at the higher end (c) by waves and subaerial waste ; 
 /, southern point of the island, likewise cut away into a cliff. 
 
 of volcanic islands to be destroyed unless replenished by 
 continual additions to their surface. At St. Helena lofty 
 cliffs of volcanic rocks 1000 to 2000 feet high bear witness 
 to the enormous denudation whereby masses of basalt two 
 or three miles long, one or two miles broad, and 1000 
 to 2000 feet thick, have been entirely removed. 1 " 
 
 ii. Fissure (Massive) Eruptions 
 
 Under the head of massive or homogeneous volcanoes 
 some geologists have included a great number of bosses or 
 dome-like projections of once-melted rock which, in regions 
 of extinct volcanoes, rise cpnspicuously above the surface 
 without any visible trace of cones or craters of fragmentary 
 material. They are usually regarded as protrusions of lava, 
 which, like the Puy de Dome in Auvergne, assumed a 
 dome-form at the surface without spreading out in sheets 
 
 132 Darwin, "Volcanic Islands," p. 104. For a more detailed account of this 
 island, see J. C. Melliss' "St. Helena," London, 1875.
 
 DYNAMICAL GEOLOGY 433 
 
 over the surrounding country, and with no accompanying 
 fragmentary discharges. But the mere absence of ashes and 
 scorisB is no proof that these did not once exist, or that the 
 present knob or boss of lava may not originally have so- 
 lidified within a cone of tuff which has been subsequently 
 removed in denudation. The extent to which the surface 
 of the ground has been changed by ordinary atmospheric 
 waste, and the comparative ease with which loose volcanic 
 dust and cinders might have been entirely removed, require 
 to be considered. Hence, though the ordinary explanation 
 is no doubt in some cases correct, it may be doubted 
 whether a large proportion of the examples cited from the 
 Rhine, Bohemia, Hungary, and other regions, ought not 
 rather to be regarded, like the "necks" so abundant in the 
 ancient volcanic districts of Britain (Book IV. Part VII.), 
 as the remaining roots of ordinary volcanic cones. If the 
 tuff of a cone, up the funnel of which lava rose and solidi- 
 fied, were swept away, we should find a central lava plug 
 or core resembling the volcanic "heads" (vulkanische Kup- 
 peri) of Germany. Unquestionably, lava has in innumer- 
 able instances risen in this way within cones of tuff or 
 cinders, partially filling them without flowing out into the 
 surrounding country. 183 
 
 But while, on either explanation of their origin, these 
 volcanic "heads" find their analogues in the emissions of 
 lava in modern volcanoes, there are numerous cases in old 
 volcanic areas where the eruptions, so far as can now be 
 judged, were not attended with the production of any 
 central cone or crater. Such emissions of lava may have 
 
 is y on geebach, Z. Deutsch. Geol. Ges. xviii. p. 643. F. von Hochstetter, 
 Neues Jahrb. 1871, p. 469. Reyer, Jahrb. K. K. Geol. Reichsanstalt, 1878, 
 p. 81; 1879, p. 463. 
 
 GEOLOGY Vol. XXIX 19
 
 484 TEXT-BOOK OF GEOLOGY 
 
 resembled those which in recent times have occurred at the 
 Hawaiian volcanoes, where enormous accumulations of lava 
 have gradually been built up into flat domes, of which 
 Mauna Loa rises to a height of 13,675 feet. Vast floods of 
 remarkably liquid basic lava have from time to time flowed 
 out tranquilly without explosion or earthquake, and with 
 no accompaniment of fragmental discharges. These cur- 
 rents of molten rock have spread out into wide sheets, 
 sloping at so low an angle that they look horizontal. The 
 lower and older portions of them have been eroded by 
 streams so as to present escarpments and outliers not un- 
 like those of western North America or the older basaltic 
 plateaus of Britain and India. 1 ' 4 
 
 The most stupendous modern basaltic-floods of Iceland 
 issued from vents along a fissure. According to Thorodd- 
 sen the post-glacial lava-fields of Odadahraun, covering 
 an area of about 4390 square kilometres, have issued from 
 about 20 distinct vents, while in the east of Iceland the 
 lava has flowed from the lips of fissures. 186 It would seem 
 that for the discharge of such wide and flat sheets of lava, 
 great mobility and tolerably complete fusion of the molten 
 mass is necessary. The phenomenon occurs among the 
 more basic lavas (basalts, etc.) rather than among the more 
 lithoid acid lavas (trachytes, rhyolites, etc.). 
 
 In former geological ages, extensive eruptions of lava, 
 without the accompaniment of scoriae, with hardly any 
 fragmentary materials, and with, at the most, only flat 
 
 134 For a graphic account of the Hawaiian lava-fields, see Captain Button, 
 Fourth Annual Report, U. S. Geol. Survey for 1882-83. See also Dana's 
 "Characteristics of Volcanoes." 
 
 135 See W. L. Watts' "Across the Vatna Jokull," Proc. Roy. Geog. Soc. 
 1876. W. G. Lock, Geol. Mag. 1881, p. 212; and papers by Thoroddsen and 
 Holland, quoted ante, p. 345.
 
 DYNAMICAL GEOLOGY 435 
 
 dome-shaped cones at the points of emission, have taken 
 place over wide areas from scattered vents, along lines or 
 systems of fissures. Vast sheets of lava have in this man- 
 ner been poured out to a depth of many hundred feet, com- 
 pletely burying the previous surface of the land and forming 
 wide plains or plateaus. These truly "massive eruptions" 
 have been held by Bichthofen 136 and others to represent the 
 grand fundamental character of volcanisrn, ordinary vol- 
 canic cones being regarded merely as parasitic excrescences 
 on the subterranean lava-reservoirs, .very much in the rela- 
 tion of minor cinder cones to their parent volcano. 1 " 
 
 Though a description of these old fissure or massive 
 eruptions ought properly to be included in Book IV., the 
 subject is so closely connected with the dynamics of exist- 
 ing active volcanoes that an account of the subject may be 
 given here. Perhaps the most stupendous example of this 
 type of volcanic structure occurs in Western North America. 
 The extent of country which has been flooded with basalt in 
 Oregon, Washington, California, Idaho, and Montana has 
 not yet been accurately surveyed, but has been estimated 
 to cover a larger area than France and Great Britain com- 
 bined, with a thickness averaging 2000 but reaching in some 
 places to 3700 feet. 138 The Snake River plain in Idaho (Fig. 
 70) forms part of this lava-flood. Surrounded on the north 
 and east by lofty mountains, it stretches westward as an ap- 
 parently boundless desert of sand and bare sheets of black 
 basalt. A few streams descending into the plain from the 
 lulls are soon swallowed up and lost. The Snake River, 
 however, flows across it, and has cut out of its lava-beds a 
 
 138 Trans. Akad. Sci. California, 1868. 
 
 137 Proc. Roy. Phys. Soc. Edin. v. 236; Nature, xxiii. p. 3. 
 
 138 J. LeConte. Amer. Journ. Sci. 3d ser. vii. (1874), 167, 259.
 
 436 
 
 TEXT-BOOK OF GEOLOGY 
 
 series of picturesque gorges and rapids. Looked at from 
 any point on its surface, it appears as a vast level plain 
 like that of a lake-bottom, though more detailed examina- 
 tion may detect a slope in one or more directions, and may 
 thereby obtain evidence as to the sites of the chief openings 
 from which the basalt was poured forth. The uniformity 
 of surface has been produced either by the lava flowing 
 over a plain or lake-bottom, or by the complete effacement 
 of an original and undulating contour of the ground under 
 
 
 Fig. 70. View of the great Basalt-plain of the Snake River, Idaho, with recent cones. 
 
 hundreds of feet of volcanic rock in successive sheets. The 
 lava rolling up to the base of the mountains has followed 
 the sinuosities of their margin, as the waters of a lake follow 
 its promontories and bays. The author crossed the Snake 
 River plain in 1879, and likewise rode for many miles along 
 its northern edge. He found the surface to be everywhere 
 marked with low hummocks or ridges of bare black basalt, 
 the surfaces of which exhibited a reticulated pavement of 
 the ends of columns. In some places, there was a percep-
 
 DYNAMICAL GEOLOGY 437 
 
 tible tendency in these ridges to range themselves in one 
 general northeasterly direction, when they might be likened 
 to a series of long, low waves, or ground-swells. In many 
 instances the crest of each ridge had cracked open into a 
 fissure which presented along its walls a series of tolerably 
 symmetrical columns (Fig. 70). That these ridges were 
 original undulations of the lava, and had not been pro- 
 duced by erosion, was indicated by the fact that the col- 
 umns were perpendicular to their surface, and changed 
 in direction according to the form of the ground which 
 was the original cooling surface of the lava. Though the 
 basalt was sometimes vesicular, no layers of slag or scoriae 
 were anywhere observed, nor did the surfaces of the ridges 
 exhibit any specially scoriform character. 
 
 There are no great cones whence this enormous flood 
 of basalt could have flowed. It probably escaped from 
 orifices or fissures still concealed under the sheets which 
 issued from them, the points of escape being marked only 
 by such low domes as could readily be buried under the suc- 
 ceeding eruptions from other vents. 189 That it was not the 
 result of one sudden outpouring of rock is shown by the dis- 
 tinct bedding of the basalt, which is well marked along the 
 river ravines. It arose from what may have been, on the 
 whole, a continuous though locally intermittent welling-out 
 of lava, probably from vents on many fissures extending 
 over a wide tract of Western America during a late Tertiary 
 period, if, indeed, the eruptions did not partly come within 
 the time of the human occupation of the continent. The 
 discharge of lava continued until the previous topography 
 was buried under some 2000 feet of lava, only the higher 
 
 139 Captain Dutton has remarked the absence of any conspicuous feature at 
 the sources from which some of the largest lava-streams of Hawaii have issued.
 
 438 TEXT-BOOK OF GEOLOGY 
 
 summits still projecting above the volcanic flood. 140 At a 
 few points on the plain and on its northern margin, the 
 author observed some small cinder cones (Fig. 70). These 
 were evidently formed during the closing stages of volcanic 
 action, and may be compared to the minor cones on a modern 
 volcano, or better, to those on the surface of a recent lava- 
 stream. 
 
 In Europe, daring older Tertiary time, similar enormous 
 outpourings of basalt covered many hundreds of square 
 miles. The most important of these is that which occupies 
 a large part of the northeast of Ireland, and in disconnected 
 areas extends through the Inner Hebrides and the Faroe 
 Islands into Iceland. Throughout that region, the paucity 
 of evidence of volcanic vents is truly remarkable. So ex- 
 tensive has been the denudation, that the inner structure of 
 the volcanic plateaus has been admirably revealed. The 
 ground beneath and around the basalt-sheets has been rent 
 into innumerable fissures which have been filled by the rise 
 of basalt into them. A vast number of basalt-dikes ranges 
 from the volcanic area eastward across Scotland and the 
 north of England and the north of Ireland. Toward the 
 west the molten rock reached the surface and was poured 
 out there, while to the eastward it does not appear to have 
 overflowed, or, at least, all evidence of the outflow has been 
 removed in denudation. When we reflect that this system 
 of dikes can be traced from the Orkney Islands southward 
 into Yorkshire and across Britain from sea to sea, over a 
 total area of probably not less than 100,000 square miles, 
 we can in some measure appreciate the volume of molten 
 basalt which in older Tertiary times underlay large tracts of 
 
 140 Prof. J. LeGonte believes that the chief fissures opened in the Cascade 
 and Blue Mountain Ranges. Amer. Journ. Sci. 3d series, v. (1874), p. 168.
 
 DYNAMICAL GEOLOGY 439 
 
 the site of the British Islands, rose up in so many thousands 
 of fissures, and poured forth at the surface over so wide an 
 area in the northwest. 141 
 
 In Africa, basaltic plateaus cover large tracts of Abys- 
 sinia, where by the denuding effect of heavy rains they have 
 been carved into picturesque hills, valleys, and ravines. 14 * 
 In India, an area of at least 200,000 square miles is covered 
 by the singularly horizontal volcanic plateaus of the ."Dec- 
 can Traps" (lavas and tuffs), which belong to the Cretaceous 
 period and attain a thickness of 6000 feet or more. 14 ' The 
 underlying platform of older rock, where it emerges from 
 beneath the edges of the basalt table-land, is found to be in 
 many places traversed by dikes; but no cones and craters 
 are anywhere visible. In these, and probably in many other 
 examples still undescribed, the formation of great plains or 
 plateaus of level sheets of lava is to be explained by "fis- 
 sure-eruptions" rather than by the operations of volcanoes 
 of the familiar "cone and crater" type. 
 
 4. Geographical and geological distri- 
 bution of volcanoes 
 
 Adequately to trace the distribution of volcanic actiom 
 over the globe, account ought to be taken of dormant and 
 extinct volcanoes, likewise of the proofs of volcanic out- 
 breaks during earlier geological periods. When this is 
 done, we learn, on the one hand, that innumerable dis- 
 tricts have been the scene of prolonged volcanic activity, 
 where there is now no underground commotion, and on the 
 other, that volcanic outbursts have been apt to take place 
 
 141 Trans. Roy. Soc. Edin. xxxv. (1888), p. 21. 
 
 I4S Blanford's "Abyssinia," 1870, p. 181. 
 
 143 Medlicott and Blanford, "Geology of India," p. 299.
 
 440 TEXT-BOOK OF GEOLOGY 
 
 again and again after wide intervals on the same ground, 
 some modern active volcanoes being thus the descendants 
 and representatives of older ones. Some of the facts re- 
 garding former volcanic action have been already stated. 
 Others will be given in Book IV. Part VII. 
 
 Confining attention to vents now active, of which the 
 total number may be about 300, 144 the chief facts regarding 
 their distribution over the globe may be thus summarized. 
 (1) Volcanoes occur along the margins of the ocean-basins, 
 particularly along lines of dominant mountain-ranges, which 
 either form part of the mainland of the continents or extend 
 as adjacent lines of islands. The vast hollow of the Pacific 
 is girdled with a wide ring of volcanic foci. (2) Volcanoes 
 rise, as a striking feature, from the submarine ridges that 
 traverse the ocean basins. All the oceanic islands are either 
 volcanic or formed of coral, and the scattered coral-islands 
 have in all likelihood been built upon the tops of submarine 
 volcanic cones. (3) Volcanoes are situated not far from the 
 sea. The only exceptions to this rule are certain vents in 
 Mantchuria and in the tract lying between Thibet and Si- 
 beria; but of the actual nature of these vents very little is 
 yet known. (4) The dominant arrangement of volcanoes 
 is in series along subterranean lines of weakness, as in the 
 chain of the Andes, the Aleutian Islands, and the Malay 
 Archipelago. A remarkable zone of volcanic vents girdles 
 the globe from Central A.merica eastward by the Azores and 
 Canary Islands to the Mediterranean, thence to the Red Sea, 
 
 144 This number is probably below the truth. Prof. J. Milne has enumerated 
 in Japan alone no fewer than fifty-three volcanoes which are either active or 
 have been active within a recent period. He remarks that, "if we were in a 
 position to indicate the volcanoes which had been in eruption during the last 
 4,000 years, the probability is that they would number several thousands rather 
 than four or five hundred." "Earthquakes and other Earth-movements," 1886, 
 p. 227. Compare Fisher, "Physics of Earth's Crust," 2d ed. chap. xxiv.
 
 DYNAMICAL GEOLOGY 441 
 
 and through the chains of islands from the south of Asia to 
 New Zealand and the heart of the Pacific. (5) On a smaller 
 scale the linear arrangement gives place to one in groups, 
 as in Italy, Iceland, and the volcanic islands of the great 
 oceans. 
 
 In the European area there are six active volcanoes 
 Vesuvius, Etna, Stromboli, Volcano, Santorin, and Nisyros. 
 Asia contains twenty-four, Africa ten, North America 
 twenty, 1 " Central America twenty-five, and South Amer- 
 ica thirty-seven. 14 * By much the larger number, however, 
 occur on islands in the ocean. In the Arctic Ocean rises the 
 solitary Jan Mayen. On the ridge separating the Arctic 
 and Atlantic basins, the group of Icelandic volcanoes is 
 found. Along the great central ridge of the Atlantic bot- 
 tom, numerous volcanic vents have risen above the surface 
 of the sea the Azores, Canary Islands, and the extinct de- 
 graded volcanoes of St. Helena, Ascension and Tristan 
 d'Acunha. On the eastern border lie the volcanic vents 
 of the islands off the African coast, and to the west those of 
 the West India Islands. Still more remarkable is the de- 
 velopment of volcanic energy in the Pacific area. From the 
 Aleutian Islands southward, a long line of volcanoes, num- 
 bering upward of a hundred active vents, extends through 
 Kamtschatka and the Kurile Islands to Japan, 147 whence 
 
 145 For an account of the remarkable extinct volcanoes of Northern Califor- 
 nia, Oregon and Washington Territory, see A. Hague and J. P. Iddings. Amer. 
 Joura. Sci. xiYi. (1883), p. 222. On Volcanoes of Mexico see H. Lenk, "Bei- 
 trage zur Geologic und Palseontologie der Republik Mexico," Leipzig, 1890; of 
 Central America, A. Dolfuss and E. de Monserrat, " Voyage Geologique, ' ' Paris, 
 folio, 1868; K. ron Seeback, Abh. Kon. Ges. Wiss. Gottingen, xxxviii. (1892). 
 
 141 For a recent account of the volcanoes of the Andes of the Equator see 
 Whymper's "Travels amongst the Great Andes." 
 
 141 For the volcanoes of Japan, besides papers quoted on p. 364, see W. J. 
 Holland, Appalachia, vi. (1890), 109. E. Naumann, Zeitsch. Deutsch. Geol. 
 Ges. 1877, p. 364. Mr. Milne enumerates 100 active vents from the Kuriles t 
 Kinshu (2,000 miles).
 
 442 TEXT-BOOK OF GEOLOGY 
 
 another numerous series carries the volcanic band far south 
 toward the Malay Archipelago, which must be regarded as 
 the chief centre of the present volcanic activity of our planet. 
 In Sumatra, Java, and adjoining islands, no fewer than fifty 
 active vents occur. The chain is continued through New 
 Guinea and the groups of islands to New Zealand. 148 Even 
 in the Antarctic regions, Mounts Erebus and Terror are 
 cited as active vents; while in the centre of the Pacific 
 Ocean rise the great lava cones of the Sandwich Islands. la 
 the Indian Ocean, the Bed Sea, and off the east coast of 
 Africa a few scattered vents appear. 
 
 Besides the existence of extinct volcanoes which have 
 obviously been active in comparatively recent times, the 
 geologist can adduce proofs of the former presence of active 
 volcanoes in many countries where cones, craters, and all 
 the ordinary aspects of volcanic mountains, have long dis- 
 appeared, but where sheets of lava, beds of tuff, dikes, and 
 necks representing the sites of volcanic vents have been rec- 
 ognized abundantly (Book IV. Part VII.). These manifes- 
 tations of volcanic action, moreover, have as wide a range 
 in geological time as they have in geographical area. Every 
 great geological period, back into pre-Cambrian time, seems 
 to have had its volcanoes. In Britain, for instance, there 
 were probably active volcanic vents in pre-Cambrian ages. 
 The Archaean gneiss of N. W. Scotland includes a remark- 
 able series of dikes presenting some points of resemblance 
 to the great Tertiary system. The Torridon sandstone of 
 the same region, which is now known to be pre-Cambrian, 
 
 148 The great eruption of Tarawera, New Zealand, in 1886, is described by 
 Prof. A. P. W. Thomas, "Report on the Eruption of Tarawera," published 
 by the Government in 1888: also Prof. Button's "Report on the Tarawera 
 Tolcanic District, Wellington, 1887," Quart. Journ. Geol. Soc. xliii. (1887), 
 p. 178.
 
 DYNAMICAL GEOLOGY 443 
 
 contains pebbles of various finely vesicular porphyrites, and 
 in one place includes a band of true tuff. In the lower Cam- 
 brian period the tuffs and diabases of Pembrokeshire were 
 erupted. Still more vigorous were the volcanoes in the 
 Lower Silurian period, when the lavas and tuffs of Snow- 
 don, Aran Mowddwy, and Cader Idris were ejected. Dur- 
 ing the deposition of the Upper Silurian rocks a few vol- 
 canoes were active in the west of Ireland. The Lower Old 
 Red Sandstone epoch was one of prolonged activity in cen- 
 tral Scotland. The earlier half of the Carboniferous period 
 likewise witnessed two distinct epochs of volcanic activity 
 over the same region. In the earlier of those, lavas (ande- 
 Bites and trachytes) were poured out in wide level plateaus 
 from many vents, while in the later, groups of minor cones 
 like the puys of Auvergne were dispersed among the lagoons. 
 During Permian time, more than a hundred small vents rose 
 in scattered groups across the centre and southwest of Scot- 
 land, while a few similar points of eruption appeared in the 
 southwest of England. No trace of any British Mesozoic 
 volcanoes has been met with. The vast interval between 
 Permian and older Tertiary time appears to have been a 
 period of total quiescence of volcanic activity. The older 
 Tertiary ages were distinguished by the outpouring of the 
 enormous basaltic plateaus of Antrim and the Inner Heb- 
 rides. >4t 
 
 In France and Germany, likewise, Palaeozoic time was 
 marked by the eruption of many diabase, porphyrite, and 
 quartz-porphyry lavas. In Brittany, for example, Dr. Bar- 
 rois has found a remarkable series of older Palaeozoic dia- 
 
 149 For a detailed summary of the volcanic history of Britain, see Presidential 
 addresses to the Geological Society, Quart. Journ. Geol. Soc. xlvii., xlviii. 
 (1891-92).
 
 444 TEXT-BOOK OF GEOLOGY 
 
 bases and porphyrites with tuft's and agglomerates. He dis- 
 tinguishes four principal periods of eruption 1. Cambrian 
 and Lower Silurian; 2. Middle and Upper Silurian ; 3. Up- 
 per Devonian; 4. Carboniferous. 160 The Permian period 
 was marked in G-ermany and also in the south of France 
 by the discharge of great masses of various quartz-porphy- 
 ries. The Triassic period likewise witnessed numerous erup- 
 tions. But from that period onward the same remarkable 
 quiescence appears to have reigned all over Europe which 
 characterized the geological history of Britain during Meso- 
 zoic time. 161 In Tertiary time a prodigious outpouring of 
 lavas, both acid and basic, continued from the Miocene 
 epoch down even perhaps to the historic period. Examples 
 of this great series are met with in Central France, the Eifel, 
 Italy, 162 Bohemia, and Hungary, almost to the existing 
 period. 163 Recent research has brought to light evidence of 
 a long succession of Tertiary and post-Tertiary volcanic out- 
 bursts in Western America (Nevada, Oregon, Idaho, Utah, 
 etc.). Contemporaneous volcanic rocks are associated with 
 Palaeozoic, Secondary, and Tertiary formations in New Zea- 
 land, and volcanic action there is not yet extinct. 
 
 Thus it can be shown that, within the same compara- 
 tively limited geographical space, volcanic action has been 
 
 150 Carte Geol. detaill. Prance, No. 7, T889. 
 
 161 Some trifling exceptions to this general statement are said to occur. 
 C. E. M. Eohrbach describes Cretaceous teschenites and diabases in Silesia 
 (Tschermak's Min. Mittheil. vii. (1885, p. 15). P. Choffat refers to Cenoma- 
 nian eruptions in Portugal (Journ. Sciencias Math. Phys. Natur, Lisbon, 1884). 
 A. B. Lagorio has found in the Crimea a series of sheets, dikes and bosses, 
 ranging from nevadites to basalts. 
 
 168 For early and classical accounts of the Italian volcanic districts, see Spal- 
 lanzani's "Voyages dans les deux Siciles," and Breislak's "Voyages Physiques 
 et Lythologiques dans la Campanie." Consult also Mercalli's "Vulcani," etc., 
 and Johnston-Lavis' "South Italian Volcanoes," already cited. 
 
 168 For a recent attempt to give a stratigraphical and geographical view of 
 the distribution of igneous rocks in Europe, see M. Bertrand, Bull. Soc. Geol. 
 France, xvi. (1888), p. 573.
 
 DYNAMICAL GEOLOGY 446 
 
 rife at intervals during a long succession of geological ages. 
 Even round the sites of still active vents, traces of far older 
 eruptions may be detected, as in the case of the existing ac- 
 tive volcanoes of Iceland, which rise from amid Tertiary 
 lavas and tuffs. Volcanic action, which now manifests itself 
 so conspicuously along certain lines, seems to have contin- 
 ued in that linear development for protracted periods of 
 time. The actual vents have changed, dying in one place 
 and breaking out in another, yet keeping on the whole along 
 the same tracts. Taking all the manifestations of volcanic 
 action together, both modern and ancient, we see that the 
 subterranean forces have operated along great lines in the 
 earth's crust, and that the existing volcanoes form but a 
 small proportion of the total number of once active vents. 
 
 Looking broadly at the geological history of volcanic ac- 
 tion we observe that, while there is evidence of the protru- 
 sion of both acid and basic materials from the remotest 
 periods, the earlier discharges were preponderantly acid. In 
 Britain, for example, the vast piles of lavas ejected during 
 the Silurian period were mainly of a felsitic character, though 
 considerable accumulations of andesites were not wanting. On 
 the other hand, the wide sheets of lava poured out in this 
 country during Tertiary time were chiefly basalts, the acid 
 protrusions occurring mostly as dikes and bosses. A similar 
 broad sequence has been observed in other countries. 
 
 When, however, we proceed to consider more closely the 
 nature of the successive eruptions during the continuance of 
 one of the volcanic periods of which records are preserved 
 among the geological formations, we discover proofs of a re- 
 markable variation in the character of the lavas. 184 Various 
 
 154 In some volcanoes (e.g. Tenerifie) the lower lavas are heavier and more 
 basic than the upper.
 
 446 TEXT-BOOK OF GEOLOGY 
 
 observers have noticed that volcanic rocks have succeeded 
 each other in a certain order in different regions. Baron 
 von Richthofen deduced from observations in Europe and 
 America a general sequence of volcanic succession, which 
 he arranged in the following order: 1. Propylite; 2. Ande- 
 site; 3. Trachyte; 4. Rhyolite; 5. Basalt. 1 " This sequence 
 he believed to be seldom or never complete in any one local- 
 ity; sometimes only one member of the series may be found; 
 but when two or more occur they follow, in his opinion, this 
 sequence, basalt being everywhere the latest of the series. 
 The subject has been more recently discussed by M. Ber- 
 trand, who remarks that in Europe each of the great areas 
 of plication has given rise to the formation of eruptive rocks 
 of every composition and structure. He recognizes a recur- 
 rence of the phenomena in successive geological periods, and 
 speaks of a definite order of eruptions in the same series. 16 ' 
 The great volcanic series of Auvergne presents a marvel- 
 lous succession of varied eruptions within a limited region 
 during what was probably a single volcanic period. The 
 first eruptions appear to have been basalts, and rocks of 
 similar character reappeared again and again in later stages 
 of the history, the intervening eruptions consisting of phono- 
 lites, trachytes, rhyolites, or andesites. The latest lavas 
 were scoriaceous basalts. 1 " Among the later Palaeozoic vol- 
 canic eruptions of Britain a more definite and regular recur- 
 rence of rocks appears to be traceable. The earlier lavas of 
 the Old Bed Sandstone and Carboniferous series were gen- 
 erally either intermediate or basic, sometimes remarkably 
 basic, while the late protrusions were decidedly acid. At 
 
 155 "The Natural System of Volcanic Rocks," Californ. Acad. Sci. 1868. 
 
 156 Bull. Soc. Geol. France, xvi. (1888), p. 611. 
 
 151 Carte Geol. detail!. France, Feuille 166 (Ciermont Ferraud).
 
 DYNAMICAL GEOLOGY 447 
 
 the one end we find basalts or diabases and picrites, fol- 
 lowed sometimes by copious outpourings of andesites, while 
 at the other end come intrusions of felsites and quartz-por- 
 phyries. Again, among the Tertiary lavas, the basalts of 
 the great plateaus are pierced by bosses and dikes of grano- 
 phyre and allied acid rocks. In these various examples the 
 facts point to some gradual change in the composition of the 
 subterranean magma during the lapse of a single volcanic 
 period a change in which there was a separation of basic 
 constituents and the discharge of more basic lavas, leaving 
 a more acid residuum to be erupted toward the end of the 
 activity. 168 
 
 5. Causes of Volcanic Action 
 
 The modus operandi whereby the internal heat of the 
 globe manifests itself in volcanic action is a problem to 
 which as yet no satisfactory solution has bee_n found. Were 
 this action merely an expression of the intensity of the heat, 
 we might expect it to have manifested itself in a far more 
 powerful manner in former periods, and to exhibit a regular- 
 ity and continuity commensurate with the exceedingly slow 
 diminution of the earth's temperature. But there is no geo- 
 logical evidence in favor of greater volcanic intensity in an- 
 cient than in more recent periods; on the contrary, it may 
 be doubted whether any of the Palaeozoic volcanoes equalled 
 in magnitude those of Tertiary and perhaps even post-Ter- 
 tiary times. On the other hand, no feature of volcanic ac- 
 tion is more conspicuous than its spasmodic fitfulness. 169 
 
 As physical considerations negative the idea of a com- 
 
 158 Quart. Journ. Geol. Soc. vol. xlviii. (1892), p. 177. 
 
 159 "Consult Dana, "Characteristics of Volcanoes," p. 15 et seq. Button, 
 U. S. Greol. Rep. 1882-83, p. 183 et seq. Prestwicb, Proc. Roy. Soc. xli. 
 (1886), p. 117. Lowl, Jahrb. Geol. Reichsanst. 1886, p. 315.
 
 448 TEXT-BOOK OF GEOLOGY 
 
 paratively thin crust, surmounting a molten interior whence 
 volcanic energy might be derived (ante, p. 100), geologists 
 have found themselves involved in great perplexity to ex- 
 plain volcanic phenomena, for the production of which a 
 source of no great depth would seem to be necessary. Some 
 have supposed the existence of pools or lakes of liquid lava 
 lying beneath the crust, and at an inconsiderable depth from 
 the surface. Others have appealed to the influence of the 
 contraction of the earth's mass, assuming the contraction to 
 be now greater in the outer than in the inner portions, and 
 that the effect of this external contraction must be to squeeze 
 out some of the internal molten matter through weak parts 
 of the crust. '" 
 
 That volcanic action is one of the results of terrestrial 
 contraction can hardly be doubted, though we are still with- 
 out satisfactory data as to the connection between the cause 
 and the effect. It will be observed that volcanoes occur 
 chiefly in lines along the crests of terrestrial ridges. There 
 is probably, therefore, a connection between the elevation of 
 these ridges and the extravasation of molten rock at the sur- 
 face. The formation of continents and mountain-chains has 
 already been referred to as probably consequent on the sub- 
 sidence and readjustment of the cool outer shell of the planet 
 upon the hotter and more rapidly contracting nucleus. 
 Every such movement, by relieving pressure on regions 
 below the axis of elevation, will tend to bring up molten 
 rock nearer the surface, and thus to promote the formation 
 and continued activity of volcanoes. 
 
 180 Cordier, for example, calculated that a contraction of only a single milli- 
 metre (about l-26th of an inch) would suffice to force out to the surface lav* 
 enough for 500 eruptions, allowing 1 cubic kilometre (al-out 1300 million cubic 
 yards) for each eruption. Prof. Prestwich invokes a alight contraction of the 
 crust as the initial cause of volcanic action. Brit. Assoc. 1881, Sects, p. 610.
 
 DYNAMICAL GEOLOGY 449 
 
 The fissure-eruptions, wherein lava has risen in innu- 
 merable rents in the ground across the whole breadth of a 
 country, and has been poured out at the surface over areas 
 of many thousand square miles, flooding them sometimes to 
 a depth of several thousand feet, undoubtedly prove that 
 molten rock existed at some depth over a large extent of 
 territory, and that, by some means still unknown, it was 
 forced out to the surface (ante, p. 434). In investigating 
 this subject, it would be important to discover whether any 
 evidence of great terrestrial crumpling or other movement 
 of the crust can be ascertained to have taken place about the 
 same geological period as a stupendous outpouring of lava 
 whether, for example, the great lava-fields of Idaho may 
 have had any connection with contemporaneous flexure of 
 the North American mountain-system, or whether the basalt- 
 plateaus of Antrim, Scotland, Faroe, and Iceland may pos- 
 sibly have been in their origin sympathetic with the post- 
 Eocene upheaval of the Alps or other Tertiary movements 
 in Europe. The most striking instance of an apparent con- 
 nection between such terrestrial disturbances and volcanic 
 phenomena is that supplied by the great semicircle of erup- 
 tions that sweeps from Central France by the Eifel, Hoch- 
 gau, and Bohemia into Hungary, and which has been re- 
 ferred to the dislocations consequent on the upheaval of 
 the Alps. 161 
 
 In the ordinary phase of volcanic action, marked by the 
 copious evolution of steam and the abundant production 
 of dust, slags, and cinders, from one or more local vents, 
 the main proximate cause of volcanic excitement is obvi- 
 
 161 Sueas, "Antlitz der Erde," i. p. 358, pi. iii. ; Julion. Annuaire du Club 
 Alpin, 1879-80, p. 446; Michel-Levy, Bull. Soc. Geol. France, xviii. (1890), 
 pp. 690, 841.
 
 450 TEXT-BOOK OF GEOLOGY 
 
 ously the expansive force exerted by vapors dissolved in 
 the molten magrna from which lavas proceed. Whether 
 and to what extent these vapors are parts of the aboriginal 
 constitution of the earth's interior, or are derived by de- 
 scent from the surface, is still an unsolved problem. The 
 abundant occlusion of hydrogen in meteorites, and the 
 capacity of many terrestrial substances, notably melted 
 metals, to absorb large quantities of gases and vapors 
 without chemical combination, and to emit them on cool- 
 ing with eruptive phenomena, not unlike those of volca- 
 noes, have led some observers to conclude that the gaseous 
 ejections at volcanic vents are portions of the original con- 
 stitution of the magma of the globe, and that to their escape 
 the activity of volcanic vents is due. Prof. Tschermak in 
 particular has advocated this opinion, and it is meeting with 
 increasing acceptance." 2 
 
 On the other hand, since so large a proportion of the 
 vapor of active volcanoes consists of steam, many geolo- 
 gists have urged that this steam has in great measure been 
 supplied by the descent of water from above ground. The 
 floor of the sea and the beds of rivers and lakes are all 
 leaky. Moreover, during volcanic eruptions and earth- 
 quakes, fissures no doubt open under the sea, as they do 
 on land, and allow the oceanic water to find access to the 
 interior. 1 " Again, rain sinking beneath the surface of 
 the land, percolates down cracks and joints, and infil- 
 
 169 He has suggested that if 190 cubic kilometres, of the constitution of cast- 
 iron, be supposed co solidify annually, and to give off 50 times its volume of 
 gases, it would suffice to maintain 20,000 active volcanoes. Sitz. Akad. Wissen. 
 Wien, lixv. (1877), p. 151. Reyer ("Beitrag zur Physik der Eruptionen," 
 Vienna, 1877) adyocates the same view. 
 
 us prof. Moseley mentions that during a submarine eruption off Hawaii in 
 1877 "a fissure opened on the coast of that island, from a few inches to three 
 feet broad, and in some places the water was seen pouring down the opening 
 into the abyss below." "Notes by a Naturalist on the 'Challenger,' " p. 503.
 
 DYNAMICAL GEOLOGY 451 
 
 trates through the very pores of the rocks. The presence 
 of nitrogen among the gaseous discharges of volcanoes may 
 indicate the decomposition of water containing atmospheric 
 gases. The abundant sublimations of chlorides are such as 
 might probably result from the decomposition of sea-water. 
 To some extent surface-waters doubtless do reach the vol- 
 canic magma. 
 
 Whatever may be its source, we cannot doubt that to 
 the enormous expansive force of superheated water (or of 
 its component gases, dissociated by the high temperature), 
 in the molten magma at the roots of volcanoes, the explo- 
 sions of a crater and the subsequent rise of a lava-column 
 are mainly due. The water or gas dissolved in the lava is 
 retained there by the enormous overlying pressure of the 
 lava-column, but when the molten material is brought up 
 to the surface the pressure is relieved and the water vapor- 
 izes and escapes. Where the relief is rapid the effect may 
 be to froth up the lava into a pasty mass of pumice, while 
 where it is sudden and extreme the escape of the water- 
 vapor may be by an explosive discharge. 
 
 It has been supposed that, somewhat like the reservoirs 
 in which hot water and steam accumulate under geysers, 
 reservoirs of molten rock receive a constant influx of water 
 from the surface, which cannot escape by other channels, 
 but is absorbed by the internal magma at an enormously 
 high temperature and under vast pressure. In the course 
 of time, the materials filling up the chimney are unable to 
 withstand the upward expansion of this imprisoned vapor 
 or water-substance, so that, after some premonitory rum- 
 blings, the whole opposing mass is blown out, and the 
 vapor escapes in the well-known masses of cloud. Mean- 
 while, the removal of the overlying column relieves the
 
 52 TEXT-BOOK OF GEOLOGY 
 
 pressure on the lava underneath, saturated with vapors or 
 superheated water. This lava therefore begins to rise in 
 the funnel until it forces its way through some weak part 
 of the cone, or pours over the top of the crater. After a 
 time, the vapor being expended, the energy of the volcano 
 ceases, and there comes a variable period of repose, until 
 a renewal of the same phenomena brings on another erup- 
 tion. By such successive paroxysms, the forms of the in- 
 ternal reservoirs and tunnels may be changed; new spaces 
 for the accumulation of superheated water being opened, 
 whence in time fresh volcanic vents issue, while the old 
 ones gradually die out. 
 
 An obvious objection to this explanation is the difficulty 
 of conceiving that water should descend at all against the 
 expansive force within. .But Daubree's experiments have 
 shown that, owing to capillarity, water may permeate rocks 
 against a high counter-pressure of steam on the further side, 
 and that so long as the water is supplied, whether by minute 
 fissures or through pores of the rocks, it may, under pres- 
 sure of its own superincumbent column, make its way into 
 highly heated regions. 164 Experience in deep mines, how- 
 ever, rather goes to show that the permeation of water 
 through the pores of rocks gets feebler as we descend. 
 
 Reference may be made here to a theory of volcanic 
 action in which the influence of terrestrial contraction as 
 the grand source of volcanic energy was insisted upon by the 
 late Mr. Mallet. 185 He maintained that all the present mani- 
 
 164 Daubree, "G-eologie Experimental, " p. 274 (criticised adversely by Fisher, 
 "Physics of Earth's Crust," 2d ed. p. 144). Tschermak, cited on page 450. 
 Reyer, "Beitrag zur Physik der Eruptionen," 1. 
 
 1M Phil. Trans. 1873. See also Daubree's experimental determination of the 
 quantity of heat evolved by the internal crushing of rocks. "Geologie Experi- 
 mentale," p. 448. For an adverse criticism of Mallet's view, see Fisher, op. cit. 
 chap. xxii.
 
 DYNAMICAL GEOLOGY 453 
 
 festations of hypogene action are due directly to the more 
 rapid contraction of the hotter internal mass of the earth 
 and the consequent crushing in of the outer cooler shell. 
 He pointed to the admitted difficulties in the way of con- 
 necting volcanic phenomena with the existence of internal 
 lakes of liquid matter, or of a central ocean of molten rock. 
 Observations made by him, on the effects of the earthquake 
 shocks accompanying the volcanic eruptions of Vesuvius 
 and of Etna, showed that the focus of disturbance could 
 not be more than a few miles deep; that, in relation to the 
 general mass of the globe, it was quite superficial, and could 
 not possibly have lain under a crust of 800 miles or upward 
 in thickness. The occurrence of volcanoes in lines, and 
 especially along some of the great mountain-chains of the 
 planet, was likewise dwelt upon by him as a fact not satis- 
 factorily explicable on any previous hypothesis of volcanic 
 energy. But he contended that all these difficulties disap- 
 pear when once the simple idea of cooling and contraction 
 is adequately realized. "The secular cooling of the globe," 
 he remarks, "is always going on, though in a very slowly 
 descending ratio. Contraction is therefore constantly pro- 
 viding a store of energy to be expended in crushing parts 
 of the crust, and through that providing for the volcanic 
 heat. But the crushing itself does not take place with uni- 
 formity; it necessarily acts per saltum after accumulated 
 pressure has reached the necessary amount at a given 
 point, where some of the pressed mass, unequally pressed 
 as we must assume it, gives way, and is succeeded perhaps 
 by a time of repose, or by the transfer of the crushing 
 action elsewhere to some weaker point. Hence, though 
 the magazine of volcanic energy is being constantly and 
 steadily replenished by secular cooling, the effects are
 
 454 TEXT-BOOK OF GEOLOGY 
 
 intermittent." He offered an experimental proof of the 
 sufficiency of the store of heat produced by this internal 
 crushing to cause all the phenomena of existing volca- 
 noes. 18 " The slight comparative depth of the volcanic foci, 
 their linear arrangement, and their occurrence along lines 
 of dominant elevation become, he contended, intelligible 
 under this hypothesis. For since the crushing in of the 
 crust may occur at any depth, the volcanic sources may 
 vary in depth indefinitely; and as the crushing will take 
 place chiefly along lines of weakness in the crust, it is 
 precisely in such lines that crumpled mountain-ridges and 
 volcanic funnels should appear. Moreover, by this expla- 
 nation its author sought to harmonize the discordant obser- 
 vations regarding variations in the rate of increase of tem- 
 perature downward within the earth, which have already 
 been cited and referred to unequal conductivity in the crust 
 (p. 97). He pointed out that in some parts of the crust the 
 crushing must be much greater than in other parts; and 
 since the heat "is directly proportionate to the local tan- 
 gential pressure which produces the crushing and the re- 
 sistance thereto," it may vary indefinitely up to actual 
 fusion. So long as the crushed rock remains out of reach 
 of a sufficient access of subterranean water, there would, of 
 course, be no disturbance. But if, through the weaker 
 parts, water enough should descend and be absorbed by 
 the intensely hot crushed mass, it would be raised to a very 
 high temperature, and, on sufficient diminution of pressure, 
 
 166 The elaborate and careful experimental researches of this observer will 
 reward attentive perusal. Mallet estimates from experiment the amount of heat 
 given out by the crushing of different rocks (syenite, granite, sandstone, slate, 
 limestone), and concludes that a cubic mile of the crust taken at the mean den- 
 sity would, if crushed into powder, give out heat enough to melt nearly 3$ cubic 
 miles of similar rock, assuming the melting-point to be 2000 Fahr.
 
 DYNAMICAL GEOLOGY 455 
 
 would flash into steam and produce the commotion of a vol- 
 canic eruption. 
 
 This ingenious theory requires the operation of sudden 
 and violent movements, or at least that the heat generated 
 by the crushing should be more than can be immediately 
 conducted away through the crust. Were the crushing slow 
 and equable, the heat developed by it might be so tran- 
 quilly dissipated that the temperature of the crust would 
 not be sensibly affected in the process, or not to such an 
 extent as to cause any appreciable molecular rearrange- 
 ment of the particles of the rocks. But an amount of 
 internal crushing insufficient to generate volcanic action 
 may have been accompanied by such an elevation of tem- 
 perature as to induce important changes in the structure 
 of rocks, such as are embraced under the term "meta- 
 morphic." 
 
 There is, indeed, strong evidence that, among the conse- 
 quences arising from the secular contraction of the globe, 
 masses of sedimentary strata, many thousands of feet in 
 thickness, have been crumpled and crushed, and that the 
 crumpling has often been accompanied by such an amount 
 of heat and evolution of chemical activity as to produce 
 an interchange and rearrangement of the elements of the 
 rocks this change sometimes advancing perhaps to the 
 point of actual fusion. (See postea, p. 506, and Book IV. 
 Part VIII.) There is reason to believe that some at least of 
 these periods of intense terrestrial disturbance have been 
 followed by periods of prolonged volcanic action in the 
 disturbed areas. Mr. Mallet's theory is thus, to some ex- 
 tent, supported by independent geological testimony. The 
 existence, however, of large reservoirs of fused rock, at a 
 comparatively small depth beneath the surface, may be con-
 
 456 TEXT-BOOK OF GEOLOGY 
 
 ceived as probable, apart from the effects of crushing. The 
 connection of volcanoes with lines of elevation, and conse- 
 quent weakness in the earth's crust, is what might have 
 been anticipated on the view that the nucleus, though 
 practically solid, is at such a temperature and pressure 
 that any diminution of the pressure, by corrugation of the 
 crust or otherwise, will cause the subjacent portion of 
 the nucleus to melt. Along lines of elevation the pressure 
 is relievecf, and consequent melting may take place. On 
 these lines of weakness and fracture, therefore, the condi- 
 tions for volcanic excitement may be conceived to be best 
 developed, whether arising from the explosive energy of 
 water dissolved in the magma or from water descending 
 to the intensely heated materials underneath the crust. 
 The periodicity of eruptions may thus depend upon the 
 length of time required for the storing up of sufficient 
 steam, and on the amount of resistance in the crust to be 
 overcome. In some volcanoes, the intervals of activity, 
 like those of many geysers, return with considerable regu- 
 larity. In other cases, the shattering of the crust, or the 
 upwelling of vast masses of lava, or the closing of subter- 
 ranean passages for the descending water, or other causes 
 may vary the conditions so much, from time to time, that 
 the eruptions follow each other at very unequal periods, 
 and with very discrepant energy. Each great outburst ex- 
 hausts for a while the vigor of the volcano, and an interval 
 is needed for the renewed accumulation of vapor. 
 
 But besides the mechanism by which volcanic eruptions 
 are produced, further problems are presented by the varie- 
 ties of materials ejected, by the differences which these ex- 
 hibit at neighboring vents, even sometimes in successive 
 eruptions from the same vent, by the alternation or recur-
 
 DYNAMICAL GEOLOGY 457 
 
 rence of lavas from basic to acid in the continuance of a sin- 
 gle volcanic period, and by the repetition of a similar cycle 
 in successive periods. Observations are yet needed from a 
 larger number of ancient volcanic districts and in greater de- 
 tail, before these problems can be satisfactorily discussed 
 and solved. It is obvious that in such a great series of 
 eruptions as that of Central France, where over a compara- 
 tively limited area an alternation of basic and acid lavas has 
 been many times repeated, the subterranean magma must 
 have undergone a succession of changes in composition. 
 Perhaps a definite cycle of such alternations may be made 
 out. The sequence from basic to acid protrusions, observ- 
 able among the British Palaeozoic volcanic rocks, is sugges- 
 tive of a separation of the more basic constituents of the 
 magma with consequent increasing acidity of the residue. 
 The earliest lavas mark the more basic condition of the 
 magma, while the latest felsite and quartz-porphyry intru- 
 sions show its impoverishment in bases at the close of a vol- 
 canic period. During the interval before the next period 
 the magma had in some way been renewed, for when erup- 
 tions began anew they were once more basic. But by the 
 close of the volcanic activity the magma had again lost a 
 large proportion of its basic constituents and had become 
 decidedly acid. 
 
 Reference has already (p. 114) been made to the specula- 
 tion of Durocher as to the existence within the crust of an 
 upper siliceous layer with a mean of 71 per cent of silica, 
 and a lower basic layer with about 51 per cent of silica. 
 Bunsen also came to the conclusion that volcanic rocks are 
 mixtures of two original normal magmac the normal tra- 
 chytic (with a mean of 76*67 silica), and the normal pyroxenic 
 (with a mean of 47-48 silica). The varying proportions 
 
 GEOLOGY Yol. XXIX 20
 
 458 TEXT-BOOK OF GEOLOGY 
 
 in which these two original magmas have been combined 
 are, in Bunsen's view, the cause of the differences of vol- 
 canic rocks. We may conceive these two layers to be 
 superposed upon each other, according to relative densities, 
 and the composition of the last material erupted at the sur- 
 face to depend upon the depth from which it has been de- 
 rived. 1 " The earliest explosions may be supposed to have 
 taken place usually from the upper lighter and more sili- 
 ceous layer, and the lavas ejected would consequently be in 
 general acid, while later eruptions, reaching down to deeper 
 and heavier zones of the magma, brought up such basic 
 lavas as basalt. Certainly the general similarity of the vol- 
 canic rocks all over the globe would appear to prove that 
 there must be considerable uniformity of composition in the 
 zones of intensely hot material from which volcanic rocks 
 are derived. 1 " 
 
 Many difficulties, however, remain yet to be explained 
 before our knowledge of volcanic action can be regarded as 
 more than rudimentary. In Book IV. Part VII. a descrip- 
 tion is given of the part volcanic rocks have played in build- 
 ing up what we see of the earth's crust, and the student will 
 there find other illustrations of facts and deductions which 
 have been given in the previous pages. 
 
 141 See R. Bunsen, Fogg. Ann. Ixiii. (1851), p. 204; Sarlorius von Walters- 
 hausen, "Sicilien und Island," p. 416; Reyer, "Beitrag zur Physik der Erup- 
 tionen," iii. Scrope had long before suggested a classification of volcanic rock* 
 into Trachyte, Graystone, and Basalt, Jouru. Science, xxi. 
 
 168 In the memoir by Captain Button, cited in a previous note, the hypothe- 
 sis is maintained that the order of appearance of the lavas is determined by their 
 relative density and fusibility, the most basic and heaviest, though most easily 
 fused, requiring the highest temperature to diminish their density to such an 
 extent as to permit them to be erupted.
 
 DYNAMICAL GEOLOGY 469 
 
 Section ii. Earthquakes 169 
 
 By the more delicate methods of observation which have 
 been invented in recent years, it has been ascertained that 
 the ground beneath our feet is apparently everywhere sub- 
 ject to continual slight tremors and to minute pulsations of 
 longer duration. The old expression "terra firma" is not 
 only not strictly true, but in the light of modern research 
 seems singularly inappropriate. Rapid changes of tempera- 
 ture and atmospheric pressure, the fall of a shower of rain, 
 the patter of birds' feet, and still more the tread of larger 
 animals, produce tremors of the ground which, though ex- 
 ceedingly minute, are capable of being made clearly audible 
 by means of the microphone and visible by means of the 
 galvanometer. Some tremors of varying intensity and ap- 
 parently of irregular occurrence, may be due to minute 
 movements or displacements in the crust of the earth. Less 
 
 169 On the phenomena of earthquakes consult Mallet, Brit. Assoc. 1847, part 
 ii. p. 30; 1850, p. 1; 1851, p. 272; 1852, p. 1; 1858, p. 1; 1861, p. 201; "The 
 Great Neapolitan Earthquake of 1857," 2 vols., 1862; D. Milne, Edin. New 
 Phil. Journ. xxxi. xxxvt ; A. Perrey, Mem. Couronn. Bruxelles, xviii. (1844), 
 Comptes rendus, lii. p. 146; Otto Volger, "Untersuchungen fiber die Phano- 
 mene der Erdbeben in der Schweiz," Gotha, 1857-58; Z. Deutsch. Geol. Ges. 
 xiii. p. 667; R. Falb, "Grundziige einer Theorie der Erdbeben und Vulkanens- 
 ausbriiche," Graz, 1871; "Gedanken und Studien iiber den Vulkanismus, etc.," 
 1874; Pfaff, "Allgemeine Geologie als exacte Wissenschaft," Leipzig, 1873, 
 p. 224. Records of observed earthquakes will be found in the memoirs of 
 Mallet and Perrey; also in papers by Fuchs in Neues Jahrb. 1865-1871, and 
 in Tschermak's Mineralog. Mittheilungen,-1873 and subsequent years. See also 
 Schmidt, "Studien iiber Erdbeben," 2d edit. 1879; "Studien iiber Vulkane und 
 Erdbeben," 1881 ; Dieffenbach, Neues Jahrb. 1872, p. 155; M. S. di Rossi, "La 
 Meteorologia Endogena," 2 vols. 1879 and 1882; M. Gatta, "L'ltalia, su vul- 
 cani e terremoti," 1882; J. Milne, "Earthquakes and other Earth -movements, " 
 1886, and his beautifully illustrated volume on the Japan Earthquake of Octo- 
 ber, 1891. G. Mercalli, in his "Vulcani e Fenomeni Vulcanici in Italia" (1883), 
 gives an account of the Italian earthquakes from 1450 B.C. to A.D. 1881; he 
 separately describes the great Ischian earthquake of 1883; "L'Isola d'Ischia," 
 Milan, 1884. Much interesting information will be found in the Bulletino del 
 Vulcanismo Italiano, which began to be published in 1874; also in the Transac- 
 tions of the Seismological Society of Japan a society instituted in the year 1880 
 for the invescigation of earthquake phenomena, especially in Japan, where they 
 are of frequent occurrence. Other papers are quoted in the following pages.
 
 460 TEXT-BOOK OF GEOLOGY 
 
 easily traceable are the slow pulsations of the crust, which 
 in many cases are periodic, and may depend on such causes 
 as the diurnal oscillation of the thermal or barometric con- 
 ditions of the atmosphere, the rise and fall of the tides, etc. 
 So numerous and well marked are these tremors and pulsa- 
 tions, that the delicate observations which were set on foot 
 to determine the lunar disturbance of gravity had to be 
 abandoned, for it was found that the minute movements 
 sought for were wholly eclipsed by these earth tremors. 1 " 
 
 The term Earthquake denotes any natural subterranean 
 concussion, varying from such slight tremors as to be hardly 
 perceptible up to severe shocks, by which houses are lev- 
 elled, rocks dislocated, landslips precipitated, and many 
 human lives destroyed. The phenomena are analogous to 
 the shock communicated to the ground by explosions of 
 mines or powder-works. They may be most intelligibly 
 considered as wave-like undulations propagated through 
 the solid crust of the earth. In Mr. Mallet's language, an 
 earthquake may be denned as "the transit of a wave of 
 elastic compression, or of a succession of these, in parallel 
 or intersecting lines through the solid substance and sur- 
 face of the disturbed country." Mr. Milne has since re- 
 marked that the disturbance may also be due to the transit 
 of waves of elastic distortion. The passage of the wave of 
 shock constitutes the real earthquake. 
 
 Besides the wave of shock transmitted through the solid 
 
 110 A. d'Abbadie, "Etudes sur la verticale," 1872. Plantamour, Comptes 
 rend. June, 1878, February, 1881; Archives Sciences Phys. Nat. Geneva, ii. p. 
 641; v. p. 97; vii. p. 601; viii. p. 551; x. p. 616; xii. (1884), p. 388. G. H. 
 Darwin, Brit. Assoc. 1882, p. 95. In this paper Prof. Darwin discusses the 
 amount of disturbance of the vertical near the coasts of continents, caused by 
 the rise and fall of the tide. J. Milne, Trans. Seismological Soc. Japan, vi. 
 (1883), p. 1; Geol. Mag. 1882, p. 482; Nature, xxvi. p. 125. The numerous 
 observations made by Rossi in Italy are summarized by G. Mercalli in his work 
 cited above, p. 332.
 
 DYNAMICAL GEOLOGY 461 
 
 crust, waves are also propagated through the air, and, where 
 the site of the impulse is not too remote, through the ocean. 
 Earthquakes originating under the sea are numerous and 
 specially destructive in their effects. They illustrate well 
 the three kinds of waves associated with the progress of an 
 earthquake. These are, 1st, The true earth-wave through 
 the earth's crust; 2d, A wave propagated through the air, to 
 which the characteristic sounds of rolling wagons, distant 
 thunder, bellowing oxen, etc., are due; 3d, Two sea- waves, 
 one of which travels on the back of the earth- wave and 
 reaches the land with it, producing no sensible effect on 
 shore; the other an enormous low swell, caused by the first 
 sudden blow of the earth-wave, but travelling at a much 
 slower rate, and reaching land often several hours after the 
 earthquake has arrived. 
 
 Amplitude of earth-movements. The popular conception 
 of the extent to which the ground moves to and fro or up 
 and down during an earthquake is a great exaggeration of 
 the truth. As the result of very careful measurement with 
 delicate instruments, there appears to be reason to believe 
 that the horizontal motion at the time of a small earthquake 
 is usually only the fraction of a millimetre, and seldom ex- 
 ceeds three or four millimetres. When the motion rises to 
 five or six millimetres brick and stone chimneys are shat- 
 tered. Yet even with such an intensity of shock a person 
 walking in an open place might be quite unconscious of any 
 perceptible movement of the ground. The vertical motion 
 also appears to be exceedingly small. 1 " 
 
 Velocity* Experiments have been made to determine the 
 
 111 Milne "Earthquakes," pp. 75, 76. An ingenious model in wire has been 
 made by Prof. Sekiya to illustrate the highly complex path pursued by a parti- 
 cle on the surface of the ground during an earthquake at Tokio, Japan, on 15th 
 January, 1887.
 
 462 TEXT-BOOK OF GEOLOGY 
 
 velocity of the earth-wave, and its variation with the nature 
 of the material through which it is propagated. Mr. Mallet 
 found that the shock produced by the explosion of gunpow- 
 der travelled at the rate per second of 825 feet in sand ; 1088 
 feet in schists, slates, and quartzites; 1306 feet in friable 
 granite; and 1664 feet in solid granite. General Abbot, by 
 observing the effects of the explosion of dynamite and gun- 
 powder, found the velocity of transmission of the shock to 
 vary from 1240 to 8800 feet per second, and to be greatest 
 where the shock is most violent. 172 Observations of the 
 time at which an earthquake has successively visited the 
 different places on its track have shown similar variations in 
 the rate of movement. Thus in the Calabrian earthquake 
 of 1857, the wave of shock varied from 658 to 989 feet per 
 second, the mean rate being 789 feet. The earthquake at 
 Viege in 1855 was estimated to have travelled northward 
 toward Strasburg at the rate of 2861 feet per second, and 
 southward toward Turin at a rate of 1398 feet, or less than 
 half the northern speed. The earthquake of 7th October, 
 1874, in northern Italy, travelled at rates varying from 273 
 to 874 feet per second. That of 12th March, 1873, showed 
 a velocity per second of 2734 feet between Eagusa and 
 Venice; 4101 feet from Spoleto to Venice; 601 feet from 
 Perugia to Orvieto; 1640 feet from Perugia to Ancona; and 
 1640 (or 2188) feet from Perugia to Kome. The rate of the 
 central European earthquake of 1872 was estimated to have 
 been 2433 feet, that of Herzogenrath, June 24, 1877,- 1555 
 feet, that of an earthquake at Travancore, in Southern Hin- 
 
 112 Amer. Journ. Sci. xv. (1878). Prof. J. Milne, experimenting in Japan, 
 has likewise ascertained that a close relation exists between the initial violence 
 of the shock and the velocity of propagation, and that there is a progressive 
 diminution in speed as the wave of shock travels outward from the centre of 
 disturbance. "Earthquakes," p. 65.
 
 DYNAMICAL GEOLOGY 463 
 
 dustan, 656 feet in a second." 8 The most accurate measure- 
 ments and computations of the velocity of earthquake move- 
 ments are probably those made by Prof. J. Milne and his 
 associates in Japan. The rates of movement during the 
 Tokio earthquake of 25th October, 1881, are estimated to 
 have ranged between 4000 and 9000 feet per second. As the 
 result of prolonged observation, Prof. Milne concludes that 
 "different earthquakes, although they may travel across the 
 same country, have very variable velocities, varying be- 
 tween several hundreds and several thousands of feet per 
 second; that the same earthquake travels more quickly 
 across districts near to its origin than it does across dis- 
 tricts which are far removed; and that the greater the inten- 
 sity of the shock, the greater is the velocity." "* 
 
 Duration* The number of shocks in an earthquake 
 varies indefinitely, as well as the length of the intervals 
 between them. Sometimes the whole earthquake only lasts 
 a few seconds; thus the city of Caracas, with its fine 
 churches and 10,000 of its inhabitants, was destroyed in 
 about half a minute; Lisbon was overthrown in five min- 
 utes. But a succession of shocks of varying intensity may 
 continue for days, weeks, or months. The Calabrian earth- 
 quake, which began in February, 1783, was continued by 
 repeated shocks for nearly four years until the end of 1786. 
 
 Modifying influence of geological structure. In its pas- 
 
 " K. von Seebach, "Das Mitteldeutsche Erdbeben von 6 Marz, 1872," 
 Leipzig, 1873. Hofer, Sitzb. Akad. Wien, Dec. 1876; A. von Lasaulx, "Das 
 Erdbeben von Herzogenrath, 22d Oct., 1873," Bonu, 1874. "Das Erdbebea 
 von Herzogenrath, 24 Juni, 1877," Bonn, 1878. G. C. Laube, on Earthquake 
 of 31st January, 1883, at Trautenau, Jahrb. Geol. Reichs. 1883, p. 331. H. 
 Credner, on the Earthquakes of the Erzgebirge and Vogtiand from 1878 to 1884, 
 Zeitsch. fur Naturwiss. vol. Ivii. (1884). P. Wahner, on Agram earthquake of 
 9 Nov. 1880, Sitz. Akad. Wien, Izxxviii. (1883), p. 15. Di Rossi, "Meteorologia 
 Endogena," i. p. 306; P. Serpieri, Institute Lombardo, 1873. 
 
 " 4 "Earthquakes," p. 94.
 
 464 TEXT-BOOK OF GEOLOGY 
 
 sage through the solid terrestrial crust from the focus of 
 origin, the earth-wave must be liable to continual deflec- 
 tions and delays, from the varying geological structure of 
 the rocks. To this cause, no doubt, must be in large meas- 
 ure ascribed the marked differences in the rate of propaga- 
 tion of the same earthquake in different directions. The 
 wave of disturbance, as it passes from one kind of rock to 
 another, and encounters materials of very different elas- 
 ticity, or as it meets with joints, dislocations, and curva- 
 
 Fig 71. Plan of Port Royal, Jamaica, showing the effects of the Earthquake 
 of 1692 (B.). 
 
 P C, Portions of the town built on limestone and left standing after the earthquake; 
 a a, L, the boundary of the town prior to the earthquake; N N, Ground gained by 
 the drifting of sand up to the end of last century; I L, H, Additions from the same 
 cause during the first quarter of the present century. 
 
 tures in the same rock, must be liable to manifold changes 
 alike in rate and in direction of movement. Even at the 
 surface, one effect of differences of material may be seen 
 in the apparently capricious demolition of certain quarters 
 of a city, while others are left comparatively scathless. 
 In such cases, it has often been found that buildings 
 erected on loose inelastic foundations, such as sand and 
 clay, are more liable to destruction than those placed upon 
 solid rock. In illustration of this statement the accom- 
 panying plan (Fig. 71) of Port Royal, Jamaica, was given
 
 DYNAMICAL GEOLOGY 465 
 
 by De la Beche 175 to show that the portions of the town 
 which did not disappear during the earthquake of 1692 
 were built upon solid white limestone, while the parts 
 built on sand were shaken to pieces. 178 
 
 It has been observed that an earthquake shock will 
 pass under a limited area without disturbing it, while the 
 region all around has been affected, as if there were some 
 superficial stratum protected from the earth-wave. Hum- 
 boldt cited a case where miners were driven up from below 
 ground by earthquake shocks not perceptible at the surface, 
 and on the other hand, an instance where they experienced 
 no sensation of an earthquake which shook the surface with 
 considerable violence. 177 Such facts bring impressively be- 
 fore the mind the extent to which the course of the earth- 
 wave must be modified by geological structure. In some 
 instances, the shock extends outward from a common 
 centre, so that a series of concentric circles may be drawn 
 round the focus, each of which will denote a certain ap- 
 proximately uniform intensity of shock ("coseismic lines" 
 of Mallet), this intensity, of course, diminishing with dis- 
 tance from the focus. The Calabrian earthquake of 1857 
 and that of Central Europe in 1872 may be taken in illus- 
 tration of this central type. In other cases, however, the 
 earthquake travels chiefly along a certain band or zone 
 (particularly along the flanks of a mountain-chain) without 
 advancing far from it laterally. This type of linear earth- 
 quake is exemplified by the frequent shocks which traverse 
 
 1 "Geological Observer," p. 426. 
 
 "' The opposite effect has been observed on the island of Ischia, the houses 
 built on loose subsoil generally having suffered much less than the others. There 
 appears, indeed, to be a considerable conflict of testimony on this subject. See 
 Milne, "Earthquakes," p. 130. 
 
 m "Cosmos," Art. Earthquakes.
 
 466 TEXT-BOOK OF GEOLOGY 
 
 Chile, Peru, and Ecuador, between the line of the Andes 
 and the Pacific coast. 178 
 
 Extent of country affected. The area shaken by an earth- 
 quake varies with the intensity of the shock, from a mere 
 local tract where a slight tremor has been experienced, up 
 to such catastrophes as that of Lisbon in 1755, which, be- 
 sides convulsing the Portuguese coasts, extended into the 
 north of Africa on the one hand and to Scandinavia on 
 the other, and was even felt as far as the east of North 
 America. Humboldt computed that the area shaken by 
 this great earthquake was four times greater than that of 
 the whole of Europe. The South American earthquakes 
 are remarkable for the great distances to which their effects 
 extend in a linear direction. Thus the strip of country in 
 Peru and Ecuador severely shaken by the earthquake of 
 1868 had a length of 2000 miles. 
 
 Depth of source* According to Mallet's observations, 
 over the centre of origin the shock is felt as a vertical 
 up-and-down movement (Seismic vertical); while, receding 
 from this centre in any direction, it is felt as an undulatory 
 movement, and comes up more and more obliquely. The 
 angle of emergence, as he termed it, was obtained by him by 
 taking the mean of observations of the rents and displace- 
 ments of walls and buildings. In Fig. 72, for example, the 
 wall there represented has been rent by an earthquake 
 which emerged to the surface in the path marked by the 
 arrow. 
 
 By observations of this nature, Mallet estimated the ap- 
 
 118 For a list of Peruvian earthquakes from A.D. 1570 to 1875, see Geograph. 
 Mag. iv. (1877), p. 206. The earthquake of 9 May, 1877, at Iquique, and its 
 ocean -wave are described by E. Geinitz, Nova Act! Ac. Cses. Leopold. Car. xl. 
 (1878), pp. 383-444.
 
 DYNAMICAL GEOLOGY 
 
 467 
 
 proximate depth of origin of an earthquake. Let Fig. 73, 
 for example, represent a portion of the earth's crust in 
 which at a an earthquake arises. The wave of shock will 
 travel outward in successive spherical shells. At the point 
 
 Pig. 78. Wall shattered by an Earthquake, of which the "path of emergence" 
 has been in the direction shown by the arrow. (After Mallet.) 
 
 e it will be felt as a vertical movement and loose objects, 
 such as paving-stones, may be jerked up into the air, and 
 descend bottom uppermost on their previous sites. At c?, 
 however, the wave will emerge at a lower angle, and will 
 give rise to an undulation of the ground, and the oscillation 
 of objects projecting above the surface. In rent buildings, 
 
 Fig. 73. Mallet's mode of estimation of depth of source of Earthquake movements. 
 
 the fissures will be on the whole perpendicular to the path 
 of emergence. By a series of observations made at different 
 points, as at g and/, a number of angles are obtained, and 
 the point where the various lines cut the vertical (a) will
 
 468 TEXT-BOOK OF GEOLOGY 
 
 mark the area of origin of the shock. By this means, 
 Mallet computed that the depth at which the impulse of 
 the Oalabrian earthquake pf 1857 was given was about five 
 miles. As the general result of his inquiries, he concluded 
 that, on the whole, the origin of earthquakes must be sought 
 in comparatively superficial parts of the crust, probably 
 never exceeding a depth of 30 geographical miles. Follow- 
 ing another method of calculation, Von Seebach computed 
 that the earthquake which affected Central Europe in 1872 
 originated at a depth of 9'6 geographical miles; that of 
 Belluno in the same year was estimated by Hofer to have 
 had its source rather more than 4 miles deep; while that 
 of Herzogenrath in 1873 was placed by Von Lasaulx at a 
 depth of about 14i miles, and that of 1877 in the same 
 region at about 14 miles. 179 
 
 Geological Effects* These are dependent not only on 
 the strength of the concussion but on the structure of the 
 ground, and on the site of the disturbance, whether under- 
 neath land or sea. They include changes superinduced on 
 the surface of the land, on terrestrial and oceanic waters, 
 and on the relative levels of land and sea. 
 
 1. Effects upon the soil and general sur- 
 face of a country. The earth-wave or wave of shock 
 underneath a country may traverse a wide region and 
 affect it violently at the time, without leaving permanent 
 traces of its passage. Blocks of rock, however, already 
 disengaged from their parent masses on declivities, may 
 be rolled down into the valleys. Landslips are produced, 
 
 119 See papers by Hofer and A. von Lasaulx, cited on p. 463. For an account 
 of the various methods employed in estimating the depth of origin of earth- 
 quakes, see Milne's "Earthquakes," chapters x. and xi. Consult also the 
 Trans. Seismolog. Soc. Japan.
 
 DYNAMICAL GEOLOGY 4o9 
 
 which may give rise to considerable subsequent changes 
 of drainage. In some instances, the surfaces of solid rocks 
 are shattered as if by gunpowder, as was particularly 
 noticed to have taken place among the Primary rocks in 
 the Concepcion earthquake of 1835. 18 It has often been 
 observed also that the soil is rent by fissures which vary 
 in size 1 from mere cracks, like those due to desiccation, up 
 to chasms a mile or more in length and 200 feet or more 
 in depth. Permanent modifications of the landscape may 
 thus be produced. Trees are thrown down, and buried, 
 wholly or in part, in the rents. These superficial effects 
 may, indeed, be soon effaced by the levelling power of the 
 atmosphere. Where, however, the chasms are wide and 
 deep enough to intercept rivulets, or to serve as channels 
 for heavy rain -torrents, they are sometimes further exca- 
 vated, so as to become gradually enlarged into ravines and 
 valleys, as has happened in the case of rents caused by 
 the earthquakes of 1811-12 in the Mississippi Valley. In 
 the earthquake which shook the South Island of New Zea- 
 land in 1848, a fissure was formed, averaging 18 inches in 
 width and traceable for a distance of 60 miles parallel 
 to the axis of the adjacent mountain-chain. The subse- 
 quent earthquake of 1855, in the same region, gave rise 
 to a fracture which could be traced along the base of a 
 line of cliff for a distance of about 90 miles. Dr. Oldham 
 has described a remarkable series of fissurings which ran 
 parallel with the river of Calhar, Eastern British India, 
 varying with it to every point of the compass and trace- 
 able for 100 miles. 181 The great Japanese earthquake of 
 
 80 Darwin, "Journal of Researches," 1845, p. 303. 
 
 181 Q. J. Geol. Soc. xxviii. p. 257. For a catalogue of Indian Earthquakes 
 down to the end of 1869, see T. Oldham, Mem. Geol. Surv. India, xix. part 2.
 
 470 
 
 TEXT-BOOK OF GEOLOGY 
 
 28th October, 1891, gave rise to some remarkable fractures 
 of the ground, in one of which one side was placed per- 
 manently at a different level from the other (Fig. 74). 
 
 Eemarkable circular cavities have been noticed in Cala- 
 bria and elsewhere, formed in the ground during the passage 
 of the earth-wave. In many cases, these holes serve as fun- 
 nels of escape for an abunbant discharge of water, so that 
 
 Fig. 74. Fissure or fault caused by the earthquake of 28th October, 1891, 
 in the Neo Valley, Japan. 
 
 when the disturbance ceases they appear as pools. They 
 are believed to be caused by the sudden collapse of subter- 
 ranean water-channels and the consequent forcible ejection 
 of the water to the surface. Besides water, discharges of 
 various gases and vapors, sometimes combustible, have been 
 noted at the fissures formed during earthquakes. 
 
 2. Effects upon ter res tri al waters. 189 Springs 
 
 Kluge, Neues Jahrb. 1861, p. 777.
 
 DYNAMICAL GEOLOGY 471 
 
 are temporarily affected by earthquake movements, becom- 
 ing greater or smaller in volume, sometimes muddy or dis- 
 colored, and sometimes increasing in temperature. Brooks 
 and rivers have been observed to flow with an interrupted 
 course, increasing or diminishing in size, stopping in their 
 flow so as to leave their channels dry, and then rolling for- 
 ward with increased rapidity. Lakes are still more sensi- 
 tive. Their waters occasionally rise and fall for several 
 hours, even at a distance of many hundred miles from the 
 centre of disturbance. Thus, on the day of the great Lisbon 
 earthquake, many of the lakes of central and northwestern 
 Europe were so affected as to maintain a succession of 
 waves rising to a height of 2 or 3 feet above their usual 
 level. Cases, however, have been observed where, owing 
 to excessive subterranean movement, lakes have been 
 emptied of their contents and their beds have been left 
 permanently dry. On the other hand, areas of dry ground 
 have been depressed, and have become the sites of new 
 lakes. 
 
 Some of the most important changes in the fresh water 
 of a region, however, are produced by the fall of masses of 
 rock and earth, which, by damming up a stream, may so 
 arrest its water as to form a lake. If the barrier be of suffi- 
 cient strength, the lake will be permanent; though, from the 
 usually loose, incoherent character of its materials, the dam 
 thrown across the pathway of a stream runs a great risk of 
 being undermined by the percolating water. A sudden giv- 
 ing way of the barrier allows the confined water to rush 
 with great violence down the valley, and to produce per- 
 haps tenfold more havoc there than may have been caused 
 by the original earthquake. When a landslip is of sufficient 
 dimensions to divert a stream from its previous course, the
 
 472 TEXT-BOOK OF GEOLOGY 
 
 new channel thus taken may become permanent, and a val- 
 ley may be cut out or widened. 
 
 3. Effects upon the sea. The great sea-wave 
 propagated outward from the centre of a sub-oceanic earth- 
 quake and reaching the land after the earth-wave has ar- 
 rived there, gives rise to much destruction along the mari- 
 time parts of the disturbed region. When it approaches a 
 low shore, the littoral waters retreat seaward, sucked up, as 
 it were, by the advancing wall of water, which, reaching a 
 height of sometimes 60 feet or more, rushes over the bare 
 beach and sweeps inland, carrying with it everything which 
 it can dislodge and bear away. Loose blocks of rock are 
 thus lifted to a considerable distance from their former posi- 
 tion, and left at a higher level. Deposits of sand, gravel, 
 and other superficial accumulations are torn up and swept 
 away, while the surface of the country, as far as the limit 
 reached by the wave, is strewn with debris. If the district 
 has been already shattered by the passage of the earth- wave, 
 the advent of the great sea-wave augments and completes 
 the devastation. The havoc caused by the Lisbon earth- 
 quake of 1755, and by that of Peru and Ecuador in 1868, 
 was much aggravated by the co-operation of the oceanic 
 wave. Where the wave breaks on land rising out of deep 
 water little damage may be done. 
 
 4. Permanent changes of level. It has been 
 observed, after the passage of an earthquake, that the 
 level of the disturbed country has sometimes been changed. 
 Thus after the terrible earthquake of 19th November, 1822, 
 the coast of Chile, for a long distance, was found to have 
 risen from 3 to 4 feet, so that along shore, littoral shells 
 were exposed still adhering to the rocks, amid multitudes 
 of dead fish. The same coast-line has been further upraised
 
 DYNAMICAL GEOLOGY 473 
 
 by subsequent earthquake shocks. On the other hand, 
 many instances have been observed where the effect of an 
 earthquake has been to depress permanently the disturbed 
 ground. For example, by the Bengal earthquake of 1762, 
 an area of 60 square miles on the coast near Chittagong sud- 
 denly went down beneath the sea, leaving only the tops of 
 the higher eminences above water. The succession of earth- 
 quakes which in the years 1811 and 1812 devastated the 
 basin of the Mississippi, gave rise to widespread depressions 
 of the ground, over some of which, above alluded to, the 
 river spread so as to form new lakes, with the tops of the 
 trees still standing above the surface of the water. 
 
 Distribution of Earthquakes, 183 While no large space of 
 the earth's surface seems to be free from at least some de- 
 gree of earthquake-movement, there are regions more es- 
 pecially liable to the visitation. As a rule, earthquakes are 
 most frequent in volcanic districts, the explosions of a vol- 
 cano being generally preceded or accompanied by tremors of 
 greater or less intensity. In the Old "World, a great belt of 
 earthquake disturbance stretches in an east and west direc- 1 
 tion, along that tract of remarkable depressions and eleva- 
 
 188 For European earthquakes an alphabetical catalogue has been compiled 
 by Prof. O'Reilly, Trans. Roy. Irish Academy, xxviii. (1886), p. 489. Cata- 
 logue of British earthquakes, op. cit. xxviii. (1884), p. 285. C. Davidson, Geol. 
 Mag. 1891, p. 450. Quart. Journ. Geol. Soc. xlvii. (1891), p. 618. Detailed 
 observations of the effects of some recent European earthquakes will be found 
 in the following Memoirs. The Andalusian earthquake of 25th Dec. 1884, T. 
 Taramelli and G. Mercalli, Real. Accad. Lincei, 1885-86, p. 116, Hubert, Compt. 
 Rend. 1885, Fouque", ibid. 20th April, 1885, and the large quarto volume of re- 
 ports by the mission specially sent to study the phenomena of this earthquake, 
 Memoires Acad. Sci. 1889; the Ligurian earthquake of 23d Feb. 1887, T. Tara- 
 melli and G. Mercalli, Ann. Ufficio Centrale Meteorolog. Geodinam. part iv. vol. 
 viii. (1888), Real. Accad. Lincei, iv. (1888); the Agram earthquake of 9th Nov. 
 1880, "Grundzuge der Abyssodynamik, " etc., by S. Pilar, Agram, 1881; the 
 middle German earthquake of 6th March, 1872, "Das Mitteldeutsche Erdbeben 
 von 6 Marz, 1872," by K. von Seebach, Leipzig, 1873. See also the papers 
 cited on pp. 459-465.
 
 474 TEXT-BOOK OF GEOLOGY 
 
 tions lying between the Alps and the mountains of northern 
 Africa, and spreading eastward so as to inclose the basins of 
 the Mediterranean, Black Sea, Caspian, and Sea of Aral, 
 and to rise into the great inountain-ridges of Central Asia. 
 In this zone lie numerous volcanic vents, both active and 
 extinct or dormant, from the Azores on the west to the basal- 
 tic plateaus of India on the east. The Pacific Ocean, sur- 
 rounded with a vast ring of volcanic vents, has its borders 
 likewise subject to frequent earthquake shocks. Some of 
 the most terrible earthquakes within human experience have 
 been those which have affected the western seaboard of 
 South America. 1 " It is worthy of notice that the coasts 
 of the Pacific Ocean more specially liable to convulsions of 
 this nature plunge steeply down into deep water with slopes 
 of one in twenty to one in thirty, while shore-lines such as 
 those of Australia, Scandinavia, and the east of South Amer- 
 ica, where the slope is no more than from one in fifty to one 
 in two hundred and fifty, are hardly ever affected by earth- 
 quakes. It should also be remarked that while earthquakes 
 are apt to occur along the flanks of mountain-chains and to 
 travel along these lines of elevation, they seldom cross a 
 large mountain-chain. In some regions the site of disturb- 
 ance is not on land but under the sea. This has been clearly 
 established for Japan. 186 
 
 Origin of Earthquakes. Though the phenomena of an 
 earthquake become intelligible as the results of the trans- 
 mission of waves of shock arising from a centre where some 
 
 184 The Charleston Earthquake of 31st August, 1886, has been fully dis- 
 cussed by Captain Button, Ninth Ann. Report U. S. Geol. Survey, 1887-88, p. 
 209. The earthquakes of Central America are discussed by F. de Montesstis 
 de Ballore in a Memoir rewarded by the Acad. Sci. Nat. Saone et Loire, and 
 published at Dijon, 1888. 
 
 185 Milne, "Earthquakes," p. 227.
 
 DYNAMICAL GEOLOGY 475 
 
 sudden and violent impulse has been given within the ter- 
 restrial crust, the origin of this sudden blow can only be 
 conjectured. Various conceivable causes may, at different 
 times and under different conditions, communicate a shock 
 to the subterranean regions. Such are the sudden flashing 
 into steam of water in the spheroidal state, the sudden con- 
 densation of steam, the explosions of a volcanic orifice, the 
 falling in of the roof of a subterranean cavity, or the sudden 
 snap of deep-seated rocks subjected to prolonged and in- 
 tense strain. 
 
 In volcanic regions, the frequent earthquakes which pre- 
 cede or accompany eruptions are doubtless traceable to ex- 
 plosions of elastic vapors, and notably of steam. As earth- 
 ~quakes originate also in districts remote from any active 
 volcano, and, so far as observation shows, at comparatively 
 shallow depths, these cannot be connected with ordinary 
 volcanic action, though it is possible that by movements of 
 molten or highly-heated matter within the crust and its in- 
 vasion of the upper layer, to which meteoric water in con- 
 siderable quantities descends, sudden and extensive genera- 
 tion of steam may occasionally take place. 18 ' In minor 
 cases, where the tremor is comparatively slight and local, 
 we may conceive that the collapse of the roof or sides of 
 some of the numerous tunnels and caverns dissolved out of 
 underground rocks by permeating water may suffice to pro- 
 duce the observed shocks. 187 The copious discharge of ma- 
 terials from a volcanic vent may produce a cavity within the 
 
 88 Pfaff, "Allgemeiue Geologic als exacte Wissenschaft," p. 230. 
 181 In the Visp Thai, Canton Wallis, for example, where there are some 
 twenty springs carrying up gypsum in solution (one of them to the extent of 
 200 cubic metres annually), continued rumblings and sharp shocks are from 
 time to time experienced. In July and August, 1855, these movements lasted 
 upward of a month, and gave rise to the ftssuring of buildings and the precipita- 
 tion of landslips. In the honeycombed limestone tract of the Karst, also, earth- 
 quakes of varied intensity are of constant occurrence.
 
 i76 TEXT-BOOK OF GEOLOGY 
 
 earth, the crushing in of which will give rise to earthquakes. 
 There appears reason to believe that the most convulsive 
 earthquakes originate under the sea, as in the cases of the 
 great Lisbon earthquake and those of Peru, Chile, and 
 Japan. For these it is as yet difficult to imagine an ade- 
 quate cause. Prof. Milne believes that they may be partly 
 due to disturbances of the nature of volcanic explosions, 
 because they originate beneath the sea, and the vibrations 
 have the peculiar rapid inward motion characteristic of the 
 discharge of an explosive like dynamite. 168 
 
 An obvious source of disturbance within the earth is the 
 rupture of rocfcs within the crust under the intense strain 
 produced by subsidence upon the more rapidly contracting 
 inner hot nucleus. This cause may conceivably affect 
 mountainous areas; but we do not know how it would 
 affect the sea-floor. In mountainous districts, many dif- 
 ferent degrees of shock, from mere tremors up to important 
 earthquakes, have been observed, and these are not im- 
 probably due to sudden more or less extensive fractures of 
 rocks still under great strain. 189 Hoernes, from a study of 
 European earthquake phenomena, concludes that though 
 some minor earth-tremors may be due to the collapse of 
 underground caverns, and others of local character to vol- 
 canic action, the greatest and most important earthquakes 
 are the immediate consequences of the formation of moun- 
 tains, and he connects the lines followed by earthquakes 
 with the structural lines of mountain-axes. 190 
 
 From what was stated at the beginning of the present sec- 
 tion, it is evident that where the earth's crust in any region 
 
 iss "Earthquakes," p. 281. 
 
 189 See postea, p. 630. Suess, "Entstekung der Alpen." Vienna, 1875. 
 
 190 "Erdbeben Studien," Jahrb. Geol. Reichs. xxviii. (1878), p. 448.
 
 DYNAMICAL GEOLOGY 477 
 
 is in a critical condition of equilibrium, some connection 
 may be expected to be traceable between the frequency of 
 earthquakes and the earth's position with regard to the moon 
 and sun, on the one hand, and changes of atmospheric condi- 
 tions, on the other. A comparison of the dates of recorded 
 earthquakes seems to bear out the following conclusions: 
 1st. An earthquake maximum occurs about the time of new 
 moon; 2d. Another maximum appears two days after the 
 first quarter; 3d. A diminution of activitj r occurs about the 
 time of full moon; 4th. The lowest earthquake minimum 
 is on the day of the last quarter. 191 There is likewise ob- 
 servable a seasonal maximum and minimum, earthquakes 
 over most of the northern hemisphere occurring most fre- 
 quently in winter, and least frequently in summer. 1 " Out of 
 656 earthquakes chronicled in France up to the year 1846, 
 three-fifths took place in the winter, and two-fifths in the 
 summer months. In Switzerland they have been observed 
 to be about three times more numerous in winter than in 
 summer. The same fact is remarked in the history even of 
 the slight earthquakes in Britain. A daily maximum ap- 
 pears to occur about 2.30 A.M., and a minimum about three- 
 quarters of an hour after noon. No connection has yet been 
 satisfactorily established between the occurrence of earth- 
 quakes and sun-spots. The greater frequency of earthquakes 
 in winter might be expected to indicate a relation between 
 their occurrence and atmospheric pressure, and possibly 
 earthquakes are more frequent with a low than with a high 
 barometer. "* 
 
 191 J. F. J. Schmidt, "Studien fiber Erdbeben," 2d ed. (1879), p. 18. 
 
 m Ibid. p. 20. See the works of Perrey cited on p. 459. 
 
 m Schmidt, op. cit. p. 23. F. Groger, Neues Jahrb. 1878, p. 928. There 
 doea not appear to be any marked connection between the state of the barometer 
 and the occurrence of earthquakes in Japan J. Milne, "Earthquakes," p. 268.
 
 478 TEXT-BOOK OF GEOLOGY 
 
 Section iii. Secular Upheaval and Depression 
 
 Besides scarcely perceptible tremors and more or less vio- 
 lent movements due to earthquake-shocks, the crust of the 
 earth is generally believed to undergo in many places oscil- 
 lations of an extremely quiet and uniform character, some- 
 times in an upward, sometimes in a downward direction. 
 So tranquil may these changes be, as to produce from day 
 to day no appreciable alteration in the aspect of the ground 
 affected, so that only after the lapse of several generations, 
 and by means of careful measurements, can they really be 
 proved. Indeed, in the interior of a country nothing but a 
 series of accurate levellings from some unmoved datum-line 
 might detect the change of level, unless the effects of the 
 terrestrial disturbance showed themselves in altering the 
 drainage. Only along the sea-coast is a ready measure 
 afforded of any such movement. 
 
 It is customary in popular language to speak of the sea 
 rising or falling relatively to the land. We cannot conceive 
 of any possible augmentation of the oceanic waters, nor of 
 any diminution, save what may be due to the extremely slow 
 processes of abstraction by the hydration of minerals and 
 absorption into the earth's interior. Any changes, there- 
 fore, in the relative levels of sea and land must be due to 
 some readjustment in the form either of the solid globe or of 
 its watery envelope or of both. Play fair argued at the be- 
 ginning of this century that no subsidence of the sea-level 
 could be local, but must extend over the globe. 1 ' 4 But it is 
 now recognized that what is called the sea-level cannot pos- 
 
 m "Illustrations of the Hmtonian Theory," 1802. The same conclusion 
 was announced by L. von Buch, "Reise durch Norwegen und Lapland," 1810.
 
 DYNAMICAL GEOLOGY 479 
 
 sess the uniformity formerly attributed to it; that on the 
 contrary it must be liable to local distortion from the attrac- 
 tive influence of the land. Not only so, but the level of the 
 surface of large inland sheets of water must be affected by 
 the surrounding high lands. 
 
 Mr. E. S. Woodward, whose recent memoir on this sub- 
 ject has been cited (p. 68), has calculated that in a lake 140 
 miles broad and 1000 feet deep in the middle, the difference 
 of level of the water-surface at the centre and at the margin 
 may amount to between three and four feet." 6 As already 
 stated he has further computed that the effect of the conti- 
 nents of Europe and Asia at the centre in disturbing the 
 sea-level must amount to about 2900 feet, if we suppose that 
 there is no deficiency of density underneath the continent, 
 and to only about 10 feet if we suppose that the very exist- 
 ence of the continent implies such a deficiency. "' 
 
 Various suggestions have been made regarding possible 
 causes of alteration of the sea-level. (1) A shifting of the 
 present distribution of density within the nucleus of the 
 planet would affect the position and level of the oceans (ante, 
 p. 89). (2) As permanent snow and ice represent so much 
 removed from the general body of water on the globe, any 
 large increase or diminution in the extent and thickness of 
 the polar ice-caps must cause a corresponding variation in 
 the sea-level (ante, p. 44). (3) A change in the earth's cen- 
 tre of gravity, such as might result from the accumulation 
 of large masses of snow and ice as an ice-cap at one of the 
 poles, has been already referred to (p. 43) as tending to raise 
 the level of the ocean in the hemisphere so affected, and to 
 
 196 Bull. U. S. Geol. Surv. No. 48 (1888), p. 59. 
 
 196 Op. cit. p. 85. See Stokes, Trans. Camb. Phil. Soc. viii. (1849), p. 672; 
 Sci. Proc. Roy. Dublin Soc. v. (1887), p. 652.
 
 480 TEXT- BOOK OF GEOLOGY 
 
 diminish it in a corresponding measure elsewhere. The re- 
 turn of the ice into the state of water would produce an op- 
 posite effect. The attractive influence of the ice-sheets of 
 the Glacial Period upon the sea-level over the northern 
 hemisphere has been discussed by various mathematicians, 
 especially by Croll, Pratt, Heath, and Lord Kelvin. Con- 
 siderable differences appear in their results, according to 
 the conditions which they postulate, but they agree that a 
 decided elevation of the sea-level must be attributed to the 
 accumulation of thick masses of snow and ice. The rise of 
 the sea-level along the border of an ice-cap of 38 angular 
 radius and 10,000 feet thick in the centre is estimated at 
 from 139 to 573 feet. 1 * 7 (4) A still further conceivable 
 source of geographical disturbance is t to be found in the 
 fact that, as a consequence of the diminution of centrifugal 
 force owing to the retardation of the earth's rotation caused 
 by the tidal wave, the sea-level must have a tendency to 
 subside at the equator and rise at the poles." 9 A larger 
 amount of land, however, need not ultimately be laid bare 
 at the equator, for the change of level resulting from thig 
 cause would be so slow that, as Dr. Croll has pointed out, 
 
 i" See Croll, "Climate and Time," chaps, xxiii., xxiv. Geol. Mag, 1874. 
 Pratt, "Figure of the Earth," D. D. Heath, Phil. Mag. xxxi. (1866), pp. 201, 
 323, xxxii. (1866), p. 34. Thomson (Lord Kelvin), op. cit. xxxi. p. 305. A. 
 Penck, Jahrb. Geograph. Gesel. Munich, vii. De Lapparent, Bull. Soc. Geol. 
 France, xiv. 1886, p. 368, Revue Generate des Sciences, May, 1890. R. S. 
 Woodward, Bull. U. S. Geol. Survey, No. 48. Von Drygalski, "Bewegungen 
 der Kontinente zur Eiszeit," Berlin, 1889. Prof. Suess believes that the limits 
 of the dry land depend upon certain large indeterminate oscillations of the stati- 
 cal figure of the oceanic envelope; that not only are "raised beaches" to be thus 
 explained, but that there are absolutely no vertical movements of the crust save 
 such as may form part of the plication arising from secular contraction ; and 
 that the doctrine of secular fluctuations in the level of the continents is merely 
 a remnant of the old "Erhebungstheorie," destined to speedy extinction. "Ant- 
 lit/ der Erde," Leipzig, 1883. Pfaff defends the general opinion against these 
 views in Zeitsch. Deutsch. Geol. Ges. 1884. 
 
 198 Croll, Phil. Mag. 1868, p. 382. Thomson, Trans. Geol. Soc. Glasgow, 
 iii. p. 223.
 
 DYNAMICAL GEOLOGY 481 
 
 the general degradation of the surface of the land might 
 keep pace with it and diminish the terrestrial area as much 
 as the retreat of the ocean tended to increase it. The same 
 writer has further suggested that the waste of the equatorial 
 land, and the deposition of the detritus in higher latitudes, 
 may still further counteract the effects of retardation and 
 the consequent change of ocean-level. (5) Some geologists 
 have supposed that where the earth's crust is loaded with 
 thick deposits of sediment or massive ice-sheets it will tend 
 to sink, while on the other hand denudation by unloading it 
 promotes upheaval. 
 
 The balance of evidence at present available seems ad- 
 verse to any theory which would account for ancient and 
 modern changes in the relative level of sea and land by 
 variations in the figure of the oceanic envelope, save to a 
 limited extent by the attraction caused by extensive masses 
 of upraised land, and possibly in northern and southern lati- 
 tudes by the attractive influence of large accumulations of 
 snow and ice. Such changes are rather to be regarded as 
 due to movements of the solid crust. The proofs of up- 
 heaval and subsidence, though sometimes obtainable from 
 wide areas, are marked by a want of uniformity and a local 
 and variable character, indicative of an action local and vari- 
 able in its operations, such as the folding of the terrestrial 
 crust, and not regular and widespread, such as might be 
 predicated of any alteration of sea-level. While admitting 
 therefore that, to a certain extent, oscillations of the rela- 
 tive level of sea and land may have arisen from some of the 
 causes above enumerated, we may hold that, on the whole, 
 it is the land which rises and sinks rather than the sea. 1 " 
 
 1M For the arguments against the view above adopted and in favor of the 
 doctrine that the increase of the land above sea- level is due to the retirement of 
 GEOLOGY Vol. XXIX 21
 
 482 TEXT-BOOK OF GEOLOGY 
 
 1. Upheaval. Various maritime tracts of land have 
 been ascertained to have undergone in recent times, or to be 
 still undergoing, what appears to be a gradual elevation 
 above the sea. On the coast of Siberia, for 600 miles to 
 the east of the river Lena, round the islands of Spitzbergen 
 and Novaja Zemlja, along the shores of the Scandinavian 
 peninsula with the exception of a small area at its southern 
 apex, and along a maritime strip of western South America, 
 it has been proved that the sea stands now at a lower level 
 with regard to the land than it formerly did. In searching 
 for proofs of such movements the student must be on his 
 guard against being deceived by any apparent retreat of the 
 sea, which may be due merely to the deposit of gravel, sand, 
 or mud along the shore, and the consequent gain of land. 
 Local accumulations of gravel, or "storm beaches," are 
 often thrown up by storms, even above the level of ordi- 
 nary high-tide mark. In estuaries, also, considerable tracts 
 of low ground are gradually raised above the tide-level by 
 the slow deposit of mud. The following proofs of actual rise 
 of the land are chiefly relied on by geologists. 800 
 
 Evidence from dead organisms. Bocks cov- 
 ered with barnacles or other littoral adherent animals, or 
 pierced by lithodomous shells, afford presumptive proof 
 of the presence of the sea. A single stone with these 
 creatures on its surface would not be satisfactory evidence, 
 for it might have been cast up by a storm; but a line of 
 large bowlders, which had evidently not been moved since 
 the cirripeds and mollusks lived upon them, and still more 
 a solid cliff with these marks of littoral or sub-littoral life 
 
 the sea, see H. Trautschold, Bulletin Societe Imp. des Naturaliates de Moscou, 
 xlii. (1869), part i. p. 1 ; 1883, No. 2, p. 341 ; Bull. Soc. Geol. France (3), viii. 
 (1879), p. 134; but more especially Sueas, in his great work the "Antlitz der 
 Erde." 
 
 900 See ''Earthquakes and Volcanoes" (A. G.), Chambers's Miscellany of 
 Tracts.
 
 DYNAMICAL GEOLOGY 483 
 
 upon its base, now raised above high-water mark, would be 
 sufficient to demonstrate a change of level. The amount 
 of this change might be pretty accurately determined by 
 measuring the vertical distance between the upper edge of 
 the barnacle zone upon the upraised rock, and the limit 
 of the same zone on the present shore. By this kind of 
 evidence, the recent uprise of the coast of Scandinavia has 
 been proved. The shell-borings on the pillars of the temple 
 of Jupiter Serapis in the Bay of Naples prove first a depres- 
 sion and then an elevation of the ground to the extent of 
 more than twenty feet. 201 Raised coral-reefs, formed by 
 living species of corals, are a conspicuous feature of the 
 geology of the West Indian Region. The terraces of Bar- 
 badoes are particularly striking. In Cuba, a raised coral- 
 reef occurs at a height of 1000 or 1100 feet above the sea. 108 
 In Peru, modern coral-limestone has been found 2900 or 
 3000 feet above sea-level.* 08 Again, in the Solomon Islands, 
 evidence of recent uprise is furnished by coral-reefs lying 
 at a height of 1100 feet," 04 and similar evidence occurs among 
 the New Hebrides at 1500 feet. 
 
 The elevation of the sea-bottom can in like manner be 
 proved by dead organisms fixed in their position of growth 
 beneath high-water mark. Thus dead specimens of Mya 
 truncata occur on some parts of the coast of the Firth 
 of Forth in considerable numbers, still placed with their 
 siphuncular end uppermost in the stiff clay in which they 
 burrowed. The position of these shells is about high-water 
 mark, but as their existing descendants do not live above 
 low-water mark, we may infer that the coast has been raised 
 by at least the difference between high- and low-water 
 mark, or eighteen feet." * Dead shells of the large Pholas 
 dactylus occur in a similar position near high-water mark 
 on the Ayrshire coast. Even below low-water, examples 
 have been noted, as in the interesting case observed by Sara 
 on the Drobaksbank in the Christiania Fjord, where dead 
 stems of Oculina prolifera (L.) occur at depths of only ten 
 or fifteen fathoms. This coral is really a deep-sea form, 
 living on the western and northern coasts of Norway, at 
 
 m Babbage, Edin. Phil. Journ. xi. (1824), p. 91. J. D. Forbes, Edin. Journ. 
 
 Sci. i. (1829), p. 260. Lyell, "Principles," ii. p. 164. 
 
 ios A. Agassiz, Amer. Acad. xi. (1882), p. 119. 
 
 803 A. Agassiz, Bull. Mus. Comp. Zool. vol. iii. 
 
 904 H. B. Guppy, Nature, 3d Januarv, 1884. 
 
 805 Hugh Miller's "Edinburgh and it's Neighborhood," p. 110.
 
 484 TEXT-BOOK OF GEOLOGY 
 
 depths of one hundred and fifty to three hundred fathoms 
 in cold water. It must have been killed as the elevation 
 of the area brought it up into upper and warmer layers of 
 water." 06 It has even been said that the pines on the edges 
 of the Norwegian snow-fields are dying in consequence of 
 the secular elevation of the land bringing them up into 
 colder zones of the atmosphere. 
 
 Any stratum of rock containing marine organisms which 
 have manifestly lived and died where their remains now lie, 
 may be held to prove a change of level between sea and 
 land. In this way it can be shown that most of the solid 
 
 Fig. 76. View of a line of ancient sea-cliff pierced at the base with sea-worn 
 caves and fronted by a Raised Beach. 
 
 land now visible to us has once been under the sea. High 
 on the flanks of mountain-chains (as in the Alps and Hima- 
 layas), undoubted marine shells occur in the solid rocks. 
 
 Sea-worn Caves. A line of sea- worn caves, now 
 standing at a distance above high-water mark beyond the 
 reach of the sea, affords evidence of recent change of level. 
 In the accompanying diagram (Fig. 75) examples of such 
 caves are seen at the base of the cliff, once the sea-margin, 
 now separated from the tide by a platform of meadow- 
 land. 
 
 Raised Beaches furnish one of the most striking 
 
 206 Quoted by Vom Rath in a paper entitled "Aus Norwegen," Neues Jahrb. 
 1869, p. 422. For another example, see G-wyn Jeffreys, Brit. Assoc. 1867, 
 p. 431.
 
 DYNAMICAL GEOLOGY 
 
 485 
 
 proofs of change of level. A beach or space between tide- 
 marks, where the sea is constantly grinding down sand and 
 gravel, mingling with them the remains of shells and other 
 organisms, sometimes piling the deposits up, sometimes 
 sweeping them away out into opener water, forms a famil- 
 iar terrace or platform on coast- lines skirting tidal seas. 
 When this margin of littoral deposits has been placed 
 above the reach of the waves, the flat terrace thus elevated 
 is known as a "raised beach" (Figs. 75, 76, 77, 78). The 
 former high- water mark then lies inland, and while its sea- 
 worn caves are in time hung with ferns and mosses, the 
 beach across which the tides once flowed furnishes a plat- 
 form on which meadows, fields, gardens, roads, houses, 
 villages, and towns spring up, while a new beach is made 
 below the margin of the uplifted one. A series of raised 
 beaches may occur at various heights above the sea. Each 
 terrace marks a former lower level 
 of the land with regard to the sea, 
 and probably a lengthened stay of 
 the land at that level, while the in- 
 tervals between them represent the 
 vertical amount of each variation 
 in the relative levels of sea and 
 land, and show that the interval 
 between the changes was too brief 
 for the formation of terraces. A 
 
 Succession of raised beaches, rising Fig.76. -Section of a Raised Beach 
 
 above the present sea-level, may ^S^ ^S&^^ 
 therefore be taken as pointing to a y^SStSSSfff^ 
 former intermittent upheaval of the cay of abundant iand-sheiis. 
 country, interrupted by long pauses, Fistra11 Bay ' Cornwa11 (B ' ) ' 
 during which the general level did not materially change, 
 unless in regions where there is reason to believe that the 
 surface of the sea has undergone a change of level from 
 the accumulation or melting of large masses of snow and 
 ice (ante, p. 43). 
 
 Raised beaches abound in the higher latitudes of the 
 northern and southern hemispheres, and this distribution 
 has been claimed as a strong argument in favor of the view 
 that they are due to a fall of the local level of the sea- 
 surface from the disappearance or diminution of former 
 ice-caps. That some at least of the raised beaches in these 
 regions may be due to this cause may be granted. The 
 gradual rise of level of the beaches when traced up the 
 fjords, which has been repeatedly asserted for some dis-
 
 486 TEXT-BOOK OF GEOLOGY 
 
 tricts, would be the natural effect of the greater mass of 
 ice in the interior. In the exploration of the lake regions 
 of North America numerous instances have been described 
 of a slope upward of the former water-levels toward the 
 main, ice-fields. A remarkable example is furnished by 
 the terraces of the vanished glacial sheet of water callea 
 Lake Agassiz which once filled the basin of the Bed Eiver 
 of the North. Mr. Warren Upham has found that these 
 ancient lines of water-level gradually rise from south to 
 north and from west to east, in the direction of the former 
 ice-fields, the amount of slope ranging from zero to 1-3 feet 
 per mile." 01 Mr. Gr. K. Gilbert has noticed a rise of as much 
 as 5 feet in a mile among the old terraces of Lake Ontario. 1 "" 
 Raised beaches occur round many parts of the coast-line 
 of Britain. De la Beche gives the subjoined view (Fig. 77) 
 
 Fig. 77. View of Raised Beach, Nelly's Cave, Cornwall (B.). 
 
 of a Cornish locality where the existing beach is flanked by 
 a cliff of slate, &, continually cut away by the sea so that 
 the overlying raised beach, a, c, will ere long disappear. 
 The coast line on both sides of Scotland is likewise fringed 
 with raised beaches, sometimes four or five occurring above 
 each other at heights of 25, 40, 50, 60, 75, and 100 feet 
 above the present high- water mark. 809 Others are found on 
 both sides of the English Channel. 810 The sides of the 
 
 *" Bui]. U. S. Geol. Surv. No. 39 (1887), pp. 18, 20. 
 
 208 Science, i. p. 222. 
 
 409 For accounts of some British raised beaches, see De la Beche, Report 
 on Geology of Devon and Cornwall," chap. xiii. ; C. Maclaren, "Geology of 
 Fife and the Lothians," 1839; R. Chambers, "Ancient Sea Margins"; Prest- 
 wich, Q. J. Geol. Soc. xxviii. p. 38 ; xxx. p. 29 ; R. Russell and T. Y. Holmes, 
 Brit. Assoc. 1876, Sects, p. 95; Ussher, Geol. Mag. 1879, p. 166. 
 
 " On the raised beach of Sangatte, near Calais, see Prestwich, Bull. Soc.
 
 DYNAMICAL GEOLOGY 4.87 
 
 mountainous fjords of Northern Norway, up to more than 
 600 feet above sea-level, are marked with conspicuous lines 
 of terraces (Fig. 78). These terraces are partly ordinary 
 beach deposits, partly notches cut out of rock, probably 
 with the aid of drifting coast-ice. 811 Proofs of recent eleva- 
 tion of the shores of the Mediterranean are furnished by 
 raised beaches at various heights above the present water- 
 level. In Corsica such terraces occur at heights of from 
 15 to 20 metres. " 
 
 On the west coast of South America, lines of raised ter- 
 race containing recent shells have been traced by Darwin 
 
 Pig. 78. View of Terraces, Alten Fjord, Norway. 
 
 as proofs of a great upheaval of that part of the globe in 
 modern geological time. The terraces are not quite hori- 
 zontal but rise toward the south. On the frontier of Bolivia, 
 they occur at from 65 to 80 feet above the existing sea-level, 
 
 Geol. France (3), viii. (1880), p. 547 ; on those of Finisterre, C. Barroia, Ann. 
 Soc. Geol. Nord. ix. (1882). 
 
 511 See R. Chambers, "Tracings of the North of Europe" (1850), p. 172 et seq. 
 Bravais, "Voyages de la Commission Scientifique du Nord," etc., translated in 
 Q. J. Geol. Soc. i. p. 534. Kjerulf, Z. Deutsch. Geol. Ges. xxii. p. 1 ; "Die 
 Geologie des siid. und mittl. Norwegen," 1880, p. 7; Geol. Mag. viii. p. 74. 
 S. A. Sexe, "On Rise of Land in Scandinavia," Index Scholarum of University, 
 Christiania, 1872. H. Mohn. Nyt. Mag. Nat. xxii. p. 1. Dakyns, Geol. Mag. 
 1877, p. 72. K. Pettersen, Arch. Math. Nat. Christiania, 1878, p. 182, x. (1885); 
 Geol. Mag. 1879, p. 298; Tromso Museums Aarshefter, III. 1880. Sitz. Akad. 
 Wien. xcviii. (1889). Lehmann, "Ueber-ehemalige Strandliiiier, " etc., Halle, 
 1879; Zeitsch. ges. Naturwiss. 1880, p. 280. A. G. Hogbom, Geol. For. For- 
 handl. Stockholm, ix. 1887, p. 19. C. Sandier, Petermann's Mittheil. xxxvi. 
 >p. 209, 235. 
 ull. Soc. Geol. France (3), iv. p. 86.
 
 438 TEXT-BOOK OF GEOLOGY 
 
 but nearer the higher mass of the Chilean Andes they are 
 found at 1000, and near Valparaiso at 1300 feet. That some 
 of these ancient sea-margins belong to the human period 
 was shown by Mr. Darwin's discovery of shells with bones 
 of birds, ears of maize, plaited reeds and cotton thread, in 
 one of the terraces opposite Callao at a height of 85 feet. 313 
 Eaised beaches occur in New Zealand, and indicate a greater 
 change of level in the southern than in the northern part of 
 the country.* 14 It should be observed that this increased 
 rise of the terraces poleward occurs both in the northern 
 and southern hemispheres, and is one of the chief facts 
 insisted upon by those who would explain the terraces by 
 displacements of the sea Tather than of the land. 
 
 *Human Records and Traditions. In countries 
 which have been long settled by a human population, it is 
 sometimes possible to prove, or at least to render probable, 
 the fact of recent change of level by reference to tradition, 
 to local names, and to works of human construction. Piers 
 and harbors, if now found to stand above the upper limit 
 of high-water, furnish indeed indisputable evidence of a 
 rise of land or fall of sea-level since tneir erection. Numer- 
 ous proofs of a recent change of level in the coast of the 
 Arctic Ocean from Spitzbergen eastward have been ob- 
 served. The Finnish coast is reported to have risen 6 feet 
 4 inches in 127 years.* 1 * At Spitzbergen itself, besides its 
 raised beaches, bearing witness to previous elevations, small 
 islands which existed two hundred years ago are now joined 
 to larger portions of land. At Novaja Zemlja, where six 
 raised beaches were found by Nordenskiold, the highest 
 being 600 feet above sea-level, 918 there seems to have been 
 a rising of the sea-bottom to the extent of 100 feet or more 
 since the Dutch expedition of 1594. On the north coast 
 of Siberia the island of Diomida, observed in 1760 by 
 Chalaourof to the east of Cape Sviatoj, was found by 
 Wrangel sixty years afterward to have been united to the 
 mainland. 117 From marks made on the coast in the middle 
 
 213 "Geological Observations," chap. x. See Geol. Mag. 1877, p. 28. 
 
 214 Haast's "Geology of Canterbury," 1879, p. 366. 
 
 215 Nature, xxvi. p. 231. 816 Ibid. xv. p. 123. 
 
 217 Grad, Bull. Soc. Geol. France, 3d ser. ii. p. 348. Traces of oscillations 
 of level within historic times have been observed in the Netherlands, Flanders 
 and Upper Italy. Bull. Soc. Geol. France, 2d ser. xix. p. 556; 3d ser. ii. pp. 
 46, 222 ; Ann. Soc. Geol. Nord. v. p. 218. For alleged changes of level in the 
 estuary of the Garonne, see Artigues, Act. Soc. Linu. Bordeaux, xxxi. (1876), 
 p. 287, and Delfortrie, ib. xxxii. p. 79.
 
 DYNAMICAL GEOLOGY 489 
 
 of last century it appears that the north of Sweden has risen 
 about 7 feet in the last 154 years, but that the movement 
 has lessened southward until in Scania it has been replaced 
 by one in a downward direction (see p. 493). 
 
 2. Subsidence. It is more difficult to trace a downward 
 movement of land, for the evidence of each successive sea- 
 margin is carried down and washed away or covered up. 
 The student will take care to guard himself against being 
 misled by mere proofs of the advance of the sea on the land. 
 In the great majority of cases, where such an advance is 
 taking place, it is due not to subsidence of the land, but to 
 erosion of the shores. It is, indeed, the converse of the 
 deposition above mentioned (p. 482) as liable to be mistaken 
 for proof of upheaval. The results of mere erosion by the 
 sea, however, and those of actual depression of the level of 
 the land, cannot always be distinguished without some care. 
 The encroachment of the sea upon the land may involve the 
 disappearance of successive fields, roads, houses, villages, 
 and even whole parishes, without any actual change of level 
 of the land. Certain causes, however, referred to below, 
 may come into operation, producing an actual submergence 
 of land without any real subsidence of the land itself. The 
 following kinds of evidence are usually cited to prove sub- 
 sidence. 
 
 Submerged Forests. As the land is brought 
 within reach of the waves, and its characteristic surface- 
 features are effaced, the submerged area may retain little or 
 no evidence of its having been a land-surface. It will be 
 covered, as a rule, with sea-worn sand or silt. Hence, no 
 doubt, the reason why, among the marine strata which form 
 so much of the stratified portion of the earth's crust, and 
 contain so many proofs of depression, actual traces of land- 
 surfaces are comparatively rare. It is only under very 
 favorable circumstances, as, for instance, where the area 
 is sheltered from prevalent winds and waves, and where,
 
 490 TEXT-BOOK OF GEOLOGY 
 
 therefore, the surface of the land can sink tranquilly under 
 the sea, that fragments of that surface may be preserved 
 under overlying marine accumulations. It is iii such places 
 that "submerged forests" occur (Fig. 79). These are stumps 
 of trees still in their positions of growth in their native soil, 
 often associated with beds of peat, full of tree-roots, hazel- 
 nuts, branches, leaves, and other indications of a terrestrial 
 surface. There is sometimes, however, considerable risk of 
 deception in regard to the nature and value of such evidence 
 of depression. Where, for instance, shingle or sand is 
 banked up against a shore or river-mouth, considerable 
 spaces may be inclosed and filled with fresh-water, the bot- 
 tom of which may be some way below high- water mark. In 
 such lagoons terrestrial vegetation and debris from the land 
 
 t^^^^J^s^yi^MM^ 
 
 Fig. 79. Section of Submerged Forest (B.). 
 
 A platform of older rocks (e e) has been covered with soil (d d) on which trees (a a a) 
 have established themselves. In course of time, after some of the trees had 
 fallen (6), and a quantity of vegetable soil had accumulated, inclosing here and 
 there the bones of deer and oxen (c c), the area sank, and the sea overflowing it 
 threw down upon its surface saudy or muddy deposits (//). 
 
 may be deposited. Eventually, if the protecting barriers 
 should be cut away the tides may flow over the layers of 
 terrestrial peat, giving a false appearance of subsidence. 
 Again, owing to removal of subterranean sandy deposits by 
 springs, overlying peat-beds may sink below sea-level. 
 
 De la Bee he has described, 'round the shores of Devon, 
 Cornwall, and western Somerset, a vegetable accumulation, 
 consisting of plants of the same species as those which now 
 grow freely on the adjoining land, and occurring as a bed at 
 the mouths of valleys, at the bottoms of sheltered bays, and 
 in front of and under low tracts of land, of which the sea- 
 ward side dips beneath the present level of the sea."* Over 
 
 * 18 "Geolopy of Devon and Cornwall," Mem. Gteol. Survey. For further 
 accounts oi -Britisii submerged forests, see Q. J. Geol. iSoc. ixii. p. 1. ; xxxiv.
 
 DYNAMICAL GEOLOGY 491 
 
 this submerged land-surface, sand and silt containing estua- 
 rine shells have generally been deposited, whence we may 
 infer that, in the submergence, the valleys first became 
 estuaries, and then sea-bays. If now, in the course of ages, 
 a series of such submerged forests should be formed one 
 over the other, and if, finally, they should, by upheaval of 
 the sea-bottom, be once more laid dry, so as to Ibe capable 
 of examination by boring, well-sinking, or otherwise, they 
 would prove a former long-continued depression, with inter- 
 vals or rest. These intervals would be marked by the 
 buried forests, and the progress of depression by the 'strata 
 of sand and mud lying between them. In short, the evi- 
 dence would be strictly on a parallel with that furnished by 
 a succession of raised beaches as to a former protracted in- 
 termittent elevation. 
 
 Along the coasts of Holland and the north of France, 
 submerged beds of peat have been regarded as proofs of 
 submergence during historic times. The amount of change 
 varies considerably in different places, and here and there 
 can hardly be appreciated. The sinking during the 350 
 years preceding 1850 is estimated to have amounted in the 
 polders of Grroningen to a mean annual rate of 8 millime- 
 tres. 218 In the north of France numerous examples of sub- 
 merged forests have been observed. In 1846, in digging the 
 harbor of St. Servan, near St. Malo, a Gaulish cemetery 
 containing ornaments and coins, and resting on a still more 
 ancient prehistoric cemetery, was met with at a level of 6 
 metres below the level of high tide, so that the submergence 
 must have been at least to that extent." 
 
 Coral-islands.- Evidence of widespread depression, 
 over the area of the Pacific and Indian Oceans, has been 
 adduced from the structure and growth of coral-reefs and 
 islands. Mr. Darwin, many years ago, stated his belief 
 that, as the reef-building corals do not live at deaths of 
 more than 20 to 30 fathoms, and yet their reefs rise out 
 of deep water, the sites on which they have formed these 
 structures must have subsided, the rate of subsidence being 
 
 p. 447. Geol. Mag. vi. p. 76; vii. -p. 64; iii. 2d ser. p. 491; vi. pp. 80, 251. 
 Mr. D. Pidgeon has argued in favor of the submerged forest of Torbay having 
 been formed without subsidence of the land. Quart. Journ. Geol. Soc. xli. 
 (1885), p. 9. See also W. Shone, op. cit. xlviii (1892), p. 96. 
 
 219 Lorie, Archives du Musee Teyler, ser. ii. vol. iii. Part 5 (1890), p. 421. 
 
 ' m Lorie, ibid. p. 438, and papers cited postea, p. 494. But see Suess, 
 "Antlitz der Erde," ii. p. 547.
 
 492 TEXT-BOOK OF GEOLOGY 
 
 so slow that the upward growth of the reefs has on the 
 whole kept pace with it." 1 More recent researches, how- 
 ever, show that the phenomena of coral-reefs are in 
 some cases, at least, capable of satisfactory explanation 
 without subsidence, and hence that their existence can 
 no longer be adduced by itself as a demonstration of the 
 subsidence of large areas of the ocean." 2 4 The formation of 
 coral-reefs is described in Book III. Part II. Section iii., 
 and Mr. Darwin's theory is there more fully explained. 
 
 Distribution of plants and animals. Since 
 the appearance of Edward Forbes's essay upon the connec- 
 tion between the distribution of the existing fauna and flora 
 of the British Isles, and the geological changes which have 
 affected that area," 3 much attention has been given to the 
 evidence furnished by the geographical distribution of plants 
 and animals as to geological revolutions. In some cases, 
 the former existence of land now submerged has been in- 
 ferred with considerable confidence from the distribution of 
 living organisms, although, as Mr. Wallace has shown in the 
 case of the supposed "Lemuria, " some of the inferences 
 have been unfounded and unnecessary." 4 The present dis- 
 tribution of plants and animals is only intelligible in the 
 light of former geological changes. As a single illustration 
 of the kind of reasoning from present zoological groupings 
 as to former geological subsidence, reference may be made 
 to the fact, that while the fishes and mollusks living in the 
 seas on the two sides of the Isthmus of Panama are on the 
 whole very distinct, a few shells and a large number of fishes 
 are identical; whence the inference has been drawn that 
 though a broad water-channel originally separated North 
 and South America in Miocene times, a series of elevations 
 and subsidences has since occurred, the most recent submer? 
 sion having lasted but a short time, allowing the passage of 
 locomotive fishes, yet not admitting of much change in the 
 comparatively stationary mollusks. 2 " 
 
 321 See Darwin's "Coral Islands," Dana's "Corals and Coral Islands," 
 and the works cited postea, Book III. Part II. Section iii. 3, under "Coral- 
 reefs." The various theories on the subject are discussed by R. Langenbeck 
 in his "Theorien viber die Entstehung dec Koralleninseln und Korallennffe," 
 1890. 
 
 224 See Proc. Roy. Phys. Soc. Edinburgh, viii. p. 1. 
 223 Mem. Geol. Survey, vol. i. 1846, p. 336. 
 
 " 24 "Island Life," 1880, p. 394. In this work the question of disiribution 
 in its geological relations is treated with admirable lucidity and fulness. 
 
 225 A. R. "Wallace, "Geographical Distribution of Animals," i. pp. 40, 76.
 
 DYNAMICAL GEOLOGY 493 
 
 Fjords. An interesting proof of an extensive depres- 
 sion of the northwest of Europe is furnished by the fjords 
 or sea-locks by which that region is indented. A fjord is a 
 long, narrow, and often singularly deep inlet of the sea, 
 which terminates inland, at the mouth of a glen or valley. 
 The word is Norwegian, and in Norway fjords are charac- 
 teristically developed. The English word "firth," however, 
 is the same, and the western coasts of the British Isles fur- 
 nish many excellent examples of fjords, such as the Scottish 
 Loch Hourn, Loch Nevis, Loch Fyne, Gareloch; and the 
 Irish Lough Foyle, Lough Swilly, Bantry Bay, Dunmanus 
 Bay. Similar indentations abound on the west coast of Brit- 
 ish North America and of the South Island of New Zealand. 
 Some of the Alpine lakes (Lucerne, Grarda, Maggiore, and 
 others), as well as many in Britain, are inland examples of 
 fjords. 
 
 There can be little doubt that, though now filled with 
 salt water, fjords have been originally land -valleys. The 
 long inlet was first excavated as a valley or glen. The ad- 
 jacent valley exactly corresponds in form and character with 
 the hollow of the fjord, and must be regarded as merely its 
 inland prolongation. That the glens have been excavated 
 by subaerial agents is a conclusion borne out by a great 
 weight of evidence, which will be detailed in later parts of 
 this work. If, therefore, we admit the subaerial origin of 
 the glen, we must also grant a similar origin to its seaward 
 prolongation. Every fjord will thus mark the site of a sub- 
 merged valley. This inference is confirmed by the fact that 
 fjords do not, as a rule, occur singly, but, like glens on land, 
 lie in groups; so that, when found intersecting a long line 
 of coast, such as that of the west of Norway or the west of 
 Scotland, they show that the sea now runs far up and fills 
 submerged glens. 
 
 Human constructions and historical rec- 
 ords. Should the sea be observed to rise to the level of 
 roads and buildings which it never used to touch, should 
 former half-tide rocks cease to be visible even at low water, 
 and should rocks, previously above the reach of the highest 
 tide, be turned first into shore-reefs, then into skerries and 
 islets, we infer that the coast-line is sinking. Such kind of 
 evidence is found in Scania, the most southerly part of 
 Sweden. Streets, built of course above high-water mark, 
 now lie below it, with older streets lying beneath them, so 
 that the subsidence is of some antiquity. A stone, the posi- 
 tion of which had been exactly determined by Linnaeus in
 
 494 TEXT-BOOK OF GEOLOGY 
 
 1749, was found after 87 years to be 100 feet nearer the 
 water's edge. 226 The west coast of Greenland, for a space 
 of more than 600 miles, is perceptibly sinking. It has there 
 been noticed that, over ancient buildings on low shores, as 
 well as over entire islets, the sea has risen. The Moravian 
 settlers have been more than once driven to shift their boat- 
 poles inland, some of the old poles remaining visible under 
 water.*" Historical evidence likewise exists of the subsi- 
 dence of ground in Holland and Belgium. 288 On the coast 
 of Dalmatia, Roman roads and villas are said to be visible 
 below the sea. 229 
 
 3. Causes of Upheaval and Depression of Land, These 
 movements must again be traced back mainly to conse- 
 quences of the internal heat of the earth. There are various 
 ways in which this cause may have acted. As rocks expand 
 when heated, and contract on cooling, we may suppose that, 
 if the crust underneath a tract of land has its temperature 
 slowly raised, as no doubt takes place round areas of nascent 
 volcanoes, a gradual uprise of the ground above will be the 
 result. The gradual transference of the heat to another 
 quarter may produce a steady subsidence. Basing on the 
 
 826 According to Erdmann, the subsidence has now ceased, or has even been 
 exchanged for an upward movement (Geol. For. Stockholm Forhandl. i. p. 93). 
 Nathorst also thinks that Scania is now sharing in the general elevation of 
 Scandinavia (ibid. p. 281). It appears that the zero of movement now passes 
 through Bornholm and Laaland. 
 
 821 These observations, which have been accepted for at least a generation 
 past (Proc. Geol. Soc. ii. 1835, p. 208), have recently been called in question, 
 but the alleged disproof is not convincing, and they are here retained as wor- 
 thy of credence. See Suess, Verhand. Geol. Reichsanstalt, 1880, No. 11, and 
 "Antlitz der Erde," ii. p. 415 et seq. 
 
 228 Besides the paper of Lorie, quoted on p. 491, consult Lavaleye, "Affais- 
 sement du sol et envasement des fleuves, survenus dans les temps historiqties, " 
 Brussels, 1859. Grad, Bull. Soc. Geol. France, ii. 3d ser. p. 46. Arends, 
 "Physische Geschichte der Nordseekiiate," 1833. Compare also R. A. Pea- 
 cock on "Physical and Historical Evidences of vast Sinkings of land on the 
 North and "West Coasts of France," etc., London, 1868. For submerged peat- 
 beds on French coast, see A. Gaspard, Ann. Soc. Geol. Nord, 1870-74, p. 40. 
 On oscillations of French coast, T. Girard, Bull. Soc. Geograph. Paris, ser. 6, 
 vol. x. p. 225; E. Delfortrie, Act. Soc. Linn. Bordeaux, ser. 4, vol. i. p. 79. 
 
 229 Boll. Com. Geol. Italiauo, 1S74, p. 57.
 
 DYNAMICAL GEOLOGY 495 
 
 calculations of Colonel Totten, cited on p. 508, Lyell esti- 
 mated that a mass of red sandstone one mile thick, having 
 its temperature augmented 200 Fahr., would raise the over- 
 lying rocks 10 feet, and that a portion of the earth's crust 
 of similar character 50 miles thick, -with an increase of 600 
 or 800, might produce an elevation of 1000 or 1500 feet. m 
 But this computation, as Mr. Mellard Eeade has pointed 
 out, takes account only of linear expansion. If from any 
 cause the mass of rock whose temperature was augmented 
 could not expand horizontally it would rise vertically, and 
 unless some of the surplus volume could be disposed of by 
 condensation of the rock, the uprise would be three times as 
 much as the linear extension. Taking this view of the case, 
 we find that a mass of the earth's crust twenty miles thick, 
 heated 1000 Fahr., and prevented from extending laterally, 
 would rise 1650 feet. 881 
 
 Again, rocks expand by fusion and contract on solidifi- 
 cation. Hence, by the alternate melting and solidifying of 
 subterranean masses, upheaval and depression of the surface 
 may possibly be produced (see pp. 508, 516). 
 
 But evidently processes of this nature can only effect 
 changes of level limited in amount and local in area. When 
 we consider the wide tracts over which terrestrial move- 
 ments are now taking place, or have occurred in past time, 
 the explanation of them must manifestly be sought in some 
 far more widespread and generally effective force in geo- 
 logical dynamics. It must be confessed, however, that no 
 altogether satisfactory solution of the problem has yet been 
 given, and that the subject still remains beset with many 
 difficulties. 
 
 230 "Principles," u. p. 235. 
 
 * 31 Mellard Reade, "Origin of Mountain Ranges" (1886), pp. 112, 114.
 
 496 TEXT-BOOK OF GEOLOGY 
 
 Professor Darwin, in one of his memoirs already cited 
 (ante, p. 46), has suggested a possible determining cause of 
 the larger features of the earth's surface. Assuming for 
 his theory a certain degree of viscosity in the earth, he 
 points out that, under the combined influence of rotation 
 and the moon's attraction, the polar regions tend to outstrip 
 the equator, and to acquire a consequent slow motion from 
 west to east relatively to the equator. The amount of dis- 
 tortion produced by this screwing motion he finds to have 
 been so slow, that 45,000,000 years ago, a point in lat. 30 
 would have been 4f, and a point in lat. 60 14J' further 
 west, with reference to the equator, than they are at 
 present. This slight transference shows us, he remarks, 
 that the amount of distortion of the surface strata from 
 this cause must be exceedingly minute. But it is conceiv- 
 able that, in earlier conditions of the planet, this screwing 
 action of the earth may have had some influence in deter- 
 mining the surface features of the planet. In a body not 
 perfectly homogeneous it might originate wrinkles at the 
 surface running perpendicular to the direction of greatest 
 pressure. "In the case of the earth, the wrinkles would 
 run north and south at the equator, and would bear away 
 to the eastward in northerly and southerly latitudes, so 
 that at the north pole the trend would be northeast, and 
 at the south pole northwest. Also the intensity of the 
 wrinkling force varies as the square of the cosine of the 
 latitude, and is thus greatest at the equator and zero at 
 the poles. Any wrinkle, when once formed, would have 
 a tendency to turn slightly, so as to become more nearly 
 east and west than it was when first made." 
 
 According to the theory, the highest elevations of the 
 earth's surface should be equatorial, and should have a gen-
 
 DYNAMICAL GEOLOGY 497 
 
 eral north and south trend, while in the northern hemi- 
 sphere the main direction of the masses of land should 
 bend round toward northeast, and in the opposite hemi- 
 sphere toward southeast. Prof. Darwin thinks that the 
 general facts of terrestrial geography tend to corroborate 
 his theoretical views, though he admits that some are very 
 unfavorable to them. In the discussion of such a theory, 
 however, we must remember that the present mountain- 
 chains on the earth's surface are not aboriginal, but arose 
 at many successive and widely-separated epochs. Now it 
 is quite certain that the younger mountain-chains (and 
 these include the loftiest on the surface of the globe) 
 arose, or at least received their chief upheaval, (luring the 
 Tertiary periods a comparatively late date in geological 
 history. Unless we are to enlarge enormously the limits 
 of time which physicists are willing to concede for the 
 evolution of the whole of that history, we can hardly sup- 
 pose that the elevation of the great mountain-chains took 
 place at an epoch at all approaching an antiquity of 45,000,- 
 000 years. Yet, according to Prof. Darwin's showing, the 
 superficial effects of internal distortion must have been ex- 
 ceedingly minute during the past 45,000,000 years. We 
 must either therefore multiply enormously the periods re- 
 quired for geological changes, or find some cause which 
 could have elevated great mountain-chains at more recent 
 intervals. 
 
 But it is well worth consideration whether the cause 
 suggested by Prof. Darwin may not have given their initial 
 trend to the masses of land, so that any subsequent wrink- 
 ling of the terrestrial surface, due to any other cause, would 
 be apt to take place along the original lines. To be able 
 to answer this question, it is necessary to ascertain the
 
 498 TEXT- BOOK OF GEOLOGY 
 
 dominant line of strike of the older geological formations. 
 But information on this subject is still scanty. In Western 
 Europe, the prevalent line along which terrestrial plications 
 took place during Palaeozoic time was certainly from S.W. 
 or S.S.W. to N.B. or N.N.E., and the same direction is 
 recognizable in the eastern States of North America. But 
 the trend of later formations is more varied. The striking 
 contradictions between the actual direction of so many 
 mountain-chains and masses of land, and what ought to 
 be their line according to the theory, seem to indicate that 
 while the effects of internal distortion may have given the 
 first outlines to the land-areas of the globe, some other cause 
 has been at work in later times, acting sometimes along the 
 original lines, sometimes across them. 
 
 The main cause to which. geologists are now disposed 
 to refer the corrugations of the earth's surface is secular 
 cooling and consequent contraction." 3 If our planet has 
 been steadily losing heat by radiation into space, it must 
 have progressively diminished in volume. The cooling 
 implies contraction. According to Mallet, the diameter 
 of the earth is less by at least 189 miles since the time 
 when the planet was a mass of liquid." 3 But the contrac- 
 tion has not manifested itself uniformly over the whole 
 surface of the planet. The crust varies much in structure, 
 in thermal resistance, and in the position of its isogeother- 
 mal lines. As the hotter nucleus contracts more rapidly 
 by cooling than the cooled and hardened crust, the latter 
 must sink down by its own weight, and in so doing re- 
 quires to accommodate itself to a continually diminishing 
 
 539 For an able criticism of this view see Fisher's "Physics of Earth's Crust,' 
 2d Edit. Consult also Mr. Reade's "Origin of Mountain Ranges. " 
 233 Phil. Trans. 1873, p. 205.
 
 DYNAMICAL GEOLOGY 499 
 
 diameter. The descent of the crust gives rise to enormous 
 tangential pressures. The rocks are crushed, crumpled, and 
 broken in many places. Subsidence must have been the 
 general rule, but every subsidence would doubtless be ac- 
 companied with upheavals of a more limited kind. The 
 direction of these upheaved tracts, whether determined, as 
 Prof. Darwin suggests, by the effects of internal distortion, 
 or by some original features in the structure of the crust, 
 would be apt to be linear. The lines, once taken as lines 
 of weakness or relief from the intense strain, would prob- 
 ably be made use of again and again at successive parox- 
 ysms or more tranquil periods of contraction. Mallet 
 ingeniously connected these movements with the linear 
 direction of mountain-chains, volcanic vents, and earth- 
 quake shocks. If the initial trend to the land-masses were 
 given as hypothetically stated by Prof. Darwin, we may 
 conceive that after the outer parts of the globe had attained 
 a considerable rigidity and could then be only slightly in- 
 fluenced by internal distortion, the effects of continued 
 secular contraction would be seen in the intermittent sub- 
 sidence of the oceanic basins already existing, and in the 
 successive crumpling and elevation of the intervening stiff- 
 ened terrestrial ridges. 
 
 This view, variously modified, has been widely accepted 
 by geologists as furnishing an explanation of the origin of 
 the upheavals and subsidences of which the earth's crust 
 contains such a long record. But it is not unattended with 
 objections. The difficulty of conceiving that a globe pos- 
 sessing on the whole a rigidity equal to that of glass or 
 steel could be corrugated as the crust of the earth has been, 
 has led some writers to adopt the hypothesis already de- 
 scribed (ante, p. 105), of an intermediate viscous layer be-
 
 500 TEXT-BOOK OF GEOLOGY 
 
 tween the solid crust and the solid nucleus, while others 
 have suggested that the observed subsidence may have 
 been caused, or at least aggravated, by the escape of vapors 
 from volcanic orifices. But with modifications, the main 
 cause of terrestrial movements is still sought in secular 
 contraction. 
 
 Some observers, following an original suggestion of 
 Babbage," 4 have supposed that upheaval and subsidence, 
 together with the solidification, crystallization, and meta- 
 morphism of the layers of the earth's crust, may have been 
 in large measure due to the deposition and removal of 
 mineral matter on the surface. There can be no doubt 
 that the lines of equal internal temperature (isogeothermal 
 lines) for a considerable depth downward, follow approxi- 
 mately the contours of the surface, curving up and down as 
 the surface rises into mountains or sinks into plains. The 
 deposition of a thousand feet of rock will, of course, cause 
 a corresponding rise in the isogeotherms, and if we assume 
 the average rise of temperature to be 1 Fahr. for every 
 50 feet, then the temperature of the crust immediately 
 below this deposited mass of rock will be raised 20. But 
 masses of sediment of much greater thickness have been 
 laid down, and we may admit that a much greater increase 
 of temperature than 20 has been effected by this means. 
 On the other hand, the denudation of the land must lead 
 to a depression of the isogeotherms, and a consequent cool- 
 ing of the upper layers of the crust. 
 
 It may be conceded that in so far as the internal struc- 
 ture of rocks may be modified by such progressive increase 
 of temperature as would arise from superficial deposit, this 
 
 234 Journ. Geol. Soc. iii. (1834), p. 206.
 
 DYNAMICAL GEOLOGY 501 
 
 cause of change must have a place in geological dynamics. 
 But it has been urged that besides this effect, the removal 
 of rock by denudation from one area and its accumulation 
 upon another affects the equilibrium of the crust; that the 
 portions where denudation is active, being relieved of 
 weight, rise, while those where deposition is prolonged, 
 being on the contrary loaded, sink.*" This hypothesis 
 has recently been strongly advocated by some of the geolo- 
 gists who have been exploring the Western Territories of 
 America, and who point in proof of its truth to evidence 
 of continuous subsidence in tracts where there was pro- 
 longed deposition, and of the uprise and curvature of 
 originally horizontal strata over mountain ranges like the 
 Uinta Mountains in Wyoming and Utah, which have been 
 for a long time out of water. To suppose, however, that 
 the removal and deposit of a few thousand feet of rock 
 should so seriously affect the equilibrium of the crust as 
 to cause it to sink and rise in proportion, would evince 
 such a mobility in the earth as could not fail to manifest 
 itself in a far more powerful way under the influence of 
 lunar and solar attraction. That there has always been the 
 closest relation between upheaval and denudation on the 
 one hand, and subsidence and deposition on the other, is 
 undoubtedly true. But denudation has been one of the 
 consequences of upheaval, and deposition has been kept 
 up only by continual subsidence. 
 
 We are concerned in the present part of this work only 
 with the surface features of the land in so far as they 
 
 235 Similarly it has been contended that the accumulation of a massive ice- 
 sheel on the land would cause a depression of the terrestrial surface. N. S. 
 Shaler, Proc. Boston Nat. Hist. Soc. xvii. p. 288. T. F. Jamieson, Quart. 
 Journ. Geol. Soc. 1882, and Geol. Mag. 1882, pp. 400, 526. Fisher, "Physics 
 of Earth's Crust," p. 223.
 
 502 TEXT-BOOK OF GEOLOGY 
 
 bear on questions of geological dynamics. The history of 
 these features will be more conveniently treated in Book 
 VII. after the structure and history of the crust have been 
 described. Before quitting the subject, however, we may 
 observe that the larger terrestrial features, such as the 
 great ocean basins, the lines of submarine ridge surmounted 
 here and there by islands chiefly of volcanic materials, the 
 continental masses of land, and at least the cores of most 
 great mountain-chains, are in the main of high antiquity, 
 stamped as it were from the earliest geological ages on the 
 physiognomy of the globe, and that their present aspect has 
 been the result not merely of original hypogene operations, 
 but of long-continued superficial action by the epigene 
 forces described in Book 111. Part II. 
 
 END OP FIRST PART OP "TEXT-BOOK OF GEOLOGf"
 
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