CLASS-BOOK OF GEOLOGY CLASS-BOOK OF GEOLOGY ARCHIBALD GEIKIE, LL.D., F.R.S. n DIRECTOR-GENERAL OF THE GEOLOGICAL SURVEY OF THE UNITED KINGDOM, AND DIRECTOR OF THE MUSEUM OF PRACTICAL GEOLOGY, JERMYN STREET, LONDON ; FORMERLY MURCHISON PROFESSOR OF GEOLOGY AND MINERALOGY IN THE UNIVERSITY OF EDINBURGH ILLUSTRATED WITH WOODCUTS,-,;" 1 . LONDON MACMILLAN AND CO. 1886 [ The RigJit of Translation and Reproduction is reserved. J fNiO OKFV, PREFACE. THE present volume completes a series of educational works on Physical Geography and Geology, projected by me many years ago. In the Primers, published in 1873, the most elementary facts and principles were presented in such a way as I thought most likely to attract the learner, by stimulating at once his faculties of observation and reflection. The continued sale of large editions of these little books in this country and in America, and the trans- lation of them into most European languages, leads me to believe that the practical methods of instruction adopted in them have been found useful. They were followed in 1877 by my Class-Book of Physical Geography, in which, upon as far as possible the same line of treatment, the sub- ject was developed with greater breadth and fulness. This volume was meant to be immediately succeeded by a corre- sponding one on Geology, but pressure of other engage- ments has delayed till now the completion of this plan. So many introductory works on Geology have been written that some apology or explanation seems required from an author who adds to their number. Experience of the practical work of teaching science long ago con- vinced me that what the young learner primarily needs is a class-book which will awaken his curiosity and interest. There should be enough of detail to enable him to under- stand how conclusions are arrived at. All through its VI PREFACE. chapters he should see how observation, generalisation, and induction go hand in hand in the progress of scientific research. But it should not be overloaded with technical details which, though of the highest importance, cannot be adequately understood until considerable advance has been made in the study. It ought to present a broad, luminous picture of each branch of the subject, necessarily, of course, incomplete, but perfectly correct and intelligible as far as it goes. This picture should be 'amplified in detail by a skilful teacher. It may, however, so arrest the attention of the learner himself as to lead him to seek, of his own accord, in larger treatises, fuller sources of infor- mation. To this ideal standard of a class-book I have striven in some measure to approach. Originally, I purposed that this present volume should be uniform in size with the Class-Book of Physical Geography. But, as the illustrations were in progress, the advantage of adopting a larger page became evident, and with this greater scope and my own enthusiasm for the subject the book has gradually grown into what it now is. With few excep- tions, the woodcuts have been drawn and engraved expressly for this volume. Mr. Sharman has kindly made for me most of the drawings of the fossils. The landscape sketches are chiefly from my own note-books. I have to thank Messrs. J. D. Cooper and M. Lacour for the skill with which they have given in wood-engraving the expres- sion of the originals. 2&th December 1885. CONTENTS. CHAPTER I. PAGE INTRODUCTORY I PART I. THE MATERIALS FOR THE HISTORY OF THE EARTH. CHAPTER II. THE INFLUENCE OF THE ATMOSPHERE IN THE CHANGES OF THE EARTH'S SURFACE . . 13 CHAPTER III. THE INFLUENCE OF RUNNING WATER IN GEOLOGICAL CHANGES, AND HOW IT IS RECORDED . . 3! CHAPTER IV. THE MEMORIALS LEFT BY LAKES . . . .56 CHAPTER V. HOW SPRINGS LEAVE THEIR MARK IN GEOLOGICAL HIS- TORY 66 via CONTENTS. CHAPTER VI. ICE-RECORDS . 82 CHAPTER VII. THE MEMORIALS OF THE PRESENCE OF THE SEA . 94 CHAPTER VIII. HOW PLANTS AND ANIMALS INSCRIBE THEIR RECORDS IN GEOLOGICAL HISTORY . . . . IO8 CHAPTER IX. THE RECORDS LEFT BY VOLCANOES AND EARTHQUAKES 124 PART II. ROCKS, AND HOW THEY TELL THE HISTORY OF THE EARTH. CHAPTER X. THE MORE IMPORTANT ELEMENTS AND MINERALS OF THE EARTH'S CRUST 153 CHAPTER XI. THE MORE IMPORTANT ROCKS AND ROCK-STRUCTURES IN THE EARTH'S CRUST . . .185 CONTENTS. ix PART III. THE STRUCTURE OF THE CRUST OF THE EARTH. CHAPTER XII. PAGE SEDIMENTARY ROCKS THEIR ORIGINAL STRUCTURES 226 CHAPTER XIII. SEDIMENTARY ROCKS STRUCTURES SUPERINDUCED IN THEM AFTER THEIR FORMATION . . . 244 CHAPTER XIV. ERUPTIVE ROCKS AND MINERAL VEINS IN THE ARCHI- TECTURE OF THE EARTH'S CRUST . . .263 CHAPTER XV. HOW FOSSILS HAVE BEEN ENTOMBED AND PRESERVED, AND HOW THEY ARE USED IN INVESTIGATING THE STRUCTURE OF THE EARTH'S CRUST, AND IN STUDYING GEOLOGICAL HISTORY . . .279 PART IV. THE GEOLOGICAL RECORD OF THE HISTORY OF THE EARTH. CHAPTER XVI. THE EARLIEST CONDITIONS OF THE GLOBE THE ARCHAEAN PERIODS . . .298 CONTENTS. CHAPTER XVII. PAGE THE PALAEOZOIC PERIODS SILURIAN . . -312 CHAPTER XVIII. DEVONIAN AND OLD RED SANDSTONE . . -337 CHAPTER XIX. CARBONIFEROUS ....... 348 CHAPTER XX. PERMIAN . , . . . . . . -370 CHAPTER XXI. THE MESOZOIC PERIODS TRIASSIC . . . -379 CHAPTER XXII. JURASSIC ... 389 CHAPTER XXIII. CRETACEOUS 46 CHAPTER XXIV. TERTIARY OR CAINOZOIC EOCENE OLIGOCENE . 423 CONTENTS. xi CHAPTER XXV. PAGE MIOCENE PLIOCENE ...... 439 CHAPTER XXVI. POST-TERTIARY OR QUATERNARY PERIODS PLEIS- TOCENE OR POST-PLIOCENE RECENT . . -455 APPENDIX . 479 INDEX 499 LIST OF ILLUSTRATIONS. 1. Weathering of rock, as shown by old masonry. (The "false- bedding " and other original structures of the stone are revealed by weathering) . . . . . . . . .16 2. Passage of sandstone upwards into soil ..... 20 3. Passage of granite upwards into soil ..... 20 4. Talus-slopes at the foot of a line of cliffs ..... 26 5. Section of rain-wash or brick-earth ...... 26 6. Sand-dunes .......... 28 7. Erosion of limestone by the solvent action of a peaty stream, Durness, Sutherlandshire ....... 33 8. Pot-holes worn out by the gyration of stones in the bed of a stream 40 9. Grand Canon of the Colorado ...... 44 10. Gullies torn out of the side of a mountain by descending torrents, with cones of detritus at their base ..... 47 11. Flat stones in a bank of river-shingle, showing the direction of the current that transported and left them .... 49 12. Section of alluvium showing direction of currents ... 50 13. River-terraces . . . . . . . . .51 14. Alluvial terraces on the side of an emptied reservoir . . 58 15. Parallel roads of Glen Roy . . . . . -59 1 6. Stages in the filling up of a lake ...... 60 17. Piece of shell-marl containing shells of Limna'a peregra . . 62 18. View of Axmouth landslip as it appeared in April 1885 . . 69 19. Section of cavern with stalactites and stalagmite ... 72 20. Section showing successive layers of growth in a stalactite . . 75 21. Travertine with impressions of leaves ..... 79 22. Glaciers and moraines ........ 85 23. Perched blocks scattered over ice-worn surface of rock . . 86 24. Stone smoothed and striated by glacier-ice . . . .89 25. Ice-striation on the floor and side of a valley .... 90 26. Duller of Buchan a caldron-shaped cavity or blow-hole worn out of granite by the sea on the coast of Aberdeenshire . . 9^ 27. The Stacks of Duncansby, Caithness, a wave-beaten coast-line . 97 28. Section of submarine plain ....... 100 29. Storm-beach ponding back a stream and forming a lake ; west coast of Sutherlandshire . . . . . . .103 xiv LIST OF ILLUSTRATIONS. FIG. PAGE 30. Section of a peat-bog . . . . . . . .no 31. Diatom -earth from floor of Antarctic Ocean, magnified 300 diameters. . . . . . . . . .112 32. Recent limestone (cockle, etc. ) . . . . . .114 33. Globigerina ooze magnified . , . , . . -115 34. Section of a coral-reef . . . . . . . .116 35. Cellular lava with a few of the cells filled up with infiltrated mineral matter (Amygdules) . . . . . .130 36. Section of a lava-current . . . . . . -131 37. Elongation of cells in direction of flow of a lava-stream . . 132 38. Volcanic block ejected during the deposition of strata in water . 137 39. Volcanoes on lines of fissure . . . . . . .139 40. Outline of a volcanic neck ....... 142 41. Ground-plan of the structure of the Neck shown in Fig. 40 . 142 42. Section through the same Neck as in Figs. 40 and 41 . 143 43. Volcanic dykes rising through the bedded tuff of a crater . . 145 44. Group of quartz-crystals (Rock-crystal) . . . . 157 45. Calcite (Iceland spar) showing its characteristic rhombohedral cleavage . . . . . . . . . .166 46. Cube, octahedron, dodecahedron . . . . . .167 47. Tetragonal prism and pyramid . . . . . .167 48. Orthorhombic prism . . . . . . . .167 49. Hexagonal prism, rhombohedron, and scalenohedron . . 168 50. Monoclinic prism. Crystal of Augite ..... 168 51. Triclinic prism. Crystal of Albite felspar .... 168 52. Section of a pebble of chalcedony . . . . . .171 53. Piece of haematite, showing the nodular external form and the internal crystalline structure . . . . . .172 54. Octahedral crystals of magnetite in chlorite schist . . .173 55. Dendritic markings due to arborescent deposit of earthy manganese oxide .......... 174 56. Cavity in a lava, filled with zeolite which has crystallised in long slender needles . . . . . . . . .176 57. Hornblende crystal . . . . . . . 177 58. Olivine crystal . . . . . . . . .178 59. Calcite in the form of nail-head spar . . . . .179 60. Calcite in the form of dog-tooth spar . . . . .180 61. Sphaerosiderite or Clay-ironstone concretion enclosing portion of a fern . . . . . . . . . .181 62. Gypsum crystals . . . . . . . . .182 63. Group of fluor-spar crystals . . . . . . .183 64. Concretions . . . . . . . . . .187 65. Section of a septarian nodule, with coprolite of a fish as a nucleus 188 66. Piece of oolite 189 67. Piece of pisolite ......... 189 68. Cavities in quartz containing liquids (magnified) . . . 190 69. Crystallites 191 70. Porphyritic structure . . . . . . . .192 71. Spherulites and fluxion-structure . . . . . .192 LIST OF ILLUSTRATIONS. xv FIG. PAGE 72. Schistose structure . . . . . I 94 73. Brecciated structure volcanic breccia, a rock composed of angular fragments of lava, in a paste of finer volcanic debris 197 74. Conglomerate . . . . . .198 75. Concretionary forms assumed by Dolomite, Magnesian Lime- stone, Durham ........ 206 76. Weathered surface of crinoidal limestone . . . .210 77. Group of crystals of felspar, quartz, and mica, from a cavity in the Mourne Mountain granite . . . . . .216 78. Columnar basalts of the Isle of Staffa, resting upon tuff (to the right is Fingal's Cave) ....... 219 79. Section of stratified rocks ....... 227 80. Section showing alternation of beds ..... 230 8 1. False-bedded sandstone . . . . . . .231 82. Ripple-marked surface ....... 232 83. Cast of a sun-cracked surface preserved in the next succeeding layer of sediment ........ 233 84. Rain-prints on fine mud ....... 234 85. Vertical trees (Sigillaria) in sandstone, Swansea (Logan) . 238 86. Hills formed out of horizontal sedimentary rocks . . . 239 87. Section of overlap ........ 240 88. Unconformability . . . . . . . .241 89. Joints in a stratified rock ....... 246 90. Dip and Strike ......... 248 91. Clinometer. ......... 248 92. Dip, Strike, and Outcrop ....... 249 93. Inclined strata shown to be parts of curves .... 250 94. Curved strata (anticlinal fold), near St. Abb's Head . .251 95. Curved strata (synclinal fold), near Banff .... 252 96. Anticlines and Synclines ....... 253 97. Section of the Grosse Windgalle (10,482 feet), Canton Uri, Switzerland, showing crumpled and inverted strata (after Heim ) 254 98. Distortion of fossils by the shearing of rocks . . . -255 99. Curved and cleaved rocks. Coast of Wigtonshire . . .256 100. Examples of normal Faults . . . . . . .257 101. Sections to show the relations of Plications to reversed Faults 258 102. Throw of a Fault ........ 259 103. Ordinary unaltered red sandstone, Keeshorn, Ross-shire . 260 104. Sheared red sandstone forming now a micaceous schist, Kee- shorn, Ross-shire ........ 260 105. Outline and section of a Boss traversing stratified rocks . . 265 1 06. Ground-plan of Granite-boss with ring of Contact -Meta- morphism ......... 267 107. Intrusive Sheet ......... 268 108. Interstratified or contemporaneous Sheets .... 269 109. Section to illustrate evidence of contemporaneous volcanic action .......... 270 no. Map of Dykes near Muirkirk, Ayrshire . .... 273 xvi LIST OF ILLUSTRATIONS. FIG. PAGE in. Section of a volcanic neck . . . . . . 274 112. Section of a mineral vein ....... 276 113. Common Cockle (Card-turn edule] ..... 285 114. Fragment of crumpled Schist ...... 309 115. A, Fucoid-like impression (Eophyton Linneanuni) from Cam- brian rocks (g). B, An Upper Silurian sea-weecl (Clwndrites verisimilis], natural size . . . . . . .319 116. Oldhamia radiata (natural size), Ireland . . . .321 117. Graptolites . .....'.... 323 118. Hydrozoon from the Cambrian rocks ..... 324 119. Silurian Rugose Coral ........ 325 1 20. Silurian Alcyonarian Coral . . . . . . .325 121. Silurian Cystidean ........ 326 122. Silurian Star-fish . ........ 326 123. Filled-up Burrows or Trails left by a sea-worm on the bed of the Silurian sea . . . . . . . .327 124. Trilobites (Primordial or Cambrian) ..... 328 125. Trilobites (Lower and Upper Silurian) ..... 329 126. Silurian Phyllopod Crustacean . . . . . 330 127. Silurian Brachiopods . . . . . . . -331 128. Silurian Lamellibranch . . . . . . .332 129. Silurian Gasteropod ........ 332 130. Silurian Cephalopods ........ 334 131. Plants of the Devonian period ...... 339 132. Overlapping scales of an Old Red Sandstone fish . . . 340 133. Scale-covered Old Red Sandstone fishes .... 341 134. Plate-covered Old Red Sandstone fishes . . . 341 135. Devonian Eurypterid Crustacean ...... 343 136. Devonian trilobites . . . . . . . . 344 137. Devonian corals ......... 345 138. Devonian Brachiopods ....... 346 139. Devonian Lamellibranch and Cephalopod .... 347 140. Section of part of the Cape Breton coalfield, showing a succes- sion of buried trees and land-surfaces . . . 352 141. Carboniferous Ferns ........ 355 142. Carboniferous Lycopod . . . . . ... 356 143. Carboniferous Equisetaceous Plants ..... 357 144. Sigillaria with Stigmaria roots . . . . . -357 145. Cordaites alloidius . . . . . . . .358 146. Carboniferous Foraminifer . . . . . . .361 147. Carboniferous Rugose Corals . . . . . .361 148. Carboniferous Sea-Urchin ....... 362 149. Carboniferous Crinoid ....... 362 150. Carboniferous Blastoid ....... 362 151. Carboniferous Trilobite ....... 363 152. Carboniferous Polyzoon ....... 364 153. Carboniferous Brachiopods ....... 365 154. Carboniferous Lamellibranchs ...... 366 LIST OF ILLUSTRATIONS. xvil FIG. P AGE 155. Carboniferous Gasteropods ....... 366 156. Carboniferous Pteropod ....... 367 157. Carboniferous Cephalopods ...... 367 158. Carboniferous Fishes ........ 368 159. Permian Plants . ........ 373 160. Permian Brachiopods ........ 374 1 6 1. Permian Lamellibranchs ....... 375 162. Permian Ganoid Fish ........ 375 163. Permian Labyrinthodont ....... 376 164. Triassic Plants 382 165. Triassic Crinoid . . . . . . . . -383 1 66. Triassic Lamellibranchs . . . . '. . -384 167. Triassic Cephalopods ........ 384 168. Triassic Lizard 385 169. Triassic Crocodile Scutes . . . . . . -385 170. Triassic Marsupial Teeth ....... 386 171. Jurassic Cycad ......... 390 172. Jurassic reef-building Coral ....... 391 173. Jurassic Crinoid ........ 392 174. Jurassic Sea-urchin ........ 393 175. Jurassic Lamellibranchs ....... 394 176. Jurassic Ammonites ........ 395 177. Jurassic Belemnite . . . . . . . 395 178. Jurassic Crustacean ........ 396 179. Jurassic Fish ......... 396 1 80. Jurassic Sea-lizard ........ 397 181. Jurassic Pterosaur, or flying reptile ..... 398 182. Jurassic Bird ......... 400 183. Jurassic Marsupial Teeth and Jaw ..... 400 184. Cretaceous Plants ........ 409 185. Cretaceous Foraminifera ....... 410 1 86. Cretaceous Sponge . . . . . . . .410 187. Cretaceous Sea-urchins . . . . . . .411 1 8 8. Cretaceous Lamellibranchs . . . . . . .412 189. Cretaceous Lamellibranchs . . . . . . .413 190. Cretaceous Cephalopods ....... 414 191. Cretaceous Fish ......... 415 192. Cretaceous Deinosaur . 416 193. Eocene Plant ......... 428 194. Eocene Molluscs ........ 429 195. Eocene Mammal . . . . . . . .431 196. Skull of Tinoceras ingens . 432 197. Oligocene Molluscs ........ 436 198. Miocene Plants ......... 441 199. Mastodon augustidens ....... 442 200. Skull of Deinotherium giganteum . . . . .442 20 1. Pliocene Plants ......... 449 202. Pliocene Marine Shells ....... 450 xvm LIST OF ILLUSTRATIONS. FIG. PAGE 203. Helladotherium Duvernoyi a gigantic animal intermediate in structure between the giraffe and the antelope, Pikermi, Attica . . . . . . . . . . 453 204. Pleistocene or Glacial Shells . . . . . .462 205. Mammoth from the skeleton in the Muse"e Royal, Brussels . 463 206. Back view of skull of Musk-sheep, Brick-earth, Crayford, Kent 463 207. Palaeolithic Implements ....... 470 208. Antler of Reindeer found at Bilney Moor, East Dereham, Norfolk 473 209. Neolithic Implements 475 CHAPTER I. INTRODUCTORY. THE main features of the dry land on which we live seem to remain unchanged from year to year. The valleys and plains familiar to our forefathers are still familiar to us, bear- ing the same meadows and woodlands, the same hamlets and villages, though generation after generation of men has meanwhile passed away. The hills and mountains now rise along the sky-line as they did long centuries ago, catching as of old the fresh rains of heaven and gathering them into the brooks and rivers which, through unknown ages, have never ceased to flow seawards. So steadfast do these features appear to stand, and so strong a contrast do they offer to the shortness and changeableness of human life, that they have become typical in our minds of all that is ancient and durable. We speak of the firm earth, of the everlasting hills, of the imperishable mountains, as if, where all else is fleeting and mutable, these forms at least remain unchanged. And yet attentive observation of what takes place from day to day around us shows that the surface of a country is not now exactly as it used to be. We notice various changes B 2 , t c t c c t , 1 1 , INTRODUCTORY. [CHAP. of its topography going on now, which have doubtless been in progress for a long time, and the accumulated effect of which may ultimately transform altogether the character of landscapes. A strong gale, for instance, will level thousands of trees in its pathway, turning a tract of forest or woodland into a bare space, which becomes perhaps a quaking morass, or may be changed into arable ground by the farmer. A flooded river will in a few hours cut away large slices from its banks, and spreading over fields and meadows, will bury many acres of fertile land under a covering of barren sand and shingle. A long-continued, heavy rain, by loosening masses of earth or rock on steep slopes, causes destructive landslips. A hard frost splinters the naked fronts of crags and cliffs, and breaks up bare soil. In short, every shower of rain and gust of wind, if we could only watch them narrowly enough, would be found to have done something towards modifying the surface of the land. Along the sea- margin, too, how ceaseless is the progress of change ! In most places, the waves are cutting away the land, sometimes even at so fast a rate as two or three feet in a year. Here and there, on the other hand, they cast sand and silt ashore so as to increase the breadth of the dry land. These are ordinary everyday causes of alteration, and though singly insignificant enough, their united effect after long centuries cannot but be great. From time to time, however, other less frequent but more powerful influences come into play. In most large regions of the globe, the ground is often convulsed by earthquakes, many of which leave permanent scars upon the surface of the land. Vol- canoes, too, in many countries pour forth streams of molten rock and showers of dust and cinders that bury the surround- ing districts and greatly alter their appearance. i.] GEOLOGICAL CHANGES WITNESSED BY MAN. 3 Turning to the pages of human history, we find there the records of similar changes in bygone times. Lakes, on which our rude forefathers paddled their canoes and built their wattled island -dwellings, have wholly disappeared. Bogs, over whose treacherous surface they could not follow the chase of red deer or Irish elk, have become meadows and fields. Forests, where they hunted the wild boar, have been turned into grassy pastures. Cities have been entirely destroyed by earthquakes or have been entombed under the piles of ashes discharged from a burning mountain. So great have been the inroads of the sea that, in some instances, the sites of what a few hundred years ago were farms and hamlets, now lie under the sea half a mile or more from the modern shore. Elsewhere the land has gained upon the sea, and the harbours of an earlier time are now several miles distant from the coast-line. But man has naturally kept note only of the more im- pressive changes, in other words, of those which had most influence upon his own doings. We may be certain, how- ever, that there have been innumerable minor alterations of the surface of the land within human history, of which no chronicler has made mention, either because they seemed too trivial, or because they took place so imperceptibly as never to be noticed. Fortunately, in many cases, these mutations of the land have written their own memorials, which can be as satisfactorily interpreted as the ancient manuscripts from which our early national history is com- piled. In illustration of the character of these natural chronicles, let us for a moment consider the subsoil beneath cities that have been inhabited for many centuries. In London, for example, when excavations are made for drainage, building, 4 INTRODUCTORY. [CHAP. or other purposes, there are sometimes found, many feet below the level of the present streets, mosaic pavements and foundations, together with earthen vessels, bronze implements, ornaments, coins, and other relics of Roman time. Now, if we knew nothing, from actual authentic history, of the exist- ence of such a people as the Romans, or of their former presence in England, these discoveries, deep beneath the surface of modern London, would prove that long before the present streets were built, the site of the city was occupied by a civilised race which employed bronze and iron for the useful purposes of life, had a metal coinage, and showed not a little artistic skill in its pottery, glass, and sculpture. But down beneath the rubbish wherein the Roman remains are embedded, lie gravels and sands from which rudely- fashioned human implements of flint have been obtained. Whence we further learn that, before the civilised metal-using people appeared, an earlier race had been there, which employed weapons and instruments of roughly chipped flint. That this was the order of appearance of the successive peoples that have inhabited the site of London is, of course, obvious. But let us ask ourselves why it is obvious. We observe that there are, broadly speaking, three layers or deposits from which the evidence is derived. The upper layer is that which contains the foundations and rubbish of modern London. Next comes that which encloses the relics of the Roman occupation. At the bottom lies the layer that preserves the scanty traces of the early flint-folk. The upper deposit is necessarily the newest, for it could not be laid down until after the accumulation of those below it, which must, of course, be progressively older, as they are traced deeper from the surface. By the mere fact that the layers lie one above another, we are furnished with a simple i.] GEOLOGICAL METHODS. 5 clue which enables us to determine their relative time of formation. We may know nothing whatever as to how old they are. But we can be absolutely certain of what is termed their "order of superposition," and this order marks their chronological sequence, that is, it shows that the bottom layer came first and the top layer last. This kind of observation and reasoning will enable us to detect almost everywhere proofs that the surface of the land has not always been what it is to-day. In some districts, for example, when the dark layer of vegetable soil is turned up which supports the plants that keep the land so green, there may be found below it sand and gravel, full of smooth well- rounded stones. Such materials are to be seen in the course of formation where water keeps them moving to and fro, as on the beds of rivers, the margins of lakes, or the shores of the sea. Wherever smoothed rolled pebbles occur, they point to the influence of moving water ; so that we conclude, even though the site is now dry land, that the sand and gravel underneath it prove it to have been formerly under water. Again, below the soil in other regions, lie layers of oysters and other sea-shells. These remains, spread out like similar shells on the beach or bed of the sea at the present day, enable us to infer that where they lie the sea once rolled. Pits, quarries, or other excavations that lay open still deeper layers of material, bring before us interesting and impressive testimony regarding the ancient mutations of the land. Suppose, by way of further illustration, that under- neath a bed of sand full of oyster-shells, there lies a dark . brown band of peat. This substance, composed of mosses and other water-loving plants, is formed in boggy places by the growth of marshy vegetation. Below the peat, there 6 INTRODUCTORY. [CHAP. might occur a layer of soft white marl full of lake-shells, such as may be observed on the bottoms of many lakes at the present time. These three layers oyster-bed, peat, and marl would present a perfectly clear and intelligible record of a curious series of changes in the site of the locality. The bottom layer of white marl with its peculiar shells would show that at one time the place was occupied by a lake. The next layer of peat would indicate that, by the growth of marshy vegetation, the lake was gradually changed into a morass. The upper layer of oyster-shells would prove that the ground was then submerged beneath the sea. The present condition of the ground shows that subsequently the sea retired and the locality passed into dry land as it is to-day. It is evident that by this method of examination in- formation may be gathered regarding early conditions of the earth's surface, long before the authentic dates of human history. Such inquiries form the subject of Geology, which is the science that investigates the history of the earth. The records in which this history is chronicled are the soils and rocks under our feet. It is the task of the geologist so to arrange and interpret these records as to show through what successive changes the globe has passed, and how the dry land has come to wear the aspect which it presents at the present time. Just as the historian would be wholly unable to decipher the inscriptions of an ancient race of people unless he had first discovered a key to the language in which they are written, so the geologist would find himself baffled in his efforts to trace backward the history of the earth if he were not provided with a clue to the interpretation of the records in which that history is contained. Such a clue is furnished T.] GEOLOGICAL METHODS. 7 to him by a study of the operations of nature now in pro- gress upon the earth's surface. Only in so far as he makes himself acquainted with these modern changes, can he hope to follow intelligently and successfully the story of earlier phases in the earth's progress. It will be seen that this truth has already been illustrated in the instances above given of the evidence that the surface of the land has not been always as it is now. The beds of sand and gravel, of oyster-shells, of peat and of marl, would have told us nothing as to ancient geography had we not been able to ascertain their origin and history by finding corresponding materials now in course of accumulation. To one ignorant of the peculiarities of fresh-water shells, the layer of marl would have conveyed no intelligible meaning. But knowing and recognising these peculiarities, we feel sure that the marl marks the site of a former lake. Thus the study of the present supplies a key that unlocks the secrets of the past. In order, therefore, to trace back the history of the earth, the geologist must begin by carefully watching the changes that now take place upon the earth, and by observing how nature elaborates the materials that preserve more or less completely the record of these changes. In the following pages, I propose to follow this method of inquiry, and, as far as the subject will permit, to start with no assumptions which the learner cannot easily verify for himself. We shall begin with the familiar everyday operations of the air, rain, frost, and other natural agents. As these have been fully described in my Class-Book of Physical Geography, it will not be needful here to consider them again in detail. We shall rather pass on to inquire in what various ways they are engaged in contributing to the formation of new mineral accumulations, and in thereby providing fresh materials for 8 INTRODUCTORY. [CHAP. the preservation of the facts on which geological history is founded. Having thus traced how new rocks are formed, we may then proceed to arrange the similar rocks of older time, marking what are the peculiarities of each and how they may best be classified. If the labours of the geologist were concerned merely with the former mutations of the earth's surface, how sea and land have changed places, how rivers have altered their courses, how lakes have been filled up, how valleys have been excavated, how mountains, peaks, and precipices have been carved, how plains have been spread out, and how the story of these revolutions has been written in enduring characters upon the very framework of the land, he would feel the want of one of the great sources of interest in the study of the present face of nature. We naturally connect all modern changes of the earth's surface with the life of the plants and animals that flourish there, and more especially with their influence on the progress of man himself. If there were no similar connection of the ancient changes with once living things if the history of the earth were merely one of dead inert matter it would lose much of its interest for us. But happily that history includes the records of successive generations of plants and animals which, from early times, have peopled land and sea. The remains of these organisms have been preserved in the deposits of different ages, and can be compared and contrasted with those of the modern world. To realise how such preservation has been possible, and how far the forms so retained afford an adequate picture of the life of the time to which they belonged, we must turn once more to watch how nature deals with this matter at the present time. Of the millions of flowers, shrubs, and i.] NATURAL CHRONICLES. 9 trees which year after year clothe the land with beauty, how many relics are preserved ? Where are the successive gener- ations of insect, bird, and beast which have appeared in this country since man first set foot upon its soil ? They have utterly vanished. If all their living descendants could suddenly be swept away, how could we tell that such plants and animals ever lived at all ? It must be confessed that of the vast majority not a trace remains. Nevertheless we should be able to recover relics of some of them by searching in the comparatively few places where, at the present day, we see that the remains of plants and animals are entombed and preserved. From the alluvial terraces of rivers, from the silt of lake-bottoms, from the depths of peat-mosses, from the floors of subterranean caverns, from the incrustations left by springs, we might recover traces of some at least of the plants and animals. And from these fragmentary and in- complete records we might conjecture what may have been the general character of the life of the time. By searching the similar records of earlier ages the geologist has brought to light many profoundly interesting vestiges of vegetation and of animal life belonging to types that have long since passed away. It must be evident, however, that if we confine our inquiries merely to its surface we shall necessarily gain a most imperfect view of the general history of the earth. Beneath that surface, as volcanoes show, there lies a hot interior, which must have profoundly influenced the changes of the outer parts or crust of the planet. The study of vol- canoes enables us to penetrate, as it were, a little way into that interior, and to understand some of the processes in progress there. But our knowledge of the inside of the earth can obviously be based only to a very limited extent 10 INTRODUCTORY. [CHAP. on direct observation, for man cannot penetrate far below the surface. The deepest mines do not go deep enough to reach materials differing in any essential respect from those visible above ground. Nevertheless, by inference from such observations as can be made, and by repeated and varied experiments in laboratories, imitating as closely as can be devised what may be supposed to be the conditions that exist deep within the globe, some probable conclusions can be drawn even as to the changes that take place in those deeper recesses that lie for ever concealed from our eyes. These conclusions will be stated and the rocks will be described, on the origin of which they appear to throw light. I have compared the soils and rocks with which geology deals to the records out of which the historian writes the chronicles of a nation. We might vary the simile by liken- ing them to the materials employed in the construction of a great building. It is of course interesting enough to know what kinds of marble, granite, mortar, wood, brass, or iron, have been chosen by an architect. But much more import- ant is it to inquire how these various substances have been grouped together so as to form such a building. In like manner, besides the nature and mode of origin of the various rocks of which the visible and accessible part of the earth consists, we ought to know how these varied substances have been arranged so as to build up what we can see of the terrestrial crust. In short, we should try to trace the archi- tecture of the globe, noting how each variety of rock occupies its own characteristic place, and how they are all grouped and braced together to form the solid framework of the land. This then will be the next subject for consideration. But in a great historical edifice, like one of the minsters of Europe, for example, there are often several different i.] PRINCIPLES FOR THE LEARNER S GUIDANCE, n styles. A student of architecture can detect these distinc- tions, and by their means can show that a cathedral has not been completed in one age ; that it may even have been partially destroyed and rebuilt during successive centuries, only finally taking its present form after many political vicissitudes and changes of architectural taste. Each edifice has thus a separate history, which is recorded by the way the materials have been shaped and put together in the various parts of the masonry. So it is with the architecture of the earth. We have evidence of many demolitions and rebuildings, and the story of their general progress can still be deciphered among the rocks. It is the business of geology to trace out that story, to put all the scattered materials together, and to make known by what a long succession of changes the earth has reached its present state. An outline of what geology has accomplished in this task will form the last and concluding part of this volume. In the following chapters I wish two principles to be kept steadily in view. In the first place, looking upon geology as the study of the earth's history, we need not at first concern ourselves with any details, save those that may be needed to enable us clearly to understand what the general character and progress of this history have been. In a science which embraces so vast a range as geology, the multiplicity of facts to be examined and remembered may seem at first to be almost overwhelming. But a selection of the essential facts is sufficient to give the learner a clear view of the general principles and conclusions of the science, and to enable him to enter with intelligence and interest into more detailed treatises. In the second place, geology is essentially a science of observation. The facts with which it deals should, as far as possible, be verified by our own 12 INTRODUCTORY. [CHAP. i. personal examination. We should lose no opportunity of seeing with our own eyes the actual progress of the changes which it investigates, and the proofs which it adduces of similar changes in the far past. To do this will lead us into the fields and hills, to the banks of rivers and lakes, and to the shores of the sea. We can hardly take any country walk, indeed, in which with duly observant eye we may not detect either some geological operation in actual progress, or the evidence of one which has now been completed. Having learnt what to look for and how to interpret it when seen, we are as it were gifted with a new sense. Every land- scape comes to possess a fresh interest and charm, for we carry about with us everywhere an added power of enjoy- ment, whether the scenery has long been familiar or presents itself for the first time. I would therefore seek at the outset to impress upon those who propose to read the following pages, that one of the main objects with which this book is written is to foster a habit of observation, and to serve as a guide to what they are themselves to look for, rather than merely to relate what has been seen and determined by others. If they will so learn these lessons, I feel sure that they will never regret the time and labour they may spend over the task. PART I. THE MATERIALS FOR THE HISTORY OF THE EARTH. CHAPTER II. THE INFLUENCE OF THE ATMOSPHERE IN THE CHANGES OF THE EARTH'S SURFACE. IN the history of mankind, no sharp line can be drawn between the events that are happening now or have happened within the last few generations, and those that took place long ago, and which are sometimes, though inaccurately, spoken of as historical. Every people is enacting its history to-day just as fully as it did many cen- turies ago. The historian recognises this continuity in human progress. He knows that the feelings and aspira- tions which guided mankind in old times were essentially the same influences that impel them now, and therefore that the wider his knowledge of his fellowmen of the present day, the broader will be his grasp in dealing with the trans- actions of former generations. So too is it with the history of the earth. That history is in progress now as really as it has ever been, and its events are being recorded in the 14 GEOLOGICAL WORK OF THE AIR. [CHAP. same way and by the same agents as in the far past. Its continuity has never been broken. Obviously, therefore, if we would explore its records "in the dark backward and abysm of time," we should first make ourselves familiar with the manner in which these records are being written from day to day before our eyes. In this first Part, attention will accordingly be given to the changes in progress upon the earth at the present time, and to the various ways in which the passing of these changes is chronicled in natural records. We shall watch the actual transaction of geological history, and mark in what way its incidents inscribe themselves on the page of the earth's surface. 1 Every day and hour some geological event, trifling and transient or stupendous and durable, comes and goes, leaving sometimes only an imperceptible trace of its passage, at other times graving itself almost imperishably in the annals of the globe. Tracing the origin and develop- ment of these geological annals of the present time, we shall best qualify ourselves for deciphering the records of the early revolutions of the planet. We are thus led to study the various chronicles compiled respectively by the air, rain, rivers, springs, glaciers, the sea, plants and animals, volcanoes, and earthquakes in other words, all the deposits left by the operations of these agents, the scars or other features made by them upon the earth's surface, and all other memorials of geological change. Having learnt how modern deposits are produced, and how they preserve the story of their origin, we shall then be able to group with them the corresponding deposits of earlier times, and to 1 For descriptions of the ordinary operations of geological agents the reader is referred to my Class- Book of Physical Geography. My object now is to direct attention to what is most enduring in these operations, and in what various ways they form permanent geological records. ii.] UNIVERSAL DECAY OF EARTH S SURFACE. 15 embrace all the geological records, ancient as well as modern, in one general scheme of classification. Such a scheme will enable us to see the continuity of the materials of geological history, and will fix definitely for us the character and relative position of all the chief rocks out of which the visible part of the globe is composed. The gradual change that overtakes everything on the face of the earth is expressed in all languages by familiar phrases which imply that the mere passing of time is the cause of the change. As Sir Thomas Browne quaintly said more than two hundred years ago, " time antiquates antiquities, and hath an art to make dust of all things." We speak of the dust of antiquity and the gnawing tooth of time. We say that things are time-eaten, worn with age, crumbling under a weight of years. Nothing suggests such epithets so strikingly as an old building. We know that the masonry at first was smooth and fresh ; but now we describe it as weather-beaten, decayed, corroded. So distinctive is this appearance that it is always looked for in an ancient piece of stone-work ; and if not seen, its absence at once suggests a doubt whether the masonry can really be old. No matter of what varieties of stone the edifice may have been built, a few generations may be enough to give them this look of venerable antiquity. The surface that was left smoothly polished by the builders grows rough and uneven, with scars and holes eaten into it. Portions of the original polish that may here and there have escaped, serve as a measure of how much has actually been removed from the rest of the surface. Now, if in the lapse of time, stone which has been artificially dressed is wasted away, we may be quite certain that the same stone in its natural position on the slope of i6 GEOLOGICAL WORK OF THE AIR. [CHAP. a hill or valley, or by the edge of a river or of the sea, must decay in a similar way. Indeed, an examination of any crumbling building will show that, in proportion as the chiselled surface disappears, the stone puts on the ordin- ary look which it wears where it has never been cut by man, and where only the finger of time has touched it. Could we remove some of the de- cayed stones from the build- FIG. i. Weathering of rock, as, in g and insert them into a shown by old masonry. (The natural crag or cliff of the "false-bedding" and other ori- same kind of st their gmal structures of the stone are revealed by weathering.) peculiar time -worn aspect would be found to be so ex- actly that of the rest of the cliff that probably no one would ever suspect that a mason's tools had once been upon them. From this identity of surface between the time-worn stones of an old building and the stone of a cliff we may confidently infer that the decay so characteristic of ancient masonry is as marked upon natural faces of rock. The gradual disappearance of the artificial smoothness given by the mason, and its replacement by the ordinary natural rough surface of the*stone, shows that this natural surface must also be the result of decay. And as the peculiar crumbling character is universal, we may be sure that the decay with which it is connected must be general over the globe. But the mere passing of time obviously cannot change anything, and to say that it does is only a convenient IT.] CAUSES OF WEATHERING. 17 figure of speech. It is not time, but the natural processes which require time for their work, that produce the wide- spread decay over the surface of the earth. Of these natural processes, there are four that specially deserve consideration, changes of temperature, saturation and desiccation, frost, and rain. (i.) Changes of Temperature. In countries where the days are excessively hot, with nights correspondingly cool, the surfaces of rocks heated sometimes, as in parts of Africa, up to more than 130 Fahr. by a tropical sun, undergo con- siderable expansion in consequence of this increase of temperature. At night, on the other hand, the rapid radia- tion quickly chills the stone and causes it to contract. Hence the superficial parts, being in a perpetual state of strain, gradually crack up or peel off. The face of a cliff is thus worn slowly backward, and the prostrate blocks that fall from it are reduced to smaller fragments and finally to dust. Where, as in Europe and the settled parts of North America, the contrasts of temperature are not so marked, the same kind of waste takes place in a less striking manner. (2.) Saturation and Desiccation. Another cause of the decay of the exposed surfaces of rocks is to be sought in the alternate soaking of them with rain and drying of them in sunshine, whereby the component particles of the stone are loosened and fall to powder. Some kinds of stone freshly quarried and left to this kind of action are rapidly disintegrated. The rock called shale (see p. 202) is pecu- liarly liable to decay from this cause. The cliffs into which it sometimes rises show at their base long trails of rubbish entirely derived from its waste. (3.) Frost. Athird and familiar source of decay in stone c 1 8 GEOLOGICAL WORK OF THE AIR. [CHAP. exposed to the atmosphere is to be found in the action of Frost. The water that falls from the air upon the surface of the land soaks into the soil and into the pores of rocks. When the temperature of the air falls below the freezing point, the imprisoned moisture expands as it passes into ice, and in expanding pushes aside the particles between which it is entangled. Where this takes place in soil, the pebbles and the grains of sand and earth are separated from each other by the ice that shoots between them. They are all frozen into a solid mass that rings like stone under our feet ; but, as soon as a thaw sets in, the ice that formed the binding cement passes into water which converts the soil into soft earth or mud. This process, repeated winter after winter, breaks up the materials of the soil and enables them to be more easily made use of by plants and more readily blown away by wind or washed off by rain. Where the action of frost affects the surface of a rock, the particles separated from each other are eventually blown or washed away, or the rock peals off in thin crusts or breaks up into angular pieces, which are gradually disintegrated and removed. (4.) Rain. One further cause of decay may be sought in the remarkable power possessed by Rain of chemically corroding stones. In falling through the atmosphere, rain absorbs the gases of the air, and with their aid attacks sur- faces of rock. With the oxygen thus acquired, it oxidises those substances which can still take more of this gas, causing them to rust. As a consequence of this alteration, the cohe- sion of the particles is usually weakened, and the stone crumbles down. With the carbon-dioxide or carbonic acid, it dissolves and removes some of the more soluble ingre- dients in the form of carbonates, thereby also usually loosen- ir.] CAUSES OF WEATHERING. 19 ing the component particles of the stone. In general, the influence of rain is to cause the exposed parts of rocks to rot from the surface inward. Where the ground is protected with vegetation, the decay is no doubt retarded ; but in the absence of vegetation, the outer crust of the decayed layer is apt to be washed off by rain, or when dried to powder may be blown away and scattered by wind. As fast as it is removed from the surface, however, it is renewed under- neath by the continued soaking of rain into the stone. Hence one of the first lessons to be learnt when from the common evidence around us we seek to know what has been the history of the ground on which we live is one of ceaseless decay. All over the land, in all kinds of climates, and from various causes, bare surfaces of soil and rock yield to the influences of the atmosphere or weather. The decay thus set in motion is commonly called " weathering." That it may often be comparatively rapid is familiarly and in- structively shown in buildings or open-air monuments of which the dates are precisely known. Marble tombstones in the graveyards of large towns, for example, hardly keep their inscriptions legible for even so long as a century. Before that time, the surface of the stone has crumbled away into a kind of. sand. Everywhere the weather-eaten surfaces, the crumbling crust of decayed stone, and the scattered blocks and trains of rubbish, tell their tale of universal waste. It is well to take numerous opportunities of observing the process of this decay in different situations and on various kinds of materials. We can thus best realise the important part which weathering must play in the changes of the earth's surface, and we prepare ourselves for the con- sideration of the next question that arises, What becomes 2O GEOLOGICAL WORK OF THE AIR. [CHAP. of all the rotted material? a question to answer which leads us into the very foundations of geological history. . Openings from the soil down into the rock underneath often afford instructive lessons re- garding the decay of the surface of the land. Fig. 2, for instance, is a drawing of one of these sections, in which a gradual passage may be traced from solid sandstone (^under- neath up into broken-up sandstone (b\ and thence into the earthy layer (c) that supports the vegetation of the surface. Traced from below up- wards, the rock is found to become FIG. 2. -Passage of sand- more and more broken and crum . stone upwards into soil. blmg, with an increasing number of rootlets that strike freely through it in all directions, until it passes insensibly into the uppermost dark layer of vegetable soil or humus. This dark layer owes its characteristic brown or black colour to the decaying re- mains of vegetation diffused through it. Again, granite in its unweathered state is a hard, compact, crystalline rock that may be quarried out in large solid blocks (a FIG. 3. -Passage of granite upwards in Fig> ^ yt when traced upward to within a few feet from the surface it may be seen to have been split by in- ii.] SOIL AND SUBSOIL. 21 numerable rents into fragments which are nevertheless still lying in their original position. As these fragments are attacked by percolating moisture, their surfaces decay, leaving the still unweathered parts as rounded blocks (b\ which might at first be mistaken for transported boulders. They are, however, parts of the rock broken up in place, and not fragments that have been carried from a distance. The little quartz veins that traverse the solid granite can be recognised running through the decayed and fresh parts alike. But besides being broken into pieces, the granite rots away and loses its cohesion. Some of the smaller pieces can be crumbled down between the fingers, and this decay increases towards the soil until the rock be- comes a mere sand or sandy clay in which a few harder kernels are still left. Into this soft layer roots may descend from the surface and, like the sandstone, it passes upwards into the overlying soil (c). Soil and Subsoil. In such sections as the foregoing, three distinct layers can be recognised which merge into each other. At the bottom lies the rock, either undecayed or at least still fresh enough to show its true nature. Next comes the broken-up crumbling layer through which stray roots descend. This is known as the subsoil. At the top lies the dark band, crowded with rootlets and forming the true soil. These three layers obviously represent successive stages in the decay of the surface of the land. The soil is the layer of most complete decay. The subsoil is an inter- mediate band where the progress of decomposition has not advanced so far, while the shattered rock underneath shows the earlier stages of disintegration. Vegetation sends its roots and rootlets through the rotted rock. As the plants die, they are succeeded by others, and the rotted remains of 22 GEOLOGICAL WORK OF THE AIR. [CHAP. their successive generations gradually darken the uppermost decomposed layer. Worms, insects, and larger animals that may die on the surface, likewise add their mouldering remains. And thus from animals and plants there is furnished to the soil that organic matter on which its fertility so much depends. The very decay of the vegetation helps to promote that of the underlying rock, for it supplies various organic acids ready to be absorbed by percolating rain-water, the power of which to decompose rocks is thereby increased (p. 32). It is obvious, then, that in answer to the question, What becomes of the rotted material produced by weathering? we may confidently assert that, over surfaces of land protected by a cover of vegetation, this material in large measure accumulates where it is formed. Such accumulation will naturally take place chiefly on flat or gently inclined ground. Where the slope is steep, the decomposed layer will tend to travel down-hill by mere gravitation, and to be further im- pelled downward by descending rain-water. If there is so intimate a connection between the soil at the surface and the rock underneath, we can readily under- stand that soils should vary from one district to another, according to the nature of the underlying rocks. Clays will produce clayey soil, sandstones, sandy soil, or, where these two kinds of rock occur together, they may give rise to sandy clay or loam. Hence, knowing what the underlying rock is, we may usually infer what must be the character of the overlying soil, or, from the nature of the soil, we may form an opinion respecting the quality of the rock that lies below. But it will probably occur to the thoughtful observer that when once a covering of soil and subsoil has been formed over a level piece of ground, especially where there ii.] SOIL AND SUBSOIL. . 23 is also an overlying carpet of verdure, the process of decay should cease the very layer of rotted material coming event- ually to protect the rock from further disintegration. Un- doubtedly, under these circumstances, weathering is reduced to its feeblest condition. But that it still continues will be evident from some considerations, the force of which will be better understood a few pages further on. If the process were wholly arrested, then in course of time plants growing on the surface would extract from the soil all the nutriment they could get out of it, and with the increasing impoverish- ment of the soil, they would dwindle away and finally die out, until perhaps only the simpler forms of vegetation would grow on the site. Something of this kind not im- probably takes place where forests decay and are replaced by scrub and grass. But the long-continued growth of the same kind of plants upon a tract of land doubtless indicates that in some way the process of weathering is not entirely arrested, but that, as generation succeeds generation, the plants are still able to draw nutriment from fresh portions of decomposed rock. A cutting made through the soil and subsoil shows that roots force their way downward into the rock, which splits up and allows percolating water to soak downwards through it. The subsoil thus gradually eats its way into the solid rock below. Influences are at work also, whereby there is an imperceptible removal of material from the surface of the soil. Notable among these influences are rain, wind, and earth-worms. Wherever soil is bare of vegetation it is directly exposed to removal by rain and wind. Ground is seldom so flat that rain may not flow a little way along the surface before sinking underneath. In its flow, it carries off the finer particles of the soil. These may travel each time only a short way, but as the operation 24 GEOLOGICAL WORK OF THE AIR. [CHAI-. is repeated, they are in the course of years gradually moved down to lower ground or to some runnel or brook that sweeps them away seaward. Both on gentle and on steep slopes, this transporting power of rain is continually removing the upper layer of bared soil. Where soil is exposed to the sun, it is liable to be dried into mere dust, which is borne off by wind. How readily this may happen is often strikingly seen after dry weather in spring-time. The earth of ploughed fields becomes loose and powdery, and clouds of its finer particles are carried up into the air and transported to other farms, as gusts of wind sweep across. " March dust," which is a proverbial expression, may be remembered as an illus- tration of one way in which the upper parts of the soil are removed. Even where a grassy covering protects the general sur- face, bare places may always be found whence this covering has been removed. Rabbits, moles, and other animals throw out soil from their burrows. Mice sometimes lay it bare by eating the pasture down to the roots. The common earth- worms bring up to daylight in the course of a year an almost incredible quantity of it in their castings. -Mr. Darwin estimated that this quantity is in some places not less than 10 tons per annum over an acre of ground. Only the finest particles of mould are swallowed by worms and conveyed by them to the surface, and it is precisely these which are most apt to be washed off by rain or to be dried and blown away as dust by the wind. Where it remains on the ground, the soil brought up by worms covers over stones and other objects lying there, which consequently seem to sink into the earth. The operation of these animals causes the materials of the soil to be thoroughly mixed. In tropical countries, the termite or " white ant " conveys a ii.] TALUS-SLOPES. 25 prodigious amount of fine earth up into the open air. With this material it builds hills sometimes 60 feet high and visible for a distance of several miles ; likewise tunnels and chambers, which it plasters all over the stems and branches of trees, often so continuously that hardly any bark can be seen. The fine soil thus exposed is liable to be blown away by the wind or washed off by the fierce tropical rains. Although, therefore, the layer of vegetable soil which covers the land appears to be a permanent protection, it does not really prevent a large amount of material from being removed even from grassy ground. It forms the record of the slow and almost imperceptible geological changes that affect the regions where it accumulates, the quiet fall of rain, the gradual rotting away of the upper part of the rock under- neath, the growth and decay of a long succession of genera- tions of plants, the ceaseless labours of the earth-worm, the scarcely appreciable removal of material from the surface by the action of rain and wind, and the equally insensible descent of the crumbling subsoil farther and farther into the solid stone below. Having learnt how all this is told by the soil beneath our feet, we should be ready to recog- nise in the soil of former ages a similar chronicle of quiet atmospheric disintegration. Talus. Besides soil and subsoil, there are other forms in which decomposed rock accumulates on the surface of the land. Where a large mass of bare rock rises up as a steep bank or cliff, it is liable to constant degradation, and the materials detached from its surface accumulate down the slopes, forming what is known as a Talus (Fig. 4). In mountainous or hilly regions, where rocky precipices rise high into the air, there gather at their feet and down their clefts long trails or screes of loose blocks split off from them 26 GEOLOGICAL WORK OF THE AIR. [CHAP. by the weather. Such slopes, especially where they are not too steep, and where the rubbish that forms them is not too FIG. 4. Talus-slopes at the foot of a line of cliffs. coarse, may be more or less covered with vegetation, which in some measure arrests the descent of the debris. But from time to time, during heavy rains, deep gullies are torn out of them by rapidly formed torrents, which sweep down their materials to lower levels (Fig. 10). The sections laid bare in these gullies show that the rubbish is arranged in more or less distinct layers which lie generally parallel with the surface of the slope ; in other words, it is rudely stratified, and its *- layers or strata are inclined at the angle FIG. 5. -Section of rain- o f the declivity which seldom exceeds wash or brick-earth. o 7. Vegetable soil. 6. 35 ' Brick-earth. 5. White Rain-wash, Brick-earth. On sand. 4. Brick-earth, more gentle slopes, even where no bare 3. White sand. 2. rock pro j ects i nto the air the fall of rain Brick-earth, i. Gravel with seams of sand. gradually washes down the upper parts of the soil to lower levels. Hence arise thick accumulations of what is known as rain-wash soil ii.] DUST AND SAND-DUNES. 27 mixed often with angular fragments of still undecomposed rock, and not infrequently forming a kind of brick-earth (Fig. 5). Deposits of this nature are still gathering now, though their lower portions may be of great antiquity. In the south-east of England, for instance, the brick-earths con- tain the bones of animals that have long since passed away. Dust. By the action of wind, above referred to, a vast amount of fine dust and sand is carried up into the air and strewn far and wide over the land. In dry countries, such as large tracts of Central Asia, the air is often thick with a fine yellow dust which may entirely obscure the sun at mid-day, and which settles over everything. After many centuries, a thick deposit is thus accumulated on the surface of the land. Some of the ancient cities of the Old World, Nineveh and Babylon for example, after being long abandoned by man, have gradually been buried under the fine soil drifted over them by the wind and intercepted and protected by the weeds that grew up over the ruins. Even in regions where there is a large annual rainfall, seasons of drought occur, during which there may be a considerable drifting of the finer particles of soil by the wind. We probably hardly realise how much the soil is in some regions removed and in others heightened from this cause. Sand-dunes. Some of the most striking and familiar examples of the accumulation of loose deposits by the wind are those to which the name of Dunes is given. On sandy shores, exposed to winds that blow landwards, the sand is dried and then carried away from the beach, gathering into long mounds or ridges which run parallel to the coast-line. These ridges are often 50 or 60 feet, sometimes even more than 250 feet high, with deep troughs and irregular circular hollows between them, and they occasionally form a strip 28 GEOLOGICAL WORK OF THE AIR. [CHAr. several miles broad, bordering the sea. The particles of sand are driven inland by the wind, and the dunes gradually bury fields, roads, and villages, unless their progress is arrested by the growth of vegetation over their shifting surfaces. On many parts of the west coast of Europe, the dunes are marching into the interior at the rate of 20 feet in a year. Hence large tracts of land have within historic FIG. 6. Sand-dunes. times been entirely lost under them. In the north of Scot- land, for example, an ancient and extensive barony, so noted for its fertility that it was called "the granary of Moray," was devastated about the middle of the seventeenth century by the moving sands, which now rise in barren ridges more than 100 feet above the site of the buried land. In the interior of continents also, where with great dryness of climate there is a continual disintegration of the surface of rocks, wide wastes of sand accumulate, as in the deserts of Lybia and Arabia and in the heart of Australia. ii.J DECAY OF THE SURFACE OF THE LAND. 29 There can be no doubt, however, that though in the layer of vegetable soil, in the heaps of rubbish that gather on slopes and at the base of rocky banks and precipices, and in the widespread drifting of dust and sand over the land by the action of the wind, we have evidence that much of the material arising from the general decay of the surface of the land accumulates under various forms upon that surface, nevertheless its stay there is not permanent. Wind and rain are continually removing it, sometimes in vast quantities, into the sea. Every brook, made muddy by heavy rain, is an example of this transport, for the mud that discolours the water is simply the finer portions of the soil washed off by rain. When we reflect upon the multitude of streams, large and small, in all parts of the globe, and consider that they are all busy carrying their freights of mud to the sea, we can in some measure appreciate how great must be the total annual amount of material so re- moved. What becomes of this material will form the subject of the succeeding chapters. Summary. The first lesson to be learnt from an exam- ination of the surface of the land is, that everywhere decay is in progress upon it. Wherever the solid rock rises into the air, it breaks up and crumbles away under the various influences combined in the process of Weathering. The wasted materials caused by this universal disintegration partly accumulate where they are formed, and make soil. But in large measure, also, they are blown away by wind and washed off by rain. Even where they appear to be securely protected by a covering of vegetation, the common earth- worm brings the fine parts of them up to the surface, where they come within reach of rain and wind, so that on tracts permanently grassed over, there may be a continuous and 30 GEOLOGICAL WORK OF THE AIR. [CHAP. IT. not inconsiderable removal of fine soil from the surface. As the upper layers of soil are removed, roots and percolat- ing water are enabled to reach down farther into the solid rock which is broken up into subsoil, and thus the general surface of the land is insensibly lowered. Besides accumulating in situ as subsoil and soil, the debris of decomposed rock forms talus-slopes and screes at the foot of crags, and a layer of rain-wash or brick-earth over gentler slopes. Where the action of wind comes markedly into play, tracts of sand-dunes may be piled up along the borders of the sea and of lakes, or in the arid in- terior of continents ; and wide regions are in course of time buried under the fine dust which is sometimes so thick in the air as to obscure the noonday sun. But in none of these forms can the accumulation of decomposed material be regarded as permanent. So long as it is exposed to the influences of the atmosphere, this material is liable to be swept away from the surface of the land and borne outwards into the sea. CHAPTER III. THE INFLUENCE OF RUNNING WATER IN GEOLOGICAL CHANGES, AND HOW IT IS RECORDED. IT appears, then, that from various causes all over the globe, there is a continual decay of the surface of the land ; that the decomposed material partly accumulates as soil, subsoil, and sheets or heaps of loose earth or sand, but that much of it is washed off the land by rain or blown into the rivers or into the sea by wind. We have now to consider the part taken by running water in this transport. From the single rain-drop up to the mighty river, every portion of the water that flows over the land is busy with its own share of the work. When we reflect on the amount of rain that falls annually over the land, and on the number of streams, large and small, that are ceaselessly at work, we realise how difficult it must be to form any fit notion of the entire amount of change which, even in a single year, these agents work upon the surface of the earth. The influence of rain in the decay of the surface of the land was briefly alluded to in the last chapter. As soon as a drop of rain reaches the ground, it begins its appointed geological task, dissolving what it can carry off in solution, and pushing forward and downward whatever it has power 32 RECORDS OF RUNNING WATER. [CHAP. to move. As the rain-drops gather into runnels, the same duty, but on a larger scale, is performed by them ; and as the runnels unite into large streams, and these into yet mightier rivers, the operations, though becoming colossal in magnitude, remain essentially the same in kind. In the operations of the nearest brook, we see before us in minia- ture a sample of the changes produced by the thousands of rivers which, in all quarters of the globe, are flowing from the mountains to the sea. Watching these operations from day to day, we discover that they may all be classed under two heads. In the first place, the brook hollows out the channel in which it flows and thus aids in the general waste of the surface of the land ; and in the second place, it carries away fine silt and other material resulting from that waste. Rivers are thus at once agents that themselves directly de- grade the land, and that sweep the loosened detritus towards the ocean. An acquaintance with each of these kinds of work is needful to enable us to understand the nature of the records which river-action leaves behind it. Chemical Action of Running Water. We have seen that rain in its descent from the clouds absorbs air, and that with the oxygen and carbonic acid which it thus obtains it proceeds to corrode the surfaces of rock on which it falls. When it reaches the ground and absorbs the acids termed "humous," which are supplied by the decomposing vegeta- tion of the soil, it acquires increased power of eating into the stones over which it flows. When it rolls along as a runnel, brook, or river, it no doubt still attacks the rocks of its channel, though its action in this respect is not so easily detected. In some circumstances, however, the solvent influence of river-water upon solid rocks is strikingly displayed. Where the water contains a large proportion of in.] CHEMICAL ACTION OF RUNNING WATER. 33 the acids of the soil, and flows over a kind of rock specially liable to be eaten away by these acids, the most favourable conditions are presented for observing the change. Thus, a stream which issues from a peat-bog is usually dark brown in colour, from the vegetable solutions which it extracts from the moss. Among these solutions are some^of the organic acids referred to, ready to eat into the surface of the rocks or loose stones which the stream may encounter in its descent. No kind of rock is more liable than limestone FIG. 7. Erosion of limestone by the solvent action of a peaty stream. Durness, Sutherlandshire. to corrosion under such circumstances. Peaty water flowing over it eats it away with comparative rapidity, while those portions of the rock that rise above the stream escape solu- tion, except in so far as they are attacked by rain. Hence arise some curious features in the scenery of the water- course. The walls of limestone above the water, being attacked only by the atmosphere, are not eaten away so fast as their base, over which the stream flows. They are con- D 34 RECORDS OF RUNNING WATER. [CHAP. sequently undermined, and are sometimes cut into dark tunnels and passages (Fig. 7). Even where the solvent action of the water of rivers is otherwise inappreciable, it can be detected by means of chemical analysis. Thus rivers, partly by the action of their water upon the loose stones and solid rocks of their channels, and partly by the contributions they receive from springs (which will be afterwards described), convey a vast amount of dissolved material into the sea. The substance thus invisibly transported consists of various mineral salts. One of the most abundant of these carbonate of lime is the substance that forms limestone, and furnishes the mineral matter required for the hard parts of a large proportion of the lower animals. It is a matter of some interest to know that this substance, so indispensable for the formation of the shells of so large a number of sea- creatures, is constantly supplied to the. sea by the streams that flow into it. The rivers of Western Europe, for instance, have been ascertained to convey about i part of dissolved mineral matter in every 5000 parts of water, and of this mineral matter about a half consists of carbonate of lime. It has been estimated that the Rhine bears enough carbonate of lime into the sea every year to make three hundred and thirty-two thousand millions of oysters of the usual size. Another abundant ingredient of river-water is gypsum or sulphate of lime, of which the Thames is computed to carry annually past London not less than 180,000 tons. The total quantity of carbonate of lime, removed from the limestones of its basin by this river in a year, amounts, on an average, to 140 tons from every square mile, which is estimated to be equal to the lowering of the general surface to the extent of T y- of an inch from each square mile in a century, or one foot in 13,200 years. in.] MECHANICAL ACTION OF RUNNING WATER. 35 Mechanical Action of Running Water Trans- port. The dissolved material forms but a small proportion of the total amount of mineral substances conveyed by rivers from land to sea. A single shower of rain washes off fine dust and soil from the surface of the ground into the nearest brook which, though previously clear, now rolls along with a discoloured current. An increase in the volume of the water enables a stream to sweep along sand, gravel, and blocks of stone lying in its channel, and to keep these materials moving until, as the declivity lessens and the rain ceases, the current becomes too feeble to do more than lazily carry onward the fine silt that discolours it. Every stream, large or small, is ceaselessly busy transporting mud, sand, or gravel. And as the ultimate destination of all this sediment is the bottom of the sea, it is evident that if there are no compensating influences at work to repair its losses, the land must in the end be all worn away. Some of the most instructive lessons regarding the work of running water on land are afforded by the beds of moun- tain-torrents. Huge blocks, detached from the crags and cliffs on either side, may there be seen cumbering the path- way of the water, which seems quite powerless to move such masses and can only sweep round them or find a passage beneath them. But follow one of these torrents in its descent, and you will find that by degrees the blocks, losing their sharp edges, become rounded boulders, and pass first into coarse shingle and then into fine gravel. In the quieter reaches of the water, sheets of sand begin to make their appearance, and at last when the stream reaches the plains, no sediment of coarser grain than mere silt may be seen in its channel. In the constant transport maintained by watercourses, the transported materials, by being tossed 36 RECORDS OF RUNNING WATER. [CHAP. along rocky channels and continually ground against each other, diminish in size as they descend. A river flowing from a range of mountains to the distant ocean may be likened to a natural mill, into which large angular masses of rock are cast at the upper end, and out of which only fine sand and silt are discharged at the lower. Partly, therefore, owing to the fine dust and soil swept into them by wind and rain from the slowly decomposing surface of the land, and partly to the friction of the detritus which they sweep along their channels, rivers always con- tain more or less mineral matter suspended in their water or travelling with the current on the bottom. The amount of material thus transported varies greatly in different rivers, and at successive seasons even in the same river. In some cases, the rain is spread so equably through the year that the rivers flow onward with a quiet monotony, never rising much above or sinking below their average level. In such circum- stances, the amount of sediment they carry downward is proportionately small. On the other hand, where either from heavy periodical rains or from rapid melting of snow, rivers are liable to floods, they acquire an enormously increased power of transport, and their burden of sediment is proportionately augmented. In a few days or weeks of high water, they may convey to the sea a hundredfold the amount of mineral matter which they could carry in a whole year of their quieter mood. Measurements have been made of the proportions of sediment in the waters of different rivers at various seasons of the year. The results as might be expected show great variations. Thus the Garonne, rising among the higher peaks of the Pyrenees, drains a large area of the south of France, and is subject to floods by which an enormous in.] TRANSPORT OF DETRITUS. 37 quantity of sediment is swept down from the mountains to the plains. Its proportion of mud has been estimated to be as much as i part in 100 parts of water. The Durance, which takes its source high on the western flank of the Cottian Alps, is one of the rapidest and muddiest rivers in Europe. Its angle of slope varies from i in 208 to i in 467, the declivity of the great rivers of the globe being probably not more than i in 2600, while that of a navigable stream ought not to exceed 10 inches per mile or i in 6336. The Durance is, therefore, rather a torrent than a river. With this rapidity of descent is conjoined an excessive capacity for transporting sediment. In floods of exceptional severity, the proportion of mud in the stream has been estimated at one-tenth by weight of the water, while the average propor- tion for nine years from 1867 to 1875 was about -gi . Prob- ably the best general average is to be obtained from a river which drains a wide region exhibiting considerable diversi- ties of climate, topography, rocks, and soils. The Missis- sippi presents a good illustration of these diversities, and has accordingly been taken as a kind of typical river, furnishing, so to speak, a standard by which the operations of other rivers may be compared and which may perhaps be assumed as a fair average for all the rivers of the globe. Numerous measurements have been made of the proportion of sediment carried into the Gulf of Mexico by this vast river, with the result of showing that the average amount of sediment is by weight i part in every 1500 parts of water, or little more than one-third of the proportion in the water of the Durance. If now we assume that, all over the world, the general average proportion of sediment floating in the water of rivers is i part in every 1500 of water, we can readily 38' RECORDS OF RUNNING WATER. [CHAP. understand how seriously in the course of time must the land be lowered by the constant removal of so much de- composed rock from its surface. Knowing the area of the basin drained by a river, and also the proportion of sediment in its water, we can easily calculate the general loss from the surface of the basin. The ratio of the weight or " specific gravity " of the silt to that of solid rock may be taken to be as 19 is to 25. Accordingly the Mississippi conveys annually from its drainage basin an amount of sediment equivalent to the removal of g-^-Q- part of a foot of rock from the general surface of the basin. At this rate, one foot of rock will be worn away every 6000 years. If we take the general height of the land of the whole globe to be 2120 feet, and suppose it to be continu- ously wasted at the same rate at which the Mississippi basin is now suffering, then the whole dry land would be carried into the sea in 12,720,000 years. Or if we assume the mean height of Europe to be 973 feet and that this continent is degraded at the Mississippi rate of waste until the last vestige of it disappears, the process of destruction would be completed in rather less than 6,000,000 years. Such estimates are not intended to be close approximations to the truth. As the land is lowered, the rate of decay will gradually diminish, so that the later stages of decay will be enormously protracted. But by taking the rate of operation now ascertained to be in progress in such a river basin as the Mississippi, we obtain a valuable standard of com- parison, and learn that the degradation of the land is much greater and more rapid than might have been supposed. Erosion of Watercourses. But rivers are not merely carriers of the mud, sand, and gravel swept into their channels by other agencies. By keeping these materials in.] EROSION OF WATER-CHANNELS. 39 in motion, the currents reduce them in size, and at the same time employ them to hollow out the channels wherein they move. The mutual friction that grinds down large blocks of rock into mere sand and mud, tells also upon the rocky beds along which the material is driven. The most solid rocks are worn down ; deep long gorges are dug out, and the watercourses, when they have once chosen their sites, remain on them and sink them gradually deeper and deeper beneath the general level of the country. The surfaces of stone exposed to this attrition assume the familiar smoothed and rounded appearance which is known as water-worn. The loose stones lying in the channel of a stream, and the solid rocks as high up as floods can scour them, present this characteristic aspect. Here and there, where a few stones have been caught in an eddy of the current and are kept in constant gyration, they reduce each other in dimensions, and at the same time grind out a hollow in the underlying rock. The sand and mud produced by the friction are swept off by the current, and the stones when sufficiently reduced in size are also carried away. But their places are eventually taken by other blocks brought down by floods, so that the supply of grinding material is kept up until the original hollow is enlarged into a wide deep caldron, at the bottom of which the stones can only be stirred by the heaviest floods. Cavities of this kind, known as pot-holes, are of frequent occurrence in rocky watercourses as well as on rocky shores, in short, wherever eddies of water can keep shingle rotating upon solid rock. As they often coalesce by the wearing away of the intervening channel, they greatly aid in the deepening of a watercourse. In most rocky gorges, a suc- cession of old pot-holes may be traced far above the present level of the stream (Fig. 8). 40 RECORDS OF RUNNING WATER. [CHAP. That it is by means of the gravel and other detritus pushed along the bottom by the current, rather than by the mere friction of the water on its bed, that a river excavates its channel, is most strikingly shown immediately below a lake. In traversing a lake, the tributary streams are filtered. .... - 1 ';."'..:. ,"~ FIG. 8. Pot-holes worn out by the gyration of stones in the bed of a stream. Depositing their sediment on the floor of the lake, they unite in the clear transparent river which escapes at the lower end. The Rhone, for instance, flows into the Lake of Geneva as a turbid river ; it issues from that great reser- voir at Geneva as a rushing current of the bluest, most translucent water which, though it sweeps over ledges of in.] MEANDERINGS OF STREAMS. 4 1 rock, has not yet been able to grind them down into a deep gorge. The Niagara, also, filtered by Lake Erie, has not acquired sediment enough to enable it to cut deeply into the rocks over which it foams in its rapids before throwing itself over the great Falls. One of the most characteristic features of streams is the singularly sinuous courses they follow. As a rule, too, the flatter the ground over which they flow, the more do they wind. Not uncommonly they form loops, the nearest bends of which in the end unite, and as the current passes along the now straightened channel, the old one is left to become by degrees a lake or pond of stagnant water, then a marsh, and lastly, dry ground. We might suppose that in flowing off the land, water would take the shortest and most direct road to the sea. But this is far from being the case. The slightest inequalities of level have originally determined sinuosities of the channels, and trifling differences in the hardness of the banks, in the accumulation of sediment, and in the direction of the currents and eddies have been enough to turn a stream now to one side now to another, until it has assumed its present meandering course. How easily this may be done can be instructively observed on a roadway or other bare surface of ground. Seen when quite dry and smooth, hardly any depressions in which water would flow might be detected on such a surface. But after a heavy shower of rain, runnels of muddy water will be seen coursing down the slope in serpentine channels that at once recall the winding rivers of a great drainage-system. The slightest differences of level have been enough to turn the water from side to side. A mere pebble or projecting heap of earth or tuft of grass has sufficed to cause a bend. The water, though always descending, has only been able to reach 42 RECORDS OF RUNNING WATER. [CHAP. the bottom by keeping the lowest levels, and turning from right to left as these guided it. When a river has once taken its course and has begun to excavate its channel, only some great disturbance, such as an earthquake or volcanic eruption, can turn it out of that course. If its original pathway has been a winding one, it goes on digging out its bed which, with all its bends, gradu- ally sinks below the level of the surrounding country. The deep and picturesque gorge in which the Moselle winds from Treves to Coblenz has in this way been slowly eroded out of the undulating tableland across which the river originally flowed. In another and most characteristic way, the shape of the ground and the nature and arrangement of the rocks over which they flow, materially influence rivers in the forms into which they carve their channels. The Rhone and the Niagara, for instance, though filtered by the lakes through which they flow, do not run far before plunging into deep ravines. Obviously such ravines cannot have been dug out by the same process of mechanical attrition whereby river- channels in general are eroded. Yet the frequency of gorges in river scenery shows that they cannot be due to any ex- ceptional operation. They may generally be accounted for by some arrangement of rocks wherein a bed of harder material is underlain by one more easily removable. Where a stream, after flowing over the upper bed, encounters the decomposable bed below, it eats away the latter more rapidly. The overlying hard rock is thus undermined, and, as its support is destroyed, slice after slice is cut away from it. The waterfall which this kind of structure produces continues to eat its way backward or up the course of the stream, so long as the necessary conditions are maintained in.] WATERFALLS AND GORGES. 43 of hard rocks lying upon soft. Any change of structure which would bring the hard rocks down to the bed of the channel, and remove the soft rocks from the action of the current and the dash of the spray would gradually destroy the waterfall. It is obvious that, by cutting its way back- ward, a waterfall excavates a ravine. The renowned Falls of Niagara supply a striking illustra- tion of the process now described. The vast body of water which issues from Lake Erie, after flowing through a level country for a few miles, rushes down its rapids and then plunges over a precipice of solid limestone. Beneath this hard rock lies a band of comparatively easily eroded shale. As the water loosens and removes the lower rock, large portions of the face of the precipice behind the Falls are from time to time precipitated into the boiling flood below. The waterfall is thus slowly prolonging the ravine below the Falls. The magnificent gorge in which the Niagara, after its tumultuous descent, flows sullenly to Lake Ontario is not less than 7 miles long, from 200 to 400 yards wide, and from 200 to 300 feet deep. There is no reason to doubt that this chasm has been entirely dug out by the gradual reces- sion of the Falls from the cliffs at Queenstown, over which the river at first poured. We may form some conception of the amount of rock thus removed from the estimate that it would make a rampart about 12 feet high and 6 feet thick, extending right round the whole globe at the equator. Still more gigantic are the gorges or canons of the Colorado and its tributaries in Western America. The Grand Canon of the Colorado is 300 miles long, and in some places more than 6000 feet deep (Fig. 9). The country traversed by it is a network of profound ravines, at the bottom of which the streams flow that have eroded them out of the table-land. 44 RECORDS OF RUNNING WATER. [CHAP. in.] PERMANENT RECORDS OF RIVER-ACTION. 45 Permanent Records of River-Action. If, then, all the streams on the surface of the globe are engaged in the double task of digging out their channels and carrying away the loose materials that arise from the decomposition of the surface of the land, let us ask ourselves what memorials of these operations they leave behind them. In what form do the running waters of the land inscribe their annals in geological history ? If these waters could suddenly be dried up all over the earth, how could we tell what changes they had once worked upon the surface of the land ? Can we detect the traces of ancient rivers where there are no rivers now? From what has been said in this lesson it will be evident that in answer to such questions as these, we may affirm that one unmistakable evidence of the former presence of rivers is to be found in the channels which they have eroded. The gorges, rocky defiles, pot-holes, and water-worn rocks which mark the pathway of a stream would long remain as striking memorials of the work of running water. In districts, now dry and barren, such as large regions in the Levant, there are abundant channels (wadies) now seldom or never occu- pied by a stream, but which were evidently at one time the beds of active torrents. But more universal testimony to the work of running water is to be found in the deposits or alluvium which it has accumulated. Spreading out on either side, sometimes far beyond the limits of the ordinary or modern channels, these deposits, even when worn into fragmentary patches, retain their clear record of the operations of the river. Let us in imagination follow the course of a river from the mountains to the sea, and mark as we go the circumstances under which the accumulation of sediment takes place. 46 RECORDS OF RUNNING WATER. [CHAP. The power possessed by running water to carry forward sediment depends mainly upon the velocity of the current. The more rapidly a stream flows, the more sediment can it transport, and the larger are the blocks which it can move. The velocity is regulated chiefly by the angle of slope ; the greater the declivity, the higher the velocity and the larger the capacity of the stream to carry down debris. Any cause, therefore, which lessens the velocity of a current diminishes its carrying power, and if the water is bearing along gravel, sand, or mud, some of these materials will begin to drop and remain at rest on the bottom. In the course of every stream, various conditions arise whereby the velocity of the current is reduced. One of the most obvious of these is a diminu- tion in the slope of the channel. Another is the union of a rapid tributary with a more gently flowing stream. A third is the junction of a stream with the still waters of a lake or with the sea. In these circumstances, the flow of the water being checked, the sediment at once begins to fall to the bottom. Tracing now the progress of a river, for illustrations of this law of deposition, we find that among the mountains where the river takes its rise, the torrents that rush down the declivities have torn out of them such vast quantities of soil and rock as to seam them with deep clefts and gullies. Where each of these rapid streamlets reaches the valley below, its rapidity of motion is at once lessened, and with this slackening of speed and consequent loss of carrying power, there is an accompanying deposit of detritus. Blocks of rock, angular rubbish, rounded shingle, sand, and earth are thrown down in the form of a cone of which the apex starts from the bottom of the gully and the base spreads out over the plain (Fig. 10). Such cones vary in dimensions in.] FANS OF ALLUVIUM. 47 according to the size of the torrent and the comparative ease with which the rocks of the mountain-side can be loosened and removed. Some of them, thrown down by the transient runnels of the last sudden rain-storm, may not be more than FIG. 10. Gullies torn out of the side of a mountain by descending torrents, with cones of detritus at their base. a few cubic yards in bulk. But on the skirts of moun- tainous regions they may grow into masses hundreds of feet thick and many miles in diameter. The valleys in a range of mountains afford many striking examples of these alluvial cones or fans, as they are called. Where the tributary 48 RECORDS OF RUNNING WATER. [CHAP. torrents are numerous, a succession of such cones or fans, nearly or quite touching each other, spreads over the floor of a valley. From this cause, so large an amount of detritus has within historic times been swept down into some of the valleys of the Tyrol that churches and other buildings are now half-buried in the accumulation. Looking more closely at the materials brought down by the torrents, we find them arranged in rude irregular layers, sloping downwards into the plain, the coarsest and most angular detritus lying nearest to the mountains, while more rounded and water-worn shingle or sand extends to the outer margin of the cone. This grouping of irregular layers of angular and half-rounded detritus is most characteristic of the action of torrents. Hence, where it occurs, even though no water may run there at the present day, it may be re- garded as indicating that at some former time a torrent swept down detritus over that site. Quitting the more abrupt declivities, and augmented by numerous tributaries from either side, the stream whose course we are tracing loses the character of a torrent and assumes that of a river. It still flows with velocity enough to carry along not only mud and sand but even somewhat coarse gravel. The large angular blocks of the torrent part of its course, however, are no longer to be seen, and all the detritus becomes more and more rounded and smoothed as we follow it towards the plains. At many places, deposits of gravel or sand take place, more especially at the inner side of the curves which the stream makes as it winds down the valley. Sweeping with a more rapid flow round the outer side of the curve, the current lingers in eddies on the inner side and drops there a quantity of sediment. When the water is low, these strips of sand and shingle on the in.] ARRANGEMENT OF RIVER GRAVEL. 49 concave side of each curve of the river form a distinctive feature in the scenery. It is interesting to walk along one of these strips and to mark how the current has left its record there. The stones are well smoothed and rounded, showing that they have been rolled against each other along the bottom of the channel for a sufficient distance to lose their original sharp edges and to pass from the state of rough angular detritus into that of thoroughly water-worn gravel. Further, they will be found not to lie entirely at random, as might at first sight be imagined. A little examination will show that, where the stones are oblong, they are gener- ally placed with their longer axis pointing across the stream. This would naturally be the position which they would assume where the current kept rolling them forward along the channel. Those which are flat in shape will be observed usually to slope up stream. That the sloping face must look in the direction from which the current moves will be evident from Fig. n, where a current, moving in the direc- tion of the arrow and gradually di- minishing in force, FlG> x j.Flat stones in a bank of river-shingle, would no longer be showing the direction of the current (indi- able to overturn the cated ^ the arrow ) that trans P orted and left them, stones which it had so placed as to offer the least obstacle to its passage. Had the current flowed from the opposite quarter, it would have found the upturned edges of the stones exposed to it, and would have readily overturned them until they found a position in which they again presented least resistance to the water. In a section of gravel, it is thus often quite possible to tell from what quarter the current flowed that deposited the pebbles. 5 RECORDS OF RUNNING WATER. [CHAP. Yet another feature in the arrangement of the materials deserves attention. It is well seen where a digging has been made in one of the alluvial banks, but better still in a section of one of the terraces to be immediately referred to. The layers of gravel or sand in some bands may be observed to be inclined at a steeper angle than in others, as shown in the accompanying figure (Fig. 12). In such cases, it will be noticed that the slope of the more inclined layers is down the stream, and hence that their direc- tion gives a clue to that of the current which ar- ranged them. We may even catch similar layers in the act of deposition among shallow pools in to which currents are discharging sediment. The gravel or sand may be observed moving along the bottom, and then falling over the edge of a bank into the bottom of the pool. As the sediment advances by succes- sive additions to its steep slope in front, it gradually fills the pool up. Its progress may be compared to that of a railway embankment formed by the discharge of waggon -loads of rubbish down its end. A section through such an embank- ment would reveal a series of bands of variously coloured materials inclined steeply towards the direction in which the waggon-loads were thrown down. Yet the top of the em- bankment may be kept quite level for the permanent way. The nearly level bands (, c) in Fig 1 2 represent the general bottom on which the sediment accumulated, while the steeper FIG. 12. Section of alluvium showing direction of currents. in.] RIVER-TERRACES. 51 lines in the lower gravel (a) point to the existence and direc- tion of the currents by which sediment was pushed forward along that bottom. (Compare pp. 230, 231.) As the river flows onward through a gradually expanding valley, another characteristic feature becomes prominent. Flanking each side of the flat land through which the stream pursues its winding course, there runs a steep slope or bank a few feet or yards in height, terminating above in a second or higher plain, which again may be bordered with another similar bank, above which there may lie a third plain. These slopes and plains form a group of terraces, rising step by step above and away from the river, sometimes to a height of several hundred feet, and occasionally to the number of 6 or 8 or even more (Fig. 13). Here and there, FIG. 13. River-terraces. by the narrowing of the intervening strip of plain, two terraces merge into one, and at some places the river in winding down the valley has cut away great slices from the terraces, perhaps even entirely removing them and eating back into the rock out of which the valley has been exca- 52 RECORDS OF RUNNING WATER. [CHAP. vated. Sections are thus exposed showing a succession of gravels, sands, and loams like those of the present river. From the line of the uppermost terrace down to the spits of shingle now forming in the channel, we have evidently a series of river-deposits. But how could the river have flowed at the level of these high gravels, so far above its present limits? An examination of the behaviour of the stream during floods will help towards an answer to this question. When from heavy rains or melted snows the river over- flows its banks, it spreads out over the level ground on either side. The tract liable to be thus submerged during inun- dations is called the flood-plain. As the river rises in flood, it becomes more and more turbid from the quantity of mud and silt poured into it by its tributaries on either side. Its increase in volume likewise augments its velocity and con- sequently its power of scouring its bed and of transporting the coarser detritus resting there. Large quantities of shingle may thus be swept out of the ordinary channel and be strewn across the nearer parts of the flood-plain. As the current spreads over this plain, its velocity and transporting capacity diminish, and consequently sediment begins to be thrown down. Grass, bushes, and trees, standing on the flood-plain, filter some of the sediment out of the water. Fine mud and sand, for instance, adhere to the leaves and stems, whence they are eventually washed off by rain into the soil underneath. In this way, the flood-plain is gradually heightened by the river itself. At the same time, the bed of the river is deepened by the scour of the current, until, in the end, even the highest floods are no longer able to inundate the flood-plain. The difference of level between that plain and the surface of the river gradually increases ; by degrees the river begins to cut away the edges of the terrace which it in.] RIVER-TERRACES. 53 cannot now overflow and to form a new flood -plain at a lower level. In this manner, it slowly lowers its bed, and leaves on either side a set of alluvial terraces to mark suc- cessive stages in the process of excavation. If during this process the level of the land should be raised, the slope of the rivers, and consequently their scour, would be aug- mented, and they would thereby acquire greater capacity for the formation of terraces. There is reason to believe that this has taken place both in Europe and North America. It is obvious that the highest terraces must be the oldest, and that the series is progressively younger down to the terrace that is being formed at the present time. Yet, in the materials comprising any one terrace, those lying at the top must be the youngest. This apparent contradiction arises from the double action of the river in eroding its bed and depositing its sediment. If there were no lowering of the channel, then the deposits would follow the usual order of sequence, the oldest being below and the youngest above. This order is maintained in the constituents of each single terrace, for the lowermost layers of gravel must evidently have been accumulated before the deposit of those that overlie them. But when the level of the water is lowered, the next set of deposits must, though younger, lie at a lower level than those that preceded them. In no case, however, will the older beds be found really to overlie the younger. They have been formed at different levels. The gravel, sand, and loam laid down by a river are marked by an arrangement in layers or beds lying one upon another. This stratified disposition indeed is characteristic of all sedimentary accumulations, and is best developed where currents have been most active in transporting and assorting the materials (p. 227). It is the feature that first 54 RECORDS OF RUNNING WATER. [CHAP. catches the eye in any river -bank, where a section of the older deposits or alluvium is exposed. Beds of coarser and finer detritus alternate with each other, but the coarsest are generally to be observed below and the finest above. The deltas accumulated by rivers in lakes and in the sea will be noticed in chapters iv. and vii. But besides the inorganic detritus carried down by a river, we have also to consider the fate of the remains of plants and the carcases of animals that are swept down the streams, especially during floods. Swollen by sudden and heavy rains, a river will rise above its ordinary level and uproot trees and shrubs. On such occasions, too, moles and rabbits are drowned and buried in their burrows on the alluvial flood-plain. Birds, insects, and even some of the larger mammals are from time to time drowned and swept away by floods and buried in the sediment, and their remains, where of a durable kind or where sufficiently covered over, may be preserved for an indefinite period. The shells and fishes living in the river itself may also be killed during the flood and may be entombed with the other organisms in the sediment. Summary. The material produced by the universal decay of the surface of the land is washed off by rain and swept seawards by brooks and rivers. The rate at which the general level of the land is being lowered by the opera- tion of running water may be approximately ascertained by measuring or estimating the amount of mineral matter carried seaward every year from a definite region, such as a river- basin. Taking merely the matter in mechanical suspension, and assuming that the proportion of it transported annually in the water of the Mississippi may be regarded as an average proportion for the rivers of Europe, we find that this con- TIT.] SUMMARY. 55 tinent, at the Mississippi rate of degradation, might be re- duced to the sea-level in rather less than 6,000,000 years. In pursuing their course over the land, running waters gradually deepen and widen the channels in which they flow, partly by chemically dissolving the rocks and partly by rubbing them down by the friction of the transported sand, gravel, and stones. When they have once chosen their channels, they usually keep to them, and the sinuous windings, at first determined by trifling inequalities on the surface of country across which the streams began to flow, are gradually deepened into picturesque gorges. In the excavation of such ravines, waterfalls play an important part by gradually receding up stream. River -channels, especially if cut deeply into the solid rock, remain as endur- ing monuments of the work of running water. But still more important as geological records, because more frequent and covering a larger area, are the deposits which rivers leave as their memorials. Whatever checks the velocity of a current weakens its transporting power, and causes it to drop some of its sediment to the bottom. Accordingly, accumulations of sediment occur at the foot of torrent slopes, along the lower and more level ground, especially on the inner or concave side of the loops, over the flood-plains, and finally in the deltas formed where rivers enter lakes or the sea. In these various situations, thick stratified beds of silt, sand, and gravel may be formed, enclosing the remains of the plants and animals living on the land at the time. As a river deepens its channel, it leaves on either side alluvial terraces that mark successive flood-plains over which it has flowed. CHAPTER IV. THE MEMORIALS LEFT BY LAKES. ACCORDING to the law stated in last chapter, that when water is checked in its flow, it must drop some of its sedi- ment, lakes are pre-eminently places for the deposition and accumulation of mineral matter. In their quiet depths, the debris worn away from the surface of the land is filtered out of the water and allowed to gather undisturbed upon the bottom. The tributary streams may enter a large lake swollen and muddy, but the escaping river is transparent. It is evident, therefore, that lakes must be continually silt- ing up, and that when this process is complete, the site of a lake will be occupied by a series of deposits comprising a record of how the water was made to disappear. To those who know the aspect of lakes only in fine weather, they may seem places where geological operations are at their very minimum of activity. The placid surface of the water ripples upon beaches of gravel or spits of sand ; reeds and marshy plants grow out into the shallows; the few streamlets that tumble down from the surrounding hills furnish perhaps the only sounds that break the stillness, but their music and motion are at once hushed when they lose themselves in the lake. The scene might serve as a very CHAP, iv.] FILLING UP OF LAKES. 57 emblem of perfectly undisturbed conditions of repose. But come back to this same scene during an autumn storm, when the mists have gathered all round the hills, and the rain, after pouring down for hours, has turned every gully into the track of a roaring torrent. Each tributary brook, hardly visible perhaps in summer, now rushes foaming and muddy from its dell and sweeps out into the lake. The larger streams bear along on their swift brown currents trunks of trees, leaves, twigs, with now and then the carcase of some animal that has been drowned by the rising flood. Hour after hour, from every side, these innumerable swollen waters bear their freights of gravel, sand, and mud into the lake. Hundreds or thousands of tons of sediment must thus be swept down during a single storm. When we multiply this result by the number of storms in a year and by the number of years in an ordinary human life, we need not be surprised to be told that even within the memory of the present generation, and still more within historic times, conspicuous changes have taken place in many lakes. In the Lake of Lucerne, for example, the River Reuss, which bears down the drainage of the huge mountains round the St. Gothard, deposits about 7,000,000 cubic feet of sediment every year. Since the year 1714 the Kander, which drains the northern flanks of the centre of the Bernese Oberland, is said to have thrown into the lower end of the Lake of Thun such an amount of sediment as to form an area of 230 acres, now partly woodland, partly meadow and marsh. Since the time of the Romans, the Rhone has filled up the upper end of the Lake of Geneva to such an extent that a Roman harbour, still called Port Valais, is now nearly 2 miles from the edge of the lake, the intervening ground having been converted first into marshes and then into meadows and farms. 58 GEOLOGICAL RECORDS LEFT BY LAKES. [CHAP. It is at the mouths of streams pouring into a lake that the process of filling up is most rapid and striking. But it may be detected at many other places round the margin. Instructive lessons on this subject may be learned at a reservoir formed by damming back the waters of a steep- sided valley, and liable to be sometimes nearly dry (Fig. 14). In such a situation, when the water is low, it may be noticed that a series of parallel lines runs all round the sides of the reservoir, and that these lines consist of gravel, sand, or earth. Each of them marks a former level of the water, FIG. 14. Alluvial terraces on the side of an emptied reservoir. and they show that the reservoir was not drained off at once but intermittently, each pause in the diminution of level being marked by a line of sediment. It is easy to watch how these lines are formed along the present margin of the water. The loose debris from the bare slope above, partly by its own gravitation, partly by the wash of rain, slides down into the water. But as soon as it gets there, its further downward movement is arrested. By the ripple of the water it is gently moved up and down, but keeps on the whole just below the line to which the water reaches. So long as it is concealed under the water, its position and extent can hardly be realised. But as soon as the level of IV.] LAKE-TERRACES. 59 the reservoir sinks, the sediment is left as a marked shelf or terrace. In natural lakes, the same process is going on, though hardly recognisable, because concealed under the water. But if by any means a lake could be rapidly emptied, its former level would be marked by a shelf or alluvial terrace. In some cases, the barrier of a lake has been removed, and FIG. 15. Parallel roads of Glen Roy. the sinking of the water has revealed the terrace. The famous " parallel roads " of Glen Roy, in the west of Scot- land, are notable examples (Fig. 15). The valleys in that region were anciently dammed up by large glaciers. The drainage accumulated behind the ice, filled up the valleys and converted them into a series of lakes. The former levels of these lakes and the successive stages of their diminution 6o GEOLOGICAL RECORDS LEFT BY LAKES. [CHAP. and disappearance are shown by the series of alluvial shelves known as parallel roads. The highest of these is 1140 feet, the middle terrace 1059 feet, and the lowest 847 feet above the level of the sea. Thus, partly by the washing of detritus down from the adjoining slopes by rain, partly by the sediment carried into them by streams, and partly by the growth of marshy vegeta- tion along their margins, lakes are visibly diminishing in size. In mountainous countries, every stage of this dis- appearance may be observed (Fig. 16). Where the lakes FIG. 1 6. Stages in the filling up of a lake. In A two streamlets are represented as pouring their deltas into a lake. In B they have filled the lake up, converting it into a meadow across which they wind on their way down the valley. are deep, the tongues of sediment or " deltas " which the streams push in front of them have not yet been able to advance far from the shore. In other cases, every tributary has built up an alluvial plain which grows outwards and along the coast, until it unites with those of its neighbours to form a nearly continuous belt of flat meadow and marsh round the lake. By degrees, as this belt increases in width, the lake narrows, until the whole tract is finally converted into an alluvial plain, through which the river and its tribu- taries wind on their way to lower levels. The successive flat meadow -like expansions of valleys among hills and moun- iv.] DEPOSITS IN LAKES. 61 tains were probably in most cases originally lakes which have in this manner been gradually filled up. The bottoms of lakes must evidently contain many interesting relics. Dispersed through the shingle, sand, and mud that gather there, are the remains of plants and animals that lived on the surrounding land. Leaves, fruits, twigs, branches, and trunks embedded in the silt may preserve for an indefinite period their record of the vegetation of the time. The wings or wing-cases of insects, the shells of land-snails, the bones of birds and mammals, carried down into the depths of a lake and entombed in the silt there will remain as a chronicle of the kind of animals that haunted the surrounding hills and valleys. The layers of gravel, sand, and silt laid down on the floor of a lake do not essentially differ from these deposited in the terraces of a river. But they ought generally to be finer in grain, and the proportion of silt, mud, or clay among them, especially away from the margin of the lake, must usually be greater than in the alluvium of a river. They are, no doubt, further distinguished by the greater abundance of the remains of plants and animals preserved in them. But lakes likewise serve as receptacles for a series of de- posits which are peculiar to them, and which consequently have much interest and importance as they furnish a ready means of detecting the sites of lakes that have long disap- peared. The molluscs of lacustrine waters are quite dis- tinct from the snails of the adjoining shores. The shells of these animals gather on the bottoms of some lakes in such numbers as to form there a deposit of the white crumbling marl, already referred to on p. 6. If nothing occurs to interrupt this deposit, it may grow to be many feet or yards in thickness. The shells in the upper parts are quite fresh, 62 GEOLOGICAL RECORDS LEFT BY LAKES. [CHAP. some of the animals having only recently died ; but they become more and more decayed below until, towards the bottom of the deposit, the marl passes into a more compact chalk-like substance in which few or no shells may be recog- nisable (Fig. 17). On the sites of lakes that have been FIG. 17. Piece of shell-marl containing shells of Limtteea peregra. naturally filled up or artificially drained, such marl has been extensively dug as a manure for land. Besides the shells from the decay of which it is chiefly formed, it sometimes yields the bones of deer, oxen, and other animals, whose carcases must originally have sunk to the bottom of the lake and been there gradually covered up in the growing mass of marl. Many examples of these marl-deposits are to be found among the drained lakes of Scotland and Ireland. Yet another peculiar accumulation is met with on the bottom of some lakes, particularly in Sweden. In the neighbourhood of banks of reeds and on the sloping shallows of the larger lakes, a deposit of hydrated peroxide of iron takes place, in the form of concretions varying in size from small grains like gunpowder up to cakes measuring 6 inches across. The iron is no doubt dissolved out of the rocks of the neighbourhood by water containing organic iv.] SALT-LAKES. ' 63 acids or carbonic acid. In this condition, it is liable to be oxidised on exposure, and can then no longer be retained in solution. It is accordingly precipitated to the bottom where it collects in grains which by successive additions to their surface become pellets, balls, or cakes. Possibly some of the microscopic plants (diatoms) which abound on the bottoms of the lakes may facilitate the accumulation of the iron by abstracting this substance from the water and depositing it inside their siliceous coverings. Beds of con- cretionary brown ironstone are formed in Sweden from 10 to 200 yards long, 5 to 15 yards broad, and from 8 to 30 inches thick. During winter when the lakes are frozen over, the iron is raked up from the bottom through holes made for the purpose in the ice, and is largely used for the manufacture of iron in the Swedish furnaces. When the iron has been removed, it begins to form again, and instances are known where, after the supply had been completely exhausted, beds several inches in thickness were formed again in twenty-six years. The salt-lakes of desert regions present a wholly peculiar series of deposits. These sheets of water have no outlet ; yet there is reason to believe that most of them were at first fresh, and discharged their outflow like ordinary lakes. Owing to geological changes of level and of climate, they have long ceased to overflow. The water that runs into them, instead of escaping by a river, is evaporated back into the air. But the various mineral salts carried by it in solu- tion from rocks and soils are not evaporated also. They remain behind in the lakes, which are consequently becoming gradually salter. Among the salts thus introduced, common salt (sodium-chloride) and gypsum (calcium-sulphate) are two of the most important. These substances, as the water 64 GEOLOGICAL RECORDS LEFT BY LAKES. [CHAP. evaporates in the shallows, bays, and pools, are precipitated to the bottom where they form solid layers of salt and gypsum. The latter substance begins to be thrown down when 37 per cent of the water containing it has been evaporated. The sodium-chloride does not appear until 93 per cent of the water has disappeared. In the order of deposit, there- fore, gypsum comes before the salt. Some bitter lakes contain sodium-carbonate, in others magnesium-chloride is abundant. The Dead Sea, the Great Salt Lake of Utah, and many other salt lakes and inland seas furnish interesting evidence of the way in which they have gradually changed. In their upper terraces, 1000 feet or more above the present level of the water, fresh-water shells occur, showing that these basins were at first fresh. But their valley-bottoms are now crusted with gypsum and salt, and their waters are almost wholly devoid of life. Such conditions as these help us to understand how great deposits of gypsum and rock-salt were formed in England, Germany, and many other regions where the climate would not now permit of any such condensation of the water (chapter xxi.) Summary. The records inscribed by lakes in geologi- cal history consist of layers of various kinds of sediment. These deposits may form mere shelves or terraces along the margin of the water which, if drained off, will leave them as evidence of its former levels. By the long-continued opera- tions of rain, brooks, and rivers, continually bringing down sediment, lakes are gradually filled up with alluvium, and finally become flat meadow-land with tributary streams winding through it. The deposits that thus replace the lacustrine water consist mainly of sand or gravel near shore, while finer silt occupies the site of the deeper water. They may also include beds of marl formed of fresh-water shells, iv.] SUMMARY. 65 and sheets of brown iron ore. Throughout them all, re- mains of the plants and animals of the surrounding land are likely to be entombed and preserved. Salt lakes leave, as their enduring memorial, beds of rock-salt and gypsum, sometimes carbonate of soda and other salts. Many of them were at first fresh, as is shown by the presence of ordinary fresh-water shells in their upper terraces. But by change of climate and long -continued excess of evaporation over precipitation, the water has gradually become more and more saline, and has sometimes disappeared altogether, leaving behind it deposits of common salt, gypsum, and other chemical precipitates. CHAPTER V. HOW SPRINGS LEAVE THEIR MARK IN GEOLOGICAL HISTORY. THE changes made by running water upon the land are not confined to that portion of the rainfall which courses along the surface. Even when it sinks underground and seems to have passed out of the general circulation, the subter- ranean moisture does not remain inactive. After travelling for a longer or shorter distance through the pores of rocks, or along their joints and other divisional planes, it finds its way once more to daylight and reappears in Springs. 1 In this underground journey, it corrodes rocks, somewhat in the same way as rain attacks those that are exposed to the outer air, and it works some curious changes upon the face of the land. Subterranean water thus leaves distinct and characteristic memorials as its contributions to geological history. There are two aspects in which the work of underground water may be considered here. In the first place, portions of the substance of subterranean rocks are carried up above ground; in the second place, some of these materials are laid down again in a new form and take a conspicuous place among the geological monuments of their time. 1 Physical Geography Class-Book, p. 222, CHAP, v.] ORIGIN OF LANDSLIPS. 67 Abstraction of Material. In the removal of mineral substance, water percolating through rocks acts in two distinct ways, mechanical and chemical, each of which shows itself in its own peculiar effects upon the surface. While slowly filtering through porous materials, water tends to remove loose particles and thus to lessen the support of overlying rocks. But even where there is no transport, the water itself, by saturating a porous layer that rests upon a more or less impervious one, loosens the cohesion of that porous layer. The overlying mass of rock is thus made to rest upon a watery and weakened platform, and if from its posi- tion it should have a tendency to gravitate in any given direction, it may at last yield to this tendency and slide downwards. Along the sides of sea-cliffs, on the precipitous slopes of valleys or river-gorges, or on the declivities of hills and mountains, the conditions are often extremely favour- able for the descent of large masses of rock from higher to lower levels. Remarkable illustrations of such Landslips, as they are called, have been observed along the south coast of England, where certain porous sandy rocks underlying a thick sheet of chalk rest upon more or less impervious clays, which, by arresting the water in its descent, throw it out along the base of the slopes. After much wet weather, the upper surface of these clays becomes, as it were, lubricated by the accumulation of water, and large slices of the overlying rocks, having their support thereby weakened, break off from the solid cliffs behind and slide down towards the sea. The most memorable example occurred at Christmas time, in the year 1839, on tne coast of Devonshire not far from Axmouth. At that locality, the chalk-downs end off in a line of broken cliff some 500 feet above the sea. From the 68 GEOLOGICAL RECORDS LEFT BY SPRINGS. [CHAP. edge of the downs flanked by this cliff a tract about 800 yards long, containing not less than 30 acres of arable land, sank down with all its fields, hedgerows, and pathways. This sunken mass, where it broke away from the upland, left behind it a new cliff, showing along the crest the trun- cated ends of the fields, of which the continuation was to be found in a chasm more than 200 feet deep. While the ground sank into this defile and was tilted steeply towards the base of the cliff, it was torn up by a long rent running on the whole in the line of the cliff, and by many parallel and transverse fissures. Nearly half a century has passed away since this landslip occurred. The cliff remains much as it was at first, and the sunken fields with their bits of hedgerow still slope steeply down to the bottom of the declivity. But the lapse of time has allowed the influence of the atmosphere to come into play. The outstanding dislocated fragments with their vertical walls and flat tops, showing segments of fields, have been gradually worn into tower-like masses with sloping declivities of debris. The long parallel rent has been widened by rain into a defile with shelving sides. Everywhere the rawness of the original fissures has been softened by the rich tapestry of verdure which the genial climate of that southern coast fosters in every sheltered nook. But the scars have not been healed, and they will no doubt remain still visible for many a year to come. Along the south coast of England, many landslips, of which there is no historical record, have produced some of the most picturesque scenery of that region. Masses that have slipped away from the main cliff have so grouped them- selves down the slopes that hillocks and hollows succeed each other in endless confusion, as in the well-known v.] ORIGIN OF LANDSLIPS. 6 9 70 GEOLOGICAL RECORDS LEFT BY SPRINGS. [CHAP. Undercliff of the Isle of Wight. Some of the tumbled rocks are still fresh enough to show that they have fallen at no very remote period, or even that the slipping still con- tinues ; others, again, have yielded so much to the weather that their date doubtless goes far back into the past, and some of them are crowned with what are now venerable ruins. The most stupendous landslips on record have occurred in mountainous countries. Upwards of 150 destructive examples have been chronicled in Switzerland. Of these, one of the most memorable was that of the Rossberg, a mountain lying behind the Rigi, and composed of thick masses of hard red sandstone and conglomerate so arranged as to slope down into the valley of Goldau. The summer of the year 1806 having been particularly wet, so large an amount of water had collected in the more porous layers of rock as to weaken the support of the overlying mass ; conse- quently a large part of the side of the mountain suddenly gave way and rushed down into the valley, burying under the debris about a square German mile of fertile land, four villages containing 330 cottages and outhouses, and 457 inhabitants. To this day, huge angular blocks of sandstone lying on the farther side of the valley bear witness to the destruction caused by this landslip, and the scar on the mountain-slope whence the fallen masses descended is still fresh. But it is by its chemical action on the rocks through which it flows that subterranean water removes by far the largest amount of mineral matter, and produces the greatest geological change. Even pure water will dissolve a minute quantity of the substance of many rocks. But rain is far from being chemically pure water. In previous chapters it has been described as taking oxygen and carbonic acid out v.] SOLUTION BY SPRINGS. 71 of the air in its descent, and abstracting organic acids and carbonic acid from the soil through which it sinks. By help of these ingredients, it is enabled to attack even the most durable rocks, and to carry some of their dissolved substance up to the surface of the ground. One of the substances most readily attacked and removed even by pure water is the mineral known as carbonate of lime. Among other impurities, natural waters generally contain carbonic acid, which may be derived from the air or from the soil; occasionally from some deeper subter- ranean source. The presence of this acid gives the water greatly increased solvent power, enabling it readily to attack carbonate of lime, whether in the form of limestone, or diffused through rocks composed mainly of other substances. Even lime, which is not in the form of carbonate, but is united with silica in various crystalline minerals (silicates, p. 174), may by this means be decomposed and combined with carbonic acid. It is then removed in solution as car- bonate. So long as the water retains enough of free carbonic acid, it can keep the carbonate of lime in solution and carry it onward. Limestone is a rock almost entirely composed of car- bonate of lime. It occurs in most parts of the world, cover- ing sometimes tracts of hundreds or thousands of square miles, and often rising into groups of hills, or even into ranges of mountains (see pp. 204, 209). The abundance of this rock affords ample opportunity for the display of the solvent action of subterranean water. Trickling down the vertical joints and along the planes between the limestone beds, the water dissolves and removes the stone, until in the course of centuries these passages are gradually enlarged into wide clefts, tunnels, and caverns, The ground be- 72 GEOLOGICAL RECORDS LEFT BY SPRINGS. [CHAP. comes honeycombed with openings into dark subterranean chambers, and running streams fall into these openings and continue their course underground. Every country which possesses large limestone tracts furnishes examples of the way in which such labyrinthine tunnels and systems of caverns are excavated. In England, for example, the Peak Cavern of Derbyshire is believed to be 2300 feet long, and in some places 120 feet high. On a much more magnificent scale are the caverns of Adelsberg near Trieste, which have been explored to a distance of FIG. 19. Section of cavern with stalactites and stalagmite. between 4 and 5 miles, but are probably still more extensive. The river Poik has broken into one part of the labyrinth of chambers, through which it rushes before emerging again to the light. Narrow tunnels expand into spacious halls, beyond which egress is again afforded by low passages into other lofty recesses. The most stupendous chamber measures 669 feet in length, 630 feet in breadth, and in feet in height. From the roofs hang pendent white stalac- tites (p. 74), which, uniting with the floor, form pillars of endless varieties of form and size. Still more gigantic is the system of subterranean passages in the Mammoth Cave of v.] DEPOSITS FROM SPRINGS. 73 Kentucky, the accessible parts of which are believed to have a combined length of about 150 miles. The largest cavern in this vast labyrinth has an area of two acres, and is covered by a vault 125 feet high. Of the mineral matter dissolved by permeating water out of the rocks underground, by far the larger part is dis- charged by springs into rivers and ultimately finds its way to the sea. The total amount of material thus supplied to the sea every year must be enormous. Much of it, indeed, is abstracted from ocean-water by the numerous tribes of marine plants and animals. In particular, the lime, silica, and organic matter are readily seized upon to build up the framework and furnish the food of these creatures. But, probably, more mineral matter is supplied in solution than is required by the organisms of the sea, in which case the water of the sea must be gradually growing heavier and salter. Deposition of Material. But it is the smaller propor- tion of the material not conveyed into the sea that specially demands attention. Every spring, even the purest and most transparent, contains mineral solutions in sufficient quantity to be detected by chemical analysis. Hence all plants and animals that drink the water of springs and rivers necessarily imbibe these solutions which, indeed, supply some of the mineral salts whereof the harder parts both" of plants and animals are constructed. Many springs, however, contain so large a proportion of mineral matter, that when they reach the surface and begin to evaporate, they drop their solutions as a precipitate, which settles down upon the bottom or on objects within reach of the water. After years of undis- turbed continuance, extensive sheets of mineral material may in this manner be accumulated, which remain as enduring 74 GEOLOGICAL RECORDS LEFT BY SPRINGS. [CHAP. monuments of the work of underground water, even long after the springs that formed them have ceased to flow. Among the accumulations of this nature by far the most frequent and important are those formed by what are called Calcareous Springs. In regions abounding in limestone or rocks containing much carbonate of lime, the subterranean waters which, as we have seen, gradually erode such vast systems of tunnels, clefts, and caverns, carry away the dis- solved rock, and retain it in solution only so long as they can keep their carbonic acid. As soon as they begin to evaporate and to lose some of this acid, they lose also the power of retaining so much carbonate of lime in solution. This substance is accordingly dropped as a fine white powder or precipitate, which gathers on the surfaces over which the water trickles or flows. The most familiar example of this process is to be seen under the arches of bridges and vaults. Long pendent white stalks or stalactites hang from between the joints of the masonry, while wavy ribs of the same substance run down the piers or walls, and even collect upon the ground (stalagmite}. A few years may suffice to drape an archway with a kind of fringe of these pencil-like icicles of stone. Percolating from above through the joints between the stones of the masonry, the rain-water, armed with its minute proportion of carbonic acid, at once attacks the lime of the mortar, forming carbonate of lime which is carried downward in solution. Arriving at the surface of the arch, the water gathers into a drop which remains hanging there for a brief interval before it falls to the ground. That in- terval suffices to allow some of the carbonic acid to escape, and some of the water to evaporate. Consequently, round the outer rim of the drop a slight precipitation of white v.] STALACTITE AND STALAGMITE. 75 chalky carbonate of lime takes place. This circular pellicle, after the drop falls, is increased by a similar deposit from the next drop, and thus drop by drop the original rim or ring is gradually lengthened into a tube which may eventually be filled up inside and may be thickened irregularly outside by the trickle of calcareous water (Fig. 20). But the deposition on the roof does not ex- haust the stock of dissolved carbonate. When the drops reach the ground the same process of evaporation and precipita- tion continues. Little mounds of the same white chalky substance are built up on the floor, and, if the place remain undisturbed, may grow until they unite with the stalac- tites from the roof, forming white pillars that reach from floor to ceiling (Fig. 19, and p. 204). It is in limestone caverns that stalac- titic growth is seen on the most colossal scale. These quiet recesses having re- mained undisturbed for many ages, the process of solution and precipitation has FIG. 20. Section advanced without interruption until, in showing successive many cases, vast caverns have been trans- layer f of growth in ' a stalactite. formed into grottoes of the most marvellous beauty. White glistening fringes and curtains of carbonate of lime, or spar, as it is popularly called, hang in endless variety and beauty of form from the roof. Pillars of every dimension, from slender wands up to thick-ribbed columns like those of a cathedral, connect the roof and the pavement. The walls, projecting in massive buttresses and retiring into 76 GEOLOGICAL RECORDS LEFT BY SPRINGS. [CHAP. alcoves, are everywhere festooned with a grotesque drapery of stone. The floor is crowded with mounds and bosses of strangely imitative forms which recall some of the oddest shapes above ground. Wandering through such a scene, the visitor somehow feels himself to be in another world, where much of the architecture and ornament belongs to styles utterly unlike those which can be seen anywhere else. The material composing stalactite and stalagmite is at first, as already stated, a fine white chalky pulp-like substance which dries into a white powder. But as the deposition continues, the older layers, being impregnated with calcareous water, receive a precipitation of carbonate of lime between their minute pores and crevices, and assume a crystalline structure. Solidifying and hardening by degrees they end by becoming a compact crystalline stone (spar) which rings under the hammer. The numerous caverns of limestone districts have offered ready shelter to various kinds of wild animals and to man himself. Some of them have been hyaena-dens, and from under their hard floor of stalagmite, the bones of hyaenas and of the creatures they fed upon are disinterred in abund- ance. Rude human implements have likewise been obtained from the same deposits, showing that man was contemporary with animals which have long been extinct. The solvent action of underground water has thus been of the utmost service in geological history, first, in forming caverns that could be used as retreats, and then in providing a hard in- crustation which should effectually seal up and preserve the relics of the denizens left upon the cavern-floors. Calcareous springs, issuing from limestone or other rock abounding in lime, deposit carbonate of lime as a white pre- cipitate. So large is the proportion of mineral contained by v.] CALCAREOUS DEPOSITS. 77 some waters that thick and extensive accumulations of it have been formed. The substance thus deposited is known by the names of Calcareous Tufa, Calc-sinter^ or Travertine. It varies in texture, some kinds being loose and crumbling, others hard and crystalline. In many places it is composed of thin layers or laminae, of which sixty may be counted in the thickness of an inch, but bound together into a solid stone. These laminae mark the successive layers of deposit. They are formed parallel to the surface over which the water flows or trickles, hence they may be observed not only on the flat bottoms of the pools, but irregularly enveloping the walls of the channel as far up as the dash of water or spray can reach. Rounded bosses may thus be formed above the level of the stream, and the recesses may be hung with stalactites. The calcareous springs of Northern and Central Italy have long been noted for the large amount of their dissolved lime, the rapidity with which it is deposited, and the extensive masses in which it has accumulated. Thus at San Filippo in Tuscany, it is deposited at the rate of one foot in four months, and it has been piled up to a depth of at least 250 feet, forming a hill a mile and a quarter long, and a third of a mile broad. So compact are many of the Italian tra- vertines that they have from time immemorial been exten- sively used as a building stone, which can be dressed and is remarkably durable. Many of the finest buildings of ancient and modern Rome have been constructed of travertine. A familiar feature of many calcareous springs deserves notice here. The precipitation of calc-sinter is not always due merely to evaporation. In many cases, where the pro- portion of carbonate of lime in solution is so small that under ordinary circumstances no precipitation of it would 78 GEOLOGICAL RECORDS LEFT BY SPRINGS. [CHAP. take place, large masses of it have been deposited in a peculiar fibrous form. On examination, this precipitation will be found to be caused by the action of plants, particu- larly bog-mosses which, decomposing the carbonic acid in the water, cause the lime-carbonate to be deposited along their stems and leaflets. The plants are thus incrusted with sinter which, preserving their forms, looks as if it were com- posed of heaps of moss turned into stone. Hence the name of petrifying springs often given to waters where this process is to be seen. There is, however, no true petrifaction or conversion of the actual substance of the plants into stone. The fibres are merely incrusted with travertine, inside of which they eventually die and decay. But as the plants continue to grow outward, they increase the sinter by fresh layers, while the inner and dead parts of the mass are filled up and solidified by the deposit of the precipitate within their cavities. A growing accumulation of travertine presents a special interest to the geologist from the fact that it offers excep- tional facilities for the preservation of remains of the plants and animals of the neighbourhood. Leaves from the sur- rounding trees and shrubs are blown into pools or fall upon moist surfaces where the precipitation of lime is actively going on (Fig. 21). Dead insects, snail-shells, birds, small mammals, and other denizens of the district may fall or be carried into similar positions. These remains may be rapidly enclosed within the stony substance before they have time to decay, and even if they should afterwards moulder into dust, the sinter enclosing them retains the mould of their forms, and thus preserves for an indefinite period the record of their former existence. A second but less abundant deposit from springs is found v.] DEPOSITS FROM CHALYBEATE SPRINGS. 79 in regions where the rocks below ground contain decompos- ing sulphide of iron (p. 184). Water percolating through FIG. 21. Travertine with impressions of leaves. such rocks and oxidising the sulphur of that mineral, forms sulphate of iron (ferrous sulphate) which it removes in solu- tion. The presence of any notable quantity of this sulphate is at once revealed by the marked inky taste of the water and by the yellowish-brown precipitate on the sides and bottom of the channel. Such water is termed Chalybeate. When it mixes with other water containing dissolved car- bonates (which are so generally present in running water), the sulphate is decomposed, the sulphuric acid passing over to the lime or alkali of the carbonate, while the iron takes up oxygen and falls to the bottom as a yellowish-brown pre- cipitate (p. 172). This interchange of combinations, with the consequent precipitation of iron-oxide, may continue for a considerable distance from the outflow of the chaly- beate water. Nearest the source the deposit of hydrated 8o GEOLOGICAL RECORDS LEFT BY SPRINGS. [CHAP. ferric oxide or ochre is thickest. It encloses leaves, stems, and other organic remains, and preserves moulds or casts of their forms. It also cements the loose sand and shingle of a river-bottom into solid rock. One other deposit from spring-water may be enumerated here. In volcanic regions, hot springs (geysers) rise to the surface which, besides other mineral ingredients, contain a considerable proportion of silica (p. 157). This substance is de- posited as Siliceous Sinter round the vents whence the water is discharged, where it forms a white stone rising into mounds and terraces with fringes and bunches of coral-like growth. Where many springs have risen in the same district, their respective sheets of sinter may unite, and thus extensive tracts are buried under the deposit. In Iceland, for ex- ample, one of the sheets is said to be two leagues long, a quarter of a league wide, and a hundred feet thick. In the Yellowstone Park of North America, many valleys are floored over with heaps of sinter, and in New Zealand other extensive accumulations of the same material are to be found. It is obvious that, like travertine, siliceous sinter may readily entomb and preserve a record of the plants and animals that lived at the time of its deposition. Summary. The underground circulation of water pro- duces changes that leave durable records in geological history. These changes are of two kinds, (i) Landslips are caused, by which the forms of cliffs, hills, and mountains are per- manently altered. Vast labyrinths of subterranean tunnels, galleries, and caverns are dissolved out of calcareous rocks, and openings are made from these passages up to the surface whereby rivers are engulfed. Many of the caves thus hollowed out have served as dens of wild beasts and dwell- ing-places for man, and the relics of these inhabitants have v.] SUMMARY. 8 1 been preserved under the stalagmite of the floors. (2) An enormous quantity of mineral matter is brought up to the surface by springs. Most of these solutions are conveyed ultimately to the sea where they partly supply the substances required by the teeming population of marine plants and animals. But under favourable circumstances, considerable deposits of mineral matter are made by springs, more especi- ally in the form of travertine, siliceous sinter, and ochre. In these deposits the remains of terrestrial vegetation, also of insects, birds, mammals, and other animals are not infre- quently preserved, and remain as permanent memorials of the life of the time when they flourished. CHAPTER VI. ICE-RECORDS. ICE in various ways alters the surface of the land. By eroding even the most durable rocks, and by removing and piling up elsewhere a vast amount of loose materials, it greatly modifies the details of a landscape. As it assumes various forms, so it accomplishes its work with considerable diversity. The action of frost upon soil and bare surfaces of rock has already (p. 18) been described. We have now to consider the action of frozen rivers and lakes, snow and glaciers, which have each their own characteristic style of operation, and leave behind them their distinctive contribu- tion to the geological history of the earth. Frozen Rivers and Lakes. In countries with a severe winter climate, the rivers and lakes are frozen over and the cake of ice that covers them may be more than two feet thick. When this cake is broken up in early summer, large masses of it are driven ashore, tearing up the boulders, gravel, sand, or mud, and pushing them to a height of many feet above the ordinary level of the water. When the ice melts, huge heaps of detritus are found to have been piled up by it, which remain as enduring monuments of its power. Not only so, but large fragments of the ice that has been formed CHAP, vi.] FROZEN RIVERS SNOW. 83 along shore and has enclosed blocks of stone, gravel, and sand, are driven away and may travel many miles before they melt and drop their freight of stones. On the St. Lawrence and on the coast of Labrador, there is a constant transportation of boulders by this means. Further, besides freezing over the surface, the water not infrequently forms a loose spongy kind of ice on the bottom (Anchor-ice^ Ground- ice) which encloses stones and gravel, and carries them up to the surface where it joins the cake of ice there. This bottom of ice is formed abundantly on some parts of the Canadian rivers. Swept down by the current, it accumulates against the bars or banks, or is pushed over the upper ice, and from time to time gathers into temporary barriers the bursting of which may cause destructive floods. In the river St. Lawrence, banks and islets have been to a large extent worn down by the grating of successive ice-rafts upon them. Snow. On level or gently inclined ground, snow dis- appears where it falls. But while it remains, it exercises a protective influence upon the soil and vegetation, shielding them from the action of frost. On slopes of sufficient decliv- ity, however, the sheet of snow acquires a tendency to descend by gravitation. In many cases, it creeps or slides down the side of a hill or valley, and in so doing moves forward bare soil, loose stones, or other objects lying on the surface. By this means, the debris of weathered rock in exposed situa- tions is gradually pushed down-hill and the rock is bared for further disintegration. But it is where the declivities are steep enough to allow the snow to break off in large sheets and to rush rapidly down that the most striking changes are observable. Such descending masses are known as Ava- lanches. Varying from 10 to 50 feet or more in thickness 84 ICE-RECORDS. [CHAP. and several hundred yards broad and long, they sweep down the mountain sides with terrific force, carrying away trees, soil, houses, and even large blocks of rock. The winter of 1884-85 was especially remarkable for the number of ava- lanches in the valleys of the Alps, and for the enormous loss of life and property which they caused. Not only are the declivities bared of their trees, soil, and boulders, but huge mounds of debris are piled up on the valley below. Frequently, also, such a quantity of snow, ice, and rubbish is thrown across the course of a stream as to dam back the water, which accumulates until it overflows or sweeps away the barrier. In another but indirect way, snow may power- fully affect the surface of a district where, by rapid melting, it so swells the rivers as to give rise to destructive floods. While, therefore, the influence of snow is on the whole to protect the surface of the land, it shows itself in mountainous regions singularly destructive, and leaves as chief memorials of this destructiveness the mounds and rough heaps of earth and stones that mark where the down -rushing ava- lanches have come to rest. Glaciers and Ice-Sheets leave their record in char- acters so distinct that they cannot usually be confounded with those of any other kind of geological agent. The changes which they produce on the surface of the land may be divided into two parts: (i) the transport of materials from the high grounds to lower levels, and (2) the erosion of their beds. As a glacier descends its valley, it receives upon its surface the earth, sand, mud, gravel, boulders, and blocks of rock that roll or are washed down from the slopes on either side. Most of this rubbish accumulates on the edges of the glacier, where it is slowly borne to lower levels as the ice creeps downwards. But some of it falls into the vi.] GLACIERS AND ICE-SHEETS. 85 crevasses by which the ice is split, and may either be im- prisoned within the glacier, or may reach the rocky floor over which the ice is sliding. The rubbish borne onward upon the surface of the glacier is known as moraine-stuff. The mounds of it running along each side of the glacier form lateral moraines, those on the right hand side as we look down the length of the valley being the right lateral FIG. 22. Glacier with medial and lateral moraines. moraine, those on the other side the left lateral moraine. Where two glaciers unite, the left lateral moraine of the one joins the right lateral moraine of the other, forming what is called a medial moraine that runs down the middle of the united glacier. Where a glacier has many tributaries bearing much moraine-stuff, its surface may be like a bare plain covered with earth and stones, so that except where a yawning crevasse reveals the clear blue gleam of the ice 86 ICE-RECORDS. [CHAP. below, nothing but earth and stones meets the eye. When the glacier melts, the detritus is thrown in heaps upon the valley, forming there the terminal moraine. Glaciers, like rivers, are subject to variations of level. E ven from year to year they slowly sink below their previous limit or rise above it. The glacier of La Brenva, for example, on the Italian side of Mont Blanc, subsided no less than 300 feet in the first half of the present century. One notable consequence of such diminution is that the blocks of rock FIG. 23. Perched blocks scattered over ice-worn surface of rock. lying on the edges of a glacier are stranded on the side of the valley, as the ice shrinks away from them. Such Perched Blocks or Erratics (Fig. 23), as they are called, afford an excellent means of noting how much higher and longer a glacier has once been than it is now. Their great size (some of them are as large as good-sized cottages) and their peculiar positions make it quite certain that they could not have been transported by any current of water. They are often poised on the tops of crags, on the very edges of precipices, or on steep slopes where they could never have vr.] ERRATIC BLOCKS. 87 been left by any flood, even had the flood been capable of moving them. The agent that deposited them in such positions must have been one that acted very quietly and slowly, letting the blocks gently sink into the sites they now occupy. The only agent known to us that could have done this is glacier-ice. We can actually see similar blocks on the glaciers now, and others which have only recently been stranded on the side of a valley from which the ice has sunk. In the Swiss valleys, the scattered ice-borne boulders may be observed by hundreds far above the exist- ing level of the glaciers and many miles beyond where these now end. If the origin of the dispersed erratics is self-evident in a valley where a glacier is still busy transporting them, those that occur in valleys which are now destitute of glaciers can offer no difficulty ; they become, indeed, striking monu- ments that glaciers once existed there. Scattered erratic blocks offer much interesting evidence of the movements of the ice by which they were transported. In a glacier-valley, the blocks that fall upon the ice remain on the side from which they have descended. Hence, if there is any notable difference between the rocks of the two sides, this difference will be recognisable in the composition of the moraines, and will remain distinct even to the end of the glacier. If, therefore, in a district from which the glaciers have disappeared, we can trace up the scattered blocks to their sources among the mountains, we thereby obtain evi- dence of the actual track followed by the vanished glaciers. The limits to which these blocks are traceable do not, of course, absolutely fix the limits of the ice that transported them. They prove, however, that the ice extended at least as far as they occur, but it may obviously have risen higher and advanced farther than the space within which the blocks 88 ICE-RECORDS. [CHAP. are now confined. In Europe, some striking examples occur of the use of this kind of evidence. Thus the peculiar blocks of the Valais can be traced all the way to the site of the modern city of Lyons. There can be no doubt that the glacier of the Rhone once extended over all that interven- ing country and reached at least as far as Lyons, a distance of not less than 170 miles from where it now ends. Again, from the occurrence of blocks of some of the characteristic rocks of Southern Scandinavia, in Northern Germany, Belgium, and the east of England, we learn that a great sheet of ice once rilled up the bed of the Baltic and the North Sea, carrying with it immense numbers of northern erratics. In Britain, where there are now neither glaciers nor snow-fields, the abundant dispersion of boulders from the chief tracts of high grounds shows that this country was once in large part buried under ice, like modern Green- land. The evidence for these statements will be more fully given in a later part of this volume (chapter xxvi.) Besides the moraine-stuff carried along on the surface, loose detritus and blocks of rock are pushed onwards under the ice. When a glacier retires, this earthy and stony debris, where not swept away by the escaping river, is left on the floor of the valley. One remarkable feature of the stones in it is that a large proportion of them are smoothed, polished, and covered with fine scratches or ruts, such as would be made by hard sharp-pointed fragments of stone or grains of sand. These markings run for the most part along the length of each oblong stone, but not infrequently cross each other, and sometimes an older may be noticed partially effaced by a newer set. The striation of these stones is a most characteristic mark of the action of glaciers. The stones under the ice are fixed in the line of least resistance VI.] GLACIAL STRIATION. that is, end on. In this position, under the weight of hundreds of feet of ice, they are pressed upon the floor over which the glacier is travelling. Every sharp edge of stone or grain of sand, pressed along' the surface of a block, or over which the block itself is slowly drawn, engraves a fine scratch or a deeper rut. As the block moves onward, it is more and more scratched, losing its corners and edges, FIG. 24. Stone smoothed and striated by glacier-ice. becoming smaller and smoother till, if it should travel far enough, it might be entirely ground into sand or mud (Fig. 24). The same process is carried on upon the solid rocks over which the ice moves. These are smoothed, striated, and polished by the friction of the grains of sand, pebbles, and blocks of stone crushed against them by the slowly creeping mass of ice. Every boss of rock that looks toward the quarter from which the overlying ice is moving is ground away, while those that face to the opposite side are more or less sharp and unworn. The striation is especially note- worthy. From the fine scratches, such as are made by grains of sand, up to deep ruts like those of cart-wheels in unmended roadways, or to still wider and deeper hollows, all 9 ICE-RECORDS. [CHAP. the friction -markings run in a general uniform direction, which is that of the motion of the glacier. Such striated surfaces could only be produced by some agent with rigidity FIG. 25. Ice-striation on the floor and side of a valley. enough to hold the sand-grains and stones in position, and press them steadily onward upon the rocks. A river polishes the rocks of its channel by driving shingle and sand across them j but the currents are perpetually tossing these materials now to one side, now to another, so that smoothed and polished surfaces are produced, but with nothing at all re- sembling striation. A glacier, however, by keeping its grinding materials fixed in the bottom of the ice, engraves its characteristic parallel striae and groovings, as it slowly vi.] EROSION BY GLACIERS. 91 creeps down the valley. All the surfaces of rock within reach of the ice are smoothed, polished, and striated. Such surfaces present the most unmistakable evidence of glacier- action, for they can be produced by no other known natural agency. Hence, where they occur in glacier valleys, far above and beyond the present limits of the ice, they prove how greatly the ice has sunk. In regions also where there are now no glaciers, these rock-markings remain as almost im- perishable witnesses that glaciers once existed. By means of their evidence, for example, we can trace the march of great ice-sheets which once enveloped the whole of Scandi- navia and lay deep upon nearly the whole of Britain. The river that escapes from the end of a glacier is always muddy. The fine sand and mud that discolour the water are not supplied by the thawing of the clear ice, nor by the sparkling brooks that gush out of the mountain-slopes, nor by the melting of the snows among the peaks that rise on either side. This material can only come from the rocky floor 01 the glacier itself. It is the fine sediment ground away from the rocks and loose stones by their mutual friction under the pressure of the overlying ice. It is thus a kind of index or measure of the amount of material worn off the rocky bed by the grinding action of the glacier. We can readily see that as this erosion and transport are continually in progress, the amount of material removed in the course of time must be very great. It has been estimated, for ex- ample, that the Justedal glacier in Norway removes annually from its bed 2,427,000 cubic feet of sediment. At this rate the amount removed in a century would be enough to fill up a valley or ravine 10 miles long, 100 feet broad, and 40 feet deep. In arctic and antarctic latitudes, where the land is 92 ICE-RECORDS. [CHAP. buried under a vast ice-sheet, which is continually creeping seaward and breaking off into huge masses that float away as icebergs, there must be a constant erosion of the terrestrial surface. Were the ice to retire from these regions, the ground would be found to wear what is called a glaciated surface ; that is to say, all the bare rocks would present a character- istic ice-worn aspect, rising into smooth rounded bosses like dolphins' backs (roches moutonn'ees), and sinking into hollows that would become lake -basins. Everywhere these bare rocks would show the striae and groovings graven upon them by the ice, radiating generally from the central high grounds, and thus indicating the direction of flow of the main streams of the ice-sheet. Piles of earth, ice -polished stones, and blocks of rock would be found strewn over the country, especially in the valleys and over the plains. These materials would still further illustrate the movements of the ice, for they would be found to be singularly local in char- acter, each district having supplied its own contribution of detritus. Thus in a region of red sandstone, the rubbish would be red and sandy ; in one of black slate, it would be black and clayey (see chapter xxvi.) Summary. In this chapter we have seen that ice in various ways affects the surface of the land and leaves its mark there. Frost pulverises soil, disintegrates exposed surfaces of stone, and splits open bare rocks along their lines of natural joint. On rivers and lakes, the disrupted ice wears down banks and pushes up mounds of sand, gravel, and boulders along the shores. In the condition of ava- lanches, it causes large quantities of earth, soil, and blocks of rock to be removed from the mountain-slopes and piled up on the valleys. In the form of glaciers, it transports the debris of the mountains to lower levels, bearing along vi.] SUMMARY. 93 and sometimes stranding masses of rock as large as cottages, which no other known natural agent could transport. Moving down a valley, it wears away the rocks, giving them a peculiar smoothed and striated surface which is thoroughly characteristic. By this grinding action, it erodes its bed and produces a large amount of fine sediment, which is carried away by the river that escapes at the end of the glacier. Land-ice thus leaves thoroughly distinctive and enduring memorials of its presence in polished and grooved rocks, in masses of earth, clay, or gravel, with striated stones, and in the dispersal of erratic blocks from principal masses of high ground. These memorials may remain for ages after the ice itself has vanished. By their evidence we know that the present glaciers of the Alps are only a shrunk remnant of the great ice -fields which once covered that region; that the Scandinavian glaciers swept across what is now the bed of the North Sea as far as the mouth of the Thames ; and that Scotland, Ireland, Wales, and the greater part of Eng- land were buried under great sheets of ice which crept downwards into the North Sea on the one side, and into the Atlantic on the other. CHAPTER VII. THE MEMORIALS OF THE PRESENCE OF THE SEA. WE have now to inquire how the work of the sea is registered in geological history. This work is broadly of two kinds. In the first place, the sea is engaged in wear- ing away the edges of the land, and in the second place, being the great receptacle into which all the materials, worn away from the land, are transported, it arranges these mate- rials over its floor, ready to be raised again into land at some future time. I. Demolition of the Land. In its work of destruc- tion along the coasts of the land, the sea acts to some extent (though we do not yet know how far) by chemically dissolv- ing the rocks and sediments which it covers. Cast-iron bars, for example, are so corroded by sea-water as to lose nearly half their strength in fifty years. Doubtless many minerals and rocks are liable to similar attacks. But it is by its mechanical effects that the sea accom- plishes most of its erosion. The mere weight with which ocean-waves fall upon exposed coasts breaks off fragments of rock from cliffs. Masses, 13 tons in weight, have been known to be quarried out of the solid rock by the force of the breakers in Shetland, at a height of 70 feet above CHAP. VII.] BREAKER- ACTION. 95 sea -level. As a wave may fall with a blow equal to a pres- sure 3 tons on the square foot, it compresses the air in every cleft and cranny of a cliff, and when it drops it allows the air instantly to expand again. By this alternate compression and expansion, portions of the cliff are loosened and removed. Where there is any weaker part in the rock, a long tunnel FIG. 26. Buller of Buchan a caldron-shaped cavity 'or blow-hole worn out of granite by the sea on the coast of Aberdeenshire. may be excavated, which may even be drilled through to the daylight above, forming an opening at some distance inland from the edge of the cliff. During storms, the breakers rush through such a tunnel, and spout forth from the opening (or blow-hole) in clouds of spray. Probably the most effective part of the destructive action of the sea is to be found in the battery of gravel, shingle, 96 MEMORIALS LEFT BY THE SEA. [CHAP. and loose blocks of stone which the waves discharge against cliffs exposed to their fury. These loose materials, caught up by the advancing breakers and thrown with great force upon the rocks of a coast-line, are dragged back in the recoil of the water, but only to be again lifted and swung forward. In this loud turmoil, the loose stones are reduced in size and are ground smooth by friction against each other and upon the solid cliff. The well-rounded and polished aspect of the gravel on such storm-beaten shores is an eloquent testimony to the work of the waves. But still more striking, because more measurable, is the proof that the very cliffs themselves cannot resist the blows dealt upon them by the wave -borne stones. Above the ordinary limit reached by the tides, the rocks rise with a rough ragged face, bearing the scars inflicted on it by the ceaseless attacks of the air, rain, frost, and the other agencies that waste the surface of the land. But all along the base of the cliff, within reach of the waves, the rocks have been smoothed and polished by the ceaseless grinding of the shingle upon them, while arches, tunnels, solitary pillars, half-tide skerries, creeks, and caves attest the steady advance of the sea and the gradual demoli- tion of the shore. Every rocky coast -line exposed to a tempestuous sea affords illustrations of these features of the work of waves. Even where the rocks are of the most durable kind, they cannot resist the ceaseless artillery of the ocean. They are slowly battered down, and every stage in their demolition may be witnessed, from the sunken reef, which at some dis- tance from the shore marks where the coast -line once ran, up to the tunnelled cliff from which a huge mass was detached during the storms of last winter. But where the materials composing the cliffs are more easily removed, the progress VII.] BREAKER-ACTION. 97 H 98 MEMORIALS LEFT BY THE SEA. [CHAP. of the waves may be comparatively rapid. Thus on the east coast of Yorkshire between Spurn Point and Flamborough Head, the cliffs consist of boulder-clay, and vary up to more than 100 feet in height. At high water, the tide rises against the base of these cliffs, and easily scours away the loose debris which would otherwise gather there and protect them. Hence, within historic times, a large tract of land, with its parishes, farms, villages, and seaports, has been washed away, the rate of loss being estimated at not less than 2\ yards in a year. Since the Roman occupation a strip of land between 2 and 3 miles broad is believed to have disappeared. It is evident that to carry on effectively this mechanical erosion, the sea-water must be in rapid motion. But in the deeper recesses of the ocean, where there is probably no appreciable movement of the water, there can hardly be any sensible erosion. In truth, it is only in the upper parts of the sea, which are liable to be agitated by wind, that the conditions for marine erosion can be said to exist. The space within which these conditions are to be looked for is that comprised between the lowest depth to which the grind- ing influence of waves extends, and the greatest height to which breakers are thrown upon the land. These limits, no doubt, vary considerably in different regions. In some parts of the open sea, as off the coast of Florida, the disturbing action of the waves has been supposed to reach to a depth of 600 feet, though the average limit is probably greatly less. On exposed promontories in stormy seas, such as those of the north of Scotland, breakers have been known to hurl up stones to a height of 300 feet above sea-level. But probably the zone, within which the erosive work of the sea is carried on, does not as a rule exceed 300 feet in vertical range. Within some such limits as these, the sea is engaged in VIL] RATE OF MARINE EROSION. 99 gnawing away the edges of the land. A little reflection will show us that, if no counteracting operation should come into play, the prolonged erosive action of the waves would reduce the land below the sea-level. If we suppose the average rate of demolition to be 10 feet in a century, then it would take not less than 52,800 years to cut away a strip one mile broad from the edge of the land. But while the sea is slowly eating away the coast-line, the whole surface of the land is at the same time crumbling down, and the wasted materials are being carried away by rivers into the sea at such a rate that, long before the sea could pare away more than a mere narrow selvage, the whole land might be worn down to the sea-level by air, rain, and rivers (p. 38). But there are counteracting influences in nature that would probably prevent the complete demolition of the land. What these influences are will be more fully considered in a later chapter. In the meantime, it will be enough to bear in mind that while the land is constantly worn down by the forces that are acting upon its surface, it is liable from time to time to be uplifted by other forces acting from below. And the existing relation between the amount and height of land, and the extent of sea, on the face of the globe, must be looked upon as the balance between the working of both these antagonistic classes of agencies. But without considering for the present whether the results of the erosion performed by the sea will be inter- rupted or arrested, we can readily perceive that their tend- ency is toward the reduction of the level of the land to a submarine plain (Fig. 28). As the waves cut away slice after slice from a coast-line, the portion of land which they thus overflow, and over which they drive the shingle to and fro, is worn down until it comes below the lower limit of breaker- ioo MEMORIALS LEFT BY THE SEA. [CHAP. action, where it may be covered up with sand or mud. When the abraded land has been reduced to this level, it FIG. 28. Section of submarine plain." *V. Land cut into caves, tunnels, sea-stacks, reefs and skerries by the waves, and reduced to a platform below the level of the sea (s s) on which the gravel, sand, and mud (d) produced by the waste of the coast may accumulate. reaches a limit where erosion ceases, and where the sea, no longer able to wear it down further, protects it from injury by other agents of demolition. We see, then, that the goal toward which all the wear and tear of a coast-line tends, is the formation of a more or less level platform cut out of the land. Yet an attentive study of the process will convince us that in the production of such a platform the sea has really had less to do than the atmospheric agents of destruction. An ordinary sea- cliff is not a vertical wall. In the great majority of cases it slopes seaward at a steep angle ; but if it had been formed, and were now being cut away, mainly by the sea, it ought obviously to have receded fastest where the waves attack it that is, at its base. In other words, if sea-cliffs retired chiefly because they are demolished by the sea, they ought to be most eroded at the bottom, and should therefore be usually overhanging precipices. That this is not the case shows that some other agency is concerned which causes the higher parts of a cliff to recede faster than those below. This agency can be no other than that of the atmospheric vii.] DEPOSITS FORMED BV THE SEA. J lol forces air, frost, rain, and springs. These cause the face of the cliff to crumble down, detaching mass after mass, which, piled up below, serve as a breakwater, and must be broken up and removed by the waves before the solid cliif behind them can be attacked. II. Accumulations formed by the Sea. It is not its erosive action that constitutes the most important claim of the sea to the careful study of the geologist. After all, the mere marginal belt or fringe within which this action is confined forms such a small fraction of the whole terrestrial area of the globe, that its importance dwindles down when we compare it with the enormously vaster surface over which the operations of the air, rain, rivers, springs, and glaciers are displayed. But when we regard the sea as the receptacle into which all the materials worn off the land ultimately find their way, we see what a large part it must play in geological history. During the last fifteen years great additions have been made to our knowledge of the sea-bottom all over the world. Portions of the deposits accumulating there have been dredged up even from the deepest abysses, so that it is now possible to construct charts, showing the general distribution of materials over the floor of the ocean. Beginning at the shore, let us trace the various types of marine deposits outward to the floors of the great ocean- abysses. In many places, the sea is more or less barred back by the accumulation of sediment worn away from the land. In estuaries, for example, there is often such an amount of mud in the water that the bottom on either side is gradually raised above the level of tide-mark, and forms eventually a series of meadows which the sea can no longer overflow. At the mouths of rivers with a considerable current, 102 MEMORIALS LEFT BY THE SEA. [CHAP. a check is given to the flow of the water when it reaches the sea, and there is a consequent arrest of its detritus. Hence, a bar is formed across the outflow of a river, which during floods is swept seawards, and during on-shore gales is driven again inland. Even where there is no large river, the smaller streams flowing off the surface of a country may carry down sediment enough to be arrested by the sea, and thrown up as a long bank or bar running parallel with the coast. Be- hind this bar, the drainage of the interior accumulates in long lagoons, which find an outflow through some breach in the bar, or by soaking through the porous materials of the bar itself. A large part of the eastern coast of the United States is fringed with such bars and lagoons. A space several hundred miles long on the east coast of India is similarly bordered. But the most remarkable kind of accumulation of ter- restrial detritus in the sea is undoubtedly that of river-deltas. Where the tidal scour is not too great, the sediment brought down by a large river into a marine bay or gulf gradually sinks to the bottom as the fresh spreads over and mingles with the salt water. During floods, coarse sediment is swept along, while during low states of the river nothing but fine mud may be transported. Alternating sheets of different kinds of sediment are thus laid down one upon another on the sea-floor, until by degrees they reach the surface, and thus gradually increase the breadth of the land. Some deltas are of enormous size and depth. That of the Ganges and Brahmaputra covers an area of between 50,000 and 60,000 square miles that is, about as large as England and Wales. It has been bored through to a depth of 481 feet, and has been found to consist of numerous alternations of fine clays, marls, and sands or sandstones, with occasional vii.] STORM-BEACHES. 103 layers of gravel. In all this great thickness of sediment, no trace of marine organisms was found, but land-plants and bones of terrestrial and fluviatile animals occurred. Turning now to the deposits that are more distinctively those of the sea itself, we find that ridges of coarse shingle, gravel, and sand are piled up along the extreme upper limit reached by the waves. The coarsest materials are for the most part thrown highest, especially in bays and narrow .KiG. 29. Storm-beach ponding back a stream and forming a lake j west coast of Sutherlandshire. creeks where the breakers are confined within converging shores. In such situations, during heavy gales, storm-beaches of coarse rounded shingle are formed sometimes several yards above ordinary high-tide mark (Fig. 29). Where a barrier of this kind is thrown across the mouth of a brook, the fresh water may be ponded back to form a small lake, of which the outflow usually escapes by percolation through the shingle. In sheltered bays, behind headlands, or on parts of a coast -line where tidal currents meet, detritus may 104 MEMORIALS LEFT BY THE SEA. [CHAP. accumulate in spits or bars. Islands may in this way be gradually united to each other or to the mainland, while the mainland itself may gain considerably in breadth. At Romney Marsh, on the south-east coast of England, for instance, a tract of more than 80 square miles, which in Roman times was in great part covered by the sea at high water, is now dry land, having been gained partly by the natural increase of shingle thrown up by the waves and partly by the barriers artificially erected to exclude the sea. While the coarsest shingle usually accumulates towards the upper part of the beach, the materials generally arrange themselves according to size and weight, becoming on the whole finer as they are traced towards low-water mark. But patches of coarse gravel may be noticed on any part of a beach, and large boulders may be seen even below the limits of the lowest tides. As a rule, the deposits formed along a beach, and in the sea immediately beyond, include the coarsest kinds of marine sediment. They are also marked by frequent alternations of coarse and fine detritus, these rapid interchanges pointing to the varying action of the waves and strong shore -currents. Towards the lower limit of breaker-action, fine gravel and sand are allowed to settle down, and beyond these, in quiet depths where the bottom is not disturbed, fine sand and mud washed away from the land slowly accumulate. The distance to which the finer detritus of the land is carried by ocean -currents, before it finds its way to the bottom, varies up to about 200 miles or more. Within this belt of sea, the land-derived materials are distributed over the ocean -floor. Coarse and fine gravel and sand are the most common materials in the tracts nearest the land. Beyond these, lie tracts of fine sand and silt with occasional vii.] DEPOSITS OF THE OCEAN ABYSSES. 105 patches of gravel. Still farther from the land, at depths of 600 feet and upwards, fine blue and green muds are found, composed of minute particles of such minerals as form the ordinary rocks of the land. But traced out into the open ocean, these various deposits of recognisable ter- restrial origin give place to thoroughly oceanic accumu- lations, especially to widespread sheets of red and brown exceedingly fine clay. This clay, the most generally dif- fused deposit of the deeper or abysmal parts of the sea, appears to be derived from the decomposition of volcanic fragments either washed away from volcanic islands or supplied by submarine eruptions. That it is accumulated with extreme slowness is shown by two curious and interest- ing kinds of evidence. Where it occurs farthest removed from land, great numbers of sharks' teeth, with ear-bones and other bones of whales, have been dredged up from it, some of these relics being quite fresh, others partially coated with a crust of brown peroxide of manganese, some wholly and thickly enveloped in this substance. The same haul of the dredge has brought up bones in all these conditions, so that they must be lying side by side on the red clay floor of the ocean abysses. The deposition of manganese is no doubt an exceedingly slow process, but it is evidently faster than the deposition of the red clay. The bones dredged up probably represent a long succession of generations of animals. Yet so tardily does the red clay gather over them, that the older ones are not yet covered up by it, though they have had time to be deeply encased in oxide of manganese. The second kind of evidence of the extreme slowness of deposit in the ocean abysses is supplied by minute spherules of metallic iron, which occurring in numbers dispersed through the red clay, have been identified as portions of meteorites or io6 MEMORIALS LEFT BY THE SEA. [CHAP. falling stars. These particles no doubt fall all over the ocean, but it is only where the rate of deposition of sediment is exceedingly slow that they may be expected to be detected. Besides the sediments now enumerated, the bottom of the sea receives abundant accumulations of the remains of shells, corals, foraminifera and other marine creatures ; but these will be described in the next chapter, where an account is given of the various ways in which plants and animals, both upon the land and in the sea, inscribe their records in geologi- cal history. It must also be borne in mind that throughout all the sediments of the sea-floor, from the upper part of the beach down to the bottom of the deepest and remotest abyss, the remains of the plants, sponges, corals, shells, fishes and other organisms of the ocean may be entombed and preserved. It will suffice here to remember that various depths and regions of the sea have their own characteristic forms of life, the remains of which are preserved in the sediments accumulating there, and that although gravel, sand, and mud laid down beneath the sea may not differ in any recognisable detail from similar materials deposited in a lake or river, yet the presence of marine organisms in them would be enough to prove that they had been formed in the sea. It is evident, also, that if the sea-floor over a wide area were raised into land, the extent of the deposits would show that they could not have been accumulated in any mere river or lake, but must bear witness to the former presence of the sea itself. Summary. The sea records its work upon the surface of the earth in a twofold way. In the first place, in co- operation with the atmospheric agents of disintegration, it eats away the margin of the land and planes it down. The final result of this process if uninterrupted would be to vii.] SUMMARY. 107 reduce the level of the land to that of a submarine platform, the position of the surface of which would be determined by the lower limit of effective breaker-action. In the second place, the sea gathers over its floor all the detritus worn by . every agency from the surface of the land. This material is not distributed at random; it is assorted and arranged by the waves and currents, the coarsest portions being laid down nearest the land, and the finest in stiller and deeper water. The belt of sea-floor within which this deposition takes place probably does not much exceed a breadth of 200 miles. Beyond that belt, the bottom of the ocean is covered to a large extent with deposits of red clay derived from the decomposition of volcanic material and laid down with extreme slowness. These truly oceanic accumulations are recognisably distinct from those derived from terrestrial sources within the narrow zone of deposition near the land. CHAPTER VIII. HOW PLANTS AND ANIMALS INSCRIBE THEIR RECORDS IN GEOLOGICAL HISTORY. BROADLY considered, there are two distinct ways in which plants and animals leave their mark upon the surface of the earth. In the first place, they act directly by promoting or arresting the decay of the land, and by forming out of their own remains deposits which are sometimes thick and exten- sive. In the second place, their remains are transported and entombed in sedimentary accumulations of many different kinds, and furnish important evidence as to the conditions under which these accumulations were formed. Each of these two kinds of memorial deserves our careful attention, for, taken together, they comprise the most generally inter- esting departments of geology, and those in which the history of the earth is principally discussed. 1 I. We have first to consider the direct action of plants and animals upon the surface of the globe. This action is often of a destructive kind, both plants and animals taking their part in promoting the general disintegration of rocks and soils. Thus, by their decay they furnish to the soil those 1 In the Appendix a Table of the Vegetable and Animal King- doms is given, from which the organic grade of the plants and animals referred to in this and subsequent chapters may be understood. CHAP, viii.] ACTION OF PLANTS AND ANIMALS. 109 organic acids referred to on p. 32, as so important in in- creasing the solvent power of water, and thereby promoting the waste of rocks. By thrusting their roots into crevices of cliffs, plants loosen and gradually wedge off pieces of rock, and by sending their roots and rootlets through the soil, they open up the subsoil to be attacked by air and descending moisture (p. 21). The action of the common earthworm in bringing up fine soil to be exposed to the influences of wind and rain was referred to at p. 24. Many burrowing animals also, such as the mole and rabbit, throw up large quantities of soil and subsoil which are liable to be blown or washed away. On the other hand, the action may be conservative, as, for instance, where, by forming a covering of turf, vegetation protects the soil underneath from being rapidly removed, or where sand-loving plants bind together the surface of dunes, and thereby arrest the progress of the sand, or where forests shield a mountain-side from the effects of heavy rains and descending avalanches. But it is chiefly by the aggregation of their own remains into more or less extensive deposits that plants and animals leave their most prominent and enduring memorials. As examples of the way in which this is done by plants, refer- ence may be made to peat-bogs, mangrove-swamps, infusorial earth, and calcareous sea-weeds. In temperate and arctic countries, marshy vegetation accumulates in peat -bogs over areas of many square miles and to a depth of sometimes 50 feet. These deposits are largely due to the growth of bog -mosses and other aquatic plants which, dying in their lower parts, continue to grow upward on the same spot. On flat or gently-inclined moors, in hollows between hills, on valley -bottoms, and in shallow lakes, this marshy vegetation accumulates as a wet spongy no RECORDS OF PLANTS AND ANIMALS. [CHAP. fibrous mass, the lower portions of which by degrees become a more or less compact dark brown or black pulpy substance, wherein the fibrous texture, so well seen in the upper or younger parts, in large measure disappears. In a thick bed of peat, it is not infrequently possible to detect a succession of plant remains, showing that one kind of vegetation has given place to another during the accumulation of the mass. In Europe, as already mentioned on p. 6, peat-bogs often rest directly upon fresh -water marl containing remains of lacustrine shells (i in Fig. 30). In every such case, it is evident that the peat has FIG. 3 o.-Section of a accumulated on the site of a shallow peat-bog. lake which has been filled up, and converted into a morass by the growth of marsh-plants along its edges and over its floor. The lowest parts of the peat may contain remains of the reeds, sedges, and other aquatic plants which choked up the lake (2, 3). Higher up, the peat consists almost entirely of the matted fibres of different mosses, especially of the kind known as Bog-moss or Sphagnum (4). The uppermost layers (5, 6) may be full of roots of different heaths which spread over the surface of the bog. The rate of growth of peat has been observed in different situations in Central Europe to vary from less than a foot to about 2 feet in ten years ; but in more northern latitudes the growth is probably slower. Many thousand square miles of Europe and North America are covered with peat -bogs, those of Ireland being computed to occupy a seventh part of the surface of the island, or upwards of 4000 square miles. viii.] PEAT, MANGROVE-SWAMPS, DIATOM-EARTH, in As the aquatic plants grow from the sides toward the centre of a shallow lake, they gradually cover over the surface of the water with a spongy layer of matted vegetation. Animals, and 'man himself, venturing on this treacherous surface sink through it, and may be drowned in the black peaty mire underneath ; and long afterwards, when the morass has become firm ground, and openings are made in it for digging out the peat to be used as fuel, their bodies may be found in an excellent state of preservation. The peaty water so protects them from decay that the very skin and hair sometimes remain. In Ireland, numerous skeletons of the great Irish elk have been obtained from the bogs, though the animal itself has been extinct since before the beginning of the authentic history of the country. Along the flat shores of tropical lands, the mangrove tree grows out into the salt water, forming a belt of jungle which runs up or completely fills the creeks and bays. So dense is the vegetation that the sand and mud, washed into the sea from the land, are arrested among the roots and radicles of the trees, and thus the sea is gradually replaced by firm ground. The coast of Florida is fringed with such man- grove-swamps for a breadth of from 5 to 20 miles. In such regions, not only does the growth of these swamps add to the breadth of the land, but the sea is barred back, and prevented from attacking the newly -formed ground inside. A third kind of vegetable deposit to be referred to here is that known by the names of infusorial earth, diatom-earth, and tripoli-powder. It consists almost entirely of the minute frustules of microscopic plants called diatoms, which are found abundantly in lakes and likewise in some regions of the ocean (Fig. 31). These lowly organisms are remarkable H2 RECORDS OF PLANTS AND ANIMALS. [CHAP. for secreting silica in their structure. As they die, their singu- larly durable siliceous remains fall like a fine dust on the bottom of the water, and accumulate there as a pale grey or straw-coloured deposit, which, when dry, is like flour, and in its pure varieties is made almost entirely of silica (90 to 97 per cent). Underneath the peat-bogs of Britain, a layer of this material is sometimes met with. One of the most famous examples is that of Richmond, Virginia, where a bed of it occurs 30 feet thick. 'At Bilin in Bohemia also an important bed has long been known. The bottom of some FIG. 31. Diatom-earth from floor of Antarctic Ocean, magnified 300 diameters {Challenger Expedition). parts of the Southern Ocean is covered with a diatom-ooze made up mainly of siliceous diatoms, but containing also other siliceous organisms (radiolarians) and calcareous fora- minifera (Fig. 31). Yet one further illustration of plant-action in the build- ing up of solid rock may be given. Some sea-weeds abstract from sea-water carbonate of lime, which they secrete to such an extent as to form a hard stony structure, as in the case of the common nullipore. When the plants die, their re- mains are thrown ashore and pounded up by the waves, and being singularly durable they form a white calcareous sand. viii.] NULLIPORE-SAND, SHELL-BANKS. 113 By the action of the wind, this sand is blown inland and may accumulate into dunes. But unlike ordinary sand, it is liable to be slightly dissolved by rain-water, and as the por- tion so dissolved is soon redeposited by the evaporation of the moisture, the little sand-grains are cemented together, and a hard crust is formed which protects the sand under- neath from being blown away. Meanwhile rain-water per- colating through the mounds gradually solidifies them by cementing the particles of sand to each other, and thick masses of solid white stone are thus produced. Changes of this kind have taken place on a great scale at Bermuda, where all the dry land consists of limestone formed of com- pacted calcareous sand, mainly the detritus of sea-weeds. Animals are, on the whole, far more successful than plants in leaving enduring memorials of their life and work. They secrete hard outer shells and internal skeletons en- dowed with great durability, and capable of being piled up into thick and extensive deposits which may be solidified into compact and enduring stone. On land, we have an example of this kind of accumulation in the lacustrine marl already (pp. 6, 61) described as formed of the congregated remains of various shells. But it is in the sea that animals, secreting carbonate of lime, build up thick masses of rock, such as shell-banks, ooze, and coral-reefs. Some molluscs, such as the oyster, live in populous communities upon submarine banks. In the course of generations, thick accumulations of their shells are formed on these banks. By the action of currents, also, large quantities of broken shells are drifted to various parts of the sea-bottom not far from land. Such deposits of shells, in situ or transported, may be more or less mixed with or buried under sand and silt, according as the currents vary i H4 RECORDS OF PLANTS AND ANIMALS. [CHAP. in direction and force. On the other hand, they may be gradually cemented into a solid calcareous mass, as has been observed off the coast of Florida, where they form on the FIG. 32. Recent limestone (Common Cockle, etc., cemented in a matrix of broken shells). sea-bottom a sheet of limestone, made up of remains of the very same kinds of creatures that are living there. From observations made during the great expedition of the Challenger^ it has been estimated that in a square mile of the tropical ocean down to a depth of 100 fathoms there are more than 16 tons of carbonate of lime in the form of living animals. A continual rain of dead calcareous organ- isms is falling to the bottom, where their remains accumulate as a soft chalky ooze. Wide tracts of the ocean -floor are covered with a pale grey ooze of this nature, composed mainly of the remains of the shells of the foraminifer Globi- gerina (Fig. 33). In the north Atlantic this deposit prob- ably extends not less than 1300 miles from east to west, and several hundred miles from north to south. Here and there, especially among volcanic islands, por- tions of the sea-bed have been raised up into land, and masses of modern limestone have thereby been exposed to CORAL ISLANDS. H5 view. Though they are full of the same kind of shells as are still living in the neighbouring sea, they have been cemented into compact and even somewhat crystalline FIG. 33. Globigerina ooze dredged up by Challenger Expedition from a depth of 1900 fathoms in the North Atlantic (M). rock, which has been eaten into caverns by percolating water, like limestones of much older date. This cementa- tion, as above remarked, is due to water permeating the stone, dissolving from." its outer parts the calcareous matter of shells, corallines, and other organic remains, and redeposit- ing it again lower down, so as to cement the organic detritus into a compact stone. Coral islands offer an impressive example of how exten- sive masses of solid rock may be built up entirely of the aggregated remains of animals. In some of the warmer seas of the globe, and notably in the track of the great ocean -currents, where marine life is so abundant, various kinds of coral take root upon the edges and summits of sub- merged ridges and peaks, as well as on the shelving sea- bottom facing continents or encircling islands (i in Fig. 34). These creatures do not appear to flourish at a greater depth than 15 or 20 fathoms, and they are killed by exposure n6 RECORDS OF PLANTS AND ANIMALS. [CHAP. to sun and air. The vertical space within which they live may therefore be stated broadly as about 100 feet. They grow in colonies, each composed of many individuals, but all united into one mass, which at first may be merely a little solitary clump on the sea-floor, but which, as it grows, joins other similar clumps to form what is known as a reef. Each individual secretes from the sea-water a hard calcareous skeleton inside its transparent jelly-like body, and when it 'dies, this skeleton forms part of the platform upon which the FIG. 34. Section of a coral-reef. I. Top of the submarine ridge or bank on which the corals begin to build. 2.. Coral-reef. 3. Talus of large blocks of coral-rock on which the reef is built outward. 4. Fine coral sand and mud produced by the grinding action of the breakers on the edge of the reef. 5. Coral sand thrown up by the waves and gradually accumulating above their reach to form dry ground. next generation starts. Thus the reef is gradually built upward as a mass of calcareous rock (2), though only its upper surface is covered with living corals. These creatures continue to work upward until they reach low-water mark, and then their further upward progress is checked. But they are still able to grow outward. On the outer edges of the reef they flourish most vigorously, for there, amid the play of the breakers, they find the food that is brought to them by the ocean-currents. From time to time, fragments are torn off by breakers from the reef and roll down its steep front (3). There, partly by the chemical action of the sea- VIIL] CORAL ISLANDS. 117 water, and partly by the fine calcareous mud and sand (4), produced by the grinding action of the waves and washed into their crevices, these loose blocks are cemented into a firm steep slope, on the top of which the reef continues to grow outwards. Blocks of coral and quantities of coral-sand are also thrown up on the surface of the reef, where, by degrees, they form a belt of low land above the reach of the waves (5). On the inside of the reef, where the corals can- not find the abundant food -supply afforded by the open water outside, they dwindle and die. Thus the tendency of all reefs must be to grow seawards and to increase in breadth. Perhaps their breadth may afford some indication of their relative age. Where a reef has started on a shelving sea-bottom near the coast of a continent, or round a volcanic island, the space of water inside is termed the Lagoon Channel. Where the reef has been built up on some submarine ridge or peak, and there is consequently no land inside, the enclosed space of water is called a Lagoon^ and the circular reef of coral is known as an Atoll. If no subsidence of the sea-bottom takes place, the maximum thickness of a reef must be limited by the space within which the corals can thrive that is, a vertical depth of about 100 feet from the surface of the sea. But the effect of the destruction of the ocean-front of the reef, and the piling up of a slope of its fragments on the sea-bottom outside, will be to furnish a platform of the same materials on which the reef itself may grow outward, so that the united mass of calcareous rock may attain a very much greater thickness than 100 feet. It is remarkable how rapidly and completely the struc- ture of the coral-skeleton is effaced from the coral-rock, and a more or less crystalline and compact texture is put in its n8 RECORDS OF PLANTS AND ANIMALS. [CHAP. place. The change is brought about partly by the action of both sea-water and rain-water in dissolving and redeposit- ing carbonate of lime among the minute interstices of the rock, and partly also by the abundant mud and sand pro- duced by the pounding action of the breakers on the reef, and washed into the crevices. On the portion of a reef laid dry at low-water, the coral-rock looks in many places as solid and old as some of the ancient white limestones and marbles of the land. There, in pools, where a current or ripple of water keeps the grains of coral-sand in motion, each grain may be seen to have taken a spherical form unlike that of the ordinary irregularly rounded or angular particles. This arises because carbonate of lime in solution in the water is deposited round each grain as it moves along. A mass of such grains aggregated together is called oolite, from its resemblance to fish-roe. In many limestones, forming wide tracts of richly cultivated country, this oolitic structure is strikingly exhibited. There can be no doubt that in these cases it was produced in a similar way to that now in pro- gress on coral-reefs (see p. 188). In the coral tracts of the Pacific Ocean, there are nearly 300 coral islands, besides extensive reefs round volcanic islands. Others occur in the Indian Ocean. Coral-reefs abound in the West Indian Seas, where, on many of the islands, they have been upraised into dry land, in Cuba to a height of 1 100 feet above sea-level. The Great Barrier Reef that fronts the north-eastern coast of Australia is 1250 miles long, and from 10 to 90 miles broad. There are other ways in which the aggregation of animal remains forms more or less extensive and durable rocks. To some of these, references will be made in later chapters. Enough has been said here to show that by the accumula- viii.] ENTOMBMENT OF ORGANIC REMAINS. 119 tion of their hard parts animals leave permanent records of their presence both on land and in the sea. II. But it is not only in rocks formed out of their remains that animals leave their enduring records. These remains may be preserved in almost every kind of deposit, under the most wonderful variety of conditions. And as it is in large measure from their occurrence in such deposits that the geologist derives the evidence that successive tribes of plants and animals have peopled the globe, and that the climate and geography of the earth have greatly varied at different periods, we shall find it useful to observe the different ways in which the remains both of plants and animals are at this moment being entombed and preserved upon the land and in the sea. With the knowledge thus gained, it will be easier to understand the lessons taught by the organic remains that lie among the various solid rocks around us. It is evident that in the vast majority of cases, the plants and animals of the land leave no perceptible trace of their presence. Of the forests that once covered so much of Central and Northern Europe, which is now bare ground, most have disappeared, and unless authentic history told that they had once flourished, we might never know anything about them. There were also herds of wild oxen, bears, wolves, and other denizens contemporaneous with the vanished forests. But they too have passed away, and we might ransack the soil in vain for any trace of them. If the remains of terrestrial vegetation and animals are anywhere preserved it must obviously be only locally, but the favourable circumstances for their preservation, although not everywhere to be found, do present themselves in many places if we seek for them. The fundamental condition is that the relics should, as soon as possible after death, be so 120 RECORDS OF PLANTS AND ANIMALS. [CHAP. covered up as to be protected from the air and from too rapid decomposition. Where this condition is fulfilled, the more durable of them may be preserved for an indefinite series of ages. (a) On land, there are various places where the remains both of plants and animals are buried and shielded from decay. To some of these reference has already been made. Thus on the floors of lakes, amid the fine silt, mud, and marl gathering there, leaves, fruits, and branches, or tree- trunks, washed from the neighbouring shores, may be im- bedded, together with insects, birds, fishes, lizards, frogs, field-mice, rabbits, and other inhabitants. These remains may of course often decay on the lake-bottom, but where they sink into or are quickly covered up by the sediment, they may be effectually preserved from obliteration. They undergo a change, indeed, being gradually turned into stone, as will be described in chapter xv. But this conversion may be effected so gently as to retain the finest microscopic textures of the original organisms. In peat-bogs also, as already stated (p. 1 1 1), wild animals are often engulfed, and their soft parts are occasionally preserved as well as their skeletons. The deltas of river- mouths must receive abundantly the remains of animals swept off by floods. As the carcases float seawards, they begin to fall to pieces and the separate bones sink to the bottom, where they are soon buried in the silt. Among the first bones to separate from the rest of the skeleton are the lower jaws (pp. 400, 405). We should therefore expect that were excavations made in a delta these bones would occur most frequently, the rest of the skeleton being apt to be car- ried farther out to sea before its bones could find their way to the bottom. The stalagmite floor of caverns has already viii.] BONE-CAVES. 121 been referred to (p. 76) as an admirable material for enclosing and preserving organic remains. The animals that fell into those recesses, or used them as dens in which they lived or into which they dragged their prey, have left their bones on the floors, where, encased in or covered by solid stalagmite, these relics have remained for ages. Most of our knowledge of the animals which inhabited Europe at the time when man appeared, is derived from the materials disinterred from these bone-caves. Allusion has also been made to the traver- tine formed by mineral-springs and to the facility with which leaves, shells, insects, and small birds, reptiles, or mammals may be enclosed and preserved in it. Thus, while the plants and animals of the land for the most part die and decay into mere mould, there are here and there localities where their remains are covered up from decay and preserved as memorials of the life of the time. (&) On the bottom of the sea, the conditions for the pre- servation of organic remains are more general and favourable than on land. Among the sands and gravels of the shore, some of the stronger shells that live in the shallower waters near land, but often only in rolled fragments, may be covered up and preserved. It is below tide-mark, however, and more especially beneath the limit to which the disturbing action of breakers descends, that the remains of the denizens of the sea are most likely to be buried in sediment and to be preserved there as memorials of the life of the sea. It is evident that hard and therefore durable relics have the best chance of escaping destruction. Shells, corals, corallines, spicules of sponges, teeth, vertebrae, and ear -bones of fishes may be securely entombed in successive layers of silt or mud. But the vast crowds of marine creatures that have no hard parts must almost always perish without leaving any 122 RECORDS OF PLANTS AND ANIMALS. [CHAP. trace whatever of their existence. And even in the case ot those which possess hard shells or skeletons, it will be easily understood that the great majority of them must be decom- posed upon the sea-bottom, their component elements pass- ing back again into the sea -water from which they were originally derived. It is only where sediment is deposited fast enough to cover them up and protect them before they have time to decay, that they may be expected to be pre- served. In the most favourable circumstances, therefore, only a very small proportion of the creatures living in the sea at any time leave a tangible record of their presence in the deposits of the sea-bottom. It is in the upper waters of the ocean, and especially in the neighbourhood of land, that life is most abundant. The same region, also, is that in which the sediment derived from the waste of the land is chiefly distributed. Hence it is in these marginal parts of the ocean that the conditions for preserving memorials of the animals that inhabit the sea are best developed. As we recede from the land, the rate of deposit of sediment on the sea -floor gradually diminishes, until in the central abysses it reaches that feeble stage so strikingly brought before us by the evidence of the manganese nodules (p. 105). The larger and thinner calcareous organisms are attacked by the sea-water and dissolved, apparently before they can sink to the bottom ; at least their remains are comparatively rarely found there. It is such indestructible objects as sharks' teeth and vertebrae and ear-bones of whales that form the most conspicuous organic relics in those abysmal deposits. Summary. Plants and animals leave their records in geological history, partly by forming distinct accumulations viir.] SUMMARY. 123 of their remains, partly by contributing their remains to be imbedded in different kinds of deposits both on land and in the sea. As examples of the first mode of chronicling their existence, we may take the growth of marsh-plants in peat- bogs, the spread of mangrove-swamps along tropical shores, and the deposition of infusorial earth on the bottom of lakes and of the sea; the accumulation of nullipore sand into solid stone, the formation of extensive shell-banks in many seas, the wide diffusion of organic ooze over the floor of the sea, and the growth of coral reefs. As illustrations of the second method, we may cite the manner in which the remains of terrestrial plants and animals are preserved in peat-bogs, in the deltas of rivers, in the stalagmite of caverns, and in the travertine of springs ; and the way in which the hard parts of marine creatures are entombed in the sediments of the sea-floor, more especially along that belt fringing the continents and islands, where the chief deposit of sediment from the disintegration of the land takes place. Neverthe- less, alike on land and sea, the proportion of organic remains thus sealed up and preserved is probably always but an in- significant part of the total population of plants and animals living at any given moment. How the remains of plants and animals when once en- tombed in sediment are then hardened and petrified, so as to retain their minute structures, and to be capable of endur- ing for untold ages, will be treated of in chapter xv. CHAPTER IX. THE RECORDS LEFT BY VOLCANOES AND EARTHQUAKES. THE geological changes described in the foregoing chapters affect only the surface of the earth. A little reflection will convince us that they may all be referred to one common source of energy the sun. It is chiefly to the daily influ- ence of that great centre of heat and light that we must ascribe the ceaseless movements of the atmosphere, the phenomena of evaporation and condensation, the circulation of water over the land, the waves and currents of the sea, in short, the whole complex system which constitutes what has been called the Life of the Earth. Could this influence be conceivably withdrawn, the planet would become cold, dark, silent, lifeless. But besides the continual transformations of its surface due to solar energy, our globe possesses distinct energy of its own. Its movements of rotation and revolution, for example, provide a vast store of force, whereby many of the most important geological processes are initiated or modi- fied, as in the phenomena of day and night, and the seasons, with the innumerable meteorological and other effects that flow therefrom. These movements, though slowly growing feebler, bear witness to the wonderful vigour of the earlier CHAP, ix.] INTERNAL HEAT OF THE GLOBE. 125 phases of the earth's existence. Inside the globe, too, lies a vast magazine of planetary energy in the form of an in- terior of intensely hot material. The cool outer shell is but an insignificant part of the total bulk of the globe. To this cool part the name of " crust " was given at a time when the earth was believed to consist of an inner molten nucleus enclosed within an outer solid shell or crust. The term is now used merely to denote the cool solid external part of the globe, without implying any theory as to the nature of the interior. It is obvious that we are not likely ever to learn by direct observation what may be the condition of the interior of our planet. The cool solid outer shell is far too thick to be pierced through by human efforts ; but by various kinds of observa- tions, more or less probable conclusions may be drawn with regard to this problem. In the first place, it has been ascer- tained that all over the world, wherever borings are made for water or in mining operations, the temperature increases in proportion to the depth pierced, and that the average rate of increase amounts to about- one degree Fahrenheit for every 64 feet of descent. If the rise of temperature continues inward at this rate or at any rate at all approaching it, then at a distance from the surface, which in proportion to the bulk of the whole globe is comparatively trifling, the heat must be as great as that at which the ordinary materials of the crust would melt at the surface. In the second place, thermal springs in all quarters of the globe, rising sometimes with the temperature of boiling water, and occasionally even still hotter, prove that the interior of the planet must be very much hotter than its exterior. In the third place, volcanoes widely distributed over the earth's surface throw out steam and heated vapours, red-hot stones, and streams of molten rock. 126 VOLCANOES AND EARTHQUAKES. [CHAP. It is quite certain, therefore, that the interior of the globe must be intensely hot; but whether it is actually molten or solid has been the subject of prolonged discus- sion. Three opinions have found stout defenders, (i) The older geologists maintained that the phenomena of volcanoes and earthquakes could not be explained, except on the sup- position of a crust only a few miles thick, enclosing a vast central ocean of molten material. (2) This view has been opposed by physicists who have shown that the globe, if this were actually its structure, could not resist the attraction of sun and moon, but would be drawn out of shape, as the ocean is in the phenomenon of the tides, and that the absence of any appreciable tidal deformation in the crust shows that the earth must be practically solid and as rigid as a ball of glass, or of steel. (3) A third opinion has been advanced by geologists who, while admitting that the earth behaves on the whole as a solid rigid body, yet believe that many geological phenomena can only be explained by the existence of some liquid mass beneath the crust. Accord- ingly they suppose that while the nucleus is retained in the solid state by the enormous superincumbent pressure under which it lies, and the crust has become solid by cooling, there is an intermediate liquid or viscous layer which has not yet cooled sufficiently to pass into the solid crust above, and does not lie under sufficient pressure to form part of the solid nucleus below. At present, the balance of evidence and argument seems to be in favour of the practical rigidity and solidity of the globe as a whole. But the materials of its interior must possess temperatures far higher than those at which they would melt at the surface. They are no doubt kept solid by the vast overlying pressure, and any change which could relieve them of this pressure would allow them ix.] VOLCANOES. 127 to pass into the liquid form. This subject will be again alluded to in chapter xvi. Meanwhile, let us consider how the intensely hot nucleus of the planet reacts upon its surface. Rocks are bad conductors of heat. So slowly is the heat of the interior conducted upwards by them that the temperature of the surface of the crust is not appreciably affected by that of the intensely hot nucleus. But the fact that the surface is not warmed from this source shows that the heat of the interior must pass off into space as fast as it arrives at the surface, and proves that our planet is gradu- ally cooling. For many millions of years the earth has been radiating heat into space, and has consequently been losing energy. Its present store of planetary vitality, therefore, must be regarded as greatly less than it once was. VOLCANOES. Of all the manifestations of this vitality, by far the most impressive are those furnished by volcanoes. The general characters of these vents of communication between the hot interior and cool surface of the planet are doubtless already familiar to the reader of these chapters the volcano itself, a conical hill or mountain, formed mainly or entirely of materials ejected from below, having on its truncated summit the basin-shaped crater, at the bottom of which lies the vent or funnel from which, as well as from rents on the flanks of the cone, hot vapours, cinders, ashes, and streams of molten lava are discharged, till they gradually pile up the volcanic cone round the vent whence they escape. A volcanic cone, so long as it remains, bears eloquent testimony to the nature of the causes that produced it. Even many centuries after it has ceased to be active, when no vapours rise from any part of its cold, silent, and motion- 128 VOLCANOES AND EARTHQUAKES. [CHAP. less surface, its conical form, its cup-shaped crater, its slopes of loose ashes, and its black bristling lava-currents remain as unimpeachable witnesses that the volcanic fires, now quenched, once blazed forth fiercely. The wonderful groups of volcanoes in Auvergne and the Eifel are as fresh as if they had not yet ceased to be active, and might break forth again at any moment ; yet they have been quiescent ever since the beginning of authentic human history. But in the progress of the degradation which everywhere slowly changes the face of the land, it is impossible that vol- canic hills should escape the waste which befalls every other kind of eminence. We can picture a time when the volcanic cones of Auvergne will have been worn away, and when the lava-streams that descend from them will be cut into ravines and isolated masses by the streams that have even already deeply trenched them. Where all the ordinary and familiar signs of a volcano have been removed, how can we tell that any volcano ever existed ? What enduring record do vol- canoes inscribe in geological history ? Now, it must be obvious that among the operations of an active volcano, many of the most striking phenomena have hardly any importance as aids in recognising the traces of long extinct volcanic action. The earthquakes and tremors that accompany volcanic outbursts, the constant and prodigious out-rushing of steam, the abundant discharge of gases and acid vapours, though singularly impressive at the time, leave little or no lasting mark of their occurrence. It is not in phenomena, so to speak, transient in their effects, that we must seek for a guide in exploring the records of ancient volcanoes, but in those which fracture or otherwise alter the rocks below ground, and pile up heaps of material above. Keeping this aim before us, we may obtain from an ix.] VOLCANOES AND VOLCANIC PRODUCTS. 129 examination of what takes place at an active volcano such durable proofs of volcanic energy as will enable us to recog- nise the former existence of volcanoes over many tracts of the globe where human eye has never witnessed an erup- tion, and where, indeed, all trace of what could be called a volcano has utterly vanished. A method of observation and reasoning has been established, from the use of which we learn that in some countries, Britain for example, though there is now no sign of volcanic activity, there has been a succession of volcanoes during many protracted and widely separated periods, and that probably the interval that has passed away since the last eruptions is not so vast as that which separated these from those that preceded them. A similar story has been made out in many parts of the con- tinent of Europe, in the United States, India, and New Zealand, and, indeed, in most countries where the subject has been fully investigated. A little reflection on this question will convince us that the permanent records of volcanic action must be of two kinds : first and most obvious are the piles of volcanic materials which have been spread out upon the surface of the earth, not only round the immediate vents of eruption, but often to great distances from them ; secondly, the rents and other openings in the solid crust of the earth caused by the volcanic explosions, and some of which have served as channels by which the volcanic materials have been expelled to the surface. Volcanic Products. We shall first consider those materials which are erupted from volcanic vents and are heaped up on the surface as volcanic cones or spread out as sheets. They may be conveniently divided into two groups ; ist, Lava, and 2d, Fragmentary materials. K 130 VOLCANOES AND EARTHQUAKES. [CHAP. (i) Lava. Under this name are comprised all the molten rocks of volcanoes. These rocks present many varieties in composition and texture, some of the more important of which will be described in chapter xi. Most of them are crystalline that is, are made up wholly or in greater part of crystals of two or more minerals interlocked and felted together into a coherent mass. Some are chiefly composed of a dark brown or black glass, while others consist of a compact stony substance with abundant crystals imbedded in FIG. 35. Cellular Lava with a few of the cells filled up with infiltrated mineral matter (Amygdules). it. In many cases, they are strikingly cellular that is to say, they contain a large number of spherical or almond-shaped cavities somewhat like those of a sponge or of bread, formed by the expansion of the steam absorbed in the molten rock (Fig. 35 and p. 193). They vary much in weight and in colour. The heavier kinds are more than three times the weight of water; or, in other words, they have a specific gravity ranging up to 3*3 ; and are commonly dark grey to black. The lighter varieties, on the other hand, are little more than twice the weight of water, or have a specific IX.] LAVA-CURRENTS. gravity which may be as low as 2*3, while their colours are usually paler, sometimes almost white. When lava is poured out at the surface it issues at a white heat that is, at a temperature sometimes above that of melting copper, or more than 2204 Fahr. ; but its surface rapidly darkens, cools, and hardens into a solid crust which varies in aspect according to the liquidity of the mass. Some lavas are remarkably fluid, flowing along swiftly like melted iron ; others move sluggishly in a stiff viscous stream. In many pasty lavas, the surface breaks up into rough cindery blocks or scoriae like the slags of a foundry, which grind upon each other as the still molten stream underneath creeps forward (p. 193). In general, the upper part of a lava-stream is more cellular than the central portions, no doubt because the imprisoned steam can there more easily expand. The bottom, too, is often rough and slaggy, as the lava is cooled by contact with the ground, and portions of the chilled bottom-crust are pushed along or broken up and involved in the still fluid portion above. There are thus three more or less well-defined zones in ^ ~* Chlorine is a transparent gas of a greenish-yellow colour, but except possibly at active volcanic vents it does not occur in the free state ; united with the alkali metals, potassium, sodium, and magnesium, it forms the chief salts of sea-water. The most important of these salts, sodium-chloride, or com- mon salt (NaCl) contains 60*64 per cent of chlorine, and forms 2-64 per cent by weight of sea-water. This salt is found diffused in microscopic particles in the air, especially near the sea, and beds of it hundreds of feet thick occur in many parts of the world among the sedimentary rocks that constitute most of the dry land. Phosphorus does not occur free; it has so strong an affinity for oxygen that it rapidly oxidises on exposure to the air, and even melts and takes fire. Its most frequent combination is with oxygen and calcium, as calcium -phos- phate or phosphate of lime (Ca 3 (PO 4 ) 2 , p. 182). Though for the most part present in minute proportions, it is widely dif- fused in nature. It occurs in fresh and sea water, in soil and in plants, especially in their fruits and seeds ; it is supplied by plants to animals for the formation of bones, which when burnt are found to consist almost entirely of phosphate of lime. x.] METALS. 161 Fluorine also is never met with uncombined ; it never unites with oxygen, forming in this respect the sole exception among the elements ; its most frequent combination as a rock constituent is with calcium, when it forms the mineral Fluor-spar (CaF 2 ). Like phosphorus, it is widely diffused in minute proportions in the waters of some springs, rivers, and the sea, and in the bones of animals. To these metalloids we may add the colourless, tasteless gas Nitrogen, which, though not largely present in the earth's crust, constitutes four-fifths by volume or 77 per cent by weight of the atmosphere. It does not enter into com- bination so readily as the other elements above enumerated, but it is always found in the composition of plants and is a constituent of many animal tissues. It is the principal ingredient of the substance called ammonia, which is pro- duced when moist organic matter is decomposed in the air. In many rocks composed wholly or in great part of organic remains, such, for instance, as peat and coal, nitrogen is a constant constituent. METALS. Though so large a proportion of the known terrestrial elements are metals, these are much less abundant in the earth's crust than the metalloids. The most frequent are Aluminium, Calcium, and Magnesium. The substances most familiar to us as metals occupy an altogether sub- ordinate part among rocks, the most abundant of them being Iron. Aluminium never occurs in the free state, but can be artificially separated from its compounds when it is seen to be a white, light, malleable metal. It is almost always united with oxygen as the oxide of Alumina (A1 2 O 3 ) which occurs crystallised as the ruby and sapphire, but is for the most part united with silica, and in this form constitutes the basis of M 162 ELEMENTS OF EARTH'S CRUST. [CHAP. the great family of minerals known as the Silicates of Alumina, or Aluminous Silicates. These silicates generally contain some other ingredient which is more liable to de- composition, and when they decay and their more soluble parts are removed, they pass into clay, which consists chiefly of hydrated silicate of alumina. Calcium is not met with uncombined, but has been artificially isolated and found to be a light, yellowish metal, between gold and lead in hardness. It occurs in nature chiefly combined with carbonic acid as a carbonate, and with sulphuric acid as a sulphate, to both of which substances reference has already been made; it is also present in many silicates. So abundant is calcium-carbonate or car- bonate of lime in nature that it may be detected in most natural waters, which dissolve it and carry it in solution into the sea. Its presence in rocks may be detected by a drop of any mineral acid, when the liberated carbon -dioxide escapes as a gas with brisk effervescence. Calcium-sulphate is likewise a common constituent of terrestrial waters, especi- ally of those which in household management are called hard ; it constitutes not less than 3^6 per cent of the salts in ordinary sea-water, and when sea-water is evaporated this sulphate (gypsum), being least soluble, is the first to be pre- cipitated in minute crystals resembling in shape those shown in Fig. 62. Magnesium is likewise only isolated artificially, when it appears as a soft, silver-white, malleable and ductile metal. It occurs in sea-water combined with chlorine as magnesium- chloride, which constitutes io'S per cent of the total propor- tion of salts. It unites with carbonic acid as a carbonate, which with carbonate of lime forms the widely diffused rock called magnesian limestone or dolomite ; it also enters into x.] ALKALI METALS, IRON. 163 the composition of the Magnesian Silicates which are only second in importance to those of alumina. Potassium and Sodium (alkali metals) are only obtain- able in the free state by chemical processes, when they are found to be white, brittle metals that float on water, and rapidly oxidise if exposed to the air. Combined with chlorine, sodium forms the familiar chloride known as common salt, which constitutes 777 per cent of the salts of sea-water, is abundantly present in salt lakes, and occurs in extensive beds among the rocks of the dry land. Potassium-chloride like- wise occurs in the sea and may be obtained from the ashes of burnt sea-weed. Enormous deposits of it, combined with chlorides of sodium and magnesium, have been met with in Germany (Stassmrt). Potassium also exists in the sea in combination with sulphuric acid as potassium-sulphate or sul- phate of potash, which amounts to about 2 "4 of the total salts of sea-water. Sulphates of potassium, sodium, magnesium, and calcium form thick masses of rock in the Stassfurt deposits. Potassium and sodium in combination with silica form silicates which enter largely into the composition of many rocks. They are readily attacked by water contain- ing carbonic acid, giving rise to what are called carbonates of the alkalies, or alkaline carbonates, which are removed in solution. By this means, carbonate of potash is introduced into soil, where it is taken up by plants into their leaves and succulent parts. When wood is burnt, this carbonate in considerable quantity may be dissolved with water out of the ash. Iron is found in the free or native state in minute grains and large blocks in some volcanic rocks, also in granules of " cosmic dust," probably of meteoric origin, and in fragments of various size which have undoubtedly fallen upon the 164 ELEMENTS OF EARTH S CRUST. [CHAP. earth's surface from the regions of space. There is reason to believe that much of the solid interior of the globe may consist of native iron and other metals. But it is in combination that iron is chiefly of importance in the earth's crust. It has united with oxygen to form several abundant oxides. The protoxide or ferrous oxide (FeO) contains the lowest proportion of oxygen, and being, there- fore, prone to take up more, gives rise to many of the processes of decay included under the general name of weathering. It is readily dissolved by organic and other acids, and is then removed in solution, but on exposure rapidly oxidises and passes into the highest oxide, known as the peroxide or sesquioxide of iron or ferric oxide (Fe 2 O 3 ), which, being the permanent insoluble form, is found abund- antly among the rocks of the earth's crust. Iron is the great colouring matter of nature ; its protoxide compounds give greenish hues to many rocks, while its peroxide colours them various shades of red which, when the peroxide is combined "with water, pass into many tints of brown, orange, and yellow. Manganese is commonly associated with iron in minute proportion in many lavas and other crystalline rocks; its oxides resemble those of iron in their modes of occurrence. Barium and Calcium are called metals of the alkaline earths. The former can only be obtained in a free state by artificial means, when it appears as a pale yellow very heavy metal which rapidly tarnishes. In nature it chiefly occurs as the sulphate, barytes, or heavy spar (BaSO 4 ), a mineral of frequent occurrence in veins associated with metallic ores (p. 182). Passing now from the simple elements we have next to note the mineral-forms in which they appear as constituents of the earth's crust. A mineral may be defined as an in- x.] MINERALS DEFINED. 165 organic substance, having theoretically a definite chemical composition, and in most cases, also, a certain geometrical form. It may consist of only one element, for example, the diamond, sulphur, and the native metals, gold, silver, copper, etc. But in the vast majority of cases, minerals consist of at least two, usually more, elements in definite chemical proportions. In the following short list of the more important minerals of the earth's crust they are arranged chemically, according to the predominant element in them, or the manner in which the combinations of the elements have taken place, so that their leading features of composi- tion may be at once perceived. The two elements, Carbon and Sulphur, in their native or uncombined state, sometimes form considerable masses of rock. Some of the native metals, also, may be enumerated as rock-constituents when they occur in sufficient quantities to be commercially import- ant. Gold, for example, is found in grains and strings, in veins of quartz, and in irregular pieces or nuggets dispersed through the gravel deposits of regions where gold-bearing quartz-veins traverse the solid rocks. Omitting, however, the minerals formed of a single element, we may pass on to combinations of two or more elements, and consider first those in which oxygen is combined with some other element, forming what are commonly grouped together as oxides. Then will come the Silicates, or combinations of silica with one or more bases, followed by the Carbonates, or combina- tions of carbon-dioxide with some base ; the Sulphates, or compounds of sulphuric acid and a base ; the Fluorides, or compounds of fluorine and a metal ; the Chlorides, or com- pounds of chlorine and a metal ; and the Sulphides, or com- pounds of sulphur and a metal. One of the most obvious features in a crystal of any i66 ELEMENTS OF EARTH'S CRUST. [CHAP. mineral is the regular and sharply-defined edges and corners which it presents. Take a piece of rock-crystal or quartz, for example (Fig. 44), and you will find it to consist of six sides or faces, forming what is called a prism, and bevelled off at the end into a six-sided cone, called a pyramid. If you examine a large collection of similar crystals you may find no two of them exactly alike, yet they agree in pre- senting a six-sided figure. Again, procure a piece of the common mineral calcite, either a whole crystal (Figs. 59, 60), or a portion of a crystalline mass (Fig. 45) ; break it and you will find each fragment to possess the same form, that of a rhombohedron ; crush one of these fragments and you will observe that each little grain of the powder pre- serves the same shape. The rhombohedron, therefore, is FIG. 45. Calcite (Iceland spar), called the fundamental crys- ne form Qf the mineml The property so strikingly shown in calcite, of breaking along definite crystalline planes, is termed cleavage. So perfect is the cleavage of calcite that the crystallised mineral can hardly be broken, except along the planes that define the rhombohedron. Many minerals cleave more or less easily in one or more directions, and break irregularly in others. The cleavage affords a guide to the proper crystalline form of a mineral. Though there are many hundreds of varieties of crystal- line form, they may all be reduced to six primary types or systems. These are distinguished from each other by the showmg its characteristic rhom- bohedral cleavage. x.] CRYSTALLINE FORMS OF MINERALS. 167 number and position of their axes, which are mathematical straight lines, intersecting each other in the interior of a crystal, and connecting the centres of opposite flat faces of the crystal, or opposite angles or corners. The six systems, with their axes, are enumerated in the subjoined list. I. Isometric (monometric, cubical, tesseral, regular). In this system there are three axes which are of the same length, and inter- sect each other at a right angle. The cube, octahedron, and dodecahedron are examples (Fig. 46). Crystals of this system b FIG. 46. Cube (a), octahedron v ), dodecahedron (c\ are distinguished by their symmetry, their length, breadth, and thickness being equal. Common salt, fluor-spar (Fig. 64), and magnetite (Fig. 54) are illustrations. II. Tetragonal (dimetric). The axes are three in number, and intersect each other at a right angle, but one of them, called the vertical axis, is longer or shorter than the other two, which are lateral axes. Hence a crystal belonging to this system may either be oblong or squat (Fig. 47). FIG. 47. Tetragonal prism (b) FIG. 48. Orthorhom- and pyramid (a). bic prism. III. Orthorhombic (trimetric) has the three axes intersecting each other at a right angle, but all of unequal lengths. The 1 68 ELEMENTS OF EARTH S CRUST. [CHAP. rectangular and rhombic prisms, and the rhombic octahedron belong to this system (Fig. 48). IV. Hexagonal. This is the only system with four axes (Fig. 49). The lateral axes are all equal, intersect at right angles the FIG. 49. Hexagonal prism (a), rhombohedron (l>), and scalenohedron (c). vertical axis (which is longer or shorter than they are), and form with each other angles of 60. Water, for instance, crystal- lises in this system, and the six-rayed star of a snow-flake is an illustration of the way in which the lateral axes are placed. Quartz is an example (Fig. 44), also calcite (Figs. 59, 60). V. Monoclinic, with all the axes of unequal length. One of the lateral axes cuts the vertical axis at a right angle, the other intersects the vertical axis obliquely. Augite (Fig. 50), Horn- blende (Fig. 57), and Gypsum (Fig. 62) are examples. VI. Triclinic, the most unsymmetrical of all the systems, all the axes being unequal and placed obliquely to each other (Fig. 51). FIG. 50. Monoclinic prism. Crystal of Augite. FIG. 51. Triclinic prism. Crystal of Albite felspar. Every mineral that takes a crystalline form belongs to one or other of these six systems, and through all its x.] ORIGIN OF CRYSTALLISED MINERALS. 169 varieties of external form the fundamental relations of the axes remain unchanged. Some minerals have crystallised out of solutions in water. How this may take place can be profitably studied by dissolving salt, sugar, or alum in water, and watching how the crystals of these substances gradually shape them- selves out of the concentrated solution, each according to its own crystalline pattern. Other minerals have crystallised from hot vapours (sublimation), as may be observed at the fissures of an active volcano. Others have crystallised out of molten solutions, as in the case of lava. Thoroughly fused lava is a glassy or vitreous solution of all the mineral substances that enter into the composition of the rock, and when it cools, the various minerals crystallise out of it, those that are least fusible taking form first, the most fusible appearing last ; but a residue of non-crystalline glass sometimes remain- ing even when the rock has solidified. It is evident that minerals can only form perfect crystals where they have room and time to crystallise. But where they are crowded together, and where the solution in which they are dissolved dries or cools too rapidly, their regular and symmetrical growth is arrested. They then form only imperfect crystals, but their internal structure is crystalline, and if examined carefully will be found to show that in the attempt to form definite crystals each mineral has followed its own crystalline type. These characters are of much im- portance in the study of rocks, for rocks are only large aggregates of minerals, wherein definite crystals are excep- tional, though the structure of the whole mass may still be quite crystalline. But minerals also occur in various indefinite or non- crystalline shapes. Sometimes they are fibrous or disposed 170 ELEMENTS OF EARTH'S CRUST. [CHAP. in minute fibre-like threads (Fig. 56) ; or concretionary when they have been aggregated into various irregular concretions of globular, kidney -shaped, grape -like, or other imitative shapes (Figs. 61, 64, 65, 75); or stalactitic (Fig. 20) when they have been deposited in pendent forms like stalactites ; or amorphous when they have no definite shape of any kind, as, for instance, in massive ironstone. Oxides occur abundantly as minerals. The most import- ant are those of Silicon (Quartz) and Iron (Haematite, Limonite, Magnetite, Titanic Iron). QUARTZ (Silica, Silicic Acid, SiO 2 ), already alluded to, is the most abundant mineral in the earth's crust. It occurs crystallised, also in various crystalline and non-crystalline varieties. In the crystallised form as common quartz it is, when pure, clear and glassy, but is often coloured yellow, red, green, brown, or black from various impurities. It crystal- lises in the six-sided prisms and pyramids above referred to, the clear colourless varieties being rock-crystal (Fig. 44). When purple it is called amethyst ; yellow and smoke- coloured varieties, found among the Grampian Mountains, are popularly known as Cairngorm stones. In many places, silica has been deposited as chalcedony, in translucent masses with a waxy lustre, and pale grey, blue, brown, red, or black colours. Deposits of this kind are not infrequent among the cavities of rocks. The common pebbles and agates with concentric bands of different colours are examples of chalcedony, and show how the successive layers have been deposited from the walls of the cavity inwards to the centre which is often filled with crystalline quartz (Fig. 52). The dark opaque varieties are called jasper. Quartz can be usually recognised by its vitreous lustre and hardness ; it cannot be scratched with a knife, but easily x.] QUARTZ. 17 1 scratches glass, and it is not soluble in the ordinary acids. It is an essential constituent of many rocks, such as granite FIG. 52. Section of a pebble of chalcedony. The outer banded layers are chalcedony, the interior being nearly filled up with crystalline quartz. and sandstone. Silica being dissolved by natural waters, especially where organic acids or alkaline carbonates are present, is introduced by permeating water into the heart of even the most solid rocks. Hence it is found abundantly in strings and veins traversing rocks, also in cavities and replacing the forms of plants and animals imbedded in sedimentary deposits. Soluble silica is abstracted by some plants and animals and built up into their organic structures (diatoms, radiolarians, sponges). Four minerals composed of Oxides of Iron occur abund- antly among rocks. The peroxide is found in two frequent forms, one without water (Haematite), the other with water (Limonite). The peroxide and protoxide combine to form Magnetite, and a mixture of the peroxide with the per- oxide of the metal titanium gives Titanic Iron. 172 ELEMENTS OF EARTHS CRUST. [CHAP. HEMATITE or Specular Iron (Fe 2 O 3 = Fe7oO3o) occurs in rhombohedral crystals that can with difficulty be scratched with a knife; but is more usually found in a massive condition with a compact, fibrous, or granular texture, and dark steel-grey or iron-black colour, which becomes bright red when the mineral is scratched or powdered. The earthy kinds are red in colour, and it is in this earthy form FIG. 53. Piece of haematite, showing the nodular external form and the internal crystalline structure. that haematite plays so important a part as a colouring material in nature. Red sandstone, for example, owes its red colour to a deposit of earthy peroxide of iron round the grains of sand. Haematite occurs crystallised in fissures of lavas as a product of the hot vapours that escape at these places; but is more abundant in beds and concretionary masses (Fig. 53) among various rocks. LIMONITE or Brown Iron-ore differs from Haematite in being rather softer, in containing more than 14 per cent of water which is combined with the iron to form the hydrated peroxide, in being usually massive or earthy, in presenting a dark brown to yellow colour (ochre), and in giving a yellowish- brown to dull yellow powder when scratched or bruised. It may be seen in the course of being deposited at the present x.] IRON OXIDES. 173 time through the action of vegetation in bogs and lakes, hence its name of Bog-iron ore (chapter viii.), likewise in springs and streams where the water carries much sul- phate of iron. The common yellow and brown colours of sandstones and many other rocks are generally due to the presence of this mineral. MAGNETITE (Fe 3 O 4 ) occurs crystallised in octahedrons and dodecahedrons of an iron-black colour, giving a black powder when scratched. It is found abundantly in many rocks (schists, lavas, etc.), sometimes in large crystals (Fig. FIG. 54. Octahedral crystals of magnetite in chlorite schist. 54), sometimes in such minute form as can only be detected with the microscope. It also forms extensive beds of a massive structure. Its presence in rocks may be detected by its influence on a magnetised needle. By pounding basalt and some other rocks down to powder, minute crystals and grains of magnetite may be extracted with a magnet. TITANIC IRON (FeTi) 2 O 3 ) occurs in iron-black crystals like those of haematite, from which they may be distinguished by the dark colour and metallic lustre of its surface when scratched. Though it occurs in beds and veins in certain kinds of rock (schists, serpentine, syenite), its most generally ELEMENTS OF EARTH S CRUST. [CHAP. diffused condition is in minute crystals and grains scattered through many crystalline rocks (basalt, diabase, etc.) MANGANESE OXIDES are commonly associated with those of iron in rocks. They are liable to be deposited in the form of bog-manganese, under conditions similar to those FIG. 55. Dendritic markings due to arborescent deposit of earthy manganese oxide. in which bog-iron is thrown down. Earthy manganese oxide (wad) not infrequently appears between the joints of fine- grained rocks in arborescent forms that look so like plants as to have been often mistaken for vegetable remains. These plant-like deposits are called Dendrites or dendritic markings (Fig. 55). Silicates. Compounds of Silica with various bases x.] FELSPARS. 175 form by far the most numerous and abundant series of minerals in the earth's crust. They may be grouped accord- ing to the chief metallic base in their composition, the most important are the Silicates of Alumina, and the Silicates of Magnesia. Of the aluminous silicates we need consider here only the Felspars, Zeolites, and Mica. Among the magnesian silicates it will be enough to note the leading characters of Hornblende, Augite, Olivine, Talc, Chlorite, and Serpentine. When the learner has made himself so familiar with these as to be able readily to recognise them, he may proceed to the examination of others. FELSPARS. This family of minerals plays an important part in the construction of the earth's crust, for it constitutes the largest part of the crystalline rocks which, like lava, have been erupted from below ; is found abundantly in the great series of schists; and by decomposition has given rise to the clays, out of which so many sedimentary rocks have been formed. The felspars are divided into two series, according to crystalline form Orthoclase and Plagioclase. Orthoclase or potash -felspar contains about i6'Sg per cent of potash, crystallises in monoclinic or oblique rhombic prisms, but also occurs massive ; is white, grey, or pink in colour ; has a glassy lustre ; can with difficulty be scratched with a knife, but easily with quartz. Associated with quartz, it is an abundant ingredient of many ancient crystalline rocks (granite, felsite, gneiss, etc.) In the form of sanidine it is an essential constituent of many modern volcanic rocks. Plagioclase. Under this name are grouped several species of felspar which, differing much from each other in chemical composition, agree in crystallising in the same type or system, which is that of a triclinic or oblique rhomboidal prism. As abundant ingredients of rocks they commonly 1 76 ELEMENTS OF EARTH'S CRUST. [CHAP. appear as clear, colourless, or white glassy strips, on the flat faces of which a fine minute parallel ruling may be detected with the naked eye, or with a lens. This striation or lamella- tion is a distinctive character, which proves the crystals in which it occurs not to be orthoclase. The plagioclase fel- spars occur as essential constituents of many volcanic rocks, and also among ancient eruptive masses and schists. Among them are Microdine or Potash-felspar, with 15 per cent of potash; Albite or Soda-felspar, containing nearly 12 per cent of soda (Fig. 51); Anorthite or Lime-felspar, with 20*10 per cent of lime ; Soda-lime felspar. Lime-soda felspar a group of felspars containing variable proportions of soda, lime, and sometimes potash ; the chief varieties are Oligodase (Silica, 62-65 per cent), Andesine (Silica, 58-61 per cent), Labrador- ite (Silica, 50-56 per cent). ZEOLITES, a characteristic family of minerals, composed FIG. 56. Cavity in a lava, filled with zeolite which has crystallised in long slender needles. essentially of silicate of alumina and some alkali with water ; often marked by a peculiar pearly lustre, especially on certain X.] HORNBLENDE, AUGITE. 177 planes of cleavage ; usually found filling up cavities in rocks where they have been deposited from solution in water. Some of the species commonly crystallise in fine needles or silky tufts. The zeolites have obviously been formed from the decomposition of other minerals, particularly felspars. They are especially abundant in the steam-cells of old lavas in which plagioclase felspars prevail, either lining the walls of the cavities, and shooting out in crystals or fibres towards the centre (Fig. 56), or filling the cavities up entirely. MICA, a group of minerals (monoclinic) specially dis- tinguished by their ready cleavage into thin, parallel, usually elastic silvery laminae, They are aluminous silicates with potash (soda), or with magnesia and ferrous oxide, and always with water. They occur as essential constituents of granite, gneiss, and many other eruptive and schistose rocks, also in worn spangles in many sedimentary strata (micaceous sandstone). Among their varieties the two most important are Muscovite (white mica, potash -mica), and Black mica (magnesia-mica, Biotite). HORNBLENDE or AMPHIBOLE, a silicate of magnesia, with lime, iron-oxides, and sometimes alumina, occurs in mono- clinic (oblique rhombic) prisms, also colum- nar, fibrous, and massive. It is divisible into (1) a group of pale-coloured varieties, con- taining little or no alumina, white or pale green in colour, often fibrous (Tremolite, Actinolite, Asbestus\ found more particularly among gneisses and associated rocks, and (2) a dark group containing 5 to 18 per cent FlG ' 57- Horn- blende crystal, of alumina, which replaces the other bases ; dark green to black in colour, in stout, dumpy prisms (Fig. 57), and in columnar or bladed aggregates (Common horn- N 178 ELEMENTS OF EARTH'S CRUST. [CHAP. blende). Abundant in many eruptive rocks, and also forming almost entire beds of rock among the crystalline schists. AUGITE (PYROXENE), in composition resembles horn- blende ; indeed, they are only different forms of the same substance, differing slightly in crystalline form, horn- blende being the result of slow and augite of rapid crystal- lisation. Like hornblende, also, augite occurs in two groups : (l) pale non- aluminous, found more especially among gneisses, marbles, and associated rocks ; and (2) dark green or black (Fig. 50), occurring abundantly in many eruptive rocks, such as black heavy lavas (basalts, etc.) OLIVINE (PERIDOT) (SiO 2 41*01, MgO 49*16, FeO 9*83), occurs in small orthorhombic prisms and glassy grains in basalts and other lavas ; of a pale yellowish- green or olive-green colour, whence its name. These grains can often be readily detected on the black ground of the rock, through which they are abundantly dispersed. Olivine is liable to alteration, and especi- ally to conversion into serpentine by the FIG. 58 -Olivine influence of percolating water (Fig. 58). crystal ; the light portions repre- CHLORITE (SlO 2 25-28, Al^ 19-23, sent the unde- FeO 15-29, MgO 13-25, H 2 O 9-12) is a dark composed mm- o ii ve _g reen hydrated magnesian silicate, eral, the shaded . parts show the ^ 1S so so ^ as to " e easi ly scratched with conversion of the the nail, and occurs in small six-sided olivine into ser- tables, also in various scaly and tufted ag- gregations diffused through certain rocks. It appears generally to be the result of the alteration of some previous anhydrous magnesian silicate, such as hornblende. SERPENTINE (Mg 3 Si 2 O 7 +2H 2 O) is another hydrated mag- nesian silicate, containing a little protoxide of iron and x.] CALCITE. 179 alumina, usually massive, dark green but often mottled with red. It occurs in thick beds among schists, is often asso- ciated with limestones, and may be looked for in all rocks that contain olivine, of the alteration of which it is often the result. In many serpentines, traces of the original olivine crystals can be detected. Carbonates. Though these are abundant in nature, only three of them require notice here as important constituents of the earth's crust, those of lime, lime and magnesia, and iron. CALCITE (calcium-carbonate, carbonate of lime, CaCO 3 ) crystallises in the fourth system, and has for its fundamental crystalline form the rhombohedron, as already mentioned (p. 1 66). When quite pure it is transparent (Iceland spar, Fig. 45), with the lustre of glass ; but more usually is opaque and white. Its crystals, where the chief axis is shorter than the others, sometimes take the form of flat rhombo- hedrons (nail-head spar, Fig. 59) ; where, on the other hand, that axis is elongated, they present pointed pyramids (scaleno- FIG. 59. Calcite in the form of "nail-head spar." hedrons, dog-tooth spar, Fig. 60). The mineral occurs also in fibrous, granular, and compact forms. The decomposition of silicates containing lime by permeating water gives rise i8o ELEMENTS OF EARTH* S CRUST. [CHAP. to calcium-carbonate, which is removed in solution. Being readily soluble in water containing carbonic acid, it is found in almost all natural waters, by which it is introduced into the cavities of rocks. Some plants and many animals secrete large quantities of carbonate of lime, and their remains are aggregated into beds of limestone, which is a massive and more or less impure form of calcite. Calcite is easily FIG. 60. Calcite in the form of dog-tooth spar. scratched with a knife, and is characterised by its abundant effervescence when acid is dropped upon it. A less frequent and stable form of calcium-carbonate is Aragonite which crystallises in orthorhombic forms, but is more usually found in globular, dendritic, coral -like, or other irregular shapes, and is rather harder and heavier than calcite. DOLOMITE assumes a rhombohedral crystallisation, and is a compound of 54*4 of magnesium - carbonate, with 45-6 of calcium -carbonate. It is rather harder than calcite, and does not effervesce so freely with acid. It occurs in strings and veins like calcite, but also in massive beds having a prevalent pale yellow or brown colour (owing to hydrated x.] SIDERITE, GYPSUM. 181 peroxide of iron), a granular and often cavernous texture, and a tendency to crumble down on exposure. SIDERITE (chalybite, spathic iron, ferrous carbonate, FeCO 3 ), another rhombohedral carbonate, contains 62 -per cent of ferrous oxide or pro- toxide of iron. In its crystal- line form it is grey or brown, becoming much darker on ex- posure as the protoxide passes into peroxide. It also occurs mixed with clay in concre- tions and beds, frequently associated with remains of plants and animals (Sphcero- siderite, Clay-ironstone, Figs. 61, 65). Sulphates. Two sul- phates deserve notice for their importance among rock- FIG. 61. Sphserosiderite or Clay- masses those Of lime and ir nstone concretion enclosing portion of a fern, baryta. GYPSUM (hydrous calcium - sulphate, CaSO 4 +2HO 2 ) occurs in monoclinic crystals, commonly with the form of right rhomboidal prisms (Fig. 62, a), which not infrequently appear as macks or twin -crystals (Fig. 62, b\ When pure it is clear and colourless, with a peculiar pearly lustre (Selemte); it is found fibrous with a silky sheen (Satin- spar}, also white and granular (Alabaster). It is so soft as to be easily cut with' a knife or even scratched with the finger-nails. It is readily distinguished from calcite by its crystalline form, softness, and non-effervescence with acid. When burnt it becomes an opaque white powder (plaster of 182 ELEMENTS OF EARTH S CRUST. [CHAP. Paris). Gypsum occurs in beds associated with sheets of rock-salt and dolomite (pp. 63, 206) ; it is soluble in water, and is found in many springs and rivers, as well as in the sea. One thousand parts of water at 32 Fahr. dissolve 2*05 parts of sulphate of lime ; but the solubility of the substance is increased in the presence of common salt, a thousand FIG. 62. Gypsum crystals. parts of a saturated solution of common salt taking up as much as 8-2 parts of the sulphate. Anhydrous calcium-sulphate or Anhydrite is harder and heavier than gypsum, and is found extensively in beds asso- ciated with rock-salt deposits. By absorbing water, it in- creases in bulk and passes into gypsum. BARYTES (Heavy spar, barium-sulphate, BaSO 4 ), the usual form in which the metal barium is distributed over the globe, crystallises in orthorhombic prisms which are generally tabu- lar ; but most frequently it occurs in various massive forms. The purer varieties are transparent or translucent, but in general the mineral is dull yellowish or pinkish white, with a x.] FLUORITE, HALITE, PYRITE. 183 vitreous lustre, and is readily recognisable from other similar substances by its great weight ; it does not effervesce with acids. Barytes is usually met with in veins traversing rocks, especially in association with metallic ores. Phosphates. Only one of these requires to be enume- rated in the present list of minerals the phosphate of lime or Apatite. APATITE (tricalcic phosphate, phosphate of lime, p. 160) crystallises in hexagonal prisms which, as minute colourless FIG. 63. Group of fluor-spar crystals. needles, are abundant in many crystalline rocks; it also occurs in large crystals and in amorphous beds associated with gneiss. It is soluble in water containing carbonic acid, ammoniacal salts, common salt, and other salts. Hence its introduction into the soil, and its absorption by plants, as already mentioned (p. 160). Fluorides. The only member of this family which occurs conspicuously in the mineral kingdom is calcium fluoride or FLUOR-SPAR (Fluorite, CaF 2 ) which, in the form of colourless, but more commonly light green, purple, or yellow 1 84 ELEMENTS OF EARTH* S CRUST. [CHAP. x. cubes, is found in mineral veins not infrequently accompany- ing lead-ores (Fig. 63). ~ Chlorides. Reference has already been made to the only chloride which occurs plentifully as a rock -mass, the chloride of sodium, known as HALITE or Rock-salt (NaCl, chlorine 60-64, sodium 39-36). It crystallises in cubical forms, and is also found massive in beds that mark the evaporation of former salt-lakes or inland seas (p. 207). r* Sulphides. Many combinations of sulphur with the metals occur, some of them of great commercial value \ but the only one that need be mentioned here for its wide diffu- sion as a rock-constituent is the iron-disulphide (FeS 2 ), in which the elements are combined in the proportion of 46-7 iron and 53-3 sulphur. This substance assumes two crystal- line forms : (i) PYRITE which occurs in cubes and other forms of the first or monometric system, of a bronze-yellow colour and metallic lustre, so hard as to strike fire with steel, and giving a brownish-black powder when scratched. This mineral is abundantly diffused in minute grains, strings, veins, concretions (Fig. 64, c\ and crystals in many different kinds of rocks ; it is usually recognisable by its colour, lustre, and hardness ; (2) MARCASITE (white pyrite) crystallises in the tetragonal system, has a paler colour than ordinary pyrite, and is much more liable to decomposition. This form, rather than pyrite, is usually associated with the remains of plants and animals imbedded among rocks. The sulphide has no doubt often been precipitated round decaying organ- isms by their effect in reducing sulphate of iron. By its ready decomposition, marcasite gives rise to the production of sul- phuric acid and the consequent formation of sulphates. One of the most frequent indications of this decomposition is the rise of chalybeate springs (p. 79). CHAPTER XL THE MORE IMPORTANT ROCKS AND ROCK-STRUCTURES IN THE EARTH'S CRUST. FROM the distribution of the more important elements in the earth's crust and the mineral forms which they assume, we have now to advance a stage farther and inquire how the minerals are combined and distributed so as to build up the crust. As a rule, simple minerals do not occur alone in large masses ; more usually they are combined in various proportions to form what are known as Rocks. A rock may be defined as a mass of inorganic matter, composed of one or more minerals, having for the most part a variable chemical composition, with no necessarily symmetrical ex- ternal form, and ranging in cohesion from loose or feebly aggregated debris up to the most solid stone. Blown sand, peat, coal, sandstone, limestone, lava, granite, though so unlike each other, are all included under the general name of Rocks. In entering upon the study of rocks, it is desirable to be provided with such helps as are needed for determining leading external characters ; in particular, a hammer to de- tach fresh splinters of rock, a pocket-knife for trying the hardness of minerals, a small phial of dilute hydrochloric 1 86 ROCK-STRUCTURES. [CHAP. acid for detecting carbonate of lime, and a pocket lens. The learner, however, must bear in mind that the thorough investigation of rocks is a laborious pursuit, requiring qualifications in chemistry and mineralogy. He must not expect to be able to recognise rocks from description until he has made good progress in the study. He will find much advantage in procuring a set of named specimens, and making himself familiar with such of their characters as he can himself readily observe. Great light has in recent years been thrown upon the structure and history of rocks by examining them with the microscope. For this purpose, a thin chip or slice of the rock to be studied is ground smooth with emery and water, and after being polished with flour-emery upon plate-glass, the polished side is cemented with Canada balsam to a piece of glass, and the other side is then ground down until the specimen is so thin as to be transparent. Thin^ sections of rock thus prepared reveal under the microscope the minutest kinds of rock -structure. Not only can the component minerals be detected, but it is often possible to tell the order in which they have appeared, and what has been the probable origin and history of the rock. Some illustrations of this method of investigation will be given in a later part of the present chapter. It will be of advantage to begin by taking note of some of the more important characters of rocks, and of the names which geologists apply to them. Sedimentary composed of sediment which may be either a mechanically suspended detritus, such as mud, sand, or gravel ; or a chemical precipitate, as rock-salt and cal- careous tufa. The various deposits which are accumulated on the floors of lakes, in river-courses, and on the bed of the sea, are examples of sedimentary rocks. XI.] CONCRETIONS. 187 Fragmental) Clastic composed of fragments derived from some previous rock. All ordinary detritus is of this nature. Concretionary composed of mineral matter which has been aggregated round some centre so as to form rounded FIG. 64. Concretions. a, b, " Fairy stones ;" c, Pyrite, showing internal radiated structure. or irregularly -shaped lumps. Some minerals, particularly pyrite (Fig. 64, c), marcasite, siderite, and calcite, are fre- quently found in concretionary forms, especially round some organic relic, such as a shell or plant (Fig. 61). In alluvial clay, calcareous concretions which often take curious imitative shapes, are known as " fairy stones " (Fig. 64, a, b, see p. 235). 1 88 ROCK-STRUCTURES. [CHAP. When nodules of limestone, ironstone, or cement-stone are marked internally by cracks which radiate towards, but FIG. 65. Section of a septarian nodule, with coprolite of a fish as a nucleus. do not reach, the outside, and are filled up with calcite or other mineral, they are known as Septaria or septarian nodules (Fig. 65). Oolitic made up of spherical grains, each of which has been formed by the deposition of successive coatings of mineral matter round some grain of sand, fragment of shell, or other foreign particle ( Fig. 66). A rock with this structure looks like fish-roe, hence the name oolite or roe-stone ; but when the granules are like peas, the rock becomes pisolitic (pea -stone, Fig. 67). This peculiar structure is produced in water (springs, lakes, or enclosed parts of the sea), wherein dissolved mineral matter (usually carbonate of lime) is so abundant as to be deposited in thin pellicles round the grains of sediment that are kept in motion by the current (p. 118). Stratified^ Bedded arranged in layers, strata, or beds XI.] AQUEOUS, UNSTRATIFIED. 189 lying generally parallel to each other, as in ordinary sediment- ary deposits (Fig. 79, p. 227). FIG. 66. Piece of oolite. Aqueous laid down in water, comprising nearly the whole of the sedimentary and stratified rocks. FIG. 67. Piece of pisolite. Unstratified, Massive having no arrangement in definite layers or strata. Lavas and the other eruptive rocks are examples (chapter xiv.) IQO ROCK-STRUCTURES. [CHAP. Eruptive^ Igneous forced upwards in a molten or plastic condition into or through the earth's crust. All lavas are Eruptive or Igneous rocks. In the same division must be classed granite and allied masses, which have been thrust through rocks at some depth within the earth's crust. Crystalline consisting wholly or chiefly of crystals or crystalline grains which have taken their forms by crystal- lising where they are now found. Rocks of this nature may have arisen from (a) igneous fusion, as in the case of lavas, where the minerals have separated out of a molten glass, or what is called a Magma ; (b) aqueous solution, as where crystalline calcite forms stalactite and stalagmite in a cavern ; (c) sublimation, where the materials have crystallised out of hot vapours, as in the vents and clefts of volcanoes. By the aid of the microscope it can often be ascertained that the crystals or crystalline grains in a rock, as they were crystallising out of their solution, have enclosed various foreign bodies. Among the objects thus taken up are minute globules of gas, which are prodigiously abund- ant in certain minerals in some lavas; liquids, usually water, en- closed in cavities of the crystals, but not quite filling them, and FIG. 68. -Cavities in quartz leayi ft minute freely _ moving containing liquids (magnified). bubble (Fig. 68); glass, filling globular spaces and probably portions of the original glassy magma, out of which the crystals formed; crystals and crystallites (rudimentary crystalline forms, Fig. 69) of other minerals. Thus a crystal, which to the eye may appear quite free from impurities, may be found to be full of various kinds XL] GLASSY, PORPHYRITIC. I9 1 of enclosures. Obviously the study of these enclosures can- FIG. 69. Crystallites (highly magnified). not but throw light on the conditions under which the rocks enclosing them were produced. Glassy, Vitreous having a structure and aspect like that of artificial glass. Some lavas, obsidian for example, are natural glasses, and look not unlike masses of dark bottle- glass. In almost all cases, however, they contain dispersed crystals, crystallites, or other enclosures, and these substances have sometimes multiplied to such an extent as to take the place of the original glass. When a glass is thus converted into a dull, opaque, stony, or lithoid substance, it is said to be devitrified. The microscope enables us to detect traces of an original glassy condition in many crystalline eruptive rocks which, consequently, are thus ascertained to have been once molten glass that has been devitrified by the development of crystals and crystallites. Porphyritic composed of a compact or crystalline base or matrix, through which are scattered conspicuous crystals much larger than those of the base, and generally of some felspar. Many eruptive rocks have this structure and are sometimes spoken of as porphyries or as being porphyritic. The large crystals have probably been formed in the rock while still in a mobile state within the earth's crust, while the minuter crystals of the base have been developed during ROCK-STRUCTURES. [CHAP. the consolidation of the rock at the surface ; in the succes- sive zones of growth which porphyritic crystals often present, FIG. 70. Porphyritic structure. we may note by the enclosed minerals the stages of consoli- dation of the rock. Spherulitic composed of or containing small pea-like FIG. 71. Spherulites and fluxion-structure. A, Spherulites, as seen under the microscope (with polarised light). B, Fluxion structure of obsidian, as seen under the microscope. globular bodies (spherulites) which show a minutely fibrous internal structure radiating from the centre (Fig. 7 1, A). This XL] VESICULAR, AMYGDALOIDAL. 193 structure is particularly observable in vitreous rocks, where it appears to be one of the stages of devitrification. Vesicular containing spherical cavities. In many erup- tive rocks (as in modern lavas) the expansion of interstitial steam, while the mass was still in a molten condition, has produced this cellular structure (Fig. 35), the vesicles have usually remarkably smooth walls; they may form a com- paratively small part of the whole mass, or they may so increase as to make pieces of the rock capable of floating on water. Where the vesicular structure is conjoined with more solid parts, as in the irregular slags of an iron furnace, it may be called slaggy. Where, as in the scoriae of a volcano, the cellular and solid parts are in about equal pro- portions, and the vesicles vary greatly in numbers and size within short distances, the structure may be termed scoria- ceous. The lighter and more froth-like varieties that can float on water are said to be pumiceous, or to have the characters of pumice (p. 214). Exposed to the influence of percolating water, vesicular rocks have had their vesicles filled up by the deposition of various minerals from solution, especially quartz, calcite, and zeolites. These substances first begin to encrust the walls of the cells, and as layer succeeds layer they gradu- ally fill the cells up (Fig. 52); as the cells have not infre- quently been elongated in one direction by the motion of the rock before consolidation was completed (Fig. 37), the mineral deposits in them, taking their exact moulds, appear as oval or almond-shaped bodies. Hence rocks which have been treated in this way are called Amygdaloids, and the kernels filling up the cells are termed Amygdules (Fig. 35). An amygdaloidal rock, therefore, was originally a molten lava, rendered cellular by the expansion of its absorbed steam and gases, its vesicles having been subsequently filled o 194 ROCK-STRUCTURES. [CHAP. up by the deposit in them of mineral matter, often derived out of the surrounding rock by the decomposing and re- arranging action of percolating water. Fluxion -structure an arrangement of the crystallites and crystals of an eruptive rock in streaky lines, the minuter forms being grouped round the larger, indicative of the internal movement of the mass previous to its consolidation. The lines are those in which the particles moved past each other, the larger crystals giving rise to obstructions and eddies in the flow of the smaller objects past them. This structure is characteristic of many once molten rocks ; it is well seen in obsidian (Fig. 71, B). Schistose, Foliated consisting of minerals that have crystallised in approximately parallel, wavy, and irregular FIG. 72. Schistose structure. laminae, layers, or folia (Fig. 72). Such rocks are called generally schists. They have, in large measure, been formed by the alteration or metamorphism of other rocks of various kinds (chapter xiii.) Various schemes of classification of rocks are in use XL] SEDIMENTARY ROCKS. 195 among geologists, some based on mode of origin, others on mineral composition or structure. For the purpose of the learner, perhaps the most instructive and useful arrangement is one which as far as possible combines the advantages of both these systems. Accordingly, in the following account of the more important rocks which enter into the structure of the earth's crust, a threefold subdivision will be adopted into : (i) sedimentary rocks ; (ii) eruptive rocks ; (iii) schistose rocks. (i.) SEDIMENTARY ROCKS. This division includes the largest number, and to the geologist the most important of the rocks accessible to our notice. It comprises the various deposits that owe their origin to the decay of the surface of the land, and which are laid down on the land or over the bed of the sea, together with all those which are directly or indirectly due to the growth of plants and animals. It thus embraces those which constitute the main mass of the earth's crust so far as known to us, and which contain the evidence whence the geological history of the earth is chiefly worked out. It is, therefore, worthy of the earliest and closest attention of the student. Sedimentary rocks, being due to the deposition of some kind of sediment or detritus, are obviously not original or primitive rocks. They have all been derived from some source, the nature of which, if not its actual site, can usually be easily determined. In no case, therefore, can a sedi- mentary rock carry us back to the beginning of things ; it is itself derivative and presupposes the existence of some older rock or material from which it could be derived. One of their most obvious characters is that, as a rule, they 196 SEDIMENTARY ROCKS. [CHAP. are stratified. They have been deposited, usually in water, sometimes in air, layer above layer, and bed above bed, each of these strata marking a particular interval in the progress of deposition (chapter xii.) As regards their mode of origin, they may be subdivided into three great sections : (i) fragmental or clastic, composed of fragments of pre- existing rocks ; (2) chemically precipitated, as in the deposits from mineral springs ; and (3) formed of the remains of organisms, as in peat and coral-rock. (i.) Fragmental or Clastic Rocks. These are masses of mechanically -formed sediment, derived from the destruction of older rocks ; they vary in coherence from loose sand or mud up to the most compact sandstone or conglomerate ; they are accumulating abund- antly at the present time in the beds of rivers and lakes, and on the floor of the sea, and they have been formed in a similar way all over the globe from the earliest periods of known geological history. Some of the more frequent kinds are the following : CLIFF -DEBRIS coarse angular rubbish, including large blocks of stone, disengaged by the weather from cliffs and other bare faces of rock. This kind of detritus is formed abundantly in rugged and mountainous regions, especially where the action of frost is severe ; it slides down the slopes and accumulates at their foot, unless washed away by torrents. In glacier-valleys it descends to the ice, where, gathering into moraines (chapter vi.), it is transported to lower levels. The perched blocks of such valleys are some of the larger fragments of this cliff-debris left stranded by the ice, and from around which the smaller detritus has been washed away (Fig. 23). XL] SOIL, BRECCIA, GRAVEL. 197 SOIL, SUBSOIL, described in chapter ii., represent the re- sult of the subaerial decomposition of the surface of the land. BRECCIA a rock composed of angular fragments. Such a rock shows that its materials have not travelled far ; otherwise, they would have lost their edges, and would have been more or less rounded. Ordinary cliff- debris may consolidate into a breccia, more especially where it falls into water and is allowed to gather on the bottom. FIG. 73. Brecciated structure volcanic breccia, a rock composed of angular fragments of lava, in a paste of finer volcanic debris. The angular fragments shot out of a volcano often accumu- late into volcanic breccia (Fig. 73). A rock with abundant angular fragments is said to be brecciated. GRAVEL loose rounded water-worn detritus, in which the pebbles range in average size between that of a small pea and that of a walnut ; where they are larger they form shingle. They may consist of fragments of any kind of rock, though having resulted from more or less violent water- action, as a rule, pieces of only the more durable stones are found in them. Quartz and other siliceous materials, from 198 SEDIMENTARY ROCKS. [CHAP. their great hardness, are better able to withstand the grind- ing to which the detritus on an exposed sea-shore, or in the bed of a rapid stream, is subjected. Hence quartzose and siliceous pebbles are the most frequent constituents of gravel and shingle. CONGLOMERATE a name given to gravel and shingle when they have been consolidated into stone, the pebbles being bound together by some kind of paste or cement- FIG. 74. Conglomerate. ing material, which may be fine hardened sand, clay, or some calcareous, siliceous, or ferruginous cement (Fig. 74). As above remarked with regard to gravel, the component materials of conglomerate may have been derived from any kind of rock, but siliceous pebbles are of most common occurrence. Different names are given to conglomerates, according to the nature of the pebbles, as quartz-conglomer- ate, flint-conglomerate, limestone-conglomerate. SAND a name given to fine kinds of detritus, the grain of which may vary from the size of a small pea down to minute particles that can only be detected with a lens. In XL] SAND, SANDSTONE. 199 general, for the reason already assigned in the case of gravel, the component grains of sand are of quartz or of some other durable material. Examined with a good magnifying glass, they are seen to be rounded, water-worn, or sometimes angular, unworn particles of indefinite shapes which, except in their smaller size, resemble those of gravel- stones. Sand may be formed by the disintegration of the surface of rocks exposed to the weather, more especially in dry climates, where there is a great difference between the temperature of day and night (p. 17). The loosened particles are blown away by the wind, and may be heaped up into great sand-wastes, as in the tracts known as deserts. On a sea-coast, where a sandy beach is liable to be laid bare and exposed to be dried between tides by breezes blowing from the sea, the upper particles of sand are lifted up by the wind and borne away landward, to be piled up into dunes (p. 27). In some places, the materials are derived mainly from the remains of calcareous sea-weeds, shells, corallines, and other calcareous organisms exposed to the pounding action of the surf. A sand composed of such materials speedily hardens into a more or less coherent and even compact limestone, for rain falling on it dissolves some carbonate of lime which, being immediately deposited again, as the moisture evaporates, coats the grains of sand and cements them together. At Bermuda, as already stated, all the rock above sea-level has been formed in this way, and some of it is hard enough to make a good building stone (p. 112). Ordinary siliceous or quartzose sand remains loose, unless its grains are made to cohere by some kind of cement, when it becomes sandstone. SANDSTONE consolidated sand. The grains are chiefly quartz, but may include particles of any other mineral or 200 SEDIMENTARY ROCKS. [CHAP. rock ; they are bound together by some kind of cement which has either been laid down with them at the time of their deposition, or has subsequently been introduced by water permeating the sand. The cementing material may be argillaceous that is, some kind of clay ; or calcareous, consisting of carbonate of lime ; or ferruginous, composed mainly of peroxide of iron ; or siliceous, where silica has been deposited in the interstices of the mass. The colours of sandstone vary chiefly with the nature of this cementing material. The hydrous peroxide of iron colours them shades of yellow and brown; the anhydrous peroxide of iron gives them different tints of red ; the mineral glauconite gives them a greenish hue. Some varieties of sandstone are named after a conspicuous component or structure ; thus micaceous sandstone is distinguished by abundant spangles of mica deposited along the bedding planes, whereby the rock can be split up into thin layers ; freestone a thick- bedded sandstone that does not tend to split up in any one direction, and can therefore be cut into blocks of any size and form; glauconitic sandstone (green sand), containing green grains and kernels of glauconite ; quartzose sandstone, conspicuously composed of quartz-grains ; grit a sandstone formed of coarse or sharp, somewhat angular grains of quartz. GREYWACKE a greyish, compact, granular rock, com- posed of rounded or subangular grains of quartz and other minerals or rocks, cemented together in a compact paste ; it differs from sandstone chiefly in its darker colour, in the proportion of other grains than those of quartz, and in the presence of a tough cement. The rocks above enumerated represent the coarser and more durable kinds of detritus derived from the weathering XL] CLAYS. 201 of the surface of the land ; but during the progress of the decomposition from which these materials are derived some of the component ingredients of the rocks decay into clay, or what is called argillaceous sediment. This more parti- cularly occurs in the case of felspars and other aluminous silicates, the decomposition of which produces minute particles capable of being lifted up and carried a great distance by running water. Hence argillaceous sediment, being finer in grain, travels farther, on the whole, than quartz- ose sediment; and beds of clay denote, generally, deeper and stiller water than beds of sand. CLAY a fine-grained argillaceous substance, derived from the decay and hydration of aluminous silicates, white when pure, but usually mixed with impurities, which impart to it various shades of grey, green, brown, red, purple, or blue ; it usually contains interstitial water, and when wet can be kneaded between the fingers ; when dry it is soft and friable, and adheres to the tongue. Shaken with water it becomes MUD, and even a small quantity will make a glass of water turbid, so fine are the particles of which it is composed. KAOLIN the name given to the white purer forms of clay, resulting from the decomposition of the felspars of granite or similar rocks ; it is sometimes called China-clay, from its use in the manufacture of porcelain. FIRE-CLAY a white, grey, yellow, or black clay, nearly free from alkalies and iron, and capable of standing a great heat without fusing; it is abundantly found underneath coal-seams, where it represents the ancient soil on which the plants grew that have been converted into coal. BRICK-CLAY a name commonly applied to any clay, loam, or earth from which bricks can be made ; these de- posits are always more or less sandy and impure clays. 202 SEDIMENTARY ROCKS. [CHAP. MUDSTONE a compact solidified clay or clay -rock, having little or no tendency to split into thin laminae. SHALE clay that has become hard and splits into thin laminae which lie parallel with the planes of deposit. A thoroughly fissile shale can be subdivided into leaves as thin as fine cardboard. This is the common form which the clays of the older geological formations have assumed. Grada- tions can be traced from shale by additions of sand into fissile sandstones, of calcareous matter into limestone, of carbonate of iron into ironstone, and of carbonaceous matter into coal. These passages are interesting as indications of the conditions under which the rocks were formed. Where, for example, shale shades off into coral -limestone, we see that mud gathered over one part of the sea -floor, while not far off, probably in clearer water, corals flourished and built up a limestone out of their remains. LOESS a pale somewhat calcareous and sandy clay, found in regions where it has probably been accumulated by the drifting action of the wind. It is sufficiently coherent to be capable of excavation into tunnels and passages, and in China is even dug out into houses and subterranean villages. It occupies parts of the valleys of the Rhine, Danube, Mississippi, and other large rivers, but also crosses watersheds (p. 471). Fragmental rocks of volcanic origin may be enumerated here. They consist partly of materials ejected in fragment- ary form from volcanic vents, and partly of the detritus derived from the disintegration of volcanic rocks already erupted to the surface. They are comprised under the general name of TUFF. BOMBS round elliptical or discoidal pieces of lava which xi.] VOLCANIC FRAGMENTAL ROCKS. 203 have been ejected in a molten state from an active vent, and have acquired their form from rapid rotation in the air during ascent and descent. They are often very cellular or even quite empty inside. Where the large ejected stones are of irregular forms, and appear to have been thrown out in an already solidified condition, as from the consolidated crust of the lava-plug,, or from the sides of the funnel or crater, they are called VOLCANIC BLOCKS (p. 135). LAPILLI ejected pieces of lava, usually vesicular or porous, from the size of a pea to a walnut. VOLCANIC ASH the fine dust produced by the explosion of the superheated steam absorbed in molten lava. Under the microscope, it is often found to consist of minute grains of glass, and, in such cases, shows that the lava from which it was derived rose from below in the condition of a liquid glassy magma. In other instances, it is made up of the crystallites and crystals arising from the devitrification of the glass. It consolidates into a more or less coherent mass, which is known as TUFF, and which may receive some dis- tinctive name according to the nature of the lava that has supplied it, as basalt-tuff and trachyte-tuff. Most tuffs con- tain angular and vesicular pieces of lava, and sometimes pass into coarse breccias. In many cases, they enclose the remains of plants and animals which, if of terrestrial kinds, indicate that the eruptions took place on land ; if of marine species, that the volcanoes were probably submarine. AGGLOMERATE a coarse, usually unstratified accumula- tion of blocks of lava and other rocks, filling up the chimney or neck of a volcanic vent. (2.) Rocks formed by Chemical Precipitation. In chapter v. it was pointed out that all natural waters 204 CHEMICAL PRECIPITATES. [CHAP. contain in solution invisible mineral matter which they have dissolved out of the rocks of the earth's crust, and that the quantity of this material is sometimes so great that it is precipitated into visible form as the water evaporates. The substance most abundantly dissolved and deposited is car- bonate of lime. Others of frequent occurrence are sulphate of lime, chloride of sodium, silica, carbonate of magnesia, and various salts of iron. Among the rocks of the earth's crust, considerable masses of these substances have been piled up by chemical precipitation. LIMESTONE compact or crystalline calcium-carbonate, which may be nearly pure, or may contain sand, clay, or other impurity, and may consequently pass into sandstone, shale, or other sedimentary rock. Probably the great majority of the limestones in the earth's crust have been formed by the agency of animals, as more particularly referred to at p. 209. We are here concerned only with those which have been deposited from chemical solution. The most familiar example of this kind of limestone is afforded by stalactites and stalagmite, which have already been described (chapter v. and Fig. 20). Large masses of it have been deposited by calcareous springs and streams. At first, it is a fine white milky precipitate, but gradually crystals of calcite shape themselves and grow out of it, with their vertical axes usually at right angles to the surface of deposit. In a verti- cal stalactite, consequently, the prisms radiate horizontally from the centre outwards ; on a horizontal surface of stalag- mite they diverge perpendicular to the floor. A mass of limestone, not originally crystalline, may thus acquire a thoroughly crystalline internal structure by the action of in- filtrating water in dissolving and redepositing the carbonate of lime in a crystalline condition. XL] LIMESTONE, DOLOMITE. 205 Limestones vary greatly in texture and purity. Some are snow-white and distinctly crystalline; others are grey, blue, yellow, or brown, dull and compact, and full of various impurities. They may usually be detected by the ease with which they can be scratched, and their copious effervescence when a drop of weak acid is put on the scratched surface. Pure limestone dissolves entirely in hydrochloric acid, so that the amount of residue is an indication of the proportion of insoluble impurity. Among the varieties of limestone the following may be named : Oolite a limestone composed of minute spherical grains like the roe of a fish, each grain being composed of concentrically deposited layers or shells of calcite (Fig. 66) ; Pisolite a similar rock, where the grains are as large as peas (Fig. 67); Travertine or calcareous tufa a white porous crumbling rock which, by infiltration of carbonate of lime, may acquire a compact texture, and become suitable for building-stone (p. 77); Hydraulic lime- stone containing 10 to 30 per cent of fine sand or clay, and having the property, after being burnt, of hardening under water into a firm compact mortar. DOLOMITE, MAGNESIAN LIMESTONE this substance has been already referred to as a mineral (p. 180); but it also occurs in large masses as a white or yellowish crystalline or compact rock. The white varieties look like marble. The yellow and brown kinds contain various impurities, and are coloured by iron oxide. Dolomite differs from limestone in its greater hardness and feebler solubility in acid, in its frequently cellular or cavernous texture, its tendency to assume spherical, grape-shaped, or other irregular concre- tionary forms (Fig. 75), and its proneness to crumble down into loose crystals. It occurs in beds not uncommonly associated with gypsum and rock-salt, and in such conditions 206 CHEMICAL PRECIPITATES. [CHAP. it may have been deposited first as limestone which, by the chemical action of the magnesian salts in the saline water, FIG. 75. Concretionary forms assumed by Dolomite, Magnesian Limestone, Durham. had its carbonate of lime partially replaced by carbonate of magnesia. It is also found in irregular bands traversing limestone which, probably by the influence of percolating water containing carbonate of magnesia in solution, has been changed into dolomite. GYPSUM is not only a mineral (p. 181) but also a rock, white, grey, brown, or reddish in colour, granular to com- pact, sometimes fibrous or coarsely crystalline in texture, and consists of sulphate of lime. It is easily scratched with the nail, and is not affected by acids, being thus readily distin- guishable from limestone. It is found in beds or veins, xr.] GYPSUM, ROCK-SALT, IRONSTONE. 207 especially associated with layers of red clay and rock-salt, and in these cases has evidently resulted from the evapora- tion of water containing it in solution, as in sea-water. The lime-sulphate being less soluble than the other constituents is precipitated first.' Hence in a thick series of alternations of beds of gypsum (or anhydrite) and rock-salt, each layer of sulphate of lime indicates a new supply of water into the natural reservoirs where the evaporation took place. The overlying bed of salt, usually much thicker than the gypsum, points to the condensation of the water into a strong brine, from which the salt was ultimately precipitated. And the next sheet of sulphate of lime tells how, by the breaking down of the barrier, renewed supplies of salt water were poured into the basin. ROCK-SALT occurs in beds or layers, from less than an inch to hundreds or even thousands of feet in thickness. One mass of salt in Galicia is more than 4600 feet thick, and a still thicker mass occurs near Berlin. When quite pure, rock-salt is clear and colourless, but it is usually more or less mixed with impurities, particularly with red clay, as above remarked. It has been formed in inland salt lakes or basins by the evaporation and concentration of the saline water. It is being deposited at the present time in the Dead Sea, the Great Salt Lake, and the salt lakes so frequent in the desert regions of continents, where the drainage does not flow outwards to the sea. IRONSTONE. Various minerals are included under this name as large rock-masses. One of the most important of them is Hematite (p. 172), which occurs in large beds and veins, as well as filling up caverns in limestone. Limonite or bog-iron-ore is formed in lakes and marshy places (p. 62), and occurs in beds among other sedimentary accumulations. 208 ORGANICALLY DERIVED ROCKS. [CHAP. Magnetite (p. 173) is found in beds and huge wedge-shaped masses among various crystalline rocks, as in Scandinavia, where it sometimes forms an entire mountain. Carbonate of iron (Siderite, Sphaerosiderite, Clay-ironstone) occurs in concretions and beds among argillaceous deposits (Figs. 61, 65). In the Coal-measures, for example, it is largely de- veloped, much of the iron of Britain being obtained from this source. As many ironstones are largely due to the influence of plants and animals, the rock is alluded to again on p. 2 1 1 . SILICEOUS SINTER a white powdery to compact and flinty deposit from the hot water of springs in volcanic dis- tricts, consisting of 84 to 91 per cent of silica, with small proportions of alumina, peroxide of iron, lime, magnesia, and alkali, and from 5 to 8 per cent of water. It accumulates in basin-shaped cavities round the mouths of hot springs and geysers, and sometimes forms extensive terraces and mounds, as at the geyser regions of Iceland, Wyoming, and New Zealand. VEIN-QUARTZ a massive form of quartz, which occurs in thin veins and in broad dyke-like reefs, traversing especi- ally the older rocks. (3.) Rocks formed of the Remains of Plants or Animals. In chapter viii. an account was given of the manner in which extensive accumulations are now being formed of the remains of plants and animals. Similar deposits have con- stantly been accumulated from an early period in the history of the earth. Regarding them with reference to their mode of origin, we observe that in some cases they have been piled up by the unremitting growth and decay of the organisms upon the same site. In a thick coral-reef, for example, the xi.] LIMESTONE FORMED OF ORGANIC REMAINS. 209 living corals now building on the surface are the descendants of those whose skeletons form the coral -rock underneath. In other cases, the remains of the organisms are broken up and carried along by moving water, which deposits them elsewhere as a sediment. Strictly speaking these last de- posits are fragmental, and might be classed with those described at p. 196 ; they pass into ordinary sand, sandstone, clay, or shale. But it will be more convenient to class to- gether all the rocks which consist mainly of organic remains, whether they have been directly built up by the organisms, or have only been formed out of their detrital remains. LIMESTONE. As carbonate of lime is so largely secreted by animals in their hard parts which are more or less durable, it is naturally the most common substance among rocks of organic origin. By far the larger proportion of the lime- stones of the earth's crust have been formed out of the remains of marine animals. The following are some of the more important or interesting varieties : Shell-marl a soft white earthy crumbling deposit formed chiefly of fresh-water shells (p. 62). By subsequent infiltration it may be hardened into a compact stone when it is known as fresh-water lime- stone ; Calcareous sand a mass of broken-up shells, calcare- ous algae, and other calcareous organisms (p. 199), often cemented by percolating water into solid stone ; Coral-rock a limestone formed by the continuous growth of corals and cemented into a solid compact and even crystalline rock by the washing of calcareous mud into its interstices and the permeation of sea -water and rain-water through it, whereby crystalline calcite is deposited within it (p. 115); Chalk a soft, white rock, soiling the fingers, formed of a fine calcareous powder of remains of foraminifera, shells, etc. (see Ooze, p. 1 14); Crinoidal limestone composed chiefly of the calcareous 210 ORGANICALLY DERIVED ROCKS. [CHAP. joints of the marine creatures known as crinoids, with fora- minifera, shells, corals, and other organisms. A limestone composed in great part of organic remains may show little trace of its origin on a fresh fracture of the stone ; but a FIG. 76. Weathered surface of crinoidal limestone. weathered surface will often reveal its true nature, the fossils being better able to withstand the action of the atmosphere than the surrounding matrix which is accordingly removed, leaving them standing out in relief (Fig. 76). PEAT a yellow, brown, or black fibrous mass of com- pressed and somewhat altered vegetation. It occurs in boggy places in temperate latitudes where it largely consists of bog-mosses and other marshy plants (p. 109). Its upper parts are loose and full of the roots of living plants, while the bottom portions may be compact and black like clay, and with little trace of vegetable structure. LIGNITE or Brown Coal is a more compressed and chemi- cally changed condition of vegetation. It varies in colour from yellow to deep brown or black and may be regarded XL] COAL, IRONSTONE. 211 as an intermediate stage between peat and coal. It occurs in beds intercalated between layers of shale, clay, and sand- stone. COAL a compact, brittle, black, or dark brown stone, formed of mineralised vegetation, and found in beds or seams usually resting on clay, and covered with sandstone, shale, etc. There are many varieties of coal differing from each other in the relative proportions of their constituents. Caking-coal, such as is ordinarily used in England, con- tains from 75 to 80 per cent of carbon, 5 or 6 per cent of hydrogen, and 10 or 12 per cent of oxygen, with some sul- phur and other impurities. Anthracite^ the most thoroughly mineralised condition of vegetation, is a hard, brittle, lustrous substance, from which the hydrogen and oxygen have been in great measure driven away, leaving 90 per cent or more of carbon. IRONSTONE. Reference was made at pp. 62, 79, to iron- stone precipitated from chemical solution. This precipita- tion is often caused through the medium of decomposing organic matter. Organic acids, produced by the decay of plants in marshy places and shallow lakes, attack the salts of iron contained in the rocks or detritus of the bottom, and remove the iron in solution. On exposure, the iron oxidises and is thrown down as a yellow or brown precipitate of limonite or bog -iron -ore (pp. 172, 207). Clay -ironstone , composed of a mixture of carbonate of iron, with clay and carbonaceous matter, occurs abundantly with remains of plants, shells, fishes, etc., in the Coal-measures, and has, no doubt, been also formed through the agency of organic acids which, passing into carbonic acid, have given rise to the solu- tion and subsequent deposit of the iron as carbonate mingled with mud and with entombed plants and animals. 212 ERUPTIVE ROCKS. [CHAP. FLINT. Some siliceous deposits, due to organic agency, have been already referred to at p. in. Besides these, mention may be made of Flint, which occurs as dark lumps and irregular nodular sheets in chalk and other lime- stones, frequently enclosing urchins, shells, and other organisms, which are sometimes converted into flint. Its mode of origin is not yet thoroughly understood, but there is reason to regard it as due to the abstraction of silica from sea-water, either directly, by such animals as sponges, or indirectly, by the decomposition of animal remains. GUANO a brown, light, powdery deposit, formed of the droppings of sea-birds in rainless tracts of the west coasts of South America and Africa. It has a great com- mercial value as an important manure. BONE -BEDS deposits composed of fragmentary or entire bones of fish, reptiles, or higher animals. The floors of some caverns are covered with stalagmite, so full of pieces of the bones of cave-bears, hyaenas, and other extinct and living species, as to be called Bone-breccia. Layers of stone, full of the coprolites (fossil excrement) of various verte- brate animals, have, in recent Jyears, been largely worked as sources of phosphate of lime for the manufacture of arti- ficial manures. (ii.) ERUPTIVE ROCKS. Under this division are grouped all the massive rocks which have been erupted from underneath into the crust or to the surface of the earth. They are composed chiefly of silicates of alumina, magnesia, lime, potash, and soda, with different proportions of free silica, magnetic or other oxide of iron, and phosphate of lime. The principal silicate XL] CLASSIFICATION OF ERUPTIVE ROCKS. 213 is generally some felspar, the number of eruptive rocks without felspar being comparatively small. The felspar is, in different rocks, conjoined with mica, hornblende, augite, magnetite, or other minerals. No perfectly satisfactory classification of the eruptive rocks has yet been devised; they have been grouped according to their presumed mode of origin, some being classed as hypogene, from their supposed origin, deep within the earth's crust, others as volcanic, from having been ejected by volcanoes. They have, likewise, been arranged accord- ing to their chemical composition, and also with reference to their internal structure. In the following enumeration of some of the more abundant and important varieties, it may be enough to adopt an arrangement in three sections, according to the nature of the predominant silicate : viz. (i) orthoclase rocks; (2) plagioclase rocks; and (3) olivine and serpentine rocks. It has already been pointed out that the original condition of many lavas and other erup- tive rocks has been that of molten glass, their present stony structure being due to the more or less complete devitrifica- tion and disappearance of the glass by the development of crystals and crystallites out of it during the process of cool- ing and consolidation. Though there is no evidence that all crystalline eruptive rocks have once been in the state of molten glass, it may be useful to begin with the vitreous varieties, which we know to represent the earliest forms of many that are now quite crystalline. (i.) Orthoclase Rocks. In this section the prevalent silicate is orthoclase, either in its common, dull, white, or pink form, or in the glassy condition (sanidine). In many of the rocks, free quartz 214 ORTHOCLASE ROCKS. [CHAP. occurs either in irregular crystalline blebs or in definite crystals, which frequently take the form of double pyramids. Among other minerals, hornblende, white and black mica, and apatite are of common occurrence. The rocks of this division are the most acid of the eruptive series that is, they contain the largest proportion of silica or silicic acid, sometimes more than 75 per cent. Some of them (granite) are only found as masses that have consolidated deep be- neath the surface ; others (trachyte, rhyolite, obsidian) are abundant as superficial volcanic products. OBSIDIAN a black, brown, or greenish (sometimes yellow, blue, or red) glass, breaking with a shell-like or con- choidal fracture and into sharp splinters, which are trans- lucent at the edges. Examined in a thin section under the microscope, the rock is found to owe its usual blackness to the presence of minute opaque crystallites which are crowded through it, not infrequently drawn out into streaky lines, and curving round any larger crystal that may be embedded in the mass. These arrangements, called fluxion -structure (p. 194), have evidently been caused by the movement of the rock while still in a fused state, the crystallites and other objects being borne onward by the currents of molten glass. In some obsidians, little spherulites of a dull grey enamel- like substance have made their appearance as stages in the devitrification of the rock (Fig. 71); but the mass has consolidated before the stony condition could be completed. In other instances, the whole rock has passed into a stony enamel-like mass (pearlstone). Where a still molten ob- sidian has been frothed up by the expansion of steam or gas through it, so as to become a spongy cellular substance which will float on water, it is called pumice. Obsidian occurs in many volcanic regions, sometimes as streams of XL] TRACHYTE, FELSITE, SYENITE. 215 lava which have been poured forth at the surface, some- times in dykes and veins, and often in fragments ejected with the other detritus that now forms tuffs. TRACHYTE a compact porphyritic rock, consisting mainly of orthoclase (sanidine), with some plagioclase and usually with some hornblende, or with augite, mica, magne- tite, or other minerals; having a peculiar matrix which, under the microscope, is found to consist mainly of minute felspar-crystallites. Large crystals of orthoclase (sanidine) are frequent, and also scales of dark mica. This rock is found abundantly among some of the younger volcanic regions of the world, where it occurs in lava-streams and also in intrusive sheets and dykes. QUARTZ -TRACHYTE (Liparite, Rhyolite) is a rock composed of a compact, often rough and somewhat porous base, through which are scattered crystals of felspar and blebs of quartz, often also with hornblende and mica. FELSITE an exceedingly close-grained rock, composed of an intimate mixture of quartz and orthoclase. The felspar often occurs as large disseminated crystals, giving the porphy- ritic structure. Where the quartz appears as distinct blebs or crystals (sometimes double pyramids) the rock becomes QUARTZ-PORPHYRY. The felsites and quartz-porphyries play an important part among the eruptive rocks of older geo- logical time, occurring both in the form of lavas erupted to the surface and of intrusive masses that have consoli- dated below ground. Many of them can be proved to have been originally in the condition of molten glass which has been devitrified. SYENITE a thoroughly crystalline rock, consisting essen- tially of orthoclase and hornblende, and distinguished from granite chiefly by the absence or small amount of quartz. 216 ORTHOCLASE ROCKS. [CHAP. It occurs in bosses and veins which have been erupted into older rocks. GRANITE a thoroughly crystalline compound of felspar, quartz, and mica, the individual minerals being large enough to be distinctly recognised by the naked eye. Sometimes large crystals of felspar are porphyritically scattered through the rock. Granite occurs in large eruptive masses which have been intruded into many different kinds of rocks, also in smaller bosses and veins. Round the outside of a mass FIG. 77. Group of crystals of felspar, quartz, and mica, from a cavity in the Mourne Mountain granite. of granite, there frequently diverge from it dykes and veins (p. 272) which, where of great width, may be merely prolonga- tions of the granite ; but which, when of small dimensions, are apt to appear as felsite or quartz-porphyry. There can be no doubt that such fine-grained veins are actually portions of the same mass of rock as the granite, so that granite and felsite or quartz-porphyry are only different conditions of the same substance, the differences being probably due to XL] PLAGIOCLASE ROCKS. 217 variations in the circumstances under which the cooling and consolidation took place. The crystalline-granular structure is so distinctive of granite that the name granitic 01 granitoid is often applied to it. The constituent minerals have not had room to assume perfect crystallised shapes, but occa- sionally they have been able to shoot out in perfect crystals where cavities occur. Fig. 77, for example, shows a group of the ordinary crystals of this rock which have crystallised in a cavity of the granite of the Mourne Mountains, Ireland. (2.) Plagioclase Rocks. In this Section, the felspar is some variety of plagioclase, and the other most frequent silicate is either augite or horn- blende. Though free quartz occurs in some of the rocks, they contain generally so much less silica than the orthoclase rocks as to be called basic compounds. A similar range of texture can be observed in them to that characteristic of the orthoclase series, from a true glass up to a thoroughly crys- talline granitoid rock. Some of them, more especially the coarsely crystalline varieties, are probably of deep-seated origin; others (and these include the great majority) are truly volcanic ejections which have risen in volcanic pipes and fissures, and have been poured forth at the surface as actual lava-streams. BASALT-ROCKS a group of rocks consisting of plagio- clase, augite, olivine, and magnetite or titaniferous iron, to which apatite and other minerals may be added. These rocks range in texture from a black glass up to a coarsely crystalline mass wherein the component minerals are dis- tinctly visible to the naked eye. Different names are em- ployed to distinguish these varieties. Basalt-glass is a general epithet to denote the vitreous varieties. These are 218 PLAGIOCLASE ROCKS. [CHAP. particularly to be observed along the edges of dykes and other intrusive masses, where they represent the outer surface of the basalt that was suddenly chilled and consolidated by coming in contact with the cold walls of the vent into which it was injected, and where they no doubt show what was the original state of the whole basalt before devitrifica- tion converted the rock into its present crystalline structure. Basalt a black, compact, heavy, homogeneous rock, break- ing with a conchoidal fracture, showing sometimes large porphyritic crystals of plagioclase, olivine, or augite, but too fine-grained for the component minerals of the base to be determined except with the microscope. The coarser varieties, where the minerals can be recognised with the naked eye, are known as dolerite. The basalt -rocks are pre-eminently volcanic lavas, occurring both as intrusive masses that consolidated underground, and as sheets that were poured out in successive streams at the surface. The black, compact kinds (true basalt) are particularly prone to assume columnar forms (Fig. 78), whence colum- nar rocks are sometimes spoken of as basalfic. In some varieties of basalt the mineral Leucite takes the part of the plagioclase; and in others this is done by another mineral, Nepheline. DIABASE a name given to some ancient basalt-rocks in which, owing to alteration of their augite or olivine, a greenish chloritic discoloration has often taken place. The lavas of early geological time are to a large extent diabase. ANDESITE is closely allied to basalt; but contains no olivine. It sometimes includes free quartz and hornblende may be substituted in it for augite. Hornblende-andesite and augite-andesite are lavas which have been extensively erupted in later geological time. XI.] PLAGIOCLASE ROCKS. 219 220 OLIVINE AND SERPENTINE ROCKS. [CHAP. DIORITE a crystalline aggregate of plagioclase and hornblende, usually with magnetite and apatite, sometimes with augite and mica. The hornblende is black or dark green and often more or less decomposed, giving rise to a greenish chloritic discoloration of the felspar. From its prevalent green colour, the rock was formerly known as "greenstone." It occurs in intrusive masses, and seems generally if not always to have consolidated below ground instead of being poured out at the surface. GABBRO, DIALLAGE-ROCK a thoroughly crystalline grani- toid aggregate of plagioclase and the variety of augite known as diallage, which appears in distinct brown or greenish crystals, with a peculiar metalloidal or pearly lustre ; it is found in bosses associated with granite, gneiss, etc., and also sometimes with volcanic rocks. (3.) Olivine and Serpentine Rocks. In this group may be included a comparatively small number of rocks which consist principally of olivine, and which by gradual alteration pass into serpentine (Fig. 58). OLIVINE-ROCKS (Peridotites) are liable to remarkably rapid changes of texture and composition. In some places they are mainly made up of olivine, augite, or hornblende, magne- tite, and brown mica, but some of these minerals may dis- appear and some felspar may take their place. They are intrusive masses which appear to have been generally injected into the crust in connection with volcanic eruptions, rather than to have been poured out at the surface in true lava-streams. SERPENTINE a compact, dull, or faintly glimmering rock, with a general dark dirty green colour, variously mottled, greasy to the touch, easily scratched and giving a white XL] SCHISTS. 221 powder which does not effervesce with acids. It is a mass- ive form of the mineral serpentine described on p. 178; frequently containing disseminated crystals of the minerals bronzite, enstatite, and chromic iron, and veins of a delicately fibrous silky variety of serpentine known as chrysotile. Many serpentines were originally olivine- rocks which, by hydration and alteration of their magnesian silicates, have assumed their present characters. Serpentine occurs in bosses, dykes, and veins, which were evidently of eruptive origin and were at first probably olivine -rocks; it is also found in thick beds associated with limestones and crystal- line schists, where it may be a metamorphosed sedimentary rock. (iii.) THE SCHISTS AND THEIR ACCOMPANIMENTS. This section includes a remarkable series of rocks of which the leading character is the possession of a schistose or foliated character (Fig. 72). They are, in their more typical varieties, distinctly crystalline; but some of them shade off into ordinary fragmental rocks, such as shale and sandstone. Several of them agree- in chemical and mineral composition with some of the eruptive rocks already enumer- ated, but differ from these in the peculiar foliated arrange- ment of their minerals, though gradations can also be traced between them. In the schists, therefore, we see an assemblage of rocks which, though possessing distinct characters of their own, may yet be observed to shade off into fragmental rocks on the one side, and into eruptive rocks on the other. In chapter xiii. some further account of the schists will be given. 222 SCHISTS. [CHAP. and it will there be shown on what grounds they have been regarded as metamorphic or altered rocks. For the present, in taking notice of their composition and structure, it will be enough to state that in many cases they can be shown to be more or less altered and crystalline, but originally sedi- mentary rocks ; in other instances, they are crystalline erup- tive masses, which have been subjected to such enormous pressure and shearing, that a foliated structure and recrys- tallisation of minerals have been superinduced in them. CLAY-SLATE a hard fissile clay-rock, through which minute scales of mica and crystals or crystallites of other minerals have been developed; generally bluish-grey to purple or green, and splitting into thin parallel leaves. As this rock often contains remains of marine animals and plants, and is interstratified with bands of sandstone, grit, conglomerate, and limestone, it was undoubtedly, at first, in the condition of soft mud on the sea-bottom. Sometimes the organic remains in it are so curiously elongated or distorted in one general direction as to show that the rock has been drawn out by intense pressure and shearing (Figs. 98, 103, 104). The planes along which clay-slate splits are generally independent of the original surfaces of deposit, sometimes cross these at a right angle, and have been superinduced in the rock by mechanical movements, as explained in chapter xiii. Different varieties of clay-slate have received special names. Roofing-slate is the fine compact durable kind, em- ployed for roofing purposes and also for the manufacture of cisterns, chimney-pieces, writing - slates ; Alum-slate dark, carbonaceous, and pyritous, the iron disulphide oxidising into sulphuric acid, and giving rise to an efflorescence of alum ; Whet-slate, honestone exceedingly hard, fine-grained, and suitable for making hones ; sometimes owing its hardness to XL] AMPHIBOLITES, MICA-SCHIST. 223 the presence of microscopic crystals of garnet ; Chiastolite- slate containing disseminated crystals of chiastolite, and found especially around eruptive bosses of granite. By in- crease of its mica-flakes a clay-slate passes into a Phyllite, which has a more silvery sheen, and represents a farther stage of metamorphism. Phyllite, by increase of the mica, becomes Mica-slate. Clay-slate occurs extensively among the older geological formations in all parts of the world. AMPHIBOLITES rocks composed mainly of hornblende, but with quartz, orthoclase, and other minerals in minor pro- portions ; sometimes they are massive and granular (Horn- blende-rocK), and in this condition may represent original eruptive rocks; more usually they are schistose (Horn- blende-schist). Gradations can be followed from a hornblende- schist into a massive rock (diorite, diabase, etc.), so that there can be no doubt that, in these cases at least, the schistose structure is not original but has been superinduced. Am- phibolites occur among the crystalline schists in most parts of the world as occasional bands or bosses, perhaps originally of an eruptive nature, but more or less affected by the movements that have induced the schistose structure. CHLORITE-SCHIST a scaly, schistose aggregate of green- ish chlorite with quartz, and often with felspar, mica, and octahedra of magnetite (Fig. 54) ; it occurs in beds associ- ated with gneiss and other schists. MICA- SCHIST (MiCA- SLATE) a schistose aggregate of quartz and mica, the two minerals being arranged in irregular but nearly parallel wavy folia. The rock splits along the laminae of mica, so that its flat surfaces have a bright silvery sheen, and the quartz is not well seen except on the cross fracture, where only the thin edges of the mica -plates pre- sent themselves. Mica-schist is often remarkably crumpled 224 SCHISTS AND THEIR ACCOMPANIMENTS. [CHAP. or puckered a structure bearing witness to the intense compression it has undergone. It abounds in most regions where schists are extensively developed (chapter xvi.) GNEISS a schistose aggregate of orthoclase, quartz, and mica, varying in texture from a fine-grained rock up to a coarse crystalline mass which, in hand specimens, may not be distinguishable from granite. There is no difference, in- deed, as regards composition, between gneiss and granite ; gneiss may be called a foliated granite. There is good reason to believe that some gneisses at least have been made out of granite by the process of shearing above referred to. Gneiss occurs abundantly among the oldest known rocks of the earth's crust, and may be found in most large regions of crystalline schists (chapter xvi.) A few rocks which are commonly associated with the schists, or with evidence of metamorphism, may be noticed here marble, quartzite, and schistose conglomerate. MARBLE a crystalline granular aggregate of calcite, white when pure, and having the texture of loaf-sugar, but passing into various colours according to the nature of the impurities. It occurs in beds among the schists, and is no doubt a limestone, formed either by chemical precipitation or by organic agency, which has been metamorphosed by heat and pressure into its present thoroughly crystalline character. Some of the fossiliferous limestones through which the Christiania granite rises have been changed into crystalline marble, but their original corals and shells have not been wholly effaced (see chapter xiv.) QUARTZITE a hard, compact, granular rock, composed of adherent quartz-grains, and breaking with a characteristic lustrous fracture. It occurs in beds and thick masses, asso- ciated with slates, mica-schists, and limestones ; it sometimes XL] SCHISTOSE GRIT AND CONGLOMERATE. 225 contains organic remains; and has evidently at one time been a siliceous sand, which owes its present firm texture to subse- quent metamorphism. SCHISTOSE GRIT and CONGLOMERATE. Interstratified with clay-slates and mica-schists there are sometimes found beds of grit and conglomerate, the grains and pebbles of which consist of quartz or other durable material, while the paste in which these rolled fragments have been imbedded has been metamorphosed into a slate or schist, so that the original clay or mud has passed into a more or less crystal- line micaceous substance that wraps round the pebbles as a kind of glaze. The original fragmental character of such rocks admits of no doubt ; they were obviously at one time sheets of fine and coarse gravel mixed with sandy mud; and their presence among schistose rocks furnishes additional corroborative evidence of the original sedimentary character of some of these rocks. PART III. THE STRUCTURE OF THE CRUST OF THE EARTH. CHAPTER XII. SEDIMENTARY ROCKS THEIR ORIGINAL STRUCTURES. HAVING in the two foregoing chapters considered the more important elementary substances of which the earth's crust is composed and their combinations in minerals and rocks, we have to inquire how these minerals and rocks have been put together so as to build up the crust A very little examination will suffice to show us that the upper or outer parts of the solid globe consist chiefly of sedimentary rocks. All over the plains and low grounds of the earth's surface, which cover so large a proportion of the whole area of the land, some kind of sediment underlies the soil clay, sand, gravel, limestone. It is for the most part only in hilly or mountainous regions that anything has been pushed up from below, so as to indicate the nature of the materials under- neath ; but everywhere we encounter proofs that the sedi- mentary rocks do not remain as they were deposited. In the first place, most of them were laid down on the sea-floor, and they have been upraised into land. In the next place, HAP. XII.] STRATIFICATION. 227 not only have they been upheaved, they have not infre- quently been bent, broken, and crushed, until sometimes their original condition can no longer be determined. More- over they have been invaded by masses of lava and other eruptive rocks, which have been thrust in among them and have often burst through them to form volcanoes at the surface. We must now endeavour to form as clear a con- ception as possible of what, after all these changes, the present structure of the crust actually is. In this chapter, therefore, we may examine some of the leading characters of sedimentary rocks in the architecture of the crust, more particularly those which have been determined by the con- ditions under which the rocks were formed. In the next chapter we shall consider some of the more important characters which have been superinduced upon the rocks since their formation. STRATIFICATION. It has been shown (p. 53) that one of the most distinctive features in sedimentary rocks is that they are stratified that is, are ar- ranged in layers one above another. As those at the bottom must have been de- posited before those at the top, a succession of layers of stratified rocks forms a record of deposition, in which the early stages are chronicled by the lower, and the later stages by the upper layers. An illus- tration of this kind of record FIG. 79. Section of stratified rocks. has already been given in the introductory chapter. As a further example, the accompanying section (Fig. 79) may be 228 STRUCTURES OF SEDIMENTARY ROCKS. [CHAP. taken. At the bottom lies a bed (a) of dark shale or clay with fragments of crinoids, corals, shells, and other marine organ- isms. Such a bed unmistakably points to a former muddy sea- floor, on which the creatures lived whose remains have been preserved in the hardened mud or shale. The next bed () is one of limestone full of similar organic remains ; it shows that the supply of mud which had previously made the water turbid and had been slowly gathering in successive layers on the bottom now ceased. The water became clear and much better fitted for the life of the crinoids, corals, and shells. These creatures accordingly flourished abundantly, living and dying on the spot generation after generation, until their accumulated remains had built up a solid sheet of limestone several feet thick. But once more muddy currents spread over the place, and from the cloud of sus- pended mud there slowly settled down the layer of blue clay (. formation regarding the food of the animal, portions of undigested scales, teeth, and bones being traceable in it. FOSSILISATION. The process by which the remains of a plant or animal are preserved in the fossil state is termed Fossilisation. It varies greatly in details, but all these may be reduced to three leading types. 1. Entire or partial preservation of the original sub- stance. In rare instances, the entire animal or plant has been preserved, of which the most remarkable examples are those where carcases of the extinct mammoth have been sealed up in the frozen mud and peat of Siberia, and have thus been preserved in ice, every portion of the animal substance being retained, and the flesh being fresh enough to be devoured by living carnivores. Insects have been preserved in the resin of trees, and may now be seen, em- balmed like mummies, in amber. More usually, however, a variable proportion of the organic matter has passed away, and its more durable parts have been left, as in the car- bonisation of plants (peat, lignite, coal) and the disappear- ance of the organic matter from shells and bones, which then become dry and brittle and adhere to the tongue. 2. Entire removal of the original substance and internal structure, only the external form being preserved. When a dead animal or plant has been entombed, the mineral matter in which it lies hardens round it and takes a mould of its form. This may be accomplished with great perfection if the mineral is sufficiently fine-grained and solidifies before the object within has time to decay. Carbonate of lime and silica are specially well adapted for taking the moulds of organisms, but fine mud, marl, and sand, are also effective. The organism may then entirely decay, and its substance may be gradually removed by percolating water, leaving a XV.] MODES OF FOSSILISATION. 285 hollow empty space or mould of its form. Such moulds are of frequent occurrence among fossiliferous rocks, and are specially characteristic of molluscs, the shells of which are so abundant, and occur imbedded in so many different kinds of material. Sometimes it is the external form of the shell that has been taken, the shell itself having entirely disappeared; in other cases a cast of the interior of the shell has been preserved. How different these two repre- sentations of the same shell may be is shown in Fig. 113, FIG. 113. Common Cockle {Cardiitm edule)\ (a) side view of both valves ; (/>) mould of the external form of one valve taken in plaster of Paris ; (c) side view of cast in plaster of Paris of interior of the united valves. wherein a represents a side view of the common cockle, while c is a cast of the interior of the shell in plaster of Paris. The contrast between a mould of the outside and inside of the same shell is shown by the difference between b and c, which are both impressions taken in plaster. After the decay and removal of the substance of the enclosed organisms, the moulds may be filled up with mineral matter, which is sometimes different from the sur- rounding rock. The empty cavities have formed convenient receptacles for any deposit which permeating water might 286 NATURE AND USE OF FOSSILS. [CHAP. introduce. Hence we find casts of organisms in sand, clay, ironstone, silica, limestone, pyrites, and other mineral sub- stances. Of course, in such cases, though the external form of the original organism is preserved, there is no trace of internal structure. No single particle of the cast may ever have formed part of the plant or animal. 3. Partial or entire petrifaction of organic structure by molecular replacement. Plants and animals which have undergone this change have had their substance gradually removed and replaced, particle by particle, with mineral matter. This transformation has been effected by percolat- ing water containing mineral solutions, and has proceeded so tranquilly, that sometimes not a delicate tissue in the internal structure of a plant has been displaced, and yet so rapidly, that the plant had not time to rot before the con- version was completed. Accordingly, in true petrifactions, that is, plants or animals of which the structure has been more or less perfectly preserved in stone, the petrifying material is always such as may have been deposited from water. The most common substance employed by nature in the process of petrifaction is carbonate of lime, which, as we have seen, is almost always present in the water of springs and rivers. Organic structures replaced by this substance are said to be calcified. Frequently the carbonate of lime has assumed, more or less completely, a crystalline structure after its deposition, and in so doing has generally injured or destroyed the organic structure which it originally replaced. Where the calcareous matter of an organism has been removed by percolating water, the fossil is said to be decalci- fied. Another abundant petrifying medium in nature is silica, which, in its soluble form, is generally diffused in terrestrial waters, where humus acids or organic matter are xv.] FOSSILS PROVE GEOGRAPHICAL CHANGES. 287 present in solution. The replacement of organic structures by silica, called silitification, furnishes the most perfect form of petrifaction. The interchange of mineral matter has been so complete that even the finest microscopic structures have been faithfully preserved. Silicified wood is an excellent example of this perfect replacement. Sulphides, which are often produced by the reducing action of decaying organic matter upon sulphates, occur also as petrifying media, the most common being the iron sulphide, usually in the less stable form of marcasite, but sometimes as pyrite. Car- bonate of iron likewise frequently replaces organic structures ; the clay-ironstones of the Carboniferous system abound with the remains of plants, shell-fishes, and other organisms which have been converted into siderite (Fig. 62). The chief value of fossils in geology is to be found in the light which they cast upon former conditions of geography and climate, and in the clue which they furnish to the relative ages of different geological formations. I. HOW FOSSILS INDICATE FORMER CHANGES IN GEO- GRAPHY. Terrestrial plants and animals obviously point to the existence of land. If their remains are found in strata wherein most of the fossils are marine, they usually show that the deposits were laid down upon the sea-floor not far from land. But where they occur in the positions in which they lived and died, they prove that their site was formerly a land- surface. The stumps of trees remaining in their positions of growth, with their roots branching out freely from them in the clay or loam underneath, undoubtedly mark the posi- tion of an ancient woodland. If, with these remains, there are associated in the same strata wing-cases of beetles, bones of birds and of land-animals, additional corroborative evi- dence is thereby obtained as to the existence of the ancient 288 NATURE AND USE OF FOSSILS. [CHAP. land. More usually, however, it is by the deposits left on lake-bottoms that the land of former periods of geological history is known. As already pointed out (chapter iv.), the fine mud and marl of lakes receive and preserve abundant relics of the vegetation and animal life of the surrounding regions. As illustrations of lacustrine formations, from which most of our knowledge of the contemporary terrestrial life is obtained, reference may be made to the Molasse of Switzer- land, the limestones and marls of the Limagne d'Auvergne, and the vast depth of strata from which so rich an assem- blage of plant and animal remains has been obtained in the Western Territories of the United States (see chapter xxiv.) Alternations of buried forests or peat -mosses, with lake deposits, show how lakes have successively increased and diminished in volume. The frequent occurrence of a bed of lacustrine marl at the bottom of a peat -bog proves how commonly shallow lakes have been filled up and displaced by the growth of marshy vegetation. Remains of marine plants and animals almost invariably demonstrate that the locality in which they are found was once covered by the sea. Exceptions to this rule are so few as hardly to be worthy of special notice, as, for instance, when molluscs, crustaceans, and other forms of marine life are carried up by sea-birds to considerable elevations, where, after their soft parts have been eaten, their hard shells and crusts may be preserved in truly terrestrial deposits, or when sea-shells, tossed up by breakers above the tide-line, are swept inland by wind. Rolled fragments of shells, mingled in well-rounded gravel and sand, point to some former shore, where these materials were ground down by beach-waves. Fine muddy sediment, containing unbroken shells, echinoderms, crusta- xv.] FOSSILS SHOW CHANGES OF CLIMATE. 289 ceans, and other relics of the sea, indicate deeper water beyond the scour of waves, tides, and currents. Beds of limestone, full of corals and crinoids, mark the site of a clear sea, in which these organisms were allowed to flourish un- disturbed for many generations. It may often be observed that the fossils, which are abundant and large in a limestone, become few in number and small in size in an overlying bed of shale or clay; or that they wholly disappear in the argilla- ceous rock. The meaning of this can hardly be mistaken. The clear water in which the marine creatures were able to build up the limestone was at last invaded by some current carrying mud. Consequently, while the more delicate forms perished, others continued to live on in diminished numbers and dwarfed development, until at last the muddy sediment settled down so thickly that the animals, whose hard parts might have been preserved, were driven away from that area of the sea-bottom. 2. HOW FOSSILS INDICATE FORMER CONDITIONS OF CLIMATE. The remains of plants or animals characteristic of tropical countries may be taken to bear witness to a tropical climate at the time which they represent. If, for example, a deposit were found containing leaves of palms and bones of tigers, lions, and elephants, we should infer that it was formed in some tropical country, such as the warmer parts of Africa or Asia. On the other hand, were a stratum to yield leaves of a small birch and willow, with bones of reindeer, musk-ox, and lemming, we would regard it as evidence of a cold climate. Such inferences, however, should be based either upon the occurrence of the very same species as are now living, and the characteristic climate of which is known, or upon assemblages of plants or animals which may be compared with corresponding assemblages 290 NATURE AND USE OF FOSSILS. [CHAP. now living. We may be tolerably confident- that the existing reindeer has always been restricted to a cold climate, and that the living elephants have as characteristically been con- fined to warm climates. But it would be rash to assume that all deer prefer cold and all elephants choose heat. The bones of an extinct variety of elephant and one of rhinoceros, have long been known as occurring even up within the Arctic regions, and when these remains were first found the conclusion was naturally drawn that they proved the former existence of a warm climate in the far north. But the sub- sequent discovery of entire carcases of the animals covered with a thick mat of woolly hair, showed that they were adapted for life in a cold climate, and their occurrence in association with the remains of animals which still live in the Arctic regions, proved beyond doubt that the original inference regarding them was erroneous. In drawing con- clusions as to climate from fossil evidence, it is always desirable to base them upon the concurrent testimony of as large a variety of organisms as possible, and to remember that they become less and less reliable in proportion as the organisms on which they are founded depart from the species now living. 3. HOW FOSSILS INDICATE GEOLOGICAL CHRONOLOGY. As the result of careful observations all over the world, it has been ascertained that in the youngest strata the organic- remains are nearly or quite the same as species now living, but that, as we proceed into older strata, the number of exist- ing species diminishes, and the number of extinct species increases, until at last no living species is to be found. Moreover, the extinct species found in younger strata dis- appear as we trace them back into older rocks, and their places are taken by other extinct species. Every great series xv.] FOSSILS AND GEOLOGICAL CHRONOLOGY. 291 of fossiliferous rocks is thus characterised by its own peculiar assemblage of species. Not only do the species change; the genera, too, disappear one by one as we follow them into older rocks, until among the earliest strata only a few of the living genera are represented. Whole families and orders of animals which once flourished have utterly vanished from the living world, and we only know of their existence from the remains of them preserved among the rocks. A certain definite order of succession has been observed among the organic remains imbedded in the stratified rocks of the earth's crust, and this order has been found to be broadly alike all over the world. The fossils of the oldest fossiliferous rocks of Europe, for instance, are like those of the oldest fossiliferous rocks of Asia, Africa, America, and Australasia, and those of each succeeding series of rocks follow the same general sequence. It is obvious, therefore, that fossils supply us with an invaluable means of fixing the relative position of rocks in the series of geological forma- tions. Whether or not the same type of fossils was always contemporaneous over the whole planet cannot be deter- mined ; but it generally occupied the same place in the pre- cession of life. Hence stratified formations, which may be quite unlike each other in regard to the nature of their component materials, if they contain similar organic remains, may be compared with each other, and classed under the same name. Fossils characteristic of particular subdivisions of the series of geological formations are known as type-fossils, of which the following are examples : Lepidodendra and Sigillaria, characteristic of Old Red Sandstone and Carboniferous rocks (p. 354). Cycads, characteristic of Mesozoic rocks (pp. 305, 379). 292 NATURE AND USE OF FOSSILS. [CHAP. Graptolites, characteristic of Silurian rocks (p. 323). Trilobites ,, Cambrian to Carboniferous rocks (pp. 328, 344, 363). Cystideans, characteristic of Silurian rocks (Fig. 121). Blastoids ,, Carboniferous rocks (Fig. 150). Hippurites ,, Cretaceous rocks (p. 413). Orthoceratites ,, Palaeozoic rocks (Figs. 130, 157). Ammonites ,, Mesozoic rocks (Figs. 176, 190). Cephalaspid fishes ,, Silurian, Old Red Sandstone (p. 340). Ichthyosaurus and Plesiosaurus Mesozoic rocks (Fig. 180). Iguanodon Cretaceous rocks (Fig. 192). Toothed birds Cretaceous rocks (p. 417). Nummulites, Palaeotherium, Anoplotherium, Deinocerata, charac- teristic of older Tertiary rocks (pp. 427-438). Mastodon, Elephas, Equus, Cervus, Hycena, Apes, characteristic of younger Tertiary and Recent rocks (pp. 439-478). By attentive study and comparison, the fossiliferous rocks in different countries have been subdivided into sections, each characterised by its own facies or type or organic remains. Consequently, beginning with the oldest and proceeding upward to the youngest, we advance through natural chronicles of the successive tribes of plants and animals which have lived on the earth's surface. These chronicles, consisting of sandstones, shales, limestones, and the other kinds of stratified deposits, form what is called the Geological Record. In order to establish their true sequence in time, their order of superposition must first be deter- mined ; that is, it is requisite to know which lie at the bottom, and must have been formed first, and in what order the others succeed them. When this fundamental question has once been settled, then the fossils characteristic of each group of strata serve as a guide for recognising that group wherever it may be found. While fossils enable us to divide the Geological Record into chapters, they also show how strikingly imperfect this xv.] THE GEOLOGICAL RECORD. 293 record is as a history of the plants and animals that have lived on the surface of the earth, and of the revolutions which that surface has undergone. We may be sure that the pro- gress of life from its earliest appearance in lowly forms of plant or animal has been continuous up to the present con- dition of things. But in the Geological Record there occur numerous gaps. The fossils of one group of rocks are suc- ceeded by a more or less completely different series in the next group. At one time it was supposed that such breaks in the continuity of the record marked terrestrial convulsions that caused the destruction of the plants and animals of the time, and were followed by the creation of new tribes of living things. But evidence has every year been augmenting that no such general destruction and fresh creation ever took place. The gaps in the record mark no real interruption of the life of the globe. They are rather to be looked upon as chapters that have been torn out of the annals, or which never were written. We have already learnt in chapter viii. how many chances there must be against the preservation of anything like a complete record of the life of the globe at any particular time. It is also clear that even where the chronicle may have been comparatively full, it is exposed to many dangers afterwards. The rocks containing it may be hidden beneath the sea, or raised up into land and entirely worn away, or entombed beneath volcanic ejections, or so crushed and crumpled as to become no longer legible. Taking fossils as a guide, geologists have partitioned the fossiliferous rocks into what are called stratigraphical sub- divisions as follows : A bed, or limited number of beds, in which one or more distinctive species of fossils occur, is called a zone or horizon, and may be named after its most typical fossil. Thus in the Lias, the zone in which the ammonite 294 NATURE AND USE OF FOSSILS. [CHAP. known as Ammonites Jamesoni occurs, is spoken of as the "zone of Ammonites Jamesoni" or "famesom-zone." Two or more zones, united by the occurrence in them of a number of the same characteristic species or genera, form what are known as Beds or an Assise. Two or more of such beds or assises may be termed a Group or Stage. Where the number of assises in a stage is large they may be subdivided into Sub- stages or Sub-groups. The stage or group will then consist of several sub -stages, and each sub -stage or sub-group of several assises. A number of groups or stages is combined into a Series, Section, or Formation, and a number of series, sections, or formations constitute a System. A number of systems are connected together to form each of the great divisions of the Geological Record. This classification will be best understood if placed in tabular form, as in the sub- joined subdivisions, which occur in the Cretaceous System. 1 Stratigraphical Components. Descriptive Names. Examples from the Cre- taceous System. A stratum, layer, ^ seam, or bed, or a number of such minor subdivisions, r = Zone or horizon . . Zone of Peclcn aspcr. characterised by some distinctive fossil Two or more zones = Beds or an assise Warminster beds. C Group or stage, Cenomanian stage, Two or more sets of con- which may be comprising the nected beds or assises = subdivided into Rothomagian and sub-groups or Carentonian sub- I sub-stages stages. Two or more groups or Series, section, or Neocomian forma- stages formation tion. Several related forma- ) _ System Cretaceous System. tions i 1 For an account of the Cretaceous System, see chapter xxiii xv.J STRATIGRAPHICAL NOMENCLATURE. 295 The names by which the larger subdivision of the Geo- logical Record are known have been adopted at various times and on no regular system. Some of them are purely litho- logical ; that is, they refer to the mere mineral nature of the strata, apart altogether from their fossils, such as Coal- measures, Chalk, Greensand, Oolite. These names belong to the early years of the progress of geology, before the nature and value of organic remains had been definitely realised. Other epithets have been suggested by localities where the strata occur, as London Clay, Oxford Clay, Moun- tain Limestone. The more recent names for the larger divisions have, in general, been chosen from districts where the strata are typically developed, or where they were first critically studied, e.g. Silurian, Devonian, Permian, Jurassic. In some cases, the larger subdivisions have received names from some distinguishing feature in their fossil contents, as Eocene, Miocene, Pliocene. 1 But it is mainly to the minor sections that the characters of the fossil contents have sup- plied names. The designation of any particular group of strata has gradually come to acquire a chronological meaning. Thus we speak of the Oolites or Oolitic formations of England, and include under these terms a thick series of limestones, clays, sandstones, and other strata, replete with organic remains, and containing the records of a long interval of geological time. But we also speak of the Oolitic period a phrase which, in the strict grammatical use of the word, is of course incorrect, but which conveniently designates the period of geological time during which the great series of Oolites was deposited, and when the abundant life of which they contain the remains flourished on the surface of the 1 For the meanings of these names see chapter xxiv. 296 NATURE AND USE OF FOSSILS. [CHAP. earth. This chronological meaning has indeed come to be the more usual sense in which the names of the major subdivisions of the Geological Record are generally em- ployed. Such adjectives as Devonian and Jurassic do not so much suggest to the mind of the geologist Devonshire and the Jura Mountains, from which they were taken, nor even the rocks to which they are applied, as the great sections of the earth's history of which these rocks contain the memorials. He compares the Jurassic or Devonian rocks of one country with those of another, studies the organic remains contained in them, and then obtains materials for forming some conception of what were the conditions of geography and climate, and what was the general character of the vegetable and animal life of the globe, during the periods which he classes as Jurassic and Devonian. Summary. Fossils are the remains or traces of plants and animals which have been imbedded in the rocks of the earth's crust. From the exceptional nature of the circum- stances in which these remains have been entombed and preserved, only a comparatively small proportion of the various tribes of plants and animals living at any time upon the earth is likely to be fossilised. Those organisms which contain hard parts are best fitted for becoming fossils. The original substance of the organism may, in rare cases, be pre- served ; more usually the organic matter is partially or wholly removed. Sometimes a mere cast of the plant or animal in amorphous mineral matter retains the outward form without any trace of the internal structure. In other instances, true petrifaction has taken place, the organic structure being reproduced in calcite, silica, or other mineral by molecular replacement. Fossils are of the utmost value in geology, inasmuch as xv.] SUMMARY. 297 they indicate (i) former changes in geography, such as the existence of ancient land -surfaces, lakes, and rivers, the former extension of the sea over what is now dry land, and changes in the currents of the ocean ; (2) former conditions of climate, such as an Arctic state of things as far south as Central France, where bones of reindeer and other Arctic animals have been found ; (3) the chronological sequence of geological formations, and, consequently, the succession of events in geological history, each great group of strata being characterised by its distinctive fossils. This is the most important use of fossils. Having ascertained the order of superposition of fossiliferous rocks, that is, the order in which they were successively deposited, and having found what are the characteristic fossils of each subdivision, we obtain a guide by which to identify the various rock-groups from district to district, and from country to country. By means of the evidence of fossils the stratified rocks of the Geological Record have been divided into sections and sub- sections, to which names are applied that have now come to designate not merely the rocks and their fossils, but the period of geological time during which these rocks were accumulated and these fossils actually lived. PART IV. THE GEOLOGICAL RECORD OF THE HISTORY OF THE EARTH. CHAPTER XVI. THE EARLIEST CONDITIONS OF THE GLOBE THE ARCH/EAN PERIODS. THE foregoing chapters have dealt chiefly with the materials of which the crust of the earth consists, with the processes whereby these materials are produced or modified, and with the methods pursued by geologists in making their study of these materials and processes subservient to the elucidation of the history of the earth. The soils, rocks, and minerals beneath our feet, like the inscriptions and sculptures of a long-lost race of people, are in themselves full of interest, apart from the story which they chronicle ; but it is when they are made to reveal the history of land and sea, and of life upon the earth, that they are put to their noblest use. The investigation of the various processes whereby geological changes are carried on at the present day is undoubtedly full of fascination for the student of nature ; yet he is con- scious that it gains enormously in interest when he reflects CHAP, xvi.] GEOLOGICAL HISTORY. 299 that in watching the geological operations of the present day he is brought face to face with the same instruments whereby the very framework of the continents has been piled up and sculptured into the present outlines of moun- tain, valley, and plain. The highest aim of the geologist is to trace the history of the earth. All his researches, remote though they may seem from this aim, are linked together in the one great task of unravelling the successive mutations through which each area of the earth's surface has passed, and of discover- ing what successive races of plants and animals have appeared upon the globe. The investigation of facts and processes, to which the previous pages have been devoted, must accordingly be regarded as in one sense introductory to the highest branch of geological inquiry. We have now to apply the methods and principles already discussed to the elucidation of the history of our planet and its in- habitants. Within the limits of this volume only a mere outline of what has been ascertained regarding this history can be given. I shall arrange in chronological order the main phases through which the globe seems to have passed, and present such a general summary of the more important facts regarding each of them as may, I hope, convey an adequate outline of what is at present known regarding the successive periods of geological history. As the primitive stages of mankind upon the earth and the early progress of every race fade into the obscurities of mythology and archaeology, so the story of the primeval condition of our globe is lost in the dim light of remote ages, regarding which almost all that is known or can be surmised is furnished by the calculations and speculations of the astron- omer. If the earth's history could only be traced out from 300 EARLIEST CONDITIONS OF THE GLOBE. [CHAP. evidence supplied by the planet itself, it could be followed no further back than the oldest portions of the earth now accessible to us. Yet there can be no doubt that the planet must have had a long history before the appearance of any of the solid portions now to be seen. That such was the case is made almost certain by the traces of a gradual evolu- tion or development which astronomers have been led to recognise among the heavenly bodies. Our earth being only one of a number of planets revolving round the sun, the earliest stages of its separate existence must be studied in reference to the whole planetary system of which it forms a part. Thus, in compiling the earliest chapter of the history of the earth, the geologist turns for evidence to the researches of the astronomer among stars and nebulae. In recent years, more precise methods of inquiry, and, in particular, the application of the spectroscope to the study of the stars, have gone far to confirm the speculation known as the Nebular Hypothesis. According to this view, the orderly related series of heavenly bodies, which we call the Solar System, existed at one time, enormously remote from the present, as a Nebula that is, a cloudy mass of matter, like one of those nebulous, faintly luminous clouds which can be seen in the heavens. This nebula probably extended at least as far as the outermost planetary member of the system is now removed from the sun. It may have .con- sisted entirely of incandescent gases or vapours, or of clouds of stones in rapid movement, like the stones that from time to time fall through our atmosphere as meteorites, and reach the surface of the earth. The collision of these stones mov- ing with planetary velocity would dissipate them into vapour, as is perhaps the case in the faint luminous tails of comets. At all events, the materials of the nebula began to condense, xvi.] NEBULAR HYPOTHESIS. 301 and in so doing, threw off, or left behind, successive rings (like those around the planet Saturn), which, in obedience to the rotation of the parent nebula, began to rotate in one general plane around the gradually shrinking nucleus. As the process of condensation proceeded, these rings broke up, and their fragments rushed together with such force as not improbably to generate heat enough to dissipate them again into vapour. They eventually condensed into planets, sometimes with a further formation of rings, or with a dis- ruption of these secondary rings, and the consequent forma- tion of moons or satellites round the planets. The outer planets would thus be the oldest, and, on the whole, the coolest and least dense. Towards the centre of the nebula the heaviest elements might be expected to condense, and there the high temperature would longest continue. The sun is the remaining intensely hot nucleus of the original nebula, from which heat is still radiated to the furthest part of the system. When a planetary ring broke up, and by the heat thereby generated was probably reduced to the state of vapour, its materials, as they cooled, would tend to arrange themselves in accordance with their respective densities, the heaviest in the centre, and the lightest outside. In process of time, as cooling and contraction advanced, the outer layers might grow. quite cold, while the inner nucleus of the planet might still be intensely hot. Such, in brief, is the well-known Nebu- lar Hypothesis. Now the present condition of our earth is very much what, according to this hypothesis or theory, it might be expected to be. On the outside comes the lightest layer or shell in the form of an Atmosphere, consisting of gases and vapours. Below this gaseous envelope which entirely sur- 302 EARLIEST CONDITIONS OF THE GLOBE. [CHAP. rounds the globe lies an inner envelope of water, the ocean, which covers about two -thirds of the earth's surface, and is likewise composed of gases. Underneath this watery cover- ing, and rising above it in dry land, rests the solid part of the globe, which, so far as accessible to us, is composed of rocks twice to thrice the weight of pure water. But obser- vations with the pendulum at various heights above the sea show that the attraction of the earth as a whole indicates that the globe probably has a density about five and a half times that of water. Hence we may infer that its inner nucleus not improbably consists of heavy materials, and may be metallic. There is thus evidence of an arrangement of the planet's materials in successive spherical shells, the lightest or least dense being on the outside, and the heaviest or densest in the centre. Again, the outside of the earth is now quite cool ; but abundant proof exists that at no great distance below the surface the temperature is high. Volcanoes, hot springs, and artificial borings all over the world testify to the abun- dant store of heat within the earth. Probably at a depth of not more than 20 miles from the surface the temperature is as high as the melting-point of any ordinary rock at the surface. By far the largest part of the planet, therefore, is hotter than molten iron. We need have no hesitation in admitting it to be highly probable that the earth was originally in the state of incandescent vapour, and that it has ever since that time been cooling and contracting. Its present shape affords strong presumption in favour of the opinion that the globe was once in a plastic condition. The flattening at the poles and bulging at the equator, or what is called the oblately spheroidal figure of the planet, is just the shape which a plastic mass would have assumed in xvi.] PRIMEVAL SEA AND ATMOSPHERE. 303 obedience to the influence of the movement of rotation, imparted to it when detached from the parent nebula. At present a complete rotation is performed by the earth in twenty-four hours. But calculations have been made with the result of showing that originally the rate of rotation was much greater. Fifty-seven millions of years ago it was about four times faster, the length of the day being only six and three-quarter hours. The moon at that time was only about 35,000 miles distant from the earth, instead of 239,000 miles as at present. Since these early times the rate of rota- tion has gradually been diminishing, and the figure of the earth has been slowly tending to become more spherical, by sinking in the equatorial and rising in the polar regions. Of the first hard crust that formed upon the surface of the earth no trace has yet been found. Indeed, there is reason to suppose that this original crust would break up and sink into the molten mass beneath, and that not until after many such formations and submergences did a crust establish itself of sufficient strength to form a permanent solid surface. Even though solid, the surface may still have been at a glowing red-heat, like so much molten iron. Over this burning nucleus lay the original atmosphere, consisting not merely of the gases in the present atmo- sphere, but of the hot vapours which subsequently con- densed into the ocean, or were absorbed into the crust. It was a hot, vaporous envelope, under the pressure of which the first layers of water that condensed from it may have had the temperature of molten lead. As the steam passed into water, it would carry down with it the gaseous chlorides of sodium, magnesium, and other vapours in the original atmosphere, so that the first ocean was probably not only hot, but intensely saline. 304 EARLIEST CONDITIONS OF THE GLOBE. [CHAP. Regarding these early ages in the earth's history we can only surmise, for no direct record of them has been pre- served. They are sometimes spoken of as pre-geological ; but geology really embraces the whole history of the planet, no matter from what sources the evidence may be obtained. Deposits from this original hot saline ocean have been supposed to be recognisable in the very oldest crystalline schists ; but for this supposition there does not appear to be any good ground. The early history of our planet like that of man himself is lost in the dimness of antiquity, and we can only speculate about it on more or less plausible suppositions. When we come to the solid framework of the earth we stand on firmer footing in the investigation of geological history. The terrestrial crust, or that portion of the globe which is accessible to human observation, has been found to consist of successive layers of rock, which, though far from constant in their occurrence, and though often broken and crumpled by subsequent disturbance, have been recog- nised over a large part of the globe. They contain the earth's own chronicle of its history, which has already been referred to as the Geological Record, and the subdivision of which into larger and minor sections, according mainly to the evidence of fossils, was explained in the preceding chapter. Had the successive layers of rock that constitute the Geological Record remained in their original positions, only the uppermost, and therefore most recent of them would have been visible, and nothing more could have been learnt regarding the underlying layers, except in so far as it might have been possible to explore them by boring into them. But the deepest mines do not reach greater xvi.] DIVISIONS OF GEOLOGICAL RECORD. 35 depths than between 3000 and 4000 feet from the surface. Owing, however, to the way in which the crust of the earth has been plicated and fractured, portions of the bottom layers have been pushed up to the surface, and those that lay above them have been thrown into vertical or inclined positions, so that we can walk over their upturned edges and examine them, bed by bed. Instead of being restricted to merely the uppermost few hundred feet of the crust, we are enabled to examine many thousand feet of its rocks. The total mean thickness of the accessible fossiliferous rocks of Europe has been estimated at 75,000 feet or upwards of 14 miles. This vast depth of rock has been laid bare to observation by successive disturbances of the crust. The main divisions of the Geological Record and, we may also say, of geological time, are five: (i) Archaean, embracing the periods of the earliest rocks, wherein no traces of organic life occur; (2) Palaeozoic (ancient life) or Primary, including the long succession of ages during which the earliest types of life existed ; (3) Mesozoic (middle life) or Secondary, comprising a series of periods when more advanced types of life flourished ; (4) Cainozoic (recent life) or Tertiary, embracing the ages when the existing types of life appeared ; but excluding man ; and (5) Quaternary or Post-tertiary and Recent, including the time since man ap- peared upon the earth. Each of these main sections is further subdivided into systems or periods, and each system into formations as already explained. Arranged in their order of sequence, the various divisions of the Geological Record may be placed as in the accompanying Table, 306 EARLIEST CONDITIONS OF THE GLOBE. [CHAP. THE GEOLOGICAL RECORD, or, Order of Succession of the Stratified Formations of the Earth's Crust. rt _ Recent and Prehistoric. Pleistocene or Glacial. Pliocene. Miocene. Oligocene. Eocene. o Cretaceous. Danian. Senonian. Turonian. Cenomanian. Gault. Neocomian. Jurassic. Purbeckian. Portlandian. Kimmeridgian. Corallian. Oxfordian. Bathonian. Bajocian. Liassic. Triassic. Rhaetic. Keuper or Upper Trias. Muschelkalk. Bunter or Lower Trias. 2 Permian. Upper Red Sandstones, clays, and gypsum. Magnesian Limestone (Zechstein). Marl vSlate (Kupferschiefer). Lower Red Sandstones, breccias, etc. (Rothliegende). xvi.] ARCHAEAN PERIODS. 307 Carboniferous. Coal Measures. Millstone Grit. Carboniferous Limestone series. Devonian and Old Red Sandstone. n . ( Upper Cypridina and Goniatite beds. ^ -I Middle Stringocephalus (Eifel) Limestone. I Lower Spirifer Sandstone, etc. Old r d f Upper Yellow an( l Rec l Sandstones, with Holop- o if) tychius, Pterichthys major, etc. T | Lower Sandstones,flagstones, and conglomerates, ! with Cephalaspis, Coccosteus, Astcrohpis, etc. Silurian. IH / Ludlow group. od Wenlock group. V Upper Llandovery group. ./ Cavadoc and Bala group. I Lower Llandovery group. 9 | Llandeilo group. 1 Arenig group. 'rt . ( Upper Tremadoc Slates. 1 -| I Lingula Flags. S j Lower Menevian group. S M ! Harlech and Longmynd group. o v ArcliL.janTPre-Cambrian). THE ARCHAEAN PERIODS. Owing to the revolutions which the crust of the earth has undergone, there have been pushed up to the surface, from underneath the oldest fossiliferous strata, certain very ancient crystalline rocks which form what is termed the Archaean system. As already mentioned, these rocks have by some geologists been supposed to be a part of the primeval crust of the planet, which solidified from fusion. 3S ARCHAEAN. [CHAP. By others they have been thought to have been formed in the boiling ocean, which first condensed upon the still hot surface of the globe. In truth, we are still profoundly ignorant as to the conditions under which they arose. We have hardly any means of ascertaining in what order they were formed. We know no method of determining whether those of one region belong to the same period as those of another. Nor can we always be sure that what have been called Archaean rocks may not belong to a much later part of Geological Record, their peculiar crystalline structure having been superinduced upon them by some of those subterranean movements described in chapter xiii. Of Archaean rocks the most abundant is gneiss, passing on the one hand into granite, and on the other into micaceous and argillaceous schists, with interstratified bands of various hornblendic, pyroxenic, and garnetiferous rocks, limestone, dolomite, serpentine, quartzite, graphite, haematite, magnetite, etc. These various materials are more or less distinctly bedded. But the beds are for the most part inconstant, swelling out into thick zones, and then rapidly diminishing and dying out. This bedding somewhat resembles that of sedimentary rocks, and the manner in which the limestone and graphite occur, recalls the way in which limestone and coal are found in the fossiliferous formations. The inference has accordingly been drawn that the Archaean crystalline bands were really deposited as chemical precipitates or mechanical sediments on the floor of the primeval ocean, and have since been more or less crystallised and disturbed. But from what has been brought forward in chapter xiii., regarding the totally- new structures which have been developed in rocks by subterranean movement, it is evident that a bedded arrange- xvi.] GNEISSES AND SCHISTS. 309 ment and a crystalline texture, like those of the Archaean gneisses and schists, have sometimes been induced in rocks by excessive crumpling, fracture, and shearing. How far, therefore, the apparent bedding of Archaean rocks is their original condition, or is the result of subsequent disturbance, is a question that cannot yet be answered. The alternations of gneiss and other crystalline masses form bands which are usually placed on end or at high angles, and are often intensely crumpled and puckered, having evi- dently undergone enormous crushing (Fig. 114). Attempts FIG. 114. Fragment of crumpled Schist. have been made to subdivide them into groups or series, ac- cording to their apparent order of succession and lithological characters. But such subdivisions, even where practicable, are probably only of local value. As a rule, those members of the system which, if the succession of beds may be trusted, are the lowest and oldest, present coarser crystalline characters than those which seem to be higher and later. They often con- sist of massive granitic gneiss, with abundant veins and bands of the coarsely crystalline variety of granite, known as peg- matite. The apparently higher rocks are less coarsely crystal- line gneiss, and often mica-schists and other schistose masses. 310 ARCHAEAN. [CHAP. No unquestionable relic of organic existence has been met with among Archaean rocks. Some of the Archsean limestones of Canada have yielded a peculiar mixture of serpentine and calcite, with a structure which is regarded by some able naturalists as that of a reef-building foraminifer. It occurs in masses, and is supposed by these writers to have grown in large, thick sheets or reefs over the sea- bottom. By other observers, however, this supposed organism (to which the name of Eozoon has been given) is regarded as merely a mineral segregation, and various un- doubted mineral structures are pointed to in illustration and confirmation of this view. The rocks in which Eozoon occurs have been so greatly mineralised by the processes of metamorphism, that any original organic structure in them could hardly be expected to have escaped destruction. Though the structure in Eozoon is in some respects peculiar, it nevertheless so much resembles some recognised mineral arrangements, that its claim to be regarded as an organism has not been satisfactorily established. Archsean rocks cover a large area in Europe. In the British Islands, they are principally developed among the Hebrides and along the north-west coasts of the Scottish Highlands, where they give rise to a singular type of scenery. Over much of that region they form hummocky bosses of naked rock, with tarns and peat-bogs lying in the hollows, seldom rising into mountains, but forming the platform which supports a singular group of red sandstone mountains. Here and there, they mount up into solitary hills or groups of hills. The highest point they reach on the mainland is at Stack, near Loch Laxford, which is 2364 feet above the sea. But in the Island of Harris they sweep upwards into rugged mountainous ground, of which the xvi.] LAURENTIAN, HURONIAN. 311 highest summits rise more than 2600 feet out of the Atlantic, and are visible far and wide as a notable landmark. On the continent of Europe, Archaean rocks have their greatest extension in Scandinavia, where they evidently be- long to the same ancient land as that of which the Hebrides and Scottish Highlands are fragments. They range through Finland far into Russia, appearing in the centre of the chain of the Ural Mountains. They form likewise the nucleus of the Carpathians and the Alps, and appear in detached areas in Bavaria, Bohemia, France, and the Pyrenees. They are estimated to occupy an area of more than 2,000,000 of square miles in the more northerly part of North America, stretching from the Arctic regions southwards to the great lakes. In this vast region they have been subdivided into an older series, termed Laurentian, and a younger series, called Huronian. It thus appears that both in the Old and New World, the Archaean rocks are chiefly exposed in the northern tracts of the continents. The areas which they there overspread were probably land at a very early geological period, and it was mainly from the waste of this land that the original materials were derived, out of which the enormous masses of stratified rocks were formed. In the southern hemisphere, also, ancient gneisses and other schists, referred to the Archaean system, rise from under the oldest fossiliferous formations. In Australia and in New Zealand they cover large tracts of country, and appear in the heart of the mountain ranges. It thus appears that all over the world the oldest known rocks are gneisses and similar or allied crystalline masses, having a remarkable uniformity of character. CHAPTER XVII. THE PAL/EOZOIC PERIODS SILURIAN. THE portion of geological history which treats of those ages in which the earliest known types of plants and animals lived is termed Palaeozoic. Of the first appearance of organic life upon our planet we know nothing. Whether plants or animals came first, and in what forms they came, are questions to which as yet no satisfactory answer can be given. The oldest discovered fossils are assuredly not vestiges of the first living things that peopled the globe. There is every reason, indeed, to hope that as researches in all parts of the world are pushed into older and yet older rocks, still more ancient organisms may be discovered. But it is in the highest degree improbable that any trace of the earliest beginnings of life will ever be found. The first plants and the first animals were probably of a lowly kind, with no hard parts capable of preservation in the fossil state. Moreover, the sedimentary rocks which may have chronicled the first advent of organised existence are hardly likely to have escaped the varied revolutions to which all parts of the crust of the earth have been exposed, but have probably been buried out of sight, or have been so crushed and broken and metamorphosed that their ori- CHAP, xvii.] PALEOZOIC ROCKS. 313 ginal condition, and any fossils they may have enclosed, are no longer to be recognised. The first chapters have been, as it were, torn out from the chronicle of the earth's history. The Palaeozoic rocks, which contain the earliest record of plant and animal life, consist mainly of the hardened mud, sand, and gravel of the sea-bottom. Here and there, they include beds or thick groups of beds of limestone composed of marine shells, crinoids, corals, and other denizens of salt water. They are thus essentially the chronicles of the sea. But they also contain occasional vestiges of shores, and even of the jungles and swamps of the land, with a few rare glimpses into the terrestrial life of the time. Everywhere they abound in evidence of shallow water ; for though chiefly marine, they appear to have been accumulated not far from land. We may believe that in the earliest periods, as at the present day, the sediment washed away from the land has been deposited on the sea-floor, for the most part at no great distance from the coast. The land from the waste of which the Palaeozoic rocks were formed lay in Europe and North America chiefly to- wards the north. It no doubt consisted of Archaean rocks, such as still rise out from under the oldest Palaeozoic forma- tions. As already mentioned, the north-west Highlands of Scotland, part of the tableland of Scandinavia, and most of North America to the north of the great lakes are probably portions of that earliest land, which, after being deeply buried under later geological accumulations, have once more been laid bare to the winds and waves. We can form some con- ception of the bulk of the primeval northern land by noting the thickness of sedimentary rocks that were formed out of its detritus during the Palaeozoic periods. The older half 3*4 PALEOZOIC PERIODS. [CHAP. of the Palaeozoic rocks in the British Islands, for example, is at least 16,000 feet or 3 miles thick, and covers an area of not less than 60,000 square miles. This material, derived from the waste of the Archaean rocks, would make a table- land larger than Spain, with an average height of 5000 feet, or a mountain chain 1800 miles long, with an average eleva- tion of 16,000 feet. Of the general form and height of the northern land that supplied this vast mass of sedimentary matter nothing, is known. Perhaps it was lofty ; but it may have been slowly uplifted, so that its rise compensated for the ceaseless degradation of its surface. Abundant evidence of volcanic action has been preserved among the Palaeozoic rocks in the form of piles of lavas and tuffs. We find also many indications of upward and down- ward movements of the crust of the earth. The mere fact of the superposition of many thousands of feet of shallow- water strata, one above another, is a proof of gradual subsi- dence. For it is evident that the accumulation of such a thickness of sediment, and the continuance of a shallow sea over the area of deposition, could only take place during a progressive subsidence (see p. 239). The life of the Palaeozoic periods, so far as known from the fossils which have been obtained from the rocks, appears to have been far more uniform over the whole globe than at any subsequent epoch in geological history. For instance, the same species of fossils are found in corresponding rocks in Britain, Russia, United States, China, and Australia. The climate of the globe at that ancient date was doubtless more uniform than it afterwards became, and was probably also generally warmer. Palaeozoic fossils, obtained from high northern latitudes, are precisely similar to those that abound in England, whence it may be inferred that not only was xvii.] LIFE OF PALEOZOIC TIME. 315 there a greater uniformity of climate, but that the great cold which now characterises the Arctic regions did not then exist, w H Y ? In the earlier Palaeozoic periods, the animal life of the globe appears to have been entirely invertebrate, the highest types being chambered shells of which our living nautilus is a representative. In the later periods vertebrate life appeared. The earliest known vertebrate forms are fishes akin to some modern sharks and to the sturgeon, the poly- pterus of the Nile, and the gar-pike of American lakes. The most highly organised forms of existence upon the earth's surface in the later Palaeozoic periods were am- phibians a class of animals represented at the present day by frogs, toads, newts, and salamanders. It is evident, however, that the number and kinds of animal remains pre- served in Palaeozoic rocks afford only an imperfect record of the animal life of these early ages. Whole tribes of creatures no doubt existed of which no trace whatever has yet been recovered. An accidental discovery may at any moment reveal the former presence of some of these vanished forms. For example, the examination of a fossil tree-trunk imbedded among the coal-strata of Nova Scotia, led to the finding of the first and as yet almost the only traces of Palaeozoic land- shells, though thousands of species of marine shells, belong- ing to the same period, had long been known. Every year is enlarging our knowledge in these respects, but from the very nature of the circumstances in which the records of the rocks are formed, we cannot expect this knowledge ever to be more than fragmentary. The Palaeozoic rocks are divided into four systems which in the order of their age have been named*: (i) Silurian; (2) Devonian ; (3) Carboniferous ; (4) Permian. 316 PALAEOZOIC PERIODS. [CHAP. SILURIAN. The strata containing the earliest organic remains were formerly known as Greywacke, from the rock which is specially abundant among them. They were also termed Transition, from the supposition that they were deposited during a transitional period, between the time when no organic life was possible on the earth's surface, and that when plant and animal life abounded. But when the late Sir Roderick Murchison explored them, and showed that they contained a series of formations, each characterised by its own assemblage of organic remains, he called them the Silurian system, after the names of the old British tribe the Silures, who lived on the borders of England and Wales, where these rocks are especially well developed. This name has now been adopted all over the world as the designation of those stratified formations which contain the same or similar organic remains to those found in the typical region described by Murchison. While the succession of the rocks and fossils was estab- lished by that geologist in South Wales, and in the border countries of Wales and England, Professor Sedgwick was at work among similar rocks in North Wales. These were at first believed to be all older than those called Silurian, and were accordingly named Cambrian, after the old name for Wales, Cambria. In the end, however, it was found that throughout a large part of them the same fossils occurred, as in the Silurian series, and they were accordingly claimed as Silurian. Much controversy has since been carried on regarding the limits and names to be assigned to these rocks, and geologists are not yet agreed upon the nomenclature that should be followed. There can be no doubt, however, SILURIAN. 317 that the first succession of organic remains established among these ancient members of the great Palaeozoic series of formations was that worked out by Murchison and named by him Silurian. It is also indisputable that among these ancient rocks there is only one great type of life, and that though more or less marked differences characterise the successive chronological subdivisions of the rocks, never- theless, the general type remains persistent. As this type was clearly recognised and described by Murchison, his name of Silurian has the claim of priority. It will accord- ingly be employed in this volume as the general designation of the earliest known assemblage of organic remains. The term Cambrian will be used to denote the oldest group of the great Silurian series. It may be well to repeat that these words, like all those adopted by geologists to distinguish the successive rock-groups of the earth's crust, have acquired a chronological meaning. We speak not only of Silurian strata and Silurian fossils, but of Silurian time. The term is used to denote that particular period in the history of the earth when Silurian strata were deposited, and when Silurian fossils were the living denizens of sea and land. The Silurian system all over the world presents more generally uniform lithological characters than later parts of the Geological Record. Its rocks consist in great part 01 hardened sand and gravel, which appear as greywacke, sand- stone, grit, and conglomerate ; also of indurated mud, which has assumed the condition of shales and slates. Among these mechanical sediments there occasionally occur, particu- larly in the higher parts of the system, bands of fossiliferous limestone ; while in some regions (Wales), at different levels in the series, lie thick intercalations of contemporaneous lavas and tuffs. In certain countries (Russia, New York) 318 PALAEOZOIC PERIODS. [CHAP. Silurian rocks have undergone little change since the time of their deposition ; but as a rule they have been more or less indurated, plicated, and dislocated; while in many places (Scotland, Scandinavia, etc.) they have been so crushed as to pass into various crystalline schists. The organic remains of the Silurian system possess a peculiar interest, inasmuch as they embrace the oldest forms of life yet discovered. Those found in the lower parts of the system differ in various respects from those met with in the higher parts. Hence, having respect to these differences, it has been found of advantage to subdivide the system into three great sections. The lowest or oldest series is the Primordial Silurian or Cambrian ; the central series is the Lower Silurian, and the highest is the Upper Silurian. A table of these sections with their subdivisions is given on P- 335- Taking the fossils of the Silurian system as a whole, we find that they include both plants and animals, in other words, a flora and a fauna. The flora, however, is ex- ceedingly meagre. It consists chiefly of sea-weeds which usually occur .in the form of fucoid-like impressions. But many plant-like markings occur among these ancient strata which are almost certainly not plants. Some of them are the tracks left by worms, crustaceans, or other creatures upon soft mud or sand ; others are casts of hollows made by trickling water on yielding sediment, or have been formed by some other process, not connected either with plant or animal life. But remains of delicate branching algae, like some living forms, occur in the higher parts of the Silurian system (Fig. 115). Among the Upper Silurian strata, also, traces of land-vegetation have been detected in the form of spores and stems of cryptogamous plants. Lycopods or XVII.] SILURIAN. 319 club-mosses, and ferns appear to have been the chief types in the earliest terrestrial floras ; at least it is remains refer- FIG. 115. A, Fucoid-like impression (Eophyton Linneanum] from Cambrian rocks (^). 1 B, An Upper Silurian sea-weed (Chondrites verisimilis\ natural size. able to them that chiefly occur in the older Palaeozoic rocks. They reached a great development in the Carboniferous Period, in the account of which a fuller description of them will be given. We can dimly picture the Silurian land, with its waving thickets of fern, above which lycopod trees raised their fluted and scarred stems, threw out their scaly moss-like branches, and shed their spiky cones. The fauna found in Silurian rocks is much more abun- 1 The numbers inserted within parentheses in the titles of the figures of fossils indicate how much the figures have been reduced or magni- fied. Thus 3 = reduced one-third ; = magnified four times. 320 PAL/EOZOIC PERIODS. [CHAP. dant in certain kinds of strata than in others. Grits and sandstones, for instance, are comparatively unfossiliferous, while fine shales, slates, and limestones are often crowded with fossils. It is not that life was on the whole more abundant at the time of the deposition of some kinds of strata, but that the local conditions for its growth and for the subsequent entombment and preservation of its remains were then more favourable. At the present time, for example, dredging operations show the most remarkable variations between different and even adjacent parts of the sea-bottom as regards the abundance of marine life. Some tracts are almost lifeless, while others are crowded with a varied and prolific fauna. We can easily understand that if, from the nature of the bottom, plants and smaller animals cannot flourish on a particular tract, the larger kinds that feed on them will also desert it. Even if organisms live and die in some numbers over a part of the sea-bed, the conditions may not be suitable there for the preservation of their remains. The rate of deposit of sediment, for instance, may be so slow that the remains may decay before there is time for them to be covered up ; or the sediment may be unfitted for effectually preserving them, even when they are buried in it. We must not lose sight of these facts in our explorations of the Geological Record. A relation has always existed between the abundance or absence of fossils in a sedimentary rock, and the circumstances under which the rock was originally formed. The oldest fossiliferous strata (Primordial or Cambrian) contain a remarkable assemblage of animal remains, which, being the earliest traces of the animal life of the globe, might have been anticipated to belong to the very lowest tribes of the animal kingdom. But they are by no means xvii.] SILURIAN. 321 of such humble organisation. On the contrary, they include no representatives of many of the groups of simpler inver- tebrates, which we may be sure were nevertheless living at the same time. Not only so, but some of the fossils belong to comparatively high grades in the scale of invertebrate life, such as chambered molluscs. From this incompleteness, and from the wide differences in the organic grade of the forms actually preserved in the rocks, we may reasonably infer that only a most meagre representation of the life of the time has come down to us in the fossil state. Some of the fossils, moreover, have been so indistinctly preserved that considerable difficulty is experienced in deciding to what sections of the animal or vegetable kingdoms they should be assigned. Among the organisms which have given rise to much discussion, allusion may be made to the puzzling form FIG. 1 1 6. Oldhamia radiata (natural size), Ireland. called Oldhamia (Fig. 1 16), which has been variously referred to the Hydrozoa, the Sertularia, the Polyzoa, and the cal- careous Algae. The simplest animal organisms yet detected in the Silu- rian system are Foraminifera and Sponges, and their remains have survived, because they contained hard parts capable of preservation in the sand and mud of the sea-floor. A fora- minifer (of which there are still many living types in the Y 322 PALEOZOIC PERIODS. [CHAP. present ocean, see Fig. 33) is a minute animal composed of a transparent jelly-like substance which, possessing no definite organs, has, in some kinds, the power of secreting a hard calcareous or horny shell, through openings or pores (foramina} in which, filaments from the jelly-like mass are protruded. By other kinds, grains of sand are cemented together to form a protecting shell. It is these calcareous and sandy coverings which occur in the fossil state and prove the presence of foraminifera in the older oceans of the globe. Sponges also are known to have lived in the Silurian seas from the remains of their hard parts preserved as fossils. A sponge is a mass of soft transparent jelly-like substance perforated by tubes or canals, and supported on an internal network of minute calcareous or siliceous spicules, or of interlacing horny fibres. Most fossil sponges are cal- careous or siliceous, and their remains being durable, they have been preserved sometimes in wonderful perfection. The common sponge, familiar in domestic use, is an ex- ample of the horny type. Some of the most characteristic Silurian organisms be- long to the Hydrozoa, and are embraced under the general title of Graptolites a name given to them from their fancied resemblance to quill-pens. They were composed of a horny or chitinous substance, and hence they commonly present themselves merely as black streaks upon the stone. Each graptolite was a colony comprising many individuals which occupied each its own cell. The cells are in some kinds placed in a row on one side of a supporting rod or axis ; in other kinds there is a row of cells on both sides. Some varieties are straight, others curved or spiral. Some are simple branches, others are composed of two or more branches, while in certain types a large number of separate XVII.] SILURIAN. 323 branches is united in one common centre. One of the most ancient hydrozoa is Dictyograptus (Dictyonema^ Fig. FIG. 117. Graptolites. A, Rastrites Linn&i. B, Monograptus prio- don. C, Diplograptus pristis. D, Phyllograptus typus. E, Didymograptus Murchisonii (all natural size). 1 1 8), found in the Cambrian rocks. Some of the more characteristic forms of graptolites are shown in Fig. 117. These organisms abound in some parts of the system, certain bands of dark shale being especially crowded with them. The double graptolites (such as C, Fig. 117) are more char- acteristic of the Lower Silurian rocks, while the single graptolites (like B, Fig. 117) run throughout the system. Corals abounded in some parts of the Silurian ocean. Their remains chiefly occur in the limestones, doubtless because these rocks were formed in comparatively clear water, in which the corals could flourish. But they differed in structure from the familiar reef- building corals of the present day. The great majority of them belonged to the family of the Rugose corals, now only sparingly represented in the waters of the present ocean. As their name denotes 3 2 4 PALEOZOIC PERIODS. [CHAP. they were particularly marked by their thick rugged walls. Many of them were single, independent individuals ;' some FIG. 1 1 8. Hydrozoon from the Cambrian rocks (Dictyograptus (Dicty- onema) sociale), natural size. lived grouped together in colonies, while others were some- times solitary and sometimes gregarious. A typical example of these rugose solitary forms is Omphyma, shown in Fig. 119. Other genera were Cyathaxoma, Cyathophyllum, and Zaphrentis. There were likewise less numerous and more delicate compound forms belonging to what are known as the Tabulate corals (Favosites, Halysites\ while another type (Hdiolites % Fig. 120) represented in ancient times the Alcyonarian corals (ffeliopora) of the present time. The Crinoids or Stone-lilies played an important part in the earlier seas of the globe. In some regions they lived in such abundance on the sea-floor that their aggregated xvii.] SILURIAN. 325 remains formed solid beds of limestone hundreds of feet thick, and covering thousands of square miles. As their FIG. 119. Silurian Rugose Coral FIG. 120. Silurian Alcyonarian (Omphyma turbinatum, ^). Coral (Heliolites interstinctus, natural size). name denotes, crinoids are lily-shaped animals, having a calcareous jointed flexible stalk fixed to the bottom and supporting at its upper end the body, which is composed of calcareous plates furnished with branched calcareous arms (see Figs. 149, 165, 173). It is these hard calcareous parts which have been so abundantly preserved in the fossil state. Remains of crinoids are found in various parts of the Silurian system (Dendrocrinus, Glyptocrinus\ chiefly in the limestones, but not in such abundance and variety as in later portions of the Palaeozoic forma- tions (see especially pp. 345 and 362, and Figs. 149, 165, and 173). Allied to the crinoids were the Cystideans, a curious order of echinoderms, with rounded or oval bodies enclosed in calcareous plates, possessing only rudimentary arms, and a comparatively small and short jointed stalk. They first appeared in the earlier part of the Silurian period, and attained their chief development in that period, there- 326 PALAEOZOIC PERIODS. [CHAP. after diminishing in numbers. They are thus characteristic- ally Silurian types of life. One of them is represented in Fig. 121. Star-fishes and Brittle-stars possess hard calcareous plates and spines, which, being imbedded in a tough leathery integument, have not infrequently been preserved as fossils. Among the fossils of the Cambrian rocks are recognisable FIG. 121. Silurian Cystidean {Pseudocrinites quad) 'ifasciatns, natural size). FIG. 122. Silurian Star-fish (Pal(zasterina stellata^ 3^). remains of star-fishes, and other forms occur in different parts of the Silurian system. Some of the genera are Palaaster, Palczasterina (Fig. 122), Palcsochoma. Brittle-stars also date from the same ancient period (Protaster). Numerous kinds of Sea-worms (Annelids) crawled over the sandy and muddy bottom and shores of the Silu- rian ocean. These creatures have left no trace of their bodies, which, like those of their representatives in the present ocean, were soft and unfitted for preservation. But the burrows which they made in wet sand or mud, and the xvii.] SILURIAN. 327 trails they left upon the soft surfaces over which they moved have been abundantly preserved. These markings afford unquestionable proof of the presence of creatures which have otherwise utterly dis- appeared. Names have been given to the different kinds of burrows (Arenicolites, ScolithuSj Lumbricaria, Fig. 123) and of trails (Palceo- chordci) Palceophycus). There were likewise representa- tives Of the familiar Serpula, FIG. 1 23. Filled up Burrows or Trails which is found on the pre- left b 7 a sea-worm on the bed of sent sea-bottom, encrusting the su " ian sea ^umbricaria an- tiqua, g). shells with a calcareous protecting tube, inside of which the annelide lives. This tube has been preserved in the fossil state in rocks of all ages. Among the most abundant and characteristic fossils of the older stratified rocks of the earth's crust are those to which the general name of Trilobites has been given. These long extinct animals were crustaceans, having a more or less distinctly three-lobed body, at one end of which was the head or cephalic shield, usually with a pair of fixed compound eyes ; at the other end the caudal shield or tail ; while between the two shields was the ringed or jointed body, the rings of which were movable, so that the animal could bring the two shields together or coil itself up. It will be seen from the different genera represented in Figs. 124 and 125 how varied were the forms which they assumed. In the shapes and relative sizes of the shield and segmented body, in the number of the body- rings, in the development of spines, and in other features, 328 PALAEOZOIC PERIODS. [CHAP. the most wonderful variety is traceable among the trilobites even of the oldest fossiliferous strata. Some of the earliest FlG. 124. Trilobites (Primordial or Cambrian) ; (a) Paradoxides Bohemicus (natural size) ; (b] Agnostus princeps (f) ; (c) Olenus micrurus (natural size) ; (d) Ellipsocephalus Hoffi (natural size). genera were also the largest, Paradoxides^ sometimes reach- ing a length of nearly two feet. Yet contemporaneous with this large creature were some diminutive forms. A few genera (among them Agnostus] were blind ; but most possessed eyes furnished with facets, which in some forms are fourteen in number, while in others they are said to amount to 15,000. The peculiar crescent -shaped eye on each side of the head is well shown in some of the forms represented in Figs. 124 and 125. The trilobites appear to XVII.] SILURIAN. 329 have particularly swarmed on sandy and muddy bottoms, for their remains are abundant in many sandstones and shales. FIG. 125. Trilobites (Lower and Upper Silurian) ; (a) Asaphus tyranmis(^}\ (b] Ogygia Buchii(^); (c) Illamts barriensis (^) ; (d) Trinucletts concentricus (natural size) ; (e) Homalonotus delphino- cephalus (^). They continued to flourish all through the Silurian period. 330 PALEOZOIC PERIODS. [CHAP. But towards the close of Palaeozoic time they died out. They are thus a distinctively Palaeozoic type of life, each great division of the Palaeozoic rocks being characterised by its own varieties of the type. Another form of crustacean life represented in the early Palaeozoic ocean was that of the Phyllopods animals fur- nished with bivalve shell-like cara- paces, which protected the head and upper part of the body, while the jointed tail projected beyond it. Most of them were of small size (Fig. 126). Of all the divisions of the animal kingdom none is so impor- FIG. 126. Silurian Phyllo- tant to the geologist as that of the pod Crustacean (Ceratio- Mollusca< When Qne walks along carts papiho). the shores of the sea at the present time, by far the most abundant remains of the marine organ- isms to be there observed are shells. They occur in all stages of freshness and decay, and we may trace even their com- minuted fragments forming much of the white sand of the beach. So in the geological formations, which represent the shores and shallow sea-bottoms of former periods, it is mainly remains of the marine shells that have been pre- served. From their abundance and wide diffusion, they supply us with a basis for the comparison of the strata of different ages and countries, such as no other kind of organic remains can afford. It is interesting and important to find that among the fossils of the oldest fossiliferous rocks the remains of mol- luscan shells occur, and that they are of kinds which can be satisfactorily referred to their place in the great series of the XVII.] SILURIAN. 331 Mollusca. The most abundant of them are representatives of the Brachiopods or Lamp-shells. Among these are species of the genera Lingula (Lingulella, Fig. 127) and Discina which FIG. 127. Silurian Brachiopods; (a) Linguklla Davisii (natural size), Cambrian ; (b] Atrypa reticularis (natural size), Caradoc beds to Lower Devonian ; (c) Orthis actoni) Trochoceras (Lituites] cornu-arietis (i). habitants of the sea. Of the land -animals of the time nothing was known until the year 1884, when, by a curi- ous coincidence, the discovery was made of the remains of scorpions in the Silurian rocks of Sweden, Scotland, and the United States, and of an insect allied to the living cockroach (Palceoblattina) in those of France. If scorpions and insects existed during this ancient period we may be sure that other forms of terrestrial life were also present. A new interest is thus given to the prosecution of the search for fossils among these older formations. Putting together the evidence furnished by the rocks and fossils of the Silurian system, we get a glimpse of the aspect of the globe during the early geological period which they represent. The rocks bring before us the sand, mud, and gravel of the bottom of the sea, and tell of some old land from which these materials were worn away. The xvii.] SILURIAN. 335 detritus carried out from the shores of that land was laid down upon the sea-bottom just as similar materials are being disposed of at the present day. The area occupied by Silurian rocks marks out the tracts then covered by the sea. Following these upon a map we perceive that vast regions of the existing continents were then parts of the ocean-floor. In Europe, for example, Silurian rocks underlie the greater part of the British Islands, whence they stretch northwards across a large part of Scandinavia and the basin of the Baltic. They rise to the surface in many places on the continent from Spain to the Ural Mountains. They are found forming parts of some of the great mountain-chains of the globe, as, for instance, in the Cordilleras of South America, in the Alps, and in the Himalayas. Even at the antipodes they are met with as thick masses in Australia and New Zealand. It is evident that the geography of the globe in Silurian times was utterly unlike what it is now. A large part of the present land was then covered with shallow seas, in which the Silurian sedimentary rocks were laid down. There would seem to have been extensive masses of land in the boreal part of the northern hemisphere con- necting the European, Asiatic, and American continents. Along the coast-line of the northern land and across the shallow seas lying to the south of it, the same species of marine organisms migrated freely between the Old and the New Worlds. The following Table shows the subdivisions which have been made in the Silurian system of Britain. ( Ludlow group (mudstone and Aymestry limestone) Kirkby g g Moor and Bannisdale flags and slates. g' J Wenlock group (shales and limestones) Denbighshire and > J7J Coniston grits and flags. ^ Upper Llandovery group May Hill sandstones. 33 6 PALAEOZOIC PERIODS. [CHAP. xvn. d f Lower Llandovery group grits and sandstones. Bala and Caradoc group sandstones, slates, and grits, with ~ j Bala (Coniston) limestone. Llandeilo group dark argillaceous and sometimes calcareous [ flagstones and shales. i-3 I Arenig group dark slates, flags, and sandstones. g ( Tremadoc group dark grey slates. c/5 1 Lingula flag group bluish and black slates, flags, and sand- 'g | J stones. Menevian group sandstones, shales, slates, and grits, g ^ Harlech and Longmynd group purple, red, and grey flags, 5 I sandstones, slates, and conglomerates. CHAPTER XVIII. DEVONIAN AND OLD RED SANDSTONE. THE Devonian system, which comes next in order, was named by Sedgwick and Murchison after the county of Devon where they studied its details. ' In Europe, and like- wise in the eastern part of North America, it occurs in two distinct types which bring before us the records of two very different conditions in the geography of these regions during the time when the rocks composing the system were being deposited. The ordinary type, which occurs all over the world, represents the tracts that were covered by the sea, and has preserved the remains of many forms of the marine life of the period. It is that to which the name Devonian is more particularly applicable. The less frequent type is characterised by thick accumulations of sandstones, flag- stones, and conglomerates laid down in lakes and inland seas, and contains a very distinct assemblage of land and fresh -water fossils. This lacustrine type is known by the name of Old Red Sandstone. In their general character the Devonian rocks resemble those of the Silurian system underneath. In Central Europe, where they attain a thickness of many thousand feet, their lower division consists mainly of sandstones, grits, grey wackes, 33 8 PALEOZOIC PERIODS. [CHAP. slates, and phyllites. The central zone contains thick masses of limestone, often full of corals and shells, while the upper portions comprise thin-bedded sandstones, shales, and limestones. These various strata represent the sedi- ments intermittently laid down upon the bottom of the sea which then covered the greater part of Europe. Here and there, they include bands of diabase and tuff, which show that submarine volcanic eruptions took place during their deposition. In the north-west of Europe, however, the floor of the Silurian sea was irregularly ridged up into land, and large lakes were formed, into which rivers from the ancient northern continent poured enormous quantities of gravel, sand, and silt. The sites of these lakes can be traced in Scot- land, the north of England, and Ireland. Similar evidence of land and lake-waters is found in New Brunswick and Nova Scotia. That some of the larger lakes were marked by lines of active volcanoes is well shown in Central Scotland, where the piles of lava and ashes left by the eruptions are more than 6000 feet thick. The occurrence of both marine and lacustrine deposits is of the highest interest, for, on the one hand, we learn what kinds of animals lived in the sea in succession to those that peopled the Silurian waters, and, on the other hand, we meet with the first abundant remains of the vegetation that covered the land, and of the fishes that inhabited the fresh waters. The terrestrial flora of the Devonian period has been only sparingly preserved in the marine beds; but occa- sional drifted specimens occur to show that land was not very distant from the tracts on which these beds were laid down. In the lacustrine strata or Old Red Sandstone of Britain more abundant remains have been met with, but xviii.] DEVONIAN, OLD RED SANDSTONE. 339 the chief sources of information regarding this flora are to be sought in New Brunswick and Gaspe, where upwards of FIG. 131. Plants of the Devonian period ; (a) Psilophyton (|) ; (b) Palaopteris (). 100 species of plants have been discovered. Both in Europe and in North America the Devonian vegetation was char- acterised by the predominance of ferns, lycopods, and cala- mites. It was essentially acrogenous that is, it consisted mainly of flowerless plants like our modern ferns, club- mosses, and horse-tail reeds. Traces of coniferous plants, however, show that on the uplands of the time pine-trees grew, the stems of which were now and then swept down by floods into the lakes or the sea. The general aspect of the flora must have been uniformly green and somewhat monotonous; yet we know that these early woodlands were not without insect life. Neuropterous and orthopter- ous wings have been preserved in the strata. Some of these indicate the existence of ancient forms of ephemera or May-fly, one of which was so large as to have a spread of wing measuring 5 inches across. There were likewise 340 PALEOZOIC PERIODS. [CHAP. millipedes, which fed on the decayed wood of these primeval forests. Traces of land-snails too have been detected among the fossil vegetation. It is evident, however, that the plant and animal life of the land has only been sparingly pre- served; and though our knowledge of it has in recent years been largely increased, we shall probably never discover more than a mere fragmentary representation of what the original terrestrial flora and fauna really were. The lake-basins of the Old Red Sandstone have yielded large numbers of remains of the fishes of the time. They are members of the remarkable order of Ganoids the earliest known type of fishes which, though so abundant in early geological time, is represented at the present day by only a few widely scattered species, such as the sturgeon, the polypterus of the Nile, and the bony pike or gar-pike of the American lakes. These modern forms are denizens of fresh water, and there is reason to believe that their early ancestors were also inhabitants of lakes and rivers, though many of them may also have been able to pass out to the sea. The ganoids are so named from the en- amelled scales and plates of bone in which they are en- cased. In some of the fossil forms, this defensive armour consisted of accurately fitting and overlapping scales (Figs. 132, 133) ; in others, the head FIG. 132. Overlapping scales of with more or less of the body an Old Red Sandstone fish (Hoi- was protected by large and optychius A ndersoni, natural size). , . . . r , /T ,. x thick plates of bone (Fig. 134). Examples of both these kinds of armature are to be observed among the fishes of the Old Red Sandstone. Some of the xviii.] DEVONIAN, OLD RED SANDSTONE. 341 most characteristic scale-covered genera are Osteolepis, Dip- lopterus, Glyptolamus, Holoptychius, Acanthodes (Figs. 132, FIG. 133. Scale-covered Old Red Sandstone fishes; (a) Osteolepis; (b) Acanthodes (both reduced). 133). The acanthodians (Fig. 133, b\ distinguished by the thorn-like spines supporting their fins, reached their greatest FIG. 134. Plate-covered Old Red Sandstone fishes ; (a) Cephalaspis ; (l>) Pterichthys (both reduced). development during the Devonian period. Of the plate- covered ganoids or placoderms some of the most character- 34 2 PALAEOZOIC PERIODS. [OHAP. istic were the curious Cephalaspis (Fig. 134, a\ with its head- buckler shaped like a saddler's awl, the Pteraspis, which, with Cephalaspis^ had already appeared in the Silurian period, the Coccosteus and Pterichthys (Fig. 134, ). Some of the contemporaries of these creatures attained a great size. Thus the Asterolepis had its head and shoulders en- cased in a buckler, which in some examples is 20 inches long by 1 6 broad. Still larger were some of its American allies, one of which, the Dinichthys, had a head -buckler 3 feet long armed with formidable teeth. One of the fishes of the Old Red Sandstone, named Dipterus, has recently been found to have a singular modern representative in the barramunda or mud-fish (Ceratodus) of the Queensland rivers in Australia. It resembled the ganoids in its external enamel and strong bony helmet, but its jaws present the characteristic teeth, and its scales have the rounded or "cycloid" form of Ceratodus. That some of these fishes swarmed in the waters of the Old Red Sandstone is shown by the prodigious numbers of their remains occasionally preserved in the sandstones and flag- stones. Their bodies lie piled on each other in such numbers, and often so well preserved, as to show that probably the animals were suddenly killed, and were covered up with sediment before their remains had time to decay and to be dispersed by the currents of water. Perhaps earthquake shocks, or the copious discharge of mephitic gases, or other sudden baneful influence may have been the cause- of the extensive destruction of life in these ancient waters. That some of the fishes found their way to the sea, as our modern salmon does, is indicated by the occasional occurrence of their remains among those of the truly marine fauna of the Devonian rocks. But the rarity of their xviii.] DEVONIAN, OLD RED SANDSTONE. 343 presence there, compared with their prodigious abundance in some parts of the Old Red Sandstone, probably serves to show that they were essentially inhabitants of the lakes and rivers of the land. Among the animals that appear to have been migratory between the sea and the terrestrial waters, were the curious forms known as Eurypterids, which, though generally classed with the crustaceans, had many affinities with the arachnids or scorpions. One of the most remarkable of these crea- tures was the Pterygotus, of which the general form is shown in Fig. 135. Most of the species are small, though one of them found in Scot- land must have attained a length of 5 or 6 feet. But it is the marine or Devonian FIG. 135. Devonian r .Eury- fauna which is most widely spread P terid Crustacean (Ptery- over the globe, and from its exten- ** us> r sive distribution is of most importance to the geologist. Taken as a whole, it presents a general resemblance to that of the Silurian period which it succeeded. Some of the Silurian species survived in it, and new species of the old genera made their appearance. But important differences are to be observed between the faunas of the two systems, showing the long lapse of time, and the changes which it brought about in the life of the globe. It is specially interesting to mark how some of the characteristic Silurian types dwindle and finally die out in the Devonian system. One of the best examples of this survival and disappearance is supplied by the graptolites. 344 PALEOZOIC PERIODS. [CHAP. It will be remembered how prodigiously abundant these creatures were in the Silurian seas. They are met with also in scattered specimens in the lower and middle divisions of the Devonian system, but their rarity there affords a striking contrast to their profusion among the Silurian strata, and they seem to have entirely died out before the end of the Devonian period, for no traces of them occur in the later parts of the system, and they have never been met with in any later geological formation. Again, triloRites, which form such a predominant and striking feature of the Silurian fauna, occur in greatly dimin- FlG. 136. Devonian Trilobites ; (a) Bronteus flabellifer (|) ; (l>) Dal- manites rugosa (^) ; (c) Homalonotus armatus (\}\ (d) Harpes macrocephalus (). ished number and variety among the Devonian rocks. Most of the Silurian genera are absent, some of the most frequent Devonian types being Phacops, Cryphceus, Homalonotus, Dalmanites, and Bronteus (Fig. 136). We shall find that this peculiarly Palaeozoic type of Crustacea finally died out XVIIL] DEVONIAN, OLD RED SANDSTONE. 345 in the next or Carboniferous period. But while the trilobites were waning, the eurypterids, already referred to, appeared and attained a great development. In the clearer parts of the sea vast numbers of rugose corals flourished, and, with other calcareous organisms, built FIG. 137. Devonian Corals; (a}Cyathophylhim ceratites (&)\ (b) Calceola sandalina (f ). up solid masses of limestone. Some of the characteristic genera were Cyathophyllum (Fig. 137), Acervularia, Cysti- phyllum, and the curious Calceola which, after being succes- sively placed among the lamellibranchs and the brachiopods, is now regarded as a rugose coral with an opercular lid. With these were likewise associated vast numbers of crinoids, of which the genera Cyathocrinus and Cupressocrinus were especially characteristic. The brachiopods reached their maximum of development in the Devonian seas, upwards of 60 genera and 1 100 species having been described from Devonian rocks. Comparing them with those of the Silurian system, we notice that some of the most characteristic Silurian types, such as forms of Or this and Strophomena^ became fewer in number, while forms of Productus and Chonetes increased. The most abundant 346 PALAEOZOIC PERIODS. [CHAP. families were those of the Spirifers ( Uncites, Cyrtia, Athyris, Atrypd) and Rhynchonellids (Fig. 138). Two distinctively Devonian brachiopods were Stringocephalus and Remsehria, FIG. 138. Devonian Brachiopods; (a)Uncites gryphus (f); (I)} Stringo- cephalus Burtini (f ) ; (c) Spirifera disjuncta ( Verneuillii) (J). allied to the still living Terebratula. The former is especi- ally characteristic of one of the Middle limestones (see Table on next page). The other mollusca appear to have been well repre- sented in the Devonian seas. Of the lamellibranchs, Pterinea is particularly abundant in the lower part of the system, Cucullcea (Fig. 139, A) in the upper part. The Devonian cephalopods included many species of the genera xviii.] DEVONIAN, OLD RED SANDSTONE. 347 Orthoceras^ Cyrtoceras^ Clymenia^ Goniatites, and Bactrites (Fig. 139, B). FIG. 139. A, Devonian Lamellibranch (Cucullaa Hardingii, |). B, Devonian Cephalopod (Clymenia Sedgwickii, f ). The Devonian system in Europe is subdivided as in the subjoined Table : Pilton and Pickwell-Down group of England Upper Old Red Sandstone of Scotland. Famennian and Frasnian sandstones, shales, and limestones Upper \ of the North of France and Belgium Psammites de Condroz. Cypridina- shales, Spirifer sandstone, Rhynchonella cuboides beds of Germany. ( Ilfracombe and Plymouth limestones, grits, and conglomerates of Devonshire. [No middle Old Red Sandstone.] Middle \ Limestone of Givet, and Calceola shales of North of France, j Stringocephalus- limestone of the Eifel Calceola-group of (^ Germany. iLinton slates and sandstones of Devon and Cornwall Lower Old Red Sandstone of Scotland and Wales. Coblenzian, Taunusian, and Gedinnian rocks of the Ardennes and Taunus. CHAPTER XIX. CARBONIFEROUS. THE next great division of the Geological Record has re- ceived the name of Carboniferous, from the beds of coal (Latin, Carbd] which form one of its most conspicuous features. The rocks of which it consists reach sometimes a thickness of fully 20,000 feet, and contain the chronicle of a remarkable series of geographical changes which suc- ceeded the Devonian period. They include limestones made up in great part of corals, crinoids, polyzoa, brachio- pods, and other calcareous organisms which swarmed in the clearer parts of the sea ; sandstones often full of coaly streaks and remains of terrestrial plants ; dark shales not infrequently charged with vegetation, and containing nodules and seams of clay-ironstone; and seams of coal varying from less than an inch to several feet or yards in thickness, and generally resting on beds of fire-clay. These various strata are disposed in such a way as to afford us clear evidence of the physical geography of large areas of the earth's surface during the Carboniferous period. The limestones attain a thickness of sometimes several thousand feet, with hardly any intermixture of sedimentary material. They consist partly of aggregated masses of CHAP, xix.] CARBONIFEROUS. 349 corals and coralloid animals, which grew on the sea- floor somewhat after the manner of modern coral-reefs ; partly of aggregated stems and joints of crinoids, which must have flourished in prodigious numbers on the bottom, mixed with fragments of other organisms, and aggregated into sheets of solid stone. This Carboniferous or Mountain Limestone stretches from the west of Ireland eastwards for a distance of 750 miles, across England, Wales, Belgium, and Rhine- land into Westphalia. In the basin of the Meuse it is not less than 2500 feet thick, and in Derbyshire, where it attains its maximum development, it exceeds 6000 feet. Such an enormous accumulation of organic remains shows that, during the time of its deposition, a wide and clear sea extended over the centre of Europe. But as the limestone is traced northwards, it is found to diminish in thickness. Beds of sandstone, shale, and coal begin to make their appearance in it, and rapidly increase in importance, as they are followed away from the chief limestone area ; while the limestone itself is at last reduced in Scotland to a few beds, each only a yard or two in thickness. From this change in the character of the rocks, the inference may be drawn that, while the sea extended from the west of Ire- land eastwards into Westphalia, land lying to the north supplied sand, mud, and drifted plants, which, being scat- tered over the sea-floor, prevented the thick limestone from extending northwards. These detrital materials now form the masses of sandstone and shale that take the place of the limestone in the north of England and in Scotland. The occasional northward extension of a limestone bed full of marine organisms serves to mark a time when, for a longer or shorter interval, the water cleared, sand and mud ceased to be carried so far southward, and the corals, crinoids, and 350 PALEOZOIC PERIODS. [CHAP. other limestone -building creatures were able to spread themselves farther over the sea-floor. But the thinness of such intercalated limestones also indicates that the inter- vals favourable for their formation were comparatively short, the sandy and muddy silt being once again borne southward from the land, killing off or driving away the limestone-builders and spreading new sheets of sand and mud over the site. There can be no doubt that, while these changes were in progress, the whole wide area of deposition in Western and Central Europe was undergoing a gradual depression. The sea-bottom was sinking, but so slowly that the growth of limestone and the deposit of sediment probably on the whole kept pace with it. The actual depth of the water may not have varied greatly even during a subsidence of several thousand feet. That this must have been the case may be inferred from the structure of the limestone itself. We have seen that this rock sometimes exceeds 6000 feet in thickness. Had there been no subsidence of the sea- floor during the accumulation of so thick a mass of organic debris, it is evident that the first beds of limestone must have been begun at a depth of at least 6000 feet below the surface of the sea, and that, by the gradual increase of calcareous matter, the sea was eventually filled up to that amount, if it was not filled up entirely. But we can hardly suppose that the same kinds of organisms could live at a depth of 6000 feet and also at or near the surface. We would expect to find the organic contents of the lower parts of the limestone entirely different from those in the upper parts. But though there are differences sufficient to admit of the limestone being separated into stages, each marked by its own distinctive assemblage of fossils, the xix.] CARBONIFEROUS. 351 general character or facies of the organisms remains so uniform and persistent throughout, as to make it quite certain that the conditions under which the creatures lived on the bottom and built up the limestone continued with but little change during the whole time when the 6000 feet of rock were being deposited. As this could not have been the case had there been a gulf of 6000 feet to fill up, we are led to conclude that the bottom slowly subsided until its original level, on which the limestone began to form, had sunk at least 6000 feet. This conclusion is borne out by many other considera- tions. Thus the sedimentary strata that replace the lime- stone on its northern margin are also several thousand feet thick. But from bottom to top they abound with evidence of shallow-water conditions of deposition. Their repeated alternations of sandstone, grit (even conglomerate), and shale ; the presence in them of constant current- bedding; the frequent occurrence of ripple -marked and sun-cracked surfaces ; the preservation of abundant remains of terrestrial vegetation some of it evidently in its position of growth prove that the mass of sediment was not laid down in a deep hollow of the sea-bottom, but in shallow waters not far from the margin of the land. But probably the most interesting evidence of long- continued subsidence during the Carboniferous period is furnished by the history of the coal-seams. Coal is com- posed of compressed and mineralised vegetation. Each layer of coal is usually underlain by a bed of fire-clay, or at least of shale, through which roots and rootlets, descending from the under surface of the coal-seam, branch out freely. There can be no doubt that each bed of fire-clay is an old soil, and the seam of coal lying upon it represents the matted 352 PALAEOZOIC PERIODS. [CHAP. growth of vegetation which that soil supported. Hence, in each case of the association of a fire-clay and a coal-seam, we have distinct evidence of a ter- restrial surface. In many regions the Carbonifer- ous system comprises a series of sandstones, shales, and other strata, many thousands of feet in thickness, throughout which, on successive plat- forms, there lie hundreds of seams of coal. If each of these seams marks a former surface of terrestrial vegeta- tion, how is this succession of buried land-surfaces to be accounted for? There is obviously but one solution of the problem. The area over which the coal-seams extend must have been slowly sinking. During this subsi- dence, sand, mud, and silt were transported from the neighbouring land, and in such quantity as to fill up the shallow waters. On the muddy flats thus formed, the vegeta- tion of the flat marshy swamps spread FIG. 140. Section of part seaward. There may not improb- of the Cape Breton coal- a foly have been pauses in the down- field, showing a succession ward movement duri which th of buried trees and land- & surfaces; (a) sandstones ; maritime jungles and forests con- (^)shales; (c) coal-seams ; tinued to flourish and to form a thick (d] under-clays or soils. matte d mass of vegetable matter. When the subsidence recommenced, this mass of living and dead vegetation was carried down beneath the water and xix.] CARBONIFEROUS. 353 buried under fresh deposits of sand and mud. As the weight of sediment increased, the vegetable matter would be gradually compressed and would slowly pass into coal. But eventually another interval of rest or of slower subsidence would allow the shallow sea once more to be silted up. Again the marsh -loving plants from the neighbouring swampy shores would creep outward and cover the tract with a new mantle of vegetation, which, on the renewal of the downward movement, would be submerged and buried. In the successive strata of a coal-field, therefore, we are presented with the records of a prolonged period of sub- sidence, probably marked by longer or shorter intervals of rest. These more stationary periods are indicated by the coal-seams, and perhaps their relative duration may be inferred from the thickness of these seams. A thick coal-bed not improbably marks a time of rest, when the vegetation was allowed to flourish unchecked, or when at least the sinking was so imperceptible that the successive generations of plants, springing up on the remains of their predecessors, contrived to keep themselves above the level of the water. In the present world there is no vegetable growth now in progress quite like that of the coal-seams of the Car- boniferous period. Perhaps the nearest analogy is supplied by the mangrove -swamps of tropical coasts (p. in). In these tracts, the mangrove trees grow seaward, dropping their roots and radicles into the shallow waters, and gradually forming a belt of swampy jungle several miles broad. That the coal -jungles extended into the sea is shown by the occurrence of marine shells and other organisms in the coal itself. But there were probably also wide swamps wherein the water was fresh. A single coal-seam may sometimes be 2 A 354 PALEOZOIC PERIODS. [CHAP. traced over an area of more than 1000 square miles, show- ing how widespread and uniform were the conditions in which it was formed. During the subterranean movements that marked the Carboniferous period, the Devonian physical geography was entirely remodelled. The lake basins of the Old Red Sand- stone were effaced, and the sea of the Carboniferous lime- stone spread over their site. Much of the Devonian marine area was upridged into land, and the rocks underwent that intense compression and plication which have given them their cleaved, crumpled, and metamorphic aspect, and in connection with which they were invaded by granite and intersected with mineral veins. It is deserving of remark that volcanic action, which played so notable a part in Devonian time, was continued, but with diminished vigour, in the Carboniferous period. During the earlier half of the period, volcanic outbursts were frequent in different parts of Britain, particularly in Derbyshire, the Isle of Man, central and southern Scotland, and the south-west of Ireland. The lava and ashes ejected in these areas during the time of the Carboniferous Limestone form conspicuous groups of hills. Of the plant and animal life of the Carboniferous period much is now known from the abundant remains which have been preserved of the terrestrial surfaces and sea-floors of the time. Beginning with the flora, we have first to notice its general resemblance to that of the Devonian period. Many of the genera of the older time survived in the Carboniferous jungles; but other forms appear in vast profusion, which have not been met with in any Devonian or Old Red Sand- stone strata. The Carboniferous flora also must have been singularly monotonous, consisting as it did almost entirely XIX.] CARBONIFEROUS. 355 of flowerless plants. Not only so, but the very same species and genera of plants appear to have then ranged over the whole world, for their remains are found in Carboniferous strata from the Equator to the Arctic Circle. Ferns, lycopods, and equisetaceae, constituted the main mass of the vegetation. The ferns recall not a few of their modern allies, some of the more abundant kinds being Sphenopteris, FIG. 141. Carboniferous Ferns ; (a) Neuropteris macrophylla (^} ; (b} Sphenopteris artemisicefolia () ; (c) Alethopteris {Pecopteris) lonchitica (). Neuropteris, and Pecopteris (Fig. 141). Among the lycopods, the most common genus is Lepidodendron, so named from the scale-like leaf- scars that wind round its stem (Fig. 142). Its smaller branches, closely covered with small pointed leaves, and bearing at their ends little cones or spikes (Lepidostrobus\ remind one of the club- mosses of our moors and mountains ; but instead of being low-growing or creeping plants, like their modern represent- atives, they shot up into trees, sometimes 50 feet or more in height. Equisetacese abounded in the Carboniferous swamps, the most frequent genus being Calamites, the jointed and 356 PALAEOZOIC PERIODS. [CHAP. finely-ribbed stems of which are frequent fossils in the sand- stones and shales (Fig. 143, a). This plant probably grew in dense thickets in the sandy and muddy lagoons, and bore as its foliage slim branches, with whorls of pointed leaves set round the joints (Aster ophyllites^ Fig. 143, b). The Sigillarioids were among the most abundant, and, at the same time, most puzzling members of the Carboni- ferous flora. They do not appear to have any close modern allies, and their place in the botanical scale has been a subject of much controversy. The stem of these trees, sometimes reaching a height of 50 feet or more, was fluted, each of the par- allel ribs being marked by a row of leaf- scars, hence the name Sigillaria, from the FIG. i42.^Carboni- seal-like impressions of these scars (Fig. ferous Lycopod 1 44). These surface-markings disappeared (Lepidodendron ^ thg tree and Jn the 1()wer Qf Sternbergii) J). . . the trunk they passed down into the pitted and tubercular surface characteristic of the roots (Stigmaria), so abundant still in their position of growth in fire-clay, and also as drifted broken specimens in sandstones and shales. Another plant that took a prominent part in the Carboniferous flora was that named Cordaites (Fig. 145). Its true botanical place is still matter of dispute; some writers placing it with the lycopods, others with the cycads, or even among the conifers. It bore parallel -veined leaves somewhat like those of a yucca, which, when they fell off, left prominent scars on the stem, and it also carried spikes or buds (Carpolithes, Fig. 145). All the plants now enumer- xix.] . CARBONIFEROUS. 357 ated probably grew on the lower grounds and swamps. But FIG. 143. Carboniferous Equisetaceous Plants ; (a) Calamites Lind- leyi ( = C. Mougeoti, Lindl., 5) ; (b] Asterophyllites densifolius (^). on the higher and drier tracts of the interior there grew arau- FlG. 144. Sigillaria with Stigmaria roots (much reduced). carian pines (Dadoxylon, Araucarioxylon\ the trunks of which, PALEOZOIC PERIODS. [CHAP. swept down by floods, were imbedded in some of the sands of the time and now appear petrified in the sandstones. FIG. 145. Cordaites alloidius (), with Carpolithes attached. While the terrestrial vegetation of the Carboniferous period has been so abundantly entombed, the fauna of the land has been but scantily preserved. That air-breathers existed, however, has been made known by the rinding of specimens of scorpions, myriapods, true insects, and amphi- bians. Within the last few years vast numbers of the remains of scorpions have been discovered in the Carbon- iferous rocks of Scotland. These ancient forms (Eoscorpius) presented a remarkably close resemblance to the living scorpion, and so well have they been preserved among the shales that even the minutest parts of their structure can be recognised. They possessed stings like their modern descendants, whence we may infer the presence of other forms of life which they killed. The Carboniferous wood- lands had plant-eating millipedes, and their silence was broken by the hum of insect-life ; for ancestral forms of dragon-flies (Libellula), May-flies (Ephemtrida)> stone-flies ) white -ants (Termidce), cockroaches (Blattida), xix.] CARBONIFEROUS. 359 spectre -insects (Phasmida\ crickets (Gryllida), locusts (Acrydiidcz), and other curious transitional forms between modern types that are quite distinct have been detected chiefly among the shales and coals of the Coal-measures. Some of these insects attained a great size ; a single wing of one of them (Mcgaptilus] must have measured between 7 and 8 inches in length. While detached wings and more or less complete bodies have been found as rare and precious discoveries in many coal-fields in Europe and America, it is at Commentry in France that remains of insects have been met with in largest numbers no fewer than 1300 specimens having there been disinterred, most of them admirably preserved. In the interior of decaying trees early forms of land-snails lived, having a striking resemblance to some kinds that are still to be found in our present woodlands (Pupa). The lagoons in which the coal-growths flourished were tenanted by numerous forms of animal life. Among these were various mussel-like molluscs (Anthracomya, Fig. 154, Anthracosia), which were possibly restricted to fresh water. But wherever the sea-water penetrated, it carried some of its characteristic life with it, particularly Lingula, Distinct, Aviculopecten, Goniatites, and other marine shells. The fishes of the lagoons were chiefly ganoids (Megalichthys, Rhizodus, Fig. 158, Cheirodus, Strepsodus, etc.). But some of the rays and sharks from the sea made their way into these waters, for their spines are occasionally found among the coal-seams and shales (Gyracanthus, Pleuracanthns, Fig. 158). That the larger fishes lived upon the smaller ones is shown by a curious and interesting piece of evidence. Many of the shales are full of small oblong bodies which contain in their interior the broken and undigested scales 360 PALEOZOIC PERIODS. [CHAP. and bones of small fishes. From their contents, their peculiar external form and markings, and their phosphatic composition, these bodies (coprolites) are recognised as the excrement of some of the larger fishes, and the teeth and scales within them serve to show what were the smaller forms on which these fishes lived (see Fig. 65). During the Carboniferous period, and indeed through- out the later parts of the Palaeozoic ages, the most highly organised creatures living on the globe, so far as we at present know, belonged to the Amphibia the great class which includes our modern frogs, toads, and salamanders. They belonged, however, to an order that has long been entirely extinct the Labyrinthodonts, so named from the labyrinthine folds of the internal substance of their teeth. They were somewhat like the existing salamander in form, with weak limbs and a long tail. Their skulls were en- cased in strong plates of bone, and they likewise carried protective bony scutes on the under sides of their bodies. Those found in Carboniferous rocks are mostly small in size, but some of them, measuring perhaps 7 or 8 feet in length, must have been the monsters of the lagoons, in which they lived. Some of the leading genera are Archegosaurus, Anthracosaurus, Loxomma, Dendrerpeton, Baphetes. The marine life of the Carboniferous period has been extensively preserved in the Carboniferous Limestone, which, as already stated, consists of little else than aggregated remains of organisms. In walking over the surface of the beds of this limestone, one treads upon the floor of the sea in Carboniferous times, with its corals, crinoids, and shells crowded and crushed upon each other. Beginning with the most lowly of these organisms, we may observe abun- XIX.] CARBONIFEROUS. dant remains of foraminifera, which in some portions of the limestone constitute the greater part of the rock. One of their most characteristic forms is named Fusulina (Fig. 146), which enters largely into the structure of the limestone across the Old World from FIG. 146. Carboniferous Russia to China and Japan, and like- Foraminifer (Fusulina cylindrica> f ). wise in North America. Corals have been preserved in prodigious numbers ; indeed, some parts of the limestone are almost entirely made up of them. Most of them are rugose kinds, characteristic genera being Zaphrentis, Lithostrotion (Fig. 147), Clisiophyllum, Lonsdakia. With FIG. 147. Carboniferous Rugose Corals ; (a) Zaphrentis Enniskilleni (2) ; (V) Lithostrotion juncettm (natural size). these there occur also tabulate forms, including Chcetetes, Alveolites, Favosites, etc. Of the sea-urchins, the plates and spines of the genus Archizocidaris (Fig. 148) are 362 PALAEOZOIC PERIODS. [CHAP. specially frequent. But the most common echinoderms are members of the great order of crinoids, which must tl b FIG. 148. Carboniferous Sea - Urchin (Archao- cidaris Urei, natural size) ; (a] Single plate ; (b} Portion of spine. FIG. 149. Carboniferous Crinoid ( Woodocrimis expansus, g). have grown in thick groves over many square miles of the sea-bottom. So prodigiously numerous were they that their remains have been aggre- gated into beds of limestone hundreds of feet in thick- ness, hence known as crin- oidal or encrinite limestone (Fig. 76). The general FIG. 150. - Carboniferous Blastoid P lant - like form of these (Cup of Pentremite, magnified), animals is shown in Fig. (a) View from above ; (b} Side view. I4 g. But usually the cal- careous joints and plates fell asunder. Frequent genera are named PZatycrinus, Poteriocrinus, Cyathocrinus. The Carboniferous seas were tenanted by a peculiar extinct order xix.] CARBONIFEROUS. 3 6 3 of echinoderms known as Blastoids or Pentremites (Fig. 150), distinguished from true crinoids by the want of free arms, and by the arrangement of the plates forming the cup. These creatures are characteristically Carboniferous, though they are found also in the higher part of the Silurian system and in Devonian rocks. The Crustacea of the Carboniferous period presented a strong contrast to those of earlier geological time. In par- ticular, the great family of the Trilobites, so characteristic of the older Palaeozoic systems, now died out altogether. Instead of its numerous types in the Silurian and Devonian rocks, it is represented in the Carboniferous system by only four genera, all the species of which are small (Phillipsia^ Fig. 151, Griffithides, Brachymetopus\ and none of which rises into the next succeeding system. The most abundant crustaceans were ostracods an order still abundantly represented at the present day. They are minute forms enclosed within a bivalve shell or carapace which en- tirely invests the body. Many of these live in fresh water- the Cypris, for example, being abundant in ponds and F f.^\' -Coniferous _. , Trilobite (/%/// (b] Streptorhynchus crenistria (^) ; (c] Spirifera striata (3). latus, Productus longispinus, Streptorhynchus crenistria, Spiri- fera glabra, Terebratula hastata. Some of the more common lamellibranch molluscs (Fig 154) belong to the genera Aviculopecten, Leda, Nucula, Edmondia, Modiola, Anthracomya. Among the gasteropods Euomphalus, Pleurotomaria, Loxonema, and Bellerophon (Fig. 155) are not infrequent. A pteropod (Conularia, Fig. 156) may be gathered in great numbers in some parts of the Carboniferous Limestone. The cephalopods were repre- sented by numerous species of Orthoceras^ Nautilus, and Goniatites (Fig. 157). Remains of fishes are not infrequent in the Carboni- ferous Limestone. But they present a striking contrast to those of the black shales and ironstones of the Coal-measures. 3 66 PALAEOZOIC PERIODS. [CHAP. They consist for the most part of teeth or of spines belong- ing to large predatory sharks. These teeth were placed as FIG. 154. Carboniferous Lamellibranchs ; (a) Edmondia sulcata (|) ; (b) Anthracomya Adamsii (f ) ; (c) Aviculopecten fallax (f ). a kind of pavement and roof in the mouth, and were used as effective instruments for crushing the hard parts of the FlG. 155- Carboniferous Gasteropods ; (a) Euomphalus pent- angulatus (J) ; (b) Bellerophon tenuifascia (f ). animals on which those larger creatures preyed. If, as is probable, the sharks fed upon the ganoid fishes of the time, they must have required a powerful apparatus of teeth for xix.] CARBONIFEROUS. 367 crushing the hard, bony armour in which these fishes were encased. Of the commoner genera of sharks which have FIG. 156. Carbon- FIG. 157. Carboniferous Cephalo- iferous Pteropod pods ; (a) Orthoceras goldfussianum ( Conularia quad- (^) ; (ti) Goniatites sphcericus (na- risulcata, ). tural size). been named from the forms of their teeth the only hard parts of their structure that have survived the following may be mentioned : Cochliodus, Orodus, Psammodus, Petalodus. The small ganoids that so abound in the black shales, iron- stones, and coal-seams, which represent the deposits of the sheltered lagoons of the coal-jungles, are hardly to be found in the thick limestone, whence we may infer that they were inhabitants of the quiet shore -waters, and did not venture out into the open sea, where the sharks found their congenial element. But the occasional occurrence of the teeth and spines of sharks in the Coal-measure shales and coal-seams shows that these monsters now and then made their way into the inland waters, where they would find abundant food. The Carboniferous system in Europe presents at least two well-marked subdivisions. In the lower section the strata are in large measure marine, for they include the 3 68 PALAEOZOIC PERIODS. [CHAP. Carboniferous Limestone ; in the upper part they consist mainly of sandstones, shales, fire-clays, and coal-seams, con- FIG. 158. Carboniferous Fishes ; (a) Tooth of Rhizodus Hibberti ($) ; (b] Tooth of Orodus ramosus (|) ; (c) Ichthyodorulite or Fin-spine of Pleuracanthus l&vissimus (^). stituting what are called the Coal-measures, or coal-bearing division of the system. The subjoined Table shows the order of succession of the rocks in Britain : / Coal-Measures. At the top, red and grey sand- stones, clays, and thin limestone, resting upon a great thickness of white, grey, and yellow sandstones, clays, shales, and fire-clays, with numerous workable coal-seams, and with a lower subdivision of coal-bearing beds, among which there occur marine fossils (Orthoceras, Posidonomya, etc.) Thickness in South Wales, 12,000 feet ; South Lancashire, 8000 feet ; I Central Scotland, 3000 feet. Lagoon type. XIX.] CARBONIFEROUS. Marine type, but passing north- wards into that of the lagoons. Millstone Grit. Grits, flagstones, sandstones, and shales, with thin seams of coal and occasional bands containing marine fossils. Thickness 400-1000 feet, increasing in Lancashire to 5500 feet. Carboniferous Limestone. Consisting typically of massive marine limestones and shales, but pass- ing laterally into sandstones and shales, with thin coal-seams, which indicate alternations of marine and brackish water conditions. Thickness in South Wales, 500 feet, increasing northwards to more than 4000 feet in Derbyshire, and to upwards of 6000 feet in Lancashire, but dimin- ishing northwards into Scotland. The base of the Carboniferous Limestone series passes down conformably into the Upper I Old Red Sandstone. The Carboniferous system occupies a number of detached areas on the European continent. Its largest tract extends from the north of France, through Belgium, into Westphalia. The most important coal-fields of Europe belonging to this system are those of Belgium, Westphalia, the north of France, Saarbriicken, St. Etienne in Central France, Bohemia, and the Donetz in Southern Russia. In North America, the Coal-measures of the eastern United States reach a thick- ness of 4000 feet in Pennsylvania, and contain many valu- able seams of coal. They increase in thickness northwards, reaching a maximum of 8000 feet in Nova Scotia. They are underlain by bands of conglomeratic strata, answering to the English Millstone grit, below which comes a group of beds with marine fossils (sub -carboniferous), probably representing the Carboniferous Limestone of Europe. In Australia and New Zealand, also, thick masses of sedimentary strata contain recognisable Carboniferous organic remains. In New South Wales they include a valuable succession of coal-seams. 2 B CHAPTER XX. PERMIAN. THE prolonged subsidence during which the Coal-measures were accumulated was at last brought to an end by a series of great terrestrial disturbances, whereby the lagoons and coal-growing swamps were in great measure effaced from the geography of Europe. So abrupt in some regions is the discordance between the Coal-measures and the next series of strata, that geologists have naturally been led to regard this break as one of great chronological importance, serving as the boundary between two distinct systems. Nevertheless, so far as the evidence of fossils goes, there is no such interrup- tion of the Geological Record as might be supposed from this stratigraphical unconformability, many of the Carboni- ferous types of life having survived the terrestrial disturb- ances. Again, though the discordance among the strata is, in many parts of Europe, particularly in England, most striking, yet it is by no means universal. On the contrary, some localities (Autun in France, and the Bohemian coal- field, for example) escaped the upheaval and prolonged denudation which elsewhere have produced so marked a hiatus in the chronicle. And in these places a gradual passage can be traced from the strata and fossils of the Coal-measures CHAP, xx.] PERMIAN. 37 1 into those of the next succeeding division of the series, no sharp line being there discoverable, nor any evidence to warrant the separation of the overlying strata as an inde- pendent system distinct from the Carboniferous. Hence, by many geologists, the rocks now to be described are regarded as the upper part of the Carboniferous system. To these overlying rocks the name of Permian was given, from the Russian province of Perm, where they are well developed. They consist of red sandstones, marls, conglomerates, and breccias, with limestones and dolomites. In Germany they are often called Dyas, because they are easily grouped in two great divisions : the lower consisting of sandstones, conglomerates, and other fragmental deposits ; the upper of limestones and dolomites. The coarsest strata breccias and conglomerates are composed of rounded and angular fragments of granite, diorite, gneiss, grey- wacke, sandstone, and other crystalline and older Palaeozoic rocks, which must have been upheaved and exposed to denudation before Permian time. The sandstones are usually bright brick-red in colour, owing to the presence of earthy peroxide of iron which serves to cement the particles of sand together. The shales or marls are coloured by the same pigment. So characteristic indeed is the red colour of the rocks that they form part of a great series of strata, originally known as the New Red Sandstone. Generally, greenish or whitish spots and streaks occur in the red beds, marking where the iron-oxide has been reduced and removed by decaying organic matter. Red strata are, as a rule, singularly barren of organic remains, probably because the water from which the iron-peroxide was precipitated must have been unfitted for the support of life. The red Permian rocks are therefore generally unfossiliferous. Among them, 372 PALEOZOIC PERIODS. [CHAP. however, occur dark shales or " marl-slate," which have yielded numerous remains of fishes. The limestones too are fossiliferous, but they are associated with unfossiliferous dolomite, gypsum, anhydrite, and rock-salt. In some places seams of coal also occur. These various rocks tell distinctly the story of their origin. They could not have been deposited in the open sea, but rather in basins more or less shut off from it, wherein the water was charged with iron and was liable to concentration, with the consequent precipitation of its solutions. The beds of anhydrite, gypsum, and rock-salt are memorials of these processes. The dolomite may at first have been laid down as limestone which afterwards was converted into dolomite by the action of the magnesian salts in the concentrated water. In such intensely saline and bitter solutions, animal life would not be likely to flourish, and hence, no doubt, the poverty of fossils in the Permian series of rocks. But it is observable that where evidence occurs of the cessa- tion of ferruginous, saliferous, and gypseous deposition, there fossils not infrequently appear. The brown Marl -Slate, for example, and the thick beds of limestone are sometimes abundantly fossiliferous, and indeed are almost the only bands of rock in the whole series where organic remains occur. They were probably deposited during intervals when the barriers of the inland seas or salt-lakes were broken down, or, at least, when from some cause the waters came to be con- nected with the open sea, and when a portion of the ordinary marine fauna swarmed into them. Hence, from the very circumstances in which their remains have been entombed and preserved, the flora and fauna of Permian times are comparatively little known. The total number of species and genera obtained from Permian XX.] PERMIAN. 373 rocks, hardly more than 300 in all, forms a singular contrast to the rich assemblages which have been recovered from the FIG. 159. Permian Plants; (a) Callipteris conferta () ; (b) Walchia piniformis (|). older systems. But that the land of these times was still richly clothed with vegetation and the sea abundantly stocked with animal life, there can be no doubt. The flora appears to have closely resembled that of the Carboniferous period, a considerable proportion of the species of plants being sur- vivals from the Carboniferous jungles and forests. The Lepidodendra, Sigillarise, and Calamites, which had been such conspicuous members of all the Palaeozoic floras, now appear in diminishing number and variety, and finally die out. 374 PALEOZOIC PERIODS. [CHAP. With their cessation, new features arise in the vegetation. Among these may be mentioned the abundance of tree- ferns, which, though they sparingly existed even as far back as Devonian times, now attained a conspicuous development (PsaroniuSy Caulopteris). The genus of ferns called Callip- teris likewise played a prominent part in the Permian woodlands (Fig. 159, a). But perhaps the most remarkable FIG. 160. Permian Brachiopods ; (a) Prodtictus horridus (reduced); (b] Strophalosia Goldfussi ; (c] Camarophoria Jmmbktonensis (f). feature in the flora was the abundance of its conifers, and the appearance of the earliest forms of cycads. The yew- like conifer Walchia (Fig. 159, b\ if we may judge from the abundance of its remains, flourished in great profusion on the drier grounds, mingled with others that bore cones (Ullmannid}. The cycads, which now made their advent, continued during Mesozoic time to give the leading character to the vegetation of the globe. XX.] PERMIAN. 375 The scanty relics of the Permian fauna, as above stated, have been almost wholly preserved in those strata which were FIG. 161. Permian Lamellibranchs; (a] Bakevellia tumida (natural size) ; (/>) Schizodiis Schlotheimi (natural size). deposited during temporary irruptions of the open sea into the inland salt-basins of the time. Some of the Carboniferous genera of brachiopods still survived Productus, Spirifera, and Strophalosia being conspicuous (Fig. 160). Among the lamellibranchs Axinus^ Bakevellia, and Schizodus are fre- quent forms (Fig. 1 6 1 ). Among the higher molluscs, which FIG. 162. Permian Ganoid Fish (Platysomus striatus, have been but sparingly preserved in the rocks, the old types of Orthoceras, Cyrtoceras, and Nautilus are still to be 376 PALAEOZOIC PERIODS. [CHAP. noticed. In Europe, the fishes of the time have been chiefly sealed up in the marl-slate or copper-shale (Kup- ferschiefer); two of the most frequent genera being Palczon- iscus and Platysomus (Fig. 162). Labyrinthodonts continued to abound in the waters. Some of the Carboniferous genera still survived, but with these were associated many new forms, most of which have been discovered in the strata overlying the true Coal- FiG. 163. Permian Labyrinthodont (Branchiosaunts salamandroides, natural size). measures of Bohemia (Fig. 163). But a great onward step in the advance of animal organisation was made in Permian time by the appearance of the earliest known lizard Pro- torosaurus, which, like the living crocodile, had its teeth implanted in distinct sockets. In Britain the Permian strata rest unconformably on the Carboniferous system, which must have been greatly disturbed and enormously denuded before they were deposited. They consist of the following subdivisions : Upper red sandstones, clays, and gypsum (50 to 100 feet thick in the east of England, but swelling out west of the Pennine Chain to 600 feet). Magnesian limestone a mass of dolomite ranging up to 600 feet in thickness, and the chief repository of the Permian fossils ; remark- xx.] PERMIAN. 377 able for the curious concretionary forms assumed by many of its beds on the coast of Durham (Fig. 75). Marl-slate a hard brown shale with occasional limestone bands. Lower red and variegated sandstones with conglomerates and breccias. This division attains a thickness of 3000 feet in Cumberland, but is hardly represented in the east of England. In Germany, where the Dyas or twofold development of the Permian rocks is so well displayed, the lower sub- division, called Rothliegende, consists of great masses of conglomerate with sandstones, shales, thin limestones, and important intercalations of contemporaneous volcanic rocks, both lavas and tuffs. The upper section is composed chiefly of limestone called Zechstein, and answering to the Magnesian limestone of England. With it are associated beds of anhydrite, gypsum, rock-salt, and bituminous lime- stone, and underneath it lies the celebrated Kupferschiefer or copper-shale a black bituminous shale, about 2 feet thick, which has long been extensively worked on the flanks of the Harz Mountains for the ores of copper with which it is im- pregnated, and which is the great repository for the fossil fishes of the Permian period. This remarkable band of rock was probably deposited in one of the inland basins, which at first may have maintained a free communication with the open sea. But eventually mineral springs, not improbably connected with the volcanic action of the time, brought up such an abundant supply of dissolved metallic salts as to kill the fish and render the water unsuitable for their exist- ence. The metallic salts were reduced and precipitated as sulphides round the organisms, and impregnated the sur- rounding mud. In the overlying succession of strata, we see how the area was once more overspread by the clearer and opener water, which brought in the fauna of the Zech- stein, and then how the basin gradually came to be shut off 378 PALAEOZOIC PERIODS. [CHAP. xx. once more, until from its concentrated waters the various beds of anhydrite, gypsum, and rock-salt were thrown down. In the heart of France at Autun, the Coal-measures pass upward into Permian strata, as already stated. That area appears to have escaped the disturbance which in Western Europe placed the Permian unconformably upon the Car- boniferous rocks. It presents a mass of sandstones, shales, coal-seams, and some bands of magnesian limestone, the whole having a thickness of more than 3000 feet referred to the Permian period. The plants in the lower part of this group of strata are unmistakably Carboniferous, but Permian forms appear in increasing numbers as the beds are followed upwards until the highest stage presents a predominant Permian flora. Besides the characteristic Permian fishes, these strata have yielded remains of several frog -like animals (Protriton, Pleuronoura), and of some saurians (Actinodon, Euchirosaurus, etc.) CHAPTER XXI. THE MESOZOIC PERIODS TRIASSIC. THE great series of red strata referred to in the foregoing chapter as overlying the Carboniferous system in England was called " New Red Sandstone," to distinguish it from the " Old Red Sandstone " which underlies that system. But the progress of geology on the European continent eventually proved that, notwithstanding their general simi- larity of lithological character, two series of rocks had been comprised under the general title of New Red Sandstone. The older of these, separated from the rest under the name of Permian, was placed at the top of the great succes- sion of Palaeozoic formations. The younger division (still sometimes spoken of as New Red Sandstone) was called Trias, and was regarded as the first system in the great Mesozoic or Secondary succession. Essentially the Permian strata form merely the upper part of the Carboniferous system. Their types of life are fundamentally Palaeozoic, but, as we have seen, both the flora and fauna are marked by a decrease in the numbers and variety of old forms, and by the advent of the pre- cursors of a new order of things. Conifers and cycads now begin to replace the early types of lepidodendron and 380 MESOZOIC PERIODS. [CHAP. sigillaria ; amphibians become more abundant, and saurians now take their place at the head of the animal world. But when we ascend into the Trias, though in Europe the physical conditions of deposition remained much the same as in Permian time, we meet with a decided contrast in the organic remains. A new and more advanced phase of development presents itself in that richer and more varied assemblage of plant and animal life which character- ised Mesozoic or Secondary time. The word TRIAS has reference to the marked threefold division of the rocks of this system in Germany. In that country, and generally in Western Europe, the rocks consist of bright red sandstones and marls or clays, with beds of gypsum, anhydrite, rock-salt, dolomite, and limestone. These rocks, so closely resembling the Permian series below, had evidently a similar origin. They were deposited in inland seas or salt-lakes, wherein, by evaporation and concentration of the water, the dissolved salts were precipitated upon the bottom, and where, consequently, the conditions must have been extremely unfavourable for the presence of living things. The sites of these inland basins can still be partially traced. They extended at least as far west as the north of Ireland. One or more of them lay across the site of the plains of Central England. Others were dotted over the Lowlands of middle Europe. The largest of them occupied an extensive area now traversed by the Rhine, and stretched, on the one hand, from Basel to the plains of Hanover, and, on the other, from the highlands of Saxony and Bohemia across the site of the Vosges Mountains westward to the flank of the Ardennes. The continent must then have been somewhat like the steppes of Southern Russia a region of sandy wastes and salt-lakes, with a warm and dry climate. Probably xxi.] TRIASSIC. 381 higher land rose to the north, as in earlier geological times, for traces of its vegetation have been found in Sweden. But southwards lay the more open sea, spreading over part at least of the site of the modern Alps, and thence probably across much of Asia to the Indian and Pacific Oceans. So long as only the deposits of the salt-basins had been explored, it was but natural that comparatively little should be known of the flora and fauna of the Triassic period. The climate around these lakes was perhaps not a very salubrious one, and hence there may have been a scanty terrestrial fauna in their immediate vicinity, while the waters of the lakes themselves were entirely unsuited for the support of life. It is not surprising, therefore, that the strata deposited in these tracts are on the whole unfossiliferous; that, indeed, fossils only abound where there are indications that, owing to some temporary depression or breaking down of the barriers, the open sea spread into these basins, and carried with it the organisms whose remains gathered into beds of limestone. But over the tracts that lay under the open sea, a more abundant marine fauna lived and died. It is in the records of that sea-bottom, rather than in those of the salt-basins, that we must seek for the evidence of the general character of the life over the globe, and for the fossil data with which to compare together the Triassic rocks of distant regions. The flora of the Triassic period has been preserved chiefly in the dark shales and thin coal-seams formed in some of the inland basins. So far as known to us it con- sisted chiefly of ferns, equisetums or horse-tails, conifers, and cycads. Among the ferns a few Carboniferous genera still survived, but some of the most characteristic forms were tree-ferns. The oldest known true horse-tails are met with in the Trias (Fig. 164, a). The most abundant conifer 382 MESOZOIC PERIODS. [CHAP. is the cypress-like Voltzia (Fig. 164, b}. Cycads, already a feature in the vegetation of the Permian system, now increase FIG. 164. Triassic Plants ; (a) Horse-tail Reed (Equisetum colum- nare, ) ; (V) Conifer ( Voltzia heterophylla, ) ; (c) Cycad (Ptero- phyllum Blasii, ). in number and variety. During the Mesozoic ages they continued to be the most characteristic members of the terrestrial flora, insomuch that this division of geological time is sometimes spoken of as the "Age of Cycads." Some of the more common cycads in the Triassic rocks are Ptero- phyllum, Zamites, Podozamites (Fig. 164, c). The red gypseous and saliferous strata, for the reason already given, are on the whole unfossiliferous. Here and there, footprints of amphibians, preserved on the sandstones, give us a glimpse of the higher forms of life that moved about on the margin of the salt-lakes. The beds of lime- stone, which represent intervals when, for a time, the sea xxi.] TRIASSIC. 3 8 3 overspread the lakes, contain sometimes abundant fossils. But they are numerous in individuals rather than in species or genera, as if the conditions for life in those waters were still somewhat unfavour- able. On the other hand, the limestones laid down in the opener sea are crowded with a varied fauna. One of the most typical fossils of the Trias is the crinoid Encrinus liliiformis (Fig. 165), one of the most familiar fossils of the limestones (Mus- chelkalk) which in Germany form the cen- tral division of the system. Among the lamellibranchs, Myophoria, Avicula Pecten, Cardium, Pullastra, Daonella, and Monotis FlG - 165. Triassic . . /T -,. , ,^ Crinoid (Encrinus are characteristic genera (Fig. 166), some species such as Avicula contorta, Pecten valoniensis, and Cardium rhceticum being eminently useful in tracing the upper parts of the Trias (Rhaetic) all over Europe from Italy to Scandinavia. One of the most dis- tinctive features of the Triassic fauna is its development of cephalopod life. In the limestones of the middle subdivision of Germany, a few species of cephalopods occur, the two prevalent forms being species of Nautilus and the ammonite Ceratites (Fig. 167). But when we turn to the Trias of the Eastern Alps, which represents the deposits of the more open sea, we meet with a remarkable abundance and variety of cephalopods, and with a striking admixture of ancient and more modern types. For example, the vener- able genus Orthoceras, which occurs even down in the Cambrian rocks, is found also here as a survival from Palaeozoic time. But new types now appeared. In par- ticular, the tribe of Ammonites, so pre-eminently typical of 384 MESOZOIC PERIODS. [CHAP. the molluscan life of the Mesozoic seas, is represented by numerous genera and species (Arcestes, Trachyceras, Pina- FlG. 1 66. Triassic Lamellibranchs ; (a) Avicula contorta (natural size) ; (b) Pecten valoniensis (J) ; (c) Cardittm rhceticum (natural size) ; (d) Myophoria milgaris (^). coceras, Phylloceras, besides Ceratites above referred to). FiG. 167. Triassic Cephalopods ; (a) Nautilus bidorsattts () ; (b) Ceratites nodostis (reduced). Among the fishes of the Trias, the genera Acrodus, Ceratodus, Gyrolepis, Hybodus^ and Pholidophorus may be mentioned. XXL] TRIASSIC. 385 Labyrinthodonts still haunted the lagoons and sandy shores (Mastodonsaurus, Trematosaurus) \ but they no longer re- mained the most important members of the animal world. Various early types of lizards now took their places in the ranks of creation (Hyperodapedon, Tekrpeton^ Fig. 168). A strange order of Triassic reptiles was characterised by the jaws having the form of a beak, somewhat like that of a turtle ; Dicynodon, one of these forms, carried two huge tusks in the upper jaw. A remarkable and long extinct order of reptiles, that of the Deinosaurs, made its first appearance FIG. 168. Triassic Lizard FIG. 169. Triassic Crocodile (Scutes (Telerpeton elginense, ^). of Stagonolepis elginensis^ J). in Triassic time. These creatures were marked by peculi- arities of structure that linked them both with true reptiles and with birds, while in size they resembled elephants and rhinoceroses. They seem to have walked mainly on their hind feet, the three-toed or five-toed bird-like imprints of which are numerous on some beds of sandstone. They are characteristically Mesozoic types of life. Another not less typically Mesozoic form, that of the Plesiosaurs, likewise began in Triassic time ; but it will be more particularly alluded to in the following chapter. The earliest known crocodiles have been found in Triassic rocks ; some of the 2 c 386 MESOZOIC PERIODS. [CHAP. scutes or scales of one of these animals are shown in Fig. 169. But the most important advance in the fauna of the globe during the Triassic period was the first appearance of mammalian life. Detached teeth and lower a \$3f J aws nave been met in the uppermost parts of the Triassic system which have been identified as possessing structures like those of some of the marsupial animals of Australia (Microlestes^ Fig. 170). It is in- T . . teresting to know that the earliest repre- Marsupial (Micro- sentatives of the great class of the mam- lestes Moorei} ; (a) malia belonged to one of its lowest divi- Lower molar tooth, sions> They were small creatures, probably outer side (f) ; (b) ... J ir J Ditto (nat. size); (c) resembling the Myrmecobms or Banded |Ditto, front side (f). Ant-eater of New South Wales. The Triassic strata of the inland basins (England, Germany, France, etc.) have been subdivided into the following groups : 'Red, green, and grey marls, black shales, sand- stones, bone-beds, and in Germany sometimes thin seams of coal. Characteristic fossils are ^Rhsetic. Cardium rhcBticum^ Avicula contorta, Pecten valoniensis, Pullastra arenicola, Acrodus, Ceratodus, Hybodus, Saurians, Microlestes. SRed, grey, and green marls, with beds of rock- salt and gypsum. Red sandstones and marls (England) ; grey sand- stones and dark marls and clays, with thin seams of earthy coal (Germany). ! Limestones and dolomites, with bands ol anhy- drite, gypsum, and rock-salt. The limestones are the great repository of the fossils. This subdivision is absent, or only feebly represented in England. Bunter or Lower / Mottled red and green sandstones, marls, and Trias. I sometimes pebble-beds. xxi.] TRIASSIC. 387 The salt-beds of Cheshire have long been worked for commercial purposes. The lower bed is sometimes more than 100 feet thick; but the salt deposits of Germany are much more important. Thus at Sperenberg, 20 miles south of Berlin, a boring was put down through about 290 feet of gypsum, and then through upwards of 5000 feet of rock-salt, without reaching the bottom of the deposit. The alternation of bands of rock-salt with thin layers of anhydrite or of gypsum no doubt marks successive periods of desiccation and inflow; in other words, each seam of the sulphate of lime (which is the least soluble salt, and is therefore thrown down first) seems to indicate a renewed supply of salt water from outside, probably from the open sea, while the overlying rock-salt shows the con- tinued evaporation during which the water became a con- centrated solution, and deposited the thicker layer of sodium chloride. Sometimes the concentration continued until still more soluble salts, such as chlorides of potassium and magnesium, were also eliminated. These phenomena are well displayed at the great salt-mines of Stassfurt, on the north flank of the Harz Mountains. The lowest rock there found is a mass of pure, solid, crystalline rock-salt of still unknown thickness, but which has been pierced for about 1000 feet. This rock is separated into layers, averaging about 3! inches in thickness, by partings of anhydrite inch thick or less. If each of these "year rings," as the German miners call them, represented the deposit formed during the dry season of a single year, then the mass of 1000 feet would have taken more than 3000 years for its formation. But there do not appear to be any good grounds for believing that each band marks one year's accumulation. Above the rock-salt lie valuable deposits of the more soluble MESOZOIC PERIODS. [CHAP. xxi. salts, particularly chlorides of potassium and magnesium, with sulphates of lime and magnesia. The compound known as Carnallite (a double chloride of potassium and magnesium) is now the chief source of the potash salts of commerce. In the Rhaetic beds of England, one of the most interest- ing bands is the so-called " bone-bed " a thin layer of dark sandstone, charged with bones, teeth, and scales of fishes and saurians. A thin seam of limestone in the same group contains wings and wing-cases of insects. The Trias of the Eastern Alps reaches a thickness of many thousand feet, and forms great ranges of mountains. The lower division runs throughout the Alps with consider- able uniformity of character, so that it forms a useful platform from which to investigate the complicated geological struc- ture of these mountains. The Upper Trias consists of several thousand feet of shales, marls, limestones, and dolomites, while the Rhaetic group swells out into a great succession of limestones and dolomites. During the time when the Triassic sea stretched over the site of the Alps there were evidently considerable oscillations ot level, and there likewise occurred extensive volcanic eruptions, whereby large masses of lavas and tuffs were ejected. These rocks now form conspicuous hills in the Tyrol. Triassic rocks have been traced in Beloochistan, the Salt Range of the Punjab, Northern Kashmir, and Western Thibet. They cover a large area of North America, and have been recognised in Australia and New Zealand. Rocks which have been assigned to the same geological period occur in South Africa, and have there yielded a remarkable series of reptilian remains. CHAPTER XXII. JURASSIC. THE Jurassic system which follows the Trias has received its name from the Jura Mountains, where it is well developed. It contains the record of a great series of geographical changes which in Europe entirely effaced the inland basins and sandy wastes of the previous period, and during which sedimentary rocks were accumulated that now extend in a broad belt across England, from the coasts of Dorset to those of Yorkshire, cover an enormous area of France and Germany, and sweep along both sides of the Alps and the Apennines. These strata vary greatly in composition and thickness as they are traced from country to country. In one district they present a series of limestones which, as they are followed into another, pass into shales or sand- stones. The widespread uniformity of lithological character, so marked among the Palaeozoic systems, gives place in the Mesozoic series to greater variety. Sandstones, shales, and limestones alternate more rapidly with each other, and are more local in their extent. They indicate greater vicissi- tudes in the process of deposition, more frequent alternations of sea and land, and not improbably greater differences of climate than in Palaeozoic time. 39 MESOZOIC PERIODS. [CHAP. The flora of the Jurassic period is marked by the same general characters as that of the Trias ferns, equisetums, conifers, and cycads, being its distinguishing elements. Cycads now abound (Pterophyllum, Zamites, Cycadites, and many others, Fig. 171). Among the conifers are the remote FIG. 171. Jurassic Cycad (Cycadeoidea microphylla, ). ancestors of our " Puzzle-monkeys," introduced from Chili and now so common as ornamental garden shrubs (Arau- caria imbricatd), and of our pines and firs. This vegetation flourished luxuriantly over the area of Britain ; on the site of Yorkshire it grew so densely as to give rise to thick peaty accumulations, which now form beds of coal. It went far northward, for its remains have been abundantly preserved in Spitzbergen, where numerous cycads have been found among them. These plants unquestionably grew and flourished within the Arctic Circle, so that, though the cli- mates of the globe were already beginning to emerge from the greater uniformity of Palaeozoic time, the Arctic regions still enjoyed a temperature like that of sub-tropical countries at the present time. xxii.] JURASSIC. 391 The animal world during the Jurassic period, if we may judge of it from its fossil remains, must have been much more varied alike on land and in the sea than during the previous ages of the earth's history. From the circumstances in which the strata were deposited, relics of the life of the land are frequently met with, besides abundant records of that of the sea. A characteristic feature of the period was the profusion of corals, which at different times spread over much of the site of modern Europe. They were no longer the rugose forms so distinctive of the Palaeozoic seas, but true reef-building astraeids, belonging to the genera Isastrcea^ Thamnastrcza, Montlivaltia, Thecosmilia, etc. (Fig. 172). FIG. 172. Jurassic reef- building Coral (Isastrcea explanata^ ). From the Cornbrash. Crinoids were still abundant, though less so than in the Carboniferous limestone sea ; the old forms were now re- placed by others, among which the most conspicuous was the Pentacrinite (Fig. 173) a genus still living in our present seas. Sea-urchins swarmed on some parts of the sea-floor; among their more frequent genera are Cidaris (Fig. 174), Diadema, Hemicidaris^ Acrosalenia, Glyptichus^ Pygaster. Of the contrasts between the Mesozoic and Palaeozoic faunas one of the most marked is to be found 392 MESOZOIC PERIODS. [CHAP. among the brachiopods. Except the persistent inarticulate types which have lived on from Cambrian time to the FIG. 173. Jurassic Crinoid (I^entacrinusfasciculosus, -f). present day (Crania, Lingula, Discind), the numerous and varied forms which played so important a part in the life of the Palaeozoic seas died out almost entirely at the close of the Palaeozoic period. The ancient Spirifers and Leptaenids lingered on until the Jurassic period, and then disappeared. On the other hand, the genera Rhynchonella and Terebratula, which occupied a subordinate place in earlier ages, now became the chief representatives of the xxii.] JURASSIC. 393 brachiopods. They abounded throughout Mesozoic time, but they have gradually diminished in number since then, and at the present day each genus survives only in a small number of species. With the decay of the brachiopods, the other divi- sions of the mollusca proportion- ately advanced. The lamelli- branchs attained a great develop- ment in Mesozoic time, some FlG - I 74- -Jurassic Sea-urchin , . . \_-s-i (Cidans florigemma. ), characteristic genera being Ger- Corallian villia, Exogyra, Lima, Ostrea, Pecten, Pinna, Astarte, Hippopodium, Trigonia (Fig. 175). Some of the oysters were particularly abundant, Gryphcea^ for instance, being so plentiful in some bands of lime- stone as to give the name of "Gryphite limestone" to them. But undoubtedly the distinctive feature of the mol- luscan fauna of Mesozoic time was the great development of the cephalopods. The chambered division was repre- sented by an extraordinary variety of Ammonites (Fig. 176) and the cuttle-fishes by many species of Belemnite (Fig. 177). The ammonites have been made use of to mark off the formations into distinct zones ; for, as a rule, the vertical range of each species is comparatively small. The band of strata characterised by a particular species of ammonite is called the zone of that species, e.g. Zone of Amm. Murchisoncz. Another striking contrast is presented by the Jurassic Crustacea when compared with those of the Palaeozoic ages. The ancient order of trilobites, so abundant in the seas of the older time, had now entirely disappeared; the eury- pterids, which took their place upon the scene as the trilo- 394 MESOZOIC PERIODS. [CHAP. bites were on the wane, had likewise vanished. In their stead there now came abundant ten-footed Crustacea, in- FIG. 175. Jurassic Lamellibranchs ; (a] Trigonia monilifera{\\ Kim- meridge Clay; (b) Plicatula spinosa (), Middle Lias; (c] Gryphcca arcuata (incurva) (-), Lower Lias. eluding both long-tailed forms the ancestors of our lobsters, prawns, shrimps, and cray-fish and short-tailed forms that heralded the coming of our living crabs (Fig. 178). Among the Jurassic strata there occasionally occur thin bands, which have received the name of " insect-beds " from the numer- ous insect -remains which they contain. The neuroptera are most frequent, but orthoptera and coleoptera also occur. Among these remains are forms of dragon-fly, May-fly, grass- hopper, and cockroach. The wing-cases of beetles also are not uncommon ; and there has been found the wing of a XXII.] JURASSIC. 395 butterfly the oldest example of a lepidopterous insect yet known. FIG. 176. Jurassic Ammonites ; (a) Ammonites striatus (), Middle Lias ; (b} A. communis (--), Upper Lias ; (c) A. cordatus (), Lower Calcareous Grit ; (d) A. Jason ( J), Oxford Clay. Fishes abounded in the waters of the Jurassic time. FIG. 177. Jurassic Belemnite (B. hastatus, nat. size), Middle Oolite. Those of which the remains have been preserved are in large measure small ganoids (Pholidophorus^ Dapedius, 396 MESOZOIC PERIODS. [CHAP. Lepidotus, Pycnodus, Fig. 179), with no representatives of the huge bone -cased placoderms of earlier time. There FIG. 178. Jurassic Crustacean (Scaphetts ancylochelis). were likewise various tribes of sharks and rays (ffybodus, Acrodus, Squaloraia). But taking the Jurassic fauna as a whole, undoubtedly its most striking character was given by the extraordinary FlG. 179. Jurassic Fish {Pholidophorus Bechei> ), Lower Lias. development of its reptiles. So remarkably varied was the reptilian life throughout the Mesozoic period that this part of the earth's history has been called the " Age of Reptiles." There were forms which haunted the sea, others that fre- quented the rivers, some that lived on the land, some that flew through the air. Never before or since has there been such a profusion of reptilian types. Some of these are still xxii.] JURASSIC. 397 represented at the present time. The Jurassic Teleosaurus and SteneosauriiS) for example, have their counterparts now in the living crocodile and alligator. The modern turtles, too, are descendants of those which lived in Jurassic times. But it is the long-extinct types that fill us with astonish- ment. One of the most abundant of them is that of the enaliosaurs or sea-lizards, of which the two leading forms were the Ichthyosaurus and Plesiosaums. The former crea- ture, occasionally more than 24 feet long, somewhat re- sembled a whale in shape and bulk, its head being joined by no distinct neck to the body, which tapered into a long FIG. 1 80. Jurassic Sea-lizard (Ichthyosaurus communis> -g^), Lias. tail. It swam by means of two pairs 01 strong paddles, and probably steered itself by a fin on the tail. Its eyes were large, and had a ring of bony plates round the eyeball, which remain distinct in the fossil state. Its jaws were armed with numerous strong pointed teeth, not set in dis- tinct sockets. This reptile probably lived chiefly in the sea, feeding there upon the abundant ganoid fishes which its huge protected eyes enabled it to track even into the deeper water. But it, no doubt, also sought the land, and was able to waddle along the shore or to lie there basking in the sunshine. The Plesiosaurus, in many respects like the Ichthyosaurus, was distinguished by its proportionately shorter tail, longer neck, smaller head, larger paddles, and the insertion of the teeth in distinct sockets. It probably haunted the lagoons, rivers, and shallow seas of the time. 398 MESOZOIC PERIODS. [CHAP. Its long swan-like neck enabled it to lie at the bottom and raise its head to the surface to breathe, or, when at the surface, to send down its powerful jaws and catch its prey at the bottom. Still more extraordinary were the Pterosaurs or flying reptiles strange bat-like creatures with disproportionately FIG. 181. Jurassic Pterosaur, or flying reptile (Pterodactylus crassi- rostris\ Middle Oolite. large heads, and large bone-cased eyes like those of Ichthyo- saurus. The outermost finger of each forefoot was pro- longed to a great length, and supported a membrane with which the animals could fly. The bones were hollow and filled with air like those of birds. Various forms of these winged lizards are found in the Jurassic rocks, the most common being Pterodactylus (Fig. 1 8 1 ) ; others are Dimor- phodon and Rhamphorhynchus. The Deinosaurs, which have already been noticed as having appeared in Triassic time, attained a far greater development in the Jurassic period. These huge ostrich- like reptiles now reached their maximum in size and variety. XXIL] JURASSIC. 399 One of their genera, Megalosaurus, is believed to have been 25 feet long, and to have walked on its massive hind legs along the margins of the shallow waters in search of the smaller animals on which it preyed. Another form, Ceteo- saurus, which may have been as much as 50 feet from the snout to the tip of the tail, and stood some 10 feet high, fed on the vegetation that shaded the rivers and lagoons where it lived. Still more gigantic were some deinosaurs, of which the remains have been found in the Jurassic rocks of North America. Brontosaurus^ about 50 feet or more in length, with a short body, long neck and tail, and small head, had enormous feet, each of which made an imprint measuring about a square yard in area. Stegosaurus, another sluggish deinosaur, was protected by numerous spines and huge plates of bone on its back, some of them more than 3 feet across. The largest of all these monsters, and, so far as yet known, the most colossal animal that ever walked on the earth, was the Atlantosaurus, which is believed to have been not much less than 100 feet in length, and 30 feet or more in height. In another respect, the fauna of the Jurassic period stands out from those that preceded it ; it contained the earliest known birds. These interesting prototypes differed much from modern birds, more particularly in the possession of certain peculiarities of structure that linked them with reptiles. They had teeth in their jaws, and some of them carried long lizard -like tails, each vertebra of which bore a pair of quill-feathers. The best known genus is Archce- opteiyx> found in the lithographic limestone of Solenhofen. Other genera have been obtained in North America. Marsupials, which, so far as yet known, made their appearance in Triassic time, continued to be the only repre- 400 MESOZOIC PERIODS. [CHAP. sentatives of the mammalia during the Jurassic period, FIG. 182. Jurassic Bird (Archaopteryx macrotira, about \\ Solenhofen Limestone, Middle Jurassic. at least no other types have yet been discovered among 5 FIG. 183. Jurassic Marsupial (Phascolotherium Bucklandi); (a) Teeth magnified ; (b] Jaw, natural size. the fossils. Lower jaws and detached teeth have been XXII.] JURASSIC. 401 obtained from two distinct platforms in England the Stonesfield Slate and Purbeck beds and have been referred to a number of genera which find their nearest modern representatives in the Australian phalangers and kangaroos and in the American opossums (Phascolotherium, Stereo- gnathus, Spalacotherium^ Plagiaulax). The Jurassic system has been arranged in the following subdivisions : I Upper fresh-water beds (Purbeck). Middle marine beds ,, Lower fresh- water beds ,, Limestones and calcareous freestones (Portland Stone). Sandstones and marls (Portland Sand). Dark shales and clays (Kimmeridge Clay). Coral rag (limestone with corals), clays, and cal- careous grits. Blue and brown clay (Oxford Clay). Calcareous sandstone (Kellaways Rock Callo- vian). Shelly limestones, clays, and sands (Cornbrash, Bradford Clay, and Forest Marble). Shelly limestones (Great or Bath Oolite) Stones- field Slate. Fuller's Earth. Marine calcareous freestones and grits (Chelten- ham), represented in Yorkshire by 800 feet or more of estuarine sandstones, shales, and lime- stones, with beds of coal. Sandy beds and clays (Upper Lias, Toarcian). Limestones, sands, clays, and ironstones (Middle Lias, Marlstone). Thin blue and brown limestones, and dark shales (Lower Lias, Sinemurian and Hettangian). The Lias, so called originally by the Somerset quarry- men from its marked arrangement into " layers," extends completely across England from Lyme Regis to Whitby. 2 D Purbeckian . Portlandian . Kimmeridgian Corallian Oxfordian . Bathonian . Bajocian (Inferior Oolite) Liassic . 402 MESOZOIC PERIODS. [CHAP. It can be divided into three distinct sections : (a) A lower group of thin blue limestones and dark shales with lime- stone nodules, the limestones being largely used for making cement. This is one of the chief platforms for the reptilian remains, entire skeletons of ichthyosaurus, plesiosaurus, etc., having been exhumed at Lyme Regis; (b) Marlstone or Middle Lias hard argillaceous or ferruginous limestones which form a low ridge or escarpment rising from the plain of the Lower Lias. In Yorkshire it contains a thick series of beds of earthy carbonate of iron, which are exten- sively mined as a source for the manufacture of iron ; (c) Clays and shales surmounted by sandy beds (Upper Lias Sands). The organic remains of the Lias are abundant and well preserved. They are chiefly marine; but that the rocks containing them were deposited near land is indicated by the numerous leaves, branches, and fruits im- bedded in them, and by the various insect-remains that have been obtained from them. In Germany, where the Lias is well developed and presents a general resemblance to the English type, it is known as the Lower or Black Jura. It is still better shown in France, where its three stages attain in Lorraine a united thickness of more than 600 feet. To the south, however, in Provence, it reaches the great thick- ness of 2300 feet. The Bajocian stage, so named from Bayeux in Normandy, where it is well displayed, has long been known in England under the name of Inferior Oolite. It presents two distinct types in this country, being a thoroughly marine formation in the south-western counties and passing northward into a series of strata which were accumulated in an estuary, and which contain the chief repositories of the Jurassic flora. Among the estuarine beds of Yorkshire a few thin XXIL] JURASSIC. 403 coal-seams occur, which have been worked to some extent. On the continent, this division is characteristically marine ; it reaches its greatest development in Provence, where it is 950 feet thick. It runs through the Jura Mountains, where it is made up of more than 300 feet of strata, chiefly lime- stone. In Germany the strata from the top of the Lias to the base of the Callovian group that is, the two stages of Bajocian and Bathonian are classed together as the Middle or Brown Jura, or Dogger. The Bathonian stage is named from Bath, where its subdivisions are admirably exposed. At its base is a local argillaceous band known as Fuller's Earth, because long used for fulling cloth. The chief member of the stage in the south-west of England is the Great or Bath Oolite, a succession of limestones, often oolitic, with clays and sands. The Stonesfield Slate is the name locally given to some thin-bedded limestones and sands forming the lower part of the Great Oolite, and of high geological interest from having supplied among their fossils remains of land-plants, numerous insects, bones of enaliosaurs and deinosaurs, and of small marsupials. The Great Oolite abounds in corals, and contains numerous genera of mollusca, fishes, and reptiles. The Cornbrash (so named from its friable (brashy) character, and forming good soil for corn) is one of the most persistent bands in the English Jurassic system, re- taining its characters all the way from the south-western counties to near the Humber. The Oxfordian stage, sometimes called the Middle or Oxford Oolite, consists of a lower zone of calcareous sand- stone known as the Kellaways rock or Callovian, from the name of a place in Wiltshire, and of a thick upper stiff blue and brown clay, called, from the locality where it 404 MESOZOIC PERIODS. [CHAP. is well developed, the Oxford Clay, and containing numerous ammonites, belemnites, and oysters, but no corals. In Ger- many, the strata from the base of the Callovian to the top of the Purbeckian group are known as the Malm or White Jura. The Corallian stage, so named from the corals with which it abounds, is one of the most distinctive in the Jurassic system. It is traceable across the greater part of England, over the continent of Europe from Normandy to the Mediterranean, through the east of France, and along the whole length of the Jura Mountains and the flank of the Swabian Alps. While it was being formed, the greater part of Europe lay beneath a shallow sea, the floor of which was clustered over with reefs of coral. The Kimmeridgian group or stage is typically displayed at Kimmeridge on the coast of Dorsetshire, whence its name. It there consists of dark shales, some of which are so highly bituminous as to burn readily, and which will prob- ably be eventually of commercial value as a source for the distillation of mineral oil. This group of strata has yielded a larger number of reptilian genera and species than any other in the Mesozoic systems of Britain plesiosaurs, ichthyosaurs, pterosaurs, deinosaurs, turtles, and crocodiles. It is well developed in France and Germany. The Portlandian stage, so called from the Isle of Port- land where it is well seen, consists of a lower set of sandy beds (Portland Sand), and a higher and thicker series of limestones and calcareous freestones, some of the beds containing abundant nodules and layers ot flint. These rocks are prolonged into France near Boulogne-sur-Mer. The Purbeckian group or stage is best seen in the Isle of Purbeck, hence its name. It lies on an upraised surface of Portlandian beds, showing that after the deposition of xxii.] JURASSIC. 405 these beds there was some disturbance of the sea -bed, portions of which were uplifted partly into land and partly into shallow brackish and fresh waters. The Purbeck beds are subdivided into three sub-stages : the lowest consisting of fresh-water limestones, with layers of ancient soil (" dirt- beds "), in which the stumps of cycadaceous trees still stand in the positions in which they grew (Fig. 171) ; the middle sub-stage contains oysters and other marine shells which prove that owing to subsidence the area sank under the sea ; while in the higher subdivision fresh-water fossils re- appear. Among the more interesting organisms yielded by the Purbeck beds are the remains of numerous insects and of the marsupials already referred to, which chiefly occur as lower jaws in a stratum about 5 inches thick. When the bodies of dead animals float out to sea the first bones likely to drop out of the decomposing carcases are the lower jaws ; hence the greater frequency of these bones in the fossil state. Strata belonging to the Purbeckian stage and including red and green marls, with dolomite and gypsum, are found in north-western Germany, showing in that region also the elevation of the floor of the Jurassic sea into detached basins. In India, a mass of strata 6300 feet is found in Cutch, and from its fossils is believed to represent the European Jurassic system from the Bajocian up to the top of the Portlandian stage. In Australia and New Zealand, recog- nisable Jurassic fossils have also been found, showing the extension of the Jurassic system even to the Antipodes. In North America, Jurassic rocks are not largely developed ; but in Colorado they have yielded an abundant series of organic remains including fishes, tortoises, pterosaurs, deinosaurs, crocodiles, and marsupials. CHAPTER XXIII. CRETACEOUS. THE Cretaceous system received its name in Western Europe, because in England and in Northern France its most conspicuous member is a thick mass of white chalk (Latin, Creta). It covers a far more extensive area of the surface of this continent than any of the preceding systems. Its western extremity reaches to the north of Ireland and the Western Islands of Scotland. It covers a large part of the east and south of England, stretching thence into France, where it forms a broad band, encircling the tertiary basin of Paris. It sweeps across Belgium into Westphalia, underlies the vast plain of Northern Germany and Denmark, whence it is prolonged into Southern Russia, where it over- spreads many thousands of square miles. It flanks most of the principal mountain -chains of Europe the Pyrenees, Alps, Apennines, and Carpathians. It spreads far and wide over the basin of the Mediterranean Sea, extending across vast tracts of Northern Africa, and from the Adriatic across Greece and Turkey into Asia Minor, whence it is prolonged through the Asiatic continent. As most of the rocks of the system are of marine origin, we at once perceive how entirely different the Cretaceous CHAP. XXIIL] CRETACEOUS. 47 geography must have been from that of the present day, and to what a great extent the existing land of the Old World lay then below the sea. But in tracing out the distribution of the rocks, geologists have found that the Cretaceous sea did not extend continuously across Europe. On the contrary, as they have ascertained, the old northern land still rose over the site of Northern Britain and Scan- dinavia, while to the south of it a wide depression extended across the area of Southern Britain, Northern France, Belgium, and the North German plain, eastwards to Bohemia and Silesia. This vast northern basin was the theatre of a remarkable succession of geological revolutions. While its eastern portions, during the earlier part of the Cretaceous period, were submerged under the sea, its western tracts were the site of the delta of a great river, probably descending from the land that still lay massed towards the north. During the later ages of the period, the whole basin was filled by a broad and long gulf or inlet, the southern margin of which seems to have been defined by the ridge of old rocks that run from the headlands of Brittany through Central France, the Black Forest, and the high grounds of Bohemia. South of that ridge lay the open ocean which extended all over Southern Europe and the north of Africa, and spread eastwards into Asia. Bearing in mind this peculiar disposition of sea and land, we can understand why the development of the Cretaceous system, alike in regard to its deposits and its fossils, should be so different in the area of the northern basin from that of the southern regions. In the one case, we meet with the local and changing accumulations of a com- paratively shallow and somewhat isolated portion of the sea-bed, wherein are mingled abundant traces of the prox- 408 MESOZOIC PERIODS. [CHAP. imity of land. In the other, we are presented with evidence of a wide open sea, where the same kinds of deposits and the same forms of marine life extend with little change over vast distances. Obviously, it is not the local type of the northern basin, but the more general and widespread type of Southern Europe that should be taken for the distinctive characteristics of the Cretaceous system. But the northern basin was the first to be systematically explored, and is still the best known, and hence its features have not unnaturally usurped the place of importance which ought properly to be assigned to the other wider area. Regarding the period as a whole, let us first consider the general character of its distinguishing flora and fauna, and then pass on to trace the history of the period as revealed by the succession of strata. The plants of the Cretaceous system show that the vegetable kingdom had now made a most important advance in organisation. In the lower half of the system the fossil plants yet found are on the whole like those of the Jurassic rocks that is, they- include some of the same genera of ferns, cycads, and coni- fers which these rocks contained. But already the ancestors of our common trees and flowering plants must have made their appearance, for in the upper half of the system their remains occur in abundance. This earliest dicotyledonous flora numbered among its members species of maple, alder, aralia, poplar, myrica, oak, fig, walnut, beech, plane, sassafras, laurel, cinnamon, ivy, dogwood, magnolia, gum-tree, ilex, buckthorn, cassia, credneria, and others. The modern aspect of this assemblage of plants is in striking contrast to the more antique look of all the older floras. There were likewise species of pine (Pinus), Californian pine (Sequoia), juniper (Juniperus\ and other conifers, various cycads, XXIII.] CRETACEOUS. 409 forms of screw -pine (Pandanus), palms (Sofa/), and numerous ferns (Gleichenia, Asplenmm, etc.) This flora FIG. 184. Cretaceous Plants; (a) Quercus rinkiana () ; (b) Cinna- momum sezannense (|) ; (c) Ficus atavina (f) ; (d] Sassafras re- ctirvata (f ) ; (e]Juglans arctica (J). spread over the land, surrounding the northern Cretaceous basin, and extended northwards even as far as North Green- land, from which nearly 200 species of Cretaceous plants have been obtained. The inference may be deduced that the climate of the globe must then have been much warmer than at present. The luxuriant vegetation disinterred from 4io MESOZOIC PERIODS. [CHAP. the Cretaceous rocks of North Greenland includes more than forty kinds of ferns, besides laurels, figs, magnolias, and other plants, which show that, though the winters were no FlG. 185. Cretaceous Foraminifera ; (a) Textulariabaudouiniana(^}\ (6} Globigerina cretacea ( 7 T ) ; (c] Rotalina voltziana ( 3 T )' doubt dark, they must have been extremely mild. There could have been no perpetual frost and snow in these Arctic latitudes in Cretaceous times. Foraminifera abound in some of the Cretaceous limestones, indeed, in some places they form almost the only constituent of these rocks. They are plentiful in the white chalk of England, France, and Belgium, one of the more frequent genera being Globigerina (Fig. 185) which still lives in enormous numbers in the Atlantic, and forms at the FIG^ 186,- Cretaceous Sponge bottom Qf ^ ocean ( Ventnciihtes decurrens, |). ' ooze not unlike chalk (Fig. 33). Sponges lived in great numbers in the Cretaceous sea. Their minute siliceous spicules are abundant in the Chalk, and even entire sponges enveloped in flint are not un- XXIII.] CRETACEOUS. 411 common (Ventriculites, Fig. 186). Sea-urchins are among the most familiar fossils of the Chalk, and must have lived FIG. 187. Cretaceous Sea-urchins; (a) Echinocomis coniats, f ( = Galerites albo-galertis\ under surface and side view ; (b] Ananchytes ovatus (^), side view and under surface ; (c] Micraster cor-arguinum (i)> upper and under surface. in great numbers on the Cretaceous sea-bottom. Some of their genera are still living, and have been dredged up in recent years from great depths in the ocean. Among the 412 MESOZOIC PERIODS. [CHAP. more characteristic Cretaceous types are Ananchytts, Hoi- aster, Micraster, and Echinoconus (Fig. 187). The brachio- pods were still represented chiefly by the ancient genera Terebratula and Rhynchomlla. Lamellibranchs abounded, especially the genera Ostrea, Exogyra, Inoceramus (Fig. 1 8 8), Lima, Pecten, and the various forms of Hippuritids. These last (Hippurites, Radiolites, Caprina, etc., Fig. 189) are specially characteristic, being, so far as we know, con- FIG. 188. Cretaceous Lamellibranchs ; (a] Trigonia aliformis () ; (b} Inoceramus sulcatus () ; (c] Nucula bivirgata (natural size). fined to the Cretaceous system ; hence their occurrence serves to indicate the Cretaceous age of the rock containing them. They have been imbedded in such numbers in the limestones of the south of Europe as to give the name of hippurite-limestone to these rocks. They are comparatively infrequent in the strata of the northern Cretaceous basin. Probably the most distinctive feature in the molluscan life of the Cretaceous seas was the extraordinary variety in the development of the cephalopods. This is all the more remarkable from the fact that before the next geological period the great majority of these types appear to have become extinct. The ammonites and belemnites, which played so important a part in the fauna of Mesozoic time, died out about the close of that long succession of periods. At least in Europe, while their remains continue to present XXIII.] CRETACEOUS. 413 themselves up to the top of the Cretaceous system, they disappear entirely from the overlying strata. It is curious FIG. 189. Cretaceous Lamellibranchs (Hippurites) ; \(a) Radiolites acuticostata () ; (b] Hippurites toucasiana (|) ; (c) Caprina Aguilloni (^) ; (d) Caprotina toucasianus (-5-). to observe that while these important tribes were about to vanish other cephalopods of new and varied types flourished contemporaneously with them. Never before or since, in- deed, have the cephalopodan types been so manifold (Fig. 190). For instance, Baculites was a straight chambered shell reminding us of the ancient Orthoceras. In Toxoceras the shell was bent into the form of a bow. In Hamttes it was long, tapering, and curved upon itself like a hook. In Ancyloceras it is coiled at the posterior end, while the 414 MESOZOIC PERIODS. [CHAP. FIG. 190. Cretaceous'Cephalopods ; (a) Baculites anceps () ; (b} Ply- choceras emericianus (|) ; (c) Toxoceras bituberculatus () ; (d) Hamites rotundus (J-) ; () Alnus glutinosa ; (E] Platamis aceroides (all natural size except E> which is ^). They consequently supply an excellent basis for comparison 2 G 45 TERTIARY PERIODS. [CHAP. with the existing distribution of the same species. When the deposits containing them are examined with reference to the present habitats of the species, it is found that the percentage of what are now northern shells increases from the lower to FIG. 202. Pliocene Marine Shells ; (a} Rhynchonella psittacea (natural size) ; (b) Panopcea norvegica (|) ; (c ) Purpura lapilhis (|) ; (d] Trophon antiqiium (^). the higher parts of the series. In Pliocene time, each species no doubt flourished only in that part of the sea where it found its congenial temperature and food. We infer that its requirements are still the same at the present day, in other words, that the temperature of the regions within which the species is now confined afford, on the whole, XXV.] PLIOCENE. 45 1 an indication of the temperature of the areas within which it lived in the Pliocene seas. On this basis of comparison, the inference has been drawn that the climate in the northern hemisphere, after becoming temperate, passed on to a more rigorous stage. In the end thoroughly Arctic conditions spread over most of Europe and a large part of North America, during the period that succeeded the Pliocene (p. 456). In Britain, Pliocene deposits are almost entirely confined to the counties of Norfolk and Suffolk. They consist of various shelly sands, gravels, and marls, which have long been known as " crag." Arranged in descending order, the following are the recognised subdivisions : Fresh-water, estuarine, and marine sand and silts, with layers of peat, having a total depth of 10 to 70 feet. Among the terrestrial plants are cones of Scotch fir (Pimts sylvestris) and spruce (Abies) , leaves of water-lily \Nympkaa alba), yellow pond -lily (Nuphar luteum}, hornwort (Ceratopkyttum\ blackthorn (Prunus spinosa), bog -bean (Meny- anthes trifoliata), oak, and hazel, with land and Forest-Bed Group \ fresh-water shells, and many mammals, including species of wolf, fox, machairodus, hyaena, glutton, bear, seal, horse, rhinoceros, hippopotamus, pig, ox, musk-sheep, deer, beaver, trogontherium (a huge extinct kind of beaver), mole, elephant (E. antiquus, E. meridionalis, E. primigenitts], etc. This group of strata is found at the base of the sea-cliff of boulder-clay in Norfolk, and extends under the present sea. ( Sands and clays occurring as a thin local deposit in Suffolk, 6 to 1 6 feet thick, with marine shells, Chillesford Beds -{ about two -thirds of which still live in Arctic waters ( Mya truncata, Cyprina islandica, Astarte 1. borealisy Tellina obliqua). 452 TERTIARY PERIODS. [cHAr. Norwich (fluvio- marine or mam--{ maliferous)Crag f Shelly sand and gravel, 5 to 10 feet thick, contain- ing 93 P er cent f still living species of shells, and bones and teeth of mastodon, elephant (E. meridionalis, E. antiquus), hippopotamus, rhino- ceros, etc. The proportion of northern shells is 14-6 per cent, and the following species are in- cluded Rhynchonella psittacea, Scalaria grocn- landica^ Panopcea norvegica, -Astarte borealis. About twenty species of land or fresh-water shells also occur. - A local and inconstant accumulation, 25 feet thick, of red and dark brown ferruginous shelly sand, with numerous species of shells of which 107 per cent are northern forms. Some of the character- istic shells of the deposit are Trophon antiquum, Valuta Lamberti, Purpura lapillus^ Pectunculus Carditi?n edule, Red Crag ( Shelly sands and clays containing 84 per cent of still living shells, whereof 5 per cent are northern White (Suffolk or J species. One of the characteristics of the deposit coralline) Crag . is the large number (140 species) of coral -like polyzoa (corallines or bryozoa), whence one of i. the names given to this subdivision. On the Continent, the youngest Tertiary deposits cover comparatively small areas and mark some of the last tracts occupied by the sea. Thus, in the Vienna basin, there is evidence that the sea, shut off from the main ocean, and partly converted into an inland sea, like the Caspian, was gradually filled up with sediment and raised into land. Along the northern borders of the Mediterranean Sea, thick masses of marine Pliocene strata show the prolonged depres- sion of that region during Pliocene time, and its subsequent elevation. In the south of France, these strata, lying uncon- formably on everything older than themselves, reach a height of 1150 feet above the sea. Along both sides of the Apen- nine chain, Pliocene blue marls, clays, and sands, known as xxv.] PLIOCENE. 453 the sub-Apennine beds, have been uplifted into a range of low hills. These deposits swell out southwards, reaching their greatest thickness (2000 feet or more) in Sicily, which was probably the region of maximum subsidence during Pliocene time. Here and there, in the Italian strata of this period, remains of terrestrial vegetation and land-animals are abundantly preserved. One of the most noted localities for these fossils is the upper part of the valley of the Arno. Perhaps the most curious and interesting assemblage of the land-fauna of Europe during Pliocene time has been found in some hard red clays, alternating with gravels at Pikermi in Attica. Thirty-one genera of mammals have there been obtained, of which twenty-two are extinct. The ruminants, specially well represented among these remains, include species of giraffe, helladotherium (Fig. 2 03), antelopes, FIG. 203. Helladotherium Dttvernoyi ( r ^) a gigantic animal inter- mediate in structure between the giraffe and the antelope, Pikermi, Attica. gazelles, and other forms allied to, but distinct from, any liv- ing genera. There are, likewise, the bones of gigantic wild 454 TERTIARY PERIODS. [CHAP. xxv. boar, several species of rhinoceros, mastodon, deinotherium, porcupine, hyaena, various extinct carnivores, and a monkey. In India, a somewhat similar fauna has been obtained from a massive series of fresh-water sandstones, known as the Siwalik group. A large proportion of the remains belong to existing genera of animals, such as macaque, bear, ele- phant, horse, hippopotamus, giraffe, ox, porcupine, goat, sheep, and camel. Various extinct types were contemporary with these animals, two of the most extraordinary of them being the Sivatherium and Bramatherium colossal, four- horned creatures allied to our living antelopes and prong- bucks. CHAPTER XXVI. POST-TERTIARY OR QUATERNARY PERIODS PLEISTOCENE OR POST-PLIOCENE RECENT. WE have now arrived at the last main division of the Geo- logical Record, that which is named Post-tertiary or Quater- nary, and which includes all the formations accumulated from the close of the Tertiary periods down to the present day. But no sharp line can be drawn at the top of the Tertiary groups of strata. On the contrary, it is often diffi- cult, or indeed impossible, satisfactorily to decide whether a particular deposit should be classed among the younger Tertiary or among the Post-tertiary groups. In the latter, all the molluscs are believed to belong to still living species, and the mammals, although also mostly still of existing species, include some which have become extinct. These extinct forms are numerous in proportion to the antiquity of the deposits in which they have been preserved. Accord- ingly, a classification of the Quaternary strata has been adopted, in which the older beds, containing a good many extinct mammals, have been formed into what is termed the Pleistocene, Post-pliocene, or Glacial group, while the younger beds, containing few or no extinct mammals, are termed Recent. 45 6 POST-TERTIARY PERIODS. [CHAP. The gradual refrigeration of climate which is revealed to us by the shells of the crag was prolonged and intensified in Post-tertiary time. Ultimately the northern part of the northern hemisphere was covered with snow and ice, which extended into the heart of Europe and descended far south- ward in North America. The previous denizens of land and sea were in large measure driven out or even in many cases wholly extirpated by the cold, while northern forms advanced southward to take their places. The reindeer, for instance, roamed in great numbers across Southern France, and Arctic vegetation spread all over Northern and Central Europe, even as far as the Pyrenees. After the cold had reached its climax, the ice-fields began to retreat, and the northern flora and fauna to retire before the advance of the plants and animals which had been banished by the increasingly severe temperature. And at last the present conditions of climate were reached. The story of this Ice Age is told by the Pleistocene or Post-pliocene formations, while that of the changes which immediately led to the establishment of the present order of things is made known in the Recent deposits. PLEISTOCENE, POST-PLIOCENE, OR GLACIAL. The evidence from which geologists have unravelled the history of the Ice Age or cold episode which came after the Tertiary periods in the northern hemisphere may here be briefly given. All over Northern Europe and the northern part of North America the solid rocks, where of hardness sufficient to retain it, are found to present a characteristic smoothed, polished, and striated surface. Even on crags and rocky bosses that have remained for long periods ex- posed to the action of the weather, this peculiar worn surface xxvi.] PLEISTOCENE OR GLACIAL. 457 may be traced ; but where they have been protected by a covering of clay, these markings are often as fresh as when they were first made. The groovings and fine striae do not occur at random, but in every district run in one or more determinate directions. The faces of rock that look one way are rounded off, smoothed, and polished ; those that face to the opposite quarter are more or less rough and angular. The quarter to which the worn faces are directed corresponds with that to which the striae and grooves on the rock-surfaces point. There can be no doubt that all this smoothing, polishing, grooving, and striation has been done by land-ice; that the trend of the striae marks the direction in which the ice moved, those faces of rock which looked towards the ice being ground away, while those that looked away from it escaped. By following out the direc- tions of the rock-striae we can still trace the march of the ice across the land (see chapter vi.) As the ice travelled, it carried with it more or less detritus, as a glacier does at the present day. Some of this material may have lain on the surface, but probably most of it was pushed along at the bottom of the ice. Accord- ingly, above the ice-worn surfaces of rock, there lies a great deposit of clay and boulders, evidently the debris that accumulated under the ice-sheet and was left on the surface of the ground when the ice retired. This deposit, called boulder-clay or till, bears distinct corroborative testimony to the movement of the ice. It is always more or less local in origin, but contains a variable proportion of stones which have travelled for a greater or less distance, sometimes for several hundred miles. When these stones are traced to their places of origin, which are often not hard to seek, they are found to have come from the same quarter indicated 458 POST-TERTIARY PERIODS. [CHAP. by the striation of the rocks. If, for example, the ice-worn bosses of rock show the ice to have crept from north to south, the boulders will be found to have a northern origin. The height to which striated rock- surfaces and scattered erratic blocks can be traced affords some measure of the depth of the ice-sheet. From this kind of evidence it has been ascertained that the whole of Northern Europe, amounting in all to probably not less than 770,000 square miles, was buried under one vast expanse of snow and ice. The ice-sheet was thickest in the north and west, whence it thinned away southward and eastward. Upon Scandinavia it was not improbably between 6000 and 7000 feet thick. It has left its mark at heights of more than 3000 feet in the Scottish Highlands, and over North-Western Scotland it was probably not less than 5000 feet thick. Where it abutted upon the range of the Harz Mountains, it appears to have been still not far short of 1500 feet in thickness. This vast mantle of ice was in continual motion, creeping outward and downward from the high grounds to the sea. The direction taken by its principal currents can still be followed. In Scandinavia, as shown by the rock -striae and the transport of boulders, it swept westward into the Atlantic, eastward into the Gulf of Bothnia, which it com- pletely filled up, and southward across Denmark and the low grounds of Northern Germany. The basin of the Baltic was completely choked up with ice ; so also was that of the North Sea as far south as the neighbourhood of London. From the same evidence we know that the ice which streamed off the British Islands moved eastward from the slopes of Scotland into the hollow of the North Sea, part of it turning to the left to join the south-western margin of the Scandi- xxvi.] PLEISTOCENE OR GLACIAL. 459 navian sheet, and move with it northwards across the Orkney Islands into the Atlantic, and another branch bending southwards and moving with the southerly expansion of the Scandinavian ice along the floor of the North Sea and the low grounds of the east of England ; and that on the west side of Scotland the ice filled up and crept down all the fjords, burying the Western Islands under its mantle and marching out into the Atlantic. The western margin of the ice-fields, from the south-west of Ireland to the North Cape of Norway, must have presented a vast wall of ice some 2000 miles long, and probably several hundred feet high, breaking of! into icebergs which floated away with the prevailing currents and winds. The Irish Sea was likewise filled with ice, moving in a general southerly direction. Northern Europe must thus have presented the aspect of North Greenland at the present time. The evidence of rock -striae and ice -borne blocks enables us to determine approximately the southern limit to which the great ice-cap reached. As even the southern coast of Ireland is in- tensely ice-worn, the edge of the ice must have extended some distance beyond Cape Clear, rising out of the sea with a precipitous front that faced to the south. Thence the ice-cliff swung eastwards, passing probably along the line of the Bristol Channel and keeping to the north of the valley of the Thames. That the northern ice moved down the bed of the North Sea is shown by the boulder-clays and transported stones of the eastern counties of England, among which fragments of well-known Norwegian rocks are recognisable. Its southern margin ran across what is now Holland, skirted the high grounds of Westphalia, Hanover, and the Harz, which probably there arrested its southward extension, for there is evidence that the ice swept round 460 POST-TERTIARY PERIODS. [CHAP. into the Lowlands of Saxony up to the chain of the Erz, Riesen, and Sudeten Mountains, whence its southern limit turned eastward across Silesia, Poland, and Gallicia, and then swung round to the north, passing across Russia by way of Kieff and Nijni Novgorod to the Arctic Ocean. In Europe no distinct topographical feature appears to mark the southern limit reached by the ice -sheet ; this limit can only be approximately fixed by the most southerly localities where striated rocks and transported blocks have been observed. In North America, however, the margin of the great ice-cap is prominently defined by a mound or series of mounds of detritus which seem to have been pushed in front of the ice. These mounds, beginning on the coast of Massachusetts, run across the Continent with a wonderful persistence for more than 3000 miles. They form what American geologists call the " terminal moraine." The detritus left by the ice-sheet consists of earthy, sandy, or clayey material, more or less charged with stones of all sizes up to blocks weighing many tons. For the most part it is unstratified, and bears witness to the irregular way in . which it was tumbled down by the ice. In some districts, it has been more or less arranged in water, and then assumes a stratified character. The stones in the detritus, more especially where they are hard and are imbedded in a clayey matrix, present smooth striated surfaces, the striae usually running along the length of the stone, but not infrequently crossing each other, the older being partially effaced by a newer set (Fig. 24). This characteristic striation points unmistakably to the slow creeping motion of land-ice. But the boulder-clays, earths, and gravels left by the great ice-sheet are not simply one continuous deposit. On the contrary, they contain intercalations of stratified sand, xxvi.] PLEISTOCENE OR GLACIAL. 461 clay, and even peat. In these included strata organic remains occur, for the most part those of terrestrial plants and animals, showing that the ice again and again retreated, leaving the country to be covered with vegetation, and to be tenanted by land animals ; but that after longer or shorter periods of diminution it once more advanced south- ward over its former area. These intervals of retreat are known as " interglacial periods." Probably they were of pro- longed duration, the climate becoming comparatively mild and equable while they lasted. The occurrence of boulder- clays above the interglacial deposits shows a subsequent lowering of the temperature, with a consequent renewal of glacial conditions. The Pleistocene deposits thus reveal to us a prolonged period of cold broken up by shorter intervals of milder climate. The fossils which they contain throw curious and interesting light on these oscillations of temperature. Among the plants, leaves of Arctic species of birch and willow are found far to the south of their present limits ; on the other hand, remains of plants now confined to temperate latitudes are found fossil in Siberia, and others, now living in more genial climates than those of Central Europe, are associated in interglacial deposits, with the remains of the still indi- genous vegetation. To the same effect, but still more striking, is the testi- mony of the Pleistocene fauna, with its strange mingling of northern and southern forms. The marine shells imbedded in the glacial clays of Scotland, though chiefly belonging to species that still live in the adjoining seas, include a few that are now restricted to more northern latitudes (Pecten islandicus, Leda lanceolata^ Tettina lata, etc., Fig. 204). Turning to the terrestrial mammals we find among the Pleis- 462 POST-TERTIARY PERIODS. [CHAP. tocene deposits the remains of the last of the huge pachy- derms which, through Tertiary time, had been so striking a FIG. 204. Pleistocene or Glacial Shells ; (a) Pecten islandicus () ; (b] Leda truncata () ; (c] Leda lanceolata (|) ; (d) Tellina lata () ; (e] Saxicava rugosa (|) ; (/) Natica clausa (^) ; (g) Trophon scalariforme (). feature of the animal population of Europe. The hairy mammoth (Elephas primigenius, Fig. 205) and the woolly rhinoceros (R. tichorhinus] now roamed all over the Continent and across Britain, which had not yet become an island. During the retreat of the snow and ice, they found their XXVI.] PLEISTOCENE OR GLACIAL. 463 way into the forests and pastures of Northern Siberia. Driven southwards when the cold increased, they were accompanied FIG. 205. Mammoth (Elephas primigenitis} from the skeleton in the Musee Royal, Brussels. by numerous Arctic animals which have not yet become extinct. Herds of reindeer (Cervus tarandus] sought the pastures of Central France and Switzerland ; the glutton (Gulo luscus] came to the south of England and to Auvergne ; the musk-sheep (Ovibos mos- chatus, Fig. 206) and Arctic fox (Cam's lagopus) wandered southward to the Pyrenees. But as each oscillation of FIG. 206. Back view of skull of climate slowly brought in a Musk-sheep (Ovibos moschatus, . , , , ), Brick-earth, Crayford, Kent, milder temperature, and pushed the snow and ice northward, animals of southern types made their way into Southern and Central 464 POST-TERTIARY PERIODS. [CHAI>. Among these immigrants were the porcupine (Hystrix}, leopard (Felts pardus\ African lynx (Felts pardina)^ lion (Felts leo\ hyaena, elephant, and hippopotamus, the bones of which have been found in the Pleistocene deposits. After the height of the cold period or Ice Age had been reached and the general temperature of the northern hemi- sphere began to rise again, the ice retreated from the low grounds, but still continued among the mountains. The existing snow-fields and glaciers of the Alps, the Pyrenees, and Scandinavia are the lineal descendants of those vaster ice-sheets which formerly overspread so much of Europe. The glaciers of the Alps, large though they are, can be shown to be merely the relics of their former size. The glacier of the Rhone, for example, as is proved by rock-striae and transported blocks, once extended 170 miles in direct distance from its modern termination, and rose hundreds of feet above its present surface, burying the valleys and overflowing considerable ridges of hills. The glacier of the Aar stretched once as far as Berne a distance of about 70 miles from its present termination; and judging from the marks it has left on the mountains, it must have been not less than 4000 feet thick at the Lake of Brienz. Though elsewhere in Europe the glaciers have long ago vanished from most of the high grounds, they have left unmistakable traces of their former presence. Thus in hundreds of valleys among the Highlands of Scotland, in the Lake District, and North Wales, admirably ice-worn bosses of rock and beautifully perfect moraines may be seen. We can even trace, in the succession of moraines that become smaller as they approach the head of a valley, the stages of retreat of the original glacier as it shrank before xxvi.] PLEISTOCENE OR GLACIAL. 465 the increasing warmth, till at last it disappeared together with the snow-basin that fed it. Other relics of the retirement of the ice-sheet are supplied by the long mounds and heaps of gravel and sand, so abun- dantly strewn over many Lowlands of Northern Europe. These sometimes form ridges, rising 20 or 30 feet above the ground on either side of them, and running for a number of miles. Elsewhere they are heaped together irregularly, often enclosing pools of water. They are known as osar in Sweden, kames in Scotland, and eskers in Ireland. During the later stages of the Ice Age the level of the land in Western Europe was lower than it is now. When elevation began, the upward movement continued with long intervals of rest until the land reached its present position. These pauses during the prolonged upheaval are marked by lines of raised beach (p. 149), well seen along both sides of Scotland, and also along the sea margin of Norway. So slowly and gradually did the great cold disappear that the Ice Age insensibly passed into the Recent or existing period. There can be no doubt that man appeared in Europe before the climate had become as mild as it now is, for his flint-flakes and bone implements are found associated with the bones of Arctic animals in Central France, and traces of his presence in rudely chipped stone instruments occur in deposits which point to frozen rivers. Indeed, in a certain sense, it may be said that the Ice Age still exists among the snow-fields and glaciers of Europe. Arranged in chronological order, the evidence from which the history of the Pleistocene period is determined may be given as follows : Last traces of local glaciers ; terminal and lateral moraines. Marine terraces or raised-beaches, sometimes with moraines resting 2 H 466 POST-TERTIARY PERIODS. [CHAP. upon them ; rock-shelves cut probably by waves and floating ice, and marking former levels of the sea. These beaches and shelves indicate pauses during the last upheaval of the land. Marine clays with Arctic shells. Erratic blocks chiefly transported by the great ice-sheet, but partly also by floating ice during the rise of the land. Sands and gravels (kames) arranged in heaps, mounds, and ridges, and due in some way to the melting of the edges of the ice- sheet, often associated with lacustrine deposits formed in their hollows, and containing lake-shells and terrestrial plants and animals. Boulder-clay, till, or bottom-moraine of the great ice-sheet ; the upper part sometimes rudely stratified, and in some regions separated from the lower part by a series of ' ' middle sands and gravels ; " the lower part quite unstratified and full of transported stones and boulders. Finely laminated clays, sands, layers of peat, and traces of terrestrial surfaces occur at different levels in the boulder clay, and mark " interglacial periods " of milder climate. Polished and striated surfaces of rock, ground down by the move- ment of the ice-sheet. RECENT. The insensible gradation of what is termed the Pleistocene into the Recent series of deposits affords a good illustration of the true relations of the successive geological formations to each other. We can trace this gradual passage because it is so recent that there has not yet been time for those geological revolutions, which in the past have so often removed or concealed the evidence that would otherwise have been available to show that one period or group of forma- tions merged insensibly into that which followed it. The Recent formations are those which have been accumulated since the present general arrangement of land and sea, the present distribution of climate, and the present floras and faunas of the globe were established. They are particularly distinguished by traces of the existence of man. xxvi.] RECENT. 4 6 7 Hence the geological age to which they belong is spoken of as the Hum: n Period. But, as has already been pointed out, there is good evidence that man had already appeared in Europe during Pleistocene time, so that the discovery of human relics does not afford certain evidence that the deposit containing them belongs to the Recent series. Nevertheless, it is in this series that vestiges of man become abundant, and that the proofs of his advancing civilisation are contained. Man differs in one notable respect from the other mammals whose remains occur in a fossil state. Com- paratively seldom are any of his bones discovered as fossils ; but he has left behind him other far more enduring monu- ments of his presence in the form of implements of stone, metal, bone, or horn. These relics are in a sense more valuable than his bones would have been, for while they afford us certain testimony to his existence, they give at the same time some indication of his degree of civilisation and his employments. His handiwork thus comes to possess much geological value ; his stone-hatchets, flint-flakes, bone- needles, and other pieces of workmanship are to be regarded as true fossils, from which much regarding his early history has to be determined. In the river-valleys of the north-west of France and south-east of England human implements have been found in the higher alluvial terraces. After careful exploration, it has been ascertained that these objects have not been buried there subsequently, but must have been covered up at the time the gravel was being formed. The higher terraces are of course the older deposits of the rivers, which have since deepened their valleys until they now flow at a much lower level (p. 51). The excavation of valleys must 468 POST-TERTIARY PERIODS. [CHAP. have been a slow process. Within a human lifetime it is impossible to detect any appreciable lowerir ^ of the ground from this cause. Even during the many centuries of which we have authentic human records, we can hardly anywhere detect proof of such a change. How vast then must have been the interval between the time when the rivers flowed at the level of the upper terraces and the present day ! Other evidence of the great age of these higher alluvia is to be found in the number of extinct animals whose remains are buried in them. The human implements likewise bear their testimony in support of the antiquity of the terraces, for they are extremely rude in design and construction, indicative of a race not yet advanced beyond the early stages of barbarism. In the lower and therefore younger terraces, and in other deposits which may also be regarded as belonging to a later date, the articles of human fabrication exhibit evidence of much higher skill and more tasteful design, whence they have been inferred to be the workman- ship of a subsequent period when men had made consider- able progress in the arts of life. Accordingly, a classification has been adopted, based upon the amount of finish in the stone weapons and implements, the ruder workmanship being assumed to mark the higher antiquity. The older deposits, with coarsely chipped and roughly finished human stone implements, are termed Palaeolithic, and the younger deposits with more artistically finished works in stone, bone, or metal are known as Neolithic. It will be understood that this arrangement is one rather for convenience of description than for a determination of true chronological sequence. It is quite probable, for example, that some of the palaeolithic gravels date back to the Pleistocene Ice Age, while other deposits containing similar weapons and a xxvi.] RECENT. 469 similar assemblage of extinct mammals may belong to a much later time, when the ice had long retreated to the north. It is obvious, too, that we know nothing of the relative progress made in the arts of life by the early races of man. One race may have continued fashioning the palaeolithic type of implement long after another race had already learnt to make use of the neolithic type. Even at the present day we see some barbarous races employing rude weapons of stone not unlike those of the palaeolithic gravels, while others fabricate stone arrow-heads and imple- ments of bone exactly resembling those of the neolithic deposits. It would hardly be incorrect to say that in some respects certain tribes of mankind are still in the palaeolithic or neolithic condition of human progress. i. Palaeolithic. The formations included under this term are distinguished by containing the rudest shapes of human stone implements, associated with the remains of mammals, some of which are entirely extinct, while others have disappeared from the districts where their remains have been found. These deposits may be conveniently classed under the heads of alluvium, brick-earth, cavern-beds, calcareous tufas, and loess: Alluvium. Reference has just been made to the upper river-terraces, which, rising sometimes 80 or 100 feet above the present level of the rivers, belong to a very ancient period in the history of the excavation of the valleys, and yet contain rude human implements. The mammalian bones, found in the sands, loams, and gravels of these terraces, include extinct species of elephant, rhinoceros, hippopotamus, and other animals. The human tools are roughly chipped pieces of flint or other hard stone, and their abundance in 470 POST-TERTIARY PERIODS. [CHAP. some river-gravels has suggested the belief that they were employed when the rivers were frozen over, for breaking FIG. 207. Palaeolithic Implements ; (a] Flint implement, Reculver (^) chipped out of a rounded pebble ; (b) Flint implement () from old river-gravel at Biddenham, Bedford, where remains of cave-bear, reindeer, mammoth, bison, hippopotamus, rhinoceros, and other mammalia have been found ; (f) Bone harpoon-head (^) from the red cave-earth underlying the stalagmite floor of Kent's Cavern (a and b reduced from Mr. Evans' "Ancient Stone Implements"). the ice and other operations connected with fishing. The high river-gravels of the Somme and of the valleys in the south-east of England have been specially prolific in these traces of early man. Brick-earths. On gentle slopes and on plains, the slow drifting action of wind and rain transports the finer particles of soil and accumulates them as a superficial layer of loam or brick-earth. In the south-east of England, con- xxvi.] RECENT. 47 1 siderable tracts of country have been covered with a deposit of this nature. It is still in course of accumulation, but, as already stated (p. 27), its lower parts must date back to a high antiquity, for they contain the bones of extinct mam- mals, together with human implements of palaeolithic type. Cave-earth and stalagmite. The origin of caverns in limestone districts was described in chapter v., and refer- ence was made to the formation of stalagmite on their floors, and to the remarkably perfect preservation of animal remains in and beneath that deposit. Many of these caves were dens tenanted by hyaenas or other beasts of prey (p. 76). Some of them were inhabited by man. In certain cases, they have communicated with the ground above by openings in their roofs, through which the bodies of animals have fallen or been washed by floods. The stalagmite, by cover- ing over the bones left on the floor of the caverns, or in the earth deposited there by water, has preserved them as a singularly interesting record of the life of the time. Calcareous Tufa. Here and there, the incrustation of tufa formed round the outflow of calcareous springs has preserved the remains of the vegetation and of the land- animals of palaeolithic time (compare Fig. 21). Loess. This is the name given to a remarkable accumu- lation of pale yellowish calcareous sandy earth which occurs in some of the larger river valleys of Central Europe, especi- ally in those of the Rhine and the Danube ; it likewise covers vast regions of China, and is found well developed in the basin of the Mississippi. It is unstratified and tolerably compact, so that it presents steep slopes or vertical walls along some parts of the valleys, and can be excavated into chambers and passages. In China subterranean villages have been dug out of it, along the sides of the valleys which 472 POST-TERTIARY PERIODS. [CHAP. it has filled up. It contains remains of terrestrial plants and snail-shells, also occasional bones of land-animals. It bears little or no relation to the levels of the ground, for it crosses over from one valley to another, and even mounts up to heights several thousand feet above the sea and far above the surrounding valleys. Its origin has been the subject of much discussion among geologists and travellers. But the result of much careful investigation bestowed upon it goes to show that the loess is probably a subaerial deposit formed by the long-continued drifting of fine dust by the wind. It was probably accumulated during a comparatively dry period when the climate of Central Europe, after the disappearance of the ice-sheet, resembled that of the steppes of the south-east of Russia. The assemblage of animals whose bones have been found in it closely resembles that of these steppes at the present time ; for it includes species of jerboa, porcupine, wild horses, antelopes, etc. Among its fossils, however, there occur also the bones of the mam- moth, woolly rhinoceros, musk-sheep, hare, wolf, stoat, etc., together with palaeolithic stone implements. Thus the association of animals in the palaeolithic forma- tions shows a commingling of the denizens of warmer and colder climates, like that already noticed as characteristic of the Ice Age, and hence the inference above alluded to that the palaeolithic gravels may themselves be interglacial. Among the animals distinctively of more southern type mention may be made of the lion, hyaena, hippopotamus, lynx, leopard, Caffer cat ; while among the northern forms are the glutton, Arctic fox, reindeer, Alpine hare (Lepus variabilis), Norwegian lemming (Myodes torquatus\ and musk-sheep. The animals which then roamed over Europe, but are now wholly extinct, included the mammoth, woolly xxvi.] RECENT. 473 rhinoceros, and other species of the genus, Irish elk (Mega- ceros hibernicus\ and cave-bear ( Ursus spelceus). The traces FIG. 208. Antler of Reindeer (^) found at Bilney Moor, East Dereham, Norfolk. of man consist almost entirely of pieces of his handiwork ; only rarely are any of his bones to be seen. Besides the rude chipped flints, he has left behind him, on tusks of the mammoth and horns and bones of the reindeer and other animals, preserved in the stalagmite of cavern-floors, vigorous incised outline sketches and carvings representing the species of animals with which he was familiar, and some of which have long died out. He was evidently a hunter and fisher, living in caves and rock -shelters, and pursuing with flint-tipped arrow and javelin the bison, reindeer, horse, 474 POST-TERTIARY PERIODS. [CHAP. mammoth, rhinoceros, cave-bear, and other wild beasts of his time. 2. Neolithic. In this division, the human implements indicate a con- siderable advance in the arts of life, and the remains of the mammoth, rhinoceros, and other prevalent extinct forms of the palaeolithic series are absent. The deposits here included consist of river -gravels, cave -floors, peat- bogs, lake -bottoms, raised beaches, sand-hills, pile-dwell- ings, shell-mounds, and other superficial accumulations in which the traces of human occupation have been preserved. After the extinction of the huge pachyderms, the European fauna assumed the general character which it now presents, but with the presence of at least one animal, the Irish elk, that has since become extinct, and of others, such as the reindeer, elk, wild ox or urus, grizzly bear, brown bear, wolf, wild boar, and beaver, which, though still living, have long been extirpated from many districts wherein they were once plentiful. This local extinction has, no doubt, in many if not in most cases, been the result, directly or indirectly, of human interference. But man not only drove out or annihilated the old native animals. As tribe after tribe of human population migrated into Europe from some region in Asia, they carried with them the animals they had domesticated the hog, horse, sheep, goat, shorthorn, and dog. The remains of these creatures never occur among the palaeolithic deposits; they make their appearance for the first time in the neolithic accumulations, whence the inference has been drawn that they never formed part of the aboriginal fauna of Europe, but were introduced by the human races of the neolithic period.- XXVI.] RECENT. 475 The stone articles of human workmanship found in neolithic deposits consist of polished celts and other FIG. 209. Neolithic Implements ; (a) Stone axe-head (\) ; (/>) Barbed flint arrow-head (natural size) ; (c) Roughly-chipped flint celt ();; (d) Polished celt (|), with part of its original wooden hand still attached, found in a peat-bog, Cumberland ; (e) Bone-needle (natural size), Swiss Lake Dwellings ; a, b> c, d, reduced from Mr. Evans' "Ancient Stone Implements." weapons, hammers, knives, and many other implements of domestic use. Knives, needles, pins, and other objects were made out of bone or horn. There is evidence also that the arts of spinning, weaving, and pottery-making were not unknown. The discovery of several kinds of grain shows that the neolithic folk were also farmers. Vast numbers of these various relics have been found at the pile- dwellings of Switerzland and other countries. For purposes 47 6 POST-TERTIARY PERIODS. [CHAP. of security these people were in the habit of construct- ing their wooden dwellings in lakes on foundations of beams, wattled-work, stones, and earth. Sometimes these erections were apt to be destroyed by fire, as well as to decay by age. And their places were taken by new con- structions of a similar kind built on their site. Hence, as generation after generation lived there, all kinds of articles dropped into the lakes were covered up in the silt that slowly gathered on the bottom. And now, when the lakes are drained, or when their level is lowered by prolonged drought, these accumulated droppings are laid open for the researches of antiquaries and geologists. Many im- portant relics of neolithic man have likewise been obtained from the floors of caverns and rock -shelters places that from their convenience would continue to be used as in palaeolithic time. Interesting evidence, also, of the succes- sive stages of civilisation reached by early man in Europe, is supplied by the older Danish peat-bogs, in the lower parts of which remains of the Scotch fir (Ptmis sylvestris\ a tree that had become extinct in that country before the historic period, are associated with neolithic implements. In a higher layer of the peat, trunks of common oak are found, together with bronze implements, while in the upper- most portion, the beech-tree and iron weapons take their place. Between the neolithic and the present period no line can be drawn. They shade insensibly into each other, and the materials from which the history of their geographical and climatal vicissitudes, their changes of fauna and flora, and their human migrations and development, form a common ground for the labours of the archaeologist, the historian, and the geologist. xxvi.] RECENT. 477 During the Recent period the same agencies have been and are at work as those which have been in progress during the vast succession of previous periods. In the foregoing pages, we have followed in brief outline each of these great periods, and after this survey we are led back again to the world of to-day with which the first chapters of this book began. In this circle of observation no trace can anywhere be detected of a break in the continuity of the evolution through which our globe has passed. Everywhere in the rocks beneath our feet, as on the surface of the earth, we see proofs of the operations of the same laws and the working of the same processes. Such, however, have been the disturbances of the ter- restrial crust that, although undoubtedly there has been no general interruption of the Geological Record, local interrup- tions have almost everywhere taken place. The sea-floor of one period has been raised into the dry land of another, and again, the dry land, with its chronicles of river and lake, has been submerged beneath the sea. Each hill and ridge thus comes to possess its own special history, which it will readily reveal if questioned in the right way. We are surrounded with monuments of the geological past. But these monuments are being slowly destroyed by the very same processes to which they owed their origin. Air, rain, frost, springs, rivers, glaciers, waves, and all the other connected agents of demolition are ceaselessly at work wherever land rises above the sea. It is in the course of this demolition that the characteristic features of the scenery of the land are carved out. The higher and harder parts are left as mountains and hills, the softer parts are hollowed out into valleys, and the materials worn away from them are strewn over plains. And as it is now, so doubtless has it POST-TERTIARY PERIODS. [CHAP. xxvi. been through the long ages of geological history. Decay and renovation in never-ending cycles have followed each other since the beginning of time. But amid these cycles there has been a marvellous up- ward progress of organic being. It is undoubtedly the greatest triumph of geological science to have demonstrated that the present plants and animals of the globe were not the first inhabitants of the earth, but that they have appeared only as the descendants of a vast ancestry, the latest comers in a majestic procession which has been marching through an unknown series of ages. At the head of this procession we ourselves stand, heirs of all the progress of the past and moving forward into the future wherein progress towards something higher and nobler must still be for us, as it has been for all creation, the guiding law. APPENDIX. THE VEGETABLE KINGDOM. I. CRYPTOGAMS OR FLOWERLESS PLANTS. 1 THESE bear spores that differ from true seeds in consisting only of one or more cells without an embryo. They include the following classes : Algae Fungi. These embrace the smallest and simplest forms of vegetation fresh- water confervae, desmidiae, mushrooms, lichens, sea-weeds, etc. Some of them secrete carbonate of lime and form a stony crust, as in the case of the marine nu Hi pores (p. 112), others secrete silica, as in the frustules of diatoms (p. in). These hard parts are most likely to occur as fossils ; but impressions of some of the larger kinds of sea-weeds may be left in soft mud or sand (p. 319). Fungi are not well adapted for preservation, but traces of them have been noticed even in rocks of the Carboniferous period. Characese are fresh-water plants, some of which abstract carbonate of lime from the water and deposit it as an incrustation on their surface. Hence their calcified nucules or spiral seed-like bodies [gyrogonites] and stems may accumulate at the bottom of lakes. Muscinese, mosses, and liverworts afford little facility for fossilisa- tion. But some of the mosses (sphagnum, etc. ) form beds of peat (p. 109). Filices, ferns, bearing fronds on which are placed the sporangia or spore-cases. Many of them possess a tough tissue which can for some time resist decomposition. Traces of ferns are conse- quently abundant among the fossiliferous rocks. 1 Names placed within square brackets ([ ]) are fossil forms. 480 APPENDIX. Ophioglossacese, adder's tongues and moonworts. Rhizocarpese, pepperworts. Equisetaceae, horse-tails, with hollow striated siliceous jointed stems or shoots. These stems possess considerable durability, and where buried in mud or marl may retain their forms for an indefinite period. Allied plants [Catamites] have been abun- dantly preserved among some of the older geological formations (Old Red Sandstone, Carboniferous, Permian). Lycopodiacese, club-mosses, plants with leafy branches like mosses, growing in favourable conditions into tree-like shrubs that might be mistaken for conifers. Their dichotomous stems and their fertile branches, which resemble cones and bear spore-cases, offer themselves for ready preservation as fossils. The spores are highly inflammable, and it is worthy of notice that similar spores have been detected in enormous abundance in the Carboniferous system. Lycopodiiim and Selaginella are familiar giving genera. (For extinct forms see p. 355.) II. PHANEROGAMS OR FLOWERING PLANTS. i. GYMNOSPERMS or plants with naked seeds ; that is, seeds not enclosed in an ovary. Cycadese, small plants resembling both palms and tree-ferns. The pinnate leaves are hard and leathery, and have been frequently preserved as fossils. Cycas and Zamia are two typical genera (see pp. 382, 390). Coniferse, the Pine family. The stiff hard leaves and the hard seed-cones may be looked for in the fossil state. The resinous wood also sometimes long resists decomposition and may be gradually petrified. Trunks of pine are often met with in peat- mosses. The Coniferae have been subdivided into the following families : 1. Cupressineae, cypresses, including Juniperus (Juniper), Libocedrus, Thuja, Thujopsis^ Cupressus, Taxodium, Glyptostrobus, 2. Abie tine se, pines and firs, including Pinus, Abies, Cedrus, Araucaria (p. 390), Dammara, Cunninghamia, Sequoia. 3. Podocarpeae, trees growing in New Zealand, Java, China, Japan, etc., bearing a succulent fruit or a thick fleshy stalk. 4. Taxinese, yews, plants with fleshy fruit, including the genera Taxus, Salisburia, Phyllocladus. APPENDIX. 481 Gnetaceae, joint-firs, small trees or shrubs with jointed stems (Gneltim, Ephedra, Welwitschia). ii. ANGIOSPERMS, or plants bearing their seed within an ovary. They are subdivided into two great classes the Monocotyledons or Endogens, and the Dicotyledons or Exogens. Monocotyledons, so called from their having only one cotyledon or seed-lobe. They are also known as " Endogens," from the fact that they chiefly increase in diameter by growth in the interior, whereby the exterior layers are pushed outwards. Their seeds are usually enclosed in strong sheaths or shells, of which the cocoanut is a striking example. The following are some of the families : Lemnacese (duck-weeds)j Potamogetoneae (pond-weeds) ; Pandanacese (screw-pines) ; Palmaceae (palms) ; Typhaceae (typhads, marshy plants) ; Cyperaceae (sedges) ; Graminese (grasses) ; Juncacese (rushes) ; Liliacese (lilies) ; Irideae (irises) ; Dioscoreaceae (yams) ; Taccaceae (tacca) ; Musa- ceae (plantains and bananas) ; Zingiberaceae (gingerworts) ; Orchideae (orchids). Dicotyledons or plants that have two cotyledons or seed-lobes ; also called " Exogens," because their stems increase by successive layers added to the exterior. This division includes the most highly organised members of the vegetable kingdom. Our common flowers and hardwood trees belong to it. The sections, orders, and families into which it has been partitioned are so numerous that only some of the more interesting or important to the geologist can be inserted here. It is chiefly the leaves and seeds that occur in the fossil condition and furnish means of recognising the plants. Urticaceae (nettles) ; Platanaceoe (planes) ; Cannabineae (hemps) ; Ulmaceae (elms) ; Betulaceae (birches) ; Nelum- biaceae (lotus plants) ; Nymphaeaceae (water-lilies) ; Ranun- culaceae (crowfoots) ; Anonaceae (custard-apples) ; Berberi- deae (barberries) ; Laurineae (laurels) ; Myristicaceae (nut- megs) ; Papaveraceae (poppies) ; Fumariaceae (fumitories) ; Cruciferae (plants with cross-shaped flowers, such as wall- flower, Brassica, which is the original genus from which our cultivated cabbage, cauliflower, broccoli, and turnip are derived, Sinapis or mustard, cress, radish, etc.); Convol- vulaceae (bindweeds) ; Solanacese (nightshades, potato) ; Bignoniaceae (trumpet flowers) ; Plantagineae (ribworts or 2 I 482 APPENDIX. plantains) ; Labiatse (plants with labiate flowers, such as mint, sage, lavender) ; Oleaceae (olives) ; Jasminiacese (jas- mines) ; Gentianacese (gentians) ; Valerianacere (valerians) ; Cucurbitacese (cucumbers and gourds) ; Campanulaceae (bell- flowers) ; Compositse (plants with compound flowers) ; Primulacese (primroses) ; Ericaceae (heaths) ; Rhamnaceoe (buckthorns) ; Sapindaceas (soap-trees) ; Balsaminese (bal- sam-tribe) ; Geraniaceoe (cranesbills, geraniums) ; Euphor- biaceos (spurge tribe) ; Araliacese (ivy tribe) ; Cornaceae (dog- wood tribe) ; Saxifragacese (saxifrage tribe) ; Proteaceae (found principally in Australia and Cape of Good Hope) ; Papilionaceae (plants bearing flowers like those of the pea, bean, clover, etc. ) ; Pomeae (apple tribe) ; Rosaceae (rose tribe) ; Amygdalese (almond tribe) ; Myrtaceae (myrtle tribe) ; Cactaceae (Indian figs, cactus tribe) ; Myricacece (galewort tribe) ; Juglandeae (walnut tribe). THE ANIMAL KINGDOM. I. INVERTEBRATES. I. PROTOZOA. Animals simple in structure and usually minute in size, with bodies composed of a structureless jelly-like substance (sar- code) which, in some cases, secretes siliceous or calcareous needles, or shells which serve to protect them. It is only these hard parts which have any chance of being preserved as fossils. CLASS i. RHIZOPODS, having generally a calcareous shell or silice- ous skeleton ; divided into the following three orders : Foraminifera having usually a calcareous shell pierced with fine pores through which slender thread-like processes pro- trude from the jelly-like body. These minute creatures live in enormous abundance in various parts of the ocean, and their shells gather as a deposit of ooze at the bottom (Globigerina, Lagena, Nummulina}. Their remains are also found in the geological formations, sometimes constitut- ing masses of limestone (Figs. 146, 195). Heliozoa fresh-water forms sometimes with a radial siliceous skeleton (Acanthocystis, Clathrulina}. Radiolaria marine creatures with radial siliceous skeleton, which usually consists of small siliceous needles or spicules united together. They occur in vast numbers on some parts APPENDIX. 483 of the sea-floor, where their remains form a siliceous ooze (Thalassicolla, Polycistina, p. 112). CLASS ii. INFUSORIA protozoa living chiefly in fresh water, and having a definite form enclosed within an external membrane, and usually with a mouth and anus. From their perishable nature these animals are not met with in a fossil state. II. SPONGIDA (sponges), chiefly marine forms possessing an internal skeleton of horny fibres or of calcareous or siliceous spicules. The horny sponges are illustrated by the common sponge of domestic use which is the skeleton of a Mediterranean genus, composed of a close network of horny fibres. Such forms are too perishable to be looked for as fossils. The siliceous sponges secrete minute siliceous spicules which are dis- persed in a network of sponge-fibres, sometimes in a glassy framework of six-rayed spicules (Hexactinellida). The calcareous sponges, as their name implies, secrete carbonate of lime as the substance of which their spicules consist (see Fig. 186). III. CCELENTERATA (zoophytes), radially symmetrical animals with a body composed of cells arranged in an outer and an inner layer enclosing a body-cavity. Hydrozoa, including the fresh-water Hydra, and the marine jelly- fishes, millepores, C amp amd aria, Sertularia, etc. Most of these animals offer little facility for preservation as fossils ; but some of them possess horny or calcareous structures which have been preserved in sedimentary deposits. Among these an extinct type of Hydrozoa, known as Graptolites, occurs abundantly in some of the older parts of the Geological Record (p. 322). Ctenophora spherical or cylindrical Medusae, including the Venus Girdle of the Mediterranean, and the Beroe of Northern waters. Actinozoa (corals), Polypes having a cavity in the body divided by vertical partitions into a number of compartments. The common Actinia or sea-anemone is an example ; but it is an exception to the general rule that the internal parts are strengthened by a secretion of carbonate of lime. It is this calcareous skeleton which forms the familiar part of corals. Rugosa (Tetracoralla), the older forms of coral in which the calcareous partitions are arranged in multiples of four with transverse partitions \Zaphrentis, Cyathophyllum, Am- plexitS) etc., Figs. 119, 137, 147], Alcyonaria (Octocoralla), including Alcyoma, Pennat^da, and Gorgonia, animals with eight-plumed tentacles and cal- 484 APPENDIX. careous bodies (sclerodermites) which form the foundation of a calcareous or horny skeleton (Fig. 120). Zoantharia (Hexacoralla), including the more modern forms of corals, wherein the tentacles are either six or some mul- tiple of six. Among the families comprised in this Order are the soft -bodied Actinidcc, Turbinolidcc, Oculinidcc, AstrcBidce or star -corals, Ftmgidce or mushroom corals, Madreporida (Fig. 172). IV. ECHINODERMATA animals possessing usually a symmetrical fivefold grouping of parts, and enclosed in a skin which is strengthened by hard calcareous granules, spicules, or close-fitting plates. Crinoidea globular or cup-shaped, with jointed arms, and usually fixed by a jointed calcareous stalk. Most of the Crinoids are now extinct. Among the living forms are Pentacrinus, Rhizo- crtmts, Bathycrinus, and Comatula (see Figs. 149, 165, 173). Allied to the Crinoids are the extinct Cystideans (Fig. 121) and the Blastoids (Fig. 150), found in Palaeozoic formations. Asteroidea (star-fishes). The parts of these animals that may be most readily preserved as fossils are the calcareous plates which run along the five rays of the star. These have been found in the marine deposits of many geological periods (Fig. 122). In the Brittle Stars (Ophiuroidea) the arms are flexible, cylindri- cal, and quite sharply marked off from the central disc. Echinoidea (sea-urchins), spherical, heart-shaped, or disc-shaped, with a usually immovable skeleton of calcareous plates which encloses the body like a shell and bears calcareous movable spines. They comprise the regular echinoids (Cidaris, Echinus, etc., Fig. 174) and the irregular echinoids either compressed into the form of a shield ( Clypeaster] or of a heart-shape (Spatangus, Fig. 187). Holothuroidea worm -like elongated animals, with a leathery body in which the calcareous secretion is confined to isolated particles, scales, or spicules. These calcareous bodies have been found abundantly in the Carboniferous system, and are the only evidence of the existence of this division of the echino- derms at so ancient a period. V. VERMES, comprising the various forms of worms. Compara- tively few of these animals occur in the fossil state. Many of them are worms or flukes living in the intestines or other parts of the body. The only important class to the student of geological history are the anne- lides or segmented worms. APPENDIX. 485 Errantia, free-swimming predaceous sea-worms. The only hard parts of these creatures capable of surviving as fossils are the horny jaws which have been met with in some numbers even in ancient geological formations. But as many of the species live in and crawl over mud they leave behind them in their burrows and trails evidence of their presence. Such markings remain abundantly in many ancient rocks (p. 327). Tubicolae sedentary worms, living within tubes within which they can withdraw for protection. This tube may remain as the only permanent relic of their existence. Sometimes it is a leathery substance ; in other cases it consists of grains of sand or other particles cemented by a glutinous secretion, or of solid carbonate of lime. The most familiar example of this Order is the Serpula which may so frequently be seen encrusting dead shells thrown up upon the beach. Oligochseta earth-worms and aquatic worms are not found as fossils. But the common earth-worm is as an important agent in mixing the soil and bringing up its fine particles within reach of rain and wind (p. 24). VI. ARTHROPODA ( Articulata), these differ from the worms in having jointed appendages attached to the body, which serve as organs of loco- motion. They possess a hard chitinous skin which usually becomes hardened by the deposit of calcareous matter. The articulate animals are divided into four great classes as follows : i. Crustacea chiefly aquatic forms with two pairs of antennae and numerous paired legs. They include the Phyllopods, remark- able for their compressed bivalve shell which is frequently found in the fossil state (Fig. 126) ; the Ostracods, small forms enclosed in a bivalve shell, and with seven pairs of appendages, the minute shells being abundant in the fossil state (Cypris) ; the Cirripedes or barnacles, so commonly seen encrusting shore rocks ; the Amphipods ; the Isopods ; the Decapods, which are either macrurous (long-tailed), as in prawns and lobsters (Fig. 178), or brachyurous (short-tailed), as in sea-crabs and land-crabs. A remarkable group of extinct Crustaceans is comprised in the Order Eurypterida (Fig. 135). The Xiphosura are still found living in the form of Limulus or King-crab ; but they date back to the Carboniferous period. The earliest forms of Crustaceans belong to another extinct Order, the Trilobites (Figs. 124, 125, 136, ISO- ii. Arachnida air-breathing arthropods, with two pairs of jaws 486 APPENDIX. ' and four pairs of ambulatory legs, including mites, spiders, and scorpions. Some of these animals (scorpions) have chitinous integuments which resist decomposition, and have been abun- dantly preserved in the rocks (pp. 283, 334). iii. Myriapoda, including the chilopods or centipedes, feeding entirely on animals which they bite and kill with their poisonous secretion ; and the chilognaths or millipedes and galley-worms which live in damp places and feed on vegetable and dead animal matters. iv. Insecta. Among the orders of insects of most interest in geological history are Orthoptera, with two usually unequal pairs of wings (earwigs, cockroaches, praying-insect, grasshoppers, locusts, crickets, book-lice, termites or white ants, ephemeridae or may-flies, dragon-flies). Neuroptera, insects with wings in which the nervures form a network (Corydalis, camel-neck flies, ant-lions, phryganidos or spring-flies). Hemiptera, including lice, cochineal insect, plant-lice, cicadas, bugs, water-bugs, water-scorpions. Diptera, with large glassy front wings, including the various kinds of flies, such as the house-fly, dung-fly, gad-fly, gnat, gall-fly, and flea. Lepidoptera, butterflies and moths. Coleoptera, beetles, the most durable parts of which are the horny wing-covers (elytra), so often to be found in woods and peat-mosses. They include lady-birds, stag-beetles, tiger-beetles, etc. Hymenoptera, with four membranous wings, having few nervures, comprising ants, wasps, bees. VII. MOLLUSCOIDA under this division may be grouped the Tunicaries, Polyzoa, and Brachiopoda. i. Tunicata, sea-squirts simple or compound, fixed or free organisms which have been named from the leathery integument within which they are enclosed. Though some of them abstract carbonate of lime from sea- water, they present no hard parts for fossilisation, and the class is not known in the fossil state. ii. Polyzoa (Bryozoa), sea-mats and sea-mosses composite animals, each enclosed in a horny or calcareous case, and united into colonies which are generally attached to some foreign body and often resemble plants in outer form. The calcareous colonies APPENDIX. 487 form durable objects which have been abundantly preserved as fossils. Polyzoa are met with among the oldest fossiliferous formations and still abound in the present sea. The common lace-like Flustra, so frequently to be seen encrusting the fronds of sea-weeds or dead shells, is a familiar example of them. Among the fossil forms (many of which have long been extinct) some of the most important genera axeFenestella ("lace-coral," Fig. 152), Polypora, Retepora, Glauconome, Hippothoa, Heteropora, Fasci- cularia. iii. Brachiopoda, lamp-shells molluscous animals, having bivalve, calcareous, or horny shells, one valve placed on the back, the other on the front of each individual, and taking their name from two long ciliated arms which proceed from the sides of the mouth and create the currents that bring their food. They are grouped in two Orders : (i) The Inarticulata, in which the two valves are not united along the hinge line (Lingula^ Fig. 127, Discina, Crania) ; and (2) the Articulata, in which the two valves are hinged together with teeth ( Terebratula, Rhynchonella^ Figs. 127, 138, 153, 160, 202). The brachiopods attained their chief development during the earlier periods of geological time, and are now represented by comparatively few living forms. The shells are equal sided, but the ventral is usually larger than the dorsal valve, and is prolonged into a prominent beak, by which it fixes itself, or through which the pedicle passes whereby it is attached to the sea-floor. The following are characteristic genera : Terebratula (still living), Stringocephalus (Devonian), Thecidiwn (Trias to present time), Spirifera (chiefly Palaeozoic), Atrypa (Palaeozoic), Rhynchonella (Lower Silurian to present time), Pentamerus (Silurian), Orthis, Strophomena, Productus (Palaeozoic), Leptcena (Palaeozoic to Lias), Crania, Discina, Lingula (from early Palaeozoic to present time). VIII. MOLLUSCA animals, with soft bodies, enclosed in a muscular envelope which is usually covered with a strong calcareous shell. These hard shells are durable objects, and when covered up in sediment remain for an indefinite period as evidence of the existence of the animals to which they belonged. The great abundance of the mollusca also in the sea and in terrestrial waters gives a peculiar value to their remains. They (with the Brachiopoda) furnish by far the most valuable data to the geologist for the identification and comparison of marine sedimentary deposits of all ages. They are divided into the following classes : 488 APPENDIX. i. Lamellibranchiata ordinary bivalves like the cockle, mussel, and oyster, in which the valves are placed on the right and left sides of the body. The following are the more important families : Ostreidse, oysters including among other genera Ostrea (Fig. 197), Anomia, Pec ten (Figs. 166, 204), Lima, Plicatula ( Fig. 175) [Gryphtza, Fig. 175, Exogyra, Aviculopeclen, Fig. 154], Aviculidse, wing-shells Avicula (Fig. 166) [Ptsubftemya, Bakevellia, Fig. 161, Gervillia\ Perna [Inoceramus, Fig. 1 88], Pinna. Mytilidse, mussels Mytilus (mussel), Modiola (horse-mussel), Lithodomus, Dreissena [Orthonota, Fig. 128]. Arcadae, including among other genera Area, Cticullaza (Fig. 139), Pectunculus, Nttcula (Fig. 188), Leda (Fig. 204). Trigoniadae Trigonia (Figs. 175, 188) [Myophoria, Fig. 166, Schizodus, Fig. 161, Aximts\. Unionidee Unio (river-mussel), Anodon \_Anthracosid\. Chamidse Chama [Diceras, Requienid\. [Hippuritidse, Rudistes Hippurites, Radiolites, Caprina, Caprotina, all confined to the Cretaceous system, Fig. 189]. Tridacnidas Tridacna, Cardiadse Cardium (cockle), Fig. 166. Lucinidas Ludna, Corbis. Cycladidse Cyclas, Cyrena, fluviatile and estuarine shells. Cyprinidse Cyprina, Astarte, Isocardia, Cypricardia \_Mega- lodon, Cardinia\ Cardita. Veneridse Venus, Cytherea, Artemis, Tapes. Mactridse Mactra, Lutraria. Tellinidae Tellina (Fig. 204), Psammobia, Sanguinolaria, Donax. Solenidse Solen (razor-shell). Myacidae Mya (gaper), Corbula (Fig. 197), Panopaa (Fig. 202), Glycimeris. Anatinidae Anatina, Thracia, Pholadomya (almost now extinct) \_Myacites, Edmondia, Fig. 154]. Gastrochaenidse Saxicava (Fig. 204). Pholadidse Pholas, Xylophaga, Teredo. ii. Gasteropoda or snails are named from the way in which they creep on the broad foot-like expansion of the lower part of the body. They are almost all protected by a univalve shell which is usually spiral. APPENDIX. 489 The carnivorous gasteropods possess a respiratory siphon and are all marine. Among them are the genera Strombus, Rostel- laria, Murex, Pyrula, Fusus, Buccinum (whelk), Nassa (dog- whelk), Purpura (Fig. 202), Cassidaria, Oliva (Fig. 194), Comis, Pleurotoma, Valuta (Fig. 194), Mitra, Cyprcea (cowry). There is another group possessing no respiratory siphon, which mostly live on plants, among their more prominent genera are Natica (Fig. 204), Chemmtzia \_Loxonema\, Cerithium (Fig. 194), Potamides, Nerincza, Aporrhais, Melania, 7^urri- tella, Scalaria, Littorina (periwinkle), Rissoa, Paludina (Fig. 197), Nerita, Neritina, Turbo, Trochus [Euomphalus, Fig. 155]? Haliotis\_Plenrotomaria, Murchisonid}, Fissurella, Calyp- trcea, Pileopsis, Patella (rock-limpet), Dentaliiim, Chiton. The pulmoniferous or air-breathing gasteropods include the land-snails, and have a broad foot and usually a spiral shell. The following are among the more important genera : Helix, Vilrina, Succinea, Biditmis, Pupa, Clausilia, L,imax (slug), Limncea (pond-snail), Ancylus (river-limpet), Planorbis, Cyc- lostoma, Cyclophorus. The sea-slugs possess either no shell or one so small and thin as to be wholly or partially concealed by the animal, and therefore unlikely to be preserved in a fossil state. Some, however, occur as fossils, particularly the genera Tornatella, Bulla, and Cylichna. The heteropod gasteropods are animals inhabiting the open sea, in which they are fitted to swim by a peculiar expansion of the foot into a fin-like tail or a fan-shaped ventral fin. Their more important living genera are Firola, Carinaria, Atlanta, while of extinct genera Bellerophon (Figs. 129, 155), Machirea, and Ophileta may be mentioned. iii. Pteropoda a small group of molluscs swarming in the open sea, in which they swim by means of two wing-like fins proceed- ing from the sides of the mouth. They are all small, but extinct forms of much larger size are found fossil in rocks of all ages, even in some of the most ancient. The shell when present in the living forms is glassy and translucent ; but in some of the fossil genera it is thicker. The living genera Hyalea and Cleodora occur also fossil among the Tertiary rocks. The more important fossil genera are Theca, Pterotheca, Hyolithes, Tentactdites, and Conularia (Fig. 156), all of which occur in the Palaeozoic forma- tions. 49 APPENDIX. iv. Cephalopoda the cuttle-fishes, squids, and pearly nautilus are the highest division of the mollusca, being distinguished by the long muscular arms placed round their mouth, and the plume-like gills by which they breathe. The living forms are nearly all desti- tute of an outer shell, which, however, is possessed by the pearly nautilus and argonaut or paper nautilus. Some of them have a horny or calcareous internal bone (cuttle-bone). They also possess powerful horny or partly calcareous jaws like a parrot's beak. It is only these hard parts that can be expected to occur as fossils. In former periods the cephalopods were enormously more abundant than they are now, and as most of them possessed an outer shell their remains have been abundantly preserved among the rocks. During the earlier ages of geological history, they appear to have been the magnates of the sea. According to the number of their breathing gills, cephalo- pods are grouped into Dibranchiate or two-gilled, and Tetra- branchiate or four-gilled. The Dibranchiate forms now living include the Argo- nauta or paper-sailor, the Octopus, the calamaries or squids (Loligo, Sepiola, Sepioteuthis, etc.), Sepia, and Spirula. Some of these occur also in the fossil state, but the family of the Belemnites (Fig. 177), so abundant in Jurassic and Cretaceous time, died out at the close of Secondary time. The Tetrabranchiate genera are protected by an ex- ternal chambered shell with siphuncle. They attained their chief development in Palaeozoic and Mesozoic time, and are now almost extinct. The shell in the fossil forms is some- times quite straight (Orthoceras), and from this simplest form successive stages of curvature may be observed till it becomes a flat coil (Ammonites). The more important fossil genera are Nautilus (Fig. 167), the only living genus, found also fossil as far back as early Palaeozoic deposits, Lituites (Fig. 130), Clymenia (Fig. 139), Orthoceras (Figs. 130, 157), Phragmoceras, Cyrtoceras, Ammonites (Fig. 176), Goniatites (Fig. 157), Ceratites (Fig. 167), Crioceras, Toxoceras, Ancy- loceras, Scaphites, Turrilites, Hamites, Baculites. (For some of the leading varieties of type, see Fig. 190.) VERTEBRATA. PISCES, fishes. The parts most likely to be preserved in a fossil state are the bones of the skeleton, especially the teeth ; also bony APPENDIX. 491 scales and external plates and spines. Those types of fishes which possess these hard parts have accordingly been abundantly preserved among the stratified rocks of the earth's crust. The following are the four sub-classes into which the great class of fishes has been divided : i. Leptocardii animals possessing neither brain nor heart, ribs nor jaws, and so lowly an organisation that their claim to be ranked among the vertebrata is disputed. ii. Cyclostomata animals with a cartilaginous skeleton, the skull not separate from the body and no real jaws or ribs. Some of them have horny denticles on the mouth, and these are the only hard parts that offer any facilities for fossilisation. The living Lamprey and Hag-fish are examples. Certain tooth-like bodies called " Conodonts," which occur in Silurian rocks, have been supposed to be teeth of Cyclostomes. iii. Teleostei embrace the vast majority of the living fishes of the present day. They possess a bony skeleton, and hence are often spoken of as the osseous fishes. The vertebrae are usually biconcave, each face showing a deep conical hollow. Most of them possess teeth which are usually isolated and pointed. They are for the most part covered with overlapping horny scales, but sometimes they have dermal plates of true bone, or are encased in a calcareous cuirass. As examples may be cited the perch, mullet, bream, sword-fish, John Dory, sun-fish, mackerel, tunny, lump-sucker, goby, blenny, stickleback, wrass, cod, whiting, haddock, hake, ling, flat-fishes, carp, pike, salmon, trout, herring, pilchard, eel. iv. Palseichthyes including Elasmobranchs, Chimaeroids, and Ganoids. 1. Elasmobranchs, with a cartilaginous skeleton and a skin which may be naked and is never covered with scales as in the osseous fishes, but may bear small prominences which harden by the secretion of carbonate of lime and become tooth-like, or where small and close -set form shagreen. These calcareous portions sometimes form dermal plates or tubercules, and also spines which commonly rise in front of the dorsal fins. It is these dermal defences which are so common in the fossil state under the name of Ichthyodorulites (Fig. 158). The Elasmobranchs include the Sharks and Rays. 2. Chimaeroids, represented in the living Chimaera and by the fossil Rhynchodus (Devonian), Ischiod^ls (Mesozoic), Eda- phodon (Cretaceous and Eocene). 49 2 APPENDIX. 3. Ganoids these fishes have a cartilaginous or ossified skeleton. They are usually covered with bony plates or scales. At present this order is almost extinct, only a few forms having survived. But in former times it embraced by far the largest part of the vertebrate life of the globe, and from the durability of the external bony plates and scales the remains of the extinct genera have been plentifully pre- served. Eight sub-orders have been recognised, viz. (i) Placoderms, entirely extinct, but well represented by Scaph- aspis, Cephalaspis, Coccosteus, and other Palaeozoic genera (Fig. 134) ; (2) Acanthodians, also extinct, Acanthodes (Fig. 133), Cheiracanthns ; (3) Dipnoi, represented by the living Lepidosiren of the Amazon and Ceratodus of Queensland rivers ; and by Dipterus, and Phaneropleuron of the Old Red Sandstone ; (4) Chondrosteans, of which the living sturgeon is a type, and which is represented in the fossil state by Palaoniscus (Permian) and Chondrosterts (Lias) ; (5) Poly- pteroideans, represented by the modern Polypterus of the Nile, and by many extinct genera, as the Palaeozoic Diplo- pterus, Megalichthys, Osteolepis (Fig. 341), Calacanthtts, Holoptychius, Strepsodtis, etc. ; (6) Pycnodontoideans, en- tirely extinct, represented among the Mesozoic formations by Pleurolepis, Gyrodus, Pycnodus, and other genera ; (7) Lepidosteans, of which the living lepidosteus or "gar-pike " of North America is the type. Large numbers of extinct genera have been met with in Palaeozoic and Mesozoic rocks. Among these are Pholidophorus (Fig. 1 79), Lepidotus, Cheirolepis, Amblypterus, Eurynottis, Platysomus (Fig. 162); (8) Amiodians, represented by the living Amia, a mud-fish of the fresh waters of the United States, and by several Meso- zoic and Tertiary extinct genera, as Caturus^ Leptolepis. AMPHIBIA newts, frogs, salamanders, etc., are divisible into four orders as follows : i. Urodela or tailed amphibians animals with elongated bodies and relatively short limbs, devoid of scales or pectoral plates. They comprise the living newts ( Triton], salamanders, and mud- eels (Siren). Traces of forms supposed to be allied to some of these animals have been met with in Permian rocks, but it is only in Tertiary strata that undoubted remains of Urodela have been found, ii. Anura or tailless amphibians animals with relatively short and APPENDIX. 493 broad bodies and two pairs of limbs of which the hinder are longer and stronger. Though there are no scales nor pectoral plates, portions of the skin of the back are in some cases ossified. They are typified by the frogs and toads. They are only found fossil in younger Tertiary deposits, iii. Peromela or snake-like amphibians animals with serpentiform bodies without limbs. They are not known as fossils, iv. Labyrinthodonta animals now entirely extinct which pos- sessed bodies somewhat like those of the salamanders, with rela- tively weak limbs and long tail. The head was defended by hard plates of bone, the breast by three sculptured bony plates, and the lower side of the body by an armour of oval plates or scales. The feet appear to have been five-toed. The footprints of these creatures were first found ; but more or less perfect skeletons of them have since been obtained. Their name is taken from the labyrinthine structure of their large teeth. The earliest known Labyrinthodonts are from the Carboniferous system ; they formed the magnates of the world until they were supplanted in early Mesozoic time by the great development of Reptilians (Fig. 163). REPTILIA or true reptiles, with horny scales or bony scutes, are represented now by turtles, tortoises, snakes, lizards, and crocodiles, but flourished formerly in many remarkable forms which have long been extinct. Embracing all the known living and extinct types in one view we may group them into the following orders : i. Chelonia the tortoises and turtles, distinguished for the most part by the bony case or box in which the body is enveloped. As many of these animals are of aquatic habits their hard parts must often be covered up and preserved in sedimentary deposits. They are not uncommon in the fossil state, as far back as the Jurassic series. ii. Ophidia snakes and serpents, covered with horny scales, and remarkable for the number of their vertebrae (which, in some pythons, amount to more than 400), and for the want of limbs. They are not known fossil except in the Tertiary formations, iii. Lacertilia lizards, chameleons. The oldest forms occur in the Permian system {Protorosaurus} ; in the Triassic period lived the Rhyncosaurus, Hyperodapedon, and Telerpeton (Fig. 168) ; in the Cretaceous, the long-necked Dolichosaurus and the gigantic Mosasaurus. iv. Crocodilia the crocodiles, alligators, and gavials form the highest type of living reptiles. The earliest trace of them in a 494 APPENDIX. fossil state is in the Stagonolepis of the Trias (Fig. 169). They abounded in the Jurassic seas, the genera Teleosaicrus and Steneo- saurtts being conspicuous, while in Cretaceous time the Gonio- pholis abounded. None of the modern crocodiles, however, are truly marine. The following orders of reptiles are now wholly extinct : v. Ichthyosauria animals somewhat resembling whales in shape, the head being joined to the body with no distinct neck and the body tapering away behind. It appears to have been covered merely with skin and moved through the water by means of two pairs of paddles. In the huge head the most conspicuous feature was the large eye-orbits filled with a circle of bony plates that remain often well preserved. There is only one known genus, Ichthyosaurus, abundant in the Lias (Fig. 180). vi. Plesiosauria distinguished for the most part by the dispro- portionate length of the neck and the smallness of the head. Like the ichthyosaurs, the plesiosaurs appear to have had no bony covering upon their skin ; they had two pairs of paddles, those behind being largest, and a comparatively short tail. The earliest plesiosaurus are found in Triassic rocks (Nothosaurus, Simosaurus, Pistosaurus], but they are most characteristic of the Jurassic formations (Plesiosatirus, Pliosaurus}. vii. Dicynodontia lizard-like animals with crocodilian vertebrae and tortoise-like jaws which were probably cased in a horny beak. They have been found in certain supposed Triassic strata in Scotland, South Africa, and India (Dicynodon, Oudmodon). viii. Pterosauria, flying reptiles or Pterodactyls distinguished by the length of their heads and necks, and the proportionately great size of their fore-limbs, on which the outer ringer was enormously elongated to support a wing-like membrane. These animals no doubt flew from tree to tree and hopped or shuffled along the ground. They appear to have been confined to the Mesozoic periods. The important genera are Pterodactylus (Fig. 181), RhamphorhynchuS) Dimorphodon, and Pteranodon. ix. Deinosauria a remarkable group of animals, mostly of enor- mous size, which presented structures linking them with birds. Some of them had a covering only of naked skin, others pos- sessed an armour of bony plates like those of the crocodile. The hind-limbs are usually enormously developed in comparison with the fore-limbs, showing that the animals probably walked on their hind feet. The deinosaurs abounded during the Mesozoic APPENDIX. 495 ages and in many diverse types. Some of the more important genera are Iguanodon (Fig. 192), Hylaosaurus^ Cetiosattrus, Megalosaurns, and Compsognathus. The largest animal yet known, the Atlanta >saurus, has been found in the Jurassic rocks of North America (p. 399). x. Thecodontia a group of carnivorous reptiles, remarkable for possessing teeth which have been classed by Professor Owen as incisor, canine, and molar. Their remains have been found in supposed Triassic rocks in South Africa. AVES, birds. This important section of the animal kingdom has been but sparingly found in the fossil state. The facility with which birds can escape by flight from the destruction that befalls other land-animals will no doubt suffice to explain why their fossil remains should be so infrequent. The oldest known birds had curious reptilian affinities, being furnished with jaws and teeth. Taking all the known forms of birds, recent and fossil, they may be grouped in the following sub- divisions : I. Saururse in this singular extinct group the vertebral column was prolonged into a long lizard -like tail, each vertebra of which, however, bore a couple of quill - feathers. The only known example is the Arck&opteryx of the Jurassic system the oldest bird yet discovered (Fig. 182). II. Odontornithes or toothed birds. Some of these (Odontolcse) were diving birds with rudimentary wings, ratite sternum, power- ful legs, a strong tail for steering, and jaws with numerous coni- cal teeth sunk in a deep continuous groove (Hesperornis\ Others (Odontotormse) were provided with strong wings and carinate sternum, and had their teeth sunk in separate sockets, as in the crocodiles. The toothed birds have long been extinct. They have been found most abundantly in the Cretaceous rocks of Kansas. III. Ratitae the cursores or running birds, such as the ostrich, cassowary, rhea, emeu, and apteryx. These have not with cer- tainty been found fossil in strata older than the Tertiary series. The gigantic extinct Dinornis of New Zealand belongs to this class. IV. Carinatse generally possessing powers of flight. These in- clude most of the birds of the present day. The arrangement of this great sub-class into definite orders has not been yet satis- factorily accomplished. The student, however, may find some advantage in making himself acquainted with the following names which, though in process of being superseded, are still in 49 6 APPENDIX. common use. Nat at ores swimmers or palmipeds, with short legs placed behind and provided with webbed feet. These include gulls, penguins, geese, ducks, swans, cormorants, etc. Remains of this order are found in Cretaceous and Tertiary strata. Grallatores waders, chiefly found by the shores of rivers, lakes, or the sea, distinguished by the length of their legs which are not completely webbed. They include plovers, cranes, flamingoes, storks, herons, snipes, etc. They have been found fossil in Cretaceous and Tertiary rocks. Rasores scratchers or gallinaceous birds, including the various tribes of fowls and pigeons. They are found fossil in Tertiary strata. Scansores climbers, including the parrots, cuckoos, toucans, and trogons. They are only found fossil in Tertiary and Post - tertiary rocks. Insessores perchers, passerine birds include by far the largest number of living birds, and all the ordinary song-birds. They have not been met with in a fossil state in rocks older than the Tertiary series. Raptores or birds of prey comprising birds with strong, curved, sharp-edged and pointed bills, and strong talons, as the eagles, hawks, falcons, vultures, and owls. This order also has not been obtained fossil except in Tertiary and Post-tertiary rocks. MAMMALIA. The highest class of the vertebrata is represented chiefly on the land, the marine representatives being few in number, though often of large size (whales, dolphins, porpoises, manatee, seals, morse). In marine deposits, therefore, we need not expect to find mam- malian remains abundant at the present time. Doubtless from the time of their first appearance mammals have always been on the whole terrestrial animals ; their fossil remains consequently occur but sparingly among ancient geological formations. The earliest known examples belong to the Marsupial type, and have been found in the Triassic and Jurassic rocks of Europe and North America. I. PROTOTHERIA or ORNITHODELPHIA including the two types of Ornithorhynchus and Echidna. II. METATHERIA or DIDELPHIA Marsupial animals. Comprising the Opossums (Didelphidoe), Dasyures, Myrmecobius, Perameles, Kangaroos, and Wombats. As just mentioned, it is representatives of this section of the vertebrates that occur fossil among the Mesozoic rocks [Microlesles, Fig. 170, Drotnatheritttn, Amphitherium, Phascolotherium, Fig. 183]. III. EUTHERIA or MONODELPHIA including the vast majority of living and extinct mammalia. They may be grouped as follows : APPENDIX. 497 Edentata sloths, ant-eaters, armadilloes, pangolins, and African ant-eaters. Some enormous extinct types of Edentates have been found in America [Megatherium, Mylodon, Moropus, Glyptodon}. Sirenia aquatic fish-like animals including the manatee, dugong, sea-cow. The last-named is now extinct, the last having been killed so recently as 1768. Numerous fossil remains of Sirenians occur in Miocene and Pliocene deposits of Europe \HaUtherium^ Cetacea whales including the Balsenidse or whalebone whales, Delphinoidea or toothed whales (Sperm whale, Ziphius, Narwhal, Porpoise, Ca'ing whale, Grampus, Dolphin). Cetacean ear- bones and other bones are not infrequent in Tertiary and Post- tertiary strata. Insectivora small terrestrial mammals like the shrews, moles, myogale. No fossil insectivores older than Eocene times are known, except perhaps Stereognathus of the Stonesfield slate. Cheiroptera animals with the fore-limbs adapted for flight, in- cluding the tribe of bats. Fossil representatives are found as far back as Eocene rocks. Rodentia small terrestrial plant-eating mammals, distinguished by their large chisel-shaped incisor teeth, specially adapted for gnawing, and by the absence of canines. Among them are squirrels, marmots, beavers, dormice, rats, mice, voles, lemmings, jumping mice, jerboas, porcupines, chinchillas, cavies, rabbits, and hares. Fossil rodents belonging to most of the existing families have been met with in Tertiary and Recent strata, together with some extinct types. Ungulata or hoofed animals include the Hyrax, the Proboscideans (elephants, Fig. 205, and the extinct types of Mastodon, Fig. 199, Deinotherium, Fig. 200, etc. ) and the extinct type of the Deino- cerata (Fig. 196) ; the perissodactyl or odd-toed group (tapirs, rhinoceroses, horses {Palceotherium~\, Fig. 195), and the artio- dactyl or even-toed group (hippopotamus, peccary, swine, llama, camel, chevrotains, the true ruminants, such as deer, antelopes, giraffes [Helladotheritim, Fig. 203], and all bovine animals). The earliest known forms are of Eocene age. Carnivora, so named from the majority of them subsisting on animal food and being eminently beasts of prey. They are divided into (i) Fissipedes or true_ carnivores, generally adapted for life on land, comprising (a) the ^Eluroids or cat-like forms (lions, tigers, cats, puma, jaguar, cheetah, civet-cat, ichneumon, hyoena, and various fossil forms found in Tertiary 2 K 49 8 APPENDIX. and Post-tertiary deposits) ; (3) the Cynoids or dog-like forms (dogs, wolves, foxes) ; and (c) the Arctoids or bears and their allies (otters, badgers, weasels, raccoons, panda) ; (2) Pinni- pedes or aquatic carnivores, divisible into three well-marked families : (a) Otariids or sea-bears ; (b) Trichechids or walruses ; (c) Phocids or true seals. Primates, the highest division of vertebrate life, comprising (i) the Lemuroid animals ; (2) the Hapalids or marmosets ; (3) the Cebids or American monkeys ; (4) the Cercopithecids, the monkeys of the Old World, exclusive of the apes ; (5) the Simiids or man-like apes ( Troglodytes, Gorilla, Simla, and Hylobates] ; (6) Man. INDEX. An asterisk (*) denotes that a figure of the subject will be found on the page indicated. ABYSMAL deposits, 105, 122 Acacia, fossil, 440 Acanthodes, 341* Acer, 429, 448 Acervularia, 345 Acid rocks, 214 Acid-test for carbonates, 205 Acids, 155 Acids, organic influence of, 22, 32, 62, 71, 109, 211 Acrodus, 384, 396 Acrosalenia, 391 Acrydiidce (locusts), 359 Actinodon, 378 Actinolite, 177 Agave, 428 Agglomerate, 203 Agnopterus, 430 Agnostus, 328* Alabaster, 181 Albatross, ancestral form of, 430 Albian, 417, 419 Albite, 176 Alder, fossil, 408, 448 Alethopteris, 355* Algae, fossil, 318 Alkali metals, 160, 163 Alkaline carbonates, 163 Alkaline earths, 164 Alligator, fossil forms of, 397 Alluvium, 45, 469 ; fans or cones of, 47 ; stratification of, 49, 50, 53 Almond-tree, fossil, 429 Alnus, 448, 449* Aloe, fossil, 428 Alps, glaciers of, 86, 88, 464 Alps, history of, 439, 445, 447 Alum Bay, leaf-beds of, 433 Alum-slate, 222 Aluminium, 155, 161 Aluminous silicates, 162, 175 Alveolites, 361 Amber, 281, 438 Amethyst, 170 Ammonia, 161 Ammonites, 292, 383, 384,* 393, 395,* 414* Amorphous minerals, 170 Amphibians, fossil, 358, 360 Amphibole, 177 Amphibolites, 223 Amygdaloidal structure, 193 Amygdalus, 429 Amygdules, 130, 193 Ananchytes, 411 Anchitherinm, 431 Anchor ice, 83 Ancyloceras, 413, 414* Andesine, 176 Andesite, 218 Anhydrite, 182, 207, 372, 380, 386 Animals, destructive geological action of, 108 ; deposits formed by, 113; preservation of remains of, onland, 1 20 ; relative chances of becoming fossils, 280 Annelids, 326 Anoplotherium, 292, 436 Anorthite, 176 Ant-eater, fossil, 443 500 INDEX. Antelopes, fossil, 441, 448, 453, 472 Anthracite, 211 Anthracomya, 359, 366* Anthracosaurus, 360 Anthracosia, 359 Anthracotherium, 436 Anthrapalcemon, 364 Anticline, 252 Apatite, 183 Apes, fossil, 292, 441 Aqueous rocks, 189 Aragonite, 180 Aralia, fossil, 408 Araucaria, 390 Araucarioxylon, 357 Area, 444 Arcestes, 384 Archaean, 305 ; described, 307 ArchcBocidaris, 361, 362* Archceopteiyx, 399, 400* Archegosaurus, 360 Arenicolites, 327 Argillaceous, 200, 201 Argillornis, 430 Asaphus,. 329* Asbestus, 177 Ascoceras, 333 Ash, volcanic, 135, 203 Asplenium, 409, 428 Assise, stratigraphical, 294 Astarte, 393, 451 Asterolepis, 342 Asterophyllites, 356, 357* Astraeid corals, 391 Astronomy, relation of, to geology, 299 Athyris, 346, 364 Atlantosaurus, 399 Atmosphere, composition of, 156, 158 ; influence of, in geological changes, 13 ; origin of, 301 ; ori- ginal state of, 303 Atoll, 117 Atrypa, 331,* 346 Auchenaspis, 333 Augite, 178 Avalanches, 83 Avicula, 383, 384* Aviculopecten, 359, 366* Axes of crystals, 167 Axinus, 375 Bactrites, 347 B acuities, 413, 414* Bagshot Sands, 433 Bajocian, 401, 402 Bakevellia, 375* Bala Group, 336 Bamboo, fossil, 448 Bannisdale Flags, 335 Baphetes, 360 Barium, 155, 164 Barnacles as evidence of upheaval, 149 Bars of rivers, 102 Barton Clay, 433 Barytes, 164, 182 Basalt rocks, 217 Basaltic structure, 218 Bases, 155 Basic rocks, 217 Bath oolite, 401 Bathonian, 401, 403 Bats, fossil, 431, 436 Beaches, raised, 149, 465 Bears, fossil, 441, 448, 451, 473 Beavers, fossil, 443, 451, 474 Bed in stratigraphy, 229, 293 Bedded structure, 188, 229 Beech, fossil, 408, 440, 428 Beetles, fossil, 394, 441 Belemnites, 393, 395* Bellerophon, 332,* 365, 366* Bembridge Beds, 437 Beryx, 415* Betula, 448 Biotite, 177 Birch, fossil, 448, 461 Birds, earliest known, 399 ; Eocene, 430 ; Oligocene, 436 Birds with teeth, 292, 417, 430 Bison, fossil, 470 Blackthorn (Prunus), fossil, 451 Blastoids, 292, 362* Blattidce (cockroaches), fossil, 334, 358 Blocks, erratic, 86, 457 ; volcanic, 135. 203 INDEX. 501 Boar, wild, fossil, 453, 474 Bogs, disappearance of, 3 Bog-bean, fossil, 451 Bog-iron, 62, 173, 207, 211 Bog-manganese, 174 Bog-mosses, precipitation of lime- carbonate by, 78 ; form peat, 109, 210 Bombs, volcanic, 202 Bone-beds, 212, 388 Bone-breccia, 212 Bone-caves, 121 Boulders, transport of, by river-ice, 83 Boulder-clay, 457, 466 Bosses, 264 Brachiopods, fossil, 331,* 346,* 365.* 374.* 392, 450* Brachiopods," "Age of, 332 Brachymetopus, 363 Bracklesham Beds, 433 Bradford Clay, 401 Bramatherium, 454 Breccia, 197, 203 Brick-clay, 201 Brick-earth, 26, 470 Brittle-stars, fossil, 326 Bronteus, 344* Brontosaurus, 399 Brown coal, 210, 438 Bruxellian, 433 Buckthorn, fossil, 408, 448 Bunter or Lower Trias, 386 Burrows of animals, 279, 327 Butterfly, earliest form of, 395 Buzzard, ancestral forms of, 430 CAINOZOIC, 305, 423 Cairngorm stones, 170 Catamites, 339, 355, 357,* 373 Calc-sinter, 77 Calcaire grossier, 433 Calcareous tufa, 77, 471 Calceola, 345* Calceola-shales, 347 Calcification, 286 Calcite, 166, 179, 187 Calcium, 155, 162 Calcium-carbonate, 159, 179; detec- tion of, 162 ; removed in solution by natural waters, 71, 74, 113, 115, 180 ; deposits of, 74, 204 ; secreted by plants, 78, 112 ; secreted by animals, 113, 159, 1 80, 209 ; amount of, in living organisms in the sea, 114 ; rocks formed of, 204, 209 Calcium-phosphate, 160, 183 Calcium-silicates, solution of, 71 Calcium-sulphate, 156, 162, 181 ; deposits of, in salt-lakes, 63 Callipteris, 373,* 374 Callitris, 428 Callovian, 401, 403 Camarophoria, 374* Cambrian, 318, 320, 336 Camels, fossil, 446, 454 Cancellaria, 444 Canis, 463 Caprina, 412, 413* Caprotina, 413* Caradoc Group, 336 Carbon, 155, 158, 159, 165 Carbon-dioxide, 158 Carbonates, 165, 179 Carbonic acid, 159; solvent power of, 18, 32, 63, 70, 71, 74, 122 Carboniferous system, 348 Carbonisation, 284 Cardita, 444 Cardium, 383, 384,* 444, 452 Carnallite, 388 Carpinus, 428 Carpolilhes, 356 Carya, 448 Cassia, fossil, 408 Castanea, 428 Casts of organic remains, 286 Cat, fossil, wild, 441, 448 ; Caffer, 472 Caulopteris, 374 Cave-bear, 473 Cave-earth, 471 Caverns, origin of, 72 Caves, sea-formed, as evidence of upheaval, 149 Cellular structure, 130 502 INDEX. Cellulose, 282 Cementing materials of rocks, 200, 244 Cenomanian, 417, 419 Cephalaspid fishes, 292, 333 Cephalaspis, 333, 341* Cephalopods, fossil, 334,* 349,* 367-* 375, 383- 384.*393- 412, 414* Ceratiocaris, 330* Ceratites, 383 Ceratodus, 342, 384 Ceratophylhim, 451 Cervus, 292 Ceteosaurus, 399 Ch&ropotamus, 436 Chcetetes, 361 Chalcedony, 170 Chalk, 209, 295, 420 Chalk formation, 419, 420 Chalk-marl, 419 Chalybeate water, 79, 184 Chalybite, 181, 208 Chama, 429 Chamcerops, 428 Cheirodus, 359 Chemical solution in geology, 18, 32 Chestnut, fossil, 428 Chevrotains, fossil, 436 Chiastolite slate, 223, 267 Chillesford Beds, 451 China clay, 201 Chitin, 283 Chlorides, 155, 163, 165, 184, 387 Chlorine, 155, 160 Chlorite, 178 Chlorite-schist, 223 Chloride Marl, 419 Chondrites, 319* Chonetes, 345, 364 Chrysotile, 221 Cidaris, 391, 393* Cinnamon, fossil, 408, 409,* 429, 448 Civets, fossil, 436 Clastic rocks, 196 Clay, 162, 201 Clay-ironstone, 181, 208, 211, 235 Clay-slate, 222 Cleavage, 255 Cliff-debris, 196 Climate, indicated by fossils, 289 ; Palaeozoic, 314 ; Mesozoic, 389, 390 ; Tertiary, 425, 429, 440, 444, 445, 448, 449 ; Post-tertiary, 456, 461, 472 Clinometer, 248 Clisiophyllum, 361 Club-mosses, earliest fossil, 318, 339 Clymenia, 347 Coal, 211 ; formation of, 228, 351 Coal-measures, 295, 368 Coblenzian rocks, 347 Coccosteus, 342 Cochliodus, 367 Cockroach, Silurian, 334 ; Carboni- ferous, 358 ; Jurassic, 394 Coleoptera, fossil, 394 Colorado River, gorges 'of, 43 Colouring materials of rocks, 200 Compression, effects of, in rocks, 247, 254 Conchoidal fracture, 214 Concretionary minerals, 170 ; struc- ture in rocks, 187, 235 Conformability, 240 Conglomerate, 198 ; origin of, 229 ; area overspread by, 237 ; schistose, 225 Coniferas, fossil, 339, 357, 374, 379, 382,* 390, 428, 435, 440 Coniston Grits and Flags, 335, 336 Contemporaneous sheets, 269 Continents, geological date of, 424, 434- 447 Conularia, 365, 367* Conus, 429 Copper-slate (Permian), 377 Coprolites, 212, 283, 360 Coral-reefs, 115 ; upraised, 118 Coral-rock, 209 Corals, Silurian, 323, 325*; Devon- ian, 345*; Carboniferous, 349, 361* ; Jurassic, 391* Corallian, 401, 404 Coralline crag, 452 Corbula, 436* INDEX. 53 Cordaites, 356, 358* Cornbrash, 401, 403 Corylus, 428 Corypfiodon, 431 Cotoneaster, fossil, 429 Crabs, early forms of, 394 Crag deposits, 451 Cranes, fossil, 436 Crania, 392 Cray-fish, early forms of, 394 Credneria, fossil, 408 Cretaceous system, 406 Crevasses in glaciers, 85 Crickets, fossil, 359 Crinoidal limestone, 209, 348, 362 Crinoids, fossil, 324, 345, 362,* 383.* 392* Crioceras, 414* Crocodile, fossil forms of, 385,* 397, 416, 430, 443 Crustaceans, fossil, 327, 344, 363, 396* Crust of the earth, 125; structure of, 226 Cryphceus, 344 Cryptogams, earliest fossil, 318 Crystalline forms of minerals, 166, 167 Crystalline rocks, 190 Crystalline structure of rocks, 169 Crystallites, 190, 214 Ctenodonta, 332 Cubical system of crystals, 167 Cucullea, 346, 347* Cupressinites, 428 Cupressocrinus, 345 Curvature of strata, 251 Cuttle-fishes, fossil, 393 Cyathaxonia, 324 Cyathocrinus, 362 Cyathophyllum, 324, 345* Cycadeoidea, 390* Cycadites, 390 Cycads, fossil, 291, 374, 379, 382,* 390* Cyprcea, 444 Cypridina-shales, 347 Cyprina, 451 Cypris, 363 Cyrena, 429, 436 Cyrtia, 346 Cyrtoceras, 333, 347, 375 Cystideans, 292, 325 Cystiphyllum, 345 Cytherea, 429, 444 Dadoxylon, 357 Dalmanites, 344* Danian, 417, 421 Daonella, 383 Dapedius, 395 Dasorius, 430 Dead Sea, originally fresh, 64 Decalcification, 286 Deer, fossil, 444, 448, 451 Deinocerata, 292, 432* Deinosaurs, Triassic, 385 ; Jurassic, 398 ; Cretaceous, 415 Deinotherium, 441, 442,* 448 Deltas in lakes, 57 ; in the sea, 102 ; animal remains in, 120 Denbighshire Grits, 335 Dendrerpeton, 360 Dendritic markings, 174 Dendrocrinus, 325 Denudation (see weathering, Rain, Springs, Rivers, Glaciers, Sea), 241. 443 Deserts, 28, 199 Desiccation, effects of, 17 Detritus, transport of, by rivers, 35 ; deposit of, 46 Devitrification, 191, 192, 213, 214, 215- 273 Devonian system, 337 Diabase, 218 Diallage rock, 220 Diatoms, in, 282 Diatom-earth, in, 283 Dichobune, 431 Dicotyledonous plants, earliest known, 408 Dictyograptus , 323, 324* Dictyonema, 323, 324* Dicynodon, 385 Didymograptus, 323* Dimetric crystals, 167 Dimorphodon, 398 54 INDEX. Dinichthys, 342 Dinornis, 430 Diorite, 220 Dip, 247 Diplograptus , 323* Diplopterus, 341 Dipterus, 342 Dirt-beds of Portland, 405 Discina, 331, 359, 364, 392 Discosaurus, 417 Dislocation of rocks, 256 Dodecahedron, 167 Dogs, fossil, 436, 474 Dog-tooth spar, 179 Dogwood, fossil, 408 Dolerite, 218 Dolomite, 162, 180, 205, 372, 380 Dolphin, fossil, 443 Dragon-flies, fossil, 358, 394 Dryopithecus> 441 Dunes, 27, 109, 113, 199 Durance, sediment in water of, 37 Dust, transport of, by wind, 24, 27 Dyas, 371, 377 Dykes, 216, 266, 274 EAGLES, fossil, 436 Earth, geological energy of, 124 ; crust of, 125, 303, 304 ; internal heat of, 125, 302 ; condition of interior of, 126 ; geological struc- ture of, 301 ; density of, 302 ; shape of, 302 ; changes in rate of rotation of, 303 Earth-tremors, 147 Earthquakes, 147 Earth-worms, co-operation of, in formation and removal of soil, 24 Echini, fossil, 361,* 393* Echinoconus, 411* Echinoderms, fossil, 326, 345, 361, 383- 391 Edmondia, 365, 366* Elements, most important chemical, 155 Elephants, fossil, 292, 448, 451, 469 Elephas, 292, 451, 462, 463* Elevation, proofs of, 149, 440 Elk, fossil, 474 Elk, Irish, in, 473, 474 Elm, fossil, 428, 440, 448 Enaliosaurs, 397 Enchodus, 415 Encrinus, 383* Endo-skeleton, 283 Eocene, 426, 427 Eophyton, 319* Eoscorpius, 358 Eozoon, 310 Ephemeridas, fossil, 339, 358 Equisetaceoe, fossil, 355, 357,* 382,* 39 Equus, 292 Erratics, 86, 466 Eruptive rocks, 190, 212, 263 Eskers, 465 Estuaries, deposit of mud in, 101 Etna, geological date of, 447 Eucalyptus, 429 Euchirosaiirus, 378 Euomphalus, 365, 366* Eurypterids, 343, 345, 364, 393 Evergreen trees, fossil, 435 Exogyra, 393, 412 Exo-skeleton, 283 FAGUS, 428 Fairy-stones, 187, 235 False bedding, 50, 230 Faluns of Touraine, 444 Famennian Group, 347 Faults, 257 ; influence of, on scenery, 259 Fault-rock, 257 Favosites, 324, 361 Felis, 464 Felsite, 215 Felspars, 175 Fenestella, 364* Ferns, fossil, 319, 339,* 3SS* 373,* 381, 390 Ferric oxide, 79, 164 ; as a cement of rocks, 244 Ferrous oxide, 164 Ferrous carbonate, 181, 187, 208, 211, 236, 287 Ferrous sulphate, 79 INDEX. 505 Ferruginous cement of sandstones, 200 Fibrous minerals, 169 Ficus, 409,* 441,* 448 Fig, fossil, 408, 428, 440, 448 Fire-clay, 201 ; origin of, 228, 351 Fir-trees, fossil, 390, 451 Fishes, fossil, 333, 340, 365, 375,* 384, 395, 415,* 430 Fissures, 256 ; volcanic, 138, 144 ; formed by earthquakes, 148 Flamingoes, fossil, 436 Flint, 212, 235, 421 Flint implements, 470,* 475* Fluorides, 165, 183 Fluorine, 155, 161 Fluor-spar, 161, 183 Fluxion-structure, 192, 194, 214 Foliated structure, 194 Footprints in rocks, 233, 279, 382, 385. 399 Foraminifera, Silurian, 321 ; Car- boniferous, 361* ; Cretaceous, 410,* Eocene, 429 Foraminiferal ooze, 114 Forest-Bed Group, 451 Forest-Marble, 401 Forests, influence of, 109 ; buried, 288 Formation, geological, 294 Fossilisation, 284 Fossils, definition of, 279 ; conditions for production of, 284 ; indicate geographical changes, 288 ; indi- cate changes of climate, 289 ; afford evidence of the conditions under which volcanic eruptions have occurred, 134 ; establish geological chronology, 290 ; char- acteristic, 291 ; furnish evidence as to salinity of water, 372 ; dis- tortion of, by cleavage, 255 Fox, Arctic, 463, 472 Fox, fossil, 451 Fragmental rocks, 187, 196 Frasnian Group, 347 Freestone, 200 Frogs, fossil, 440 Frost, influence of, 17 Fuller's Earth, 401, 403 Fusion, crystallisation formed from, 190 Fusulina, 361* GABBRO, 220 Galerites, 411 Ganoids, fossil, 340, 359 Gas globules in the minerals of rocks, 190 Gasteropods, 332 Gault, 417, 419 Gazelles, fossil, 448, 453 Gedinnian rocks, 347 Geographical changes indicated by fossils, 287 Geological chronology established by fossils, 290 Geological Record, 292, 304 ; im- perfections of, 293 ; subdivisions of, 293, 305, 306 ; chronological value of subdivisions of, 295 ; vary- ing fossiliferous character of rocks of, 320 ; breaks in, 425 Geology, aims of, 6, 299 Gervillia, 393 Geysers, 80 Ginko, 428 Giraffes, fossil, 444, 448, 453 Givet, limestone of, 347 Glacial group of deposits, 455 Glaciated country, 92, 456 Glaciers, 84 ; transport detritus, 84, 457 I polish and striate rocks, 88, 457 Glass, volcanic, 169, 191 ; enclosed in crystals, 190 Glassy condition of rocks, 191, 217, 273 Glauconitic, 200 Glauconitic marl, 419 Gleichenia, 409 Globigerina, 114,* 410* Glutton, fossil, 451, 463, 472 Glyptichus, 391 Glyptocrinus, 325 Glyptoleemus, 341 Glyptostrobus, fossil, 448, 449* Gneiss, 224, 308 506 INDEX. Goat, fossil, 454, 474 Goniatites, 347, 359, 367* Granite, 216 ; weathering of, 20 Granitic structure, 217 Graptolites, 292, 322, 323,* 343 Grasshopper, fossil, 394 Gravel, 197 ; extent of deposit of, 237 Great Oolite, 401, 403 Greensand, 295 Greensand, Lower, 419 ; Upper, 419 Greenstone, 220 Greywacke, 200 Greywacke rocks (Silurian), 316 Griffithides, 363 Grit, 200 ; schistose, 225 Ground-ice, 83 Group, stratigraphical, 294 Grouse, fossil, 436 Gryllidae (crickets), fossil, 359 Gryphaa, 393, 394* Gryphite limestone, 393 Guano, 212 Gum-tree, fossil, 408, 429 Gulo, 463 Gypsum, 63, 156, 162, 181, 206 ; deposits of, 63, 207, 372, 380, 386 beds of Paris, 433, 438 Gyr acanthus, 359 Gyrolepis, 384 HADE of a fault, 257 Haematite, 171, 172, 207 Halite, 184 Halysites, 324 Hamites, 413, 414* Hare, fossil, 472 Harlech group, 336 Harpes, 344* Hazel, fossil, 428, 451 Headon Beds, 437 Heavy spar, 164, 182 Hedgehogs, fossil, 431 Heersian, 433 Helicoceras, 414* Heliolites, 324, 325* Helix, 436 Helladothefium, 453* Hemicidaris, 391 Hempstead Beds, 437 Hettangian, 401 Hexagonal system of crystals, 167 Hickory, fossil, 448 Hippopodium, 393 Hippopotamus, fossil, 443, 451, 464, 469, 472 Hippurite-limestone, 412, 422 Hippurites, 292, 412, 413* Hog, fossil, 436, 451, 474 Holaster, 412 Holopcea, 333 Holoptychius, 340 Homalonotus, 329,* 344* Honestone, 222 Horizon, stratigraphical, 293 Hornbeam fossil, 428, 440 Hornbill, African, early forms of, 430 Hornblende, 177 Hornblende-rock, 223 Hornblende-schist, 223 Hornwort, fossil, 451 Horse, ancestral forms of, 431 Horse, fossil, 448, 451, 472, 474 Horse-tail reeds, 339, 382* Human period, 467 Humous acids, influence of, in weathering, 32 Huronian, 311 Hycenarctos, 441 Hyaenas, fossil, 292, 448, 451, 464, 472 Hybodus, 384, 396 Hydraulic limestone, 205 Hydrocarbons, 160 Hydrochloric acid, 160 Hydrogen, 155, 160 Hyopotamus, 431, 436 Hyperodapedon, 385 Hypogene rocks, 213 Hystrix, 464 IBIS, ancestral forms of, 430, 436 Ice Age, 456 Iceland, sinter deposits of, 80 ; spar, 166, 179 Ice-sheets, 84, 92, 456 Ichthyodorulite or fin-spine, 368* INDEX. . 57 Ichthyosaurus, 292, 397,* 415 Igneous rocks, 190, 263 Iguanodon, 292, 416* Ilex, fossil, 408 Ilfracombe limestone, 347 Illcenus, 329* Inferior Oolite, 401 Infusorial earth, in Inoceramus, 412* Insects, fossil, 281, 284, 334, 339, 358, 388, 394 Interglacial periods, 461 Intrusive sheets, 268 Iron, 155, 163 ; oxides, 164, 171 ; as a colouring agent in nature, 164 Iron, carbonate of. See Ferrous carbonate. Iron-ore of lake bottoms, 62 Ironstone, 62, 79, 172, 207, 210, 236 Iron-sulphate in natural waters, 79 Iron-sulphide, 184 ; oxidation of, by water, 79 ; precipitated by reducing action of decaying organisms up- on solutions of the sulphate, 184, 287 Isastrcea, 391* Isometric system of crystals, 167 Ivy, fossil, 408 JASPER, 170 Jerboa, fossil, 472 Joints, 245 Juglans, 409,* 428, 448 Juniper, fossil, 408 Jurassic system, 389 KAMES, 465 Kangaroos, fossil, 401 Kaolin, 201 Kellaways Rock, 401, 403 Keuper, or Upper Trias, 386 Kimmeridgian, 401, 404 Kirkby Moor Flags, 335 Kupferschiefer, 376, 377 LABYRINTHODONTS, 360, 376,* 385 Lagoons and bars on coasts, 102 Lagoons of coral-islands, 117 Lakes, disappearance of, 3, 60 ; filter rivers, 40, 56 ; memorials left by, 56, 120 ; terraces of, 58 ; marl of, 6, 61 ; iron-deposits of, 62 ; salt, 63, 207 ; traces of among lavas, 134 ; record the existence of land, 288 ; of Old Red Sandstone, 338 ; Eocene, 427 ; Oligocene, 434. 437 I Miocene, 445, 446 Lamantin, fossil, 443 Lamellibranchs, fossil, 332,* 347,* 366,* 375.* 384.* 393 Laminae, 229 Laminated, 230 Lamna, 415, 430 Land, changes in surface of the, 2 ; rate of lowering of, by chemical solution, 34 ; rate of lowering of, by mechanical transport, 38 ; - effect of elevation of, on rivers, 52 ; demolition of, by the sea, 94 Land-animals, earliest traces of, 334 Land-shells, oldest known, 315, 340, 359 Land-surfaces, how indicated among rocks, 228, 232, 287, 351 Landenian, 433 Landslips, 67 Lapilli, 203 Laurel, fossil, 408, 429, 435, 440, 448 Laurentian, 311 Laurus, 429, 448 Lava, 130 ; plains or plateaux of, 145 ; successive sheets of, 269 Layer (in stratigraphy), 294 Leda, 365, 462* Lemming, fossil, 472 Lemurs, fossil, 431 Leopard, fossil, 464, 472 Leper ditto. , 364 Lepidodendron, 291, 355, 356,* 373, 379 Lepidostrobus, 355 Lepidotus, 396 Leptcena, 332, 392 Leucite, 218 Lias, 401 Libellula, 358 508 INDEX. Life, marine and terrestrial, relative chances of preservation of remains j of, 280 ; succession of, on the j earth, 291, 315 Lignite, 210 Lima, 393, 412 Lime, carbonate of, 159 ; detection of, 162 ; removed in solution in natural waters, 71, 74, 113. 180 ; deposited by mosses, 78 ; secreted by sea- weeds, 112 ; secreted by animals, 113, 159, 180, 209 ; amount of, in living organisms in the sea, 114; by crystallising within the pores of calcareous sedi- ment forms limestone, 76, 113, 115, 117, 204, 209; oolitic de- position of, 1 1 8, 204 ; rocks formed of, 205 ; as a cement of rocks, 244 ; as a petrifying medium, 286 Lime, phosphate of, 160, 183, 212 Limestone, 204, 209 ; dissolved by natural waters, 71 ; formation of, 74, 77, 113, 114, 115, 117, 199, 204, 209, 228, 289, 348, 350, 360, 364 Lime, sulphate of, 181, 206 Lime-tree, fossil, 446 Limncea, 436 Limonite, 171, 172, 207, 211 Lingula, 331, 359, 364, 392 Lingula Flag Group, 336 Lingulella, 331* Linton slates, 347 Lions, fossil, 441, 464, 472 Liparite, 215 Liquid-inclusions in crystals, 190 Liriodendron, 448 Lit ho sir ot ion, 361* Lituites, 333, 334* Lizards, earliest forms of, 376, 385 Lizards, fossil, 441 Llandeilo Group, 336 Llandovery Group, 335 Lobsters, early forms of, 394 Locusts, fossil, 359 Loess, 202, 471 London Clay, 433 Longmynd Group, 336 Lonsdaleia, 361 Loxomma, 360 Loxonema, 365 Ludlow Group, 335 Lumbricaria, 327* Lycopods, fossil, 318, 339, 355 Lygodium, 428 Lynx, fossil, 464, 472 MACAQUE, fossil, 454 Machairodus, 441, 451 Macles, 181 Macrotherium, 443 Magma, 190 Magnesian limestone, 162, 180, 205, 376 Magnesian silicates, 163, 175 Magnesium, 155, 162 Magnesium-chloride, 162, 163, 387 ; in salt lakes, 64 Magnetite, 171, 173, 208 Magnolia, fossil, 408, 428, 440, 441,* 448 Mammalian life, earliest traces of, 386,* 399 Mammaliferous Crag, 452 Mammals, Age of, 425, 430 Mammoth, fossil, carcases of, 284, 290 ; range of, 462 ; man coeval with, 470, 472, 473 Man, earliest traces of, 465, 466 Manganese, 155, 164 ; oxides of, 174 Manganese, peroxide of, deposited in ocean abysses, 105 Mangrove-swamps, in, 353 Manures, sources of, 212 Maple, fossil, 408, 429, 446, 448 Maraboots, fossil, 436 Marble, 224 Marcasite, 184, 187 Marl, lacustrine, 6, 61, 113, 288 Marl-slate (Permian), 372 Marlstone, 401, 402 Marmots, fossil, 436 Marsupials, fossil, 386, 399, 400,* 43i Martens, fossil, 436 Masonry, weathering of, 15 INDEX. 509 Massive rocks, 189 Mastodon, 292, 441, 442,* 448, 452 Mastodonsaurus, 385 May-fly, fossil, 339, 358, 394 May Hill sandstone, 335 Mediterranean stage (Miocene), 445 Megaceros, 473 Megalichthys, 359 Megalosaurus, 399 Megaptilus, 359 Menevian Group, 336 Menyanthes, 451 Mesozoic, 305, 379 Metalloids, 155, 156 Metals, 155, 161 Metamorphic rocks, 221, 261 Metamorphism, contact, 224, 266, 269, 272, 275 Metamorphism, regional, 260, 318 Meteoric dust in ocean abysses, 105 Mica, 177, 261 Mica-schist, 223, 261, 309 Micaceous, 200 Mice, co-operation of, in removal of soil, 24 Micr aster, 411 Microcline, 176 Microlestes, 386* Microscope, use of, in study of rocks, 1 86, 190, 191 Millipedes, fossil, 340, 358 Millstone Grit, 369 Mimosa, fossil, 440 Mineral, definition of a, 164 ; modes of origin, 169 Mineral veins, 275 Miocene, 426, 439 Mitra, 429, 444 Moa, 430 Modiola, 365 Modiolopsis, 332 Molasse, 288 Moles, co-operation of, in removal of soil, 24, 109 Moles, fossil, 436, 451 Mollusca, importance of, in geology, 330 Monkeys, fossil, 441, 448, 454 Monoclinic system of crystals, 168 Monographs, 323* Monometric crystals, 167 Monotis, 383 Moon, changes in distance of, from the earth, 303 Moraines, 84, 464 Moraine stuff, 84, 196 Morse, fossil, 443 Mosasaurus, 416 Moselle, gorge of the, 42 Moulds of organic remains, 285 Mountain -chains of different ages, 150, 242, 425, 434, 439, 447, 453 Mountain Limestone, 349 Mud, 20 1 ; extent of deposition of, 237 ; inimical to some forms of marine life, 289 Mudstone, 202 Murchison, R. I., 316, 337 Murchisonia, 333 Murex, 444 Muschelkalk, 383, 386 Muscovite, 177, 287 Musk-rats, fossil, 436 Musk-sheep, fossil, 451, 463,* 472 My a, 451 Myliobates, 430 Myophoria, 373, 384* Myriapods, fossil, 358 Myrica, fossil, 408 Myrtle, fossil, 440 NAIL-HEAD spar, 179 Natica, 462* Nautilus, 333, 365, 375, 383, 429 Nebular hypothesis, 300 Necks, volcanic, 142, 273 Nelumbium, 428 Neocomian, 418 Neolithic deposits, 468, 474 Nepheline, 218 Neuroptera, fossil, 339, 394 Neuropteris, 355* New Red Sandstone, 371, 379 Nipa, 428 Nitrogen, 156, 161 Norwich Crag, 452 Nucula, 365, 412* Nummulites, 292, 429 INDEX. Nummulitic Limestone, 427, 433 Nuphar (pond-lily), 451 Nymphcea, 451 OAK, evergreen, 435, 440, 448 Oak, fossil, 408, 440, 441,* 448, 4Si Obsidian, 214 Ochre, 79, 172 Octahedron, 167 Odontopteryx, 430 Odontornithes, 417 CEningen Beds, 446 Ogygia, 329* Oldhamia, 321* Oldhaven Beds, 433 Old Red Sandstone, 337 Olenus, 328* Oligocene, 426, 434 Oligoclase, 176 Oliva, 429* Olivine, 178 Olivine rocks, 220 Omphyma, 324, 325* Oolite, 188, 205 ; formed on coral- reefs, 1 1 8 ; as a stratigraphical name, 295 Oolites (Great, Bath, etc. ), 401, 403 Ooze, 114, 410 Ophileta, 333 Opossum, fossil, 431 Oreopithecus, 441 Ores, metallic, 275 Organic acids, 22, 32, 62, 71, 109, 211 Orodus, 367, 368* Orthis, 331,* 345 Orthoceras, 333, 334,* 347, 365, 367.* 375. 383 Orthoceratites, 292 Orthoclase, 175, 213 Orthoclase rocks, 213 Orthonota, 332* Orthoptera, fossil, 339, 394 Orthorhombic system of crystals, 167 Osar, 465 Osborne and St. Helen's Beds, 437 Osmeroides, 415 Osteolepis, 341* Ostracods, 363 Ostrea, 393, 411, 436* Olodus, 415, 430 Otters, fossil, 443 Outcrop, 249 Overlap, 240 Ovibos, 463* Oxen, fossil, 448, 451, 474 Oxfordian, 401, 403 Oxidation, 18, 79 Oxides, 155, 170 Oxygen, 155, 156 Oxy rhino,, 415 PACHYDERMS, fossil, 431, 436, 441, 448, 462 Pal&aster, 326 Pal&asterina, 326* Palcsoblattina, 334 Palceochoma, 326 Palceochorda, 327 Palaocrangon, 364 Palaeolithic deposits, 468 Palceoniscus, 376 Palceophycus, 327 Pal&opteris, 339* PalcBotherium, 292, 431,* 436 Palaeozoic, 305, 312 Palaeozoic life, 314 Palaeozoic rocks, thickness of, in Britain, 314 Palms, fossil, 409, 428, 435 Paludina, 436* Pandamis, fossil, 409, 428 Paniselian, 433 Panopcsa, 450* Paradoxides, 328* Parallel roads of Glen Roy, 59 Paroquets, fossil, 436 Pearlstone, 214 Peat, nature and origin of, 5, 109, 210 ; antiseptic influence of, in ; preservation of animal remains in, in, 120 ; contains early human relics, 476 Peccaries, fossil, 436 Pecopteris, 355 Pecten, 383, 384,* 393, 412, 444, 462 INDEX. Pectunculus, 444, 452 Pegmatite, 309 Pelican, ancestral forms of, 430, 436 Pentacrinite, 391, 392* Pentamerus, 331* Pentremites, 362* Perched blocks, 86, 196 Peridot, 178 Peridotites, 220 Perlidas (stone -flies), fossil, 358 Permian system, 370 Petalodus, 367 Petrifaction, 286 "Petrifying" springs, 78 Petrophiloides, 428* Phacops, 344 Phascolotherium, 400* Phasmidse (spectre-insects), fossil, 359 Phillipsia, 363 Pholidophorus, 384, 395, 396* Phosphates, 183 Phosphorus, 155, 160 Phyllite, 223 Phylloceras, 384 Phyllograptus, 323* Phyllopods, 330 Pigs, fossil, 436, 451, 474 Pile-dwellings, 475 Pilton and Pickwell - Down Group, 347 Pinacoceras, 384 Pine-trees, fossil, 339, 357, 390, 408, 428 Pinna, 393 Pinus, 451 Pisolitic structure, 188, 205 Placoderm fishes, 333, 341 Plagiaulax, 401 Plagioclase, 175 Plagioclase rocks, 217 Plane-tree, fossil, 408, 428, 446, 448 Planorbis, 436 Plants, precipitation of carbonate of lime by, 78 ; destructive geological action of, 108 ; deposits formed by, 109 ; preservation of remains of, 1 20, 280 ; essential parts of Structure of, 282 Plaster of Paris, 181 Platanus, 428, 448, 449* Platycrinus, 362 Platyschisma, 333 Platysomus, 375* Pleistocene, 426, 455 Plesiosaurs, 385, 415 Plesiosaurus, 292, 397 Pleuracanthus, 359, 367* Pleiironoura, 378 Pleurotomaria, 365 Plication, 253 ; connexion with faults, 258 Pliocene, 426, 447 Pliopithecus, 441 Plum-tree, fossil, 429, 448 Plymouth limestone, 347 Polyzoa, fossil, 364, 452 Pond-lily (Nuphar], 451 Pondweed, fossil, 440 Poplar, fossil, 408, 428, 440, 448 Populus, 409, 428, 448, 449* Porcupine, fossil, 454, 464, 472 Porphyritic structure, 191 Portlandian, 401, 404 Posidonomya, 368 Post- pliocene, 455 Post-tertiary, 305, 455 Potash, sulphate and carbonate of, 163 Potassium, 155, 163 Potassium-chloride, 163, 387 Potassium-sulphate, 163 Poteriocrinus, 362 Pot-holes, 39 Prawns, early forms of, 394 Present explains the Past, 7, 14 Pressure, effects of, on rocks, 244 Prestwichia, 364 Primary, 305 Primordial strata, 318, 336 Pristis, 430 Productus, 345, 364, 365,* 375* P rot aster, 326 Proteaceas, fossil, 428,* 435, 440 Protorosaurus, 376 Protriton, 378 Prunus, 429, 448, 451 Psammites de Condroz, 347 INDEX. Psammodus, 367 Psaronius, 374 Pseudocrinites, 326* Psilophyton, 339* Pteraspis, 333 Pterichthys, 341* Pterinea, 346 Pterodactylus, 398* Pterophyllum, 382, 390 Pteropods, fossil, 365 Pterosaurs, 398, 415 Pterygotus, 343* Piychoceras, 414* Pullastra, 383 Pumice, 214 Pumiceous structure, 193 Ptt^z, 359 Purbeckian, 401, 404 Purpura, 450* Pycnodus, 396 Pygaster, 391 Pyrite, 184, 187, 287 Pyroxene, 178 Pythonomorphs, 417 QUAIL, ancestral forms of, 430 Quartz, 157, 166, 170, 208 ; in con- glomerates and breccias, 198 Quartzite, 224 Quartzose, 200 Quartz-porphyry, 215 Quartz-trachyte, 215 Quaternary, 305, 455 Quercus, 409,* 441,* 448 RABBITS, co-operation of, in weather- ing of surface, 24, 109 Radiolites, 412, 413* Rain, effects of, 18, 23, 31, 113 Rain-prints, 233 Rain-wash, 26 Rastrites, 323* Rays, fossil, 396 Recent period, 305, 496 Red crag, 452 Red strata, generally unfossiliferous, 382 Reindeer, fossil, 456, 463, 472, 473* Rensseleria, 346 Reptiles, Age of, 396! Rhaetic group of rocks (Trias), 383, 386 Rhamnus (buckthorn), 448 Rhamphorhynchus, 398 Rhinoceros, fossil, 442, 448, 451, 462, 469, 472 Rhizodus, 359, 368 Rhombohedron, 166 Rhus (Sumach), 441,* 448 Rhynchonella, 331,* 346, 364, 392, 412, 450* Rhyolite, 215 Ripple-marks, 231 Rivers, chemical work of, 32 ; mechanical work of, 35, 46 ; de- clivity of, 37 ; erosion, 38 ; mean- derings of, 41 ; excavation of gorges by, 42 ; permanent records of, 45 ; velocity of, 46 ; terraces of, 51, 467, 469 ; flood-plain of, 52 ; frozen, 81 ; bars of, 102 ; deltas of, 102 Roches moutonne'es, 92 Rock-crystal, 157 Rock-salt, 184, 207, 372, 380, 387 Rocks defined, 185 ; sedimentary, 186, 195, 226, 244 ; fragmental or clastic, 187, 196 ; formed by chemical precipitation, 203 ; formed from the remains of plants and animals, 208 ; eruptive, 190, 212, 263 ; fossiliferous, thickness of, in Europe, 305 Roe-stone, 188 Roofing-slate, 222 Rotalina, 410* Rothliegende, 377 Rugose corals, 324, 345, 361 Rust, nature of, 156 SABAL, 409, 428 Sable Moyens, 433 Salisburia, 428 Salix, 428, 448, 449* Salt, common. See sodium chloride. Salt-lakes, 63, 207 ; Permian, 372 ; Triassic, 380 Salts, 156 INDEX. 513 Sand, 198 ; extent of deposit of, 237 ; calcareous, hardened into limestone, 112, 199, 209 Sand-dunes. See Dunes Sandstone, 199; weathering of, 20; relative extent of area of, 237 Sanidine, 175, 213 Sarmatian stage (Miocene), 445 Sarsaparilla, fossil, 428, 448 Sassafras, fossil, 408, 409^448 Satin-spar, 181 Saturation, effects of, 17 Saurians, fossil, 378, 380 Saxicava, 462* Scalaria, 452 Scaphceus, 396 Scaphites, 414* Schistose structure, 194, 221 Schists, 221, 261 ; regarded as part of earliest terrestrial crust, 304, 307, 308 ; formed from Silurian strata, 318 Schizodus, 375* Scolithus, 327 Scoriaceous structure, 193 Scorpions, fossil, 283, 334, 358 Screes, 25 Screw-pine, fossil, 428 Sea, proofs of former presence of, on land, 5 ; erosion of land by, 94 ; limits of erosion by, 98 ; rate of erosion by, 98, 99 ; forms by erosion a submarine plain, 98 ; accumulations formed in, 101 ; distribution of sediment in, 101, 122 ; solvent power of carbonic acid in, 118, 122; preservation of remains of plants and animals on floor of, 121 ; abundant volcanic detritus of bottom of, 136 ; evi- dence of presence of, 288 ; original composition of, 304 Sea-calf, fossil, 443 Sea-serpents, fossil, 417, 430 Sea-urchins, fossil, 361, 391, 393,* 411* Sea-water, salts in, 160, 162, 163 Sea- weeds, calcareous, 112; Silurian, Secondary rocks, 305 Secretary birds, fossil, 436 Section, stratigraphical, 294 Sedgwick, A., 316, 337 Sedimentary rocks, 186, 195, 226, 244 Selenite, 181 Senonian, 417, 420 Septaria, 188 Septaria-clay (Oligocene), 438 Sequoia, fossil, 408, 428, 448 Series, stratigraphical, 294 Serpentine, 179 ; from decomposi- tion of olivine, 178, 221 Serpentine rocks, 220 Serpula, 327 Shale, 202 ; weathering of, 17 Shallow water, evidence of, 351 Sharks, Carboniferous, 366 ; Jurassic, 396 ; Cretaceous, 415 Shearing of rocks, 222, 253, 260 Sheep, fossil, 454, 474 Sheets of eruptive rock, 268 Shell-banks, 113 Shell-marl, 6, 61, 113, 209 Shingle, 197 Shores, evidence of former, 231, 234 Shorthorn, fossil, 474 Shrews, fossil, 436 Shrimps, early forms of, 394 Siderite, 186, 187, 208, 287 Sigillaria, 238,* 291, 356, 357,* 373. 379 Silica or silicic acid, 157 ; deposited by hot springs, 80 ; secreted by plants and animals, in, 157, 171, 283 ; solubility of, 171 ; as a cement of rocks, 244 ; as a petri- fying medium, 286 Silicates, 165, 174 Silicates of alumina, 162, 175 Silicates of magnesia, 163 Siliceous cement of sandstones, 200 Siliceous sinter, 80, 208 Silicifkation, 287 Silicon, 155, 157 Silurian period, 316 Sinemurian, 401 Sinter, calcareous, 77 ; siliceous, 80, 208 2 L INDEX. Sivatherium, 454 Siwalik group, 454 Slaggy structure, 193 Smilax, 428, 448 Snakes, fossil, 441 Snow, 83 Sodium, 155, 163 Sodium-carbonate in salt -lakes, 64 Sodium - chloride, 160, 163, 184; deposits of, in salt lakes, 63 ; in- creases solubility of gypsum, 182 Soil, origin of, 21, 109, 197 Solar system, 300 Solution, crystallisation formed from, 169, 190 Solution of minerals and rocks, 18, 22, 32, 62, 63, 71 Spalacotherium, 401 Spar, 75, 76 Spathic iron, 181 Specular iron, 172 Speeton Clay, 418 Sphaerosiderite, 181, 208 Sphagnum or bog-moss, no Sphenopteris, 355* Spherulitic structure, 192, 214 Spirifera, 346,* 364, 365,* 375, 392 Spondylus, 444 Sponges, fossil, 322, 410* Springs, work done by, 66 ; abstract mineral matter, 67 ; deposit do. , 73 ; calcareous, 74 ; hot, 80 Squaloraia, 396 Squirrels, fossil, 431 Stage, stratigraphical, 294 Stagonolepis, 385* Stags, fossil, 446 Stalactites, 72, 74, 204 Stalactitic minerals, 170 Stalagmite, 74, 204, 471 Stalagmite floor of caverns, 76, 120 Star-fishes, Silurian, 326 Steam, effect of expansion of, in molten lava, 193 Stegosaurus, 399 Steneosaurus , 397 Stereognathus , 401 Stigmaria, 356, 357* Stoat, fossil, 472 Stone-lilies, 324 Stonesfield slate, 401, 403 Storm-beaches, 103 Strata, alternations of, 230 ; associa- tion of, 236 ; chronological value of, 237 ; afford evidence of de- pression, 239 ; consolidation of, 244 ; original horizontally of, 247 Stratified rocks, geological history told by, 227 Stratified structure, 53, 188, 196, 227 Stratigraphical subdivisions, 293 Stratigraphy, 230 Stratum, 229, 294 Strepsodus, 359 Streptorhynchus, 364, 365* Striation by ice, 89 Strike, 249 Stringocephalus , 346* Stringocephalus Limestone, 347 Strombus, 444 Strophalosia, 374 Strophomena, 345 Sub-Apennine Beds (Pliocene), 453 Sublimation, 169, 172, 190 Subsidence, how proved by strata, 239, 314, 350, 352 Subsidence, proofs of, 149 Subsoil, origin of, 21, 109, 197 Sub-stage or sub-group, 294 Sulphate of iron, 79 Sulphates, 159, 165, 181 Sulphide of iron, 79 Sulphides, 155, 159, 165, 184, 287 Sulphur, 155, 159, 165 Sulphuretted hydrogen, 160 Sumach, fossil, 440, 448 Sun-cracks, 233 Sun, influence of, in terrestrial changes, 124 ; history of, 301 Superposition, order of, 5, 292 Syenite, 215 SyllcBmus, 415 Syncline, 253 System" in stratigraphy, 294 TABULATE corals, 324, 361 INDEX. 5*5 Talus, formation of, 25 Tapes, 444 Tapiroid animals of Tertiary time, 431,* 436, 443 Taunusian rocks, 347 Taxodium, 448 Teleosaurus, 397 Tekrpeton, 385* Tellina, 444, 451, 462* Temperature, effect of changes of, i7 Terebratula, 364, 392, 412 TermidEe, fossil, 358 Termite, influence of, in removal of soil, 24 Terrestrial surfaces, 232 Tertiary rocks, 305 Tertiary systems, 423 Tesseral system of crystals, 167 Tetragonal system of crystals, 167 Textularia, 410* Thamnastr&a, 391 Thecosmilia, 391 Throw of faults, 258 Tillodonts, 431 Till or boulder clay, 457 Time, as measured by strata, 237 Time, influence of, in geology, 15 Tinoceras, 432* Titanic iron, 171, 173 Toads, fossil, 441 Toarcian, 401 Torsion, effects of, 247 Tortoises, fossil, 415, 430 Toxoceras, 413, 414* Trachyceras, 384 Trachyte, 215 Trails of worms, 279, 326 Transition, 316 Travertine, 77, 78, 121, 205 Tree-ferns, fossil, 374, 381 Trematosaurus, 385 Tremolite, 177 Triassic system, 379 Triclinic system of crystals, 168* Trigonia, 393, 412* Trilobites, 292, 327, 328, * 329, * 344,* 363,* 393 Trimetric system of crystals, 167 Trinucleus, 329 Tripoli powder, in, 283 Trochoceras, 334* Trogons, fossil, 436 Trogontheriiim, 451 . Trophon, 450,* 462* Tropical climate, evidence for, 289 Tufa, calcareous, 77, 471 Tuffs, 135, 202 Tulip tree, fossil, 448 Turf, conservative influence of, 109 Turonian, 417, 420 Turrilites, 414* Turtles, ancestral forms of, 397, 415, 43. 443 Type-fossils, 291 Ullmannia, 374 Ulmus, 428, 448 Uncites, 346* Unconformability, 240 Unio, 436 Unstratified rocks, 189 Upheaval. See Elevation Urus, fossil, 474 VASCULOSE, 282 Vein -quartz, 208 Veins, igneous, 216, 266, 272 ; mineral, 275 Veinstones, 275 Ventriculites, 410* Vents, volcanic, 138 Vesicular structure, 193 Victoria^ 428 Vitreous structure, 191 Volcanic action in Palaeozoic time, 314 ; Silurian, 317 ; Devonian, 338 ; Old Red Sandstone, 338 ; Carboniferous, 354 ; Permian, 377 ; Triassic, 388 ; Cretaceous, 422 ; Oligocene, 435, 437 ; Plio- cene, 447 Volcanic products, 129, 134, 202 ; vents and fissures, 138 ; craters, 127, 139 ; necks, 142, 273 ; dykes, 144, 272, 274 Volcanoes and volcanic action, 127 ; records left by, 128, 269, 270 INDEX. Voltzia, 382* Valuta, 429,* 452 WAD, 174 Wadies of the Levant, 45 Walchia, 373,* 374 Walnut, fossil, 408, 428, 446, 448 Water-bean, fossil, 428 Water, composition of, 156, 160 ; "hard," 162 Waterfalls, recession of, 43 Water-lily, fossil, 428, 451 Waves, effects of, 94 Wealden, 418 Weather, indications of, among rocks, 235 Weathering, 19-29, 164, 210 Wemmelian, 433 Wenlock group, 335 Whet-slate, 222 White-ant, influence of, in removal of soil, 24 White ants, fossil, 358 White Crag, 452 Willow, fossil, 428, 448, 461 Wind, effects of, 2, 24, 27 Wolf, fossil, 451, 472, 474 Woodcock, ancestral forms of, 430 Woodocrinus, 362* Woolwich and Reading Beds, 433 Worms, agency of, in soil-making, 24 ; traces left by marine, 279 ; earliest vestiges of, 326 YEWS, fossil, 428 Ypresian, 433 Zamites, 382, 390 Zaphrentis, 324, 361* Zechstein, 377 Zeolites, 176 // Zones, stratigraphical, 293, 393 THE END. Printed by R. & R. CLARK, Edinburgh. -X YB 24055 OF CALIFORNIA LIBRARY