Text Book of Geology for schools and colleges

Front matter

TEXT-BOOK OF GEOLOGY FOR SCHOOLS AND COLLEGES BT A/ H. ALLEYXE KICJiOLSOU, M.D., D.Sc, M.A., Ph.D., F.E.8.E., F.G.S., Etc, PROFESSOR OF NATURAL HISTORY AUD BOTANY IN UNIVERSITY COLLEGE TORONTO FORMERLY LECTURER ON NATURAL HT8TOEY IN THE MEDICAL SCHOOL OF EDIN- BURGH: AUTHOR OF ''MANUAL OF ZOOLOGY FOR THE USE OF STUDENTS'1 "TEXT-BOOK OF GEOLOGY FOE SCHOOLS AND COLLEGES." "GEOLOGY ' OF CUMBERLAND AND WESTMORELAND," ETC, ETC. TORONTO : ADAM, STEVENSON & CO. :_*:ew yobk: d. appleton &, co., 549 & 501 buoadway. 1872. Entered, according to Act of Congress, in the year 1871, by D. APPLETON & CO , In the Office os the Librarian of Congress, ut Washington. Entered, according* to the act of the Parliament of Canada, in the year one thousand eiirht hundred and seventy-one. by II. ALLEYNE NICHOLSON, in the Office of lhe Minister of Agriculture.

PREE AC E The object of the present work is to present to the learner the leading principles and facts of geological science in as brief a compass as is compatible with clearness and accuracy. No science stands less alone, or is less independent of the other sciences, than Geology. Its foundation, as a science, is upon Physical Geography, and this subject has, therefore, been treated in the earlier portion of this work as fully as space would allow. No adequate knowledge, again, of the facts and generalizations of Palæontology can be acquired without some previous acquaintance with Zoology and Botany, the former more especially. A brief outline of the classification of the animal kingdom has, therefore, been here introduced ; but the progress of the learner would be much facilitated by a more extensive study of Natural History than can possibly be presented in a work primarily devoted to Geology.

Palæontology, however, is to such an extent an independent science, and embraces such an extended area, that it can only be properly handled in a special treatise ; and such a work is now in course of preparation by the author. As to the plan of the work, it is sufficient to state that it is not based primarily upon American geology. Many important formations are not represented at all, or only in a very incomplete form, in America ; while the types of the great geological formations are at present to be sought for in TREFACE. Europe. The author is far from saying that there is any reason in Nature why this should be so ; but the vast Ameri can Continent has as yet been very imperfectly explored ; and there can be no question but that for many years to come European names and European types will hold their ground in geological literature.

Part I. Physical Geography

CHAPTEE I. Genera] Scope of Geology — Modern Geology founded upon Physical Geography — Scope of Physical Geography — Planetary Relations of the Earth — Figure and Dimensions ofthe Earth — Primitive Condition of the Earth — Internal Temperature of the Earth — Question as to the Fluidity of the Interior of the Earth— Surface of the Earth— Origin of Dry Land — Distribution of the Dry Land, Pages 1-9 CHAPTEE II. Mountains — Mountains of Circumdenudation — of Uptilting — of Ejection — Volcanoes — General Phenomena of Volcanic Eruptions — Geographical Distribution of Volcanoes — General Structure of a Volcanic Cone — Exciting Causes of Volcanic Eruptions— Earthquakes — Their Connection with Volcanic Action, and their General Phenomena, p. 10-22 CHAPTEE III. Origin of Valleys— Denudation denned — Action of Rain as a Denuding Agent— Eivers— The fiea as a Denuding Agent, P- 23-30 CHAPTEE TV.

Ice as a Denuding Agent— Line of Perpetual Snow— Glaciers — Formation of Glaciers — Moraines—Polished and Striated Blocks — Striation of the Fundamental Eocks— Eoches Moutonnees — Perched Blocks — Erratics — Continental Ice — Icebergs — Transportation of Erratic Blocks by Icebergs— Frost, p. 31-41 CHAPTEE V. Action of the Atmosphere upon the Earth's Surface — Weathering — Sand-dunes — Organic Agencies — Accumulations of Vegetable Matter — Peat — Shell-beds — Atlantic Ooze — Coral-reefs — Bearing of the Facts ofPhysical Geography on Geological Doctrine, p. 42-47 TABLE OF CONTENTS. VI

Part II. G E O L 0 6 Y

CHAPTER VI.

Definition os Geology— Successive Formation of the Crust ofthe Earth— Definition of tho term " Rock "—Classification of Rocks— Aqueous Rocks— Stratification— Fossils— Characters and Origin of the Volcanic Rocks — Granitic Rocks— Metamorphic Rocks, p. 4S-59

CHAPTER VII.

Aqueous Rocks— Mechanically-formed Rocks— Arenaceous Rocks— Sands, Sandstones, Grits, and Conglomerates— Argillaceous Rocks— Clays, Marl, Loam— Chemically and Organically formed Rocks— Chalk— Limestone— Gypsum— Rock-salt— (Joal— Gradations between tho Groups of Aqueous Rocks, P*

CHAPTER VIII.

Volcanic Rocks— Lava-flows and Traps— Dikes— Ashes and Scoriæ— Mineral Composition of the Volcanic and Trappean Rocks — Felspathic and Augitic Lavas and their Mechanical Accompaniments— Felspathic and Hornblendic Traps and their Mechanical Accompaniments — Porphyry and Amygdaloid, P- 60-74 CHAPTEE IX. Plutonic Rocks — Composition, Characters, and Mode of Formation of Granite — Syenite — Protogine — Eurite — Passage of the Granitic Rocks into Trap, . . .p. 75-77

CHAPTER X.

Metamorphic Rocks — Gneiss — Hornblende-schist — Mica-schist — Clay-slate — Quartzite — Metamorphic Limestone, p. 78-80

CHAPTER XI.

Divisional Planes of Rocks— Planes of Stratification and Lamination— Planes of Jointing- Columnar Traps and Lavas— Cleavage — Origin of Cleavage — Foliation, . . p. 81-88

CHAPTER XII.

Lateral Extent of Beds — Original Horizontality of Strata — Diagonal Stratification — Ripplemarks, Desiccation-cracks, Rain-prints — Inclined Strata — Dip — Strike — Contortions of Strata — Anticlinal and Synclinal Curves — Causes of Contortions and Curves, p. 89-98

CHAPTER XIII.

Unconformability — Overlap — Faults — Denudation of Faults — Lateral Shift produced by Faults— Repetition of Strata by Faults, p. 99-108 CHAPTEE XIV. The Relative Ages of the Aqueous Eocks — Test of Age by Superposition — Test by Mineral Characters — Test by included Organic Remains — Outlines of Zoological and Botanical Classifications — Chronological Succession of the Aqueous Rocks, . . p. 109-123 TABLE OF CONTENTS. VII

CHAPTER XV.

Laurentian Series — Life of the Laurentian Series — Huronian Formation — Cambrian Series — Life of the Cambrian Rocks, page 12U30

CHAPTER XVI.

Silurian Formation — Origin of the name Silurian — Silurian Rocks of Britain — Silurian Rocks of North America— life of the Silurian Period, p. 131-141

CHAPTER XVII.

Old Red Sandstone — Name Devonian — Old Red Sandstone and Devonian Rocks of Britain — Devonian Rocks of North America — Life of the Devonian Period . .p. 142-149

CHAPTER XVIIL

Carboniferous Rocks — Carboniferous Slates and Limestone — MUlstone Grit— Coal-measures —Plants of the Coal — Other Fossils of the Coal-measures — Origin of Coal — Life of the Carboniferous Period, p. 150-162

CHAPTER XIX.

Permian Rocks — Development in Russia and Germany — Permians of Britain — Permian Rocks of North America— Life ofthe Permian Period, .... p. 163-166

CHAPTER XX

Triassic Eocks— Origin of Eock-salt^Life ofthe Triassic Period, . . . p. 167-173

CHAPTER XXI.

Jurassic Rocks — Lias — Lower Oolites — Middle Oolites — Upper Oolites — Jurassic Rocks of North America— Life of the Jurassic Period, p. 174-184

CHAPTER XXII.

Cretaceous Series— Wealden— Lower Greensand— Gault— Upper Greensand— Chalk— Maestricht and FaxOe Beds— Chalk of the South of Europe— Cretaceous Rocks of North America— Origin of Chalk— Origin of Flints— Life ofthe Cretaceous Period, p. 185-196 CHAPTEE XXin. Relations of the Kainozoic to the Mesozoic Rocks — Classification of the Kainozoic Rocks — Eocene Rocks — Life of the Eocene Period, p. 197-204 CHAPTEE XXIV. Miocene Eocks — Miocene Deposits of Europe — American Miocene — Life of the Miocene Period— Vegetation of the Miocene Period, p. 205-210 CHAPTEE XXV. Pliocene Eocks — Coralline Crag — Eed Crag — Pliocene Deposits of Europe and America — Life ofthe Pliocene Period, p. 211-216 TABLE OF CONTENTS.

CHAPTER XXVI.

Post-Tertiary Formations— Cromer Forest-bed— Glacial Period, . . . page 216-221

CHAPTER XXVII.

Meaning of the Term Alluvium — Brick-earths — Loess — High-level and Low-level VaUey Gravels— Cavern-deposits, p. 222-229

CHAPTER XXVIII.

Recent Period — Age of Stone — Ago of Bronze— Ago of Iron — Kitcbcn-mlddens — Danish Peat — Swiss Lake-dwellings — General Scarcity of Human Remains — Types of Skull, p. 230-283

CHAPTER XXIX.

Volcanic and Trappean Rocks, their Modes of Occurrence — Tests of Agn — Contemporaneous and Intrusive Traps — Trap-dikes, p. 284-239

CHAPTER XXX.

Granitic Eocks — Ages of the Granitic Rocks — Granite Veins — Metamorphism produced by Granite — Metamorphic Rocks — Age of the Metamorphic Rocks, . . .p. 240-243

CHAPTER XXXI.

Mineral Veins — Mode of Occurrence of Metallic Ores — Connection of Veins with Faults — Mode of Deposition of Metals in Veins — Ages of Veins, . . . .p. 244-246 LIST OF ILLUSTRATIONS. FIG. PAGE 1. Mountains of Circumdenudation, . . . • .11 2. Mountains of Uptilting, ...... 11 3. Vesuvius from the West, . . . . • .13 4. Diagram of a Quiescent Volcano, ..... 14 5. Section of the Island of Palma . . . . . .19 6. Section of a Volcanic Cone, ...... 20 *_*. Section of Niagara Falls, . . . . . .25 8. Section of a Eiver Valley (Schoharie), .... 26 9. Mer de Glace, . ... . . ■ .32 10. Diagram of the Moraines of a Glacier, .... 34 11. Polished and Striated Limestone, . . . ■ .36 12. Eoche Moutonn^e, ....... 37 13. Erratic Block on the Jura, . . . . . .37 14. Structure of Coral-reefs, ...... 45 15. Section of Stratified Deposits, . . . . . .51 16.

Volcanic Dike in Madeira, ...... 70 17. Joints in Limestone, ....... 81 18. Isle of Cyclops, 82 19. Diagram of Columnar Trap, . . . . . .83 20. Diagram to illustrate Slaty Cleavage, ... . . 85 21. Striped and Faulted Slate, . . . ... .85 22. Diagram to illustrate the Thinning out of Beds, ... 90 23. False-bedding, ........ 91 24. Diagram to illustrate the Formation of Eipple-mark, . . 92 25. Eipplemarked Sandstone, . . . . . .93 26. Diagram to illustrate the Dip of Inclined Strata, . . 94 27. Inclined Strata, ........ 95 28. Contorted Strata, ....... 95 29. Diagram of an Anticlinal Curve, . . . . . .96 30. Diagram of a Synclinal Curve, ..... 97 31. Diagram to illustrate the Production of Contortions, . . 97 32. Section of Unconformable Strata, ..... 99 33. Ground-plan and Section of Unconformable Strata, . . . 100 34.

Diagram of Overlap, ....... 102 35. Diagram to illustrate the Production of Faults, . . . 103 36. Section of Faulted and Shifted Strata, .... 104 37. Diagram of a Fault, . . . . . . .104 38. Denudation of Faults, ...... 105 39. Diagram to illustrate the Lateral Shift of Beds, . . . 107 40. B?petition of Beds by Faults, . . . . .108 41. Foraminifera, ........ 112 42. Eecent Corals, . . . . . . .113 43. Bhizocrinus Lofotensis, ....... 114 44. Sea-urchin ( Cidaris), ...... LIST OF ILLUSTRATIONS. X no. pace 45. Pterygotus Anglicus, ....... 110 46. Lingula anatina, ....... 47. Shells of Univalve Mollusks, . . . . . .117 48. Pearly Nautilus, ....... 49. Skeleton of Beaver, ....... 119 50. Ideal Section of the Earth's Crust, . .... 122 51. Section of the Pre-Cambrian Rocks of Scotland, . . . 125 52.

Section ofthe Cambrian Rocks ofthe Longmynd, . „ 127 53. PUradoxidts Mofumicus, ....... 128 54. Dikelocephalus Minnesotensis, ..... 55. Oldhamia antiqua^ ........ 128 56. Hymenocaris vermicauda, ) 57. Lingula Davisiiy /■ ..... 129 58. Olenus micrvrvs^ ) 59. Tttragrapsus bryonoides^ ....... 129 60. Generalized Section ofthe Silurian Eocks of Britain, • . 132 61. Lldymograpsus patulus^ ....... 132 62. Asaphus tyrannus, I 63. Ogygia Buchii, f 64. Orthis tricenariay 1 65. Orthis vespertilio, > . . . . . . • 133 66. Strophomena grandis,) 67. Pt-ntamerus oolongusy ...... 134 68. Halysites catenularius, ) 69. Favorites Gothlandica, > ...... 135 70. Omphymaturbinatum, ) 71. Ortfiis eL-gantula, 1 72. Rhynchonella navicula, > , . . . . 135 73. Rhynchonella Wihoni, ) 74. Pttraspis /Jauks-ii, . ..... 136 75.

Onchus tenuistriatus. \ 1osi 76. Shagreen -scales of Thelndus. ) ' * 77. Generalized Section ofthe Silurians of North America, . . 137 78. Echinosphœrites Balticus, ...... 140 79. Trinucteus concentricus, ....... 140 80. Troc/ioceras giganteus, \ -4_ 81. Orthnccras Ludense, ) 82. Cephalaspis Lyellii, . . . . . . .143 83. Aaianlites (Cyclopttris) Ifibernicus, .... 144 84. Holoptychius, restored, . + . . . . . 144 85. Spinf^r disjunctus, ....... 145 86. Calceola sandalina, ....... 145 87. Phacops latifronsy ....... 145 87i. Section ofthe Devonian Eocks of North America, . . . 146 88. Sigillaria Chemvngensis, ...... 148 89. Megalodon cucullatus, . . . . , . .148 90. Clymenia linearis, . . . . . . .149 91. General Section ofthe Carboniferous Eocks, . . » . 150 92. Lit?iostrotion basaltiforme, \ „_, 93.

Lonsdaleiafioriformis^ ) ..... lea 94. Cyathocrinus planvs^ ....... 152 95. Producta semireticulata, ...... 152 96. Goniatites crenistria, ....... 153 97. Teeth of Cochliodvs contortus, ..... 153 98. Stem of Lepidodendron, \ 99. Frae^nent of Lepido'dendron Sternbergii, ["•••- I63 100. Calamites cannœformis, ] 101. Calamites Sucowii, r ■ • • • • 156 102. Eoot termination of Calamite, j 103. Sigillaria lœvigata, ....... LIST OF ILLUSTRATIONS. XI PIG. PAGE 104. Stigmaria ficoides, ....... 157 105. Erect Fossil Trees, Coal-measures, ..... 158 106. Archegosaurus minor, . . . . . .160 107. Limulus rotundatus, ....... 161 108. Cythere inflata, . . . . . . .161 109. Spirifer trigonalis, 1 , fil 110. Spirifer glaber, j 111. Generalized Section of the Permian Eocks, . . . 164 112. Walchia piniformis, ....... 165 113.

Produeta horrida, j 114. Lingula Crednerii, >• ...... 165 115. Spirifer undulatus, J 116. Outline of Palceoniscus, ....... 166 117. Generalized Section of the Triassio Eocks, . 167 118. Footprints of Cheirotherium, ...... 168 119. Voltsia heterophylla, ...... 168 120. CeratUes nodosus, ....... 169 121. Encrinus liliiformis, ...... 169 122. Cardium Bhœticum, \ 123. Pecten Valoniensis, > . . . . . . .170 124. Avicula contorta, \ 125. Teeth of Hybodus plicatilis, ) 126. Tooth of Saurichthys apicalis, > . . . . . 170 127. Scale of Gyrolepis tenuistnatus, ) 128. Molar tooth of Jficrolestes antiquus, ..... 170 129. Footprints in Triassic Sandstones of the Connecticut Valley, . 171 130. Generalized Section of the Jurassic Eocks, .... 174 131. G-ryphea incur _a, ....... 175 132. Belemnites elongatus, . ...... 175 133.

Ammonites Buchlandi,} -*- 134. Ammonites planorbis, f * 135. Extracrinus Briare-is, . ...... 176 136. Hybodus reticulatus, ....... 176 137. Pterophyllum comptum, ....... 177 138. Ammonites HumpTiresianus, ..... 177 139. Apiocrinus rotundas, ....... 178 140. P/tascolotAerium Buchlandi, ..... 178 141. Belemnites hastatus, ....... 179 142. Thecosmilia annularis, ...... 179 143. Ostrea distorta, ........ 179 144. Pterodactylus erassirostris, ...... 180 145. Plagiaulax minor, ....... 180 146. Cycadeoidea megalophylla, I 147. Zamites spiralis, ' J ..... 181 148. Ichthyosaurus communis, j 149. Plesiosaurus dolichodeirus, j •'•••• ■ I83 150. Archæopterux macrura, ...... 184 151. Generalized Section of the Cretaceous Eocks, . . . 185 152. Teeth of Iguanodon Mantelli, ..... 186 153. Ancyloceras gigas, \ 154.

Nautilus phcatus, j . 188 155. Ancyloceras spinigerum, . . . _ . . . 189 156. Ventriculites radialus, ....... 189 157. Iiwceramus Lamarckii, ...... 190 158. Baculites Faujasii, I _.qs, 159. Baculites anceps, \ ..... 160. Turrilites costatus, \ . qst 161. Scaphites æqualis, \ ' . . . r 162. Micraster cor.-anguinum, ) .„. 163. Galerites albogalerus, \ ' ' ' * " LIST OF ILLUSTRATIONS. PIG. ■"«■ 164. Mosasaurus Camperi, . . . . . • • ' 165. Hippurites organisam, ...... 166-170. Organic Bodies in the Atlantic Ooze, .... 194 171. Generalized Section of the Eocene Eocks, ... 199 172. Voluta nodosa, . . . . . . . .200 173. Calcarina rarispina, | 200 174. Spirolina stenosto-ma, ) 175. Nummulites Puschi, ..... .201 176. Carcharodon heterodon, I 201 177. Otodus obliquus, ) 178. Teeth of Zeuglodon. cetoidet, \ 202 179.

Vertebra of the same, j ' 180. Falœotherium magnum, ...... 203 181. Deinotherium giganteum, ...... 206 182. Voluta Lamberti, ....... 206 183. Fulgur canaliculatus, 1 oost 184. Fusus guadricostatus, ) ' 185. Vanessa Pluto, 209 186. Chamœrops Helvetica, ) . 209 187. Sabal major, J 188. Platanus aceroides, \ 210 189. Cinnamomum polymorplium, \ 190. Section of the Suffolk Crag, . . . . . .211 191. Voluta Lamberti, 1 192. Pyrula reticulata, V ...... tit 193. Temnechinus excavatus, ) 194. Fusus contrarius, 1 195. Purpura tetraqona, t „,„ 196. Nassa granulata, s ....-• 197. Cypræa Europœa, ) 198. Molar of Mastodon Arvernensis, ..... 213 199. Molar of Elephas meridionalis, ...... 217 200-205. Shells of the Drift of Scotland, . . 220 206. Eecent and Post-Pliocene Alluvial Deposits, .... 223 207. Molar of Elephas antiquus, ...... 224 208.

Section of Cavern and Eiver-valley, ..... 227 209. Molar of Mammoth, ....... 228 210. Lower Jaw of Cave-Hyæna, ...... 229 211. Short-headed Skull, ) oro 212. Long-headed Skull, ) 213. Trap overlying Sandstone. ...... 234 214. Diagram of the Granitic, Aqueous, and Volcanic Formations, . 234 215. Step-like appearance of Traps, . ..... 235 216. Diajram of Trap-dike, ...... 235 217. Section of an Intrusive Trap, ...... 235 218. Trap intruded in Limestone and Shale, .... 237 219. Granitic Veins, ........ 241 220. Section of Mineral Veins, ...... GEOLOGY

PART I. PHYSICAL GEOGRAPHY.

Chapter I

Geology (Gr. ge, the earth ; logos, discourse) is the science which is concerned with the investigation of the materials which compose the earth, the manner in which these materials have been arranged, and the causes and mode of origin of these arrangements. The forms, properties, chemical compo sition, and local distribution of the materials which compose the crust of the earth, constitute the separate science of Min eralogy, which is, indeed, closely related to Geology, but which, nevertheless, is not essential to the study of the iatter science. A limited knowledge, however, of Mineralogy is es sential to a due comprehension of the phenomena of Geology, and such details as are thus requisite will be introduced in their proper place. Palæontology (Gr. palaios, ancient ; onta, beings ; logos, discourse) is a branch of Geology which treats of the past life of the globe and is concerned with those animals and plants which — as will be seen hereafter — have peo pled the earth at successive periods, and have died out, to be replaced by others different in their character and structure.

Here, therefore, Geology comes closely in contact with the sciences of Zoology and Botany, the sciences which treat of the various animals and plants which inhabit the earth at the present day. Palæontology, in fact, is nothing more or less than the Zoology and Botany of the past, and it is only especially connected with Geology in so far that by its study the observer is enabled to determine the historical succession of the materials which compose the globe. For the study of Palæontology, some knowledge of the fundamental facts of Zoology and Botany is requisite, and an outline of some of these necessary facts will be subsequently given. Physical Geography, finally, comprises a knowledge of the figure and motion of the earth, of the composition, form, and distribution of the dry land, and of the forces which tend to modify its surface, of the character and distribution of the rivers and lakes which are placed on the land-masses, of the sea and at mosphere, and lastly, of the animal and vegetable life of dif ferent portions of the surface of the earth.

Modern Geology rests, as a science, upon Physical Geography ; and it is abso lutely necessary that the student should acquire some knowledge of the fundamental facts of the latter science, as a pre liminary to his commencing the study of the former. It is hardly necessary, however, to say that only the leading facts of Physical Geography can be touched upon here, in the very briefest manner, and only so far as they have a direct bearing upon the study of Geology. The points which require to be alluded to in this connection are, the form and planetary relations of the earth, the distribution of the dry land, and the agencies which tend to alter the characters of the earth's surface, especially the effects produced by rivers, ice, the at mosphere, and volcanoes. Planetary Relations of the Eartu. — The earth is one of the smaller of the planets which compose our system.

It performs an annual revolution round the sun, in an elliptical orbit, at a mean distance of 95,000,000 miles. It also rotates in twenty-four hours about its own axis, this axis being in clined a little more than twenty-three degrees to the plane of its orbit. The moon is a satellite of the earth, revolving round it at a mean distance of 240,000 miles, and causing by its attraction certain terrestrial phenomena, of which the most important are the tides. Figure and Dimensions of the Earth. — Astronomy teaches us that the earth has the form of what is technically called an " oblate spheroid." That is to say, it is not a per fect sphere or globe, all the diameters of which are equal ; but it is flattened at the poles, like an orange, one diameter being longer than the other. The earth revolves about an imaginary axis, the two extremities of which constitute the poles — the North pole and the South pole.

This, the polar axis of the earth, is, roughly speaking, 7,900 miles in length ; while the equatorial or greatest diameter of the earth is 7,926 miles in length, exceeding the polar diameter by 26 miles. PRIMITIVE CONDITION OF THE EARTH. 3 The earth, therefore, is flattened at the poles, and bulges out at the equator. That the earth, being such an oblate spheroid, with unequal axes, should revolve about its shortest axis is in obedience to woll-known mechanical laws. Whether the earth has ever revolved about an axis different to its present one, or ever will do so, we do not know. As things stand, however, the earth's circumference at the equator is greater by some eighty-three miles than its circumference at the poles, and it is, therefore, practically in the condition of a sphere revolving on its shortest axis, and having a thick belt surrounding it in a plane perpendicular to the axis of rotation.

Any disturbing force applied to the earth, with a tendency to alter its present axis of rotation, would have to overcome the excess of centrifugal force re siding in the matter of this equatorial belt. This excess of centrifugal force is so enormous in amount that we may safely say we know of no force capable at the present day of effecting this change. It is worthy of notice that the present form of the earth — the form, namely, of an oblate spheroid — is precisely the form which any body composed of fluid or semi-fluid sub stances would tend to assume, if it were set free in space after a preliminary movement of revolution had been impart ed to it. In other words, the centrifugal force developed by the rotation would tend to accumulate the component parti cles of such a body along a zone lying at right angles to its axis of revolution.

Any revolving body, of whatever original form, the particles of which are free to move, would ulti mately assume the form of an oblate spheroid, revolving upon its shortest axis, and having its longest diameter at right angles to its axis of revolution. The present figure of the earth is, therefore, a strong argument in favor of the belief that the whole earth was at one time composed of melted ma terials, the particles of which were free to move in any direction toward which they might be impelled. Primitive Condition of the Earth. — With regard to the original and primitive state of the earth, it is sufficient to state that all known facts support the theory that the earth has been, and is still, a gradually-cooling body. Upon this theory, the materials composing the earth were at one time in a state of vapor or gas, in which condition, of course, they would occupy enormously more space than they do at present.

As the loss of temperature went on, the gaseous matters of the primitive earth would radiate their heat from the periph ery, and would contract and ultimately become fluid. Fi nally, as the cooling process proceeded further and further, solidification would at last commence, either at the surface, or at the centre, or at both simultaneously. This is certainly to a great extent a mere theory, but it is supported by two facts : One of these is the fact that the present form of the earth is exactly that which it would have assumed supposing it to have been formerly fluid or semi-fluid, and to have been revolving at its present velocity. The second of these is the undoubted high temperature of parts, at any rate, of the interior of the earth at the present day ; and here we get upon tolerably firm ground.

Internal Temperature of TnE Earth. — As to the present temperature of the interior of the globe, the following facts enable us to come to some definite conclusion : 1. The phenomena exhibited at the present day — to be spoken of more particularly hereafter — prove beyond a doubt that large portions, at any rate, of the interior of the earth are in a state of complete fluidity, the fusion being the result of heat. At present the traces of direct volcanic action are only partially distributed over the globe ; but we have ample and abundant proofs that volcanic action has taken place everywhere over the earth's surface at some time or other. Further, the universal presence at the surface of rocks which can be shown to have been originally melted and fluid, is quite sufficient proof that there has always existed — as there still exists — in the interior of the earth some powerful and general source of heat. 2.

It is well known that the heating effect of the sun's rays upon the soil extends to but a very limited depth below the surface, and that a point is soon reached at which no per ceptible effect is produced by any external source of heat. Nevertheless, it has been shown, as the result of direct experi ment and observation, that there is a gradual and tolerably regular increase of heat as we recede from the surface of the earth and approach its centre. The exact ratio of this increase of temperature does not appear to be absolutely constant, but some increase there always is. In the case of mines, the or dinary rise of temperature, as we descend, is usually stated to be 1° Fahrenheit for every fifty or sixty feet of descent, after the first hundred. It is probable, however, that this increase would be found to be much more rapid than this at great depths.

The same fact, and pretty nearly the same rate of increase, is shown by the phenomena of artesian wells, in which the water always comes from great depths, and always has a temperature considerably higher than the air. In the same way, such natural hot springs as are known to us, are either in the neighborhood of volcanoes, or can generally be FLUIDITY OF THE INTERIOR OF THE EARTH. 5 shown to be situated on lines of "fault," i. e., on the line of great cracks or fissures which penetrate through the crust of the earth to a greater or less depth. 3. Whenever we can study at the surface rocks which can be shown by geological evidence to have been formerly buried at great depths in the earth, these show unmistakable marks of having been subjected to the action of heat. The above are the chief direct proofs of the internal heat of the globe, .but there is other equally forcible evidence to be drawn indirectly from the mean density or weight of the earth.

By numerous experiments it has been shown that the entire earth has a mean density or specific gravity of between five and six. That is to say, the earth is in the same condition as regards its density, as an imaginary globe would be of the same size, and composed throughout of a simple homogeneous substance weighing between five and six times as much as water. The earth, however, is not homogeneous, com posed of one uniform substance, but heterogeneous, composed of different materials having different densities. Taking the average of the rocks which compose the crust of the earth, we find their average density to be only 2.5 to 3.0. The mean density, therefore, of the earth, is at least twice what it ought to be if it were made of any known rock, as that rock appears at the surface. At first sight, it might be thought that this would prove the pres ence in the interior of the earth of some materials much heavier than ordi nary rocks, such as the metals.

And this would be so, if the effect of gravity were left out of consideration. The earth is truly twice as heavy as it would be if it were entirely composed of any known rock, as that rock appears at the surface. Say the earth were composed of granite, which weighs about 2.5, and which represents, therefore, the average weight of rocks. Granite weighs about two and a half times as much as water at the suiface of the earth, but by the effects of gravity, as we recede from the surface, its density would gradually go on increasing, till at the centre of the earth its density would be about eight times greater than at the surface, or at least twenty times as heavy as water, supposing the ratio of compression produced by gravity to be uniformly the same from the surface to the centre of the earth.