THE ANCIENT LIFE-HISTORY OF THE EARTH
A COMPREHENSIVE OUTLINE OF THE PRINCIPLES AND LEADING FACTS OF PALAEONTOLOGICAL SCIENCE
BY H. ALLEYNE NICHOLSON
M.D., D.SC., M.A., PH. D. (GOeTT), F.R.S.E, F.L.S.
PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY OF ST ANDREWS
The study of Palaeontology, or the science which is concerned with the living beings which flourished upon the globe during past periods of its history, may be pursued by two parallel but essentially distinct paths. By the one method of inquiry, we may study the anatomical characters and structure of the innumerable extinct forms of life which lie buried in the rocks simply as so many organisms, with but a slight and secondary reference to the time at which they lived. By the other method, fossil animals are regarded principally as so many landmarks in the ancient records of the world, and are studied historically and as regards their relations to the chronological succession of the strata in which they are entombed. In so doing, it is of course impossible to wholly ignore their structural characters, and their relationships with animals now living upon the earth; but these points are held to occupy a subordinate place, and to require nothing more than a comparatively general attention.
In a former work, the Author has endeavoured to furnish a summary of the more important facts of Palaeontology regarded in its strictly scientific aspect, as a mere department of the great science of Biology. The present work, on the other hand, is an attempt to treat Palaeontology more especially from its historical side, and in its more intimate relations with Geology. In accordance with this object, the introductory portion of the work is devoted to a consideration of the general principles of Palaeontology, and the bearings of this science upon various geological problems—such as the mode of formation of the sedimentary rocks, the reactions of living beings upon the crust of the earth, and the sequence in time of the fossiliferous formations. The second portion of the work deals exclusively with Historical Palaeontology, each formation being considered separately, as regards its lithological nature and subdivisions, its relations to other formations, its geographical distribution, its mode of origin, and its characteristic life-forms.
In the consideration of the characteristic fossils of each successive period, a general account is given of their more important zoological characters and their relations to living forms; but the technical language of Zoology has been avoided, and the aid of illustrations has been freely called into use. It may therefore be hoped that the work may be found to be available for the purposes of both the Geological and the Zoological student; since it is essentially an outline of Historical Palaeontology, and the student of either of the above-mentioned sciences must perforce possess some knowledge of the last. Whilst primarily intended for students, it may be added that the method of treatment adopted has been so far untechnical as not to render the work useless to the general reader who may desire to acquire some knowledge of a subject of such vast and universal interest.
In carrying out the object which he has held before him, the Author can hardly expect, from the nature of the materials with which he has had to deal, that he has kept himself absolutely clear of errors, both of omission and commission. The subject, however, is one to which he has devoted the labour of many years, both in studying the researches of others and in personal investigations of his own; and he can only trust that such errors as may exist will be found to belong chiefly to the former class, and to be neither serious nor numerous. It need only be added that the work is necessarily very limited in its scope, and that the necessity of not assuming a thorough previous acquaintance with Natural History in the reader has inexorably restricted its range still further. The Author does not, therefore, profess to have given more than a merely general outline of the subject; and those who desire to obtain a more minute and detailed knowledge of Palaeontology, must have recourse to other and more elaborate treatises.
UNITED COLLEGE, ST ANDREWS.
October 2, 1876.
PRINCIPLES OF PALAEONTOLOGY.
The general objects or geological science—The older theories of catastrophistic and intermittent action—The more modern doctrines of continuous and uniform action—Bearing of these doctrines respectively on the origin or the existing terrestrial order—Elements or truth in Catastrophism—General truth of the doctrine of Continuity—Geological time.
Definition of Palaeontology—Nature of Fossils—Different processes of fossilisation.
Aqueous and igneous rocks—General characters of the sedimentary rocks—Mode or formation of the sedimentary rocks—Definition of the term "formation"—Chief divisions of the aqueous rocks—Mechanically-formed rocks, their characters and mode of origin—Chemically and organically formed rocks—Calcareous rocks—Chalk, its microscopic structure and mode of formation—Limestone, varieties, structure, and origin—Phosphate of lime—Concretions—Sulphate of lime—Silica and siliceous deposits of various kinds—Greensands—Red clays—Carbon and carbonaceous deposits.
Chronological succession of the fossiliferous rocks—Tests or age of strata—Value of Palaeontological evidence in stratigraphical Geology—General sequence of the great formations.
The breaks in the palaeontological and geological record—Use of the term "contemporaneous" as applied to groups of strata—General sequence of strata and of life-forms interfered with by more or less extensive gaps—Unconformability—Phenomena implied by this—Causes of the imperfection of the palaeontological record.
Conclusions to be drawn from fossils—Age of rocks—Mode of origin of any fossiliferous bed—Fluviatile, lacustrine, and marine deposits—Conclusions as to climate—Proofs of elevation and subsidence of portions of the earth's crust derived from fossils.
The biological relations of fossils—Extinction of life-forms—Geological range of different species—Persistent types of life—Modern origin of existing animals and plants—Reference of fossil forms to the existing primary divisions of the animal kingdom—Departure of the older types of life from those now in existence—Resemblance of the fossils of a given formation to those of the formation next above and next below—Introduction of new life-forms.
The Laurentian and Huronian periods—General nature, divisions, and geographical distribution of the Laurentian deposits—Lower and Upper Laurentian—Reasons for believing that the Laurentian rocks are not azoic based upon their containing limestones, beds of oxide of iron, and graphite—The characters, chemical composition, and minute structure of Eozooen Canadense—Comparison of Eozooen with existing Foraminifera—Archoeosphoerinoe—Huronian formation—Nature and distribution of Huronian deposits—Organic remains of the Huronian—Literature.
The Cambrian period—General succession of Cambrian deposits in Wales—Lower Cambrian and Upper Cambrian—Cambrian deposits of the continent of Europe and North American—Life of the Cambrian period—Fucoids—Eophyton—Oldhamia—Sponges—Echinoderms—Annelides —Crustaceans—Structure of Trilobites—Brachiopods—Pteropods, Gasteropods, and Bivalves—Cephalopods—Literature.
The Lower Silurian period—The Silurian rocks generally—Limits of Lower and Upper Silurian—General succession, subdivisions, and characters of the Lower Silurian rocks of Wales—General succession, subdivisions, and characters of the Lower Silurian rocks of the North American continent—Life of the period—Fucoids—Protozoa—Graptolites—Structure of Graptolites—Corals—General structure of Corals—Crinoids— Cystideans—General characters of Cystideans—Annelides— Crustaceans—Polyzoa—Brachiopods—Bivalve and Univalve Molluscs—Chambered Cephalopods—General characters of the Cephalopoda—Conodonts.
The Upper Silurian period—General succession of the Upper Silurian deposits of Wales—Upper Silurian deposits of North America—Life of the Upper Silurian—Plants—Protozoa—Graptolites—Corals— Crinoids—General structure of Crinoids—Star-fishes—Annelides— Crustaceans—Eurypterids—Polyzoa—Brachiopods—Structure of Brachiopods—Bivalves and Univalves—Pteropods—Cephalopods— Fishes—Silurian literature.
The Devonian period—Relations between the Old Red Sandstone and the marine Devonian deposits—The Old Red Sandstone of Scotland—The Devonian strata of Devonshire—Sequence and subdivisions of the Devonian deposits of North America—Life of the period—Plants—Protozoa—Corals-Crinoids—Pentremites— Annelides—Crustaceans—Insects—Polyzoa—Brachiopods—Bivalves— Univalves—Pteropods—Cephalopods—Fishes—General divisions of the Fishes—Palaeontological evidence as to the independent existence of the Devonian system as a distinct formation—Literature.
The Carboniferous period—Relations of Carboniferous rocks to Devonian—The Carboniferous Limestone or Sub-Carboniferous series—The Millstone-grit and the Coal-measures—Life of the period—Structure and mode of formation of Coal—Plants of the Coal.
Animal life of the Carboniferous period—Protozoa—Corals— Crinoids—Pentremites—Structure of Pentremites—Echinoids— Structure of Echinoidea—Annelides—Crustacea—Insects— Arachnids—Myriapods—Polyzoa—Brachiopods—Bivalves and Univalves—Cephalopods—Fishes—Labyrinthodont Amphibians— Literature.
The Permian period—General succession, characters, and mode of formation of the Permian deposits—Life of the period— Plants—Protozoa—Corals—Echinoderms—Annelides—Crustaceans— Polyzoa—Brachiopods—Bivalves-Univalves—Pteropods— Cephalopods—Fishes—Amphibians—Reptiles—Literature.
The Triassic period—General characters and subdivisions of the Trias of the Continent of Europe and Britain—Trias of North America—Life of the period—Plants—Echinoderms—Crustaceans— Polyzoa—Brachiopods—Bivalves—Univalves—Cephalopods— Intermixture of Palaeozoic with Mesozoic types of Molluscs— Fishes—Amphibians—Reptiles—Supposed footprints of Birds— Mammals—Literature.
The Jurassic period—General sequence and subdivisions of the Jurassic deposits in Britain—Jurassic rocks of North America—Life of the period—Plants—Corals—Echinoderms—Crustaceans—Insects— Brachiopods—Bivalves—Univalves-Pteropods—Tetrabranchiate Cephalopods—Dibranchiate Cephalopods—Fishes—Reptiles—Birds— Mammals—Literature.
The Cretaceous period—General succession and subdivisions of the Cretaceous rocks in Britain—Cretaceous rocks of North America—Life of the period—Plants—Protozoa—Corals—Echinoderms— Crustaceans—Polyzoa—Brachiopods—Bivalves—Univalves— Tetrabranchiate and Dibranchiate Cephalopods—Fishes—Reptiles— Birds—Literature.
The Eocene period—Relations between the Kainozoic and Mesozoic rocks in Europe and in North America—Classification of the Tertiary deposits—The sequence and subdivisions of the Eocene rocks of Britain and France—Eocene strata of the United States—Life of the period—Plants—Foraminifera—Corals—Echinoderms—Mollusca—Fishes— Reptiles—Birds—Mammals.
The Miocene period—Miocene strata of Britain—Of France—Of Belgium—Of Austria—Of Switzerland—Of Germany—Of Greece—Of India—Of North America—Of the Arctic regions—Life of the period—Vegetation of the Miocene period—Foraminifera—Corals— Echinoderms—Articulates—Mollusca—Fishes-Amphibians—Reptiles— Mammals.
The Pliocene period—Pliocene deposits of Britain—Of Europe—Of North America—Life of the period—Climate of the period as indicated by the Invertebrate animals—The Pliocene Mammalia—Literature relating to the Tertiary deposits and their fossils.
The Post-Pliocene period—Division of the Quaternary deposits into Post-Pliocene and Recent—Relations of the Post-Pliocene deposits of the northern hemisphere to the "Glacial period"—Pre-Glacial deposits—Glacial deposits—Arctic Mollusca in Glacial beds—Post-Glacial deposits—Nature and mode of formation of high-level and low-level gravels—Nature and mode of formation of cavern-deposits—Kent's Cavern-Post—Pliocene deposits of the southern hemisphere.
Life of the Post-Pliocene period—Effect of the coming on and departure of the Glacial period upon the animals inhabiting the northern hemisphere—Birds of the Post-Pliocene—Mammalia of the Post-Pliocene—Climate of the Post-Glacial period as deduced from the Post-Glacial Mammals—Occurrence of the bones and implements of Man in Post-Pliocene deposits in association with the remains of extinct Mammalia—Literature relating to the Post-Pliocene period.
The succession of life upon the globe—Gradual and successive introduction of life-forms—What is meant by "lower" and "higher" groups of animals and plants—Succession in time of the great groups of animals in the main corresponding with their zoological order—Identical phenomena in the vegetable kingdom—Persistent types of life—High organisation of many early forms—Bearings of Palaeontology on the general doctrine of Evolution.
APPENDIX.—Tabular view of the chief Divisions of the Animal Kingdom.
LIST OF ILLUSTRATIONS
FIG. 1. Cast of Trigonia longa. 2. Microscopic section of the wood of a fossil Conifer. 3. Microscopic section of the wood of the Larch. 4. Section of Carboniferous strata, Kinghorn, Fife. 5. Diagram illustrating the formation of stratified deposits. 6. Microscopic section of a calcareous breccia. 7. Microscopic section of White Chalk. 8. Organisms in Atlantic Ooze. 9. Crinoidal marble. 10. Piece of Nummulitic limestone, Pyramids. 11. Microscopic section of Foraminiferal limestone—Carboniferous, America. 12. Microscopic section of Lower Silurian limestone. 13. Microscopic section of oolitic limestone, Jurassic. 14. Microscopic section of oolitic limestone, Carboniferous. 15. Organisms in Barbadoes earth. 16. Organisms in Richmond earth. 17. Ideal section of the crust of the earth. 18. Unconformable junction of Chalk and Eocene rocks. 19. Erect trunk of a Sigillaria. 20. Diagrammatic section of the Laurentian rocks. 21. Microscopic section of Laurentian limestone. 22. Fragment of a mass of Eozooen Canadense. 23. Diagram illustrating the structure of Eozooen. 24. Microscopic section of Eozooen Canadense. 25. Nonionina and Gromia. 26. Group of shells of living Foraminifera. 27. Diagrammatic section of Cambrian strata. 28. Eophyton Linneanum. 29. Oldhamia antiqua. 30. Scolithus Canadensis. 31. Group of Cambrian Trilobites. 32. Group of characteristic Cambrian fossils. 33. Fragment of Dictyonema sociale. 34. Generalised section of the Lower Silurian rocks of Wales. 35. Generalised section of the Lower Silurian rocks of North America. 36. Licrophycus Ottawaensis. 37. Astylospongia proemorsa. 38. Stromatopora rugosa. 39. Dichograptus octobrachiatus. 40. Didymograptus divaricatus. 41. Diplograptus pristis. 42. Phyllograptus typus. 43. Zaphrentis Stokesi. 44. Strombodes pentagonus. 45. Columnaria alveolata. 46. Group of Cystideans. 47. Group of Lower Silurian Crustaceans. 48. Ptilodictya falciformis. 49. Ptilodictya Schafferi. 50. Group of Lower Silurian Brachiopods. 51. Group of Lower Silurian Brachiopods. 52. Murchisonia gracilis. 53. Bellerophon argo. 54. Maclurea crenulata. 55. Orthoceras crebriseptum. 56. Restoration of Orthoceras. 57. Generalised section of the Upper Silurian rocks. 58. Monograptus priodon. 59. Halysites catenularia and H. agglomerata. 60. Group of Upper Silurian Star-fishes. 61. Protaster Sedgwickii. 62. Group of Upper Silurian Crinoids. 63. Planolites vulgaris. 64. Group of Upper Silurian Trilobites. 65. Pterygotus Anglicus. 66. Group of Upper Silurian Polyzoa. 67. Spirifera hysterica. 68. Group of Upper Silurian Brachiopods. 69. Group of Upper Silurian Brachiopods. 70. Pentamerus Knightii. 71. Cardiola interrupta, C. fibrosa, and Pterinoea subfalcata. 72. Group of Upper Silurian Univalves. 73. Tentaculites ornatus. 74. Pteraspis Banksii. 75. Onchus tenuistriatus and Thelodus. 76. Generalised section of the Devonian rocks of North America. 77. Psilophyton princeps. 78. Prototaxites Logani. 79. Stromatopora tuberculata. 80. Cystiphyllum vesiculosum. 81. Zaphrentis cornicula. 82. Heliophyllum exiguum. 83. Crepidophyllum Archiaci. 84. Favosites Gothlandica. 85. Favosites hemisphoerica. 86. Spirorbis omphalodes and S. Arkonensis. 87. Spirorbis laxus and S. Spinulifera. 88. Group of Devonian Trilobites. 89. Wing of Platephemera antiqua. 90. Clathropora intertexta. 91. Ceriopora Hamiltonensis. 92. Fenestella magnifica. 93. Retepora Phillipsi. 94. Fenestella cribrosa. 95. Spirifera sculptilis. 96. Spirifera mucronata. 97. Atrypa reticularis. 98. Strophomena rhomboidalis. 99. Platyceras dumosum. 100. Conularia ornata. 101. Clymenia Sedgwickii. 102. Group of Fishes from the Devonian rocks of North America. 103. Cephalaspis Lyellii. 104. Pterichthys cornutus. 105. Polypterus and Osteolepis. 106. Holoptychius nobilissimus. 107. Generalised section of the Carboniferous rocks of the North of England. 108. Odontopteris Schlotheimii. 109. Calamites cannoeformis. 110. Lepidodendron Sternbergii. 111. Sigillaria Groeseri. 112. Stigmaria ficoides. 113. Trigonocarpum ovatum. 114. Microscopic section of Foraminiferal limestone—Carboniferous, North America. 115. Fusulina cylindrica. 116. Group of Carboniferous Corals. 117. Platycrinus tricontadactylus. 118. Pentremites pyriformis and P. conoideus. 119. Archoeocidaris ellipticus. 120. Spirorbis Carbonarius. 121. Prestwichia rotundata. 122. Group of Carboniferous Crustaceans. 123. Cyclophthalmus senior. 124. Xylobius Sigillarioe. 125. Haplophlebium Barnesi. 126. Group of Carboniferous Polyzoa. 127. Group of Carboniferous Brachiopoda. 128. Pupa vetusta. 129. Goniatites Fossoe. 130. Amblypterus macropterus. 131. Cochliodus contortus. 132. Anthracosaurus Russelli. 133. Generalised section of the Permian rocks. 134. Walchia piniformis. 135. Group of Permian Brachiopods. 136. Arca antiqua. 137. Platysomus gibbosus. 138. Protorosaurus Speneri. 139. Generalised section of the Triassic rocks. 140. Zamia spiralis. 141. Triassic Conifers and Cycads. 142. Encrinus liliiformis. 143. Aspidura loricata. 144. Group of Triassic Bivalves. 145. Ceratites nodosus. 146. Tooth of Ceratodus serratus and C. Altus. 147. Ceratodus Fosteri. 148. Footprints of Cheirotherium. 149. Section of tooth of Labyrinthodont. 150. Skull of Mastodonsaurus. 151. Skull of Rhynchosaurus. 152. Belodon, Nothosaurus, Paloeosaurus, &c. 153. Placodus gigas. 154. Skulls of Dicynodon and Oudenodon. 155. Supposed footprint of Bird, from the Trias of Connecticut. 156. Lower jaw of Dromatherium sylvestre. 157. Molar tooth of Microlestes antiquus. 158. Myrmecobius fasciatus. 159. Generalised section of the Jurassic rocks. 160. Mantellia megalophylla. 161. Thecosmilia annularis. 162. Pentacrinus fasciculosus. 163. Hemicidaris crenularis. 164. Eryon arctiformis. 165. Group of Jurassic Brachiopods. 166. Ostrea Marshii. 167. Gryphoea incurva 168. Diceras arietina. 169. Nerinoea Goodhallii. 170. Ammonites Humphresianus. 171. Ammonites bifrons. 172. Beloteuthis subcostata. 173. Belemnite restored; diagram of Belemnite; Belemnites canaliculata. 174. Tetragonolepis. 175. Acrodus nobilis. 176. Ichthyosaurus communis. 177. Plesiosaurus dolichodeirus. 178. Pterodactylus crassirostris. 179. Ramphorhynchus Bucklandi, restored. 180. Skull of Megalosaurus. 181. Archoeopteryx macrura. 182. Archoeopteryx, restored. 183. Jaw of Amphitherium Prevostii. 184. Jaws of Oolitic Mammals. 185. Generalised section of the Cretaceous rocks. 186. Cretaceous Angiosperms. 187. Rotalia Boueana. 188. Siphonia ficus. 189. Ventriculites simplex. 190. Synhelia Sharpeana. 191. Galerites albogalerus. 192. Discoidea cylindrica. 193. Escharina Oceani. 194. Terebratella Astieriana. 195. Crania Ignabergensis. 196. Ostrea Couloni. 197. Spondylus spinosus. 198. Inoceramus sulcatus. 199. Hippurites Toucasiana. 200. Voluta elongata. 201. Nautilus Danicus. 202. Ancyloceras Matheronianus. 203. Turrilites catenatus 204. Forms of Cretaceous Ammonitidoe. 205. Belemnitella mucronata. 206. Tooth of Hybodus. 207. Fin-spine of Hybodus. 208. Beryx Lewesiensis and Osmeroides Mantelli. 209. Teeth of Iguanodon. 210. Skull of Mosasaurus Camperi. 211. Chelone Benstedi. 212. Jaws and vertebrae of Odontornithes. 213. Fruit of Nipadites. 214. Nummulina loevigata. 215. Turbinolia sulcata. 216. Cardita planicosta. 217. Typhis tubifer. 218. Cyproea elegans. 219. Cerithium hexagonum. 220. Limnoea pyramidalis. 221. Physa columnaris. 222. Cyclostoma Arnoudii. 223. Rhombus minimus. 224. Otodus obliquus. 225. Myliobatis Edwardsii. 226. Upper jaw of Alligator. 227. Skull of Odontopteryx toliapicus. 228. Zeuglodon cetoides. 229. Paloeotherium magnum, restored. 230. Feet of Equidoe. 231. Anoplothelium commune. 232. Skull of Dinoceras mirabilis. 233. Vespertilio Parisiensis. 234. Miocene Palms. 235. Platanus aceroides. 236. Cinnamomum polymorphum. 237. Textularia Meyeriana. 238. Scutella subrotunda. 239. Hyalea Orbignyana. 240. Tooth of Oxyrhina. 241. Tooth of Carcharodon. 242. Andrias Scheuchzeri. 243. Skull of Brontotherium ingens. 244, Hippopotamus Sivalensis. 245. Skull of Sivatherium. 246. Skull of Deinotherium. 247. Tooth of Elephas planfrons and of Mastodon Sivalensis. 248. Jaw of Pliopithecus. 249. Rhinoceros Etruscus and R. megarhinus. 250. Molar tooth of Mastodon Arvernensis. 251. Molar tooth of Etephas meridionalis. 252. Molar tooth of Elephas antiquus. 253. Skull and tooth of Machairodus cultridens. 254. Pecten Islandicus. 255. Diagram of high-level and low-level gravels. 256. Diagrammatic section of Cave. 257. Dinornis elephantopus. 258. Skull of Diprotodon. 259. Skull of Thylacoleo. 260. Skeleton of Megatherium. 261. Skeleton of Mylodon. 262. Glyptodon clavipes. 263. Skull of Rhinoceros tichorhinus. 264. Skeleton of Cervus megaceros. 265. Skull of Bos primigenius. 266. Skeleton of Mammoth. 267. Molar tooth of Mammoth. 268. Skull of Ursus speloeus. 269. Skull of Hyoena speloea. 270. Lower jaw of Trogontherium Cuvieri.
PRINCIPLES OF PALAEONTOLOGY.
THE LAWS OF GEOLOGICAL ACTION.
Under the general title of "Geology" are usually included at least two distinct branches of inquiry, allied to one another in the closest manner, and yet so distinct as to be largely capable of separate study. Geology, in its strict sense, is the science which is concerned with the investigation of the materials which compose the earth, the methods in which those materials have been arranged, and the causes and modes of origin of these arrangements. In this limited aspect, Geology is nothing more than the Physical Geography of the past, just as Physical Geography is the Geology of to-day; and though it has to call in the aid of Physics, Astronomy, Mineralogy, Chemistry, and other allies more remote, it is in itself a perfectly distinct and individual study. One has, however, only to cross the threshold of Geology to discover that the field and scope of the science cannot be thus rigidly limited to purely physical problems. The study of the physical development of the earth throughout past ages brings us at once in contact with the forms of animal and vegetable life which peopled its surface in bygone epochs, and it is found impossible adequately to comprehend the former, unless we possess some knowledge of the latter. However great its physical advances may be, Geology remains imperfect till it is wedded with Palaeontology, a study which essentially belongs to the vast complex of the Biological Sciences, but at the same time has its strictly geological side. Dealing, as it does, wholly with the consideration of such living beings as do not belong exclusively to the present order of things, Palaeontology is, in reality, a branch of Natural History, and may be regarded as substantially the Zoology and Botany of the past. It is the ancient life-history of the earth, as revealed to us by the labours of palaeontologists, with which we have mainly to do here; but before entering upon this, there are some general questions, affecting Geology and Palaeontology alike, which may be very briefly discussed.
[Footnote 1: Gr. ge, the earth; logos, a discourse.]
[Footnote 2: Gr. palaios, ancient; onta, beings; logos, discourse.]
The working geologist, dealing in the main with purely physical problems, has for his object to determine the material structure of the earth, and to investigate, as far as may be, the long chain of causes of which that structure is the ultimate result. No wider or more extended field of inquiry could be found; but philosophical geology is not content with this. At all the confines of his science, the transcendental geologist finds himself confronted with some of the most stupendous problems which have ever engaged the restless intellect of humanity. The origin and primaeval constitution of the terrestrial globe, the laws of geologic action through long ages of vicissitude and development, the origin of life, the nature and source of the myriad complexities of living beings, the advent of man, possibly even the future history of the earth, are amongst the questions with which the geologist has to grapple in his higher capacity.
These are problems which have occupied the attention of philosophers in every age of the world, and in periods long antecedent to the existence of a science of geology. The mere existence of cosmogonies in the religion of almost every nation, both ancient and modern, is a sufficient proof of the eager desire of the human mind to know something of the origin of the earth on which we tread. Every human being who has gazed on the vast panorama of the universe, though it may have been but with the eyes of a child, has felt the longing to solve, however imperfectly, "the riddle of the painful earth," and has, consciously or unconsciously, elaborated some sort of a theory as to the why and wherefore of what he sees. Apart from the profound and perhaps inscrutable problems which lie at the bottom of human existence, men have in all ages invented theories to explain the common phenomena of the material universe; and most of these theories, however varied in their details, turn out on examination to have a common root, and to be based on the same elements. Modern geology has its own theories on the same subject, and it will be well to glance for a moment at the principles underlying the old and the new views.
It has been maintained, as a metaphysical hypothesis, that there exists in the mind of man an inherent principle, in virtue of which he believes and expects that what has been, will be; and that the course of nature will be a continuous and uninterrupted one. So far, however, from any such belief existing as a necessary consequence of the constitution of the human mind, the real fact seems to be that the contrary belief has been almost universally prevalent. In all old religions, and in the philosophical systems of almost all ancient nations, the order of the universe has been regarded as distinctly unstable, mutable, and temporary. A beginning and an end have always been assumed, and the course of terrestrial events between these two indefinite points has been regarded as liable to constant interruption by revolutions and catastrophes of different kinds, in many cases emanating from supernatural sources. Few of the more ancient theological creeds, and still fewer of the ancient philosophies, attained body and shape without containing, in some form or another, the belief in the existence of periodical convulsions, and of alternating cycles of destruction and repair.
That geology, in its early infancy, should have become imbued with the spirit of this belief, is no more than might have been expected; and hence arose the at one time powerful and generally-accepted doctrine of "Catastrophism." That the succession of phenomena upon the globe, whereby the earth's crust had assumed the configuration and composition which we find it to possess, had been a discontinuous and broken succession, was the almost inevitable conclusion of the older geologists. Everywhere in their study of the rocks they met with apparently impassable gaps, and breaches of continuity that could not be bridged over. Everywhere they found themselves conducted abruptly from one system of deposits to others totally different in mineral character or in stratigraphical position. Everywhere they discovered that well-marked and easily recognisable groups of animals and plants were succeeded, without the intermediation of any obvious lapse of time, by other assemblages of organic beings of a different character. Everywhere they found evidence that the earth's crust had undergone changes of such magnitude as to render it seemingly irrational to suppose that they could have been produced by any process now in existence. If we add to the above the prevalent belief of the time as to the comparative brevity of the period which had elapsed since the birth of the globe, we can readily understand the general acceptance of some form of catastrophism amongst the earlier geologists.
As regards its general sense and substance, the doctrine of catastrophism held that the history of the earth, since first it emerged from the primitive chaos, had been one of periods of repose, alternating with catastrophes and cataclysms of a more or less violent character. The periods of tranquillity were supposed to have been long and protracted; and during each of them it was thought that one of the great geological "formations" was deposited. In each of these periods, therefore, the condition of the earth was supposed to be much the same as it is now—sediment was quietly accumulated at the bottom of the sea, and animals and plants flourished uninterruptedly in successive generations. Each period of tranquillity, however, was believed to have been, sooner or later, put an end to by a sudden and awful convulsion of nature, ushering in a brief and paroxysmal period, in which the great physical forces were unchained and permitted to spring into a portentous activity. The forces of subterranean fire, with their concomitant phenomena of earthquake and volcano, were chiefly relied upon as the efficient causes of these periods of spasm and revolution. Enormous elevations of portions of the earth's crust were thus believed to be produced, accompanied by corresponding and equally gigantic depressions of other portions. In this way new ranges of mountains were produced, and previously existing ranges levelled with the ground, seas were converted into dry land, and continents buried beneath the ocean—catastrophe following catastrophe, till the earth was rendered uninhabitable, and its races of animals and plants were extinguished, never to reappear in the same form. Finally, it was believed that this feverish activity ultimately died out, and that the ancient peace once more came to reign upon the earth. As the abnormal throes and convulsions began to be relieved, the dry land and sea once more resumed their relations of stability, the conditions of life were once more established, and new races of animals and plants sprang into existence, to last until the supervention of another fever-fit.
Such is the past history of the globe, as sketched for us, in alternating scenes of fruitful peace and revolutionary destruction, by the earlier geologists. As before said, we cannot wonder at the former general acceptance of Catastrophistic doctrines. Even in the light of our present widely-increased knowledge, the series of geological monuments remains a broken and imperfect one; nor can we ever hope to fill up completely the numerous gaps with which the geological record is defaced. Catastrophism was the natural method of accounting for these gaps, and, as we shall see, it possesses a basis of truth. At present, however, catastrophism may be said to be nearly extinct, and its place is taken by the modern doctrine of "Continuity" or "Uniformity"—a doctrine with which the name of Lyell must ever remain imperishably associated.
The fundamental thesis of the doctrine of Uniformity is, that, in spite of all apparent violations of continuity, the sequence of geological phenomena has in reality been a regular and uninterrupted one; and that the vast changes which can be shown to have passed over the earth in former periods have been the result of the slow and ceaseless working of the ordinary physical forces—acting with no greater intensity than they do now, but acting through enormously prolonged periods. The essential element in the theory of Continuity is to be found in the allotment of indefinite time for the accomplishment of the known series of geological changes. It is obviously the case, namely, that there are two possible explanations of all phenomena which lie so far concealed in "the dark backward and abysm of time," that we can have no direct knowledge of the manner in which they were produced. We may, on the one hand, suppose them to be the result of some very powerful cause, acting through a short period of time. That is Catastrophism. Or, we may suppose them to be caused by a much weaker force operating through a proportionately prolonged period. This is the view of the Uniformitarians. It is a question of energy versus time and it is time which is the true element of the case. An earthquake may remove a mountain in the course of a few seconds; but the dropping of the gentle rain will do the same, if we extend its operations over a millennium. And this is true of all agencies which are now at work, or ever have been at work, upon our planet. The Catastrophists, believing that the globe is but, as it were, the birth of yesterday, were driven of necessity to the conclusion that its history had been checkered by the intermittent action of paroxysmal and almost inconceivably potent forces. The Uniformitarians, on the other hand, maintaining the "adequacy of existing causes," and denying that the known physical forces ever acted in past time with greater intensity than they do at present, are, equally of necessity, driven to the conclusion that the world is truly in its "hoary eld," and that its present state is really the result of the tranquil and regulated action of known forces through unnumbered and innumerable centuries.
The most important point for us, in the present connection, is the bearing of these opposing doctrines upon the question, as to the origin of the existing terrestrial order. On any doctrine of uniformity that order has been evolved slowly, and, according to law, from a pre-existing order. Any doctrine of catastrophism, on the other hand, carries with it, by implication, the belief that the present order of things was brought about suddenly and irrespective of any pre-existent order; and it is important to hold clear ideas as to which of these beliefs is the true one. In the first place, we may postulate that the world had a beginning, and, equally, that the existing terrestrial order had a beginning. However far back we may go, geology does not, and cannot, reach the actual beginning of the world; and we are, therefore, left simply to our own speculations on this point. With regard, however, to the existing terrestrial order, a great deal can be discovered, and to do so is one of the principal tasks of geological science. The first steps in the production of that order lie buried in the profound and unsearchable depths of a past so prolonged as to present itself to our finite minds as almost in eternity. The last steps are in the prophetic future, and can be but dimly guessed at. Between the remote past and the distant future, we have, however, a long period which is fairly open to inspection; and in saying a "long" period, it is to be borne in mind that this term is used in its geological sense. Within this period, enormously long as it is when measured by human standards, we can trace with reasonable certainty the progressive march of events, and can determine the laws of geological action, by which the present order of things has been brought about.
The natural belief on this subject doubtless is, that the world, such as we now see it, possessed its present form and configuration from the beginning. Nothing can be more natural than the belief that the present continents and oceans have always been where they are now; that we have always had the same mountains and plains; that our rivers have always had their present courses, and our lakes their present positions; that our climate has always been the same; and that our animals and plants have always been identical with those now familiar to us. Nothing could be more natural than such a belief, and nothing could be further removed from the actual truth. On the contrary, a very slight acquaintance with geology shows us, in the words of Sir John Herschel, that "the actual configuration of our continents and islands, the coast-lines of our maps, the direction and elevation of our mountain-chains, the courses of our rivers, and the soundings of our oceans, are not things primordially arranged in the construction of our globe, but results of successive and complex actions on a former state of things; that, again, of similar actions on another still more remote; and so on, till the original and really permanent state is pushed altogether out of sight and beyond the reach even of imagination; while on the other hand, a similar, and, as far as we can see, interminable vista is opened out for the future, by which the habitability of our planet is secured amid the total abolition on it of the present theatres of terrestrial life."
Geology, then, teaches us that the physical features which now distinguish the earth's surface have been produced as the ultimate result of an almost endless succession of precedent changes. Palaeontology teaches us, though not yet in such assured accents, the same lesson. Our present animals and plants have not been produced, in their innumerable forms, each as we now know it, as the sudden, collective, and simultaneous birth of a renovated world. On the contrary, we have the clearest evidence that some of our existing animals and plants made their appearance upon the earth at a much earlier period than others. In the confederation of animated nature some races can boast of an immemorial antiquity, whilst others are comparative parvenus. We have also the clearest evidence that the animals and plants which now inhabit the globe have been preceded, over and over again, by other different assemblages of animals and plants, which have flourished in successive periods of the earth's history, have reached their culmination, and then have given way to a fresh series of living beings. We have, finally, the clearest evidence that these successive groups of animals and plants (faunae and florae) are to a greater or less extent directly connected with one another. Each group is, to a greater or less extent, the lineal descendant of the group which immediately preceded it in point of time, and is more or less fully concerned with giving origin to the group which immediately follows it. That this law of "evolution" has prevailed to a great extent is quite certain; but it does not meet all the exigencies of the case, and it is probable that its action has been supplemented by some still unknown law of a different character.
We shall have to consider the question of geological "continuity" again. In the meanwhile, it is sufficient to state that this doctrine is now almost universally accepted as the basis of all inquiries, both in the domain of geology and that of palaeontology. The advocates of continuity possess one immense advantage over those who believe in violent and revolutionary convulsions, that they call into play only agencies of which we have actual knowledge. We know that certain forces are now at work, producing certain modifications in the present condition of the globe; and we know that these forces are capable of producing the vastest of the changes which geology brings under our consideration, provided we assign a time proportionately vast for their operation. On the other hand, the advocates of catastrophism, to make good their views, are compelled to invoke forces and actions, both destructive and restorative, of which we have, and can have, no direct knowledge. They endow the whirlwind and the earthquake, the central fire and the rain from heaven, with powers as mighty as ever imagined in fable, and they build up the fragments of a repeatedly shattered world by the intervention of an intermittently active creative power.
It should not be forgotten, however, that from one point of view there is a truth in catastrophism which is sometimes overlooked by the advocates of continuity and uniformity. Catastrophism has, as its essential feature, the proposition that the known and existing forces of the earth at one time acted with much greater intensity and violence than they do at present, and they carry down the period of this excessive action to the commencement of the present terrestrial order. The Uniformitarians, in effect, deny this proposition, at any rate as regards any period of the earth's history of which we have actual cognisance. If, however, the "nebular hypothesis" of the origin of the universe be well founded—as is generally admitted—then, beyond question, the earth is a gradually cooling body, which has at one time been very much hotter than it is at present. There has been a time, therefore, in which the igneous forces of the earth, to which we owe the phenomena of earthquakes and volcanoes, must have been far more intensely active than we can conceive of from anything that we can see at the present day. By the same hypothesis, the sun is a cooling body, and must at one time have possessed a much higher temperature than it has at present. But increased heat of the sun would seriously alter the existing conditions affecting the evaporation and precipitation of moisture on our earth; and hence the aqueous forces may also have acted at one time more powerfully than they do now. The fundamental principle of catastrophism is, therefore, not wholly vicious; and we have reason to think that there must have been periods—very remote, it is true, and perhaps unrecorded in the history of the earth—in which the known physical forces may have acted with an intensity much greater than direct observation would lead us to imagine. And this may be believed, altogether irrespective of those great secular changes by which hot or cold epochs are produced, and which can hardly be called "catastrophistic," as they are produced gradually, and are liable to recur at definite intervals.
Admitting, then, that there is a truth at the bottom of the once current doctrines of catastrophism, still it remains certain that the history of the earth has been one of law in all past time, as it is now. Nor need we shrink back affrighted at the vastness of the conception—the vaster for its very vagueness—that we are thus compelled to form as to the duration of geological time. As we grope our way backward through the dark labyrinth of the ages, epoch succeeds to epoch, and period to period, each looming more gigantic in its outlines and more shadowy in its features, as it rises, dimly revealed, from the mist and vapour of an older and ever-older past. It is useless to add century to century or millennium to millennium. When we pass a certain boundary-line, which, after all, is reached very soon, figures cease to convey to our finite faculties any real notion of the periods with which we have to deal. The astronomer can employ material illustrations to give form and substance to our conceptions of celestial space; but such a resource is unavailable to the geologist. The few thousand years of which we have historical evidence sink into absolute insignificance beside the unnumbered aeons which unroll themselves one by one as we penetrate the dim recesses of the past, and decipher with feeble vision the ponderous volumes in which the record of the earth is written. Vainly does the strained intellect seek to overtake an ever-receding commencement, and toil to gain some adequate grasp of an apparently endless succession. A beginning there must have been, though we can never hope to fix its point. Even speculation droops her wings in the attenuated atmosphere of a past so remote, and the light of imagination is quenched in the darkness of a history so ancient. In time, as in space, the confines of the universe must ever remain concealed from us, and of the end we know no more than of the beginning. Inconceivable as is to us the lapse of "geological time," it is no more than "a mere moment of the past, a mere infinitesimal portion of eternity." Well may "the human heart, that weeps and trembles," say, with Richter's pilgrim through celestial space, "I will go no farther; for the spirit of man acheth with this infinity. Insufferable is the glory of God. Let me lie down in the grave, and hide me from the persecution of the Infinite, for end, I see, there is none."
THE SCOPE AND MATERIALS OF PALAEONTOLOGY.
The study of the rock-masses which constitute the crust of the earth, if carried out in the methodical and scientific manner of the geologist, at once brings us, as has been before remarked, in contact with the remains or traces of living beings which formerly dwelt upon the globe. Such remains are found, in greater or less abundance, in the great majority of rocks; and they are not only of great interest in themselves, but they have proved of the greatest importance as throwing light upon various difficult problems in geology, in natural history, in botany, and in philosophy. Their study constitutes the science of palaeontology; and though it is possible to proceed to a certain length in geology and zoology without much palaeontological knowledge, it is hardly possible to attain to a satisfactory general acquaintance with either of these subjects without having mastered the leading facts of the first. Similarly, it is not possible to study palaeontology without some acquaintance with both geology and natural history.
Palaeontology, then, is the science which treats of the living beings, whether animal or vegetable, which have inhabited the earth during past periods of its history. Its object is to elucidate, as far as may be, the structure, mode of existence, and habits of all such ancient forms of life; to determine their position in the scale of organised beings; to lay down the geographical limits within which they flourished; and to fix the period of their advent and disappearance. It is the ancient life-history of the earth; and were its record complete, it would furnish us with a detailed knowledge of the form and relations of all the animals and plants which have at any period flourished upon the land-surfaces of the globe or inhabited its waters; it would enable us to determine precisely their succession in time; and it would place in our hands an unfailing key to the problems of evolution. Unfortunately, from causes which will be subsequently discussed, the palaeontological record is extremely imperfect, and our knowledge is interrupted by gaps, which not only bear a large proportion to our solid information, but which in many cases are of such a nature that we can never hope to fill them up.
Fossils.—The remains of animals or vegetables which we now find entombed in the solid rock, and which constitute the working material of the palaeontologist, are termed "fossils," or "petrifactions." In most cases, as can be readily understood, fossils are the actual hard parts of animals and plants which were in existence when the rock in which they are now found was being deposited. Most fossils, therefore, are of the nature of the shells of shell-fish, the skeletons of coral-zoophytes, the bones of vertebrate animals, or the wood, bark, or leaves of plants. All such bodies are more or less of a hard consistence to begin with, and are capable of resisting decay for a longer or shorter time—hence the frequency with which they occur in the fossil condition. Strictly speaking, however, by the term "fossil" must be understood "any body, or the traces of the existence of any body, whether animal or vegetable, which has been buried in the earth by natural causes" (Lyell). We shall find, in fact, that many of the objects which we have to study as "fossils" have never themselves actually formed parts of any animal or vegetable, though they are due to the former existence of such organisms, and indicate what was the nature of these. Thus the footprints left by birds, or reptiles, or quadrupeds upon sand or mud, are just as much proofs of the former existence of these animals as would be bones, feathers, or scales, though in themselves they are inorganic. Under the head of fossils, therefore, come the footprints of air-breathing vertebrate animals; the tracks, trails, and burrows of sea-worms, crustaceans, or molluscs; the impressions left on the sand by stranded jelly-fishes; the burrows in stone or wood of certain shell-fish; the "moulds" or "casts" of shells, corals, and other organic remains; and various other bodies of a more or less similar nature.
[Footnote 3: Lat. fossus, dug up.]
Fossilisation.— The term "fossilisation" is applied to all those processes through which the remains of organised beings may pass in being converted into fossils. These processes are numerous and varied; but there are three principal modes of fossilisation which alone need be considered here. In the first instance, the fossil is to all intents and purposes an actual portion of the original organised being—such as a bone, a shell, or a piece of wood. In some rare instances, as in the case of the body of the Mammoth discovered embedded in ice at the mouth of the Lena in Siberia, the fossil may be preserved almost precisely in its original condition, and even with its soft parts uninjured. More commonly, certain changes have taken place in the fossil, the principal being the more or less total removal of the organic matter originally present. Thus bones become light and porous by the removal of their gelatine, so as to cleave to the tongue on being applied to that organ; whilst shells become fragile, and lose their primitive colours. In other cases, though practically the real body it represents, all the cavities of the fossil, down to its minutest recesses, may have become infiltrated with mineral matter. It need hardly be added, that it is in the more modern rocks that we find the fossils, as a rule, least changed from their former condition; but the original structure is often more or less completely retained in some of the fossils from even the most ancient formations.
In the second place, we very frequently meet with fossils in the state of "casts" or moulds of the original organic body. What occurs in this case will be readily understood if we imagine any common bivalve shell, as an Oyster, or Mussel, or Cockle, embedded in clay or mud. If the clay were sufficiently soft and fluid, the first thing would be that it would gain access to the interior of the shell, and would completely fill up the space between the valves. The pressure, also, of the surrounding matter would insure that the clay would everywhere adhere closely to the exterior of the shell. If now we suppose the clay to be in any way hardened so as to be converted into stone, and if we were to break up the stone, we should obviously have the following state of parts. The clay which filled the shell would form an accurate cast of the interior of the shell, and the clay outside would give us an exact impression or cast of the exterior of the shell (fig. 1). We should have, then, two casts, an interior and an exterior, and the two would be very different to one another, since the inside of a shell is very unlike the outside. In the case, in fact, of many univalve shells, the interior cast or "mould" is so unlike the exterior cast, or unlike the shell itself, that it may be difficult to determine the true origin of the former.
It only remains to add that there is sometimes a further complication. If the rock be very porous and permeable by water, it may happen that the original shell is entirely dissolved away, leaving the interior cast loose, like the kernel of a nut, within the case formed by the exterior cast. Or it may happen that subsequent to the attainment of this state of things, the space thus left vacant between the interior and exterior cast—the space, that is, formerly occupied by the shell itself—may be filled up by some foreign mineral deposited there by the infiltration of water. In this last case the splitting open of the rock would reveal an interior cast, an exterior cast, and finally a body which would have the exact form of the original shell, but which would be really a much later formation, and which would not exhibit under the microscope the minute structure of shell.
In the third class of cases we have fossils which present with the greatest accuracy the external form, and even sometimes the internal minute structure, of the original organic body, but which, nevertheless, are not themselves truly organic, but have been formed by a "replacement" of the particles of the primitive organism by some mineral substance. The most elegant example of this is afforded by fossil wood which has been "silicified" or converted into flint (silex). In such cases we have fossil wood which presents the rings of growth and fibrous structure of recent wood, and which under the microscope exhibits the minutest vessels which characterise ligneous tissue, together with the even more minute markings of the vessels (fig. 2). The whole, however, instead of being composed of the original carbonaceous matter of the wood, is now converted into flint. The only explanation that can be given of this by no means rare phenomenon, is that the wood must have undergone a slow process of decay in water charged with silica or flint in solution. As each successive particle of wood was removed by decay, its place was taken by a particle of flint deposited from the surrounding water, till ultimately the entire wood was silicified. The process, therefore, resembles what would take place if we were to pull down a house built of brick by successive bricks, replacing each brick as removed by a piece of stone of precisely the same size and form. The result of this would be that the house would retain its primitive size, shape, and outline, but it would finally have been converted from a house of brick into a house of stone. Many other fossils besides wood—such as shells, corals, sponges, &c.—are often found silicified; and this may be regarded as the commonest form of fossilisation by replacement. In other cases, however, though the principle of the process is the same, the replacing substance may be iron pyrites, oxide of iron, sulphur, malachite, magnesite, talc, &c.; but it is rarely that the replacement with these minerals is so perfect as to preserve the more delicate details of internal structure.
THE FOSSILIFEROUS ROCKS.
Fossils are found in rocks, though not universally or promiscuously; and it is therefore necessary that the palaeontologist should possess some acquaintance with, at any rate, those rocks which yield organic remains, and which are therefore said to be "fossiliferous." In geological language, all the materials which enter into the composition of the solid crust of the earth, be their texture what it may—from the most impalpable mud to the hardest granite—are termed "rocks;" and for our present purpose we may divide these into two great groups. In the first division are the Igneous Rocks—such as the lavas and ashes of volcanoes—which are formed within the body of the earth itself, and which owe their structure and origin to the action of heat. The Igneous Rocks are formed primarily below the surface of the earth, which they only reach as the result of volcanic action; they are generally destitute of distinct "stratification," or arrangement in successive layers; and they do not contain fossils, except in the comparatively rare instances where volcanic ashes have enveloped animals or plants which were living in the sea or on the land in the immediate vicinity of the volcanic focus. The second great division of rocks is that of the Fossiliferous, Aqueous, or Sedimentary Rocks. These are formed at the surface of the earth, and, as implied by one of their names, are invariably deposited in water. They are produced by vital or chemical action, or are formed from the "sediment" produced by the disintegration and reconstruction of previously existing rocks, without previous solution; they mostly contain fossils; and they are arranged in distinct layers or "strata." The so-called "aerial" rocks which, like beds of blown sand, have been formed by the action of the atmosphere, may also contain fossils; but they are not of such importance as to require special notice here.
For all practical purposes, we may consider that the Aqueous Rocks are the natural cemetery of the animals and plants of bygone ages; and it is therefore essential that the palaeontological student should be acquainted with some of the principal facts as to their physical characters, their minute structure and mode of origin, their chief varieties, and their historical succession.
The Sedimentary or Fossiliferous Rocks form the greater portion of that part of the earth's crust which is open to our examination, and are distinguished by the fact that they are regularly "stratified" or arranged in distinct and definite layers or "strata." These layers may consist of a single material, as in a block of sandstone, or they may consist of different materials. When examined on a large scale, they are always found to consist of alternations of layers of different mineral composition. We may examine any given area, and find in it nothing but one kind of rock—sandstone, perhaps, or limestone. In all cases, however, if we extend our examination sufficiently far, we shall ultimately come upon different rocks; and, as a general rule, the thickness of any particular set of beds is comparatively small, so that different kinds of rock alternate with one another in comparatively small spaces.
As regards the origin of the Sedimentary Rocks, they are for the most part "derivative" rocks, being derived from the wear and tear of pre-existent rocks. Sometimes, however, they owe their origin to chemical or vital action, when they would more properly be spoken of simply as Aqueous Rocks. As to their mode of deposition, we are enabled to infer that the materials which compose them have formerly been spread out by the action of water, from what we see going on every day at the mouths of our great rivers, and on a smaller scale wherever there is running water. Every stream, where it runs into a lake or into the sea, carries with it a burden of mud, sand, and rounded pebbles, derived from the waste of the rocks which form its bed and banks. When these materials cease to be impelled by the force of the moving water, they sink to the bottom, the heaviest pebbles, of course, sinking first, the smaller pebbles and sand next, and the finest mud last. Ultimately, therefore, as might have been inferred upon theoretical grounds, and as is proved by practical experience, every lake becomes a receptacle for a series of stratified rocks produced by the streams flowing into it. These deposits may vary in different parts of the lake, according as one stream brought down one kind of material and another stream contributed another material; but in all cases the materials will bear ample evidence that they were produced, sorted, and deposited by running water. The finer beds of clay or sand will all be arranged in thicker or thinner layers or laminae; and if there are any beds of pebbles these will all be rounded or smooth, just like the water-worn pebbles of any brook-course. In all probability, also, we should find in some of the beds the remains of fresh-water shells or plants or other organisms which inhabited the lake at the time these beds were being deposited.
In the same way large rivers—such as the Ganges or Mississippi—deposit all the materials which they bring down at their mouths, forming in this way their "deltas." Whenever such a delta is cut through, either by man or by some channel of the river altering its course, we find that it is composed of a succession of horizontal layers or strata of sand or mud, varying in mineral composition, in structure, or in grain, according to the nature of the materials brought down by the river at different periods. Such deltas, also, will contain the remains of animals which inhabit the river, with fragments of the plants which grew on its banks, or bones of the animals which lived in its basin.
Nor is this action confined, of course, to large rivers only, though naturally most conspicuous in the greatest bodies of water. On the contrary, all streams, of whatever size, are engaged in the work of wearing down the dry land, and of transporting the materials thus derived from higher to lower levels, never resting in this work till they reach the sea.
Lastly, the sea itself—irrespective of the materials delivered into it by rivers—is constantly preparing fresh stratified deposits by its own action. Upon every coast-line the sea is constantly eating back into the land and reducing its component rocks to form the shingle and sand which we see upon every shore. The materials thus produced are not, however, lost, but are ultimately deposited elsewhere in the form of new stratified accumulations, in which are buried the remains of animals inhabiting the sea at the time.
Whenever, then, we find anywhere in the interior of the land any series of beds having these characters—composed, that is, of distinct layers, the particles of which, both large and small, show distinct traces of the wearing action of water—whenever and wherever we find such rocks, we are justified in assuming that they have been deposited by water in the manner above mentioned. Either they were laid down in some former lake by the combined action of the streams which flowed into it; or they were deposited at the mouth of some ancient river, forming its delta; or they were laid down at the bottom of the ocean. In the first two cases, any fossils which the beds might contain would be the remains of fresh-water or terrestrial organisms. In the last case, the majority, at any rate, of the fossils would be the remains of marine animals.
The term "formation" is employed by geologists to express "any group of rocks which have some character in common, whether of origin, age, or composition" (Lyell); so that we may speak of stratified and unstratified formations, aqueous or igneous formations, fresh-water or marine formations, and so on.
CHIEF DIVISIONS OF THE AQUEOUS ROCKS.
The Aqueous Rocks may be divided into two great sections, the Mechanically-formed and the Chemically-formed, including under the last head all rocks which owe their origin to vital action, as well as those produced by ordinary chemical agencies.
A. MECHANICALLY-FORMED ROCKS.—These are all those Aqueous Rocks of which we can obtain proofs that their particles have been mechanically transported to their present situation. Thus, if we examine a piece of conglomerate or puddingstone, we find it to be composed of a number of rounded pebbles embedded in an enveloping matrix or paste, which is usually of a sandy nature, but may be composed of carbonate of lime (when the rock is said to be a "calcareous conglomerate"). The pebbles in all conglomerates are worn and rounded by the action of water in motion, and thus show that they have been subjected to much mechanical attrition, whilst they have been mechanically transported for a greater or less distance from the rock of which they originally formed part. The analogue of the old conglomerates at the present day is to be found in the great beds of shingle and gravel which are formed by the action of the sea on every coast-line, and which are composed of water-worn and well-rounded pebbles of different sizes. A breccia is a mechanically-formed rock, very similar to a conglomerate, and consisting of larger or smaller fragments of rock embedded in a common matrix. The fragments, however, are in this case all more or less angular, and are not worn or rounded. The fragments in breccias may be of large size, or they may be comparatively small (fig. 6); and the matrix may be composed of sand (arenaceous) or of carbonate of lime (calcareous). In the case of an ordinary sandstone, again, we have a rock which may be regarded as simply a very fine-grained conglomerate or breccia, being composed of small grains of sand (silica), sometimes rounded, sometimes more or less angular, cemented together by some such substance as oxide of iron, silicate of iron, or carbonate of lime. A sandstone, therefore, like a conglomerate is a mechanically-formed rock, its component grams being equally the result of mechanical attrition and having equally been transported from a distance; and the same is true of the ordinary sand of the sea-shore, which is nothing more than an unconsolidated sandstone. Other so-called sands and sandstones, though equally mechanical in their origin, are truly calcareous in their nature, and are more or less entirely composed of carbonate of lime. Of this kind are the shell-sand so common on our coasts, and the coral-sand which is so largely formed in the neighbourhood of coral-reefs. In these cases the rock is composed of fragments of the skeletons of shellfish, and numerous other marine animals, together, in many instances, with the remains of certain sea-weeds (Corallines, Nullipores, &c,) which are endowed with the power of secreting carbonate of lime from the sea-water. Lastly, in certain rocks still finer in their texture than sandstones, such as the various mud-rocks and shales, we can still recognise a mechanical source and origin. If slices of any of these rocks sufficiently thin to be transparent are examined under the microscope, it will be found that they are composed of minute grains of different sizes, which are all more or less worn and rounded, and which clearly show, therefore, that they have been subjected to mechanical attrition.
All the above-mentioned rocks, then, are mechanically-formed rocks; and they are often spoken of as "Derivative Rocks," in consequence of the fact that their particles can be shown to have been mechanically derived from other pre-existent rocks. It follows from this that every bed of any mechanically-formed rock is the measure and equivalent of a corresponding amount of destruction of some older rock. It is not necessary to enter here into a minute account of the subdivisions of these rocks, but it may be mentioned that they may be divided into two principal groups, according to their chemical composition. In the one group we have the so-called Arenaceous (Lat. arena, sand) or Siliceous Rocks, which are essentially composed of larger or smaller grains of flint or silica. In this group are comprised ordinary sand, the varieties of sandstone and grit, and most conglomerates and breccias. We shall, however, afterwards see that some siliceous rocks are of organic origin. In the second group are the so-called Argillaceous (Lat. argilla, clay) Rocks, which contain a larger or smaller amount of clay or hydrated silicate of alumina in their composition. Under this head come clays, shales, marls, marl-slate, clay-slates, and most flags and flagstones.
B. CHEMICALLY-FORMED ROCKS.—In this section are comprised all those Aqueous or Sedimentary Rocks which have been formed by chemical agencies. As many of these chemical agencies, however, are exerted through the medium of living beings, whether animals or plants, we get into this section a number of what may be called "organically-formed rocks." These are of the greatest possible importance to the palaeontologist, as being to a greater or less extent composed of the actual remains of animals or vegetables, and it will therefore be necessary to consider their character and structure in some detail.
By far the most important of the chemically-formed rocks are the so-called Calcareous Rocks (Lat. calx, lime), comprising all those which contain a large proportion of carbonate of lime, or are wholly composed of this substance. Carbonate of lime is soluble in water holding a certain amount of carbonic acid gas in solution; and it is, therefore, found in larger or smaller quantity dissolved in all natural waters, both fresh and salt, since these waters are always to some extent charged with the above-mentioned solvent gas. A great number of aquatic animals, however, together with some aquatic plants, are endowed with the power of separating the lime thus held in solution in the water, and of reducing it again to its solid condition. In this way shell-fish, crustaceans, sea-urchins, corals, and an immense number of other animals, are enabled to construct their skeletons; whilst some plants form hard structures within their tissues in a precisely similar manner. We do meet with some calcareous deposits, such as the "stalactites" and "stalagmites" of caves, the "calcareous tufa" and "travertine" of some hot springs, and the spongy calcareous deposits of so-called "petrifying springs," which are purely chemical in their origin, and owe nothing to the operation of living beings. Such deposits are formed simply by the precipitation of carbonate of lime from water, in consequence of the evaporation from the water of the carbonic acid gas which formerly held the lime in solution; but, though sometimes forming masses of considerable thickness and of geological importance, they do not concern us here. Almost all the limestones which occur in the series of the stratified rocks are, primarily at any rate, of organic origin, and have been, directly or indirectly, produced by the action of certain lime-making animals or plants, or both combined. The presumption as to all the calcareous rocks, which cannot be clearly shown to have been otherwise produced, is that they are thus organically formed; and in many cases this presumption can be readily reduced to a certainty. There are many varieties of the calcareous rocks, but the following are those which are of the greatest importance:—
Chalk is a calcareous rock of a generally soft and pulverulent texture, and with an earthy fracture. It varies in its purity, being sometimes almost wholly composed of carbonate of lime, and at other times more or less intermixed with foreign matter. Though usually soft and readily reducible to powder, chalk is occasionally, as in the north of Ireland, tolerably hard and compact; but it never assumes the crystalline aspect and stony density of limestone, except it be in immediate contact with some mass of igneous rock. By means of the microscope, the true nature and mode of formation of chalk can be determined with the greatest ease. In the case of the harder varieties, the examination can be conducted by means of slices ground down to a thinness sufficient to render them transparent; but in the softer kinds the rock must be disintegrated under water, and the debris examined microscopically. When investigated by either of these methods, chalk is found to be a genuine organic rock, being composed of the shells or hard parts of innumerable marine animals of different kinds, some entire, some fragmentary, cemented together by a matrix of very finely granular carbonate of lime. Foremost amongst the animal remains which so largely compose chalk are the shells of the minute creatures which will be subsequently spoken of under the name of Foraminifera (fig. 7), and which, in spite of their microscopic dimensions, play a more important part in the process of lime-making than perhaps any other of the larger inhabitants of the ocean.
As chalk is found in beds of hundreds of feet in thickness, and of great purity, there was long felt much difficulty in satisfactorily accounting for its mode of formation and origin. By the researches of Carpenter, Wyville Thomson, Huxley, Wallich, and others, it has, however, been shown that there is now forming, in the profound depths of our great oceans, a deposit which is in all essential respects identical with chalk, and which is generally known as the "Atlantic ooze," from its having been first discovered in that sea. This ooze is found at great depths (5000 to over 15,000 feet) in both the Atlantic and Pacific, covering enormously large areas of the sea-bottom, and it presents itself as a whitish-brown, sticky, impalpable mud, very like greyish chalk when dried. Chemical examination shows that the ooze is composed almost wholly of carbonate of lime, and microscopical examination proves it to be of organic origin, and to be made up of the remains of living beings. The principal forms of these belong to the Foraminifera, and the commonest of these are the irregularly-chambered shells of Globigerina, absolutely indistinguishable from the Globigerinoe which are so largely present in the chalk (fig. 8). Along with these occur fragments of the skeletons of other larger creatures, and a certain proportion of the flinty cases of minute animal and vegetable organisms (Polycystina and Diatoms). Though many of the minute animals, the hard parts of which form the ooze, undoubtedly live at or near the surface of the sea, others, probably, really live near the bottom; and the ooze itself forms a congenial home for numerous sponges, sea-lilies, and other marine animals which flourish at great depths in the sea. There is thus established an intimate and most interesting parallelism between the chalk and the ooze of modern oceans. Both are formed essentially in the same way, and the latter only requires consolidation to become actually converted into chalk. Both are fundamentally organic deposits, apparently requiring a great depth of water for their accumulation, and mainly composed of the remains of Foraminifera, together with the entire or broken skeletons of other marine animals of greater dimensions. It is to be remembered, however, that the ooze, though strictly representative of the chalk, cannot be said in any proper sense to be actually identical with the formation so called by geologists. A great lapse of time separates the two, and though composed of the remains of representative classes or groups of animals, it is only in the case of the lowly-organised Globigerinoe, and of some other organisms of little higher grade, that we find absolutely the same kinds or species of animals in both.
Limestone, like chalk, is composed of carbonate of lime, sometimes almost pure, but more commonly with a greater or less intermixture of some foreign material, such as alumina or silica. The varieties of limestone are almost innumerable, but the great majority can be clearly proved to agree with chalk in being essentially of organic origin, and in being more or less largely composed of the remains of living beings. In many instances the organic remains which compose limestone are so large as to be readily visible to the naked eye, and the rock is at once seen to be nothing more than an agglomeration of the skeletons, generally fragmentary, of certain marine animals, cemented together by a matrix of carbonate of lime. This is the case, for example, with the so-called "Crinoidal Limestones" and "Encrinital Marbles" with which the geologist is so familiar, especially as occurring in great beds amongst the older formations of the earth's crust. These are seen, on weathered or broken surfaces, or still better in polished slabs (fig. 9), to be composed more or less exclusively of the broken stems and detached plates of sea-lilies (Crinoids). Similarly, other limestones are composed almost entirely of the skeletons of corals; and such old coralline limestones can readily be paralleled by formations which we can find in actual course of production at the present day. We only need to transport ourselves to the islands of the Pacific, to the West Indies, or to the Indian Ocean, to find great masses of lime formed similarly by living corals, and well known to everyone under the name of "coral-reefs." Such reefs are often of vast extent, both superficially and in vertical thickness, and they fully equal in this respect any of the coralline limestones of bygone ages. Again, we find other limestones—such as the celebrated "Nummulitic Limestone" (fig. 10), which sometimes attains a thickness of some thousands of feet—which are almost entirely made up of the shells of Foraminifera. In the case of the "Nummulitic Limestone," just mentioned, these shells are of large size, varying from the size of a split pea up to that of a florin. There are, however, as we shall see, many other limestones, which are likewise largely made up of Foraminifera, but in which the shells are very much more minute, and would hardly be seen at all without the microscope.
We may, in fact, consider that the great agents in the production of limestones in past ages have been animals belonging to the Crinoids, the Corals, and the Foraminifera. At the present day, the Crinoids have been nearly extinguished, and the few known survivors seem to have retired to great depths in the ocean; but the two latter still actively carry on the work of lime-making, the former being very largely helped in their operations by certain lime-producing marine plants (Nullipores and Corallines). We have to remember, however, that though the limestones, both ancient and modern, that we have just spoken of, are truly organic, they are not necessarily formed out of the remains of animals which actually lived on the precise spot where we now find the limestone itself. We may find a crinoidal limestone, which we can show to have been actually formed by the successive growth of generations of sea-lilies in place; but we shall find many others in which the rock is made up of innumerable fragments of the skeletons of these creatures, which have been clearly worn and rubbed by the sea-waves, and which have been mechanically transported to their present site. In the same way, a limestone may be shown to have been an actual coral-reef, by the fact that we find in it great masses of coral, growing in their natural position, and exhibiting plain proofs that they were simply quietly buried by the calcareous sediment as they grew; but other limestones may contain only numerous rolled and water-worn fragments of corals. This is precisely paralleled by what we can observe in our existing coral-reefs. Parts of the modern coral-islands and coral-reefs are really made up of corals, dead or alive, which actually grew on the spot where we now find them; but other parts are composed of a limestone-rock ("coral-rock"), or of a loose sand ("coral-sand"), which is organic in the sense that it is composed of lime formed by living beings, but which, in truth, is composed of fragments of the skeletons of these living beings, mechanically transported and heaped together by the sea. To take another example nearer home, we may find great accumulations of calcareous matter formed in place, by the growth of shell-fish, such as oysters or mussels; but we can also find equally great accumulations on many of our shores in the form of "shell-sand," which is equally composed of the shells of molluscs, but which is formed by the trituration of these shells by the mechanical power of the sea-waves. We thus see that though all these limestones are primarily organic, they not uncommonly become "mechanically-formed" rocks in a secondary sense, the materials of which they are composed being formed by living beings, but having been mechanically transported to the place where we now find them.
Many limestones, as we have seen, are composed of large and conspicuous organic remains, such as strike the eye at once. Many others, however, which at first sight appear compact, more or less crystalline, and nearly devoid of traces of life, are found, when properly examined, to be also composed of the remains of various organisms. All the commoner limestones, in fact, from the Lower Silurian period onwards, can be easily proved to be thus organic rocks, if we investigate weathered or polished surfaces with a lens, or, still better, if we cut thin slices of the rock and grind these down till they are transparent. When thus examined, the rock is usually found to be composed of innumerable entire or fragmentary fossils, cemented together by a granular or crystalline matrix of carbonate of lime (figs. 11 and 12). When the matrix is granular, the rock is precisely similar to chalk, except that it is harder and less earthy in texture, whilst the fossils are only occasionally referable to the Foraminifera. In other cases, the matrix is more or less crystalline, and when this crystallisation has been carried to a great extent, the original organic nature of the rock may be greatly or completely obscured thereby. Thus, in limestones which have been greatly altered or "metamorphosed" by the combined action of heat and pressure, all traces of organic remains become annihilated, and the rock becomes completely crystalline throughout. This, for example, is the case with the ordinary white "statuary marble," slices of which exhibit under the microscope nothing but an aggregate of beautifully transparent crystals of carbonate of lime, without the smallest traces of fossils. There are also other cases, where the limestone is not necessarily highly crystalline, and where no metamorphic action in the strict sense has taken place, in which, nevertheless, the microscope fails to reveal any evidence that the rock is organic. Such cases are somewhat obscure, and doubtless depend on different causes in different instances; but they do not affect the important generalisation that limestones are fundamentally the product of the operation of living beings. This fact remains certain; and when we consider the vast superficial extent occupied by calcareous deposits, and the enormous collective thickness of these, the mind cannot fail to be impressed with the immensity of the period demanded for the formation of these by the agency of such humble and often microscopic creatures as Corals, Sea-lilies, Foraminifers, and Shell-fish.
Amongst the numerous varieties of limestone, a few are of such interest as to deserve a brief notice. Magnesian limestone or dolomite, differs from ordinary limestone in containing a certain proportion of carbonate of magnesia along with the carbonate of lime. The typical dolomites contain a large proportion of carbonate of magnesia, and are highly crystalline. The ordinary magnesian limestones (such as those of Durham in the Permian series, and the Guelph Limestones of North America in the Silurian series) are generally of a yellowish, buff, or brown colour, with a crystalline or pearly aspect, effervescing with acid much less freely than ordinary limestone, exhibiting numerous cavities from which fossils have been dissolved out, and often assuming the most varied and singular forms in consequence of what is called "concretionary action." Examination with the microscope shows that these limestones are composed of an aggregate of minute but perfectly distinct crystals, but that minute organisms of different kinds, or fragments of larger fossils, are often present as well. Other magnesian limestones, again, exhibit no striking external peculiarities by which the presence of magnesia would be readily recognised, and though the base of the rock is crystalline, they are replete with the remains of organised beings. Thus many of the magnesian limestones of the Carboniferous series of the North of England are very like ordinary limestone to look at, though effervescing less freely with acids, and the microscope proves them to be charged with the remains of Foraminifera and other minute organisms.
Marbles are of various kinds, all limestones which are sufficiently hard and compact to take a high polish going by this name. Statuary marble, and most of the celebrated foreign marbles, are "metamorphic" rocks, of a highly crystalline nature, and having all traces of their primitive organic structure obliterated. Many other marbles, however, differ from ordinary limestone simply in the matter of density. Thus, many marbles (such as Derbyshire marble) are simply "crinoidal limestones" (fig. 9); whilst various other British marbles exhibit innumerable organic remains under the microscope. Black marbles owe their colour to the presence of very minute particles of carbonaceous matter, in some cases at any rate; and they may either be metamorphic, or they may be charged with minute fossils such as Foraminifera (e.g., the black limestones of Ireland, and the black marble of Dent, in Yorkshire).