LIBRARY UNIVERSITY OF CALIFORNIA RIVERSIDE . ^ '.v v BIED'S-EYE VIEW or MARBLE CxSoN FBOM THE VERMILION CLIFFS, NEAR THE MOUTII OF THE PABIA. In the distance the Colorado River is seen to turn to the west, where its gorge divides the Twin Plateaus. On the right are seen the Eastern Karbab Displacements appearing as folds, and farther in the distance as faults. ELEMENTS GEOLOGY A TEXT-BOOK FOE COLLEGES AND FOR THE GENERAL READER. BY JOSEPH LE CONTE, '/' MirnoE OF "BELIGIOX AND SCIENCE," ETC., AXD PBOFESSP* OF GEOLOGY ASD NATTOEAL IN TUB UXIVEBSITY OF CAilFOESIA. NEW YORK : D. APPLETON AND COMPANY, 549 AND 551 BROADWAY. 1878. COPYRIGHT BY D. APPLETON AND COMPANY, 1877. P E E F A O E . IN preparing the following work I have not attempted to make an exhaustive manual to be thumbed by the special student ; for, even if I felt able to write such a work, Prof. Dana's is already in the field, and is all that can be desired in this respect. I have endeavored only to present clearly to the thoroughly cultured and intelligent student and reader whatever is best and most interesting in Geological Science. I have attempted to realize what I conceive to be comprised in the word elements, as contradistinguished from manual. I have attempted to give a really scientific presentation of all the departments of the wide field of geology, at the same time avoiding too great multiplica- tion of detail. I have desired to make a work which shall be both interesting and profitable to the intelligent general reader, and at the same time a suitable text-book for the higher classes of our colleges. In the selection of material and mode of presentation I have been guided by long experience, as to what it is possible to make interest- ing to a class of young men, somewhat advanced in general culture and eager for knowledge, but not expecting to become special geolo- gists. In a word, I have tried to give such knowledge as every thor- oughly cultured man ought to have, and at the same time is a suitable foundation for the further prosecution of the subject to those who so desire. The work is the substance of a course of lectures to a senior class, organized, compacted, and disencumbered of too much detail, by re-presentation for many successive years, and now for the first time reduced to writing. Most text-books now in use in this country are, in my opinion, either too elementary on the one hand, or else adapted as manuals for the specialists on the other. I wish to fill this gap to supply a want iv PREFACE. felt by many intelligent students and general readers, who desire a really scientific general knowledge of geology. Lyell's " Elements " comes nearest to supplying this want ; but there are two objections to this admirable work : 1. The principles (dynamical geology) are sepa- rated from the elements (structural and historical geology), and treated in a different work ; 2. Its treatment of American geology is of course meagre. I have treated several subjects in dynamical and structural geology e. g., rivers, glaciers, volcanoes, geysers, earthquakes, coral-reefs, slaty cleavage, metamorphism, mineral veins, mountain-chains, etc. more fully than is common. I feel hopeful that many geologists and physicists will thank me for so doing. I am confident that I give somewhat fairly the present condition of science on these subjects. In the historical part I have found much more difficulty in being scientific without being tiresome, and in being interesting without being superficial and wordy. I have attempted to accomplish this diffi- cult task by making evolution the central idea, about which many of the facts are grouped. I have tried to keep this idea in view, as a thread running through the whole history, often very slender some- times, indeed, invisible but reappearing from time to time to give consistency and meaning to the history. If this work have any advantage over others already before the public, it is chiefly in the two points mentioned above, viz., in a fuller presentation of some subjects in dynamical and structural geology, and in the attempt to keep evolution in view, and to make it the central idea of the history. Another advantage, I believe, is, that it does not seek to compete with the best works now before the public, but occu- pies a distinct field and supplies a distinct want. I have confined myself mostly, though not entirely, to American geology, especially in giving the distribution of the rocks and the physical geographv of the different periods. In only one case have I made American geology subordinate, viz., in the Jura-Trias period, and that only because of the meagreness of the record of this period in this country. In a science so comprehensive and many-sided as geology, it is simply impossible, as every teacher knows, to avoid anticipations in one part of what strictly belongs to a subsequent part. It is for this reason that the order of presentation of the different departments, and PREFACE. v of the various subjects under each department, is so different in the hands of different writers. The order which I have adopted I know is not free from objection on this score, but it seemed to me, on the whole, the best. In preparing the work I have, of course, drawn largely from many sources, both text-books and works of original research ; for whatever of merit there be in a work of this kind must consist not so much in the novelty of the matter as in the selecting, grouping, and presenta- tion. Such obligations are acknowledged in the pages of the work. I cannot forbear, however, making here a special acknowledgment of my indebtedness, in the historical part, to the invaluable manual of Prof. Dana. I must also acknowledge especial indebtedness to Profs. Marsh and Newberry, and the geologists and paleontologists of the United States Surveys, in charge of Prof. Hayden and Lieutenant Wheeler, not only for valuable materials, but also for much personal aid. CONTENTS. INTRODUCTORY. PAGE DEFINITION OF GEOLOGY, AND OF ITS DEPAKTNENTS 1-2 GEOLOGY, 1 ; Principal Departments, 2 ; Order of Treatment, 2. PART I. DYNAMICAL GEOLOGY. CHAPTER I. ATMOSPHERIC AGENCIES 3-8 Soils, 4 ; General Explanation, 6 ; Granite, Gneiss, Volcanic Rocks, etc., 7 ; Lime- stone, 1 ; Sandstones, 7 ; Slate, 7. MECHANICAL AGENCIES OF THE ATMOSPHERE. Frost, 8 ; Winds, 8. CHAPTER II. AQUEOUS AGENCIES 9-V6 SECTION 1. EIVERS, 9. Erosion of Rain and Rivers, 9 ; Hydrographical Basin, 10 ; Kate of Erosion of Continents, 10 ; Law of Variation of Erosive Power, 11. Ex- amples of Great Erosion now going on : Waterfalls, 12 ; Niagara, General Description, 12 ; Recession of the Falls, 12 ; Other Falls, 13 ; Time necessary to excavate Niagara Gorge, 14 ; Ravines, Gorges Canons, 15 ; Time, 17. Trans- portation and Distribution of Sediments, 18 ; Experiments, 18 ; Law of Vari- ation, 18. 1. Stratification, 20. 2. Winding Course of Rivers, 21. 3. Flood- Plain Deposits, 22 ; River-Swamp, 22 ; Natural Levies, 23 ; Artificial Levees, 23. 4. Deltas, 24 ; Process of Formation, 26 ; Rate of Growth, 27 ; Age of River- Deposits, 27. 5. Estuaries, 29 ; Mode of Formation, 29 ; Deposits in Estuaries, 30. 6. Bars, 30. SECTION 2. OCEAN. Waves and Tides. Waves, 31 ; Tides, 32 ; Examples of the Action of Waves and Tides, 33 ; Transporting Power, 36 ; Deposits, 36. Oceanic Currents, 37 ; Theory of Oceanic Currents, 37 ; Application, 38 ; Geo- logical Agency of Oceanic Currents, 39 ; Submarine Banks, 40 ; Land formed by Ocean Agencies, 42. SECTION 3. ICE, 43. Glaciers. Definition, 43 ; Necessary Conditions, 43 ; Rami- fications of Glaciers, 44 ; Motion of Glaciers, 44 ; Graphic Illustration, 46 ; Line of the Lower Limit of Glaciers, 46 ; General Description, 47 ; Earth and Stones, etc., 49. Moraines, 50. Glaciers as a Geological Agent, 51 ; Erosion, 51 ; Trans- portation, 52 ; Deposit Balanced Stones, 52 ; Material of the Terminal Moraine, i CONTENTS. PAG! 53 ; Evidences of Former Extension of Glaciers, 53 ; Glacial Lakes, 54. Mo- tion of Glaciers and its Laws. Evidences of Motion, 54; Laws of Glacier- Motion, 55. Theories of Glacier-Motion, 57. Viscosity Theory of Forbes. Statement of the Theory, 57 ; Argument, 57. Regelation Theory of Tyndall, 58 ; Eegelation, 59 ; Application to Glaciers, 59 ; Comparison of the Two Theories, 60 ; Conclusion, 60. Structure of Glaciers, 60 ; Veined Structure, 60 ; Fissures, 61. Theories of Structure. Fissures, 62 ; Veined Structure, 62 ; Physical Theory of Veins, 63. Floating Ice Icebergs, 64 ; General Descrip- tion, 65 ; Icebergs as a Geological Agent Erosion, 66 ; Deposits, 66. Shore- Ice, 67. Comparison of the Different Forms of the Mechanical Agencies of Water, 67. SECTION 4. CHEMICAL AGENCIES OF WATER. Subterranean Waters, Springs, etc., 68 ; Springs, 68 ; Artesian Wells, 69 ; Chemical Effects of Subterranean Waters, 70 ; Limestone Caves, 70. Chemical Deposits in Springs. Deposits of Carbon- ate of Lime, 71 ; Explanation, 71 ; Kinds of Materials, 71 ; Deposits of Iron, 72 ; Deposits of Silica, 72 : Deposits of Sulphur and Gypsum, 73. Chemical De- posits in Lakes. Salt Lakes and Alkaline Lakes, 73 ; Conditions of Salt-Lake Formation, 74 ; Deposits in Salt Lakes, 75. Chemical Deposits in Seas, 76. CHAPTER III. IGNEOUS AGENCIES 76-132 SECTION 1. INTERIOR HEAT OF THE EAETH. Stratum of Invariable Temperature, 76 ; Increasing Temperature of the Interior of the Earth, 77 ; Constitution of the Earth's Interior, 78 ; 1. Rate of Increase not uniform, 78 ; 2. Fusing-Point not the same for all Depths, 79 ; Astronomical Reasons, 80. SECTION 2. VOLCANOES. Definition, 81 ; Size, Number, and Distribution, 81 ; Phe- nomena of an Eruption, 82 ; Monticules, 83 ; Materials erupted, 83 ; Stones, 83 ; Lava, 83 ; Liquid Lava, 84 ; Hardened Lava, 85 ; Gas, Smoke, and Flame, 85 ; Kinds of Volcanic Cones, 86 ; Mode of Formation of a Volcanic Cone, 86 ; Comparison between a Volcanic Cone and an Exogenous Tree, 89 ; Estimate of the Age of Volcanoes, 89. Thtory of Volcanoes, 90 ; Force, 90 ; The Heat, 91 ; Internal Fluidity Theory, 91 ; Objections, 91 ; Chemical Theory, 92 ; Re- cent Theories, 92. Subordinate Volcanic Phenomena, 94 ; General Explanation, 94. Geysers, 94 ; Description, 94 ; Phenomena of an Eruption, 94; Yellowstone Geysers, 96 ; Theories of Geyser-Eruption, 99 ; Mackenzie's Theory, 99 ; Bun- sen's Investigations, 100 ; Theory of Geyser-EruptionPrinciples, 101 ; Appli- cation to Geysers, 101 ; Bunsen's Theory of Geyser-Formation, 103. SECTION 3. EARTHQUAKES, 104 ; Frequency, 104 ; Connection with other Forms of Igneous Agency, 104 ; Ultimate Cause of Earthquakes, 106 ; Proximate Cause, 106 ; Waves their Kinds and Properties, 106 ; Definition of Terms, 107 ; Ap- plication to Earthquakes, 109 ; Experimental Determination of the Velocity of the Spherical Wave, 111 ; Explanation of Earth quake- Phenomena, 111 ; Vorti- cose Earthquakes, 114 ; Explanation, 114. Earthquakes originating beneath the Ocean, 119 ; Great Sea-Wave, 119 ; Examples of the Sea-Wave, 120. Depth of Earthquake- Focus, 122; Seismometers, 122; The Determination of the Epi- centrum, 124 ; Determination of the Focus, 124 ; Effect of the Moon on Earth- quake-Occurrence, 126 ; Relation of Earthquake-Occurrence to Seasons and At- mospheric Conditions, 126. SECTION 4. GRADUAL ELEVATION AND DEPRESSION OF THE EARTH'S CRUST, 127 ; Elevation or Depression during Earthquakes, 127 ; Movements not connected with Earthquakes South America, 127 ; Italy, 127 ; Scandinavia, 129 ; Green- land, 129 ; Deltas of Large Rivers, 129 ; Southern Atlantic States, 130 ; Pacific Ocean, 130. Theories of Elevation and Depression, 131 ; Babbage's Theory, 131 ; Herschel's Theory, 132 ; General Theory, 132. CONTEXTS. i x CHAPTER IV. PAGE ORGANIC AGENCIES 133-163 SECTION 1. VEGETABLE ACCUMULATIONS. Peat-Bogs and Peat-Swamps. Descrip- tion, 133 ; Composition and Properties of Peat, 133 ; Mode of Growth, 134 ; Rate of Growth, 135 ; Conditions of Growth, 135 ; Alternation of Peat with Sediments, 136. Drift- Timber, 136. SECTION 2. BOG-!RON ORE, 136. SECTION 3. LIME ACCUMULATIONS. Coral Reefs and Islands. Interest and Im- portance, 138 ; Coral Polyp, 138 ; Compound Coral, or Corallum, 138 ; Coral Forests, 138 ; Coral Eeef, 139 ; Coral Islands, 139 ; Conditions of Coral-Growth, 140 ; Pacific Eeefs, 140 ; Fringing Eeefs, 140 ; Barrier Eeefs, 141 ; Circular Reefs, or Atolls, 142 ; Small Atolls and Lagoonless Islands, 143. Theories of Barrier and Circular Reefs, 143 ; Crater Theory, 143 ; Objections, 144 ; Sub- sidence Theory, 144 ; Proofs, 145 ; Area of Land lost, 146 ; Amount of Vertical Subsidence, 146 ; Eate of Subsidence, 147 ; Time involved, 148 ; Geological Ap- plication, 148. Rtefs of Florida, 149 ; Description of Florida, 149 ; General Process of Formation, 150 ; History of Changes, 150 ; Mangrove Islands, 151 ; Florida Eeefs compared with other Eeefs, 152 ; Probable Agency of the Gulf Stream, 152. Shell-Deposits, 153 ; Molluscous Shells, 153 ; Microscopic Shells, 154. SECTION 4. GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. Fauna and Flora, 155 ; Kinds of Distribution, 155 ; Vertical Botanical Temperature-Eegions, 156 ; Bo- tanical Temperature-Regions in Latitude, 156 ; Further Definition of Eegions, 157 ; Zoological Temperature-Eegions, 153 ; Continental Fauna and Flora, 159 ; Subdivisions, 160 ; Special Cases, 161 ; Marine Fauna Distribution in Lati- tude, 162 ; Distribution in Longitude, 162 ; Depth and Bottom, 162 ; Special Cases, 162. PART II. STRUCTURAL GEOLOGY. CHAPTER I. GENERAL FORM AND STRUCTURE OF THE EARTH .... 164-170 1. Form of the Earth, 164. 2. Density of the Earth, 165. 3. The Crust of the Earth, 166 ; Means of Geological Observation, 166. 4. General Surface Con- figuration of the Earth, 167 ; Cause of Land-Surfaces and Sea-Bottoms, 167 ; Laws of Continental Form, 168. Rocks, 169 ; CLsses of Eocks, 170. CHAPTER II. STRATIFIED OR SEDIMENTARY ROCKS 170-202 SECTION 1. STRUCTURE AND POSITION. Stratification, 170 ; Extent and Thickness, 170 ; Kinds of Stratified Eocks, 171 ; I. Stratified Eocks are more or less Con- solidated Sediments, 171 ; Cause of Consolidation, 172 ; II. Stratified Eocks have been gradually deposited, 172 ; III. Stratified Eocks were originally nearly horizontal, 173 ; Elevated, Inclined, and Folded Strata, 174 ; Dip and Strike, 175 ; Anticlines and Synclines, 177 ; Monoclinal Axes, 178 ; Unconformity, 178 ; Formation, 179. Cleavage Structure, 179 ; Sharpe's Mechanical Theory, 181 ; Physical Theory, 185 ; Sorby's Theory, 185 ; Tyndall's Theory, 186 ; Geolog- ical Application, 187. Nodular or Concretionary Structure, 188 ; Cause, 188 ; Forms of Nodules, 189 ; Kinds of Nodules found in Different Strata, 189. Fos- sils : their Origin and Distribution, 190 ; The Degrees of Preservation are very various, 191 ; Theory of Petrifaction, 192. Distribution of Fossils in the Strata, x CONTENTS. PAGE 195 ; 1. Kind of Eock, 195. 2. The Country where found, 195 ; 3. The Age, 196. SECTION 2. CLASSIFICATION OF STRATIFIED BOCKS, 197 ; 1. Order of Superposition, 198 ; 2. Lithological Character, 198 ; 3. Comparison of Fossils, 199 ; Manner of constructing a Geological Chronology, 200. CHAPTER III. UNSTRATIFIED OR IGNEOUS ROCKS 202-212 Characteristics, 202 ; Origin, 202 ; Mode of Occurrence, 202 ; Extent on the Sur- face, 203 ; Classification of Igneous Rocks, 203. 1. GRANITIC ROCKS, 203 ; Chem- ical Composition and Kinds, 204 ; Mode of Occurrence, 204. 2. TRAPPEAN OR FISSURE-ERUPTION ROCKS, 205 ; General Characteristics, 205 ; Varieties, 205 ; Mode of Occurrence, 206 ; Effect of Dikes on the Intersected Strata, 207 ; Age how determined, 208. 3. VOLCANIC ROCKS. Characteristics, 208 ; Varieties, 208. Of Certain Structures found in many Eruptive Rocks. Columnar Struct- ure, 209 ; Direction of the Columns, 210 ; Cause of Columnar Structure, 210 ; Volcanic Conglomerate and Breccia, 210; Amygdaloid, 211. Other Nodes of Classification of Igneous Bocks, 211. CHAPTER IV. METAMORPHIC ROCKS 213-219 Origin, 213 ; Position, 213 ; Extent on the Earth-Surface, 213 ; Principal Kinds, 214. Theory of Metamorphism, 215 ; Water, 215 ; Alkali, 216 ; Pressure, 216 ; Application, 216 ; Crushing, 216 ; Explanation of Associated Phenomena, 217. Origin of Granite, 217. CHAPTER V. STRUCTURE COMMON TO ALL ROCKS 220-260 SECTION 1. JOINTS AND FISSURES. Joints, 220. Fissures, or Fractures, 221 ; Cause, 221 ; Faults, 222 ; Law of Slip, 224. SECTION 2. MINERAL VEINS. Kinds, 225 ; Characteristics, 226 ; Irregularities, 226 ; Metalliferous Veins, 227 ; Contents, 227 ; Ribboned Structure, 228 ; Age, 229 ; Surface-Changes, 230 ; Cupriferous Veins, 230 ; Plumbiferous Veins, 231 ; Au- riferous Quartz-Veins, 231 ; Placer-Mines, 232. Some Important Lazes affecting the. Occurrence and the Richness of Metalliferous Veins, 232. Theory of Metal- liferous Veins, 234 ; Outline of the most Probable Theory, 235 ; Vein-Stuffs, 235 ; Metallic Ores, 236 ; Auriferous Veins of California, 237 ; Nuggets, 239 ; Illustra- tions of the Law of Circulation, 239. SECTION 3. MOUNTAIN-CHAINS: THEIR STRUCTURE AND ORIGIN, 240. Mountain- Origin, 240. General Form, and how produced, 240. Mountain-Structure, 241 ; Rate of Mountain-Formation, 244; Thickness of Mountain-Sediments, 244; Foldings and Metamorphism, 245. Mountain-Sculpture, 245 ; Resulting Forms, 246 ; The Age of Mountain-Chains, 251. Theory of the Origin of Mountain- Chains, 252 ; 1. Thick Sediments of Mountain-Chains, 254 ; Appalachian Chain, 254 ; Sierras, 256 ; Coast Range, 256 ; Alps, 256 ; 2. Position of Mountain- Chains along the Borders of Continents, 256 ; 3. Parallel Ranges, 257 ; 4. Meta- morphism of Mountain-Chains, 257 ; 5. Fissures and Slips, and Earthquakes, 258 ; 6. Fissure-Eruptions, 258 ; 7. Volcanoes, 258. CHAPTER VI. DENUDATION, OR GENERAL EROSION 260-265 Agents of Denudation, 260 ; Amount of Denudation, 261 ; Average Erosion, 263 ; Estimate of Geological Times, 264. CONTENTS. x i PART III. HISTORICAL GEOLOGY; OR, THE HISTORY OF THE EVOLUTION OF EARTH-STRUCTURE AND OF THE ORGANIC KINGDOM. CHAPTER I. PAGE GENERAL PKINCIPLES 266-271 Great Divisions and Subdivisions of Time-Eras, 268 ; Ages, 269 ; Subdivisions, 270 ; Order of Discussion, 270 ; Prehistoric Eras, 271. CHAPTER II. LAUREXTIAX SYSTEM OF ROCKS AND AECH^AX ERA .... 272-276 Rocks, 273 ; Area in North America, 274 ; Physical Geography of Archaean Times, 274 ; Time represented, 274 ; Evidences of Life, 274. CHAPTER III. PRIMARY OR PALEOZOIC SYSTEM OF ROCKS AND PALEOZOIC ERA . . 276-404 General Description, 276 ; Eocks Thickness, etc., 276; Area in the United States, 277 ; Physical Geography of the American Continent, 279 ; Subdivisions, 280 ; The Interval, 230. SECTION 1. SILURIAN SYSTEM : AGE OF INVERTEBRATES. The Eock System, 282 ; Subdivisions, 282 ; Character of the Eocks, 282 ; Area in America, 282 ; Phys- ical Geography, 283 ; Primordial Beach and its Fossils, 283 ; General Eemarks on First Distinct Fauna, 287. General Life-System of the Silurian Age, 288. Plants, 290. Animals. Protozoans, 290 ; Eadiates, Corals, 290 ; Hydrozoa, 293 ; Polyzoa, 296 ; Echinoderms, 296 ; Mollusks, 801 ; General Description of a Brachiopod, 301 ; Lamellibranchs, 304 ; Gasteropods, 305 ; Cephalopods, 305 ; Articulates, 308 ; Crustacea, 309 ; General Description, 309 ; Affinities of Trilo- bites, 312 ; Eurypterids, 313 ; Anticipations of the Next Age, 314. SECTION 2. DEVONIAN SYSTEM AND AGE OF FISHES, 314 ; Area in United States, 315 ; Physical Geography, 315 ; Subdivision into Periods, 315. Life-System of De- vonian Age Plants, 315; General Eemarks on Devonian Land-Plants, 316. Animals, 319 ; Eadiates, 319 ; Brachiopods, 320 ; Cephalopods, 320 ; Crustacea, 322 ; Insects, 322 ; Fishes, 322 ; Affinities of Devonian Fishes, 327 ; General Characteristics of Devonian Fishes, 328 ; Bank of Devonian Fishes, 331 ; Bear- ing of these Facts on the Question of Evolutiori, 332 ; Suddenness of Appear- ance, 332. SECTION 3. CARBONIFEROUS SYSTEM : AGE OF ACROGENS AND AMPHIBIANS. Eetro- spect, 333 ; Subdivisions of the Carboniferous System and Age, 334. Car- boniferous Proper Rock-System or Coal-Measures. The Name, 334 ; Thickness of Strata, 335 ; Mode of Occurrence of Coal, 335 ; Plication and Denudation, 336 ; Faults, 337 ; Thickness of Seams, 338 ; Number and Aggregate Thickness, 338 ; Coal Areas of the United States, 338 ; Extra-Carboniferous Coal, 339 ; Coal, Areas of Different Countries compared, 339 ; Eelative Production of Coal, 340. Origin of Coal and of its Varieties, 340 ; Varieties of Coal, 341 ; Varieties depending upon Purity, 341 ; Varieties of Coal depending on the Degree of Bitu- minization, 342 ; Varieties depending upon the Proportion of Fixed and Vola- tile Matter, 342 ; Origin of these Varieties, 343 ; In Contact with Air, 344 ; Out of Contact with Air, 344 ; Metamorphic Coal, 345. Plants of the Coal their Structure and Affinities, 346 ; Where found, 347 ; Principal Orders, 347 ; 1. Con- ifers, 347 ; Affinities of Carboniferous Conifers, 349 ; 2. Ferns, 349 ; 3. Lepido- dendrids, 355 ; 4. Sigillarids, 358 ; 5. Calamites, 361 ; General Conclusion, 362. xii CONTENTS. PAGE Theory of the Accumulation of Coal, 363 ; Presence of Water, 363 ; Application of the Theory to the American Coal-Fields : a. Appalachian Coal-Field, 366 ; b. Western Coal-Fields, 367 ; Appalachian Revolution, 367. Estimate of Time, 367 ; 1. From Aggregate Amount of Coal, 367 ; 2. From Amount of Sediment, 368. Physical Geography and Climate of the Coal Period. Physical Geog- raphy, 369 ; Climate, 369 ; Cause of the Climate, 370. Iron- Ore of the Coal- Measures, 373 ; Mode of Occurrence, 373 ; Kinds of Ore, 373 ; Theory of the Ac- cumulation of the Iron-Ore of the Coal-Measures, 374. Bitumen and Petro- leum, 376 ; Geological Eolations, 376 ; Oil-Formations, 377 ; Principal Oil- Horizons of the United States, 377 ; Laws of Interior Distribution, 377 ; Kinds of Rocks which bear Petroleum, 378. Origin of Petroleum and Bitumen, 379 ; Origin of Varieties, 380 ; Area of Oil-bearing Strata in the Eastern United States, 380. Fauna of the Carboniferous Age, 381 ; Vertebrates (Fishes), 388 ; Reptiles Amphibians, 390; 1. Reptilian Footprints, 390; 2. Dendrerpeton, 391 ; 3. Archegosaurus, 393 ; 4. Eosaurus, 393 ; Some General Observations on the Earliest Reptiles, 395. Some General Observations on the Whole Palaeo- zoic, 396 ; Physical Changes, 396 ; Chemical Changes, 396 ; Progressive Change in Organisms, 397 ; General Comparison of the Fauna of Paleozoic with that of Neozoic Times, 397. General Picture of Palceozoic Times, 398. Transmis- sion from the Palceozoic to the Mesozoic Permian Period. The Permian a Tran- sition Period, 400. CHAPTER IV. MESOZOIO ERA AGE OF EEPTII.ES 404-475 General Characteristics, 404 ; Subdivisions, 404. SECTION 1. TEIASSIC PERIOD, 405 ; Subdivisions, 405 ; Animals, 406 ; Fishes, 407 ; Reptiles, 409 ; Birds, 411 ; Mammals, 411. Origin of Rock-Salt, 412 ; Age of Rock-Salt, 412 ; Mode of Occurrence, 412 ; Theory of Accumulation, 413. SECTION 2. JURASSIC PERIOD, 414 ; Origin of Oolitic Limestones, 415 ; Jurassic Coal- Measures, 415 ; Dirt-BedsFossil Forest-Grounds, 415. Plants, 417. Animals, 419; Corals, 419 ; Brachiopods, 419 ; Lamellibranchs, 419 ; Cephalopods, 419'; Ammonites, 421 ; Belemnites, 423 ; Crustacea, 425 ; Insects, 425 ; Fishes, 425 ; Reptiles, 428 ; Birds, 436 ; Mammals, 438 ; Affinities of the First Mammals, 438. SECTION 3. JURA-TRIAS IN AMERICA, 439 ; Distribution of Strata, 439. Life-System, 440 ; Connecticut River Valley Sandstone the Strata, 440 ; The Record, 441 ; Reptilian Tracks, 442 ; Bird-Tracks, 443 ; Richmond and North Carolina Coal- Fields, 445 ; Other Patches, 447 ; Interior Plains and Pacific Slope, 447 ; Phys- ical Geography of the American Continent during the Jura-Trias Period, 449 ; Disturbances which closed the Period, 450. SECTION 4. CRETACEOUS PERIOD, 450 ; Rock-SystemArea in America, 451 ; Phys- ical Geography in America, 451 ; Rocks, 452 ; Chalk, 452 ; Origin of Chalk, 453 ; Extent of Chalk Seas of Cretaceous Times in Europe, 454 ; Cretaceous Coal, 455 ; Subdivisions of the Cretaceous, 456. Life-System : Plants, 456. Animals. Protozoa, 459 ; Echinoderms, 460 ; Mollusks, 461 ; Vertebrates Fishes, 465 ; Reptiles, 467 ; Birds, 470 ; Mammals, 472. Continuity of the Chalk, 473. General Observations on the Mesozoic, 474. Disturbance which closed the Mesozoic, 475. CHAPTER V. CENOZOIO ERA AGE OF MAMMALS 475-557 General Characteristics of the Cenozoic Era, 476 ; Divisions, 476. SECTION 1. TERTIARY PERIOD. Subdivisions, 477 ; Rock-SystemArea in the United States, 477 ; Physical Geography, 478 ; Character of the Rocks, 480 ; Coal, 480 ; Life-System. General Remarks, 480. Plants, 481 ; Diatoms, 483 ; Origin of In- fusorial Earths, 483. Animals, 485 ; Insects, 488 ; Fishes, 490 ; Reptiles, 492 ; CONTENTS. x iii PAGE Birds, 494 ; Mammals General Remarks, 495 ; 1. Eocene Basin of Paris, 496 ; 2. Siwalik Hills, India Miocene, 498 ; American Localities 3. Marine Eocene of Alabama, 500 ; 4. Green-River Basin "Wahsatch Beds Lower Eocene, 501 ; 5. Green-River Basin Bridger Beds Middle Eocene, 502 ; 6. Mauvaises Terres of Nebraska White River Basin Miocene, 505 ; 7. Mauvaises Terres Nio- brara Basin Pliocene, 506 ; Some General Observations on the Tertiary Mam- malian Fauna, 506 ; Genesis of the Horse, 509. General Observations on the Tertiary Period, 511. SECTION 2. QUATERNARY PERIOD. Characteristics, 513 ; Subdivisions, 513. Qua- ternary Period in Eastern North America I. Glacial Epoch. The Materials Drift, 514 ; The Bowlders, 515 ; Surface-Rock underlying Drift, 516 ; Extent, 516; Marine Deposits, 517. Theory of the Origin of the Drift, 517; State- ment of the most Probable View, 518 ; Objections answered, 518 ; Probable Condition during Glacial Times in America, 519. //. Champlain Epoch, 520 ; Evidences of Subsidence, 521 ; Sea-Margins, 521 ; Flooded Lakes, 521 ; River Terraces and Old Flood-Plain Deposits, 522. ///. Terrace Epoch, 524 ; Evi- dencesSea, 524 ; Lakes, 524 ; Rivers, 524 ; History of the Mississippi River, 525. Quaternary Period on the Western Side of the Continent, 526 ; Glaciers, 526 ; Lake-Margins, 527 ; Rivers, 529 ; Seas, 530. The Quaternary Period in Eu- rope, 530 ; 1. Epoch of Elevation First Glacial Epoch, 531 ; 2. Epoch of Sub- mergence Champlain, 532 ; 3. Epoch of Reelevation Second Glacial Epoch Terrace Epoch, 534 ; 4. Modern Epoch, 534. Home General Results of Glacial Erosion. 1. Fiords, 534; 2. Glacial Lakes, 535. Life of the Quaternary Period. Plants and Invertebrates, 535 ; Mammals, 536; 1. Bone-Caverns, 536; Origin of Cave Bone-Rubbish, 538 ; Origin of Bone-Caverns, 539 ; 2. Beaches and Ter- races, 539 ; 3. Marshes and Bogs, 539 ; 4. Frozen Soils and Ice Cliffs, 539 ; Qua- ternary Mammalian Fauna of England, 541. Mammalian fauna in North America, 542 ; Bone-Caves, 542 ; Marshes and Bogs, 542 ; River-Gravels, 544 ; Quaternary in South America, 544 ; Australia, 547 ; Geographical Fauna of Quaternary Times, 547. Some General Observations on the Whole Quaternary. 1. Cause of the Climate, 548 ; 2. Time involved in the Quaternary Period, 650 ; 3. The Quaternary a Period of Revolution a Transition between the Ceno- zoic and the Modem Eras, 550 ; 4. Drift in Relation to Gold, 554 ; Age of the River-Gravels, 556. CHAPTER VI. PsTcnozoic EBA AGE OF MAX RECENT EPOCH .... 557-570 Characteristics, 557 ; Distinctness of this Era, 557 ; The Change still in Progress Examples of Recently-Extinct Species, 558. I. ANTIQUITY OF MAN, 560. Pri- meval Man in Europe. Supposed Miocene Man Evidence unreliable, 561 ; Supposed Pliocene Man, 562 ; Quaternary Man Mammoth Age, 562 ; a. In River-Terraces, 562; b. Bone-Caves Engis Skull, 563; Neanderthal Skull, 563 ; Mentone Skeleton, 564 ; Reindeer Age or Later Palaeolithic, 564 ; Auri- gnac Cave, 5b5 ; Perigord Caves, 565 ; Conclusions, 566. Neolithic Man Refuse- Heaps Shell- Mounds Kitchen-Middens, 506 ; Transition to the Bronze Age Lake Dwellings, 566. Primeval Man in America, 567 ; Supposed Pliocene Man, 567 ; Quaternary Man, 567 ; Quaternary Man in Other Countries, 568 ; Time since Man appeared, 569. II. CHAEACTER OF PBIMEVAL MAN, 570. INTKODUCTOKY. DEFINITION OF GEOLOGY, AND OF ITS DEPARTMENTS. GEOLOGY is the physical history of the earth and its inhabitants, as recorded in its structure. It includes an account of the changes through which they have passed, the laws of these changes, and their causes. In a word, it is the history of the evolution of the earth and its inhabitants. The fundamental idea of geology, as well as its principal sub- divisions and its objects, may be most clearly brought out by compar- ing it with organic science. We may study an organism from three distinct points of view : 1. We may study its general form, the parts of which it is composed, and its minute internal structure. This is anatomy. It is best studied in the dead body. 2. We may study the living body in action, the function of each organ, the circulation of the fluids, and the manner in which all contribute to the complex phenom- ena of life. This is physiology. 3. We may study the living and growing body, by watching the process of development from the egg to the adult state, and striving to determine its laws. This is embry- ology. So, looking upon the earth as an organic unit, we may study its form, the rocks and minerals of which it is composed, and the manner in which these are arranged ; in other words, its external form and in- ternal structure. This is the anatomy of the earth, and is called struct- ural geology. Or, we may study the earth under the action of physical and chemical forces, the action and reaction of land and water, of earth and air, and the effects of these upon the form and structure. This is the physiology of the earth, and is called dynamical geology. Finally, we may study the earth in the progress of its development, from the earliest chaotic condition to its present condition, as the abode of man, and attempt to determine the laws of this development. This is the embryology of the earth, or historical geology. 1 2 INTRODUCTORY. Principal Departments. The science of geology, therefore, nat- urally divides itself into three parts, viz.: 1. Structural geology, or geognosy. 2. Dynamical geology, or physical and chemical geology. 3. Historical geology, or the history of the earth. But there are two important points of difference between geology and organic science. The central department of organic science is physiology, and both anatomy and embryology are chiefly studied to throw light on this. But the central department of geology, to which the others are subservient, is history. Again : in case of organisms especiallv animal organisms the nature of the changes producing development is such that the record of each previous condition is suc- cessively and entirely obliterated ; so that the science of embryology is possible only by direct observation of each successive stage. If this were true also of the earth, a history of the earth would, of course, be impossible. But, fortunately, we find that each previous condition of the earth has left its record indelibly impressed on its structure. Order of Treatment. The prime object of geology is to determine the history of the earth, and of the organisms which have successively inhabited its surface. The structure and constitution of the earth are the materials of this history, and the physical and chemical changes now going on around us are the means of interpreting this structure and constitution. Evidently, therefore, the only logical order of pre- senting the facts of geology is to study, first, the causes, physical and chemical, now in operation and producing structure ; then the structure and constitution of the earth which, from the beginning, have been produced by similar causes; and, lastly, from the two preceding to un- fold the history of the earth. Geology may be defined, therefore, as the history of the earth and its inhabitants, as revealed in its structure, and as interpreted by causes still in operation. There is no other science which requires for its full comprehension a general knowledge of so many other departments of science. A knowledge of mathematics, physics, and chemistry, is required to under- stand dynamical geology ; a knowledge of mineralogy and lithology is required to understand structural geology ; and a knowledge of zoology and botany is required to understand the affinities of the animals and plants which have successively inhabited the earth, and the laws which have controlled their distribution in time. PAET I. DYNAMICAL GEOLOGY. THE agencies now in operation, modifying the structure of the sur- face of the earth, may be classed under four heads, viz., atmospheric agencies, aqueous agencies, igneous agencies, and organic agencies. These agencies have operated from the beginning, and are still in operation. We study their operation now, in order that we may un- derstand their effects in previous epochs of the earth's history i. e., the structure of the earth. While all geologists agree that the nature of the agencies which have operated in modifying the earth's surface has remained the same from the beginning, they differ in their views as to the energy of their operation in different periods. Some believe that their energy has been much the same throughout the whole history of the earth, while others believe that many facts in the structure of the earth require much greater operative energy than now exists. We will attempt to show hereafter that neither of these extreme opinions is probably true, but that some of these agencies have been decreasing, while others have been increasing, with the progress of time. It is the constant change of balance between these which determines the development of the earth. CHAPTER I. ATMOSPHERIC AGENCIES. THE general effect of atmospheric agencies is the disintegration of rocks and the formation of soils. The atmosphere is composed of nitro- gen and oxygen, with small quantities of watery vapor and of carbonic acid There are but few rocks which are not gradually disintegrated 4 ATMOSPHERIC AGENCIES. under the constant chemical action of the atmosphere. The chemical agents of these changes are oxygen, carbonic acid, and watery vapor, the nitrogen being inert. To these must be added, where vegeta- tion is present, the products of vegetable decomposition, especially ammonia. Atmospheric agencies graduate so insensibly into aqueous agencies that it is difficult to define their limits. Water, holding in solution carbonic acid and oxygen, may exist as invisible vapor; or, partially condensed, as fogs ; or, completely condensed, as rain falling upon and percolating the earth. In all these forms its chemical action is the same, and, therefore, cannot be separated and treated under different classes : and yet the same rain runs off from and erodes the surface of the earth, comes out from the strata and forms springs, rivers, etc., all of which naturally fall under aqueous agencies. We shall, therefore, treat of the chemical effects of atmospheric water in the disintegration of rocks, and the formation of soils, under the head of atmospheric agencies ; and the mechanical effects of the same, in eroding the surface and carrying away the soil thus formed, under the head of aqueous agencies. In moist climates vegetation clothes and protects soil from erosion, but favors decomposition of rocks and formation of soil. Atmospheric agencies are obscure in their operation, and, therefore, imperfectly understood. Yet these are not less important than aqueous agencies, since they are the necessary condition of the operation of the latter. Unless rocks were first disintegrated into soils by the action of the atmosphere, they would not be carried away and deposited as sediments by the agency of water. These two agencies are, therefore, of equal power and importance in geology, but they differ very much in the conspicuousness of their effects. Atmospheric agencies act almost equally at all times and at all places, and their effects, at any one place or time, are almost imperceptible. Aqueous agencies, on the contra- ry, in their operation are occasional, and to a great extent local, and their effects are, therefore, more striking and easily studied. Never- theless, the aggregate effects of the former are equal to those of the latter. Soils. All soils (with the trifling exception of the thin stratum of vegetable mould which covers the ground in certain localities) are formed from the disintegration of rocks. Sometimes the soil is formed in situ, and, therefore, rests on its parent rock. Sometimes it is re- moved as fast as formed, and deposited at a distance more or less remote from the parent rock. The evidence of this origin of soils is clearest when the soil is formed in situ. In such cases it is often easy to trace every stage of gradation between perfect rock and perfect soil. This is well seen in railroad cuttings, and in wells in the gneissic or so-called primary region of our southern Atlantic slope. On examining such a ATMOSPHERIC AGENCIES. section, we find near the surface perfect soil, generally red clay ; beneath this we find the same material, but lighter colored, coarser, and more distinctly stratified; beneath this, but shading into it by imperceptible gradations, we have what seems to be stratified rock, but it crumbles into coarse dust in the hand ; this passes by imperceptible gradations into rotten rock, and finally into perfect rock. There can be no doubt that these are all different stages of a gradual decomposition. But closer observation will make the proof still clearer. In gneissic and other metamorphic regions it is not uncommon to find the rock traversed, in various directions, by veins of quartz OT flint. Now, in sections such as those mentioned above, it is common to find such a flint-vein running through the rock and upward through the superincumbent soil, until it emerges on the surface. In the slow decomposition of the rock into soil, the flint-vein has remained unchanged, because flint is not affected by atmospheric agencies. Chemical analysis, also, always shows an evident relation between the soil and the subjacent or country rock, except in cases in which the soil has been brought from a considerable distance. The depth to which soil will thus accumulate depends partly on the nature of the rock and the rapidity of decomposition, partly on the slope of the ground, and partly on climate. When the slope is considerable, as at d (Fig. 1), the rocks are bare, not because no soil is formed, but because it is removed as fast as formed, while at a the soil is deep, being formed partly by de- composition of rock in situ, and partly of soil brought down from d. Wherever perfect soil is found resting on sound rock, the soil has been shifted. If rocks were solid and impervious to water, this process would be almost inconceivably slow ; but we find that all rocks, for reasons to be discussed hereafter, are broken by fissures into irregular prismatic blocks, so that a perpendicular cliff of rock usually presents the appearance of rude gigantic masonry. These fissures, or joints, increase immensely the surface exposed to the action of atmospheric water. Again, on closer inspection, we find even the most solid parts of rocks, i. e., the blocks themselves, penetrated with capillary fissures which allow water to reach every part. Thus the rock is decomposed, or becomes rotten, to a great depth below the surface. But, while the rock is gradually changed into soil, the soil is also slowly carried away by agencies to be hereafter considered; and these changes, taking place more rapidly in FIG. 1. Ideal Section, showing Eock passing into Soil. Q ATMOSPHERIC AGENCIES. some places than in others, give rise to a great variety of forms, some of which are represented in the accompanying figure (Fig. 2). In the process of disintegration the original blocks lose their pris- matic form, and become more or less rounded, and are then called tt bowlders of disintegration. These may lie on the. surface (Fig. 2), or may be buried in the soil (Fig. 3). When of great size and very solid, so as to resist decomposition to a greater extent than the surrounding rocks, they often form huge rocMng-stones (Fig. 4). These must not be confounded with true bowlders and rocking- stones which are brought from FIG. 8. a distance, by agencies which we will discuss hereafter, and which are, therefore, entirely different from the subjacent or country rock. General Explanation. The process of rock-disintegration may be explained, in a general way, as follows : Almost all rocks are composed partly of insoluble materials, and partly of materials which are slowly dissolved by atmospheric water. In the process of time, therefore, these latter are dissolved out, and the rock crumbles into an insoluble dust, more or less saturated with water holding in solution the soluble ingredients. To illustrate : common hardened mortar may be regarded as artificial stone ; it consists of carbonate of lime and sand ; the car- bonate of lime is soluble in water containing carbonic acid (atmospheric water), while the sand is quite insoluble. If, therefore, such mortar be exposed to the air, it eventually crumbles into sand, moistened with water containing lime in solution. Again, to take a case which often occurs in Nature, it is not uncommon to find rock through which iron pyrites, FeS 2 , is abundantly disseminated. This mineral is insoluble ; but under the influence of water containing oxygen (atmospheric water) it is slowly oxidized and changed into sulphate of iron, or copperas, which, being soluble, is washed out, and the rock crumbles into an insoluble dust or soil, saturated with a solution of the iron salt. We ATMOSPHERIC AGENCIES. 7 have given these only as illustrative examples. We now proceed to give examples of the principal kinds of rocks, and of the soils formed by their disintegration. Granite, Gneiss, Volcanic Rocks, etc. Granite and gneiss are mainly composed of three minerals, quartz, feldspar, and mica, aggre- gated together into a coherent mass. Quartz is unchangeable and in- soluble in atmospheric water. Mica is also very slowly affected. Feld- spar is, therefore, the decomposable ingredient. But feldspar is, itself, a complex substance, partly soluble and partly insoluble. It is essen- tially a silicate of alumina, united with a silicate of potash or soda, although it often contains also small quantities of iron and lime. Now, while the silicate of alumina is perfectly insoluble, the other silicates are slowly dissolved by atmospheric water, with the formation of car- bonates, and the silicate of alumina is left as kaolin or clay. But, since we may regard the original rock as made up of quartz and mica, bound together by a cement of feldspar, the disintegration of the latter causes the whole rock to lose its coherence, and the final result of the process is a mass of clay containing grains of sand and scales of mica, and moistened with water containing a potash salt. If there be any iron in the feldspar, or if there be other decomposable ingredients in the rock containing iron, such, for example, as hornblende, the clay will be red. This is precisely the nature of the soil in all our primary regions. Volcanic rocks decompose into clay soils often, though not always, deeply colored with iron. Limestone. Pure limestone may be regarded as composed of gran- ules of carbonate of lime, cohering by a cement of the same. The dis- solving of the cement by atmospheric water forms a lime-soil, moistened with a solution of carbonate of lime (hard water). Impure limestone is a carbonate of lime, more or less mixed with sand or clay ; by disin- tegration it forms, therefore, a marly soil. Sandstones. Sandstones consist of grains of sand cemented togeth- er by carbonate of lime or peroxide of iron. % Where peroxide of iron is the cementing substance, the rock is almost indestructible, since this substance is not changed by atmospheric water: hence the great value of red sandstone as a building-material. But when carbonate of lime is the cementing material, this substance, being soluble in atmospheric water, is easily washed out, and the rock rapidly disintegrates into a sandy soil. Slate. In a similar manner slate-rocks disintegrate into a pure clay soil by the solution of their cementing material, which is often a small quantity of carbonate of lime. There can be no doubt that all soils are formed in the manner above indicated. We have given examples of soils formed in situ, but, as soils are often shifted, they are usually composed of a mixture formed 8 ATMOSPHERIC AGENCIES. by the disintegration of several kinds of rock. In some cases the soil has been formed in situ during the present geological epoch, and the process is still going on before our eyes. Such are the soils of the hills of the up-country or yrimary region of our Southern Atlantic States. 1 Sometimes the soil formed in the same way has been shifted to a greater or less distance. Such are the soils of our valleys and river-bottoms. In still other cases the soil has been formed by the pro- cess already described, and transported during some previous geologi- cal epoch and not reconsolidated. Such are many of the soils of the Southern low-country or tertiary region. MECHANICAL AGENCIES OF THE ATMOSPHERE. Frost. Water, penetrating rocks and freezing, breaks off huge frag- ments : these by a similar process are again broken and rebroken until the rock is reduced to dust. These effects are most conspicuous in cold climates and in mountain-regions. In cold climates huge piles of bowl- ders and earth are always seen at the * base of steep cliffs (Fig. 5). Such of the cliff above, is called a talus. In mountainous regions frost is a pow- erful agent in disintegrating the rocks, and in determining the out- lines of mountain - peaks. This is well seen in the Alps and in the Sierras. Winds. The effect of winds is seen in the phenomenon of shifting sands. At Cape Cod, for instance, the sands thrown ashore by the sea are driven by the winds inland, and thus advance upon the cultivated lands, burying them and destroying their fertility. The sands from the beach on the Pacific coast near San Francisco are driven inland in a similar manner, and are now regularly encroaching upon the better soil. Large areas of the fertile alluvial soil of Egypt, together with their cities and monuments, have been buried by the encroachments of the Sahara Desert. In Brittany and in other parts of France villages which existed during the middle ages have been overwhelmed by drift- ing sands. The same phenomena are observed on various parts of the coast of Holland and England. The rate of advance has been measured in some instances. Thus on the coast of Suffolk it is said to advance at the rate of about five miles a century; at Cape Finisterre, according ; to Ansted, at the rate of thirty-two miles per century, or 560 yards per annum. The Downs of England and Scotland are such barrens of drifting sand. Hills may be formed in this manner thirty to forty feet in height. 1 In the Northern States, in the region of the Drift, nearly all the soil has been shifted. EROSION OF KAIN AND RIVERS. CHAPTER IL AQUEOUS AGENCIES. THE agencies of water are either mechanical or chemical. The mechanical agencies of water may be treated under the threefold aspect of erosion, transportation, and sedimentary deposit. We will consider them under the heads of Rivers, Oceans, and Ice. Under chemical agencies we will consider the phenomena of chemical deposits in Springs and Lakes. Aqueous Agencies. Mechanical. I Chemical. Rivers Erosion, Transportation, Ocean Ice " " Springs Deposit in. Lakes... " " SECTION 1. RIVEKS. Under the head of river agencies we include all the effects of" cir- culating meteoric water from the time it falls as rain until it reaches the ocean : i. e., all the effects of Rain and Rivers. Water, in the form of vapor, fogs, or rain, percolating through the earth, slowly disintegrates the hardest rocks. Much of these percolat- ing waters, after accomplishing the work of soil-making, already treated in the preceding chapter, reappears on the surface in the form of springs, and gives rise to streamlets. A large portion of rain-water, however, never soaks into the earth, but runs off the surface, forming rills, which by erosion produce furrows. The uniting rills form rivulets, which exca- vate gullies. The rivulets, uniting with one another and with the streamlets issuing from springs, form torrents, which in their course excavate ravines, gorges, and canons. The vmiting torrents, finally issu- ing from their mountain-home upon the plains, form great rivers, which deposit their freight partly in their course and partly in the sea. Such is a condensed history of rain-water on its way to the ocean whence it came. Our object is to study this history in more detail. Erosion of Rain and Rivers. The whole amount of water falling on any land-surface may be divided into three parts : 1. That which rushes immediately off the surface, and causes the floods of the rivers, especially the smaller streams; 2. That which sinks into the earth, and, after doing its chemical work of soil-making, reappears as springs, and forms the regu- 10 AQUEOUS AGENCIES. lar supply of streams and rivers ; and, 3. That which reaches the sea wholly by subterranean channels. Of these, the first two are the grand erosive agents, and these only concern us at present. Of these, the for- mer predominate in proportion as the land-surface is bare ; the latter in proportion as it is covered with vegetation. Hydrographical Basin. An hydrographical basin of a river, lake, or gulf, is the whole area of land the rainfall of which drains into that river, lake, or gulf. Thus the hydrographical basin of the Mississippi River is the whole area drained by that river. It is bounded on the east and west by the Alleghany and Rocky Mountains, and on the north by a low ridge running from Lake Superior westward. The whole area of continents, with the exception of rainless deserts, may be 'regarded as made up of hydrographical basins. The ridge which sepa- rates contiguous basins is called a water-shed. It is evident that every portion of the land, with the exception of the rainless tracts already mentioned, is subject to the erosive agency of water, and is being worn away and carried into the sea. There have been various attempts to estimate the rate of this general erosion. Rate of Erosion of Continents. This is usually estimated as follows : Some great river, such as the Mississippi, is taken as the subject of experiment. By accurate measurement during every portion of the year, the average amount of water discharged into the sea per second, per hour, per day, per year, is determined. This is a matter of no small difficulty, as it involves the previous determination of the average cross- section of the river and the average velocity of the current. The aver- age cross-section X average velocity = the average discharge per sec- ond : from which may be easily obtained the annual discharge. Next, by experiment during every month of the year, the average quantity of mud contained in a given quantity of water is also determined. By an easy calculation this gives us the annual discharge of mud, or the whole quantity of insoluble matter removed from the hydrographical basin in one year. This amount, divided by the area of the river-basin, will give the average thickness of the layer of insoluble matter removed from the basin in one year. To this must be added the soluble matters, which are about $ as much as the insoluble. Estimates of this kind have been made for two great rivers, viz., the Ganges and the Mississippi. The whole amount of sediment annually carried to the sea by the Ganges has been estimated as 6,368,000,000 cubic feet. This amount, spread over the whole basin of the Ganges (400,000 square miles), would make a layer T - 7 1 - gT of a foot thick. The Ganges, therefore, erodes its basin one foot in 1,751 years. 1 The area of the Mississippi Basin is 1,244,000 square miles. The annual discharge of sediment, according to the recent and accurate 1 Philosophical Magazine, vol. v., p. 261. EROSION OF RAIN AND RIVERS. u experiments of Humphrey and Abbot, is 7,471,411,200 cubic feet, a mass sufficient to cover an area of one square mile, 268 feet deep. 1 This spread over the whole basin would cover it 4-5^5- of a foot. Therefore, this river removes from its basin a thickness of one foot in 4,640 years. The cause of the great difference in favor of the Ganges is, that this river is situated in a country subject to very great annual fall of water, the whole of which falls during a rainy season of six months. The rains are therefore very heavy, and the floods and consequent erosion pro- portionately great. The erosive power of this river is still further in- creased by the great slope of the basin, as it takes its rise in the Him- alaya, the highest mountains in the world. Now, since continents may be regarded as made up of hydrographi- cal basins, the average rate of their erosion may be determined either by making similar experiments on all the rivers of the world, or, since this is impracticable, by taking some river as an average. We believe the Mississippi is much nearer an average river than the Ganges. It can hardly be less than the average, for a considerable portion of the earth as rainless deserts is not subject to any erosion. It is probable, therefore, that the whole surface of continents is eroded at a rate not exceeding one foot in 4,640 years. For convenience, we will call it one foot in 5,000 years. We will use this estimate when we come to speak of the actual erosion which has occurred in geological times. Law of Variation of Erosive Power. The erosive power of water, or itspoicer of overcoming cohesion, varies as the square of the velocity of the current (p a v 2 ). The velocity depends upon the slope of the bed, the depth of the water, and many other circumstances, so numerous and complicated that it has been found impossible to reduce it to any simple law. The angle of slope, however, is evidently the most im- portant circumstance which controls velocity, and therefore erosive power. In the upper portions of great rivers, like the Mississippi, the erosion is very great ; while in the plains near the mouth there may be no erosion, but, on the contrary, sedimentary deposit. The high lands therefore, especially mountain-chains, are the great theatres of erosion. The general effect of erosion is leveling. If unopposed, the final effect would be to cut down all lands to the level of the sea, at an average rate of about one foot in 5,000 years. But the immediate local effect is to increase the inequalities of land-surface, deepening the fur- rows, gullies, and gorges, and increasing the intervening ridges and peaks. The effect, therefore, is like that of a graver's tool, constantly cutting at every elevation, but making trenches at every stroke. " Thus land-surfaces everywhere, especially in mountain-regions, are cut away by a process of sculpturing, and the debris carried to the low- lands and to the sea. The smaller lines and more delicate touches are 1 Humphrey and Abbot, "Report on Mississippi Riyer," pp. 148-150. 12 AQUEOUS AGENCIES. due to rain, the deeper trenches or heavier chiselings to rivers proper. The effects of the former are more universal and far greater in the aggregate, but the effects of the latter are far more conspicuous. It is only under certain conditions that rain-sculpture becomes conspicuous. These conditions seem to be a bare soil and absence of frost. Beautiful examples are found in the arid regions of Southern Utah. We now proceed to discuss the more conspicuous effects of water concentrated in river-channels. EXAMPLES OF GREAT EROSION NOW GOING ON : WATERFALLS. The erosive power of water is most easily studied in ravines, gorges, canons, and especially in great waterfalls. One of the most interesting of these is Niagara. Niagara : General Description. The plateau on which stands Lake Erie (P JV, Fig. 6) is elevated about 300 feet above that of L.ERIE. FIG- 6. Ideal Longitudinal Section through Niagara Elver from Lake Erie to Lake Ontario. Lake Ontario, and is terminated abruptly by an escarpment about 300 feet high (P)- From this point a narrow gorge with nearly perpendicular sides, and 200 to 300 feet deep, runs backward through the higher or Erie plateau as far as the falls (^V). The Niagara River runs out of Lake Erie and upon the Erie plateau as far as the falls, then pitches 167 feet perpendicularly, and then runs in the gorge for seven miles to Queenstown ( Q), where it emerges on the On- tario plateau. Long observation has proved that the position of the fall is not stationary, but slowly recedes at a rate which has been vari- ously estimated from one to three feet per annum. The process of re- cession has been carefully observed, and the reason why it maintains its perpendicularity is very elear. The surface-rock of Erie plateau is a ( firm limestone (). Beneath this is a softer shale (5). This softer rock is rapidly eroded by the force of the falling water, and leaves the harder limestone projecting as table-rocks. From time to time these project- ing tables are loosened and fall into the chasm below. This process is facilitated by the joint structure spoken of on page 5. Recession of the Falls. Now, there is every reason to believe that the fall was originally situated at Queenstown, the river falling over the escarpment at that place, and that it has worked its way backward seven miles to its present position by the process we have just described. These reasons are as follows: 1. The general configuration of the country EROSION OF RAIN AND RIVERS. 13 as already described forcibly suggests such an explanation to the most casual observer. 2. A closer examination confirms it by showing that the gorge is truly a valley of erosion, since the strata on the two sides cor- respond accurately (see Fig. 7). 3. As already seen, the falls have receded in historic times at a rate, according to Mr. Lyell, of about one foot a year. The portion of the gorge thus formed under our eyes does not differ in any essential respect from other portions farther down the stream. The evidence thus far is not perfectly conclusive that the gorge was formed by the present river during the present geologic epoch, since the gorge may have been eroded during a previous epoch, and the pres- ent river found it, appropriated it as its channel, and continued to ex- tend it. But (4.) certain stratified deposits have been found by Mr. Lyell and others on the upper margin of the ravine, containing shells, all of which are identical with the shells now living in Niagara River. On the margins of all rivers we find stratified deposits of mud and sand containing dead shell. The stratified deposits found by Mr. Lyell were such mud-banks of the Niagara River before the falls had receded so far, and therefore when the river still ran on the Erie plateau at this point. This is well seen in the subjoined figure, representing an ideal F IG . 7. Ideal Section across Chasm below the Falls. cross-section of the gorge below the falls. The dotted lines represent the former bed and level of the river ; a a represent the banks of strati- fied mud left on the margin of the gorge, as the river eroded its bed down to its present level. Other Falls. The evidence is completed by examination of other great falls. In almost all perpendicular falls we find a similar arrange- ment of strata followed by similar results. The Falls of St. Anthony, in the Mississippi River, are a very beautiful illustration. Here we find a configuration of surface very similar to that in the neighborhood of Niagara. Above the falls the Mississippi River runs on a plateau which terminates abruptly at the mouth of Minnesota River by an escarpment some fifty feet high. From this escarpment, backward through the upper plateau, runs a gorge with perpendicular sides fifty feet high for 14; AQUEOUS AGENCIES. ten miles to the foot of the falls. The river above the falls runs on a hard, silurian limestone rock, only a few feet in thickness. Beneath this is a white sandstone, so soft that it can be easily excavated with the fingers. This sandstone forms the walls of the gorge as far as the escarpment. The recession of the falls by the undermining and falling of the limestone is even more evident than at Niagara. Tributaries running into the Mississippi just below the falls are, of course, precipi- tated over the margin of the gorge. Here, therefore, the same condi- tions are repeated, and hence are formed subordinate gorges, headed by perpendicular falls. Such are the falls and gorge of Little River (Min- nehaha), which runs into the Mississippi about three miles above the mouth of the Minnesota River. Another admirable illustration of the conditions under which per- pendicular falls recede is found in the falls of the numerous tributaries of Columbia River where the great river breaks through the Cascade Range. The Columbia River gorge is 2,500 to 3,000 feet deep. The walls consist of columnar basalt underlaid near the water-level by a softer conglomerate. Every tributary at this point emerges from a deep gorge, headed two or three miles back by a perpendicular wall, over which is precipitated the water of the tributary as a fall 200 to 300 feet high. The falling water erodes the softer conglomerate, undermines the ver- tical-columned basalt, which tumbles into the stream and is carried away; and thus the fall has worked back in each case about two or three miles to its present position. 1 All of this has taken place during the present geological epoch. 3 The wonderful falls of the Yosemite Valley, of which there are six in a radius of five miles, one of them 1,600 feet, three 600 to 700 feet, and two over 400 feet high, seem to be an exception to the law given above. Their perpendicularity seems to be the result of the compara- tive recency of the evacuation of the valley by an ancient glacier, and therefore the shortness of the time during which the rivers have been falling, combined with the hardness of the granite rocks. The Yo- semite gorge was not made by the present rivers. Time necessary to excavate Niagara Gorge. All attempts to esti- mate accurately the time consumed in excavating Niagara gorge must be unreliable, since we do not yet know the circumstances which con- trolled the rate of recession at different stages of its progress. Among these circumstances, the most important are the volume of water, and especially the hardness of the rocks, and the manner in which hard and 1 Gilbert has shown (American Journal, August, 1876) that comparative freedom from detritus is another condition of the formation of perpendicular waterfalls. In mud- dy rivers commencing inequalities are filled up by sediment, and waterfalls cannot be formed. a American Journal of Science and Art, 1874, vol. vii., pp. 167, 259. EROSION OF RAIN AND RIVERS. 15 soft are superposed. The present position of the falls is apparently favorable for rapid recession. Mr. Lyell thinks, from personal observa- tion, that the average rate could not have been more than one foot per annum, and probably much less. At this rate, it would require about 36,000 years. But, whether more or less than this amount, this period must not be confounded with the age of the earth. In this part of geology we are studying causes now in operation. The work of exca- vating the Niagara chasm belongs to the present epoch, and the time is absolutely insignificant in comparison with' the inconceivable ages of which we will speak in the subsequent parts of this work. Ravines, Gorges, Canons. We have already seen (page 11) that ravines, gorges, etc., are everywhere produced in mountain- regions by the regular operation of erosive agents. Nowhere are examples more abundant or more conspicuous than in our own country, and especially in the Western portion. On the Pacific slope, the most remarkable are the gorges of the Fraser and of the Columbia Rivers, fifty miles long and several thousand feet deep ; those of the North and South Forks of the American River, 2,000 to 3,000 feet deep in solid slate ; the canon of the Tuolumne River with its Hetchhetchy Valley ; the still grander canon of the Merced, with its Yosemite Valley, with nearly vertical granite cliffs, 3,000 to nearly 5,000 feet high ; and, deepest of all, the grand canon of King's River, 3,000 to 7,000 feet deep, in hard granite. Some of these great canons have been forming ever since the forma- tion of the Sierra Range i. e., since the Jurassic period. It is possible, also, that in some of them the erosive agents have been assisted by antecedent igneous agencies, producing fissures, which have been en- larged and deepened by water and by ice. But there are some, at least, which may be proved to have been produced wholly by erosion, and that during the present or at least during very recent geological times. We refer especially to those which have been cut through lava-streams. In Middle and Northern California are.Tound lava-streams which have flowed from the crest of the Sierra. By means of the strata on which they lie, these streams are known to have flowed after the end of FIG. 8. Lava-Stream cut through by Rivers : a a, Basalt ; b b. Volcanic Ashes ; c c, Tertiary ; d d, Cretaceous Kocks. (From Whitney.) the Tertiary period. Yet the present rivers have since that time cut great canons through the lava and into the underlying rock, in some cases at least 2,000 feet deep. Such facts impress us with the immen- 16 AQUEOUS AGENCIES. Sity of geological times. This important point is discussed more fully in a subsequent part of this work. FIG. 9. Bnttes of the Cross (Powell). But nowhere in this country, or in the world, are the phenomena of canons exhibited on so grand a scale, and nowhere are they so obviously FIG. 10. Canon of the Colorado and its Tributaries (from a Drawing by Newberry). EROSION OF RAIN AND RIVERS. 17 the result of pure erosion, as in the region of the Grand Plateau of Utah, Arizona, New Mexico, and Colorado. This plateau is elevated 7,000 to 8,000 feet above the sea, and composed entirely of nearly horizontal strata, comprising nearly the whole geological series from the Tertiary downward. Through this series all the streams have cut their way downward, forming narrow canons with almost perpendicular walls sev- eral thousand feet deep, so that in many parts we have the singular phenomenon of a whole river-system running almost hidden far below the surface of the country, and rendering the country entirely impass- able in certain directions (see Frontispiece). Nor is the erosion confined to canons ; for the rain-erosion has been so thorough and general that much of the upper portion of the plateau has been wholly carried away, leaving only isolated turrets (buttes) or isolated level tables with cliff- like walls (mesas) to indicate their original height. All these facts are well shown in Fig. 10. The explanation of these deep and narrow canons is probably to be found in the predominance of stream-erosion over general disintegration and rain-erosion, which is characteristic of an arid climate (Gilbert). Chief among these canons is the Grand Canon of the Colorado, 300 miles long and 3,000 to 6,200 feet deep, forming the grandest natural geological section known. Into this the tributaries enter by side-ca- nons of nearly equal depth, and often of extreme narrowness. Fig. 11 represents the natural proportions of such a canon. Time. These remarkable canons have evidently been cut wholly by the streams which now occupy them, and which are still continuing the work. The work, prob- ably commenced in the early Tertiary with the emergence of this portion of the con- tinent, became more rapid in the latter portion of the Tertiary with the great ele- vation of the plateau, and has continued to the present time. Thus, causes now in operation are identified with geological agencies. In the Appalachian chain gorges and valleys of erosion are abundant, but the evidences of present action are less ob- vious, and therefore we defer their treat- ment to Part II., for we are now discuss- ing agencies still in operation. Among 2 FIG. 11 Section of the Virgen River (after Gilbert). 18 AQUEOUS AGENCIES. the more remarkable narrow gorges in this region, we may mention, in passing, the Tallulah River gorge, several miles long and nearly 1,000 feet deep, in Rabun Count}-, Georgia, and the gorge of the French Broad in North Carolina. The general effects of erosion will be more fully treated under " Mountain Sculpture." Transportation and Distribution of Sediments. The specific gravity of most rocks is about 2.5. Immersed in water, they, therefore, lose nearly half their weight. This fact greatly in- creases the transporting power of water. The actual transporting power of water is determined partly by experiment and partly by reason- ing on the general laws of force. By experiment we determine the transporting power under a given set of circumstances : by general rea- soning we determine its law of variation, and apply the data given by experiment to every possible case. Experiments. It has been found by experiment that a current, moving at the rate of three inches per second, will take up and carry along fine clay ; moving six inches per second, will carry fine sand ; eight inches per second, coarse sand, the siza of linseed ; twelve inches, gravel ; twenty-four inches, pebbles ; three feet, angular stones of the size of a hen's-egg. 1 It will be readily seen from the above that the carrying-power increases much more rapidly than the velocity. For instance, a current of twelve inches per second carries gravel, while a current of three feet per second, only three times greater velocity, carries stones many hundred times as large as grains of gravel. Let us investigate the law. Law of Variation. If the surface of the obstacle is constant, the force of running water varies as the velocity squared: f oc if (1). This may be easily proved. Suppose we have an obstacle like the pier of a bridge, standing in water running with any given velocity. Now, if from any cause the velocity of the current be doubled, since mo- .mentum or force is equal to quantity of matter multiplied by velocity (M ' = Q X V), the force of the current will be quadrupled, for there will be double the quantity of water strike the pier in a given time with double the velocity. If the velocity of the current be trebled, there will be three times the quantity of matter striking with three times the velocity, and the force will be increased nine times. If the velocity be quadrupled, the force is increased sixteen times, and so on. Next, if the velocity of the current remains constant, while the size of the opposing obstacle varies, then evidently the force of the current will vary as the opposing surface : if the opposing surface is doubled, the force is doubled ; if trebled, the force is trebled, etc. But in similar figures, surfaces vary as the square of the diameter. Therefore, in this case, force varies as diameter squared: f l T 1 FIG. 24.-Section of an Exposed Cliff. shown in the section (Fig. 26), in which s is the present shore-line, I the water-level, a b the platform, s' the original shore-line, and s' b c the original slope of bottom. The recession of the shore-line and the formation of the shore platform have been accurately observed in Lake Michigan (Andrews). Level platforms termi- nated by cliffs, there- fore, when found in- | land, sometimes indi- FlG 26 - cate the position of old shore-lines. Tides. The tide is a wave of immense base, and three or four feet in height in the open ocean, produced by the attractive force of the moon WAVES AND TIDES. 33 and sun on the waters of the ocean. The velocity of this wave is very great, since it travels around the earth in twenty-four hours. In the open ocean it produces very little current, only a slow transfer of the water back and forth, too slow to produce any geological effect ; * but in shallow water, where the progress of the wave is impeded, it piles up in some cases forty to fifty feet in height, and gives rise to currents of great velocity and immense erosive power. By this means bays and harbors are formed, and straits and channels are scoured out and deepened. Tides also act an important part in assisting the action of waves upon the whole coast-line. The action of waves on exposed cliffs quickly forms accumulations of d&bris at their base, composed of sand, mud, shingle, or rocky fragments (Fig 24), which receive first and greatly diminish the shock of the waves upon the cliff. The inces- sant beating of the waves upon this debris reduces it to a finer and finer condition, and the retreating waves bear much of it seaward; so that, even without the assistance of any other agent, the protection is incomplete, and the erosion therefore progresses. But if strong tidal currents run along the coast, these effectually remove such debris and leave the cliff exposed to the direct action of the waves. Examples of the Action of Waves and Tides. The coasts of the United States show many examples of the erosive action of waves and tides. The form of the whole New England coast is largely determined by this cause. The softer parts are worn away into harbors by the waves and scoured out by the tides, while the harder parts reach out like rocky arms far into the sea. Sometimes only small rocky islands, stripped of every vestige of earth, mark the position of the former coast- line. Boston Harbor and the rocky points and islands in its vicinity are good examples. The process is still going on, and its progress may be marked from year to year. On the Southern coast examples of a similar process are not want- ing. At Cape May, for instance, the coast is wearing away at a rate of about nine feet per annum. The more exposed portions about Charleston Harbor, such as Sullivan's Island, are said to be wearing away even more rapidly. As a general fact, however, the low, sandy or muddy shores of the Southern coasts are receiving accessions more rapidly than they are wearing ; while, on the contrary, the New Eng- land coast, as proved by its rocky character, is losing much more than it gains. The shores of Lake Superior (Fig. 27) furnish many beauti- ful examples of the action of waves, in this case, of course, unassisted by tides. The general form of the lake along its south shore is deter- mined by the vary ing hard ness of the rock ; the two projecting promon- tories La Pointe (a) and Keweenaw Point (c) being composed of hard, igneous rocks, while the intervening bays b and d are softer sandstone. 1 Herschel's " Physical Geography," p. 64. 3 34. AQUEOUS AGENCIES. On the south shore, about e t between La Pointe and Fond du Lac (/), the conditions of rapid erosion are beautifully seen. The shores are sand- stone cliffs, with nearly horizontal strata. These have been eroded beneath by the waves, in some places for hundreds of feet, forming FIG. 27. Lake Superior. immense overhanging table-rocks, supported by huge sandstone pillars of every conceivable shape. Among these huge pillars, and along these low arches and gloomy corridors, the waves dash with a sound like thun- der. From time to time these overhanging table-rocks, with their load of earth and primeval forests, fall into the lake. The coasts of Europe furnish examples en a more magnificent scale, and have been more carefully studied. The cliffs of Norfolk are carried away at a rate of three feet, and those of Yorkshire six feet, annually. The church of Reculver, on the coast of Kent, near the mouth of the Thames, stood, in the time of Henry VIII., one mile inland. Since that time the sea has steadily advanced until, in 1804, a portion of the church- yard fell in, and the church was abandoned as a place of worship. The church itself, ere this, would have been undermined and fallen in, had it not been protected by artificial means. There are many instances in the German Ocean of islands which have been entirely washed away during the historic period. The tidal currents through the British and Irish Channels, along the western coasts of Ireland and Scotland, among the Orkneys and Heb- rides, and especially along the coast of Norway, are very powerful. Along this latter coast it forms the celebrated Maelstrom. The erosive effects of the sea are, therefore, very conspicuous. On the south and east coasts of England the erosion is now progressing rapidly. On the west coasts of Ireland and Scotland the waste is not now so great, be- cause the softer material is all removed, but the configuration of the coast shows the waste which it has suffered. A glance at a good map WAVES AND TIDES. 35 of Ireland shows a deeply-indented western coast, composed entirely of alternating rocky promontories and deep bays. On the western coast of Scotland, and especially on the Orkney, Shetland, and Hebrides Isl- ands, the wasting effect of the sea has been still greater. Not only have we here the same character of coast as already described (as seen in the friths of Scotland), but many small islands have been eroded, until only a nucleus of the hardest rock is left ; and even these have been worn until they seem but the ghastly skeletons of once-fertile isl- ands. Figs. 28 and 29 will give some idea of the appearance of these spectral islands. The coast of Norway consists entirely of deep fiords alternating with jutting headlands of hardest rock several thousand feet high. Along this intricately -dissected coast there runs a chain of high, rocky islands, which in an accurate map is scarcely distinguishable from the coast itself, being separated only by narrow, deep fiords. Toward the 36 AQUEOUS AGENCIES. northern part of the coast the crest of the Scandinavian chain seems to run directly along the jutting promontories of the coast-line, for these headlands are the most elevated part of the country ; in fact, in some parts, it would seem that the original crest was at one time still farther west, along the line of coast-islands. If so, then the sea has not only carried away the whole western slope, but has broken through the main axis, leaving only these isolated rocky islands as monuments of its former position, and is even now carrying its ravages far inland on the eastern slope. In the case of Norway, however, and probably in case of nearly all bold, rocky coasts, the intricacy of the coast-line is not due wholly or even principally to the action of waves and tides, but also to other causes to which we shall refer hereafter. Transporting Power. The transporting power of waves is immense- ly great, often taking up and hurling on shore masses of rock hundreds of tons in weight ; but, being entirely confined to the coast-line, the dis- tance to which they carry is necessarily very limited. There are some instances, however, of materials carried to great distances by the inces- sant action of waves. Thus, according to Prof. Bache, coast-sand is carried slowly farther and farther south by the action of waves, and siliceous sand is found at Cape Sable on the extreme southern point of Florida, although the whole Florida coast as far as St. Augustine is composed of coral limestone alone. He accounts for this by supposing that the trend of the United States coast is such that waves coming from the east strike the coast obliquely and fall off toward the south, carrying each time a little sand with them. A similar phenomenon has been observed on Lake Michigan : the sands are carried steadily toward the south end, where they accumulate. Deposits. The invariable effect of waves, chafing back and forth upon coast debris, is to wear off their angles and thus to form rounded fragments and granules. Thus pebbles, shingle, and round-grained sand, though produced by all currents, are especially characteristic of wave-action. Hippie-marks are also characteristic of current-action in shallow water. They are, therefore, always formed on shore by the action of waves and tides. By means of these characteristics of shore deposit, many coast-lines of previous geological epochs have been deter- mined. We have seen that waves usually destroy land. In many cases, however, they also make land. This is the case whenever other agen- cies, such as river or tidal currents, drop sediment in shallow water, and therefore within reach of wave-action. We shall again speak of these under the head of Land formed by the Ocean Agencies. OCEANIC CURRENTS. 3f Oceanic Currents. The ocean, like the atmosphere, is in constant motion, not only on its surface, but throughout its whole mass. The general direction of the currents in the two cases is also similar, but there are disturbing and complicating causes peculiar to each, which interfere with the regularity and simplicity of the phenomena. If the currents of the atmosphere are more variable on account of the greater levity of the fluid, oceanic cur- rents have also their peculiar disturbing causes in the existence of im- passable barriers in the form of continents. In both atmosphere and sea, currents may also be deflected by submarine banks, for mountain- chains are the banks of the aerial ocean. Theory of Oceanic Currents. By some distinguished physicists, oceanic currents have been attributed entirely to the action of the trade- winds. 1 There can be no doubt that this is a real cause; yet it seems probable, nay, almost certain, that the great and controlling cause of currents of the ocean, as of the air, is difference of temperature between the equatorial and polar regions. 8 "We will, therefore, discuss the sub- ject from this point of view, although the effect would be much the same, whatever be our view of the theory. For the sake of clearness, we will take first the simplest case, and then introduce disturbing influences and show their effects. Suppose, first, the earth covered with a universal ocean, continually heated at the equator, and cooling at the poles : the difference of den- sity of the equatorial and polar seas would cause exchange or circula- tion between these regions by means of north and south currents in all longitudes, the equatorial currents being superficial because warm, and the polar currents deep-seated because cold. It is obviously impossible, however, that the principal exchange should be with the pole itself, since this is but a point, but with the northern regions. Observation shows that it is between the equator and the polar circle. In the case we are now considering, the exchange, being in all longitudes, would be scarcely, if at all, perceptible. Suppose, second, the earth be set a rotating: then the currents pass- ing from either polar to the equatorial region would be deflected more and more to the westward until, uniting at the equator, they would there form a directly westward equatorial current running around the earth. This westward-moving water would be constantly turning north- ward and southward in all longitudes as a superficial current, and finally eastward about the polar circle, to join again the deep-seated polar cur- rent going to the equator ; thus forming a series of regular ellipses lying over each other in strata, dipping eastward and outcropping 1 Herschel, " Physical Geography," p. 13 ; and Croll, " Climate and Time." 9 Guyot, " Earth and Man," p. 189. AQUEOUS AGENCIES. westward as represented in Fig. 30. As the north and south currents a a' and b b' would take place in all longitudes, they would be scarcely, if at all, perceptible ; but the east currents d d', and the westward equatorial current c c, where all these unite, would be decided. In the third place, introduce continents passing across the equator from north to south, forming impassable barriers to the east and west currents c c and d d. Then many of the lines of current aaa would be crowded against the western shore of the ocean, and of the lines bbb against the east- ern shore, forming in each case by concentration very decided currents, while in mid-ocean these currents would be still impercepti- ble. Thus the perceptible situated between continents would be repre- FIG. 80. The strong lines aaa show superficial, dotted lines b b b deep-seated currents. currents of an ocean sented by the figure (Fig. 31) taken from Dana. Besides the main currents above mentioned there would be mi- nor exchanges with the pole itself. 1 A portion of the eastward current d and d ' would turn north and south- ward, e e', and circling around would return toward the equator as a deep- seated current under , hugging the shore on account of the westward ten- dency of all currents moving toward the equator. The effect of the trade-winds would be to conspire with the cause already discussed in the formation of the equa- torial current c c', and by the reflection of this from continents, the other cur- 90" 60 30' 60 DO FIG. 81. Ideal Diagram, showing General rents SDoken of Course of Oceanic Currents. Application. We will now apply these principles in the explanation of the currents of the Atlantic Ocean, for these are best known. Currents coming from the north and south on the African coast, and corresponding to b b' in the above diagram, unite to form an equatorial current, c c', which stretches across the Atlantic until, striking (Fig. 32) against the coast of South America, it turns north and south, a a'. The 1 Dana's " Manual," p. 88. OCEANIC CURRENTS. 39 southern branch has not been accurately traced. It probably turns gradually eastward, c?', and forming a grand circle in the southern Atlantic joins again the South African current b'. The northern branch, #, runs along the coast of South America, through the Caribbean Sea and into the Gulf of Mexico, from which emerging it runs with great velocity through the narrow straits of Florida and thence under the name of the Gulf Stream along the coast of North America, turning Strc glot FIG. 32. General Course of Currents of the Atlantic. more and more eastward in obedience to the law already mentioned, until it becomes an eastward current, -'^''/ Forbes, and recently confirmed in the most perfect a/ manner by Tyndall. A line of stakes, a b c d efg, placed in a straight row across a glacier, becomes every day more and more curved, as seen in Fig. 46. The exact rate of motion for each stake is easily measured by the theodolite. The rate of the centre is often many times greater than that of the margins. 2. The Velocity of the Surface is greater than that of the Bottom. This law of currents, which is the necessary result of friction on the bed, is more difficult to prove in the case of glaciers, because it is dif- ficult to get a vertical section. The necessary observation was, how- ever, successfully accomplished by Prof. Tyndall in 1857. We have already said (page 51) that glaciers po' conform to large but not to small ; inequalities of their channels : a glacier, therefore, passing by a nar- row side-ravine will expose its whole thickness on the side. Prof. Tyn- dall, having found such a side ex- posure more than 140 feet vertical, placed three pegs in a vertical line, one near the top, one near the mid- dle, and one at the bottom (Fig. 47, a b c). The vertical line became more and more inclined daily. The daily motion at top was six inches, in the middle 4.5 inches, and at the bottom 2.5 inches. Thus, glaciers, like rivers, slide on their beds and banks, producing erosion ; but, also, the several layers, both horizontal and vertical, slide on each other. 3. The Velocity increases with the Slope. Fig. 48 represents the 56 AQUEOUS AGENCIES. surface-slope of the glacier Du Geant, G ; the Mer de Glace, M ; and the glacier De J3ois, B ; and their daily motion. The increase of ve- locity with the slope is evident. 4. The Velocity increases with the Fluidity. The daily motion of glaciers is greater in summer, when the ice is rapidly melting, than in winter ; and in mid-day than at night. 5. The Velocity increases with the Depth. In the Alps, where the thickness is 200 to 300 feet, the mean daily motion is one to three feet ; but in Greenland, where the thickness is 2,000 to 3,000 feet, the daily motion, in spite of the much lower temperature, is in some cases 60 feet. 1 6. Fluid Currents conform to the Irregularities of their Channel Glaciers, like water-currents, conform to the inequalities of the bot- tom and sides of their channels. They have their shallows and their deeps, their narrows and their lakes, their cascades, their rap- ids, and their tranquil por- tions. Fig. 49 shows a glacier running through a narrow gorge into a wide lake of ice, and again through another gorge. There is this difference, however, between a gla- cier and a water-current, viz., that, while the latter conforms to even the minutest and sharpest outlines, the former conforms only to the larger or more gentle. In this, a glacier acts like a stiff, viscous fluid. 7. The Line of Swiftest Motion is more sinuous than the Channel. We have already seen that this is true of rivers (page 21). The line of swiftest current is reflected from side to side, increasing the curves by erosion. The same has been recently proved by Tyndall to be the case with glaciers. Fig. 50 represents a portion of a sinuous glacier, like the Mer de Glace : the dotted line represents the line of swiftest motion. 1 Helland, Journal of Geological Society, vol. xxxiii., p. 142, et scq. FIG. 50. THEORIES OF GLACIER-MOTION. 57 Theories of Glacier- Motion. There are few subjects connected with the physics of the earth which have excited more interest than that of glacier-motion. The subject is one of exceeding beauty, and not without geological im- portance. Passing over several very ingenious theories which have now been abandoned, the first theory which was conceived in the true inductive spirit, and which explains the differential motion, is that of Prof. James Forbes. Viscosity Theory of Forbes. Statement of the Theory. According to Forbes, ice, though ap- parently so hard and solid, is really, to a slight extent, a viscous body. In small masses this property is not noticeable, but in large masses and under long-continued pressure it slowly yields, and will flow like a stiffly viscous fluid. In large masses like a glacier, this steady, powerful press- ure is furnished by the immense weight of the superincumbent ice. Argument. It is evident that this theory completely accounts for all the phenomena of glacier-motion, even in their minutest details. A glacier, beyond all doubt, moves like a viscous body, but it is still a question whether it does so by virtue of a property of viscosity. The proposition that ice is a viscous substance seems at first palpably ab- surd. It is necessary, therefore, to show that this proposition is not so absurd as it seems. The properties of solidity and liquidity, though perfectly distinct and even incompatible in our minds, nevertheless, in Nature, shade into one another in the most imperceptible manner. Malleability, plasticity, and viscosity, are intermediate terms of a connecting series. The idea which underlies all these expressions is that of capacity of motion of the molecules among themselves without rupture : the difference among them being the greater or less resistance to that motion. In the case of malleable bodies, like the metals, great force is required to produce motion ; in plastic bodies, like wax or clay, less force is required ; in viscous bodies, like stiff tar, motion takes place spontaneously but slowly ; while in liquids it takes place freely and with little or no resist- ance. In all of these cases, if the pressure be sufficient, the body will change its form without rupture in other words, will flow. Now, by increasing the mass we may increase the pressure to any extent. Therefore, all malleable, ductile, plastic, or viscous bodies, if in suffi- ciently large masses, will flow like water. Thus, a mass of lead, suffi- ciently thick, would certainly flow under the pressure of its own weight. But solid bodies may be divided into two great classes, viz., bodies which are malleable, plastic, or viscous, and bodies which are brittle y the very idea of brittleness being that of total incapacity of motion 58 AQUEOUS AGENCIES. among the particles without rupture. Now, ice belongs to the class of brittle bodies. Forbes attempts to remove this difficulty by showing that many apparently brittle bodies will also flow under their own weight ; for instance, pitch, so hard and brittle that it flies to pieces under a blow of the hammer, will, if the containing barrel be removed, flow and spread itself in every direction. So, also, molasses-candy, made quite hard and brittle, will still flow by standing. A remarkable pitch-lake, about three miles in circumference, occurs in Trinidad. The pitch is described as in constant, slow-boiling motion, coming up in the centre, flowing over to the circumference, and again sinking down. Yet this pitch, in small masses, would be called solid and brittle. Struck with a hammer, it flies to pieces like glass. In fact, the essen- tial peculiarity of a stiff, viscous body, in which it differs from mal- leable or plastic bodies, is, that it yields only to slowly-applied force. Forbes, therefore, thinks that glacier-ice is an exceedingly stiff, vis- cous substance, which, though apparently brittle in small quantities and to sudden force, yet, under the slow-acting but powerful pressure of its own weight, flows down the slope of its bed, squeezing through narrows and spreading out into lakes, conforming to all the larger and gentler inequalities of bed and banks, but not to the sharper ones. The velocity of motion is small in the same proportion as the viscous mass is stiff. The descent of the Mer de Glace from the cascade of the Glacier du Geant to the point of Glacier de Bois, a distance of ten miles, is 4,000 feet. Water, under these circumstances, would rush with fear- ful velocity. The glacier moves but two feet in twenty-four hours. Regelation Theory of Tyndall. If ice be indeed a viscous body, then there seems no reason why it should not yield to pressure even in small masses, if the pressure be sufficiently slowly graduated. In the hands of a skillful experiment- alist it ought to exhibit this property. Prof. Tyndall tried the ex- periment. Masses of ice of various forms were subjected to slowly- graduated, hydrostatic pressure. In every case, however slowly grad- uated the pressure, the ice broke ; but if the broken fragments were pressed together, they reunited into new forms. In this manner, ice in the hands of Prof. Tyndall proved as plastic as clay : spheres of ice (a, Fig. 51) were flattened into lenses (>), hemispheres (c) were changed into bowls (f), and bars (e) into semi-rings (./") He even asserts that ice may be moulded into any desirable form ; e. g., into vases, statuettes, rings, coils, knots, etc. Here, then, we have a power of being moulded such as was not dreamed of before ; but this power was not depend- ent on a property of viscosity, but upon another property long known, but only recently investigated by Faraday, viz., the property of rege- lation. THEORIES OF GLACIER-MOTION. 59 Regelation. If two slabs of ice be laid one atop of the other, they soon freeze into a solid mass. This will take place not only in cold weather, but in midsummer, or even if boiling 1 water be thrown over the slabs. If a mass of ice be broken to pieces, and the fragments be pressed, or even brought in contact with one another, they will quickly unite into a solid mass. Snow pressed in the warm hand, though con- FIG. 51. A B C, moulds ; a c e, original forms of the ice ; 6 df, the forms into which they were moulded. stantly melting, gradually becomes compacted into solid ice. This very remarkable but imperfectly understood property of ice completely ex- plains the phenomena of moulding ice by experiment. By this property the broken fragments reunite in a new form as solid as before. We may possibly call this property of moulding under pressure plasticity (although it is not true plasticity, since it does not mould without rupt- ure, but by rupture and regelation} ; but it cannot in any sense be called viscosity, for the true definition of viscosity is the property of yielding under tension the property of stretching like molasses-candy, or melted glass ; but ice in the experiments, according to Tyndall, did not yield in the slightest degree to tension. In the experiment, if, instead of placing the straight bar at once into the curved mould, it had been placed successively in a thousand moulds, with gradually-increased curvature, or, still better, if placed in a straight mould, and this mould, while under pressure, curved slowly, then there would have been no sudden visible ruptures, but an infinite number of small ruptures and regelations going on all the time. The ice would have behaved pre- cisely like a viscous body. Now, this is precisely what takes place in a glacier. Application to Glaciers. A glacier, on account of its immense mass, is, in its lower parts, under the immense pressure of its own weight tending to mould it to the inequalities of its own bed, and in every part under a still more powerful pressure a pressure proportioned to the height of the head of the glacier urging it down the slope of its bed. Under the influence of this pressure the mass is continually yielding by 60 AQUEOUS AGENCIES. fracture, but again uniting by regelation. By this constant process of crushing, change of form, and reunion, the glacier behaves like a plastic or viscous body; though of true plasticity or true viscosity there is, according to Tyndall, none. In fact, we have in the phenomena of glaciers the most delicate test of viscosity conceivable ; but we find the glaciers will not stand the test. For instance, the slope of the Mer de Glace at one point changes from 4 to 9 25' ' (Fig. 52), and yet the glacier, although moving but two feet a day, cannot make this slight bend without rupture ; for at this point there are always large trans- verse fissures which heal up below by pressure and regelation. In an- other place the glacier is similarly broken by passing an angle produced by a change of slope of only 2. It seems almost impossible that a body having the slightest viscositv should be fractured under these cir- cumstances. Tyndall concludes, therefore, that the motion of glaciers is viscoid, but the body is not viscous the viscoid motion being the result, not of the property of viscosity, but of fracture and regelation. Comparison Of the Two Theories. Forbes's theory supposes motion among the ultimate particles without rupture. Tyndall's supposes motion among discrete particles by rupture, change of position, and regelation. The undoubted viscoid motion is equally explained by both : by the one, by a property of viscosity ; by the other, by a prop- erty of regelation. Conclusion. It seems almost certain that both views are true, and that both properties are concerned in glacial motion. Eecent observa- tions and experiments have shown an undoubted viscosity in ice es- pecially in ice containing much inclosed and dissolved air, as is the case with glacier-ice. Ice boards supported at the two ends gradually bend into an arc under their own weight. Cylinders of snow compacted into 'ice may be bent in the hand to a semicircle without rupture. 8 Structure of Glaciers. There are two points connected with the structure of glaciers which require notice, viz., the veined structure and the fissures. Veined Structure. The ice of glaciers is not homogeneous, but con- sists of white vesicular ice (white because vesicular), banded, often very beautifully, with solid transparent blue ice (transparent blue because solid), the banding sometimes so delicate that a hand-specimen looks 1 Tyndall, " Glaciers of the Alps." 8 Altkin, American Journal of Science, vol. v., p. 305, third series. THEORIES OF GLACIER-MOTION. 61 like striped agate. These blue veins are not continuous planes, but apparently very flat lenticular in shape, varying in thickness from a line to several inches, and in length from a few inches to several feet. Their direction being parallel to one another, they give a stratified or cleavage structure to the glacier, and, in melting, the glacier often splits or cleaves along these planes. According to Prof. Forbes, look- ing upon the glacier as a whole, we may regard the strata as taking the form represented by the subjoined figures. In a section parallel to FIG. 53. Sections of a Gkcier. FIG. 5t. Ideal Diagram, showing Struct- ure of Glaciers (after Forbes). the surface (Fig. 53, a), the strata outcrop in the form of loops. A cross-section (Fig. 53, b) shows them lying in troughs, and a longi- tudinal vertical section (Fig. 53, c) shows the manner in which they dip. Fig. 54 is an ideal glacier cut in several directions, and combining in one view the three sections given above. It is generally impossible to trace the veins around from side to side. Sometimes they are most distinct on the margins, and then are called marginal veins ; some- times at the point of the loop transverse veins / sometimes tributaries running together, as in the figure (Fig. 54) the interior branches of the two loops coalesce, and are flattened against one another, and form longitudinal veins. Fissures. These are also marginal, transverse, and longitudinal. The marginal fissures are shown in Fig. 40 ; they are always at right angles to the marginal veins. 62 AQUEOUS AGENCIES. Theories of Structure. Fissures. There can be no doubt that the great fissures or crevasses are produced by tension or stretching, and that their direction is always at right angles to the line of greatest tension. Thus the transverse fissures are produced by the stretching of the glacier in passing over a salient angle. The marginal fissures are produced by the dragging or pulling of the swifter central portions upon the slower marginal por- tions. It has been proved by Hopkins, the English physicist and geolo- gist, that the line of greatest tension from this cause would be inclined 45, with the course of the glacier as shown by the arrows (Fig. 55). The fissures should be at right angles to these lines, and, therefore, also inclined 45 with the margin, and running upward and inward. The longitudinal fissures are best seen where a glacier runs through a nar- row gorge out on an open plain. The lateral spreading of the glacier causes it to crack longitudinally (Fig. 56). Fig. 57 is a longitudinal vertical section of the same. Veined Structure. Tyndall has shown conclusively that veins are always at right angles to the line of greatest pressure, and that, there- fore, they are produced by pressure. Thus fissures and veins, being produced by opposite causes one by tension and the other by pressure are found under opposite con- ditions. Thus as transverse fis- sures are produced by the longi- tudinal stretching of a glacier passing over a salient angle, so transverse veins are formed by the longitudinal compression of a glacier passing over a re'enter- ing angle. Fig. 57 is a section of the Rh6ne glacier (Fig. 56), showing the crevasses (c c c) THEORIES OF STRUCTURE. 63 produced by the steep declivity, and the veined structure (s s s) pro- duced by the compression consequent upon the change of angle on coming out on the plain. The relation of crevasses and vein-structure is still better shown in the ideal section (Fig. 58). Again, as marginal fissures are produced by the pulling of the cen- tral portions upon the lag- ging margins behind, so /n c the marginal veins are pro- duced by the crowding or pushing of the swifter cen- tral parts upon the mar- ginal parts in front (Fig. 59). The marginal veins are. therefore, inclined to the margin about 45, but pointing inward and downward, and, there- fore, at right angles to the crevasses. The relation of these to one another is shown in Fig. 60. Finally, as longitudinal fissures are produced by lateral spreading (Fig. 56), so longitudinal veins are produced by lateral compression. This is best seen where two tributaries meet at high angle (Fig. 61) for instance, where the Glacier du G6ant and the Glacier de Lechaud form the Mer de Glace (Fig. 41). All these facts have been experi- mentally illustrated by Tyndall. Physical Theory of Veins. There is little doubt that veins are formed by pressure at right angles to the direction of the veins ; but how pressure produces this structure is very imperfectly understood. Probably at least a partial explanation is contained in the following proposition : 1. White vesicular ice by powerful pressure is crushed, the air escapes, and the ice is refrozen into solid blue transparent ice. 2. Ice being a substance which expands in freezing, and, therefore, con- tracts in melting, its freezing and melting point is lowered by pressure. Therefore, ice at or near 32 Fahr. is melted by pressure. Now, the glacier is under powerful pressure of its own weight, and the stress of 64 AQUEOUS AGENCIES. this pressure is ever changing from point to point by the changing position of the particles produced by the motion. Thus the glacier in' places is ever melting under pressure, and again refreezing by relief of pressure. The melting discharges the air-bubbles, and, in refreezing, the ice is blue. 3. No substance is perfectly homogeneous, and of equal strength in all parts ; therefore, this crushing and melting, and consequent conversion of white into blue ice, take place irregularly in spots. 4. As ice of a glacier acts like a viscous substance, the final effect of pressure would be to flatten these spots, both white and blue, in the direction of greatest pressure, and extend them in a direction at right angles to the pressure, and thus create bands in this direction. 5. Differential motion would also tend to bring the veins into the direction indicated by Forbes. Floating Ice Icebergs. We have already seen (page 47) that at a certain latitude, varying from 46 in South America to about 65 in Norway, glaciers touch the surface of the ocean. Beyond this latitude, they run out to sea often to great distances. By the buoyant power of water, assisted by tides and waves, these projecting floating masses are broken off, and accumulate as immense ice-barriers in polar seas, or are drifted away by currents toward the equator. Such floating fragments of glaciers are called ice- FIG. 62. Formation of Icebergs. bergs. Fig. 62 is an ideal section, through a glacial valley, in which a g is the glacier, b the cliffs beyond, Ij the sea-level, and * an iceberg. The principal source of the icelfergs of the north Atlantic is the coast of Greenland. This country is an elevated table-land, sloping in every direction to a coast deeply indented like Norway, with alter- nate deep fiords and jutting headlands. The whole table-land is com- pletely covered with an ice-sheet, probably several thousand feet thick, moving slowly seaward, and discharging through the fiords as immense FLOATING ICE ICEBERGS. 65 glaciers, 1 which, as already explained, form icebergs. In this remarkable country no water falls from the atmosphere except in the form of snow, and all the rivers are glaciers. The geological effects of such a moving ice-sheet may be easily imagined. The whole surface of the country rock must be polished and scored, the general direction of the striae being parallel over large areas. The antarctic continent is probably similarly, and even more thick- ly, ice-sheeted, for the humid atmosphere of that region is very favorable to the accumulation of snow and ice. Captain Wilkes found an impen- etrable ice-barrier, in many places 150 to 200 .feet high, for 1,200 miles along that coast. From this ice-barrier, icebergs separate and are drifted toward the equator. The formation of icebergs in polar regions, and their drifting into warmer latitudes, to be melted there, is evidently a necessary conse- quence of the great law of circulation, for otherwise ice would accumu- late without limit in these regions. General Description. The number of icebergs accumulated about polar coasts is almost inconceivable. Scoresby counted 500 at one view. Kane counted 280 of the first magnitude at one view. They are often 200 and sometimes even 300 feet high, and the mass above water 66,000,000 cubic yards (Dr. Rink). As the specific gravity of ice is 0.918, if these were solid ice, there would be but one-twelfth above water; but as glacier-ice is somewhat vesicular, there is about one-seventh above water. The thickness of some of these icebergs must therefore be 2,000 to 3,000 feet, and their volume near 500,000,000 cubic yards, which is about equivalent to a mass one mile square, and 500 feet thick. Under the influence of the melting power of the sun un- 1 Dr. Rink, "Archives des Sciences," vol. xxvii., p. 155. 5 66 AQUEOUS AGENCIES. equally affecting different parts, they assume various and often strange forms. The accompanying figure (Fig. 63) gives the usual appearance in the northern Atlantic. Those separated from the antarctic barrier present, before they have been much acted upon by the sun, a much more regularly prismatic appearance. Fig. 64 gives the appearance of one of these prismatic blocks or tables, 180 feet high, seen by Sir James Ross in the antarctic seas. Icebergs as a Geological Agent Erosion. The polishing and scor- ing effects of the ice-sheets and of their discharging glaciers must, of course, extend over the sea-bottoms about polar coasts as far as the glaciers touch bottom, which, considering their immense thickness, must be for considerable distance (Fig. 62, j to g). This, however, is glacier agency rather than iceberg agency. On being separated they float away, and are carried by currents with their immense loads of earth and bowlders, amounting often to 100,000 tons or more, as far as 40 or even 36 latitude, where, being gradually melted, they drop their burden. If the water be not sufficiently deep, they ground, and being swayed by waves and tides they chafe and score the bottom in a somewhat irregular manner ; or, packing together in large fields, and urged onward by powerful currents, they may possibly score the bot- tom over considerable areas somewhat in the manner of glaciers. A large iceberg will ground in water 2,000 and 2,500 feet deep ; they have been found by James Ross actually aground in 1,560 feet of water off Victoria Land. A true glaciated surface, however, cannot be pro- duced by icebergs. Deposits. The bottom of the sea about polar shores is found deep- ly covered with materials brought down by glaciers and dropped by icebergs (Fig. 52). Again, similar materials are carried by icebergs as far as these are drifted by currents, and spread on the bottom of the sea everywhere in the course of these currents. Where stranded ice- bergs accumulate, as on the banks of Newfoundland, large quantities of such materials are deposited. These banks are in fact supposed to MECHANICAL AGENCIES OF WATER. 6f have been formed in this way. Such deposits have not been sufficient- ly examined ; they are probably somewhat similar to those of glaciers, exhibiting, however, some signs of the sorting power of water. Bal- anced stones or bowlders in insecure positions can hardly be left by icebergs. Shore-Ice. In cold climates the freezing of the surface of the water forms sheets of ice many inches or even feet thick, and of great extent, about the shores of rivers, bays, and seas. They often inclose stones and bowlders of considerable size, and when loosened in spring from the shore they bear these away, and again drop them at considerable distances from their parent rock. Also such sheets packed together in large masses, and driven ashore by river and tidal currents, and chafed back and forth by waves, produce effects on the shore-rocks somewhat similar to the scoring, polishing, and even the roches moutonnees of glacier-action. These effects are well seen on the shores of the St. Lawrence River and Gulf. The importance of the study of ice-agencies will be seen when we come to explain the phenomena of the Drift or Glacial period, a remark- able period in the history of the earth. Comparison of the Different Forms of the Mechanical Agencies of Water. Rivers and glaciers are constantly cutting down all lands, bearing away the materials thus gathered, and depositing them on the sea- margins. Acting alone, therefore, their effect must be to diminish the height and to extend the limits of the land. Ocean agencies, on the other hand, by tides and currents bear away to the open sea the mate- rials brought down from the land, and thus tend to prevent marginal accumulations ; and by waves and tides constantly eat away the coast- line, and thus strive to extend the domain of the sea. Thus, while river and ocean agencies are in conflict with one another at the coast-line, the one striving to extend the limits of the land, and the other of the sea, yet they cooperate with each other in destroying the inequalities of the earth's surface, and are therefore called leveling agencies. More- over, it is evident that the erosion of the land and the filling up of the seas are correlative, and one is an exact measure of the other. Now, we have seen (page 11) that the probable rate at which all continents are being cut down by rivers is about one foot in 4,500 to 5,000 years. But since the ocean is about three times the extent of the land, this spread evenly over the bottom of the sea would make a stratum about four inches thick. Therefore, we conclude that, neglecting the destruc- tive effects of waves and tides on the coast-line, which, according to 68 AQUEOUS AGENCIES. Phillips, 1 are small in amount compared with general erosion of the land-surface, we may say that stratified deposits are now forming, or the ocean-bed filling up, at the rate of about four inches in 5,000 years. SECTION 4. CHEMICAL AGENCIES OF WATER. Subterranean Waters, Springs, etc. As we have already seen (page 9), of the rain which falls on any hy- drographical basin, a part runs from the surface, producing universal ero- sion. A second part sinks into the earth, and, after a longer or shorter subterranean course, comes up as springs, and unites with the surface- water to form rivers ; while a third portion never comes up at all, but continues by subterranean passages to the sea. This last portion is removed from observation, and our knowledge concerning it is very limited. But there are numerous facts which lead to the conviction that it is often very considerable in amount. In many portions of the sea near shore, springs, and even large rivers, of fresh water, are known to well up. Thus, in the Mediterranean Sea, " a body of fresh water fifty feet in diameter rises with such force as to cause a visible con- vexity of the sea-surface." * Similar phenomena have been observed in many other places in the same sea, and also in the Gulf of Mexico near the coast of Florida, among the West India isles, and near the Sandwich Islands. Besides the last mentioned, there is still another portion of subterranean water existing permanently in every part of the earth far beneath the sea-level, filling fissures and saturating sedi- ments to great depths, and only brought to the surface by volcanic forces. This, in contradistinction from the constantly-circulating me- teoric water, may be called volcanic water. Springs. The appearance of subterranean waters upon the surface constitutes springs. They occur in two principal positions, viz. : 1. FIG. 65. Hill-side Spring. Upon hill-sides, just where porous, water-bearing strata such as sand outcrop, underlaid by impervious strata like clay ; 2. On fissures, pene- trating the country rock to great depth. Most of the small springs occurring everywhere belong to the first class. The figure (Fig. 65) represents a section of a hill composed mostly 1 Phillips, "Life on the Earth," p. 131. * Herschel's "Physical Geography." SUBTERRANEAN WATERS, SPRINGS, ETC. 69 of porous strata, #, but underlaid by impervious clay stratum, c. Wa- ter falling upon the surface sinks through b until it comes in contact with c, and then by hydrostatic pressure moves laterally until it emerges at a. Sometimes this is a geological agent of considerable importance, modifying even the forms of mountains, and producing land-slips, etc. Thus the Lookout and Raccoon Mountains, in Tennessee, are table- mountains of nearly horizontal strata, separated by erosion-valleys. These mountains are all of them capped by a sandstone stratum about 100 feet thick, underlaid by shale. The water which falls upon the mountain emerges in numerous springs all around where the sandstone cap comes in contact with the underlying shale. The sandstone is gradu- ally undermined, and falls from time to time, and thus the cliff remains always perpendicular (Fig. 66). Large springs generally is- sue from fissures. Water pass- ing along the porous stratum #, perhaps from great distance, and prevented from rising by the overlying impervious strat- um c, coming in contact with a fissure, immediately rises through it to the surface at a (Fig. 67). Artesian Wells. If subterranean streams have their origin in an ele- vated region, a d, composed of regular strata dipping under a lower flat country, c, then the subter- n. d FIG. 67. Fissure-Spring. Rc ;. Artesian Well. ranean waters passing along any porous stratum, as a (Fig. 68), and confined by two impermeable strata, b and ?, will be under power- ful hydrostatic pressure, and will, therefore, rise to the surface, perhaps with considerable force, if the stream be tapped by bor- ing at c. Borings by which water is obtained in this manner are called artesian wells, from the French province Artois, where they were first successfully attempted. The source of the water may be 100 miles or more distant from the well. Some of these wells are very deep. The Greville artesian in Paris is 2,000 feet deep. At the moment of tapping the stream, a powerful jet was thrown 112 feet high. One in West- phalia, Germany, is 2,385 feet deep; one at St. Louis, 3,843 feet; one at Louisville, Kentucky, 2,852. In parts of Alabama and California, the principal supply of water for agricultural purposes is drawn from these wells. Thus there is on all coasts a constant flowing of water, both super- ficial and subterranean, into the sea. Their relative amount it is impos- 70 AQUEOUS AGENCIES. sible to determine. Much, no doubt, depends upon the configuration of the country and the nature of the strata. The heavy hydrostatic pressure to which subterranean water is subjected, especially in ele- vated countries, brings by far the larger portion of it to the surface in the form of springs. But, in limestone regions (this rock being affected with frequent and large fissures, and open subterranean passages, as will be hereafter explained), large subterranean rivers often exist, and these, even after coming to the surface, are often reengulfed, and finally reach the sea by subterranean passages. The largest springs, there- fore, generally occur in limestone countries. From the Silver Spring, in Florida, issues a stream navigable for small steamers up to the very spring itself. The country for sixty miles around is entirely destitute of superficial streams, the whole drainage of the country being subter- ranean, and coming up in this spring. Chemical Effects of Subterranean Waters. We have already seen (page 6) how atmospheric water disintegrates rocks, dissolving out their soluble parts, and reducing their unsoluble parts to soils. Springs, therefore, always contain these soluble matters. In granite regions they contain potash ; in limestone regions they contain lime, and are called hard ; in other cases they contain salt, and are brackish ; when the saline ingredients are unusual in quantity, or in kind, they are called mineral waters. Limestone Caves. In most rocks, the insoluble part left as soil is far the largest, only a small percentage being dissolved ; but in the case of limestone the whole rock is soluble. Therefore, in limestone regions, percolating waters passing through fissures and between strata dissolve the limestone and hollow out open passages, and form immense caves. Water charged with limestone, dripping from the roofs and falling on the floors of these caves, deposit their limestone by evapora- tion, and form stalactites b b, which, meet- ing each other, form lime- stone pMars, c c. The great Mammoth Cave, in Kentucky ; Wier's Cave, i Q Virginia, and Nico- J ac k Cave, m Tennessee, are familiar examples. FIG. eO.-Limestone Cave. " As mi g ht be expected, subterranean rivers are often found running through these caves. This is the case in the Mam- .moth Cave, and in Nicojack Cave. CHEMICAL DEPOSITS IN SPRINGS. 71 Chemical Deposits in Springs. Deposits of Carbonate Of Lime. We have just seen that ordinary subterranean waters in limestone districts, and, therefore, containing small quantities of carbonate of lime, deposit this substance only very slowly by drying, as stalactites and stalagmites ; but in carbonated springs in limestone districts a very rapid deposit of lime carbonate often occurs. Explanation. In order to understand this, it is necessary to re- member : 1. That lime carbonate is insoluble in pure water, but soluble in water containing carbonic acid ; 2. That the amount of carbonate dissolved is proportionate to the amount of carbonic acid contained ; 3. That the amount of carbonic acid which may be taken in solution by water is proportionate to the pressure. Now, there are two sources of carbonic acid, viz., atmospheric and subterranean. All water contains carbonic acid from the atmosphere, and will, therefore, dissolve limestone, but this deposits only slowly by drying, as already explained. But in many districts, especially in vol- canic districts, there are abundant subterranean sources of carbonic acid. If subterranean waters come in contact with such carbonic acid, being under heavy pressure, they will take up a large quantity of this gas ; and if such water comes to the surface, the pressure being re- moved, the gas will escape in bubbles. This is a carbonated spring. If, further, the subterranean waters, thus highly charged with carbonic acid, come in contact with limestone rocks, or rocks of any kind con- taining lime carbonate, they will dissolve a proportionably large amount of this carbonate ; and when they come to the surface, the escape of the carbonic acid causes the lime carbonate to deposit abundantly. Thus around carbonated springs in limestone districts, and along the course of the streams which issue from them, are generally found ex- tensive deposits of this substance. Being found mostly in volcanic regions, these springs are commonly hot. Kinds of Materials. The material thus deposited is usually called travertine, but is very diverse in appearance. If the deposit is quiet, the material is dense ; if tumultuous, the material is spongy ; if no iron is present, it is white like marble ; but if iron be present, its oxida- tion colors it yellow, brown, or reddish. If the amount of iron be vari- able, the stone is beautifully striped. If objects of any kind, branches, twigs, leaves, are immersed in such waters, they are speedily incrusted, often in the most beautiful manner. Examples of such deposits are found in all countries. At the baths of San Vignone, Italy, a carbonated spring issuing from the top of a hill has covered the hill with a stratum of white, compact travertine 250 feet thick. In the conduit-pipe which leads the water to the baths, the AQUEOUS AGENCIES. deposit accumulates six inches thick every year. A similar deposit of travertine occurs at the baths of San Filippo. At this latter place, beautiful fac-similes of medallions, coins, etc., are formed by placing these objects of art in the spray of an artificial cascade. In Virginia, around the " Old Sweet " and the " Red Sweet " Springs, and in the course of the stream which flows from them for several miles, a brownish-yellow deposit of travertine has accumulated to the depth of at least thirty feet. The spray of Beaver Dam Falls, about three miles below the springs, incrusts every object in its reach with this deposit. In California, all about the shores of Lake Mono, abundance of beautiful and strangely- branched coralline forms are found, which have evidently been formed in a somewhat similar way. In the re- gion of the Yellowstone Park, de- posits of traver- tine from waters of hot springs run- ning down a steep incline, in a suc- cession of cas- cades, assume the most beautiful forms, as shown in the accompany- ing figure, taken from Hayden. Deposits of Iron. Iron car- bonate, like lime carbonate, is to some extent soluble in water containing carbonic acid. Subterranean waters, therefore, which always contain atmospheric car- bonic acid, when they meet this carbonate, will take up a small quan- tity in solution. Such waters are called chalybeate. On coming to the surface the iron gives up its carbonic acid, is peroxidized, becomes insoluble, and is deposited. As the presence of organic matter is usu- ally necessary to bring the iron into a soluble condition, the full dis- cussion of this very interesting subject is reserved until we take up organic agencies. Deposits of Silica. Silica is soluble in alkaline waters, especially if the waters be hot. Such waters reaching the surface and cooling, de- FIG. 70. Deposits from Carbonated Springs. CHEMICAL DEPOSITS IN LAKES. 73 posit the silica in great abundance, often at first in a gelatinous con- dition, but drying to a white porous material called siliceous sinter. Examples of such deposits are found in all geysers, as in those of Ice- land, and in the Steamboat Springs in Nevada, and especially in the wonderful geysers of Yellowstone Park. Such deposits are confined to volcanic regions, the volcanic rocks furnishing both the alkali and the heat. We will discuss these again under Igneous Agencies. Deposits of Sulphur and Gypsum. Springs containing sulphide of hydrogen (H 2 S), usually called sulphur - springs, sometimes deposit sulphur by oxidation of the hydrogen (H 2 S + O=H 2 O + S), and some- times gypsum. This latter deposit is caused by the more complete oxi- dation of the sulphide of hydrogen, forming sulphuric acid (H 2 S + 4O =H 2 SO 4 ), and the reaction of this acid on lime carbonate held in solu- tion in the same water. Chemical Deposits in Lakes. Salt Lakes and Alkaline Lakes. Salt lakes may be formed either 1. By the isolation of a portion of sea-water in the elevation of sea- bottom into land ; or, 2. By indefinite concentration of river-water in a lake without an outlet. Thus, the Dead Sea, Lake Elton, and the brine-pools of the Russian steppes, are probably concentrated remains of isolated portions of the sea, 1 for their waters are highly-concen- trated mother-liquors of sea-water, having a composition very similar to the mother-liquors of the salt-maker. The Caspian Sea, on the other hand, although elevated lake-margins show that much of its waters has dried away, is still much fresher than sea-water. This fact, to- gether with the composition of its waters, is usually supposed to indi- cate that it has been formed by the simple concentration of the waters of a once fresh lake. 2 Yet there are some evidences, as we shall see hereafter, of this sea having been once connected with the Black and with the Arctic Ocean. The composition of the waters of the Great Salt Lake of Utah would seem to indicate its origin in the isolation of sea-water ; but there are also some evidences of its once having had an outlet, in which case it must have been fresh, or at least brackish. 3 Alkaline lakes can only be formed by the second way-. Both salt and alkaline lakes, therefore, may be formed by indefinite concentra- tion of river-water in a lake without outlet. Whether the one or the other is formed depends on the composition of the river-water. If al- kaline chlorides predominate, a salt lake will be formed ; but if alka- line carbonates, an alkaline lake. Such alkaline lakes are found in Hungary, in Lower Egypt, and in Persia. In our own country, Lake Mono, fifteen miles long and twelve miles wide, and Lake Owen, of at 1 Bischof, " Chemical and Physical Geology," vol. i., p. 396. 2 Ibid., p. 91. 3 Gilbert, " Wheeler Report for 1872," p. 49. 74: AQUEOUS AGENCIES. least equal dimensions, are examples of alkaline lakes. The waters of Lake Mono consist principally of a strong solution of carbonate of soda, with a little carbonate of lime and borate of soda. 1 Conditions of Salt-Lake Formation. Spring and river waters always contain a small quantity of saline matter derived from the rocks and soils. Suppose, then, we have a lake supplied by rivers: 1. If the supply of water by rivers is greater than the loss by evaporation from the lake-surface, then the water will rise until, finding an outlet in the rim of the lake-basin, it flows into the sea. In this case the lake will remain fresh, or the quantity of saline matter will be inappreciable. But if, on the other hand, the loss by evaporation is greater than the supply by rivers, the lake will decrease in extent, and therefore in evap- orating surface, until an equilibrium is established. Now all the saline matters constantly leached from the earth accumulate in the lake with- out limit ; the lake, therefore, must eventually become saturated with saline matter, and afterward begin to deposit salt. It is evident, then, that whether a lake is fresh or salt depends upon whether or not it has an outlet, and this latter depends upon the relation of supply by rivers to loss by evaporation. Lakes are mostly fresh, because much more water falls on continents than evaporates from the same surface, the excess running back to the sea by rivers. It is only in certain parts of the continents, where the climate is very dry, that there is no such excess. In these regions alone, therefore, can salt lakes exist. Such regions occur in the interior of Asia, on the plateau of Mexico, in the basin of Utah, and in several other places. Even in case a salt lake is originally formed by the isolation of a portion of sea-water, whether it remains a salt lake or gradually becomes fresh will depend upon the conditions we have already mentioned. For example : if the Mediterranean should be separated from the At- lantic at the straits of Gibraltar, it would not only remain a salt lake, but would diminish in area, and finally deposit salt. This we conclude, because the water of the Mediterranean seems to be a little more salt than that of the Atlantic. If, on the contrary, the Black Sea were sepa- rated from the 'Mediterranean, or the Baltic from the Atlantic, or the bay of San Francisco from the Pacific, the supply by rivers, in the case of these inland seas being greater than their loss by evaporation, they would rise until they found an outlet, and then would be gradually rinsed out, and become fresh. Lake Champlain was, in very recent geological time, an arm of the sea. When first isolated it was salt. It has become fresh by this process. 1 The probability of Great Salt Lake having been produced by simple evaporation of river-water is increased by the difference in the composition of the waters of lakes in this general region. Where sedimentary rocks prevail, as in Utah, they are salt ; where volcanic rocks prevail, as about Mono and Owen, they are alkaline. CHEMICAL DEPOSITS IX LAKES. 75 Deposits in Salt Lakes. The nature of the chemical deposits in salt lakes will depend upon the manner in which these lakes have been formed. We will take the simplest case, viz., that of a lake formed by the isolation of sea-water, and its concentration by evaporation. In this case the substance first deposited would be gypsum ; for this siib- stance is insoluble in a saturated brine, and therefore always deposits first in the artificial evaporation of sea-water in salt-making. Upon the gypsum would be deposited salt. Meanwhile, however, the rivers dur- ing their flood-season would bring down sediments. During the flood- season, the supply of water being greater than loss by evaporation, the deposit of salt or gypsum would cease ; while during the dry season the deposit of sediment would cease, and the evaporation being now in ex- cess, the deposit of salt would recommence. Thus the deposits in the bottom of salt lakes probably consist of alternations of salt and sedi- ment, the whole underlaid by layers of gypsum. These views have been confirmed by observation. During the dry season Lake Elton deposits annually a considerable layer of salt. Wells dug near the margin of this lake revealed 100 alternations of salt and mud, the salt- beds being many of them eight or nine inches thick. 1 Most of the salt has already deposited ; for the water of this lake is an almost pure bit- tern. The great predominance of chloride of magnesium in Dead Sea water shows that it is a mother-liquor, from which immense quantities of common salt have already been deposited. Similar alternations, therefore, no doubt exist in the bottom of this sea. 2 The Great Salt Lake, in Utah, is also a saturated brine depositing salt, as is proved by the incrustations of salt about its margin in dry seasons ; but the de- posit has not progressed so far in this case as in the preceding. The great extent to which the waters of this lake have dried away and be- come concentrated is further shown by old lake-margins far beyond the limits, and several hundred feet above the level, of the present shore- line. Similar phenomena are observed about other salt lakes, especially about the Caspian Sea (Murchison). In the case of salt lakes, either formed entirely, or modified, by river- water, the deposits are probably much more complex and various some- times salt, sometimes carbonate of lime, and sometimes sulphate of lime. This subject, however, has been but little investigated. Deposits are also sometimes formed in lakes which are not salt. For example : the Solfatara Lake, Italy, is formed by the accumulation of the water from warm carbonated springs, similar to those of San Filippo and San Vignone. In this lake, therefore, deposits of travertine are forming. Although these deposits take place in a lake, they properly belong to deposits from springs, since they do not take place by concen- tration. a Bischof, " Chemical and Physical Geology," vol. i., p. 405. 2 Ibid., p. 400. 76 IGNEOUS AGENCIES. Chemical Deposits in Seas. Concerning these little is known. It is certain, however, that all rivers carry to the sea carbonate of lime in solution, and some of them in considerable quantities. There is scarcely any river-water which contains less carbonate of lime than sea-water ; many rivers contain four times as much. 1 This carbonate of lime thus constantly carried into the sea must eventually deposit in some form. Usually, however, sea-water is kept below the saturating point for this substance, by its constant withdrawal by shells and corals, as will be explained under Or- ganic Agency. But in shallow bays nearly cut off from the sea, or in salt lagoons on the sea-margin near the mouths of rivers in dry climates, and subject to occasional overflows by the sea and floodings by rivers, carbonate of lime and sulphate of lime may deposit by evaporation. At the mouths of many rivers, whose waters contain much carbonate of lime, as, for instance, the Rhine, the delta deposit is cemented into hard rock by means of this substance. On shores of coral seas, as upon the Keys of Florida, the coast of the West India Islands, and the islands of the Pacific, shore-material is consolidated into hard rock by the same means. On many shores in tropical regions, the waves, being driven up on flat beaches far inland, leave sea-water inclosed in shallow pools, which by evaporation give rise to calcareous deposits which are increased by the frequent alternate influx and evaporation of sea-water. Conglomerate rocks are thus forming at the present time in the Canaries and many other places. CHAPTER III. IGNEOUS AGENCIES. THE agencies thus far considered tend to reduce the inequalities of the earth by cutting down the continents and filling up the seas. Their final effect, if unopposed, would be to bring the whole surface to one level, and thus to make the empire of the sea universal. This is pre- vented by igneous agencies, which tend, by elevation of land and de- pression of sea-bottoms, to increase the inequalities of the earth-surface, and thus to increase the area and the height of the land. All the dif- ferent forms of igneous agency are connected with the interior heat of the earth. This must, therefore, be first considered. SECTION 1. INTERIOR HEAT OF THE EARTH. Stratum of Invariable Temperature. The mean surface temperature of the earth varies from 80 at the equator to nearly at the poles. 1 Bischof, " Chemical and Physical Geology," vol. i., p. 179. INTERIOR HEAT OF THE EARTH. 77 The rate of decrease in passing from the equator to the poles is not the same in all longitudes ; the isotherms, or lines joining places of equal mean temperatures, are therefore not parallel to the lines of latitude, but quite irregular. The mean temperature of the whole earth-surface is about 58. There is also in every locality a daily and an annual variation of temperature. As we pass below the surface both the daily and annual variations become less, until they cease altogether. The stratum of no daily variation is but a foot or two beneath the surface; but the stratum of no annual variation, or stratum of invariable temper- ature in temperate climates, is about sixty to seventy feet deep. The temperature of the invariable stratum is nearly the mean temperature of the place. The depth of the invariable stratum depends upon the amount of annual variation ; it is, therefore, least at the equator, and increases toward the poles. At the equator it is only one or two feet beneath the surface ; 1 in middle latitudes about sixty feet, and in high latitudes probably more than 100 feet. It is, therefore, a spheroid more oblate than the earth itself. The temperature of the earth everywhere within this spheroid is unaffected by external changes. Increasing Temperature of the Interior of the Earth. Beneath the invariable stratum the temperature of the earth everywhere increases, a. o for all depths to which it has been penetrated, at an average rate of 1 for every 53 feet. This very important fact has been determined by numerous observations on the temperature of mines and of artesian wells in almost every part of the earth. All the facts thus far stated are graphically illustrated in the accompanying figure (Fig. 71), in which the line a b represents depth below the surface, and the diverging 1 Humboldt, "Cosmos," Sabine's edition, vol. i., p. 165. 78 IGNEOUS AGENCIES. line c d the increasing heat ; m the invariable stratum ; n the line of no daily variation ; the curves p e, c e, o e, the temperatures in summer, autumn, and winter, respectively ; the space p e o the annual swing of temperature; and the smaller curves meeting on the line n, the daily variation or swing of temperature. We have given the rate of increase as about 1 in 53 feet. It varies, however, in different places, from 1 in 30 feet to 1 in 90 feet. Except in the vicinity of volcanic action, this difference is probably due to varying conductivity of the rocks. The lines, or rather surfaces, which join places in the interior of the earth, having equal tempera- tures, may be called isogeotherms. If the rate of increase were every- where the same, the isogeotherms would be regularly concentric ; but, as this is not the case, they are irregular surfaces (Fig. 72), rising nearer the earth-surface, and closing upon one another where the con- ductivity is poor, and sinking deeper and separating where the con- ductivity is greater. Constitution Of the Earth's Interior. From the facts given above it is probable that the temperature of the interior of the earth is very great. A rate of increase of 1 for every 53 feet would give us, at the depth of twenty-five or thirty miles, a temperature sufficient to fuse most rocks. Hence it has been confidently concluded by many, that the earth, beneath a comparatively thin crust of thirty miles, must be liquid. A crust of thirty miles on our globe is equivalent to a crust of less than one-tenth of an inch in a globe two feet in diameter. There are, how- ever, many objections to this conclusion. The question of the interior constitution of the earth is one of extreme difficulty and complexity, and science is not yet in a position to solve it completely. Neverthe- less, it can be proved that the solid crust must be much thicker than is usually supposed, if, indeed, there be any general interior fluid at all. The argument for the interior fluidity of the earth, beneath a crust of only thirty miles, proceeds upon two suppositions, viz. : 1. That the interior temperature increases at the same rate for all depths and, 2. That the fusing -point of rocks is the same for all depths. Now, neither of these can be true. 1. Rate of Increase not uniform. Although we have spoken of 1 for every 30 feet or 50 feet or 90 feet, yet it must not be supposed that observation gives a uniform rate of increase at any place. On the contrary, the rate is sometimes faster and sometimes slower, depending on the conductivity of the rock penetrated, and on other causes little understood. The rate given is always an average. In other words, observation gives the fact of increase, but not the law. We are thus thrown back on general reasoning. If two bars, one a good conductor, like metal, and the other a bad conductor, like charcoal, be heated red hot at one end, and the rate of INTERIOR HEAT OF THE EARTH. 79 decreasing temperature fall of heat toward the other be observed, it will be found that the rate is very rapid in the case of the charcoal ; so that a temperature of 60 is reached at the distance of two or three inches, while in the case of the metal the rate of decrease is much slower, and 60 is only reached at a distance of several feet. Con- versely, the rate of increase, or rise, in passing toward a source of heat, is rapid in the case of the bad conductor, and slow in the case of the good conductor. Now, the average density of materials at the sur- face of the earth is about 2.5, but the average density of the whole earth is more than 5.5 ; therefore the density of the central portions must be much more than 5.5. It has been estimated at 16.27. 1 There can be no doubt, therefore, that the density of the earth increases toward the centre ; and as this increase is probably largely the result of pressure, it is probably somewhat regular. Whatever be the cause, the effect would be to increase the conductivity for heat, and there- fore to diminish the rate of increasing temperature. Thus it follows that, though in an homogeneous globe the melting-point of rocks (3,000) would be reached at the depth of 30 miles, yet, in a globe increasing in density toward the centre, we must seek this temperature at a greater depth. If Alt (Fig. 73), representing depth from the surface S S, be taken as absciss, and heat be represented by ordi- nates, then, in an homogeneous earth, CD would represent uniform increase of heat, and the heat ordinate of 3,000, m m, would be reached at the depth of A m = thirty miles. But in an earth in- creasing in density, and, therefore, in conductivity, the rate would not be uni- form, but gradually decreasing. This would be represented, not by a straight line, C D, but by a curved line, C E ; D and the ordinate of 3,000 would not be reached at thirty miles, but at a much greater depth say at m', of fifty miles. 2. Fusing-Point not the same for all Depths. Nearly all substances expand in the act of melting, and contract in the act of solidifying. Only in a few substances, like ice, is the reverse true. Now, the fusing- point of all substances which expand in the act of fusing must be raised by pressure, since the expanding force of heat, in this case, must overcome not only the cohesion, but also the pressure. That this is 1 " Cosmos," vol. iv., p. 33. 80 IGNEOUS AGENCIES. true, has been proved experimentally for many substances by Hopkins. 1 But granite and other rocks have been proved to expand in fusing ; therefore the fusing-point of rocks is raised by pressure, and must be greatly raised by the inconceivable pressure to which they are sub- jected in the interior of the earth. For this reason, therefore, we must again go deeper to find the interior fluid. In the figure, m' is the point where we last found the fusing-point of 3,000. But this is the fusing-point on the surface, or under atmospheric pressure. The press- ure of 'fifty miles of rock would certainlv greatly raise the fusing- point. Suppose it is thus raised to 3,500 : to find this we must go still deeper, to m", perhaps seventy-five miles in depth. But the in- creased pressure would again raise the fusing-point ; and thus, in this chase of the increasing heat after the flying fusing-point, where the former would overtake the latter, or whether it would overtake it at all, science is yet unable to answer. From this line of reasoning, therefore, we conclude that the solid crust of the earth must be much thicker than is usually supposed, and there may be even no interior liquid at all. Astronomical Reasons. There is another and an entirely different line of reasoning which has led some of the best mathematicians and physicists to the same result. According to the thin-crust theory, the earth is still substantially a liquid globe, and therefore under the at- tractive influence of the sun and moon it ought to behave like a yielding liquid. Now, according to Hopkins, Thomson, and others, the earth in all its astronomical relations behaves like a rigid solid a solid more rigid than a solid globe of glass and the difference between the be- havior of a liquid globe and a solid globe could easily be detected by astronomical phenomena. 9 A complete exposition of the proof would be unsuitable to an elementary work. Suffice it here to say that the force of these arguments has led many of the most advanced geologists to the conclusion that the earth, if not solid to the centre, must have a crust so thick that for all purposes of the geologist it may be regarded as substantially solid ; that volcanoes are openings into local masses of liquid, not into a general interior liquid into subterranean fire-lakes, not into a universal Jire-sea in a word, that all the theories of igneous phenomena must be reconstructed on the basis of a solid earth. A few geologists, however, find a compromise in the view that there exists a semi-liquid stratum between the solid crust and a solid nucleus. The interior heat of the earth manifests itself at the surface in three principal forms, viz., volcanoes, earthquakes, and gradual oscillations of the earth's crust. 1 American Journal of Science and Art, II., vol. xxxii., p. 367. 8 Thomson has recently reaffirmed these conclusions with still greater positiveness. Nature, vol. xiv., p. 426 ; American Journal of Science and Art, vol. xii., p. 336, 1876. VOLCANOES. 81 SECTION 2. VOLCANOES. Definition. A volcano is usually a conical mountain, with a funnel- shaped, or pit-shaped, or cup-shaped opening at the top, through which are ejected materials of various kinds, always hot and often in a fused condition. The activity of volcanoes is sometimes constant, as in the case of Stromboli, in Italy, from which continuous lava-streams flow (" Cosmos," vol. iv.), but more commonly intermittent, i. e., having pe- riods of eruption alternating with periods of more or less complete re- pose. Volcanoes which have not been known to erupt during historic times are said to be extinct. It is impossible, however, to draw the line of distinction between active and extinct volcanoes. Vesuvius, until the great eruption which overthrew the ancient cities of Hercu- laneum and Pompeii, was regarded as an extinct volcano. Since that time it has been very active. Size, Number, and Distribution. Some volcanoes are among the loftiest mountains on our globe. Aconcagua, in Chili, is 23,000 feet, Cotopaxi, in Peru, 19,660 feet in height. These volcanic cones, how- ever, are situated on a high plateau ; their height, therefore, is not due to volcanic eruptions entirely. But Mauna Loa, in Hawaii, nearly 14,000 feet, and Mount Etna, 11,000 feet high, seem to be due entirely, and Mount Shasta, California, 14,440 feet, Rainier, Washington Territory, 14,444, almost entirely to this cause. The crater of Mauna Loa is two and a half miles across ; that of Kilauea three miles across and 1,000 feet deep. The number of known volcanoes, according to Humboldt, is 407, and of these 225 are known to have been active in the last 160 years. The actual number is, however, probably much greater. It has been estimated that, in the archipelago about Borneo alone, there are 900 vol- canoes. 1 The distribution of volcanoes is remarkable, (a.) They are almost entirely confined to the vicinity of the sea. Two-thirds of them are found on islands in the" midst of the sea, and the remainder, with the exception of a few in the interior of Asia, are near the sea-coast. Those on islands in the sea probably commenced, most of them, at the bottom of the sea, the islands having been formed by their agency. New islands have been suddenly formed under the eye of observers in the Mediterranean. The basin of the Pacific is the great theatre of volcanic activity, nearly seven-eighths of all known volcanoes being situ- ated on its coasts, or on islands in its midst, (b.) Volcanoes are, more- over, distributed in groups (as the Hawaiian volcanoes, the Mediter- ranean volcanoes, the West Indian volcanoes, the volcanoes of Auvergne, etc.), or along extensive lines as if connected with a great fissure of the earth's crust. The most remarkable linear series of volcanoes 1 Herschel, " Physical Geology," p. 113. 82 IGNEOUS AGENCIES. is that which belts the Pacific coast. Commencing with the Fuegian volcanoes it runs along the whole extent of the Andes, then along the Cordilleras of Mexico, the Rocky Mountains, then along the Aleutian chain of .islands, Kamtchatka, the Kurile Islands, Japan Islands, Philip- pines, New Guinea, New Zealand, to the antarctic volcanoes Mounts Erebus and Terror, thence back by Deception Island to Fuegia again, thus completely encircling the globe. (c.) Volcanoes are generally formed in comparatively recent strata. This seems to be connected with their relation to the sea ; for recent strata are abundant about the sea-coast, and the most recent are now forming in the bed of the sea. The extinct volcanoes of France and Germany are in tertiary regions. Possibly the retiring of the sea has extinguished them. In the oldest strata volcanic activity has apparently died out long ago. Phenomena of an Eruption. The phenomena of an eruption are very diverse. Sometimes there is a gradual melting of the floor of the crater, and then a rising and boiling of the liquid contents until they quietly over- flow and form immense streams of lava, extending fifty to sixty miles. After the eruption, the melted lava again sinks and cools, and solidifies, to form the floor of the crater until another eruption. This is the case with the Hawaiian and many other volcanoes in the South Seas. In other cases, as in the Mediterranean volcanoes, and especially in many in the Indian Ocean, the eruption is fearfully explosive. In such cases the eruption is usually preceded by premonitory earthquakes and sounds of subterranean explosions ; then the bottom of the crater is blown out with a violent explosion, throwing huge rocky fragments to great dis- tances, often many miles ; then the melted lava rises and overflows in streams running down the side of the mountain. The rise and overflow of lava are accompanied with violent explosions of gas which throw up immense quantities of ashes and cinders 6,000 and even 10,000 feet above the crater. 1 In the great eruption of Tomboro, in the island of Sumbawa near Java, in 1815, these explosions were heard in Sumatra, 970 miles distant. 2 The emission of gas usually continues after all other ejections cease. Violent storms and heavy rain accompany the eruption, and when the mountain reaches into the region of perpetual snow, as in many of the South American volcanoes, the fearful deluges produced by the sudden melting of the snows are often the most destructive phenomenon connected with the eruption. Volcanic eruptions, therefore, may be divided into two great types, viz., the quiet and the explosive. In the one, lava-flows predominate ; in the other, cinders and ashes, and steam and gas. The Hawaiian vol- canoes are perhaps the best examples of the former, and the Javanese volcanoes of the latter. The Mediterranean and most other volcanoes . are .mixtures of these two types in varying proportions. - 1 Dana's " Manual," p. 692. * Lyell, " Principles of Geology." VOLCANOES. 33 The quantity of materials ejected during an eruption is sometimes almost inconceivable. During the great eruption of Tomboro, already mentioned, ashes and cinders were ejected sufficient to make three Mont Blancs, or to cover the whole of Germany two feet deep. 1 The lava which streamed from Skaptar Jokull, Iceland, in 1783, has been computed to be equivalent to about twenty-one cubic miles, or to the whole quantity of water poured by the Nile into the sea in one year! These were, however, very extraordinary eruptions. In the greatest eruptions of Vesuvius the quantity of lava poured out was not more than 600,000,000 cubic feet = one square mile covered twenty-two feet deep. The volume of lava poured out by Kilauea, in 1840, is estimated by Dana as sufficient to cover one square mile of surface 800 feet deep. Great destruction of life is often produced by volcanic eruptions. The overthrow of Herculaneum and Pompeii by ejections from Vesu- vius is well known. The great eruption of Skaptar Jokull destroyed 1,300 human lives and 150,000 domestic animals. The eruption of Etna, in 1669, overwhelmed fourteen towns and villages. In the prov- ince of Tomboro, out of a population of 12,000, only twenty-six persons escaped the great eruption of 1815.' Monticules. Eruptions occur not only from the summit-crater, but also frequently from fissures in the side of the mountain. By the im- mense upheaving force necessary to raise lava to the mouth of the crater of a lofty volcano, the mountain is fissured by cracks radiating from the crater in all directions. These cracks are filled with lava, which on hardening form radiating dikes which intersect the successive layers of ejections, and bind them into a stronger mass. Through these fissures the principal streams of lava often pass. During an eruption of Mauna Loa, in 1852, the immense pressure of the lava in the princi- pal crater fissured the side of the mountain, and a fiery fountain of liquid lava, 1,000 feet wide, was projected upward through the fissure to the height of 700 feet, and continued to play for several days. Upon these fissures subordinate craters, and finally cones, are formed. These subordinate cones about the base, and upon the slopes of the principal cone, are called monticules or hornitos. There are about 600 monticules on Etna one of them over 700 feet high (Jukes). Materials erupted. As we have already stated, the materials erupted are stones, lava-streams, cinders, ashes, and gases. Stones. In explosive eruptions the solid floor of the crater is often blown out with violence, and rock-fragments, sometimes of great size, are thrown to great distances. Lava. The term lava is applied to the liquid matter poured from a volcano during eruption, and also to the same when it has hardened into rock. 1 Herschel, "Physical Geology," p. 111. 84 IGNEOUS AGENCIES. Liquid Lava, At the time of eruption the liquidity of lava varies very much, depending partly upon the heat, partly on the fusibility of the material, and partly upon the kind of fusion. In the Hawaiian volcanoes the lava is a melted' glass almost as thin as honey. In Ki- lauea this lava is often thrown into the air by the bursting of gas-bubbles, and drawn out into long threads like spun glass, which is carried by the winds, and collects in places as a soft, brownish, towy mass, called " Pele's hair." Completely fused lava, when cooled rapidly, forms vol- canic slag or volcanic glass (obsidian) ; but if cooled slowly, so that the several minerals of which it is composed have time to separate and crystallize, forms stony lava. If it is full of gas-bubbles (rock-froth), and hardens in this condition, it forms vesicular or scoriaceous lava. If the quantity of gas and steam be very great, the whole liquid mass may swell into a rock-froth, which rises to the lip of the crater, and outpours much as porter or ale from a bottle when the cork is drawn. Or the rock-froth may be thrown violently into the air, and, hardening there, may fall again in cindery or scoriaceous masses ; or, thrown with still greater violence, the rock-froth may be broken into fine rock-spray, and fall as volcanic sand and ashes'. Ashes, when consolidated by time and percolating water, form tufa. Thus, there are four physical condi- tions in which we find lava viz., stony, glassy, scoria ceous, and tufaceous. Again, the liquidity of lava and its character depend much on the kind of fusion. Daubree has shown that all siliceous rocks and glass mixtures, in the presence of superheated water even in small quanti- ties, and under pressure, will become more or less liquid, at tempera- tures far below that necessary to produce true fusion. At 400 Fahr., such rocks become pasty ; at 800, completely liquid. The same change takes place at even lower temperatures if a little alkali be pres- ent. To distinguish this liquidity from that of true igneous fusion, which requires a temperature of 2,500 to 3,000, it has been called aqueo- igneous fusion. Now, very much lava at the time of eruption is in this condition. Such lava, when the pressure is suddenly removed by break- ing up of the floor of the crater, and the contained water suddenly changed into steam, is blown into the finest dust, which is then carried to great height by the out-rushing steam, and falls again as volcanic ashes, which may consolidate into tufa. If the heat be not sufficient to produce complete aqueo-igneous fusion, the lava is outpoured as a kind of rock-broth, consisting of unfused particles in a semifused mass, which concretes into an earthy kind of rock. Or the material may pour out only as hot mud, which concretes into a kind of tufa. In fact, every variety of fusion and semifusion, depending on the degree of heat and the quantity of water, may be traced, from perfect igneous fusion through various grades of aqueo-igneous fusion, to the condition of hot mud. VOLCANOES. 85 It is evident that, of the two kinds of eruption mentioned above, the quiet type is characterized by igneous fusion, the explosive type by aqueo-igneous fusion. In the former the heat is great, but the amount of water is small ; while in the latter the heat is less, but the amount of water far greater. The rapidity of the flow of a lava-stream depends on its fluidity. In the Hawaiian volcanoes the lava, where it issues from the crater, has been seen to flow with a velocity of fifteen miles an hour ; while Vesuvian lava seldom flows at a rate of more than two or three miles an hour. Lava, like glass, passes through various grades of viscous fluidity in cooling. It gradually becomes so stiff that it may flow only a few feet per day. The froth or scum which covers the surface of a lava-stream quickly cools and hardens into a crust of vesicular lava, which may even be walked upon while the interior is still flowing beneath. In this way are often formed long galleries. Also the running together of the con- tained gas-bubbles and steam-bubbles forms huge blisters in the viscous mass, which, on hardening, form cavities often of great size. Hardened Lava. Miner alogically, lava consists essentially of feld- spar and augite, either intimately mixed, as in glassy lava, or aggre- gated in more or less distinct particles or crystals, as in the stony varie- ties. Now, feldspar is a light-colored mineral, having a specific gravity of about 2.5, while augite is usually a very dark-colored mineral, having a specific gravity of about 3.5. It is evident, therefore, that in propor- tion as feldspar predominates the lava is lighter colored and of less spe- cific gravity, and in proportion as augite predominates the rock is darker and heavier. Chemically, feldspar is a silicate of alumina and alkali (the latter being either potash or soda), with excess of silica (acid sili- cate) ; while augite is a silicate of lime, magnesia, and iron, with excess of base (basic silicate). Therefore, lavas may be divided into two classes the feldspathic or acidic lavas and the augitic or basic lavas. Further, it is seen that all lavas are multiple silicates, like glass : they are, therefore, true glass-mixtures. Now, the feldspathic or acid lavas are a more difficultly fusible, the augitic or basic lavas a more easily fusible, glass-mixture. Either of these two kinds of lava may exist in any of the conditions mentioned above viz., as stony, glassy, vesicular, or tufaceous lava. Trachyte is an example of feldspathic lava, and basalt or dolerite of augitic lava in a stony condition. Pumice is a peculiar vesicular variety of feldspathic lava. It is highly probable that the fusion and subsequent cooling of granite, or gneiss, or even of the purer varieties of mixed sandstones and clays, would make a feldspathic or trachytic lava ; while the fusion and cooling of impure slates and shales and limestones would produce an augitic or basaltic lava. Gas, Smoke, and Flame. The gases emitted by volcanoes are princi- 86 IGNEOUS AGENCIES. pally steam, sulphurous vapor (S and SOJ, hydrochloric acid, and car- bonic acid. By far the most abundant of these is steam. In violent, explosive eruptions, which eject principally cinders and ashes, it is prob- able that water, mostly in the form of steam, is one of the most abun- dant of all the ejected materials. In quiet lava-eruptions, like those of the Hawaiian volcanoes, the quantity of steam arid gases is small. It is worthy of notice, in connection with the position of volcanoes near the sea, that the gases ejected are such as might be formed from sea- water and from limestone. The so-called smoke and flame of volcanoes have no connection with combustion. The condensed vapors and the ashes suspended in the air, often in such quantities as to make midnight- darkness at high noon, form the smoke ; and the red glare of the same, reflecting the light from the incandescent lava beneath, forms the apparent flame. All volcanic ejections, except the gases, accumulate about the crater, and continue to increase with every successive eruption, forming a sort of stratified deposit. Sometimes the cone is made up of successive lay- ers of lava, as in Hawaiian volcanoes ; sometimes it is made up of suc- cessive layers of cinders or tufa ; sometimes of alternate layers of lava and tufa. Stratified materials of this kind, however, cannot be con- founded with those produced by the action of water. In the former case the stratification is not the result of the sorting of the materials. Kinds of Volcanic Cones. Volcanic cones and craters have been divided into two kinds viz., cones of elevation and cones of eruption. A cone of elevation is formed by interior forces lifting the crust of the earth at a particular point until the latter breaks and forms a crater, through which eruptions take place. It is an earth-bubble which swells and breaks at the top. A cone of eruption, on the other hand, is formed by the accumulation around a crater of its own ejections. There has been much discussion among physical geologists as to whether existing volcanic cones are formed mostly by the one method or the other. We will not enter into this discussion. It seems probable, however, that most cones are principally cones of eruption, although their height and size have been increased somewhat also by elevating forces. Mode of Formation of a Volcanic Cone. A volcano commences 1. As a simple opening in the earth's crust, in most cases with little or no FIG. 74. Section across Hawaii. VOLCANOES. elevation. Through this opening or crater are ejected, from time to time, lava, cinders, ashes, etc., which accumulate immediately about the crater, and continue to increase, by successive layers, with every erup- tion. Ejections of pure lava, particularly if the lava is very fluid, form a cone of broad base and low inclination. This is the case with the Pacific volcanoes. Fig. 74 is a section through Hawaii, show- ing the slope of the pure lava-cones of Mauna Loa (Z), nearly 14,- 000 feet high, and of Mauna Kea (K). Tufa -cones and cinder-cones (Fig. 75) take a much higher angle of slope. 2. With every eruption the powerful internal forces fissure the mountain, in lines radiating from the crater. These fissures are filled with liquid lava, which, on hardening, forms e ' FIG. 75. Section of Cinder-Cone. radiating dikes, intersecting the lay- ers of ejections, and binding them into a more solid mass. Fig. 76 shows how these dikes, rendered more visible by erosion, intersect the at the Base of the Serra del Solflzio, Etna. strata. 3. After a time, when the mountain has grown to considerable height, the force necessary to raise liquid lava to the lip of the crater becomes so great that it breaks in preference through the fissured sides of the mountain. The secondary craters thus formed immediately com- mence to make accumulations around themselves, and thus form second- 88 IGNEOUS AGENCIES. ary cones (Fig. 77, c'), or monticules, about the base and on the sides of the primary cone. If a secondary cone becomes extinct, it is finally buried (Fig. 77, c") in the layers of the primary cone. 4. Finally, in FIG. 77. Section of Volcano, showing Monticules. volcanoes of the explosive type, during great eruptions the whole top of the mountain is often blown off, and in volcanoes of the quieter type is melted and falls in in either case forming an immense crater, within which, by subsequent eruptions, another smaller cone of eruption is FIG. 78. Section of Vesuvius atod Mount Somma. built up, and in this latter often a still smaller cone is again built. This cone-within-cone structure is well illustrated by the present condi- tion, and still better by the history, of Vesuvius. Vesuvius is a double- peaked mountain, with a deep, semicircular valley between the peaks. FIG. 79. Mount Vesuvius in 1756 (after Scrope). The present active cone of Vesuvius is encircled by a rampart, very high on one side, and called Mount Somma, but traceable to some degree all around, and having the same structure as Vesuvius itself. This rampart VOLCANOES. 89 is the remains of a great crater, many miles in diameter. Fig. 78 is an ideal section through Mount Somma ($), and Vesuvius (V}. S' is the almost obliterated remains of the old crater on the other side. This is further and beautifully illustrated by the history of this mountain, which records the repeated destruction and rebuilding of these cones within cones. Fig. 79 is an outline of Vesuvius as it existed in 1756 ; 1 S is Mount Somma. Many other volcanoes are known which have similar circular ram- parts made up of layers of volcanic ejections. One of the most remark- able of these is Barren Island, in the bay of Bengal (Fig. 80). The FIG. 80. Section of Barren Island. difference between this and Vesuvius is, that the circle is more com- plete, and the valley between it and the present cone is occupied by the sea. Comparison between a Volcanic Cone and an Exogenous Tree. It is evident, then, that a cone of eruption grows by layers successively applied on the outside. Both in structure and growth it may, there- fore, be compared to an exogenous tree : 1. As the sap ascends through the centre of the shoot and descends on the outside, forming layers of wood, one outside of the other, increasing every year the height and the diameter of the tree, so in a volcano lava ascends through the centre and pours over the outside, forming also successive layers, increasing both the diameter and the height. 2. As a cross-section of a tree shows concentric rings around (Fig. 81) a central pith, and trav- ersed by pith-rays, so a cross-section of a volcano would show a central crater, with concentric layers, traversed by radiating dikes. 3. As on the pith-rays, where they emerge upon the surface, arise buds, which grow in a manner similar to the trunk, so on the radi- ating dikes are formed monticules, which grow like the principal cone. If buds die, they are covered up in the Fl - si- annual layers of the trunk ; so, in like manner, extinct monticules are buried in the layers of the principal cone. Estimate of the Age Of Volcanoes. The age of exogenous trees, as is well known, may be estimated by counting the annual rings. The age of volcanoes cannot be estimated accurately in a similar manner : 1. Because the overflows are not regularly periodical ; 2. Because 1 Scrope, Philosophical Magazine, vol. xiv., p. 139. 90 IGNEOUS AGENCIES. in the case of lava-overflows it requires many overflows to make one complete layer ; and, 3. Because it is impossible to make a complete section of the mountain. Nevertheless, Nature gives us partial sec- tions, which reveal an almost incalculable antiquity. Thus, the Val de Bove, of Etna (a huge valley reaching from near the summit to the foot, and probably formed bv an engulfment of a portion of the moun- tain), gives a perpendicular section into the heart of the mountain 3,000 feet deep. Throughout the whole of this section the mountain is composed entirely of layers of lava and cinders. It is almost cer- tain, therefore, that the whole mountain to its very base, 11,000 feet, is similarly composed. That the time necessary to accumulate this im- mense pile, 11.000 feet high and ninety miles in circumference at the base, was almost inconceivably great, is shown by the fact that Etna had already attained very nearly its present size and shape 2,500 years ago, when it was observed by the early Greek writers. The lava-stream which stopped the Carthaginians in their march against Syracuse, 396 years before Christ, may still be seen at the surface, not yet covered by subsequent eruptions. And yet Etna belongs to the most recent geo- logical epoch, for it has broken through, and is built upon, the newer tertiary strata. Theory of Volcanoes. In the theory of volcanoes there are two things to be accounted for, viz. : 1. The force necessary to raise melted lava to the crater, and even to project it with violence high into the air ; 2. The heat necessary to fuse rocks and form lava. Force. The specific gravity of lava being about 2.5 to 3, it would require the pressure of one atmosphere, or fifteen pounds to the square inch, for every eleven or twelve feet of vertical elevation of the liquid mass. The following table gives the pressure in atmospheres for four well-known volcanoes, assuming the point of hydrostatic equilibrium to be at the sea level : NAME. Height. Pressure in Atmospheres. 3 900 feet 325 Etna 11 000 " 920 13 800 " 1 150 Cotopaxi 19,660 " 1.638 The lava is often, however, in a frothy orVesicular condition. In such cases the pressure necessary to produce overflow would be much less. But, on the other hand, the force in most cases is not only sufficient to lift lava to the top of the crater, but to project it thousands of feet in the air. A rock -mass of over 2,700 cubic feet was projected from the crater of Cotopaxi to a distance of nine miles (Lyell). The agent of VOLCANOES. 91 this prodigious force is evidently gas and vapors, especially steam. The great quantity of steam issuing from all volcanoes, but especially from those of the explosive type, is sufficient proof. Thus far theorists generally agree, but from this point opinions diverge into the most op- posite directions. The Heat. There are many and diverse opinions as to the source of the heat associated with volcanic eruptions. Two prominent views, however, may be said to divide geologists. According to the one, the heat is the remains of the primal heat of the once universally incandes- cent earth ; according to the other, the heat is produced by chemical or mechanical action. According to the former, the heat is general, and only the access of water is local ; according to the latter, both the heat and the access of water are local. According to the former, volcanoes are openings through the comparatively thin crust, revealing the uni- versal interior fluid ; according to the latter, they are openings into isolated interior lakes of molten matter. The former may be called the " interior fluidity " theory ; the latter divides into two branches, which may be called respectively the " chemical " and the " mechanical " the- ory. In all, access of water to the hot interior furnishes the force. Internal Fluidity Theory. This theory supposes that the earth, from its original incandescent condition, slowly cooled and formed a surface- crust ; that this surface-crust, though ever thickening by additions to its interior surface, is still comparatively very thin, and beneath it is still the universal incandescent liquid ; that by movements of the sur- face the solid crust is fissured, and water from the sea or from other sources finds its way to the incandescent liquid mass, and develops elastic force sufficient to produce eruption. By this view the focus of volcanoes is situated at the lower limit of the solid crust. The theory seems clear and simple enough, but when closely examined there are many difficulties in the way of its accept- ance. Objections. The objections to this view are : 1. That the crust, as already shown, must be far thicker than this theory requires, probably hundreds of miles thick, if, indeed, there be any general liquid interior at all ; but volcanoes are evidently very superficial phenomena. Under the pressure of this difficulty these theorists have been driven to the acknowledgment of local thinnings of the solid crust in the region of volcanoes. 2. Pressure on a general interior liquid from any cause at any place would, by the law of hydrostatics, be transmitted equally to every part of the crust, which would, therefore, yield at the weakest point, wher- ever that may be, even though it be on the opposite side of the globe ; but the force of volcanic eruption is evidently just beneath the volcano. 3. Volcanoes belonging to the same group, and therefore near to- 92 IGNEOUS AGENCIES. gether, often erupt independently, as if each had its own reservoir of liquid matter. The pressure of these two objections has driven many to the admission of a sort of honey-combed remains of the interior liquid inclosed in the solid crust, and now isolated both from the in- terior liquid and from each other. 4. There is a limit to the descent of water into the interior of the earth ; gravity urges it downward, but the interior heat drives it back. The limit, therefore, will be where these two balance each other, i. e., where the elastic force of steam is equal to the superincum- bent columja of water. We will call this point the limit of volcanic waters. It is evident that when water was first condensed on the cool- ing earth, this limit was at the surface : water could not penetrate at all. As the earth cooled, this limit became deeper and deeper ; and, if the earth became perfectly cool to the centre, there is little doubt that the whole of the water on the earth would be absorbed. This is per- haps the case with the moon now. Now, it seems probable that at the limit of volcanic water the in- terior heat of the earth, increasing at the rate of 1 for every fifty feet, would be far short of the temperature necessary for igneous fusion of rocks. Again, the elastic force necessary to sustain the superincum- bent water would by no means be sufficient to break up the crust of the earth, or raise melted lava to the surface. But we will not pursue this subject, as it is too complex to be yet solved by science. We rely, therefore, on the first three objections. Chemical Theory. Whether or not the earth consist of solid crust covering an interior liquid, it almost certainly consists of an oxidized crust covering an unoxidized interior. Now, the oxidizing agents are water and air, and therefore the limit of the oxidized crust is the limit of volcanic water. Therefore, the oxidizing agent and the unoxidized material are in close proximity, and the former ever encroaching on the latter, and therefore liable at any moment to set up chemical action, the intensity of which would vary with the nature of the material. If the action be intense, heat may be formed sufficient to fuse the rocks and to develop elastic force necessary to produce eruption. In this general form, the chemical theory seems plausible, but many have attempted to give it more definiteness, and to explain the special forms of oxidization which cause volcanoes. The most celebrated of these definite forms is that of Sir Humphry Davy, who attributed it to the contact of water with metallic potassium, sodium, calcium and magnesium, in the interior of the earth. In such definite forms the theory seems far too hypothetical. Recent Theories. 1. Aqueo-igneous Theory. Accumulation of sediment on sea-bottoms would necessarily produce corresponding rise of isogeotherms, and thus the interior heat of the earth would invade VOLCANOES. 93 the sediments with their contained waters. The lower portion of sediments 10,000 feet thick would be raised to a temperature of about 260, and of 40,000 feet thick (sediments of this thickness and more are known) to that of 860. This temperature, or even a less temperature if alkali be present, would be sufficient in the presence of the contained water of the sediments to produce complete aqueo-igneous fusion, and probably to develop elastic force sufficient to produce eruption. This view was first brought forward by John HerscheH Observe that this temperature and the corresponding force would be gradually developed as the accumulation progressed, until sufficient to produce these effects. Observe, again, that in this case the water does not seek the heat by descending, but the heat seeks the already imprisoned water by as- cending. It seems very probable that cases of eruption of hot mud and of aqueo-igneously fused lavas may be accounted for in this way, but the temperature would not be sufficient to account for true igneous fusion. Some geologists go much further, and, supposing that the whole sur- face of the earth consists of sedimentary rocks of great thickness, im- agine that between the solid surface and a solid nucleus there exists a continuous layer of aqueo-igneously fused matter which is the seat of igneous activity. 2. Mechanical Theory. As we shall explain hereafter (p. 252),there is much reason to believe that the interior of the earth is contracting more rapidly than the exterior, and that the exterior is thus necessarily thrust upon itself by irresistible horizontal pressure. According to Mr. Mallet, the crushing of the rocky crust in places under this pressure develops heat sufficient to fuse the rocks, and to produce eruption. 3. Issuing of Super-heated Gases. Very recently Rev. O. Fisher has advanced a view which deserves attention. He thinks volcanoes are vents through which issue from the earth's interior super-heated steam and gases, melting the rocks in their course and ejecting them by their pressure. According to this view, the water is not derived from the surface, but is original and constituent. This view is independent of the condition of the earth's interior, whether solid or liquid ; for a temperature which would permit solidity at great depths would produce fusion under less pressure near the surface. 1 The sun may be regarded as a globe in an earlier and more active stage of vulcanism. From its interior gases are seen to issue in great quantity, and almost constantly. The complete development of these later theories cannot be under- taken in this part of our treatise. We will take the subject up again under the head of Mountain Formation (p. 240). Cambridge Philosophical Society, 1875. 94 IGNEOUS AGENCIES. Subordinate Volcanic Phenomena. These are hot springs, carbonated springs, solfataras, fumaroles, mud-volcanoes, and geysers. They are all secondary phenomena, i. e., formed by the percolation of meteoric water through hot volcanic ejec- tions. Or perhaps in some cases the heat may be produced by slow rock-crushing by horizontal pressure, as explained above. General Explanation. Thick masses of lava outpoured from vol- canoes remain hot in their interior for an incalculable time. Water percolating through these acquires their heat, and comes up again as hot springs ; or, if it contains carbonic acid, as carbonated springs ; or, if it contains sulphurous acid and sulphureted hydrogen, as solfataras. If condensable vapors issue in abundance so as to make an appearance of smoke, they are called fumaroles. If the hot water brings up with it mud which accumulates about the vent, then it is a mud-spring or a mud-volcano. If the heat is very great, so that violent eruption of water takes place periodically, then it becomes a geyser. This is the onty one which need detain us. Geysers. A geyser may be defined as a periodically eruptive spring. They are found only in Iceland, in the Yellowstone Park, United States, and in New Zealand. The so-called geysers of California are rather fumaroles. Those of Iceland have been long studied ; we will, therefore, describe these first. Iceland is an elevated plateau about two thousand feet high, with a narrow marginal habitable region sloping gently to the sea. The ele- vated plateau is the seat of every species of volcanic action, viz., lava- eruptions, solfataras, mud-volcanoes, hot springs, and geysers. These last exist in great numbers ; more than one hundred are found in a circle of two miles diameter. One of these, the Great Geyser, has long attracted attention. Description. The Great Geyser is a basin or pool fifty-six feet in diameter, on the top of a mound thirty feet high. From the bottom of the basin descends a funnel-shaped pipe eighteen feet in diameter at top, and seventy-eight feet deep. Both the basin and the tube are lined with silica, evidently deposited from the water. The natural inference is, that the mound is built up by deposit from the water, in somewhat the same manner as a volcanic cone is built up by its own ejections. In the intervals between the eruptions the basin is filled to the brim with perfectly transparent water, having a temperature of about 170 to 180. Phenomena Of an Eruption. 1. Immediately preceding the erup- tion sounds like cannonading are heard beneath, and bubbles rise and break on the surface of the water. 2. A bulging of the surface is then GEYSERS. 95 seen, and the water overflows the basin. 3. Immediately thereafter the whole of the water in the tube and basin is shot upward one hundred feet high, forming a fountain of dazzling splendor. 4. The eruption of water is immediately followed by the escape of steam with a roaring noise. These last two phenomena are repeated several times, so that the fountain continues to play for several minutes, until the water is sufficiently cooled, and then all is again quiet until another eruption. 96 IGXEOUS AGENCIES. The eruptions occur tolerably regularly every ninety minutes, and last six or seven minutes. Throwing large stones into the tube has the effect of bringing on the eruption more quickly. Yellowstone Geysers. In magnificence of geyser displays, however, Iceland is far surpassed by the geyser basin of Firehole River. This wonderful geyser region is situated in the northwest corner of Wyo- ming, on an elevated volcanic plateau near the head-waters of the Madi- son River, a tributary of the Missouri, and of the Snake River, a tributary of the Columbia. The basin is only about three miles wide. About it are abundant evidences of pro- digious volcanic activity in former times, and, al- though primary volcanic activity has ceased, sec- ondary volcanic phenom- ena are developed on a stupendous scale and of every kind, viz.: hot springs, carbonated springs, fumaroles, mud-volcanoes, and geysers. In this vicinity there are more than 10,000 vents of all kinds. In some places, as on Gardiner's River, the hot springs are mostly lime- depositing (p. 71) ; in others, as on Firehole River, they are geysers depositing silica. In the upper geyser basin the valley is covered with a snowy de- FIG. 83.-(After Hayden.) FIG. 84. The Turban (after Hayden). posit from the hot geyser-waters. The surface of the mound-like, chimney-like, and hive-like elevations, immediately surrounding the GEYSERS. 97 vents, is, in some cases, ornamented in the most exquisite manner by- deposits of the same, in the form of scalloped embroidery set with pearly tubercles ; in others, the siliceous deposits take the most fan- Fio. 85. Giant Geyser (after Hay den). tastic forms (Figs. 82, 83, 84). In some places the silica is deposited in large quantities, three or four inches deep, in a gelatinous condition like starch-paste. Trunks and branches of trees immersed in these wa- ters are speedily petrified. 7 98 IGNEOUS AGENCIES. We can only mention a few of the grandest of these geysers : 1. The " Grand Geyser," according to Hayden, throws up a column of water six feet in diameter to the height of 200 feet, while the steam ascends 1,000 feet or more. The eruption is repeated every thirty- two hours, and lasts twenty minutes. In a state of quiescence the temperature of the water at the surface is about 150. FIG. 86. Bee-Hive Geyser (from a Drawing by Holmes). 2. The " Giantess " throws up a large column twenty feet in diame- ter to a height of sixty feet, and through this great mass it shoots up five or six lesser jets to a height of 250 feet. It erupts about once in every eleven hours, and plays twenty minutes. GEYSERS. 99 3. The " Giant " (Fig. 85) throws a column five feet in diameter 140 feet high, and plays continuously for three hours. 4. The " Bee-Hive " (Fig. 86), so called from the shape of its mound, shoots up a splendid column two or three feet in diameter to the height by measurement of 219 feet, and plays fifteen minutes. 5. " Old Faithful," so called from the frequency and regularity of its eruptions, throws up a column six feet in diameter to the height of 100 to 150 feet regularly every hour, and plays each time fifteen minutes. FIG. ST. Forms of Geyser-Craters (after Hayden). Theories of Geyser-Eruption. The water of geysers is not volcanic water, but simple spring-water. A geyser is not, therefore, a volcano ejecting water, but a true spring. There has been much speculation concerning the cause of their truly wonderful eruptions. Mackenzie's Theory. According to Mackenzie, the eruptions of the Great Geyser may be accounted for by supposing its pipe connected by a narrow conduit with the lower part of a subterranean cave, whose walls are heated by the near vicinity of volcanic fires. Fig. 88 represents a section through the basin, tube and supposed cave. Now, if meteoric water should run into the cave through fissures more rapidly than it can evaporate, it would accumulate until it rose above, and therefore closed, the opening at a. The steam, now having no out- let, would condense in the chamber b until its pressure raised the water into the pipe, and caused it to overflow the basin. The pressure still continuing, all the water would be driven out of the cave, and partly up the pipe. Now, the pressure which sustained the whole column a d 100 IGNEOUS AGENCIES. would not only sustain, but eject with violence, the column c d. The steam would escape, the ejected water would cool, and a period of qui- escence would follow. If there were but one geyser in Iceland, this would be rightly considered a very ingenious and probable hypoth- esis, for without doubt we may conceive of a cave and conduit so con- structed as to account for the phenomena. But there are many eruptive springs in Iceland, and it is inconceivable that all of them should have caves and conduits so peculiarly constructed. This theory is therefore entirely untenable. FIG. 88. Mackenzie's Theory of Eruption. Bunsen's Investigations. The investigations of Bunsen and his theory of the eruption and the formation of geysers are among the most beautiful illustrations of scientific induction which we have in geology. We therefore give it, perhaps, more fully than its strict geological importance warrants. Bunsen examined all the phenomena of hot springs in Iceland. 1. He ascertained that geyser-water is meteoric water, containing the soluble matters of the igneous rocks in the vicinity. He formed iden- tical water by digesting Iceland rocks in hot rain-water. 2. He ascer- tained that there are two kinds of hot springs in Iceland, viz., acid springs and alkaline-carbonate springs, and that only alkaline-car- bonate springs contain any silica in solution. The reason is obvious : alkaline waters, especially if hot, are the natural solvents of silica. 3. He ascertained that only the silicated springs form geysers. Here is one important step taken one condition of geyser-formation discov- ered. Deposit of silica is necessary to the existence of geysers. The tube of a geyser is not an accidental conduit, but is built up try its own deposit. 4. Of silicated springs, only those with long tubes erupt another condition. 5. Contrary to previous opinion, the silica in solu- tion does not deposit on cooling, but only by drying. This would make the building-up of a geyser-tube an inconceivably slow process, and the time proportionally long. This, however, is not true, for the Yellow- stone geyser-waters, which deposit abundantly by cooling, evidently because they contain much more silica than those of Iceland. 6. The temperature of the water in the basin was found to be usually 170 to 180, and that in the tube to increase rapidly, though not regularly, with depth. Moreover, the temperature, both at the surface and at GEYSERS. 101 Pressure In Atmospheres. Bofflng-Point. 1 Atmos. 2 " 3 " 4 " 212 250 275 293 all depths, increased regularly as the time of eruption approached. Just before the eruption it was, at the depth of about forty-five feet, very near the boiling-point for that depth. Theory of Geyser-Eruption Principles. 1. It is well known that the boiling-point of water rises as the pressure increases. This is shown in the adjoining table. 2. It follows from the above that if water be under strong pressure, and at high temperature, though below its boiling-point for that pressure, and the pressure be diminished sufficiently, it will immediately flash into steam. 3. Water heated beneath, if the circulation be unim- peded, is very nearly the same temperature throughout. That it is never the same temperature precisely is shown by the circulation itself, which is caused by difference of temperature, producing difference in density. The phenomenon of simmering is also a well-known evidence of this difference of temperature, since it is produced by the collapse of steam-bubbles rising into the cooler water above. 4. But if the circula- tion be impeded, as when the water is contained in long, narrow, irreg- ular tubes, and heated with great rapidity, the temperature may be greater below than above to any extent, and the boiling-point may be reached in the lower part of the tube, while it is far from this point in the upper part. Application to Geysers. We will suppose a geyser to have a simple but irregular tube, without a cave, heated below by volcanic fires, or by still hot volcanic ejections. Now, we have already seen that the temperature of the water in the tube increases rapidly with the depth, but is, at every depth to which observation extends, short of the boil- ing-point for that depth. Let absciss a d (Fig. 89) represent depth in the ^ _ n| I I | 1 1 | tube, and also pressures ; and the cor- responding temperature be measured on the ordinate a n. If, then, a #, b c, c d, represent equal depths of thirty- three or more feet, which is equal to one atmospheric pressure, the curve e f passing through 210, 250, 275, and 293, at the horizontal lines, repre- senting one atmosphere, two atmos- pheres, three atmospheres, etc., would correctly represent the increasing boil- ing-points as we pass downward. We shall call this line, e /, the curve of boiling-point. The line a g commencing / / / / ' /> / 66.6 ft. 1 / / 100 ft jg at the surface at 180, 102 IGNEOUS AGENCIES. and gradually approaching the boiling-point line, but everywhere within it, would represent the actual temperature in a state of qui- escence. Now, Bunsen found that, as the time of eruption approached, the temperature at every depth approached the boiling-point for that depth, i. e., the line a g moved toward the line ef. There is no doubt, therefore, that, at the moment of eruption, at some point below the reach of observation, the line a g actually touches the line ef the boiling- point for that depth is actually reached. As soon as this occurs, a quan- tity of water in the lower portion of the tube, or perhaps even in the sub- terranean channels which lead to the tube, would be changed into steam, and the expanding steam would lift the whole column of water in the tube, and cause the water in the basin to bulge and overflow. As soon as the water overflowed, the pressure would be diminished in every part of the tube, and consequently a large quantity of water before very near the boiling-point would flash into steam and instantly eject the whole of the water in the pipe ; and the steam itself would rush out immediately afterward. The premonitory cannonading beneath is evi- dently produced by the collapse of large steam-bubbles rising through the cooler water of the upper part of the tube ; in other words, it is simmering on a huge scale. An eruption is more quickly brought on by throwing stones into the throat of the geyser, because the circula- tion is thus more effectually impeded. The theory given above is substantially that of Bunsen for the erup- tion of the Great Geyser, but modified to make it applicable to all gey- sers. In the Great Geyser, as already stated, Bunsen found a point, forty- five feet deep, where the temperature was nearer the boiling-point than at any within reach of observation, though doubtless beyond the reach of observation the temperature again approached and touched the boil- ing-point. This point, forty-five feet deep, plays an important part in Bunsen's theory. To illustrate : if e f (Fig. 90) represent again the curve of boiling-point, then the f a curve of actual temperature in the Great Geyser tube would be the irregular line a g h. At the moment of eruption, this line touched boiling-point at some depth, h, beyond the reach of observation. Then followed the lifting of the column, the overflow of the basin, the re- lief of pressure by which the point g was brought to the boil- ing-point, the instantaneous formation of steam at -transverse oscillation <-.. -circular. A wave of elasticity may have either longitudinal or transverse oscil- lation, as shown in the diagram, but those of which we shall speak will be principally the former. Waves of gravity are always of transverse vibration. Spherical waves are of longitudinal vibration, and circular waves are transverse. If a stone be thrown into still water a series of waves run in every direction from the point of disturbance, becoming lower and lower as the distance increases, until the}' become insensible. These are circular waves of transverse oscillation propagated by gravity alone. The direc- tion of propagation is along the surface of the water in direction of the radius of the circle ; the direction of oscillation is up and down, or trans- verse to the direction of propagation. Water-waves are, therefore, transverse waves of gravity, and, if propagated from a central point, are circular. They move with uniform velocity ; their height decreases as they pass outward. If, on the other hand, an impulse like an ex- plosion originate in the interior of a medium, as, for example, in the air or in the interior of the earth, the impulse acting in every direction compresses a spherical shell of matter all around itself, while the point of impulse itself passes into a state of rarefaction ; this compressed shell in expanding by its elastic force compresses the next outer shell of mat- ter, itself becoming rarefied in the act, and this last in its turn propa- gates the impulse to the next, and so on. Thus, if only a single wave were formed, there would run outward from the focal point an ever- widening spherical shell of compressed matter, followed closely by a similar shell of rarefied matter. But in every case of impulse or con- cussion there is always a series of such alternate compressed and rare- fied shells following one another. The alternate compression and rarefaction causes each particle in succession to move forth and back. This oscillatory motion is in the direction of propagation of the wave, and therefore longitudinal. All waves propagated from a point within a medium, such as sound-waves^ are elastic spherical waves of longi- tudinal oscillation. Definition Of Terms. In transverse waves, such as water-waves, the distance from wave-crest to wave-crest, or from wave-trough to wave- trough, is called the wave-length, and the perpendicular distance from trough to crest is called the wave-height. Similar terms are used in speaking of waves of longitudinal vibrations. The sense in which they 108 IGNEOUS AGENCIES. are used and their propriety are shown in the accompanying figure (Fig. 93). Let the bar A B represent a prism cut from a vibrating sphere in the direction of the radius, i. e., the direction of propagation of the wave, and let the dark and light portions represent condensation and rarefaction. Now, on the line a b, representing the natural state of the bar, draw ordinates above, to represent the degrees of compression, and below, to represent degrees of rarefaction ; then the undulating line will correctly represent the state of the bar during the transmission of elastic longitudinal waves. Thus longitudinal waves may be repre- sented in the same way as transverse waves. The most compressed portions are called crests, and the most rarefied troughs ; from crest to crest is the length, and the amount of oscillation of the particles back and forth in compression and rarefaction is the height of the wave. We shall be compelled to use these terms in speaking of earthquake- waves. Thus, then, there are two very distinct kinds of waves, both of which are common viz., circular waves of gravity, of which water- waves are the type, and spherical elastic waves, of which sound-waves are the type. We will have much to do with both of these in the ex- planation of earthquake phenomena. The velocity of water-waves depends wholly on the wave-length, and not at all on the wave-height. Therefore, water-waves run with uniform motion, since, although their height diminishes, their length remains the same. But there is one important exception to this law, and one which peculiarly concerns us in this discussion viz., when the length of waves is great in proportion to the depth of the water, then they drag bottom, and their velocity is a function of the depth of the water as well as of the length of the wave. The velocity of elastic waves, on the other hand, is not affected either by the height or the length of the wave, but only by the elasticity of the medium. Thus the harmony of a full band of music is perfect even at a great distance ; but this would be impossible unless loud sounds (high waves) and soft sounds (low waves), deep sounds (long waves) and sharp sounds (short waves), all run with the same velocity. But there is one exception here also which especially.concerns us in the EARTHQUAKES. 109 discussion of earth-waves. It is this : When the medium is very im- perfectly elastic, and the waves are high, then the medium is broken by the passage of the waves at every step, its elasticity is diminished, and the waves retarded. In order to understand clearly what follows, it is necessary to bear well in mind the distinction between velocity of oscillation and velocity of transmission or transit. These bear no relation to one another. Thus we may have a long, low water-wave moving with immense velocity along the surface, and yet communicating only a slow oscillat- ing motion up and down to a boat resting on its surface. In the case of water-waves the velocity of transit depends on the length of the wave only, the amount of vibration on the height of the wave only, while the velocity of vibration depends on the relation of the height to the length. In elastic longitudinal waves the velocity of transit de- pends on the elasticity of the medium only; the amount of vibration, as in the last case, on the height of the wave, and the velocity of vibration upon the relation of height to length of wave. Application to Earthquakes. Suppose, then, a concussion of any kind to occur at a considerable depth (x, Fig. 94), say ten or twenty miles, beneath the earth-surface, S S. A series of elastic spherical waves will be generated, consisting of alternate compressed and rarefied shells, the whole expanding with great rapidity in all directions until they reach FIG. 94. the surface at a. From this point of first emergence immediately above the focus a, the still-enlarging spherical shells would outcrop in rapidly- expanding circular waves similar in form to water-waves, but very dif- ferent in character. This we will call the surface-wave. Fig. 94 is a vertical section through the focus x and the point of first emergence (epicentrum) a, showing the series of spherical waves outcropping at a, J, c, d, etc. The circles here drawn would equally represent a series of waves, or the same wave in successive degrees of enlargement. This surface-wave would not be similar to any wave classified above. It would not be a normal wave of any kind. It would be only the out- cropping or emergence of the ever-widening spherical wave on the earth-surface. Both its velocity of transit along the surface, and the direction of its vibration in relation to the surface, will vary constantly 110 IGNEOUS AGENCIES. according to a simple law. The direction of vibration, being along the radii x a, x b, x c, etc., will be perpendicular to the surface at a, and become more inclined until it finally becomes parallel with the surface at an infinite distance. The velocity of its transit will be in- finite at a, and then gradually decrease until, if we regard the surface as a plane surface, at an infinite distance it reaches its limit, which is the velocity of the spherical wave. Between these two extremes of infinity at a, and the velocity of the spherical wave at infinite distance, the velocity of the surface-wave varies inversely as the cosine of the angle of emergence x b a, x c a, etc. For, if a a, bb, cc, del, etc., be successive positions of the spherical wave, then the radii x a, x b, x c, would be the direction both of propaga- tion and of vibration. Now, when the wave-front is at b while the spherical wave moves from b' to c, the surface-wave would move from b to c ; when the spherical wave moves from c' to d, the surface-wave moves from c to d, etc. If, therefore, be, cd, etc., be taken very small, so that b b' c, c c' d, may be considered right-angled triangles, then in every position the surface-wave moves along the hypothenuse, while the spherical wave moves along the cosine of the angle of emergence x ca,x da, etc. Letting v = velocity of the spherical wave, and v r that of the surface-wave, and E the angle of emergence, we have the proportion v' : v'.'.Rad. I cos. .Z?, and v' = =,, or if v is constant cos. J? v' oc j-. Therefore, at a, the point of first emergence, E being a right angle, and the cos. E = 0, v' = = infinity. At an infinite distance from a the angle E becomes 0, and the cosine = 1, and v' = = v. That is, at the point of first emergence the velocity of the surface-wave is infinite ; from this point it decreases as the cosine of the angle of emergence increases, until finally at an infinite distance it be- comes equal to the velocity of the spherical wave. On a spherical surface (Fig. 95) it is evident that E never becomes 0, and therefore v' never reaches the limit v. If we conceived the wave to pass through the whole earth (Fig. 96), then the velocity of the surface-wave would decrease to a certain point where E is a mini- EARTHQUAKES. HI mum, say about c, and then would again in- crease to infinity on the opposite side of the earth,>, where E becomes again a right angle. If x be near the surface, v' would become nearly equal to v at some point of its course ; but as x approaches the centre, (7, the limit of v' would be greater and greater, until, if x is at the centre, v' would become infinite every- where : i. e., a shock at the centre of the earth would reach the surface everywhere at the same moment. Experimental Determination of the Velocity of the Spherical Wave. On the supposition that earthquakes are really produced by the emer- gence on the surface of a series of elastic earth-waves, Mallet under- took to determine experimentally the velocity of such waves. Two stations were taken about a mile or more apart, and connected by tele- graphic apparatus ; a keg of gunpowder was buried at one, and at the other was placed an observatory, in which was a clock, a mercury mir- ror, and a light, the image of which reflected from the mercury mirror was thrown on a screen. The slightest tremor communicated to the mercury surface of course caused the image to dance. The moment of explosion was telegraphed ; the moment of arrival of the earth- tremor was observed. The difference gave the time of transit ; the distance, divided by the time, gave the velocity per second. In this manner Mallet found the velocity in sand 825 feet per second, or nearly nine and one-half miles per minute ; in slate, 1,225 feet per second, or fourteen miles per minute ; and in granite 1,665 feet per second, or nineteen miles per minute. As an earthquake-focus is always several miles beneath the surface, and as rocks at that depth are probably as hard as granite, nineteen miles per minute may be taken as the aver- age velocity of earth-waves as determined by these experiments. It agrees well with the observed velocity of many earthquakes. This result was unexpected, considering the law that all elastic waves in the same medium run with the same velocity, for the velocity of sound in granite or slate is probably not less than 10,000 or 12,000 feet per second. The explanation is to be found in the very imperfect coherence and elasticity of rocks. The medium is broken by the pas- sage of large and high waves of the explosion, but carries successfully the small waves of sound. Explanation of Earthquake-Phenomena Earthquakes have been divided into three kinds, viz., the explosive, the horizontally progres- sive, and the vorticose. The first kind is described by Humboldt as a violent motion directly upward, by which the earth-crust is broken up, and bodies on the surface are thrown high in the air. The shock 112 IGNEOUS AGENCIES. is extremely violent, but does not extend very far. In the second, the shock spreads on the surface like the waves on water to a great dis- tance. In the third there is a whirling motion of the earth entirely different from ordinary wave-motion. These three kinds are sometimes supposed to be essentially distinct, and possibly produced by different causes ; but we will attempt to show that the difference is wholly due to the different conditions under which the waves emerge on the sur- face. The three kinds are, iu fact, often united in the same earth- quake. The most remarkable example of explosive earthquake is that which destroyed Riobamba in 1797. In this dreadful earthquake the shock came suddenly, like the explosion of a mine. Not only was the earth broken up and rent in various places, but objects lying on the surface of the earth were thrown violently upward ; bodies of men were hurled several hundred feet in the air, and afterward were found across a river and on the top of a hill. In earthquakes of this kind 1. The impulse is very powerful and sudden, so as to make a high but not a long wave, or, in other words, the velocity of vibration or of the shock is very great ; and, 2. The focus is not deep, so that the velocity of the shock (height of the wave) does not become small be- fore it reaches the surface. At Riobamba the velocity of the shock was still very great when the wave reached the surface. From the distance bodies were thrown, Mallet supposes the velocity of the shock could not have been less than eighty feet per second (Jukes). The horizontally progressive kind may be regarded as the true type of an earthquake ; it is in fact the spreading surface-wave al- ready explained. If the elasticity of the earth, and therefore the velocity of the waves, is the same in all directions, the surface-wave will spread in concentric circles; but if the elasticity, and therefore the velocity of the waves, be greater in one direction than in another, as, for example, north and south than east and west, or the converse, then the form of the outcrop will be elliptical. In some rare cases the shock seems to run along a line. Thus progressive earthquakes have been subdivided into circular, elliptical, and linear progressive. We have already given the simple explanation of the first two ; the last may be briefly explained as follows : Let it be borne in mind : 1. That these linear earthquakes usually run along mountain-chains ; 2. That most great mountain-chains consist of a granite axis (appearing along the crest and evidently connected be- neath with the great interior rocky mass of the earth), flanked on each side with stratified rocks consisting of many different kinds ; 3. When elastic waves pass from one medium to another of different elasticity, in all cases a part of the waves passes through, but a part is always reflected. For every such change for every layer a reflection oc- EARTHQUAKES. 113 curs; and, therefore, if there are many such layers, the waves are quickly quenched. If, now, Fig. 97 represent a transverse section across such a mountain, and X the focus of an earthquake, it is evident that portion of the enlarging spherical wave which emerged along the axis a would reach the surface successfully; while those portions which FIG. 97. Diagram illustrating Linear Earthquakes. FIG. 9a struck against the strata of the flanks would be partially or wholly quenched. The mode of outcrop on the surface is shown in the map- view, Fig. 98, in which a is the epicentrum, b b the granite axis, and c c the stratified flanks. The velocity of the surface-waves, as observed in many cases of severe earthquakes, is about twenty miles a minute. This accords well with Mallet's experiments in granite. In some earthquakes the velocity has been found to be twelve to fifteen miles (Mallet's results in slate), and in some as high as thirty to thirty-five miles per minute. In no great earthquake has the velocity been found higher than the last mentioned. In some slight shocks, however, occurring recently in New England, the velocity, as determined by telegraph, is estimated as high as one hundred and forty miles per minute, or 12,000 feet per second. This amazing difference may be fully explained : It will be re- membered that the velocity of the surface- wave is infinite at the epi- centrum, and diminishes, according to a law already discussed, until it reaches, or nearly reaches, the velocity of the spherical wave. Now, if the earthquake-focus be comparatively shallow, the initial velocity of the surface-wave very rapidly approaches its minimum, and therefore the observed velocity of the surface-wave may be taken as nearly the same as that of the spherical wave ; but, if the earthquake be very deep, the diminution, even on a plane surface, is far less rapid ; and when we take into consideration the curvature of the earth-surface, it is evident that the velocity of the surface-wave is always and for all distances much greater than that of the spherical wave. This would well account for velocities of thirty to thirty-five miles, but not for one 8 114 IGNEOUS AGENCIES. hundred and forty miles. This latter is accounted for by another principle. We have seen that these high velocities occur only in slight shocks. Now, while heavy shocks (large and high waves) break the medium at every step of their passage, and are therefore retarded, as already explained, slight tremors (small and low. waves) are successfully trans- mitted without rupture, and therefore run with the natural velocity belonging to the medium, i. e., the velocity of sound. Now, the velocity of sound in granite is probably about 12,000 feet per second, or one hundred and forty miles per minute. Vorticose Earthquakes. In these cases the ground is twisted or whirled round and back, or sometimes ruptured and left in a twisted condition. The most conspicuous examples of this kind of motion occurred in the earthquake of Riobamba, and in the great Calabrian earthquake of 1783. In this latter earthquake the blocks of stone forming obelisks were twisted one on another ; the earth was broken and twisted, so that straight rows of trees were left in interrupted zig- zags. Phenomena similar to some of these were observed also in the California earthquake of 1868. Chimney-tops were separated at their junction with roofs, and twisted around without overthrow ; wardrobes and bureaus turned about at right angles to the wall, or even with their faces to the wall. Explanation. Some of these effects such as twisting of obelisks and chimney-tops, and turning about of bureaus, etc. may be ex- plained, as Lyell has shown, without any twisting motion of the earth at all or any other than the backward and forward motion common to all earthquakes. Thus, if we place one brick on another, and shake them back and forth, holding only the lower one, they are almost cer- tain to be left twisted one on the other. The reason is, that the adhe- sion is almost certain to be greater toward one end than the other the centre of friction does not coincide with the centre of gravity. This is the probable explanation of twisted obelisks and chimney -tops, etc. Also, the simple back-and-forth shaking of a wardrobe in a diagonal direction would almost certainly lift up one end and swing it around. The vorticose motion in such cases is probably not real, but only apparent. But there are other cases of undoubtedly real vorticose motion ; as, for example, straight rows of trees changed into interrupted zigzags by fissures and displacement. All such cases of real twisting are prob- ably explicable on the principle of concurrence and interference of waves. If two systems of waves of any kind meet each other, there will be points of concurrence where they reenforce each other, and points of interference where they destroy each other. Suppose, for instance, a system of water-waves, represented by the double lines i, i EARTHQUAKES. 115 (Fig. 99), running in the direction b , strike against a wall, w w: the waves would be reflected in the direction c c, and are represented by the single lines r, r. Then, if the lines represent crests, and the inter- vening space the troughs, at the places marked with crosses and dots there would be concurrence, and therefore higher crests and deeper FIG. 99. Diagram illustrating Reflection of Waves. troughs, while at the points indicated by a dash there would be inter- ference and mutual destruction, and therefore smooth water. The same takes place in earth- waves. If two systems of earth-waves meet and cross each other, we must have points of concurrence and interference in close proximity. The ground, therefore, will be thrown into violent agitation points in close proximity moving in opposite directions (twisting). If the motion be sufficient to rupture the earth, restoration is not made by counter-twisting, and the earth is left in a displaced condition. The causes of interference may be various sometimes difference of velocity of waves, already explained, by which some overrun others, concurring and interfering; more often it is the result of reflection from surfaces of different elasticity. For example, it is well known that the most violent effects of earthquakes, especially twisting of the ground, usually occur near the junction of the softer strata of the plains with the harder and more elastic strata of the mountains. Now, suppose from a shock at X (Fig. 100) a system of earth-waves should emerge at a, and run as a surface-wave toward the mountain m. The 116 IGNEOUS AGENCIES. waves, striking the hard, elastic material m, would be partly trans- mitted and partly reflected. The reflected waves, running in the direc- tion of the arrow r, would meet the ad- vancing incident waves moving in the direc- tion of the arrow i, and N / concurrence and inter- '. ference would be in- evitable. Minor Phenomena. kinds of earthquakes, but many of the minor phenomena, are explained by the wave-theory. 1. Sounds. These are usually described as a hollow rumbling, roll- ing, or grinding / sometimes as clashing, thundering, or cannonading. They are probably produced by rupture of the earth at the origin, and by the passage of the wave through the imperfectly elastic rocky medium, breaking the medium and grinding the broken parts together. But what is especially noteworthy is, that these sounds precede as well as accompany the shocks. In every earthquake there are transmitted waves of every variety of size. The great waves are sensible as shocks, or jars, or tremors ; the very small waves, too small to be appreciated as tremors, are heard as sounds. But, as already explained, these last run with greater velocity in an imperfectly coherent and elastic medium like the earth, and therefore arrive sooner than the great waves, which constitute the shock. The same was observed in Mallet's experiments. 2. Motion. As to direction, the observed motion is sometimes verti- cally up and down, sometimes horizontally back and forth, and some- times oblique to the horizon. Almost always a rocking motion, i. e., a leaning of tall objects first in one direction and then in the other, is observed. As to violence or velocity of motion, this is sometimes so great that objects are thrown into the air, and whole cities are shaken down as if they were a mere collection of card-houses, while in other cases only a slow swinging, or heaving, or gentle rocking, is observed. The difference in direction is wholly due to the position of the ob- server. At the epicentrum it is of course vertical, and thence it be- comes more and more oblique, until at great distances it is usually horizontal. The violence of the shock or velocity of ground-motion de- pends partly upon the violence of the original concussion, and partly on the distance from the origin or focus. This velocity of the ground- motion must not be confounded with the velocity of the wave already discussed. The latter is the velocity of transit from place to place ; the EARTHQUAKES. 117 former is the velocity of oscillation up and down, or back and forth. The velocity of oscillation has no relation to the velocity of transit, but depends only on the height of the wave, which constantly diminishes and becomes finally very small, though the velocity of transit remains the same, and always enormously great. The rocking motion is also easily explained. A series of waves, somewhat similar in form to water- waves (though differing in nature), actually passes beneath the observer. Of course, when an object is on the front-slope, it will lean in the direc- tion of transit ; and, when on the hind-slope, in the contrary direction. 3. Circle of Principal Destruction. In some earthquakes a certain zone at considerable distance from the point of first emergence (epicen- trum) has been observed, in which the destruction by overthrow is very great, and beyond which it speedily diminishes. This has been called the circle of principal destruction or overthrow. It is thus explained : The overthrow of buildings depends not so much on the amount of oscil- lation as upon the horizontal element of the oscillation. Now, the whole amount of oscillation is greatest at the point of first emergence, and decreases outward ; but the horizontal element is nothing at , and in- creases as the cosine E. Therefore, under the in- fluence of these two con- ditions, one decreasing the whole oscillation, the other increasing the hor- izontal element of that Oscillation, it is evident FlG ' 101 - Dia g ram illustrating Circle of Principal Disturbance. that there will be a point on every side, or, in other words, a circle, where a maximum. This is shown in Fig. \ the horizontal element will be 101, in which a a', b ', c c', etc., are the decreasing oscilation, and b b\ c c", are the horizontal element. This reaches a maximum at c. It has been found by mathematical calculation, based upon the supposition that the whole oscillation varies inversely as the square of the distance from JT, that the horizontal element will be a maximum when the angle of emergence is 54 44'. By determining by observation the circle of principal disturbance, it is easy to calculate the depth a X of the focus, for it will be the apex of a cone whose base is that circle, and whose apical angle is 70 32'. 4. Shocks more severely felt in Mines. It has been sometimes ob- served that shocks are distinctly felt in mines which are insensible at the surface. This is probably explained as follows : Let 8 S (Fig. 102) be the surface of the ground ; and let a b represent hard, elastic strata, covered with loose, inelastic materials, c c. Now, if a series of waves 118 IGNEOUS AGENCIES. come in the direction of the arrows d <#, and, passing through a b on their way to the surface, strike upon the lower surface of c c, a portion would reach the surface by refraction, but a portion would be reflected and return into a 5, concurring and interfering with the advancing waves, and producing great commotion in these strata. 5. Shocks less severe in Mines. This case is probably more common than the last. It was notably the case in the earthquake of 1872 in FIG. 102. Shocks in Mines. Inyo County, California. While the surface was severely shaken, and many houses destroyed, and large fissures formed in the earth, the miners, several hundred feet below the surface in the hard rock, scarcely felt it at all. This fact has scarcely been noticed, and no attempt has been made to explain it. 6. Bridges. In a somewhat similar manner are to be accounted for the phenomena of bridges. In the earthquake-regions of South America there are certain favored spots, often of small extent, which are partially exempt from the shocks which infest the surrounding country. The FIG. 108. earthquake-wave seems to pass under them as under a bridge to reap- pear again on the other side. The mere inspection of Fig. 103 will explain the probable cause of this exemption, viz. : reflection from the under surface of an isolated mass of soft, inelastic strata, c c. 7. Fissures. The ground-fissures, so commonly produced by earth- quakes, are sometimes of the nature of the great fissures of the crust, which are the probable cause of earthquakes. Such great fissures are EARTHQUAKES ORIGINATING BENEATH THE OCEAN. H9 usually wholly beneath the surface at great depth, but sometimes may break through and appear on the surface. This is certainly the case when decided faults occur with elevation or depression of large tracts of land. But the surface-fissures so frequently described, small in size, very numerous, and running in all directions, have an entirely different origin. They are evidently produced by the shattering of the softer, more incoherent, and inelastic surface-soil, and by the passage of the earth-wave. Even the more elastic underlying rock is broken by the same cause, but to a much less extent. Earthquakes originating beneath the Ocean. We have thus far spoken of earthquakes originating beneath the land-surface. But three-fourths of the earth-surface is covered by the sea ; and we have already seen that other forms of igneous agency are most abundant in and about the sea. As we might expect, therefore, the greater number of earthquake-shocks occur beneath the sea-bed. In such, the phenomena already described are complicated by the ad- dition of the " Great Sea - Wave." Suppose, then, an earthquake-shock to occur beneath the sea-bed : the following waves will be formed : 1. As before, a series of elastic spherical waves will spread from the focus, until they emerge on the sea-bed. 2. As before, a series of circular surface-waves, the outcrop of the spherical waves, will spread on the sea-bottom until they reach the nearest shore, and perhaps produce destructive effects there. 3. On the back of this submarine earth-wave is carried a corresponding sea-wave. This is called the "forced sea-wave," since it is not a free wave, but a forced accompaniment of the ground-wave beneath. It reaches the shore at the same time as the earth-wave. It is of little importance. 4. In addition to all these is formed the great sea-wave or tidal wave. Gfreat Sea- Wave. This common and often very destructive accom- paniment of earthquakes is formed as follows : The sudden upheaval of the sea-bad lifts the whole mass of superincumbent water to an equal extent, forming a huge mound. The falling again of this water as far beloio as it was before above its natural level generates a circular wave of gravity, which spreads like other water-waves, maintaining its origi- nal wave-length, but gradually diminishing its wave-height until it be- comes insensible. Usually, a series of such waves is formed by the motion of the sea-bottom up and down several times. These waves are often 100 to 200 miles across their base (wave-length) and fifty to sixty feet high at their origin. Their destructive effects may be inferred from the enormous quantity of water they contain. In the open sea they create no current, and are not even perceived, but, when they touch bottom near shore, they rush forward as great breakers fifty or sixty feet high, sweeping away everything in their course. 120 IGNEOUS AGENCIES. Being waves of gravity, their velocity, though very great on account of their size, is far less than that of the earth-waves, and they reach the neighboring shore, therefore, some time later, and often complete the destruction commenced by the earth-waves. Examples of the Sea-Wave. In the great earthquake which de- stroyed Lisbon in 1755, the epicentrum was on the sea-bed fifty or more miles off the coast of Portugal. From this point the surface earth- waves spread along the sea-bottom until they reached shore. It was the arrival of these waves which destroyed Lisbon. About a half-hour later, when all had become quiet, several great sea-waves, one of them sixty feet high, came rushing in, deluging the whole coast and com- pleting the destruction commenced by the earth-waves. This wave was thirty feet high at Cadiz, eighteen feet at Madeira, and five feet on the coast of Ireland. It was sensible on the coast of Norway, and even on the coast of the West Indies, after having crossed the whole breadth of the Atlantic. In 1854 a great earthquake shook the coast of Japan. Its focus was evidently beneath the sea-bed some distance off the coast, for, in about a half-hour, a series of water-waves thirty feet high rushed upcn shore and completely swept away the town of Simoda. From the same cen- tre the waves, of course, spread in the contrary direction, traversed the whole breadth of the Pacific, and in about twelve and a quarter hours struck on the coast of California at San Francisco, and swept down the coast to San Diego. These waves were thirty feet high at Simoda, fif- teen feet high at Peel's Island, about 1,000 miles off the coast of Japan, 0.65 feet, or eight inches, high at San Francisco, and six inches at San Diego. 1 On the 13th of August, 1868, a great earthquake desolated the coast of Peru. Its focus was evidently but a little way off shore, for in less than a half-hour a series of water-waves fifty or sixty feet high rushed in and greatly increased the devastation commenced by the earth-waves. These waves reached Coquiinbo, 800 miles distant, in three hours; Honolulu, Sandwich Islands, 5,580 miles, in twelve hours ; the Japan coast, over 10,000 miles, the next day. They were also observed on the coast of California, Oregon, and Alaska, over 6,000 miles in one direction, and on the Australian coast, nearly 8,000 miles in another direction. This series of waves was distinctly sensible at a distance of nearly half the circumference of the earth. Had it not been for the barrier of the South American Continent, it would have encircled the globe. 9 There are several points in the above description which we must very briefly explain : 1. The velocity of these great sea-waves, though less than that of 1 " Report of Coast Survey for 1862." 2 " Report of Coast Survey for 1869." EARTHQUAKES ORIGINATING BENEATH THE OCEAN. J21 the earth-waves, is still very great in comparison with ordinary sea- waves. The waves of the Japan earthquake crossed the Pacific to San Francisco, a distance of 4,525 miles, in a little more than twelve hours, and therefore at a rate of 370 miles per hour, or over six miles per minute. The waves of the South American earthquake of 1868 ran to the Hawaiian Islands at a rate of 454 miles per hour. This amazing velocity is the result of the great size of these waves. 2. The size of these great waves is determined by multiplying the time of oscillation by the velocity, on the well-known principle that every kind of wave runs its own length during the time of one com- plete oscillation. The velocity is obtained by observing the time at different points. The time of oscillation is determined by means of tidal gauges. The tidal gauges established by the coast survey on the Pacific coast showed that the time of oscillation of the larger waves of the Japan earthquake was about thirty-three (thirty one to thirty-five) minutes. This would give a wave-length of a little over 200 miles. It is probable that the wave-length in the case of the South American earthquake was at least equally great. 3. The distance to which the sea-waves run is far greater than that of the earth-waves. The former is distinctly sensible for 10,000 miles ; the latter very rarely more than a few hundreds. There are two rea- sons for this : 1. All waves diminish in oscillation (wave-height) as they spread from the origin, because the quantity of matter successively involved in the oscillation constantly increases. But in the one case the matter involved lies in the circumference of a fcircle ; in the other, in the surface of a sphere ; therefore, the one increases as the distance, the other as the square of the distance. Therefore, the decrease of os- cillation (height of wave) is far less rapid for water-waves than for elas- tic spherical waves. 2. A still more effective reason is this: Water- waves run in a perfectly homogeneous medium, and therefore diminish only according to the regular law just stated ; but the earth-waves run in an heterogeneous, imperfectlv elastic, and imperfectly coherent me- dium, and therefore they are rapidly quenched and dissipated by re- peated refractions and reflections, and by repeated fractures of the medium, and thus changed into other forms of force, as heat, electrici- ty, etc. Were it not for this, the destructive effects of earthquakes would be far more extensive. 4. We have said the wave-length remains unchanged. This length, therefore, represents the diameter of the original water-mound, and therefore of the original sea-bottom upheaval. In the Japan earth- quake this was 200 miles across. This shows the grand scale upon which earthquake-movements take place. 5. As already explained, earthquake sea-waves differ from all other sea-waves in that their great size makes them drag bottom even in open 122 IGNEOUS AGENCIES. deep sea. In their case, therefore, the velocity depends not only on the wave-length, but also on the depth of the sea. Knowing the size (wave-length) of these waves, and therefore what ought to be their free velocity, and also knowing their actual velocity by observation, the difference gives the retardation by dragging ; and by the retarda- tion may be calculated the mean depth of the ocean traversed. In this Avay it has been determined that the mean depth of the Pacific between Japan and San Francisco is 12,000 feet, and between Peru and Hono- lulu, Sandwich Islands, 18,500 feet. The great importance of such results is obvious. Depth of Earthquake-Focus. The great obscurity which hangs about the subject of the interior condition of the earth and the ultimate cause of igneous agencies ren- ders any positive knowledge on these subjects of peculiar interest. There can be little doubt that the phenomena of earthquake-waves, their form, their velocity, their angle of emergence, etc., if once thoroughly under- stood, would be a most delicate index of this condition, and a powerful means of solving many problems which now seem beyond the reach of science. Among problems of this kind none is more important, and at the same time more capable of solution, than the depth of the origin of earthquakes, and therefore presumably of volcanoes. Seismometers. The most direct way of determining the depth of an earthquake-focus is by means of well-constructed seismometers. These are instruments for measuring and recording earthquake-phenom- ena. They are of infinite variety of forms, depending partly upon the facts desired to be recorded, and partly upon the mode of record. As examples we will mention only two : An excellent instrument for recording slight tremors is one invented and used by Prof. Palmieri, of the Vesuvian Observatory. It consists of a telegraphic apparatus with the usual paper-slip and stile. The paper-slip, accurately divided into hours, minutes, and seconds, travels at a uniform rate by means of clock-work. The battery-circuit is closed and opened, and the recording stile worked by the shaking of a metallic bob, hung by a delicate spiral spring above a mercury-cup ; the shak- ing of the bob being determined by the tremor of the earth. Such an instrument records the exact moment of occurrence of earthquake-shocks, however slight ; also, the moment of passage of every wave and its time of oscillation ; and if there be more than one such instrument, the mo- ment of occurrence at different places gives the velocity of the surface- wave v'. It records, however, rather than measures earthquake-phe- nomena ; it is a seismograph rather than a seismometer. The best form of seismometer which we have seen described that which gives the most important information is that of Prof. Cavalleri, of Monza. It consists essentially of two pendulums, one horizontally DEPTH OF EARTHQUAKE-FOCUS. 123 and the other vertically oscillating (Figs. 104 and 105). The former (Fig. 104) is an ordinary pendulum, with a heavy bob, b, armed with a stile which touches a bed of sand, s s. The sharp point of the stile rests loosely in a slight depression in a small flat cylinder or button, c, resting lightly on the top of the firm column d. When the earthquake- shock arrives, the whole building, and therefore the attachment a, above, and the bed of sand, s s, on the floor, will move in the direction of the shock. This direction will generally be partly horizontal and partly vertical (x b,xc,x d, Fig. 94). We will consider now only the hori- V d FIGS. 104 and 105. Cavalleri's Seismometer. zontal element. The pendulum, b, will tend to retain its position, and the bed of sand will move beneath it, first in one direction and then in the other, and the stile will thus mark the sand back and forth to a dis- tance equal to the back-and-forth motion of the earth. The direction from which the impulse came is determined by the side on which the little cylinder falls. It is easy to connect the pendulum with a clock set at twelve, in such wise that the motion of the former will in- stantly set agoing the latter. The difference between this clock-time and the real time will give the instant of transit. It is clear that this pendulum does not give the whole amount of the vibration or motion of the shock, but only the horizontal element. If a b (Fig. 106) repre- sent the direction and amount of vibration, then a c is the horizontal element measured by the pendulum. This instrument, therefore, gives the moment of transit, the direction of transit, and the horizontal element of vibration. The vertical element, b c, of the vibration is FIG. 106. 124 IGNEOUS AGENCIES. given bv a vertically oscillating pendulum (Fig. 105), the point of which rests lightly on one arm, a, of a very easily-moved lever, the other arm, b, of which acts as an index by means of a graduated quadrant. When the shock moves the floor of the building upward, the heavy weight of the pendulum retaining its position by stretching of the wire spring, the arm a is pressed against the stile, and the arm b is elevated ; when the floor descends again, b is retained in its elevated position by a ratchet at "ig. a is after Wyville Thomson ; the others are after Williamson. All the figures are greatly enlarged (after Nicholson). these shells. Diatoms live also in great numbers in the hot springs of California and Nevada, and the deposits of such springs sometimes con- sist wholly of these shells. Thick strata, belonging to earlier geological times, are found wholly composed of diatoms. We are thus able to explain the formation of these strata. GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 155 Deep-sea Deposits. Over nearly all the bottom of deep seas, be- yond the reach of sedimentary deposits, we find a white, sticky ooze, composed of the carbonate-of-lime shells of microscopic animals (fora- minifers) and microscopic plants (coccospheres), Fig. 127. Some of these seem to be living, or recently dead ; some dead and empty, but still perfect ; but most of them completely disintegrated. On account of the great abundance of the shells of one form of foraminifera, this soft, white mud is called globigerina ooze. Mingled in considerable numbers among the calcareous shells are others of silica. These are also partly animals (radiolaria) and partly plants (diatoms). The extraordinary resemblance of this deep-sea ooze, both in chemical and microscopic character, to chalk, leaves no room for doubt that chalk was formed in this way. SECTION 4. GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. Fauna and Flora. The animals and plants inhabiting any country are called the fauna and flora of that country. In a more scientific sense, however, a natural fauna or flora is a group of organisms in- habiting one locality, differing conspicuously from other natural groups inhabiting other localities. All the members of such a natural group must bear certain harmonic relations to one another, and the whole group to the external physical conditions. Moreover, each group is circumscribed and separated from other neighboring groups by limiting physical conditions. Kinds of Distribution. Distribution of organisms is of two general kinds, viz., distribution in space and distribution in time, or geographi- cal distribution and geological distribution. There are, therefore, geo- graphical faunae and floras and geological faunae and florae. A geographi- cal fauna is the group of animals inhabiting any natural geographical region. Thus the animals of Australia form a distinct fauna, differing entirely from any other upon the earth's surface. A geological fauna is the whole group of animals inhabiting the earth at one epoch, and differing from that of other epochs. Thus the whole group of animals inhabiting the earth during what geologists call the secondary period form a distinct fauna, differing remarkably from all preceding or sub- sequent faunae. The flora of the coal period is very distinct from all others. The organisms of every epoch, however, were distributed over the earth's surface in separate faunas and florae. Every geological fauna and flora is, therefore, divisible into more or less distinct geographical faunae and florae. Geological faunae and florae will form the principal subject of Part III. We propose now to study only geographical dis- tribution of organisms at the present time, this portion of geology being concerned only with " causes now in operation." We study the 156 ORGANIC AGENCIES. laws of geographical distribution in the present epoch because it throws light on the geographical distribution in previous epochs, and also on the laws of geological distribution, or the history of organisms. It also, as will be shown hereafter, furnishes a key to former changes in physical geography and former migrations of species. Among physical conditions limiting the distribution of organisms, one of the most important is temperature. We will, therefore, first speak of temperature-regions, confining ourselves, for the sake of greater clearness, to plants. The principles thus established we will then ex- tend and modify. And, further, since temperature-regions may be either vertical or horizontal in latitude, we will commence with Vertical Botanical Temperature-Regions. To explain vertical dis- tribution we will take the case of a mountain, at or near the equator, because all the vertical regions are there represented. If we pass from base to summit of such a moun- tain we will traverse, first, a region of palms, so called be- cause the vegetation is char- acterized by the abundance of palms and palm-like plants, such as bananas, tree-ferns, etc. The second region traversed is char- l acterized by the prevalence of FlG - m evergreens, such as myrtles, lau- rels, etc., and ordinary deciduous trees, such as hickory, oaks, elms, poplars, etc. ; and therefore may be called the region of ordinary forest or hard-wood trees. The third region traversed is characterized by the prevalence of pines and other conifers, and therefore called the region of pines. The fourth region contains few or no trees, but only shrubs and Alpine herbaceous plants, and therefore may be called the Alpine or treeless region. The fifth, being the region of perpetual snoic, is plantless, or nearly so. Botanical Temperature-Regions in Latitude. As the regions above spoken of are determined entirely by temperature, it is evident that they must be reproduced in latitude in zones where these limiting temperatures successively reach the earth's surface. Thus, if a (Fig. 129) represents an equatorial mountain, the temper- atures which limit the botanical regions will approach the earth as we go toward either pole, as shown by the dotted lines, and successively reach the sea-level, giving rise to similar zones of temperature, and therefore to similar botan- ical regions, extending all around the earth. HARD WOOD TRESS PALMS AND TREE -FEffAIS GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 157 These zones, being temperature-zones, are not limited by parallels of latitude as represented in the figure, but by isothermal lines. In pass- ing from the equator to the poles, we traverse : 1. A region of palms, or tropical zone ; 2. A region of ordinary forest or hard-wood trees, ever- green and deciduous, or subtropical and temperate zone ; 3. A region of pines and birch, or cold temperate and subarctic zone ; 4. A treeless region, or arctic zone. The fifth or plantless region can hardly be said to exist at the sea-level in any part of the earth. Further Definition of Regions. 1. The regions we have given are characterized by the prevalence of certain orders of plants ; but the same law of limitation applies with still greater force to families, genera, and species. These smaller classification-groups are still more limited in range. Thus palms range over the whole of region No. 1 (Fig. 129) ; but one genus of palms may occupy the warmer or equatorial part, and another genus the higher or tropical parts. Thus, generally, the range of families is more restricted than that of orders, the range of genera than that of families, and the range of species than that of genera. Thus, these regions may be divided into subordinate regions, and these again into still more subordinate regions. What we further say will have reference principally, though not entirely, to species. 2. We have separated these regions by lines. It must not be supposed, how- ever, that these limits are distinctly marked. On the contrary, they shade insensjbly into each other. Some species of palms, etc., pass into the region of hard- wood trees, and vice versa. Many species of hard- wood trees pass into the region of pines, and vice versa. So, also, the sub-regions of families, genera, and species, cannot be separated by hard lines. They shade insensibly into, interpenetrate, or over- lap one another at their margins. Thus if a a' and b b' (Fig. 130) be the range, either vertical or horizontal, of two species, then in the zone b a' the two species coexist. 3. In any region or sub-region the organic forms which characterize it are most abundant in the middle portion, and become less and less abundant toward the margin, whe.re they disappear. If the line a a' (Fig. 130) represents the range of any species, then the breadth of the elliptical area will represent the relative abundance of individuals in different parts of the range. 4. Although, therefore, species, so far as numbers of individuals are concerned, come in gradually on the margin of their nat- ural region, reach their greatest abundance in the middle por- v f tion, and again gradually die out on the other margin, yet in ^ specific characters we see usually no such gradual transition. In specific character they seem to come in suddenly, to remain sub- stantially unchanged throughout their range, and pass out suddenly on the other margin. Thus, to take a single instance : in passing from 158 ORGANIC AGENCIES. the equator to the poles, at a certain latitude, the sweet-gum or liquid- amber tree first appears, few in number; it increases in number in the middle part of its range, and finally again diminishes in number and gradually disappears. But throughout its whole range this species is unmistakably the same it does not pass into any other species. It is as if the species had originated somehow (we will not now discuss how) in the area where we find it, and had extended its range as far as physical conditions and the struggle for life with other species would permit. 5. We have seen that the botanical zones in elevation and in latitude are similar to one another in the great orders which characterize them ; but they are by no means identical in genera and species. This follows from what we have said under 4. The vertical and horizontal zones No. 1 being in direct connection with one another, the species are to a large extent identical. But between zones No. 2 communication is impossible, except through zone No. 1, but this is forbidden by physical conditions ; and, therefore, although forest-trees may exist in both, the species are all different. The same is true of zones Nos. 3 and 4, and also of corresponding zones north and south of the equator. It is as if the present species had originated in the areas where we now find them, and had not been able to mingle on account of temperature barriers intervening. 6. Although, when no physical obstacle intervenes, regions or zones in latitude like those in elevation shade insensibly into one another by interpenetration, as already ex- plained, yet when physical barriers, such as an east-and-west mountain- chain, occur, no such shading is possible ; but, on the contrary, there is an abrupt change. Thus, north and south of the Himalaya Moun- tains, or north and south of the Sahara Desert, the plants are entirely different, apparently because interpenetration of contiguous flora? by spreading is impossible in this case. Zoological Temperature-Regions. We have spoken first of plants, because, being fixed to the soil, they illustrate more clearly the natural laws of distribution as determined by temperature. Animals, by their power of locomotion and migration with the seasons, interfere seriously with the simplicity of these laws. Although families, genera, and spe- cies of animals, like plants, are limited in their range, particular form* characterizing certain zones as monkeys, parrots, elephants the torrid zone, and walruses and white bears the polar zone yet it is impossible to divide the surface of the earth into zones characterized by particular orders in the same broad, general way as in the case of plants. Never- theless, all that we have said concerning the limitation of range of families, genera, and species, and the manner in which contiguous regions or sub-regions shade into one another, applies with equal force to animals. The apparent fixity of animal species within certain nar- row limits of variation is even more striking than in the case of plants. GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 159 What we shall further say will apply to animals and plants without distinction. Continental Fauna and Flora. If no physical barriers intervened, there seems to be no reason why the fauna and flora of each zone of temperature should not be continuous all around the earth. But im- passable barriers exist in the form of oceans separating continents, and on the continents in the form of north-and-south ranges of mountains. Consequently, on the supposition of the local origin of species, it fol- lows that, although the orders and sometimes families of animals and plants are similar on the two continents, the species and many of the genera are entirely different. That this diversity is the result of impas- sable barriers is sufficiently proved by the fact that most species of animals or plants, introduced from one continent to the correspond- ing zone of another, flourish well, and soon become permanent mem- bers of the fauna or flora of their adopted country. We will now dis- cuss the subject in more detail. The accompanying figure (Fig. 131) is a view of the northern hemi- sphere, the circumference being the equator and the centre the north- pole, and the circular zones being the same as already described. These, however, will not be regular circles as represented in the figure, but will be isothermal zones. They really run farther north in Europe, 160 ORGANIC AGENCIES. and farther south in North America, than represented in the figure. Now, comparing the eastern and western continents with one an- other, commencing with the arctic zone No. 4, we find that in this zone the fauna and flora are nearly identical in the two continents, the reason being, apparently, their close approximation to one an- other in this zone, and their connection by means of solid ice. In the next zone, No. 3, the species are already quite different ; and in No. 2, to which the United States and Europe mostly belong, nearly all the species, and many of the genera, and even some families, are dif- ferent. The few exceptions to the universal diversity of the fauna and flora of this country, as compared with Europe and Asia, are prin- cipally : 1. Introduced species ; 2. Species of wide range, either by reason of great hardihood or by extensive migration, and which, there- fore, belong to No. 4 as well as Nos. 3 and 2; and, 3. Alpine spe- cies, which seem to have extended, during a former cold epoch (Glacial epoch), from No. 4 to No. 3 in both continents, and with the return of milder climate have retreated, some northward, and some up the sides of the mountains of No. 3, to their appropriate zone of tempera- ture. In No. 1 the difference between the two continents is still greater, and continues without abatement into corresponding zones of the southern hemisphere, since these do not approach each other tow- ard the pole as they do at the north. Thus, we find the fauna and flora of South America and Africa as different as possible. As an illus- tration of this, we will only mention the prehensile-tailed monkeys, sloths and armadillos, llamas and toucans, humming-birds, among ani- mals, and the cacti among plants, as characteristic of South America ; and the lions, tigers, elephants, rhinoceroses, hippopotamuses, giraffes, and the tailless monkeys, of Africa. Subdivisions. The continental faunae and florae are again subdi- vided in longitude by north-and-south mountain-chains. Thus the fauna and flora of the United States are divided by the Rocky Mountain and Appalachian chains into three sub-faunae and sub-floras, viz., an Atlantic slope, an interior continental, and a Pacific slope fauna and flora. The difference between the Atlantic slope and the interior continental region is not great, because the mountain-barrier is not so high but it may be overpassed. The Rocky Mountains being a wider and higher and therefore the more impassable barrier, the fauna and flora of the Pacific slope are very distinct, almost all its species being peculiar to that region. The exceptions are mostly strong-winged birds. In a similar manner in South America the Andes chain separates faunae and florae which are very distinct, and in the eastern continent the Ural Mountains separate a European from an Asiatic fauna and flora. Sub- divisions of this kind are more marked in the case of plants and of those animals which are closely connected with plants, such as insects, than in GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 101 the case of higher animals which have a greater power of locomotion, and therefore of overcoming obstacles. Special Cases. We might mention many special cases of remarkable groups of animals and plants, especially on isolated islands. We will only mention a few by way of illustration : 1. The fauna and flora of Australia are perhaps the most remarkable in the world. Not only are the species different, but the genera, families, and orders, are peculiar to this continent. So remarkably and conspicuously is this the fact, that even the unscientific traveler is at once struck with the strange ap- pearance of the vegetation and the animals trees with narrow, rigid leaves twisted on their leaf-stalk so as to turn their edges to the sky; the mammals, about 200 species, nearly all belonging to the non- placentals, including marsupials and monotremes, a great sub-class of quadrupeds to which the kangaroos, the opossums, and the ornitho- rhynchus belong, and which are confined to Australia, with the excep- tion of several species of opossum found in America. The island of Madagascar is another remarkable zoological province. All the ani- mals on this island, with one single exception, are peculiar, being found nowhere else. This exception is that of a small quadruped sup- posed to have been introduced. On the Galapagos, a small group of islands at a considerable distance from the west coast of South Amer- ica and from all other islands, all the animals are entirely different from those of any part of the world. Reptiles of peculiar species abound, but no mammal, except one species of mouse, has yet been found. Thus we see that species are limited in one direction by tempera- ture, and in all directions by physical barriers. If we now add to these limitations also peculiar climates and soils (such, for example, as the dry plains of Utah and Arizona), which limit vegetation, and there- fore animals, we easily perceive that all these limiting causes produce groups of species confined within certain areas differing from other groups, sometimes overlapping them, sometimes trenchantly separated. Taking all causes into consideration, the whole earth has been divided into six principal faunal regions, viz. : 1. JVearctic, including North America, exclusive of Central America ; 2. Neotropic, including Central and South America ; 3. Palcearctic, including Europe, North Africa, and Asia north of the Himalayas ; 4. African, including Africa south of the Sahara ; 5. Indian or Oriental, including Asia south of the Himalayas, and the adjacent islands ; 6. Australian, including Aus- tralia, New Zealand, New Guinea, and South-Sea Islands, etc. These primary regions are subdivided into provinces and sub-provinces accord- ing to the principles already explained. For example, the neartic has been subdivided into four provinces, viz., (a) the Alleghanian, (b) the Rocky Mountain, (c) the Californian, and (d) the Canadian. 11 162 ORGANIC AGENCIES. Marine Fauna. Distribution in Latitude. In passing along the shores of Europe or of America, from south to north, we find that the species of marine animals, such as molluscous shells and fishes, gradu- ally change, one species being replaced by another in the manner al- ready explained. If the change of temperature be gradual, the change of fauna will also be gradual ; but if the change, from any cause, be sudden, the change of species will be correspondingly sudden. Thus, for example, on the coast of the United States, Cape Hatteras and Cape Cod divide the littoral fauna into three quite distinct subdivisions, changing somewhat suddenly at these points, viz., a Southern, a Middle States, and a New England, fauna. The reason is that the Gulf Stream hugs the shore as far as Hatteras, thus carrying the southern fauna northward beyond its natural limit, and then turns away from the coast. On the other hand, the arctic current hugs the New England coast as far as Cape Cod, bringing with it an arctic fauna, and then leaves the surface and goes downward. Distribution in Longitude. Both land and deep sea are impassable barriers to marine species. Hence we find that the marine species on the east and west coasts of each continent, as well as those inhabit- ing the east and west shores of the same ocean, are almost entirely dif- ferent. Thus the marine species on our Atlantic shores are not only different from those of our Pacific shores, but also from those on the Atlantic shores of Europe and Africa. The same is true of the species on the two shores of the Pacific, as compared with one another, or with those of Europe. The exceptions to the general rule, that the marine species of different shores are different, are principally arctic species of wide range, such as whales, etc. Depth and Bottom. It is found that marine species vary with the depth, so that there are littoral species, and deep-water species, and pro- found sea-bottom species. Also the species on sand-bottoms are differ- ent from those on mud-bottoms. Special Cases. The marine fauna of Australia, like its land fauna, is very peculiar, differing from all others, not only in species, but in genera and families. It is also a remarkable fact that some of its fishes belong to families once abundant in the seas everywhere, but now ex- tinct except in these waters. The marine shells of almost every isolated island in the ocean are peculiar. This is still more true of land and fresh-water shells of islands and even of different rivers of the same continent. A remarkable illustration of this is found in the species of the common river-mussels. Almost every large river in the United States has some species of shell peculiar to it. Nearly all the shells of the Altamaha River are peculiar, being found nowhere else on the face of the earth. Thus in all cases species in different localities are different in pro- GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 163 portion to the height or depth and the width of the intervening barriers, and (most important of all) also in proportion to the length of time since these barriers were established. These facts are now so well at- tested that they are used as a basis of reasoning. If two countries now separated have species identical, we are sure that they have been only very recently separated. The substantial identity of the species of Eng- land and those of contiguous Europe shows that the British Isles have been connected with the Continent at a period geologically very recent. The general resemblance, though not identity, of some plants on the Pacific coast and in Japan, produces a strong conviction that the two continents have been formerly connected in the region of the Aleutian Isles. The great distinctness of the fauna of Australia indicates a long period of isolation from all other continents. It is as if there had been a slow change of species in time, and after separation each group had taken its own way, and thus become more and more different. This subject, however, cannot be further discussed at present. PAET II. STRUCTURAL GEOLOGY. CHAPTER I. GENERAL FORM AND STRUCTURE OF THE EARTH. I. Form of the Earth. THE form of the earth is that of an oblate spheroid flattened at the poles. The polar diameter is less than the equatorial diameter by about twenty-six miles, or about -^ of the mean diameter. The highest mountains, being only five miles high, do not interfere greatly with the general form. This form, being precisely that which a fluid body revolving freely would assume, has been regarded by many of the most distinguished physicists as conclusive evidence of the former fluid condition of the earth. The argument may be stated as follows : 1. A fluid body stand- ing still, under the influence only of its own molecular or gravitating forces, would assume a perfectly spherical form ; but, if rotating, the form which it would assume, as the only form of equilibrium, is that of an oblate spheroid, with its shortest diameter coincident with the axis of rotation. Now, this is precisely the form not only of the earth, but, as far as known, of all the planetary bodies. 2. In an oblate spheroid of rotation the oblateness increases with the rapidity of rotation. Now Jupiter, which turns on its axis in ten hours, is much more oblate than the earth. The flattening of the earth is only about -^ of its diameter, while that of Jupiter is about -fa. 3. The forms of the earth and of Jupiter have been calculated ; the data of calculation being the former fluidity, the time of rotation, and an assumed rate of increasing density from surface to centre ; and the calculated form comes out nearly the same as the measured form. FORM OF THE EARTH. 165 This argument, however, only proves that the forms of the planets have been assumed under the influence of rotation, or that they are spheroids of rotation, but not that they have ever been in a fluid condi- tion. For since a rotating body, whatever be its form, always tends to assume an oblate spheroid form, and since the materials on the surface of the earth are in continual motion, being shifted hither and thither under the influence of atmospheric and aqueous agencies, it is evident that the final and total result of such motions must be in the course of infinite ages to bring the earth to the only form of equilibrium of a ro- tating body, viz., an oblate spheroid. If, for example, we have a solid and perfectly spherical earth, standing still and covered, as it would be, by a universal ocean, and then set it rotating, it is evident that the waters would gather into an equatorial ocean, and the land be left as polar continents. But this condition would not remain; for atmospheric and aqueous agencies, if unopposed, would eventually cut down the polar continents and deposit them as sediments into the equatorial seas, and the solid earth would thus become an oblate spheroid. This final effect of degrading agencies would not be opposed by igneous agencies, as the action of these is irregular, and does not tend to any particular form of the earth. Therefore, although there are many reasons, drawn both from ge- ology and from that cosmical theory (the nebular hypothesis) which ac- counts for the formation of worlds, for believing that the earth was once in an incandescent fluid condition, and that it then assumed an oblate spheroid form in obedience to the laws of equilibrium of fluids, yet the form of the earth alone is not only not conclusive proof of this former condition, as has been generally believed, but adds nothing to proofs derived from other sources, since this form is the necessary result of the forces now in operation on the earth-surface, whatever may have been its original form or condition. Further, since the earth from the time of its first consolidation has continued to cool and contract, and therefore to increase its rate of rotation, the degree of oblateness which it now has is not that which it first assumed in its incandescent fluid condition, but much greater. 2. Density of the Earth. The mean density of the earth, as determined by several independent methods, is about 5.6. The density of the materials of the earth-sur- face, leaving out water, is only about 2 to 2.5. It is evident, therefore, that the density of the central portions must be much more than 5.6. This great interior density may be the result 1. Of a difference of ma- terial. It is not improbable that the surface of the earth has become oxidized by contact with the atmosphere, and that at great depths the earth may consist largely of metallic masses. Or the great in- 166 GENERAL FORM AND STRUCTURE OF THE EARTH. terior density may be the result 2. Of condensation by the immense pressure of the superincumbent mass. In either case the tendency of increasing heat would be to diminish the increas- ing density. But how much of the greater den- sity is due to difference of material and how much to increasing pressure, and how much these are counterbalanced by expansion due to increas- ing heat, it is impossible to determine. The increase of density has been somewhat arbitrarily assumed to follow an arithmetical law. Under this condition a density equal to the mean density would be found at radius from the sur- face, and taking the surface density at 2, and the mean density at 5.5, the central density would be 16. In the diagram (Fig. 132), if a c = radius, the ordinate a x = surface density = 2, and b y = mean density = 5.5, then c z, the central density, will be = 16. It is needless to say that this result (Plana's) is unreliable. . 3. The Crust of the Earth. The surface of the earth undoubtedly differs greatly in many respects from its interior, and therefore the exterior portion may very properly be termed a crust. It is a cool crust, covering an incandescent interior ; a stratified crust, covering an unstratified interior ; probably an oxi- dized crust, covering an unoxidized interior; and many suppose a solid crust, covering a liquid interior. This last idea, which, however, we have shown (p. 79) to be very doubtful, has probably given rise to the term crust. The term, however, is used by all geologists, without ref- erence to any theory of interior condition, and only to express that por- tion of the exterior which is subject to human observation. The thick- ness which is exposed to inspection is about ten to twenty miles. Means of Geological Observation. The means by which we are en- abled to inspect the earth below its immediate surface are : 1. Artificial sections, such as mines, artesian wells, etc. These, however, do not pen- etrate below the insignificant depth of half a mile. 2. Natural sections, such as cliffs, ravines, canons, etc. These, as we have already seen (p. 17), sometimes penetrate 5,000 to 6,000 feet. 3. Tilting, and sub- sequent erosion, of the rocks, by which strata from great depths have their edges exposed. Thus, in passing along the surface from a to b (Fig. 133), lower and lower rocks are successively brought under in- spection. This is by far the most important means of observation; without it the study of geology would be almost impossible. 4. Volca- noes bring up to the surface materials from unknown but probably very great depths. GENERAL SURFACE CONFIGURATION OF THE EARTH. 167 Ten miles seem an insignificant fraction of the earth's radius, being, in fact, equivalent to less than one-thirtieth of an inch in a globe two feet in diameter. It may seem at first sight an insufficient basis for a Fio. 133. science of the earth. We must recollect, however, that only this crust has been inhabited by animals and plants on this crust only have oper- ated atmospheric, aqueous, and organic agencies and therefore on this insignificant crust have been recorded all the most important events in the history of the earth. 4. General Surface Configuration of the Earth. The earth-surface is very irregular. The hollows are occupied by the ocean, and the protuberances constitute the continents and islands. Nearly three-quarters of the whole surface is covered by the ocean. The mean heights of the continents are given by Humboldt * as follows : Europe, 671 feet ; North America, 748 feet ; South America, 1,151 feet ; Asia, 1,132 feet. The mean height of all land is probably not far from 1,000 feet. The mean depth of the ocean is probably 12,000 to 15,000 feet (Thomson). There is probably water enough in the ocean, if the ine- qualities of the earth-surface were removed, to cover the earth to a depth of at least 8,000 to 9,000 feet. The extreme height of the land above the sea-level is five miles, and the extreme depth of the ocean is at least as much. The extreme re- lief of the solid earth is therefore not less than ten miles. Cause of Land-Surfaces and Sea-Bottoms. The most usual idea among geologists as to the general constitution of the earth is that the earth is still essentially a liquid mass, covered by a solid shell of twen- ty-five to thirty miles in thickness ; and that the great inequalities, constituting land-surfaces and ocean-bottoms, are produced by the up- bending and down-bending of this crust into convex and concave arches, as shown in Fig. 134. The clear statement of this view is sufficient to 1 " Cosmos," Sabine's edition, vol. i., p. 293. 168 GENERAL FORM AND STRUCTURE OF THE EARTH. refute it ; for, when it is remembered that the arches with which we are here dealing have a span of nearly a semi-circumference of the earth, it becomes evident that no such arch, either above or below the mean level, could sustain itself for a moment. The only condition under which such inequalities could sustain themselves on a supporting liquid FIG. 135. Diagram illustrating the Conditions of Equilibrium of a Solid Crust on a Liquid Interior. is the existence of inequalities on the under surface of the crust next the liquid, similar to those on the upper surface, but in reverse, as shown in Fig. 135. And these lower or under-surface inequalities would have to be repeated not only for the largest inequalities, viz., continental surfaces and ocean-bottoms, but also for great mountain plateaus. And thus the hypothesis breaks down w r ith its own weight. Besides, we have already given good reasons (pages 79 and 80) for believing that the earth is substantially solid. Upon the hypothesis of a substantially solid earth, we explain the great inequalities constitut- ing continental surfaces and ocean-bottoms by unequal radial contrac- tion of the earth in its secular co.oling. It is evident that, in such secular cooling and contraction, unless the earth were perfectly homogeneous, some parts, being more conductive, would cool and contract more rapidly in a radial direction than others. Thus some radii would become shorter than others. The more conduc- tive, rapidly-contracting portions, with the shorter radii, would become sea-bottoms ; and the less conductive, less rapidly -contracting portions, with the longer radii, land-surfaces. In other words, the solid earth in contracting becomes slightly deformed, and the water collects in the depressions. It is only the greatest inequalities, viz., land-surfaces and sea-bot- toms, which we account for in this way. Mountain-chains are certainly formed by a different process, which we will discuss under that head (p. 240) ; and it is even possible that the causes which operate to pro- duce mountain-chains may also produce these greater inequalities. The continuance of these causes would tend constantly to increase the extent and height of the land, and to increase the depth, but dimin- ish the extent of the sea. This, on the whole, seems to have been the fact during the history of the earth, as will be shown in Part III. Nevertheless, local causes, both aqueous and igneous, as already shown in Part I., have greatly modified the general contour, both map and profile, given by secular contraction. Laws of Continental Form. That the general contour of conti- ROCKS. 169 nents and sea-bottoms has been determined by some general cause, such as secular contraction, affecting the whole earth, is further shown by the laws of continental form. The most important of these are as follows : 1. Continents consist of a great interior basin, bordered by elevated coast-chain rims. This typical form is most conspicuously seen in North and South America, Africa, and Australia. Europe- Asia is more irregu- lar, and therefore the typical form is less distinct. We give in Fig. 136, W FIG. 136. A, Section across North America (after Guyot) ; S, Section across Australia (after Guyot). A. and B, an east-and-west section of North America and of Australia, as typical examples of continental structure. The great rivers of the world, e. g., the Nile, Mississippi, Amazon, La Plata, etc., drain these interior continental basins. 2. In each continent the greatest range of mountains faces the great- est ocean. Thus in America the greatest range is on the west, facing the Pacific ; while in Africa the greatest range is on the east, facing the Indian Ocean. In Asia the Himalayas face the Indian Ocean, while the Altai face the Polar Sea. In Australia the greatest range is to the east, facing the Pacific. 3. The greatest ranges have been subjected to the greatest and most complex foldings of the strata, and are the seats of the greatest metamorphism (p. 213) and the greatest volcanic activity. 4. The outlines of the present continents have been sketched in the earliest geological times, and have been gradually developed and per- fected in the course of the history of the earth. In the case of the North American Continent this will be shown in Part III. The cause of some of these laws will be discussed under the head of Mountain-Chains. Rocks. In geology the term rock is used to signify any material consti- tuting a portion of the earth, whether hard or soft. Thus, a bed of sand or clav is no less a rock than the hardest granite. In fact, it is impossible to draw any scientific distinction between materials founded 170 STEATIFIED OR SEDIMENTARY ROCKS. upon hardness alone. The same mass of limestone may be soft chalk in one part and hard marble in another ; the same bed of clay may be hard slate in one part and good brick-earth in another ; the same bed of sandstone may be hard gritstone in one part and soft enough to be spaded in another. The same volcanic material may be stony, glassy, scoriaceous, or loose sand or ashes. Classes Of Rocks. All rocks are divided into two great classes, viz., stratified rocks and unstratified rocks. Stratified rocks are more or less consolidated sediments, and are usually, therefore, more or less earthy in structure and of aqueous origin. Unstratified rocks have been more or less completely fused, and therefore are crystalline in structure and of igneous origin. CHAPTER II. STRATIFIED OR SEDIMENTARY ROCKS. SECTION 1. STRUCTURE AND POSITION. Stratification. Stratified rocks are characterized by the fact that they are separated by parallel division-planes into larger sheet-like masses called strata, and these into smaller layers or beds, and these again into still smaller lamince. These terms are purely relative, and are therefore somewhat loosely used. Usually, however, the term stratum refers to the mineralogical character ; the term layer to sub- divisions of a stratum distinguish- able by difference of color or fine- ness ; and the term lamina to those smallest subdivisions, evidently pro- duced by the sorting power of water. For instance, in the annexed figure, , b, and c, are three strata of sand- stone, clay, and limestone, each di- visible into two layers differing in fineness or compactness of the ma- terial, and all finely laminated by the sorting power of water. The lamination, however, is not represented, except in the clay stratum, b. Extent and Thickness. Probably nine-tenths of the surface of the land, and, of course, the whole of the sea-bottom, are covered with strati- fied rocks. This proves that every portion of the surface of the earth has been at some time covered with water. The extreme thickness of STRUCTURE AND POSITION. Jfl stratified rocks is certainly not less than ten miles ; the average thick- ness is probably several miles. Kinds of Stratified Rocks, Stratified rocks are of three kinds, and their mixtures, viz., arenaceous or sand rocks, argillaceous or clay rocks, and calcareous or lime rocks. Arenaceous rocks, in their inco- herent state, are sand, gravel, shingle, rubble, etc., and in their com- pacted state are sandstones, gritstones, conglomerates, and breccias. Conglomerates are composed of rounded pebbles, and breccias of angu- lar fragments cemented together. Argillaceous rocks, in their inco- herent state, are muds and clays; partially consolidated and finely laminated they form shales, and thoroughly consolidated they form slates. Calcareous rocks are chalk, limestone, and marble. They are seldom in an incoherent state, except as chalk. These different kinds of rocks graduate into each other through intermediate shades. Thus we may have argillaceous sandstones, cal- careous sandstones, and calcareous shales or marls. The most important points connected with stratified rocks we will now, for the sake of greater clearness, bring out in the form of distinct propositions. On these propositions is based nearly the whole of geological reasoning. I. Stratified Rocks are more or less Consolidated Sediments. The evidence of this fundamental proposition is abundant and conclusive. 1. Beds of mud, clay, or sand, as already stated, may often be trace'd by insensible gradations into shales and sandstones. 2. In many places the process of consolidation is now going on before our eyes. This is most conspicuous in sediments deposited at the mouths of large rivers whose waters contain abundance of carbonate of lime in solution, or on the coasts of seas containing much carbonate of lime. Thus the sedi- ments of the Rhine are now consolidating into hard stone (p. 76), and on the coasts of Florida, Cuba, and on coral coasts generally, com- minuted shells and corals are quickly cemented into solid rock (p. 148). 3. All kinds of lamination produced by the sorting power of water which have been observed in sediments, have also been observed in stratified rocks. 4. Stratified rocks contain the remains of animals and plants, precisely as the stratified mud of our present rivers contains river-shells, our present beaches sea-shells, or the mud of our swamps the bones of our higher animals drifted from the high lands. 5. Im- pressions of various kinds, such as ripple-marks, rain-prints, footprints, etc., evidently formed when the rock was in the condition of soft mud, complete the proof. It may be considered as absolutely certain that stratified rocks are sediments. Arenaceous and argillaceous rocks are the debris of eroded land, and are therefore called mechanical sedi- ments or fragmental rocks. Limestones are either chemical deposits in lakes and seas, or are the comminuted remains of organisms. They 172 STRATIFIED OR SEDIMENTARY ROCKS. are therefore either chemical or organic sediments. Conglomerates, grits, and sandstones, indicate violent action ; shales and clays quiet action in sheltered^spots. Limestones are sometimes produced by vio- lent action e. g., coral breccia sometimes very quiet action, as in deep-sea deposits. We have already seen (p. 4) that rocks under atmospheric agen- cies are disintegrated into soils, and these soils are carried by rivers and deposited as sediments in lakes and seas. Now we see that these sediments are again in the course of time consolidated into rocks, to be again raised by igneous agencies into land, and again disintegrated into soils, and redeposited as sediments. Thus the same material has been in some cases worked over many times in an ever-recurring cycle. This is another illustration of the great law of circulation, so universal in Nature. Cause of Consolidation. The consolidation of sediments into rocks in many cases is due to some cementing principle, such as carbonate of lime, silica, or oxide of iron, present in percolating waters. In such cases the consolidation often takes place rapidly. In other cases it is due to long -continued heavy pressure, and in still others to long-con- tinued, though not necessarily very great, elevation of temperature in presence of water. In these cases the process is very slow, and there- fore it has not progressed greatly in the more recent rocks. II. Stratified Rocks have been gradually deposited. The following facts show that in many cases rocks have been deposited with extreme slowness : 1. Shales are often found the lamination of which is beau- tifully distinct, and yet each lamina no thicker than cardboard. Now, each lamina was separately formed by alternating conditions, such as the rise and fall of tide, or the flood and fall of river. 2. Again, on the interior of imbedded shells of mollusca, or on the outer surface of the shells of sea-urchins deprived of their spines, are often found attached other shells, as shown in the following figures. Now, these shells must have been dead, but not yet covered with deposit during the whole time the attached shell was growing. As a general rule, in fragmental rocks the finest FIG. 138 Serpula on Shell of an Echinoderm. FIG. 139. Serpulse on Interior of a Shell STRUCTURE AND POSITION. 173 materials, such as clay and mud, have been deposited very slowly, while coarse materials, such as sand, gravel, and pebbles, have been de- posited rapidly. Limestones, being generally formed by the accumula- tion of the calcareous remains of successive generations of organisms, living and dying on the same spot, must have accumulated with extreme slowness. The same is true of infusorial earths. It is necessary, therefore, to bear in mind that all stratified rocks were formed in previous epochs by the regular operation of agents similar to those in operation at present, and not by irregular or cataclys- mic action, as supposed by the older geologists. Thus, cceteris paribus, the thickness of a rock may be taken as a rude measure of the time consumed in its formation. III. Stratified Rocks were originally nearly horizontal. The hori- zontal position is naturally assumed by all sediments, in obedience to the law of gravity. When, therefore, we find strata highly inclined or folded, we conclude that their position has been subsequently changed. It must not be supposed, however, that the planes which separate strata were originally perfectly horizontal, or that the strata themselves were of unvarying thickness, and laid atop of each other like the sheets of a ream of paper. On the contrary, each stratum, when first deposited, must be regarded as a widely expanded cake, thickest in the middle and thinning out at the edges, and interlapping there with other similar cakes. Fig. 140 is a diagram showing the mode of interlapping. FIG. 140. Diagram showing Thinning out of Beds : a, sandstones and conglomerates ; 6, limestones. The extent of these cakes depends upon the nature of the material. In fine materials strata assume the form of extensive thin sheets, while coarse materials thin out more rapidly, and are therefore more local. The most important apparent exception to the law of original hori- zontality is the phenomenon of oblique or cross lamination. This kind of lamination is formed by rapid, shift- ing currents, bearing abundance of coarse materials, or by chafing of waves on an exposed beach. Many examples of similar lamination are found in rocks of previous epochs. Figs. 141 and 142 represent such ex- FIG. wi.-obiique Lamination, amples. In some cases oblique lami- nation may be mistaken for highly-inclined strata ; careful examination, 1Y4: STRATIFIED OR SEDIMENTARY ROCKS. however, will show that the strata are not parallel with the laminae. The strata were originally (and in the cases represented in the figures are still) horizontal, while the laminae are oblique. FIG. 142. Section on Mississippi Central Railroad at Oxford (after Hilgard) : Oblique Lamination. Elevated, Inclined, and Folded Strata. We may assume, there- fore, that strata were originally horizontal at the bottom of seas and lakes ; and, therefore, when we find them in other places and positions, they have been subsequently disturbed. Now, \ve actually do find strata in every conceivable position and place ; sometimes they retain their original horizontality, but are raised above their original level ; sometimes they have been squeezed by lateral pressure, and thrown into the most intricate contortions (Figs. 143, 144, and 145); sometimes whole groups of strata many thousand feet thick are thrown into huge parallel folds or wrinkles, forming parallel ranges of mountains (Figs. 146 and 147) ; sometimes by these movements the strata are broken, and one side of the fissure slips up, while the other FIG. 148. Contorted Strata (after Hitchcock). side drops down, thus produ- cing what is called a fault (see page 222). But whether simply elevated, or also contorted, or broken and slipped, in nearly all cases large portions of the original FIG. 144. Contorted Strata (from Logan). strata are carried away by erosion, and they are left in patches and basins, or with their upturned edges exposed on the surface, as shown STRUCTURE AND POSITION. 175 1 c FIG. 146.-Section of Appalachian Chain. FIG. 14T. Section of the Jura Mountains. ' FIG. 148. in Figs. 148, 149, and 150, in which the dotted lines show the part removed. We are thus enabled to examine strata ---I~x which would otherwise have remained forever hid from us. The exposure of the edges of strata on 1 / 1 -T 11*. -i- 1 *!?. the surface by erosion is called outcrop. There are certain terms in constant use by geologists which must be explained in this connection. Dip and Strike. The inclination of strata to an horizontal plane is 176 STRATIFIED OR SEDIMENTARY ROCKS. called the dip. Thus, in Fig. 152, the strata dip 25 toward the south. c \ d \ \ FIG. 151. Upturned and Eroded Strata, Elk Mountains, Colorado (after Hayden). The dip may vary from to 90, from horizontally to vertically. Fig-. 153 gives an example of vertical strata. When in strong foldings the strata are pushed over be- yond the perpendicular, as in Fig. 150, we have what is called an overturn dip. When strata dipping regu- larly are exposed on their edges, as in Fig. 152, their thickness may be easily calculated. If we measure the distance a b and the angle of dip c a b, then c b, the thickness of the stfata, is equal to the sine of the angle of dip, multiplied by the dis- tance a b (R = 1 : a b : : sine cab : c b and c b = a b X sine c a b). The angle of dip is ob- tained by means of an in- strument called a clinome- ter (Fig. 154). The most convenient form is a pock- et compass containing a pendulum to indicate the angle of dip. T . FIG. 153. Vertical Strata. It is rarely the case that the geologist is able to get a complete natural section of an exten- STRUCTURE AXD POSITION. 177 sive series of strata. He is usually, therefore, compelled to construct a more or less ideal section from the examination of outcrops and partial sections wherever he can find them. FIG. 154. Clinometer. The strike is the line of intersection of strata with an horizontal plane, or the direction of the outcrop of strata on a level surface. It is always at right angles to the dip. If the dip is toward the north or south, the strike is east and west. If the strata are plane, the strike is a straight line, but in folded strata the strike may become very sinu- ous. The outcrop of strata upon the actual surface is often extremely irregular, since this is affected not only by the foldings of the strata, but by the inequalities of surface produced by erosion. The intricate outcrop of rocks, under these circum- stances, can only be understood by actual examination in the field or by the use of models. 1 A comparatively simple case of such outcrop is given in Fig. 155, and the manner in which the rocks are folded and eroded is shown in the section Fig. 156. Anticlines and Synclines. Fold- ed strata, of course, usually dip al- ternately in opposite directions, forming alternate ridges and hollows, or saddles and troughs (Fig. 156). A line from which the strata dip in opposite directions on the two sides is called an anticlinal axis, or simply an anticline / a line toward which the strata dip in opposite directions on Fte. 156.-Section of Undulati'stra'ta. tllG tW SideS ' S C&lled * ^ clinal axis, or a syncline. The strata, in the case of an anticline, always form a ridge, and in the case of a syncline a trough ; but, in the actual surface, this is often entirely 1 Sopwith's Geological Models. FIG. 155. Plan of Undulating Strata. 178 STRATIFIED OR SEDIMENTARY ROCKS. reversed by erosion, so that the synclines become the ridges and the anticlines the hollows or valleys. Fig. 149 represents a section in which the anticlines or original ridges have become valleys, while the synclines or original valleys have become mountain-ridges. Examples of synclinal mountains and anticlinal valleys are by no means uncom- mon. In both anticlines and synclines the strata are repeated on each side of the axis. Monoclinal Axes. Sometimes strata over large areas are lifted bodily upward with little change of inclination, while over contiguous areas they are dropped down, the two areas being connected by a sharp bend of the strata instead of a fault. Such a bend is called a mono- clinal fold or axis (Fig. 157). Monoclinal folds pass by insensible gra- dations into faults, and are evidently produced in a similar manner the degree of flexibility of the strata determining whether the one or the other is formed. In the plateau of Colorado, where monoclinal folds are common, they may be traced into faults. Fig. 157 is taken from this region. Unconformity. We have seen (page 175) that land-surfaces are always composed of eroded, and usually of tilted, strata. We have also seen (pages 127-130) that land-surfaces are now in some places sinking and becoming sea-bottoms, while in others sea-bottoms are ris- ing and becoming land-surfaces. The same thing has happened in every geological epoch. Now, whenever an eroded land-surface sinks below the water and receives sediments, these sediments will lie in horizontal layers upon the upturned edges, and filling up the erosion hollows of the pre- vious strata. If, now, the two series of strata be again elevated into land-surface, and exposed to the inspection of the geologist, the relation of the two series to one another will be represented by the following sections (Figs. 158 and 159). When one series of strata rests thus on CLEAVAGE STRUCTURE. 179 the eroded surface or edges of another series, the two series are said to be unconformable. Of course, the whole series may be again elevated, tilted, and eroded, making the phenomena far more complex than here onformity. represented. By far the most common case is that of Fig. 158, in which the upper series rests on the upturned edges of the lower series, and there is therefore a want of parallelism between the two series ; and FIG. 159.-Unconformity. the term unconformity is usually defined as a want of parallelism ; but it should be applied also to cases like Fig. 159, where there is no want of parallelism. Conformable strata indicate a period of comparative repose, during which sediments were quietly deposited. Unconformity indicates a pe- riod of disturbance, during which the strata were elevated into a land- surface, subjected to erosion, and again subsided to receive other sedi- ments. A section like Fig. 158 or 159, one of the commonest in struct- ural geology, indicates two periods of repose and one of disturbance. The lapse of time in the periods of repose is represented by the strata ; the lapse of time in the period of disturbance is represented by the ero- sion. Every case of unconformity, therefore, indicates a gap in the history of the earth a period unrecorded by strata at that place. Formation. A group of conformable strata often constitutes what geologists call a formation. Unconformable strata usually belong to different formations. These divisions, however, are founded also upon the character of the contained fossils. This subject will be more fully explained hereafter. Cleavage Structure* We have thus far spoken only of the original and universal structure of stratified rocks, together with the tiltings, foldings, and erosion, to 1 This structure is usually treated under metamorphic rocks, as a kind of metamor- phism ; but it is found in rocks which have not undergone ordinary metamorphic changes, and it is produced by an entirely different cause. 180 STRATIFIED OR SEDIMENTARY ROCKS. which they have been subjected. There is, however, often found in stratified rocks a superinduced structure which simulates, and is often mistaken for, stratification. It is called cleavage structure, or (since it is usually found in slates) slaty cleavage. This subject has recently attracted much attention, and is an admirable example of the successful application of physics to the solution of problems in geology. Cleavage may be defined as the easy splitting of any substance in planes parallel to each other. Such definite splitting may result, in different cases, from entirely different causes. For example (a), under the influence of the sorting power of water, sedimentary materials may be so arranged as to give rise to easy splitting along the planes of lami- nation. Many rocks may be thus split into large coarse slabs called flag- stones, and are used for paving streets, or even sometimes as roofing- slates. This may be called flag-stone cleavage, or lamination cleavage. Again (5), the arrangement of the ultimate molecules of a mineral un- der the influence of molecular or crystalline forces gives rise to an ex- quisite splitting along the planes parallel to the fundamental faces of the crystal. This is called crystalline cleavage. Again (c), the ar- rangement of the wood-cells under the influence of vital forces gives rise to easy splitting of wood in the direction of the silver-grain. This may be called organic cleavage. Now, in certain slates and some other rocks is found a very perfect cleavage on a stupendous scale. Whole mountains of strata may be cleft from top to bottom in thin slabs, along planes parallel to each other. The planes of cleavage seem to have no relation to the strata, but cut through them, maintaining their parallelism, however the strata may vary in dip (Fig. 160). Usually the cleavage-planes are highly in- clined, and often nearly perpendicular. It is from the cleaving of such FIG. 160.-Cleavage-Planes cutting through Strata. slates that roofing-slates, ciphering-slates, and blackboard-slates are made. This remarkable structure has long. excited the interest of geologists, and many theories have been proposed to explain it. On cursory examination of such rocks, the first impression is, that the cleavage is but a very perfect example of flag-stone or lamination cleav- age that the cleavage-planes are in fact stratification-planes, and that we have here an admirable example of finely laminated rocks which have been highly tilted and then the edges exposed by erosion. Closer examination, however, will generally show the falseness of this view. CLEAVAGE STRUCTURE. 181 Fig. 161 represents a mass of slate in which three kinds of structure are distinctly seen, viz., joint faces, A, J3, C, J, J ; stratification-planes, FIG. 161. Strata, Cleavage-Planes, and Joints. S S S, gently dipping to the right; and cleavage-planes, highly inclined, D J), cutting through both. Cleavage-planes are therefore not stratifi- cation-planes. Again, it has been compared to crystalline cleavage, on a huge scale. It has been supposed that electricity traversing the earth in certain di- rections, while certain rocks were in a semi-fluid or plastic state through heat, arranged the particles of such rocks in a definite way, giving rise to easy splitting in definite directions. In support of this view it was urged that cleaved slates are most common in metamorphic regions; and metamorphism, as we shall see hereafter (p. 215, et seq.), indicates the previous plastic state of rocks, which is a necessary condition of the rearrangement of the particles by electricity. The great objections to this theory are 1. That the cleavage is not like crystalline cleavage, between ultimate molecules, and therefore perfectly smooth, but be- tween discrete and quite visible granules ; and, 2. That although the phenomenon is indeed most common in metamorphic rocks, yet meta- morphism is by no means a necessary condition ; on the contrary, when the real necessary conditions are present, the less the metamorphism the more perfect the cleavage. It is evident, therefore, that slaty cleavage is not due to any of the causes spoken of above. It is not flag-stone cleavage, nor crystalline cleavage, and of course cannot be organic cleavage. Sharpe's Mechanical Theory. The first decided step in the right direction was made by Sharpe. According to him, slaty cleavage is al- ways due to powerful pressure at right angles to the planes of cleavage, by which the pressed mass has been compressed in the direction of press- ure and extended in the direction of cleavage. This theory may be now regarded as completely established by the labors of Sharpe, Sorby, Haughton, Tyndall, and others. We will give a few of the most impor- tant observations which establish its truth. (a.) Distorted Shells. Many cleaved slates are full of fossils. In 182 STRATIFIED OR SEDIMENTARY ROCKS. such cases the fossils are always crushed and distorted as if by powerful pressure, their diameters being shortened at right angles to the cleavage, and greatly increased in the direction of the cleavage-planes. The fol- lowing figures (Fig. 162) are examples of distortion by pressure. In FIG. 162. -Distorted Fossils (after Sharpe). Fig. 162, Z Z gives the direction of the planes of cleavage ; Figs. 1, 2, 3, 4, represent one species ; 5, 6, 7, 8, another. In Fig. 163 still another species is represented in the natural and distorted forms. Fio. 163. Cardium Hillanum : A, natural form ; B and C, deformed by pressure. (J.) Association with Foldings. Cleavage is always associated with strong foldings and contortions of the strata. The folding of the strata FIG. 164. Cleavage-Planes intersecting Strata. is produced by horizontal pressure ; the strike of the strata, or the direction of the anticlinal and synclinal axes, being of course at right CLEAVAGE STRUCTURE. 183 angles to the direction of pressure. Now, if cleavage is produced by the same pressure which folded the strata, then in this case we ought to find the cleavage-planes highly inclined, and their strike parallel with the strike of the strata ; and such we find is usually the fact. In Fig. 164 the heavy lines represent the strata and the light lines the cleavage-planes, both outcropping on a nearly level surface, and parallel to each other. (c.) Association with Contorted Laminae. The last evidence was taken from foldings on a grand scale of the crust of the earth ; but even fine lines of lamination are often thrown into intricate foldings by squeezing together in the direction of the lamination-planes. In such case, of course the cleavage ought by theory to be at right angles to the original direction of the lami- nation, and in such direction we actually find them. Fig. 165 represents a block of rock in which three lamination-lines are visible. The lower one, f d, consists of coarse sand which could not mash, and therefore has been thrown into folds. As the specimen stands in the figure, the pressure has been horizontal ; the perpendicular lines represent the position of the cleavage-planes. Fig. 166 rep- resents a beautiful specimen of laminated slate, in which the lamination -planes have been thrown into folds by pressure. The direction of the pressure is obvious. The planes of cleavage are parallel to the face, cp, and therefore at right angles to the pressure. (d.) Flattened Nodules. In some finely-cleaved slates, such as are used for writing- slates, it is common to find small light-greenish, elliptical spots of finer material. In clay- deposits of the present day it is also common to find im- bedded little round nodules of finer material. It is probable that the greenish nodules in slates were also rounded nodules of finer clay in the original clay- deposit from which the slate was formed by consolidation. FIG. 166.-A Block of Cleayed Slate (after Jukes). But in cleaved slates these nod- FIG. 165. Cleavage-Planes (after Tyndall). 184 STRATIFIED OR SEDIMENTARY ROCKS. ules are always very much flattened in the direction at right angles to the cleavage-planes, and spread out in the direction of these planes. (e.) Apparent Diamagnetism of Cleaved Slates .under Certain Con- ditions. If a bar of iron be placed between the poles of a magnet, it will immediately place itself in the line connecting the poles (axial position) ; but if a bar of bismuth be similarly placed, it will assume a position at right angles to the axial line (equatorial position). In the former case the ends of the bar are attracted by the poles ; in the other they are repelled. Bodies which, like iron, assume the axial posi- tion, are called paramagnetic ; bodies which, like bismuth, assume the equatorial position, are called diamagnetic. But Tyndall has shown * that by strong compression a paramagnetic substance may be made to as- FIG. 167. Illustrating Behavior of Cleaved Slates in the Magnetic Field. sume an equatorial or diamagnetic position. If a cube of iron be -placed between the poles JVand S of a magnet (Fig. 167, ^4), the cube will be in- different as to position, since the attraction along any two lines, a #, c d, at right angles to one another, will be equal. But if iron-filings be made into a mass with gum, and then subjected to strong compression in one direction, and from the pressed mass a cube be cut, this cube, placed in the magnetic field, is no longer indifferent, but sets with its line of great- est compression, a b (Fig. 167, -B), axial; the attraction along tliis line being greater than along any other line, because the number and prox- imity of the particles are greater along this line. And so much greater is the magnetic attraction along this line than along any other, that this diameter may be cut away to a considerable extent, so as to make a short bar, and still the line a b will maintain its axial position (Fig. 167, (7), and the bar will seem to be diamagnetic, i. e., its long diameter will be equatorial ; not, however, because its ends are repelled, but because the attraction along the shorter diameter a b is greater than along the long diameter. If, therefore, the cutting-down of the diameter a b be continued, finally the influence of length will prevail over that of com- 1 Philosophical Magazine, third scries, vol. xxxvii., p. 1, and fourth series, vol. ii., p. 165. CLEAVAGE STRUCTURE. 185 pression, and the bar will assume its true axial position (Fig. 167, D). Now, Tyndall, while experimenting upon the magnetic properties of various bodies, 1 found that a short bar of cleaved slate, with its longer diameter in the plane of cleavage, when placed in the magnetic field, takes the equatorial position ; although, if the bar be slender, it at once shows its paramagnetism by assuming the axial position. In other words, cleaved slate behaves exactly as if it was a paramagnetic pow- der pressed in the direction at right angles to the cleavage-planes. (/.) Experimental Proof. Finally, experiments by Sorby and by Tyndall show that clay (the basis of slates), when subjected to power- ful pressure, exhibits always a cleavage, often a very perfect cleavage, at right angles to the line of pressure. Physical Theory. Cleavage is certainly produced by pressure, but the question still remains : How does pressure produce planes of easy splitting at right angles to its own direction ? What is the physical explanation of cleavage ? Sorby's Theory. 2 Mr. Sorby's view is that all cleaved rocks con- sisted, at the time when this structure was impressed upon it, of a plastic mass, with unequiaxed foreign particles disseminated through it; and that by pressure the unequiaxed particles were turned so as to bring their long diameters in a direction more or less nearly at right angles to the line of pressure, and thus determined planes of easy fracture in that direction. Usually, as in slates, the plastic material is clay, and the unequiaxed particles are mica-scales. Let A, Fig. 168, repre- sent a cube of clay with mica disseminated. If such a cube be dried and broken, the fracture will take place principally along the surfaces of the mica, which may therefore be seen glistening on the uneven surface of the fracture ; but if the cube, while still plastic, be pressed into a flattened disk, then the scales are turned with their long diameters in the direction of extension and at right angles to the line of pressure, as in B, Fig. 168, and the planes of easy fracture, being still determined by these surfaces, will be in that direction. In proof of this view, Mr. Sorby mixed clay with mica-scales or with oxide-of-iron scales, and, upon subjecting the mass to powerful compression and drying, he al- ways found a perfect cleavage at right angles to the line of pressure. Furthermore, by microscopic examination he found that both in the pressed clay and in the cleaved slates the mica-scales lay in the direc- tion of the cleavage-planes. 1 Philosophical Magazine, 4th series, vol. v., p. 303. 2 Ib., 2d series, vol. xi., p. 20. FIG. 168. Illustrating Sorby's Theory of Slaty Cleavage (after Sorby). 186 STRATIFIED OR SEDIMENTARY ROCKS. FIG. 169. Illustrating Sorby's Theory of Slaty Cleavage (after Sorby). Although cleavage is most perfect in slates, yet other rocks are sometimes affected with this structure. In a specimen of cleaved lime- stone, Sorby found under the microscope unequiaxed fragments of broken shells, corals, crinoid stems, etc. (organic particles), ^ in a homogeneous limestone-paste, lying with their long diameters in the direction of cleavage. Originally the limestone was a lime-mud with (he supposes) unequiaxed organic particles disseminated. In some cases, however, Sorby recognized the very important fact that the organic fragments, which were encrinal joints, had been flattened by pressure had changed their form instead of their position. A., Fig. 169, gives a section of the mass in the sup- posed original condition, and B the condition after pressure. This obser- vation contained the germ of the theory proposed by Tyndall. TyndalTs Theory. 1 Tyndall was led to reject Sorby's theory by the observation that cleavage structure was not confined to masses contain- ing unequiaxed particles of any kind, but, on the contrary, the cleavage is more perfect in proportion as the mass is free from all such particles. Clay, deprived of the last trace of foreign particles by the sorting power of water, when pressed, cleaved in the most perfect manner. Common beeswax, flattened by powerful pressure between two plates of glass and then hardened by cold, exhibits a most beautiful cleavage structure. Almost any substance curds, white-lead powder, plumbago subjected to powerful pressure, exhibits to some extent a similar structure. Tyn- dall explains these facts thus : Nearly all substances, except vitreous, have a granular or a crystalline structure, i. e., consist entirely of dis- crete granules or crystals, with surfaces of easy fracture between them. When such substances are broken, the fracture takes place between the crystals or granules, producing a rough crystalline or granular surface, entirely different from the smooth surface of vitreous fracture. Marble, cast-iron, earthenware, and clay, are good examples of crystalline and granular structure. Now, if a mass thus composed yield to pressure, every constituent granule is flattened into a scale, and the structure be- comes scaly / and as the surfaces of easy fracture will still be between the constituent scales, we have cleavage at right angles to the line of pressure. A mass of iron, just taken from the puddling-furnace and cooled, exhibits a granular structure ; but if drawn out into a bar, each granule is extended into a thread, and the structure becomes fibrous ; 1 Philosophical Magazine, 2d series, vol. xii., p. 35. CLEAVAGE STRUCTURE. 187 or if rolled into a sheet, each granule is flattened into a scale, and we have a cleavage structure. There can be little doubt that this is the true explanation of slaty cleavage. The change of form which, as we have seen, has taken place in the fossil-shells, encrinal joints, and rounded nodules, has affected every constituent granule of the original earthy mass, so that the struct- ure becomes essentially scaly instead of granular ; the cleavage being between the constituent scales. Sorby, it is true, in his observations on cleaved limestones, recognized the true cause of cleavage, viz., the change of form of discrete particles ; but he regarded this as subordi- nate to change of position. Besides, the particles of Sorby were for- eign, which Tyndall has shown to be unnecessary ; while the particles of Tyndall are constituent. Geological Application. It may be considered, therefore, as certain that cleaved slates have assumed their peculiar structure under the in- fluence of powerful pressure at right angles to the cleavage-planes, by which the whole squeezed mass is mashed together in one direction and extended in another. Taking any ideal sphere in the original unsqueezed mass : after mashing the diameter in the line of pressure has been short- ened, the diameter in the line of cleavage-^//? has been correspondingly extended, and the diameter in the line of cleavage-strike unaffected, since extension of this diameter in any place must be compensated by short- ening in a contiguous place right or left ; so that the original sphere has been converted into a greatly-flattened ellipsoid of three unequal diame- ters. The amount of compression and extension may be estimated in the case a by the amount of distortion of shells of known form (Figs. 162 and 163) ; in the case c by a comparison of the transverse diameter with the length of the folded line/V? (Fig. 165) ; in the case d by the relation between the diameters of the elliptic spots. By these means, but prin- cipally by the first, Haughton * has estimated that the original sphere has been changed into an ellipsoid, whose greatest and shortest diameters are to each other, in some cases, as 2 : 1, in others as 3 : 1,4: 1, 6 or 7 : 1,9:1, and in some even 11 : 1. The average in well-cleaved slates, according to Sorby, is about 6 : 1. Now, since this ratio is the result partly of compression and partly of extension, it is evident that either the compression alone or the extension alone would be the square roots of these ratios. Therefore, we may assume the average compression as 2| : 1, and the average extension as 1 : 2|. It is impossible to over-estimate the geological importance of these facts. Whole mountains of strata, whole regions of the earth's crust, are cleaved to great and unknown depths, showing that the crust has been subjected to an almost inconceivable force, squeezing it together in an horizontal direction and swelling it upward. This upward swell- 1 Philosophical Magazine, fourth series, vol. xii., p. 409. 188 STRATIFIED OR SEDIMENTARY ROCKS. ing, or thickening of the strata by lateral squeezing, is a probable cause of gradual elevation of the earth's crust, which has not been noticed by geologists. We will speak again of this important subject in our dis- cussion of mountain-formation. There are reasons for believing that the squeezing did not take place, and the structure was not formed, while the strata were in their original condition of plastic sediment, but after they had been consoli- dated into rock and the contained fossils had been completely petrified, otherwise the shells must have been broken by the pressure. Yet, on the other hand, some degree of plasticity seems absolutely necessary to account for so great a compression in one direction and extension in another without disintegration of the mass. It seems most probable that at the time the structure was produced these rocks were deeply buried beneath other rocks and in a somewhat plastic state, through the influence of heat in the presence of water. Afterward, they were exposed by erosion. Nodular or Concretionary Structure. In many stratified rocks are found nodules of various forms scattered through the mass or in layers parallel to the planes of stratification. Like slaty cleavage, this structure is the result of internal changes sub- sequent to the sedimentation ; for the planes of stratification often pass directly through the nodules (Figs. 170 and 171). The flint nodules of FIG. 170. FIG. 171. the chalk, and the clay iron-stone nodules of the coal strata and hy- draulic lime-balls, common in many clays, are familiar illustrations of this structure. Cause. Nodular concretions seem to occur whenever any substance is diffused in small quantities through a mass of entirely different mate- rial. Thus, if strata of sandstone or clay have small quantities of car- bonate of lime or carbonate of iron diffused through them, the diffused particles of lime or iron will gradually, by a process little understood, segregate themselves into more or less spherical or nodular masses, in some cases almost pure, but generally inclosing a considerable quantity of the material of the strata. In this manner lime-balls and iron-ore balls and nodules, so common in sandstones and clays, are formed. In like manner, the flint nodules of the chalk were formed by the segre- gation of silica, originally diffused in small quantities through the chalk- NODULAR OR CONCRETIONARY STRUCTURE. 189 sediment. Very often some foreign substance forms the nucleus about which the segregation commences. On breaking a nodule open, a shell or some other organism is often found beautifully preserved. These nodules, therefore, are a fruitful source of beau- tiful fossils. In most cases, probably in all cases, the segregating substance must have been to some extent soluble in water pervading, or suspensible in water percolating, the stratum. Some- times the nodules run together, form- ing a more or less continuous stratum. In such cases, the segregating material is more impure. Forms of Nodules. The typical and most common form is globular. This is well seen in lime-balls and iron-balls. Sometimes these balls are solid, sometimes they have irregular cracks in the centre (Fig. 172), sometimes they have a radiated structure (Fig. 173), sometimes they are hollow like a shell (this is common in iron-balls). They vary in size from that of a pea to six and eight feet in diameter. Often, however, instead of the spherical form, they take on various and strange and FIG. 173. Dolomite containing Concretions, Sunderland (after Jukes). fantastic shapes (Fig. 174), sometimes like a dumb-bell, sometimes a flattened disk, sometimes a ring, sometimes a flattened ellipsoid, regu- larly seamed on the surface like the shell of a turtle (turtle-stones). They are often mistaken by unscientific observers for fossils. Kinds of Nodules found in Different Strata. In sandstone strata the nodules are commonly carbonate of lime or oxide of iron (lime or iron balls). In clay strata they are carbonate of lime or carbonate of iron (clay iron-stone of coal strata), or a mixture of these (Roman cement nodules of the London clay). 190 STRATIFIED OR SEDIMENTARY ROCKS. In limestone the nodules are always silica, and conversely silica nodules are peculiar to limestone. The flint nodules of the chalk are remarkable for being arranged in planes parallel to the planes of strati- FIG. 174. Limestone Strata containing Concretions. fioation (Fig. 175). Sometimes the siliceous matter segregates in con- tinuous strata of siliceous limestone (Fig. 176). In the cases thus far spoken of, the nodules are scattered through the mass of the strata or arranged in planes parallel to planes of stra- FIG. 175. Chalk-Cliffs with Flint Nodules. tification. But in some cases the whole mass of the rock assumes a con- cretionary or concentric structure (Fig. 177). The cause of this is still more difficult to explain. FOSSILS : THEIR ORIGIN AND DISTRIBUTION. Stratified rocks, as we have already seen, are sediments accumulated in ancient seas, lakes, deltas, etc., and consolidated by time. As now, so then, dead shells were imbedded in shore-deposits ; leaves and logs of FOSSILS: THEIR ORIGIN AND DISTRIBUTION. 191 high land-plants, and bones of land-animals, were drifted into swamps and deltas and buried in mud ; and tracks were formed on flat, muddy shores by animals walking on them. These have been preserved with more or less change, and are even now found in great numbers in- closed in stratified rocks. They are called fossils. A fossil, therefore, is any evi- dence of the former existence of a living being. Fossils are the remains of the fauna and flora of previous geolog- ical epochs. Their presence is the most constant charac- teristic of stratified rocks. The Degrees of Preservation are very various. Sometimes only the tracks of animals, or impressions of leaves of plants, are preserved. More commonly the bones or shells, or other hard parts of animals, are FIG. 176.-Chalk-Cliffs. FIG. 177. Coal-measure Shale, weathering into Spheroids. preserved with various degrees of change. Sometimes even the soft and more perishable tissues are preserved. We will treat of these degrees under three principal heads : 1. Decomposition prevented and the Organic Matter more or less completely preserved. Cases of this kind are usually found in compar- atively recent strata, and imbedded either in frozen soils, or in peat, or in stiff clays ; although some cases of partial preservation of the or- 192 STRATIFIED OR SEDIMENTARY ROCKS. ganic matter are found even in old rocks. Extinct elephants have been found frozen in the river-bluffs of Siberia so perfectly preserved that dogs and wolves ate their flesh. Skeletons of men and animals are found in peat-bogs and stiff clays of a comparatively recent formation, the organic matter of which is still preserved. In clays of the Tertiary period the imbedded shells still retain the epidermis, and even in the Lias (mesozoic) shells are found retaining the nacreous lustre. Coal is vegetable matter changed but not destroyed. It is found in almost every formation, even down to the oldest. Every degree of change may be traced in different specimens of fossil wood, between perfect wood and perfect coaL 2. Petrifaction: Organic Form and Structure preserved. In t : :e last case the organic matter is more or less preserved. In the case now to be described the organic matter is entirely gone ; but the or- ganic form and the organic structure are preserved in mineral matter. This is what is usually called petrifaction or mineralization. The best example of this is petrified wood. In a good specimen of petrified wood, not only the external form of the trunk, not only the general structure of the stem viz., pith, wood, and bark not only the radiating silver-grain and the concentric rings of growth, are discernible, but even the microscopic cellular structure of the wood, and the exquisite sculpturings of the cell-walls themselves, are perfectly preserved, so that the kind of wood may often be determined by the microscope with the utmost certainty. Yet not one particle of the organic matter of the wood remains. It has been entirely replaced by mineral matter ; usually by some form of silica. The same is true of shells and bones of animals ; but as shells and bones consist naturally partly of organic and partly of mineral matter, very often it is only the organic matter which is replaced, although sometimes the original mineral matter is also replaced by silica or other mineral substance. The radiating structure of corals or the microscopic structure of teeth, bones, and shells, is often beautifully preserved. This kind of preservation for shells and corals is most common in limestones and clays ; for wood, in gravels. Theory of Petrifaction. If wood be soaked in a strong solution of sulphate of iron (copperas) and dried, and the same process be re- peated until the wood is highly charged with this salt, and then burned, the structure of the wood will be preserved in the peroxide of iron left. Also, it is well known that the smallest fissures and cavities in rocks are speedily filled by infiltrating waters with mineral matters. Now, wood buried in soil soaked with some petrifying material becomes highly charged with the same, and the cells filled with infiltrated mat- ter, and when the wood decays the petrifying material is left, retaining the structure of the wood. But this is not all, for in Nature there is FOSSILS: THEIR ORIGIN AND DISTRIBUTION. ^93 an additional process, not illustrated either by the experiment or by the example of infiltrated fillings. As each particle of organic matter passes away by decay, a particle of mineral matter takes its place, until finally the whole of the organic matter is replaced. Petrifaction, there- fore, is a process of substitution, as well as interstitial filling. Now, it so happens, probably from the different nature of the process in the two cases, that the interstitial filling always differs, either in chemical composition or in color, from the substituting mate- rial. Thus the structure is still visible, though the mass is solid. If Fig. 178 represent a cross-section of three petrified wood-cells, the matter filling the cells (b) is always different from the matter forming the cell-wall (a). The most common petrifying materials are silica, carbonate of lime, and sulphide of iron (pyrites). In the case of petri- faction by pyrites the process is quite intelligible, but the structure is usually very imperfectly preserved. If water containing sulphate of iron (FeSOJ come in contact with decaying organic matter, the salt is deoxidized by the organic matter, the latter passing off as carbonic acid and water, and the former becomes insoluble sulphide (FeS), and is deposited. Now, as each particle of organic matter passes away as CO, and H 2 O, the molecule of iron sulphate which effected the change is itself changed into insoluble sulphide, and takes its place. The process of replacement by silica (silicification) is less clear, but it is probably as follows : Silica is found in solution in many waters, being held in this condition by small quantities of alkali present in the waters. In contact with decomposing wood the alkali is neutralized by the humic, ulmic, and other acids of decomposition, and the silica therefore deposited. 3. Organic Form only preserved. In the third case organic mat- ter and organic structure are both lost, and only organic form is pre- served. This kind of fossilization is most commonly seen in shells. It may be subdivided into four subordinate cases, represented in section by , b, c, and d of Fig. 179. In this figure the horizontal lines represent the original sediment which may or may not have consolidated into rock ; the vertical lines represent a subsequent filling of different and usually finer material. In a we have a mould of the external form of 13 194 STRATIFIED OR SEDIMENTARY ROCKS. the shell preserved in sediment. The shell with the undecayed animal was imbedded, and afterward entirely dissolved away, leaving only the hollow mould. In b the same process has taken place, only the mould has been subsequently filled by infiltration of slightly soluble matters. In this case we have both the mould and the cast of the external form ; the mould being formed of sediment, and the cast of infiltrated matter. These are always of different materials, i. e., different either in chemical composition or in state of aggregation. In c we have a mould of the external form in sediment, and a cast of the internal form in the same material, with an empty space between, having the exact form and thickness of the shell. In this case, the already dead and empty shell FIG. 180. o, Cast of interior; 6, natural form. Fro. 181. a, Natural form ; &, cast of interior and mould of exterior. FIG. 182. Trigonia Longa, showing cast (a) of the exterior and (6) of the interior of the shell. was imbedded in sediment, which also filled its interior ; afterward the shell was removed, leaving an empty space. In d this empty space was subsequently filled by infiltration. In shore and river deposits of the present day it is very common. to find shells imbedded in, and filled with, sand or mud. In the more recent tertiary rocks shells are com- monly found in the same condition precisely ; but in the older rocks more commonly the original shell is removed, and the space either left empty or filled by infiltration. Cases c and d are well represented by Figs. 180, 181, and 182. Cases like a and c are most commonly found in porous rocks like sandstone ; b and c?, especially the latter, are found DISTRIBUTION OF FOSSILS IN THE STRATA. 195 in all kinds of rocks. By far the most common infiltration fillings are carbonate of lime and silica. Often we find impressions of the forms of small portions only of the original organism, as of the leaves of trees, or the feet of animals walk- ing on the soft mud of the flat shores of ancient bays. Such tracks were afterward covered up with river or tidal deposit, and thus pre- served. On cleaving the rock along the lamination-planes we have on one side a mould and on the other the cast of the foot. Between cases 1 and 2 every stage of gradation may be traced. The amount of change, as a general fact, varies with the age of the rock ; but is still more dependent on the kind of rock and the degree of metamorphism (p. 213). In an impermeable rock, like clay, the changes are much more slow than in a porous rock, like sandstone. Distribution of Fossils in the Strata. The nature of the fossil species found in rocks is determined partly by the kind of rock, partly by the country where the rock is found, and partly by the age of the rock. 1. Kind of Rock. It has been already stated (p. 162) that the species of lower marine animals vary with the depth. They also vary with the kind of bottom. Thus, along shore-lines and on sand-bottom the species differ from those in deep water and on mud-bottom. Shells are found mostly along shore-lines, corals in opener seas, and foramini- fera in deep seas. The same was true in every previous epoch. We might expect, therefore, and do find, that the lower marine fossils of sandstones, shales, and limestones, differ even when these strata belong to the same country and geological epoch. The higher marine animals, such as fishes, cuttle-fish, etc., swimming freely in the sea, are more independent of bottoms, and we find their skeletons and shells equally in all kinds of strata. Land animals perish on land, and their skeletons are drifted into bays, river-deltas, and lakes, and buried there mostly in fresh-water or brackish-w.ater deposits of sand and clay. It is, there- fore, in such strata that their remains are commonly found. 2. The Country where found. We have already seen (p. 155) that the fauna and flora of different countries at the present time differ as to species, and often as to genera and families ; the difference being generally in proportion to the difference in .climate and the physical barriers intervening. The same was true of the faunas and floras of previous epochs, and therefore of the fossils of the same age in differ- ent countries. The fossil species of the same epoch in America, and in Europe and in Asia, are not usually identical, although there may be a general resemblance. The geographical diversity, however, is small in the lowest and oldest rocks, and becomes greater and greater as we 196 STRATIFIED OR SEDIMENTARY ROCKS. pass upward into newer and newer rocks, and is greatest in the fauna and flora of the present day. 3. The Age. This introduces the subject of the laws of distri- bution of organisms in time, or of fossils vertically in the series of stratified rocks. The subject will be more fully treated in Part III., of which it constitutes the principal portion. We now bring out only so much as is necessary as a basis of classification of stratified rocks. (a.) Geological Fauna and Flora. As we pass from the oldest and lowest rocks upward to the newest and highest, we find that all the species, most of the genera, and many of the families, change many times. Now, all the species of animals and plants inhabiting the earth at one time constitute the fauna and flora of that geological time. Geological faunae, therefore, have changed many times. In a conformable series of rocks the change from one fossil fauna or flora to another succeeding is always gradual, the species of the later fauna or flora gradually replacing those of the earlier. But between two series of unconform- able strata the change is sudden and complete as if one fauna and flora had been suddenly destroyed and another introduced. It must be remembered, however, that unconformity always indicates a great lapse of time unrepresented at the place of observation by strata or fossils. It is therefore probable that the apparent suddenness of the change is only the result of our ignorance of the fauna and flora of the period unrepresented. Nevertheless, as unconformity always indi- cates changes of physical geography, and therefore of climate, it is probable that in the history of the earth there were periods of great changes, marked by unconformity of strata, during which changes of species were more rapid, separated by periods of comparative quiet, marked by conformity, during which the species were either un- changed, or changed slowly. Such a period is called a geological period or geological epoch, and the rocks formed during a geological period, or epoch, is called & formation. There are, therefore, two tests of a formation and a corresponding geological period, viz., 1. Conformity of the strata, or rock-system, and, 2. General similarity of fossils, or life-system / and two modes of separating formations and corresponding periods, viz., unconformity of the rock-system, and great and sudden change of the life-system. A geological formation, therefore, may be defined as a group of con- formable rocks containing similar fossils, usually separated from other similar groups containing different fossils by unconformity. A geo- logical period may be defined as a period of comparative quiet, during which the physical geography, climate, and fauna and flora, were sub- stantially the same, usually separated from other similar periods by changes of physical geography and climate, which resulted in changes of fauna and flora. Of these two tests, however, the life-sj^stem is CLASSIFICATION OF STRATIFIED ROCKS. 19^ usually considered the most important, and in case of disagreement must control classification. (b.) Geological Faunce and Florae differ more than Geographical Faunae and Florae. If there were no geographical diversity, species of the same age would be identical all over the earth, and therefore it would be easy to determine strata of the same age (geological horizon). On the other hand, if geographical diversity in any age were as great as the diversity between two successive ages, then it would seem im- possible to establish a geological horizon. But this law states that the difference between two successive faunas is greater than between two contiguous faunae. In other words, the species of successive periods, or fossils of successive formations, differ from each other more than species of the same period or fossils of the same formation in different parts of the earth. There is a general similarity in the species of the same period all over the surface of the earth. Kence by comparison of fossils it is possible to determine what strata, in different portions of the earth, belong to the same period (to synchronize strata). The strata all over the earth, which were formed at the same time, are said to be- long to the same geological horizon. Strata of the same horizon are determinate by similarity of fossils with considerable certainty, until we come up to the tertiary rocks. In all the newer rocks, however, the geographical diversity is so great as to interfere seriously with the ability to synchronize by means of comparison of fossils. Another method, therefore, is used for these higher rocks. (c.) By examining and comparing fossils from the lowest to the highest rocks, it has been observed that there is a steady approach of the fossil faunas and florae to the present faunae and florae, first in the families, then in the genera, and finally in the species. The species of fossil molluscous shells begin to be identical with molluscous species of the present day only in the tertiary rocks, and the proportion of iden- tical species steadily increases as we pass upward. Thus in the newer rocks, just where the other method (comparison of fossil faunas with one another) begins to fail, we mav svnchronize strata of different localities, by comparing their shell fauna with the shell fauna of the present day, in the same localities. Those are said to be of the same age which contain the same percentage of shells identical with those of the present day. SECTION 2. CLASSIFICATION OF STRATIFIED ROCKS. Geology is essentially a history. Stratified rocks are the leaves on which this history is recorded. The fundamental idea of every clas- sification is therefore relative age. The object to be attained in clas- sification is, first, to arrange all rocks in chronological order, so that the history may be read as it was written ; and then, second, to collect 198 STRATIFIED OR SEDIMENTARY ROCKS. them into larger and smaller groups, called systems, series, formations, corresponding to the great eras, periods, epochs, of the earth's history. There are several different methods of determining the relative age of rocks : 1. Order of Superposition. It is evident, from the manner in which stratified rocks are formed viz., by sedimentation that their original position indicates, with absolute certainty, their relative age, the lower being older than the higher. If, therefore, the original position of any series of strata be retained or not very greatly disturbed, and we have a good section, the relative age of the strata which compose the series may be easily determined. But the strata, as we have already seen, have in many cases been crushed and contorted and folded in the most intricate manner, sometimes even turned over ; have been broken and slipped, and large masses carried away by erosion, and often so changed by heat and other agents, that their stratification is nearly or quite obliterated. For these reasons it is often very difficult to determine the relative position, and thus to construct an ideal section of the strata of a series of rocks, even in a single locality. Nevertheless, in spite of all these difficulties, the method of superposition is conclusive, and takes precedence of all others whenever it can be applied. In spite of all these difficulties, if the whole geological series were present in any one locality, it would be comparatively easy to construct the geological chronology. But a series of rocks in any one locality cannot give us the whole history of the earth. Since sedimentation only takes place at the bot- tom of water, those places which were land-surfaces during any geo- logical epoch received no deposit, and therefore the strata representing that epoch must be wanting there. Now, as there have been frequent oscillations of land-surfaces and sea-bottoms in past times, similar to those taking place at the present time, we find that in every known local series of strata there exist many and great gaps, so many and so great that the record may be regarded as only fragmentary. Such gaps are usually indicated by unconformity. It is the task of the geolo- gist, by extensive comparison of rocks in all countries, to fill up these gaps, and make a continuous series. The leaves of the book of Time are scattered hither and thither over the surface of the earth, and it is the duty of the geologist to gather and arrange them according to their paging. This is done by comparison of rocks of different localities, partly by their lithological character, but principally by the fossils which they contain. 2. Lithological Character. At the present time, in our seas and lakes, deposits are forming composed of sand, clav, mud, and lime, of ev- ery kind, in different localities. The same has taken place in previous epochs. Sandstones, limestones, and slates, not differing greatly from CLASSIFICATION OF STRATIFIED ROCKS. 199 those forming at the present time, except in degree of consolidation, have been formed in every geological period. Lithological charac- ter, therefore, is no test of age. In comparing rocks of widely-sepa- rated localities, as, for example, the rocks of different continents, dif- ference of lithological character is no evidence of difference of age, nor similarity of lithological character of any value in determining a geo- logical horizon. But, as deposits are now being formed of a similar char- acter over considerable areas, so also we find strata (the deposits of pre- vious epochs), continuous and unchanged in lithological character, over large tracts of country. Therefore, in contiguous localities, similarity of lithological character becomes a very valuable means of identifying strata. If, in two localities not too widely separated, we find a similar rock, e. g., a sandstone of similar grain and color, we conclude that they probably belong to the same age, or are, in fact, the same stratum. 3. Comparison of Fossils. This is by far the best, and in widely- separated localities the only, method of determining the age of rocks. The principle of this method is that every geological epoch has its own fauna and flora by which it may be identified everywhere in spite of those slight differences which result from geographical diversity ; and, therefore, similarity of fossils shows similarity of age. There are, how- ever, certain limitations to the application of this method which must be borne in mind : (a.) The lower marine species are much affected by depths and bot- toms, and therefore we should expect that sandstone fossils, limestone fossils, and slate fossils, would differ in species even in the same epoch. Again, in lake and delta deposits, the entombed species would prob- ably be entirely different from those of marine deposits. We must be careful, therefore, to compare fossils of rocks formed under similar con- ditions. (b.) We must also make due allowance for geographical diversity. This, as we have already stated, becomes greater and greater as we pass up the series of rocks. In the lower or older rocks the geographi- cal diversity is small ; in strata of the same age in different countries the fossils are quite similar, most of the genera and many of the species being undistinguishable. It is therefore comparatively easy, by com- parison of fossils, to synchronize the strata and determine the geological horizon. In the middle rocks the geographical diversity is greater, but the general similarity is still considerable the difference between or- ganisms of consecutive epochs (geological faunae and florae) is still much greater than the difference between organisms of the same epoch in dif- ferent countries (geographical faunas and floras) ; and, therefore, it is still quite possible, by comparison of fossils, to synchronize the strata. In the higher or newer rocks the geographical diversity has become so great that we are compelled to determine age and synchronize strata, 200 STRATIFIED OR SEDIMENTARY ROCKS. no longer entirely by comparison of fossils of the different localities with each other, but also by the comparison of the fossils of each local- ity with the living species in the same locality. In these rocks we de- termine relative age by relative percentage of living species, and simi- larity of age (geological horizon) by similarity of this percentage. Manner of constructing a Geological Chronology. The manner in which a geological chronology has actually grown up, under the com- bined labors of the geologists of all countries, may be briefly stated as follows : First, the order of superposition, and therefore the relative ages of the strata composing the rock-series of many different countries, were determined independently ; next, by comparison of these, partly by lithological character, if the localities are contiguous, and partly by fossils, the geologist determines those which are synchronous and those which are wanting in each locality. Thus, out of several local series, by intercalation, he constructs a more complete ideal series. In case of doubt, he strives to find places where the doubtful strata come together, and observes their relative position. In Fig. 183, A and B represent FIG. 183 Diagram illustrating the Mode of determining the Chronological Order of Strata. two contiguous localities in which by independent study the relative po- sitions and ages of 6 and 7 strata respectively have been determined. By comparison, the rocks of the two series are found to consist of eleven strata of different ages, some being wanting in the one and some in the other locality. The figure represents the strata as con- nected and traceable from one locality to the' other, but the intervening portions between A and B may be removed by erosion, as shown by the dotted line, or covered with water. In such case, the actual overlapping cannot be observed, if it ever existed, but the comparison in other re- spects is the same. In widely-separated localities of course the compar- ison can only be made by means of fossils. Thus as the examination of the earth's surface progresses, with every new country examined some gaps are filled up, and the series becomes more perfect. Many gaps still remain unfilled. The series will continue to be made more perfect, and the chronology more complete, until the geological examination of the earth-surface is complete. CLASSIFICATION OF STRATIFIED ROCKS. 201 The second object to be attained by classification is the division and subdivision of the whole series into larger and smaller groups, corre- sponding to the eras, periods, and epochs of time. The following is an outline of the classification of Dana, slightly modified. Except in the uppermost part it is carried only as far as periods : EBAS. AGES. PERIODS. EPOCHS. 5. Psychozoic. 7. Age of Man. Human, 22 Recent. 4. Cenozoic. 6 The Age of Mam- mals. f Quaternary, [ Tertiary, ! Terrace. Champlain. Glacial. ( Pliocene. 20 -J Miocene. ( Eocene. 3. Mesozoic. 5. The Age of Rep- tiles. ( Cretaceous, } Jurassic, ( Triassic, 19 18 17 2. Palaeozoic. Carboniferous Age. "| 4. The Age of Aero- 1 gens and Am- f phibians. {Permian, Carboniferous, Sub-carboniferous, 16 15 14 Devonian. 3. The Age of Fishes. fCatskill, I Chemung, "I Hamilton, [Corniferous, 13 12 11 10 Sttunan. (>A 2. The Age of Inverte- brates. /, - ? . -'-^ 'Oriskany, 9 i "Helderberg, 8 Salina, 7 > Niagara,-^ ^_U- 6 ^Trenton, ^ 5 < .Canadian, 4 Primordial, /,,... 3 1. Archaean, or Eozoic. 1. Archaean. ( Huronian, | Laurentian, 2 1 As we have already stated, the gaps in the series are usually indi- cated by unconformity. Now, since unconformity always indicates move- ments of the crust, changes of the outlines of sea and land, changes of climate, and consequent changes in the fauna and flora, these gaps mark the times of great revolutions in the earth's history, and are therefore the natural boundaries of the eras, periods, etc. The whole rock-series, 202 UNSTRATIFIED OR IGNEOUS ROCKS. therefore, is divided, by means of unconformity and the character of the fossils, into larger groups called systems, and these again into smaller groups called series and formations. The largest groups are founded upon universal, or almost universal, unconformity, and a consequent very great difference in character of organisms; the smaller groups are founded upon a less general unconformity and less difference in char- acter of the organisms. Corresponding with the great divisions and subdivisions of the rock-system are the eras, ages, periods, and epochs of the history. The several terms expressing the divisions and sub- divisions, both of the rocks and of the history, are unfortunately used in a loose manner. We will try to use them in the manner indicated. It will be observed that the divisions are founded upon (a) unconformity, and (b) change in fossils. These generally accompany each other, since they are produced by the same cause, viz., change of physical geography. In some localities, however, they may be in discordance. In this case, the change of fossils is considered the more important, and controls classification. CHAPTER III. UNSTRATIFIED OR IGNEOUS ROCKS. Characteristics. The unstratified are distinguished from the strati- fied rocks a. By the absence of true stratification or lamination of sorted materials ; b. By the absence of fossils ; c. Usually by a crystal- line or else a glassy structure ; and, d. By their mode of occurrence, explained below. Origin. They have evidently been consolidated from a fused or semi-fused condition, and are therefore called igneous rocks. This ori- gin is clearly shown by their structure, by their occurrence in dikes and tortuous veins, by the effect they often produce upon the stratified rocks with which they come in contact, and by their resemblance to ordinary lavas. Mode of Occurrence. They occur a. Underlying the strata, and forming the great mass of the interior of the earth ; b. Forming the peaks and axes of many mountain-chains ; c. Filling fissures, often of great extent, in the stratified rocks, or other igneous rocks ; d. Over- lying strata, as if erupted through fissures and outpoured on the surface ; and, e. Lying conformably between the strata, as if forced between them in a melted condition, or else outpoured upon the bed of the sea, and afterward covered with sediments. All these positions are illus- trated in Fig. 184. In all these modes of occurrence, the observed rock is connected with an underlying mass, of which it is-but the extension. GRANITIC ROCKS. FIG. 184. Diagram showing Mode of Occurrence of Igneous Rocks. Extent on the Surface. The appearance of these rocks on the sur- face is far less extensive than that of the stratified rocks. Certainly not more than one-tenth of the surface of the earth is composed of them. But beneath they are supposed to constitute the great mass of the earth. Classification of Igneous Rocks. Igneous rocks are best classified, not by means of their relative age,. but partly by their lithological character, and partly by their mode of occurrence. They are thus divisible into three groups, viz., the granitic rocks, the trappean or fissure-eruption rocks, and the volcanic or crater-eruption rocks. After describing these, we will notice briefly attempts at classification on other bases. 1. GRANITIC ROCKS. The rocks of this group are characterized by a coarse-grained, speckled or mottled appearance, arising from the fact that they are formed by the aggregation of distinct crystals or masses of different FIG. 185. -Graphic Granite: A, cross-section ; J3, longitudinal section. colors. Granite, which is the type of this group, consists of quartz, feldspar, and mica. Sometimes the mica is wanting, and the quartz is in the form of curiously-bent laminae, which on cross-section resem- ble Hebrew or Arabic characters, disseminated in a mass of feld- spar. The rock is then called graphic granite (Fig. 185) ; when the 204 UNSTRATIFIED OR IGNEOUS ROCKS. mica is replaced by hornblende, the rock is called syenite y 1 when, in addition to the quartz and feldspar, both mica and hornblende occur, it is called syenitic granite. Most of the granite in this country is syenitic. The dark specks in granite are due to mica or to hornblende ; the opaque white, or reddish, or greenish, with distinct cleavage, is feldspar, and the grayish glassy is quartz. Chemical Composition and Kinds. Quartz is pure silica or silicic acid (SiO a ). Feldspar of this group is an acid silicate of alumina and alkali, potash or soda (orthoclase). Hornblende is a basic silicate of magnesia and lime, with also oxide of iron and alumina. Remembering also that hornblende is a black mineral, while quartz and feldspar are either colorless or very light-colored, it is evident that this group may be divided into two sub-groups, the one more acid, and the other more basic ; and in proportion as quartz and feldspar predominate, the rock is lighter colored, less dense, and more acid ; in proportion as hornblende predominates, it is darker, heavier, and more basic. Granite may be taken as the type of the more acid sub-group, and the darker varieties of syenite as the type of the more basic sub-group. These sub-groups graduate insensibly into each other. Since a general characteristic of the granitic group is the existence of free quartz in notable quantity, this group, taken as a whole, is usually regarded as more acid than the other two groups. Mode Of Occurrence. Granitic rocks commonly occur forming the axes and peaks of mountain-chains (Fig. 186, A), or as rounded masses, of greater or less extent, coming up through stratified rocks of the older series (Fig. 186, J3). They also sometimes occur as tor- tuous veins, running from an underlying mass into the stratified rocks above, as if forced by heavy pressure, while in a fused condition, into small, irregular fissures of the overlying strata (Fig. 187, A and _B), and some- times, though rarely, as dikes filling great fissures, as in the elvans of Cornwall; but it is doubtful whether these should be considered as true granites. They are probably a quartz-porphyry. We have no distinct evidence that granite is ever an eruptive rock, i. e., that it has ever been forced upward through great fissures of the earth's crust, and outpoured on the surface in the manner of lavas and 1 Some writers use the term syenite to designate a rock consisting of feldspar and hornblende only. In this case syenite would differ from diorite only in the form of the feldspar, which in the former is orthic (orthoclase), and in the latter clinic (plagioclase). FIG. 1S6. Diagram illustrating Mode of Occurrence of Granite. TRAPPEAN OR FISSURE-ERUPTION ROCKS. 205 traps. Hence also ashes, cinders, tufas, or other evidences of contact of a fused mass with the atmosphere, have never been found in connec- tion with granites. Most geologists, therefore, believe that granite, al- though it may be irruptive or intrusive by pressure, as explained above, FIG. 187. Granite Veins. is never an eruptive rock. Granitic rocks are most probably formed at great depths, and remain where they are formed. Whenever, there- fore, they appear on the surface, it is probable they have been exposed by extensive denudation. 2. TRAPPEAN OR FISSURE-ERUPTION ROCKS. Some geologists identify these with volcanic rocks, regarding their lithological differences, especially their more crystalline structure, as the result only of the fact that they have been subjected to erosion, which has exposed their deeper parts, while of modern lavas we see only the surface. But the distinctive character of these rocks consists not in their age, but in their mode of occurrence, having come up through fissures and spread out on the surface as extensive sheets, rather than through craters, and run off in streams. General Characteristics. Trappean rocks, differ from granitic in being usually finer grained ; in usually, though not always, wanting quartz and mica ; and in their mode of occurrence, explained below. In general appearance they vary greatly. They are sometimes minutely speckled with distinct crystals, sometimes compact or crypto-crystalline, sometimes even glassy or scoriaceous or tufaceous, like modern lavas. They consist usually of only two minerals, viz., feldspar (or other allied mineral replacing) and hornblende or augite. These may be inti- mately mixed or in distinct crystals. Varieties. The varieties are so numerous and run by such insen- sible gradations into each other, that it is useless to do more than mention the principal types. Like granitic rocks, they are divided into ACID 8EK1ES. Porphyry, Felstone, Phonolite, Trachyte, Pumice, Obsidian. BASIC SERIES. Diorite, Dolerite, Melaphyr, Basalt, Black obsidian. 206 UNSTRATIFIED OR IGXEOUS ROCKS. two sub-groups a more acid and a more basic. In proportion as feld- spar predominates, the rock is lighter colored and more acid ; in pro- portion as hornblende or augite predominates, it is darker colored and more basic. Felstone, phonolite, porphyry, trachyte, and the light- colored obsidians, are types of the acid series ; diorite, dolerite, basalt, and black obsidians, of the basic, as shown in the following table. Phonolite is a grayish, crypto-crystalline rock, composed chiefly of feldspar, which breaks or joints into thin slabs like slate, and rings under the hammer. Felstone and petrosi- lix have a composition similar to phonolite. The term porphyry is very loosely applied to a great va- riety of rocks in which large crystals are imbedded in a more compact matrix. Felsite porphyry, which may be considered the type, consists of a grayish or reddish feldspathic mass, containing large crystals of lighter- colored and purer feldspar. Quartz-porphyry contains distinct crystals of quartz, etc. But any rock is called porphyritic which contains large crystals, giving the mass a spotted appearance. Thus there may be a porphyritic diorite, porphyritic diabase, or even porphyritic granite. Trachyte is a light-colored, feldspathic rock, having a rough feel, from the presence of small crystals of feldspar. Diorite is a distinctly crystalline, and therefore a distinctly speckled, dark-colored rock, consisting of feldspar and hornblende. Dolerite has a similar general appearance, but finer grained, and a similar composi- tion, except that augite replaces hornblende as the basic ingredient. Melaphyr and diabase may be regarded as varieties of dolerite. Basalt is a very dark, crypto-crystalline variety of dolerite. Many of these more basic varieties of rock, when somewhat changed by weathering, are called green-stones. The glassy and scoriaceous varieties are found, but are not so com- mon in the trappean as in the recent lavas. These extreme varieties pass insensibly into each other by change of proportion of their mineral ingredients, and into the granitic series by the addition of quartz and mica. Some varieties of quartz-porphyry thus pass into granite, and some varieties of diorite into syenite. Mode of Occurrence. Trap-rocks usually occur in vertical sheets, filling great fissures intersecting the strata, or in extensive horizontal sheets outpoured on the surface. Sometimes similar sheets are found between the strata as if outpoured on the sea-bottom, and afterward covered with sediments. In some cases, however, such bedded traps are metamorphic, and not truly eruptive. Trap-dikes (as the fillings of fissures are called) vary in thickness from a few inches to fifty or TRAPPEAN OR FISSURE-ERUPTION ROCKS. 207 Fio. 188,-Dikes. even one hundred feet ; they are often fifty to one hundred miles long, and extend downward to unknown depth. When there is no overflow- ing portion observable, but the dike simply outcrops along the surface, then it is probable either that the overflow has been subsequently removed by erosion, or else that the liquid matter filled a fissure which originally did not reach the surface (as at/, Fig. 184), and has subsequently been exposed by erosion. In either case, such an outcropping dike is an evidence of extensive denudation. Sometimes the outcropping dike has resisted the erosion more than the country rock, and the dike is left standing, like a low wall, run- ning over the face of the country (Fig. 188, a) ; at other times the country rock has resisted more than the dike, and the place of the dike is marked by a slight depression like a ditch (Fig. 188, b). These appearances have given rise to the term dike. They are represented in section in the figure. Outpoured masses of fissure-eruption rocks are often of immense thickness and extent, forming, in some cases, the chief bulk of whole mountain-chains. There is also, .mr.nmm _, frequently, abundant evidence of repeated outflows over the same region, forming sheets, piled one atop of another, as shown in the figure. In such cases, on account of the peculiar columnar structure described on page 209, the crumbling of the rocks gives rise to somewhat regular terraces or benches. From this circumstance has arisen the term trap, from the Swedish word trappa, a stair. The great lava-flood of the Northwest covers an area of 150,000 to 200,000 square miles, and is 3,000 to 4,000 feet thick in its thickest part, where cut through by the Columbia River. In another place, at least seventy miles distant, where cut into 2,500 feet deep by the Des- chutes River, at least thirty successive sheets may be counted. Another great lava-flood, according to Gilbert, covers 25,000 square miles of Arizona, and is 3,000 feet thick in its thickest portions. 1 The scoriaceous and tufaceous conditions, though not so common as in crater-eruption rocks, are also often found in connection with trap, clearly indicating the contact of melted rock with the atmosphere. Effect of Dikes on the Intersected Strata, The strata forming the 1 Wheeler's " Report Geology," 1874. f---^^*-'""-'- ^ ^sU-ETT^ -7-J M^IJT _ - I~~ 208 UNSTRATIFIED OR IGNEOUS ROCKS. bounding walls of a dike are almost always greatly changed by the intense heat of the fused matter. Limestones and chalk are changed into crystalline marble ; clay is baked into porcelain jasper ; impure sandstones are changed into a speckled rock resembling gneiss ; seams of bituminous coal are changed into anthracite, or sometimes into coke. In all cases the original stratification and the contained fossils are more or less completely destroyed. These effects extend sometimes only a few feet, sometimes many yards, from the dike itself. Age how determined. "When two dikes intersect each other, then, of course, the intersecting must be younger than the intersected dike. In this manner the relative age of dikes intersecting the same region may often be determined. The absolute age of igneous rocks can only be determined by means of the strata with which they are associated. If a dike is found either intersecting (c) or outpoured upon the sur- face of strata of known age (t?, Fig. 184), the dike must be younger than the strata. If a dike (c'), intersecting strata and outcropping on the surface, is found overlaid by other strata through which it does not break, then the igneous injection is younger than the former and older than the latter. The series of events indicated is briefly as follows : first, the older series of sediments has been formed ; then fissures formed and filled by igneous injection ; then erosion has carried away the upper portion of the strata and its included dike, so that the dike outcrops along the eroded surface ; and, lastly, the whole has been submerged and again covered with sediment. 3. VOLCANIC ROCKS. Characteristics. Volcanic differ from trappean rocks, in being still finer grained ; in being more frequently glassy, scoriaceous, and tufa- ceous ; but especially in their mode of occurrence, having come up through craters instead of fissures, running off as streams instead of spreading as extensive sheets, and accumulating in the form of isolated cones instead of covering great areas of country. Some believe, also, that as a group they are more basic than either of the others. Varieties. These have already been described under Volcanoes. It is almost impossible to draw any clear lithologic distinction be- .tween this and the last group. Many of the varieties described under the last as, for example, trachyte and basalt, pumice and obsidian belong equally here. The rocks of this group also are divisible into two sub-groups : a feldspathic lighter-colored, more acid group ; and an augitic darker-colored and more basic group. As already explained under Volcanoes, both of these may exist under all the different physi- cal conditions of stony, glassy, and scoriaceous lava, and also as sand and ashes. Of the acid series, trachyte, the lighter-colored obsidians, STRUCTURES FOUND IN ERUPTIVE ROCKS. 209 and pumice, are good types ; and of the basic series are basalt and the black obsidian, and black scoriae. Of Certain Structures found in many Eruptive Hocks. Columnar Structure. Many kinds of eruptive rock, both trappean and volcanic, exhibit sometimes a remarkable columnar structure. This is most conspicuous in basalt, and is therefore often called basaltic structure. Sheets and dikes of this rock are often found composed wholly of regular prismatic jointed columns, closely fitting together, FIG. 190. Columnar Basalt, New South Wales (Da varying in size from a few inches to a foot or more, and in length from several feet to fifty or one hundred feet. When these columns have been well exposed on cliffs by the action of waves, or on river-banks by the erosive action of currents, or even by atmospheric disintegration, FIG. 191. Basaltic Columns on Se 2ntary Eock, Lake Superior (after Owen). they produce a very striking scenic effect (Figs. 190, 191). In Europe the Giant's Causeway, on the coast of Ireland, and Fingal's Cave, in the island of Staffa, on the west coast of Scotland, are conspicuous ex- amples. In the United States we have examples in Mount Holyoke, 14 210 UNSTRATIFIED OR IGNEOUS ROCKS. on the Connecticut River ; in the Palisades of the Hudson River ; in the traps on the shores of Lake Superior ; and especially in splendid cliffs of the Columbia and Deschutes Rivers, in Oregon. Direction Of the Columns. The direction of the columns is usually at right angles to the cooling surface. In horizontal sheets, therefore, the columns are vertical, but in dikes they are horizontal (Fig. 192). A dike left standing above the general surface of country sometimes FIG. 192. Columnar Dike, Lake Superior (after Owen). presents the appearance of a long pile of cord-wood. In some cases the columns are curved and twisted in a manner not easy to explain ; sometimes, instead of columnar, a ball structure is observed. Cause of Columnar Structure. There is little doubt that this structure is produced by contraction in the act of cooling. Many sub- stances break in a prismatic way in contracting. Masses of wet starch, or very fine mud exposed to the sun, crack in this way. In basalt the structure is more regular than in any other known substance. The subject of the cause of jointed columnar structure has been very ably discussed by Mr. Mallet. 1 Volcanic Conglomerate and Breccia. If a stream of fused rock, whether from a crater or a fissure, run down a stream-bed, it gathers up the pebbles in its course, and after solidification forms a conglom- erate which differs from a true conglomerate (p. 171) in the fact that the uniting paste is igneous instead of sedimentary. In a similar manner volcanic breccias are formed by the flowing of a lava-stream over a surface covered with rubble. 1 Philosophical Magazine, August and September, 1875. OTHER MODES OF CLASSIFICATION OF IGNEOUS ROCKS. 211 Amygdaloid. Still another structure, very common in lavas and traps, is the amygdaloidal. The rock called amygdaloid greatly resembles volcanic conglomerate, being appar- ently composed of almond-shaped peb- bles in an igneous paste, but is formed in a wholly different way. Outpoured traps, and especially lava-streams, are very often vesicular, i. e, filled with vapor-blebs, usually of a flattened el- lipsoidal form. In the course of time these cavities are filled with silica, carbonate of lime, or some other mate- rial, by infiltrated water holding these matters in solution. Sometimes the filling has taken place very slowly by FlG " 193 - successive additions of different-colored material. Thus are formed the beautiful agate pebbles, or more properly amygdules, so common in trap. The most common filling is silica, because water percolating through igneous rocks is always alkaline, and holds silica in solution. Other Modes of Classification of Igneous Hocks. There is no subject connected with geology which is in a state of greater confusion than the classification and nomenclature of igneous rocks. It seems proper to mention some of the different views enter- tained. Many geologists think that the three groups mentioned above are characteristic of different periods of the earth's history, and therefore associated with strata of different ages. They think that granites are associated only with the Archaean and Palaeozoic strata, traps with Mesozoic, and volcanic rocks with Tertiary and modern strata, and that therefore the earliest eruptions were granitic, then trappean, and last volcanic. Again, many think that erupted matters of different times have become progressively more basic. They think that, although each group may be divided into a more acid and a more basic sub-group, yet as a whole the granitic group is the most acid, the volcanic most basic, and the trappean intermediate, as shown in the diagram on page 212. Again, these two views, usually held by the same persons, are by them connected with a third view in regard to the original constitu- tion of the earth's crust. On first cooling the outer layer is supposed to have been highly oxidized and highly siliceous or acid, in other words granitic; beneath was a less oxidized and less acid layer, and so on, becoming less and less acid or more and more basic. The first erup- tions were from the outer layer, and therefore granitic. Afterward, 212 UNSTRATIFIED OR IGNEOUS ROCKS. as tlie solid crust grew thicker and thicker, the eruptions were from deeper and deeper layers, and therefore more and more basic. But as to age there can be no doubt that . granite, though most commonly found in the older rocks, is associated with strata of all ages up to the Middle Tertiary ; and trappean eruptions (if we use this term to mean fissure- Acid. fcasic. Granitic. Granite. Syenite. Trappean. Porphyry. I Diorite. , : Volcanic. Trachyte. Basalt. eruptions) have occurred until the later ter- tiary. The granite of Mont Blanc was pushed up about the end of the Eocene period (Lyell), and the lava-flood of the Northwest was out- poured from fissures in the Cascade Range at the end of the Miocene. Also, as to compo- sition^ trachyte has much the same compo- sition as granite, except that more of the silica is in combination and less of it free in the former than in the latter. Similarly, the dark, very hornblendic varieties of syenite have much the same chemical (though not miner- alogical) composition as basalt. Again, others, with great show of reason, think that much if not all the difference between the three groups, in mineralogical character and crystalline structure, is due to the different depths at which, and slow- ness with which, the solidification took place ; for slowness of cooling tends to produce a more complete separation and crystallization of different minerals from the fused glassy magma ; they think, therefore, that if trachyte could be traced down deep enough, it would be found to pass into porphyry, and finally into granite, and similarly basalt would become diorite, and finally dark syenites. 1 On this view, what we cannot do, erosion has done for us ; and granite is most commonly associated with the older rocks only because these have been most eroded, and therefore their deeper parts exposed. Similarly, a less erosion of the Mesozoic or Secondary rocks has exposed porphyries and diorites. Of the modern lavas, only the upper parts are exposed. If we take this view, then the only true distinction among igneous rocks is founded on the mode of eruption, viz., fissure-eruption rocks and crater-eruption rocks ; and each of these may assume the form of lava, or trap, or granite, according to the depth of formation. The confusion in the classification and nomenclature of igneous rocks is still further increased by the undoubted fact that nearly all the varieties of igneous rocks mentioned above are found also among meta- morphic rocks, which have not been erupted at all. This subject is further treated under the head of Metamorphism. 1 This gradual change has very recently been distinctly observed in Southeastern Eu- rope by Judd. Geological Magazine, 1876, vol. xxxii., p. 292. METAMORPHIC ROCKS. 213 CHAPTER IV. METAMORPHIC ROCKS. THERE is a third class of rocks, intermediate in character between the ordinary sedimentary and the igneous rocks, and therefore put off until these had been described. The rocks of this class are stratified, like the sedimentary, but crystalline, and usually non-fossiliferous, like the igneous rocks. They graduate insensibly on the one hand into the true unchanged sediment, and on the other into true igneous rocks. Origin. Their origin is evidently sedimentary, like other stratified rocks, but they have been subsequently subjected to heat and other agents which have changed their structure, sometimes entirely destroy- ing their fossils and even their lamination structure, and inducing in- stead a crystalline structure. The evidence of their sedimentary origin is found in their gradation into unchanged fossiliferous strata ; the evidence of their subsequent change by heat, in their gradation into true igneous rocks. For this reason they are called metamorphic rocks. Position. All the lowest and oldest rocks are metamorphic. The converse, however, viz., that metamorphic rocks are always among the oldest, is by no means true. Metamorphism is not, therefore, a test of age. Metamorphic rocks are found of all ages up to the Tertiary. The Coast Range of California is much of it metamorphic, although the strata belong to the Tertiary and Cretaceous periods. Metamorphism seems to be universal in the Laurentian, is general in the Palaeozoic, frequent in the Mesozoic, exceptional in the Tertiary, and entirely wanting in recent sediments. It is therefore less and less common as we pass up the series of rocks. The date of metamorphism is also different from that of the origin of the strata. Metamorphism has taken place in all geologi- cal periods, and is doubtless now progressing in deeply-buried strata. Metamorphism is also generally associated with foldings, tiltings, in- tersecting dikes, and other evidences of igneous agency, and is there- fore chiefly found in mountainous regions. It is also usually found only in very thick strata. Extent on the Earth-Surface. These rocks exist, outcropping on the surface, over wide regions. Nearly the whole of Canada and Labra- dor, a large strip on the eastern slope of the Appalachians, and a large portion of the mountainous regions of the western border of this con- tinent, are composed of them. Beneath the surface they probably un- derlie all other stratified rocks. Their thickness is also often immense. 214: METAMORPHIC ROCKS. The Laurentian series of Canada is probably 40,000 feet thick, and metamorphic throughout. Principal Kinds. The principal kinds of metamorphic rocks are: Gneiss, mica-schist, chlorite-schist, talcose-schist, hornblende-schist, clay-slate, quartzite, marble, and serpentine. Gneiss, the most universal and characteristic of these rocks, has the general appearance and mineral composition of granite, except that it is more or less distinctly stratified. Often, however, the stratification can only be observed in large masses. Gneiss runs by insensible grada- tions, on the one hand, into granite, and on the other, through the more perfectly stratified schists, into sandy clays or clayey sands. FIG. 194 Gneiss. The schists are usually grayish fissile rocks, made up largely of scales of mica, or chlorite, or talc. Hornblende-schist is similarly made up of scales of hornblende, and is therefore a very dark rock. The fissile structure of schists is due to the presence of these scales, and is therefore wholly different from that of slates. It is called foliation-structure. /Serpentine is a compact, greenish magnesian rock. The other varie- ties need no description. Hornblende-schists run by insensible grada- tions into clay-slates on the one hand, and into diorites and syenites on the other. All these kinds may be regarded as changed sands, limestones, and clays, the infinite varieties being the result of the difference in the original sediments and the degrees of metamorphism. Sands and limestones are often found very pure ; such when metamorphosed produce quartzite and marble. Clays, on the contrary, are almost always impure, containing sand, lime, iron, magnesia, etc. Such impure clays, if sand is in excess, produce by metamorphosis gneiss, mica-schist, and the like ; but if lime and iron are in considerable quantities they produce hornblende- schist or clay-slate ; if magnesia, talcose-schist. The origin of serpen- tine is not well understood ; but it is evidently a changed magnesian clay. All gradations between such clays and serpentine may be found in the Tertiary and Cretaceous strata of the Coast Range of California. THEORY OF METAMORPHISM. 215 Theory of Metamorphism. There are few subjects more obscure than the cause of metamor- phism, and the conditions under which it occurs. Some important light has been thrown on it, however, recently. For the sake of clearness, it will be better to divide metamorphism into two kinds, somewhat dif- ferent in their causes, viz., local and general. Local Metamorphism is that produced by direct contact with evident sources of intense heat, as when dikes break through stratified rocks. As already seen (p. 207), under these circumstances, impure sandstones are changed into schists, or into gneiss ; clays, into slates, or into porce- lain jasper; limestones, into marbles; and bituminous coal, into coke, or into anthracite. In these cases it is evident that the cause of the change is the intense heat of the incandescent, fused contents of the dike at the moment of filling. In such cases of local metamorphism, the effects usually extend but a few yards from the wall of the dike. General Metamorphism. But in many cases we cannot trace the change to any evident source of intense heat. Rocks, thousands of feet in thickness, and covering hundreds of thousands of square miles, are universally changed. The principal agents of this general metamorphism seem to be heat, water, alkali, pressure. That heat is a necessary agent is sufficiently evident from the gen- eral similarity of the results to local metamorphism. But that the heat was not intense, and therefore not sufficient of itself to produce the ef- fects, is also quite certain. For (a.) metamorphic rocks are often found interstratified with unchanged rocks. Intense heat would have affected them all alike, or nearly alike. (#.) Many minerals are found in meta- morphic rocks which will not stand intense heat. As an example, carbon has been found in contact with magnetic iron-ore, although it is known that this contact cannot exist, even at the temperature of red-heat, with- out reduction of the iron-ore, (V.) The effect of simple dry heat, as shown in cases of local metamorphism, does not extend many yards. (d.} Water- cavities are found abundantly in metamorphic rocks. This will be more fully explained farther on. Water. Heat combined with water seems to be the true agent. Recent experiments of Daubree, Senarmont, and others, prove that water at 400 C. (= 752 Fahr.) reduces to a pasty condition nearly all ordinary rocks ; moreover, that at this temperature crystals of quartz, feldspar, mica, augite, etc., are formed. Such a pasty or aqueo-fused mass slowly cooled would form a crystalline rock containing crystals of quartz, feldspar, mica, etc. ; in other words, would be metamorphic. The quantity of water necessary for these effects is shown by experiment to be very small only five to ten per cent. In other words, the in- cluded water of sediments is amply sufficient. 216 METAMORPHIC ROCKS. Alkali. Alkaline carbonates, or alkaline silicates, so common in natural waters, greatly promote the process, causing the aqueo-igneous pastiness or aqueo-igneous fusion to take place at a much lower tem- perature. Pressure. Simple pressure is not itself a direct agent, but the necessary condition for the action of the others, since it is impossible to have high temperature in the presence of water without corresponding pressure. It is evident, therefore, that while metamorphism by dry heat would require a temperature of 2,000 to 3,000, in the presence of water the same result is produced at 300 or 400 C. ; or in the presence of alkali, even in small amount, probably at 300 or 400 Fahr. Application. All these agents are found associated in deeply-buried sediments. Series of outcropping strata are often found 20,000 or even 40,000 feet thick. The lower strata of such a series, by the regular increase of interior heat alone, must have been, before uptilting, at a temperature of between 700 and 800 Fahr., a temperature sufficient, with their included water, to produce complete aqueo-igneous pasti- ness, and therefore, by cooling and crystallization, complete meta- morphism. Suppose, then, a b 5, Fig. 195, represent the contour of land and sea-bottom at the beginning of any period, and the dotted line i i the FIG. 195. Diagram illustrating the Invasion of Sediments by the Interior Heat. isogeotherm of 800. If, now, sediments 40,000 to 50,000 feet thick be deposited so that the sea-bottom is raised to V b', then the isotherm of 800 will rise to i' i' and invade the lower portions of the sediments with their included water. Such sediments would be completely changed in their lower portions, and to a less extent higher up. It is probable that even 300 to 400 Fahr. is sufficient to produce a consid- erable degree of change ; or even 200, if alkali be present. Crushing. Although simple gravitative pressure is only a condi- tion, and not a cause, of heat, horizontal pressure with crushing of the crust, by the conversion of mechanical energy into heat, becomes, as Mallet has shown, 1 an active source of this agent. Now, in all cases 1 "Philosophical Transactions," 1873, p. 147. ORIGIN OF GRANITE. 217 of metamorphism we find ample 'evidences of such horizontal crushing in the associated foldings and cleavage of the strata. Explanation Of Associated Phenomena. This theory readily ex- plains 1. Why metamorphism is always associated with great thick- ness of strata; 2. Why the oldest rocks are most commonly meta- morphic, since these have usually had the newer rocks piled upon them, and have been subsequently exposed by erosion. The newer rocks are sometimes also metamorphic, but in these cases they are very thick. 3. It also explains the interstratification of metamorphic with unchanged rocks ; since some rocks are more easily affected by heated water than others, and the composition of the included water may be also different, some containing alkali and some not. 4. It also explains its association with foldings of strata and with mountain-chains, as will be more fully explained hereafter. If metamorphism is only produced in deeply-buried sediments, then the exposure of such rocks on the surface can only result from exten- sive erosion. Origin of Granite. There is much reason to believe that most granites are npt the re- sult of simple dry fusion, as is usually supposed ; but, on the contrary, only the last term of metamorphism of highly-siliceous sediments. Ac- cording to this view, incipient pastiness by heat and water makes gneiss ; complete pastiness, completely destroying stratification, makes granite. The principal arguments for this view may be briefly stated as follows : 1 1. In many localities in mountain-regions, and nowhere better than in the Sierras of California, every stage of gradation may be observed between clayey sandstones and gneiss, and between gneiss and granite. So perfect is this gradation, that it is impossible to draw sharply the distinction. Even geologists who believe that granite is the primitive rock have been compelled to admit that there is also a metamorphic granite, scarcely distinguishable from primitive granite. 2. Not only gneiss, but even granite, is sometimes interstratified with unchanged fossiliferous rock. 3. Chemists recognize two kinds of silica, viz., an amorphous va- riety of specific gravity 2.2, and a crystallized variety, specific gravity 2.6. These two varieties differ from each other not only in density, but also in chemical properties, the former being much more easily attacked by alkalies than the latter. By solidification from fusion (dry way) only the variety of specific gravity 2.2 can be formed, while the variety 2.6 is formed only by slow deposit from solution (humid way). 1 Rose, Philosophical Magazine, xix., p. 32 ; Delesse, " Archives des Sciences," vol. vii., p. 190; Hunt, American Journal of Science and Arts, new series, vol. i., pp. 82, 182. 218 METAMORPHIC ROCKS. Now, the quartz of granite is always of the variety 2.6, and therefore must have been formed in presence of water. 4. The several minerals of which granite is composed will not sep- arate from a fused granitic magma on cooling; but, on the contrary, fused granite solidifies into a highly-siliceous glass. The only answer to this, as well as to the preceding, is that the behavior of the granitic magma, when fused on a large scale and cooled slowly in the laboratory of Nature, is possibly different from its behavior when melted in small masses and cooled less slowly in our laboratories. 5. Crystals of quartz, feldspar, and mica, are frequently formed in Nature by the humid process, as, for example, in, metamorphic rocks ; and have also been artificially formed by the same process by Daubree, Senarmont, and others, as already stated (p. 215) ; but they have never been formed artificially by the dry way. 6. In nearly all rocks and minerals microscopic cavities are found indicating the conditions under which crystallization or solidification took place. If crystals are formed by sublimation, they contain vacu- ous cavities. If they are formed by solidification from fusion (dry way), and if gases are present, they may contain air-blebs ; but, if they crystallize slowly from a glassy magma, they contain spots of glassy matter or glass cavities, as in slags and lavas. If they are formed by crystallization from solution, then they have fluid cavities. Now, not only are these fluid cavities found in metamorphic rocks, but also in the quartz and feldspar of granite. " A thousand millions of these micro- scopic cavities in a cubic inch is not at all unusual ; and the inclosed water often constitutes one to two per cent, of the volume of the quartz." * Besides these fluid cavities, however, glass cavities are also found in the quartz and feldspar of granite. These facts point plainly to the agency of both heat and water in the formation of granite. Even the temperature at which metamorphic rocks and granite solidified has been approximately determined by Mr. Sorby. The prin-- ciple on which this is done is as follows : If crystallization from solution, or solidification in the presence of water, take place at ordinary temper- atures, then the fluid cavities will be full ; but if at high temperatures, and the mass subsequently cools, then by the contraction of the con- tained liquid a vacuous space will be formed which will be larger, in proportion to the amount of contraction, and therefore to the tempera- ture of solidification. Knowing, therefore, the relative sizes of the vacuole and the contained water, and the coefficient of expansion of the water and the rock, the temperature at which the cavity would fill (which is the temperature of solidification) may be calculated. Some- times this temperature may be gotten by actual experiment, i. e., by heating until the cavity fills. By this method Mr. Sorby has calculated 1 Sorby, Quarterly Journal of the Geological Society, vol. xiv., pp. 3*29, 453. ORIGIN OF GRANITE. 219 the temperature of solidification of certain metamorphic rocks of Corn- wall as 392 Fahr., and of some granites as 482, and others only 212. It seems almost certain, therefore, that most granites have not been formed by dry, igneous fusion. Yet that this rock has been in a liquid or pasty condition is perfectly certain from its occurrence in tortuous veins. Therefore it has been rendered pasty by heat in the presence of water under great pressures, such as always exist in deeply -buried strata. The weight of the superincumbent strata, or else pressure by folding and crushing of the strata, has forced it into cracks and great fissures. What we have said of granite applies of course to the whole gra- nitic group. Granitic rocks are often only the last term of the metamor- phism cf sediments ; granite being produced from the more siliceous sediments, and dark syenites from the mere basic impure clays. But we cannot stop with this group. It is certain that many if not all the rocks of the Trappean group also may be made by metamorphism of sediments. Many bedded diorites, dolerites, and felsites, are undoubt- edly formed in this way, for the gradations can be distinctly traced into slates. Prof. Dana * has recently recognized this as so certain that he proposes the addition of the prefix metct to these to indicate their origin. Thus he recognizes a syenite and a metasyenite, a diorite and a metadiorite, dolerite and metadolerite, felsite and metafelsite, etc., and we might add granite and metagranite. Many geologists push these views so as to include also even the true lavas. Deeply-buried sediments under gentle heat in the presence of water and pressure undergo incipient change and form metamorphic rocks ; under greater heat become pasty and form granite, metasyenites, metadiorites, metafelsites, etc. ; under still greater heat, increased probably, as Mallet suggests, by mechanical energy in crushed strata being converted into heat, become completely fused, and are then out- poured upon the surface either by the elastic force of the steam gener- ated, or by the pressure and squeezing produced by the folding of the crust of the earth, so common in mountainous regions. According to this view, every portion of the earth's crust has been worked over and over again, passing through the several conditions of soil, sediment, stratified rock, metamorphic rock, and igneous rock, perhaps many times in the course of the geological history of the earth, and we look in vain for the primitive rock of the earth's crust. 1 American Journal of Science and Arts, vol. xi., p. 119, February, 1876. 220 STRUCTURE COMMON TO ALL ROCKS. CHAPTER V. STRUCTURE COMMON TO ALL ROCKS. WE have thus far given a brief description of the three classes of rocks, their structure and mode of occurrence. There are still, how- ever, several important kinds of structure which are common to all these classes of rocks, and require description. These are joints, fissures, and veins. Mountain-chains, as involving all kinds of rocks and all kinds of structure, must be taken up last. SECTION 1. JOINTS AND FISSURES. Joints. All rocks, whether stratified or igneous, are divided, by cracks or division-planes, in three directions, into separable irregularly prismatic blocks of various sizes and shapes. These cracks are called joints. In stratified rocks the planes between the bedding constitute one of these division-planes, while the other two are nearly at right angles to this and to each other, and are true joints. In igneous rocks all the division-planes are of the nature of joints. In sandstone these blocks are large and irregularly prismatic ; in slate, small, confusedly rhom- Fio. 196. Regular Jointing of Limestone. boidal ; in shale, long, parallel, straight ; in limestone, large, regular, cubic; in basalt, regular, jointed, columnar ; in granite, large, irregularly cubic or irregularly columnar. On this account a perpendicular rocky FISSURES, OR FRACTURES.. 221 cliff usually presents the appearance of huge, irregular masonry, with- out cement. The cause of joints is probably the shrinkage of the rock in the act of consolidation from sediments (lithification), as in stratified rocks, or FIG. 197. Granitic Columns. in cooling from a previous condition of high temperature, as in the igneous and metamorphic rocks. Fissures, or Fractures. These must not be confounded with joints. Joints are cracks in the individual strata or beds; fissures are fractures in the earth's crust, passing through many strata, and even sometimes through many formations. The former are produced by shrinkage ; the latter by movements of the earth's crust. Fissures, therefore, are often fifty or more miles in length, thirty to fifty feet in width, and pass downward to unknown but certainly very great depths. Cause. The cause of great fissures is evidently always movements, and usually foldings or wrinkling of the earth's crust, produced probably by contraction of the interior portions, as will be explained under Moun- tain-Chains, page 240. The natural tendency of such foldings would be to form a parallel system of fissures in the direction of the folds, and therefore at right angles to the direction of the folding force. Fissures are usually thus found in systems parallel among themselves, and to the axes of mountain-chains. Through such fissures igneous rocks in a fused condition are often forced, forming dikes and overflowing sheets. Besides the principal fissures just explained, Hopkins has shown that, in the case of the formation of mountains, there would be formed also other smaller fissures at right angles to these. Often the walls on the two sides of a fissure do not correspond with each other, but one side has been pushed up higher or dropped down lower than the other. Such a displacement is called a fault, a slip, or dislocation. This may occur in fissures in any kind of rock, but is most marked and most easily distinguished in stratified rocks. When the 222 STRUCTURE COMMON TO ALL ROCKS. strata are sufficiently flexible to admit it, they are bent instead of brok- en, and a monocline is formed instead of a fault (Fig. 198). When the fissure is filled at the moment of its formation with fused matter from beneath, it is called a dike. When it is not filled at the moment of its formation with igneous injection, but slowly afterward with other mat- ter, and by a different process, it is called a vein. FIG. 19S. -Section of Nutria-Fold, New Mexico (after Gilbert). Faults. In faults the extent of vertical displacement varies from a few inches to hundreds or even thousands of feet. In the Appalachian chain there occur faults in which the vertical dislocation is 5,000 to 20,000 feet. In Southwest Virginia, according to Rogers, there is a line of fracture extending parallel to the Appalachian chain for eighty miles, in which there is a vertical slip of 8,000 feet, 1 the Lower Silurian being brought up on one side until it comes in conjunction with the Lower Carboniferous on the other (Fig. 199). In Western Pennsylvania, FIG. 199.-Fault in Southwest Virginia : a, Silurian ; d, Carboniferous (after Lesley). according to Lesley, there is another fault extending for twenty miles, in which the lowermost of the Lower Silurian is brought up on a level with the uppermost of the Upper Silurian, the whole Silurian strata be- ing at this place 20,000 feet thick, so that one may stand astride of the fissure with one foot on the Trenton limestone (Lower Silurian), and the other on the Hamilton shales (Devonian). 2 On the north side of the Uintah Mountains there is a slip, according to Powell, of nearly 20,000 feet, 3 The Sevier Valley fault, Utah, may be traced partly as a slip, partly as a monocline, for 225 miles (Gilbert). 1 Dana's " Manual," p. 399. 5 " Manual of Coal," p. 147. 3 " Exploration of Colorado River," p. 156. FISSURES, OR FRACTURES. 223 If such slips were suddenly produced by violent convulsion, then, at the time of formation, there must have been a steep (Fig. 200), or sometimes even an overhanging, escarpment (Fig. 199), equal to the displacement. In some cases there is such an escarpment or line of steep mountain-slope corresponding to the line of slip. In the Colo- rado Plateau region the north and south cliffs are produced by faults (Powell). The Zandia Mountains, New Mexico, are produced by a Fit;. 200. drop of 11,000 feet on the western side, leaving an escarpment still 7,000 feet high (Gilbert). In the Basin Range region also many of the ridges are formed by faults. But in most cases there is no such escarpment, the two sides of the fault having been cut down to one level by subsequent erosion, so that the unpractised eye detects nothing unusual along the line of fracture and slip. In Fig. 200 the strong I FIG. 201. Strata repeated by Faults. line a a shows the present surface, while the dotted line bbb shows the surface after the displacement as it would be if unaffected by erosion. In many cases, however, it seems more probable that there never existed any such escarpment as represented in Fig. 200, but that the displacement was produced by a slow, creeping motion, or else by a succession of smaller sudden slips probably accompanied with earth- 224 STRUCTURE COMMON TO ALL ROCKS. quakes (p. 106), and thus that the slipping and the denudation have gone on together pari passu. In Fig. 243, on page 263, the upper part shows the great Uintah fault restored, while the lower part shows the actual condition of things produced by erosion. FIG. 202. Section through Portion of Plateau Region of Utah, showir (after Howell). : a Succession of Faults When faults occur in inclined outcropping strata, the same series of strata may be repeated several times, as in Fig. 201. In such a case, the observer walking over the surface of the cbuntry from A to B FIG. 203.-Fault with Change of Dip: d, dike. might suppose here a series of nine strata, whereas there are but three strata, a, b, c, three times repeated. Fig. 202 is a natural section show- ing this. Sometimes the dip of the strata on the two sides of a fault are not parallel, the change of inclination being effected at the time of the dis- placement, as shown in Fig. 203. Upon the eroded surface of such dislocated strata, by subsequent subsidence, other strata may be unconformably depos- ited (Fig. 204). FIG. 204. Unconformity on Faulted Strata. Law of Slip. In faults the plane of fracture is sometimes vertical, but much more generally it is more or less inclined. In such cases, in by far the larger number of great faults, the strata on the upper side (hanging wall) of the fracture have dropped down, while the strata on the lower side (foot-wall) have gone up, as in Figs. 205 and 206. This would probably be the case if, after the fracture, the relation of parts was adjusted by gravity alone. In some cases of strongly-folded strata, however, the hanging wall seems to have been pushed and made to slide upward over the foot-wall as if by powerful horizontal squeezing. This is the case with the great slip in Southwestern Virginia, repre- sented in Fig. 199. Examples of this kind, however, are exceptional. In several hundred cases of great fissures, examined by Phillips, in MINERAL VEINS. 225 England, nearly all followed the law given above. 1 Fig. 205 is a sec- tion across Yarrow Colliery, in which all the slips follow this law. Of FIG. 205. Section across Yarrow Colliery, showing the Law of Faults (after De la Beche). the numerous slips figured by Powell, Gilbert, and Howell, as occurring in the Plateau and Basin Range region, nearly all follow this law. Fig. 206 is a section illustrating this fact. East * FIG. 206. Section of Pahranagat Range, Nevada, showing the Law of Faults (after Gilbert). SECTION 2. MINERAL VEINS. All rocks, but especially metamorphic rocks in mountain-regions, are seamed and scarred in every direction, as if broken and again mended, as if wounded and again healed. All such seams and scars, of whatever nature and by whatever process formed, are often called by the general name of veins. It is better, however, that dikes and so- called granite-veins, or all cases of fissures filled at the moment of formation by igneous injection, should be separated from the category of veins. True veins, then, are accumulations, mostly in fissures, of certain mineral matters usually in a purer and more sparry form than they exist in the rocks. The accumulation has in all cases taken place Kinds. Thus limited, veins are of three kinds : Veins of segrega- tion, veins of infiltration, and great fissure-veins. These three, how- ever, graduate into each other in such wise that it is often difficult to determine to which we must refer any particular case. Some writers make many other kinds. 1. Veins of Segregation. In these the vein-matter does not differ greatly from the inclosing rock. Such are the irregular lines of granite in granite, the lines differing from the inclosing rock only in color or texture ; also irregular veins of feldspar in granite or in gneiss. Under the same head belong also the irregular streaks, clouds, and blotches, so 1 Phillips's " Geology," p. 35. , 15 226 STRUCTURE COMMON TO ALL ROCKS. common in marble. In these cases there seems to be no distinct line of separation between the vein and the inclosing rock no distinct wall to the vein. The reason is, these veins are not formed by the filling of a previously-existing fissure, but by the segregation of certain materials, in certain spots and along certain lines, from the general mass of the rock, either when the latter was in plastic condition from heat and water, or else by means of percolating water, somewhat as concretions of lime, clay, iron-ore, and flint, are formed in the strata (p. 188). 2. Veins of Infiltration. Metamorphic rocks have, probably in all cases, been subjected to powerful horizontal pressure. Besides the wide folds into which such rocks are thus thrown and the great fissures thus produced, the strata are often broken into small pieces by means of the squeezing and crushing. The small fissures thus produced are often filled by lateral secretion from the walls, or else by slowly-percolating waters holding in solution the more soluble matters contained in the rocks. The process is similar to the filling of cavities left by imbedded organisms (p. 193), and still more to the filling of air-blebs in traps and lavas, and the formation of agates and carnelian amygdules (p. 211). In veins of this kind, therefore, a beautiful ribbon-structure is often produced by the successive deposition of different-colored materials on the walls of the fissure. Veins of this kind also, since they are the filling of a previously-existing fissure, have distinct walls. The filling consists most commonly of silica or of carbonate of lime. 3. Fissure - Veins. These are fillings of the great fissures produced by movements of the earth's crust. When these fissures are filled at the time of formation by igneous injection, they are called dikes ; but if subsequently with mineral matter, by a different process, to be dis- cussed hereafter, they are fissure-veins. These veins, therefore, like dikes, outcrop over the surface of the country often for many miles, fifty or more. Like dikes, also, they are often many yards in width, and extend to unknown, but certainly very great, depths. Like dikes and fissures, also, they occur in parallel systems. Characteristics. The most obvious characteristics of the veins of this class are their size, their continuity for great distances and to great depths, and their occurrence in parallel systems. As the vein is a filling of a previously-existing fissure, the distinction between the vein and the wall-rock is usually quite marked. In many cases, in fact, the vein-filling is separated from the wall-rock by a layer of earthy or clayey matter called a selvage, as if the sides of the open fissure had become foul by percolating waters carrying clay, before the fissure was filled with mineral matter. The contents of fissure-veins are also far more varied than those of other classes. Irregularities. Although more regular than other kinds, yet fis- sure-veins are also often quite irregular sometimes branching, some- MINERAL VEINS. 227 times narrowing or pinching out in some parts and widening in others (Fig. 207), sometimes dividing and again coming together, and thus inclosing a portion of the wall-rock (Fig. 208). Such an inclosed mass of country rock in the midst of a vein is called a " horse" Many of these irregularities are probably the result of movements after the fis- sure was formed, or even after it was filled. Thus, it abed (Fig. 207) be one wall of an irregular vein, then it is probable that a' b' c d' was the original position of this wall ; but, before it was filled, it slipped up FIG. 20T. -Irregularities in Veins. Fto. 208. Irregularities of Veins. to its present position. Again, movements may reopen a fissure after it is filled. In such cases, if the adhesion of the filling to the wall is strong, portions of the wall- rock are torn away; and if a second filling takes place, a " horse " is formed. Thus aaa and bbb (Fig. 208) represent the two original walls of an irregular vein ; but subsequent movement reopened the fissure to b' b' b' and tore away the horse H, after which the vein was again filled. Veins, of course, usually intersect the strata; but in some cases where strata-planes, or else cleavage-planes, are highly inclined, the opening is between these planes, and the veins are, therefore, conform- able with them. Metalliferous Veins. Some metals, particularly iron, occur prin- cipally in great beds, being accumulated by a process already described (p. 136). Others, especially lead, often accumulate in flat cavities be- tween the strata, especially of limestone. But most metals occur in veins. All the kinds of veins mentioned above may contain metals, but the segregative veins are usually too irregular and uncertain, and the infiltrative veins too small, to be profitable. True, profitable metal- liferous veins are almost always great fissure-veins. We will speak, therefore, only of these, and the further description of fissure-veins is best undertaken under this head. Contents. The contents of metalliferous veins are of two general 228 STRUCTURE COMMON TO ALL ROCKS. kinds, viz., vein-stuffs and ores. The principal vein-stuffs are quartz, carbonate of lime (calc-spar), carbonate of baryta, carbonate of iron, sulphate of baryta (heavy spar), and fluoride of calcium (fluor-spar). By far the most common of these is quartz, and next is calc-spar. Often, however, the vein-stuff is an aggregate of minerals forming a true rock. Nearly the whole of a vein consists usually of vein-stuff. The ore exists in comparatively small quantities, sometimes forming a cen- tral rib or sheet, as if deposited last (Fig. 209) ; sometimes in irregular isolated masses called bunches or pockets, or in small strings, or grains, irregularly scattered through the vein-stuff and extending often a little way into the wall-rock. The chemical forms in which metals occur are very various ; some- times they occur as pure metal (as always in the case of gold and plat- inum, and sometimes in the case of silver and copper), but more com- monly in the form of metallic sulphides, metallic oxides, and metallic carbonates. Of these the metallic sulphides are by far the most com- mon. It is worthy of remark that all these forms are comparatively very insoluble. The same is true of the vein-stuffs. Ribboned Structure. The ribboned or banded structure, already spoken of under Veins of Infiltration, is very commonly found in great fissure-veins. This structure is as characteristic of veins as the colum- nar structure is of dikes. The layers on the two sides usually corre- spond to each other (Fig. 209) ; sometimes the successive layers are of different color, giving rise to a beautiful, striped appearance. Some- times the successive layers on both sides are of different materials, as in Fig. 210, in which the central rib, <7, is galena, and a or, b b, c c, are successive layers of quartz, fluor, and baryta. Sometimes, in cases of quartz-filling, the layers are agate, except the centre, which is filled up with a comb of interlocking crystals, as in Fig. 211. The same occurs often in amygdules, the last filling being crystalline. Sometimes there is evidence of successive openings and fillings, as in Fig. 212, where a represents quartz crystals, interlocking in the centre and based on agate layers, b b, while c represents quartz with disseminated copper pyrites. MINERAL VEINS. 229 In this case it seems probable that 1 and 2 were the walls when the agate and quartz filling took place, and that afterward the fissure was I CL I FIG. 211. reopened along 2, so that the walls became 2 and 3, and the new fis- sure thus formed was filled with cuprif- erous quartz. The same is well shown in Fig. 213, where a, #, c, IBONIFEROUS DEVON lf\N S I L U R JAN LAUREN T I A N SYSTEM S YS T E M Psychozoic. Cenozoic. Mesozoic. Archaean, or Eozoic. Fio. 245. will be seen that the ages correspond with the eras, except in the case of the Palaeozoic era. This long and diversified era is clearly divisible into three ages. Subdivisions. The subdivisions of eras and ages into periods and epochs are founded, as already explained (p. 201), on less general un- conformity in the rock-system, and less conspicuous changes in the life- system. The names of periods are often, and of epochs are nearly always, local, and therefore different in different countries. We will, of course, use those appropriate to American geology. The table on page 201 represents, as far as periods, the classification used in this country. We have added epochs only in the uppermost part, viz., in the Tertiary and Quaternary. We give, also (Fig. 246), an ideal diagram of the principal groups of strata, which we shall notice in the order of their superposition, indi- cating also the principal places of general unconformity. Order of Discussion. Many geologists take up the several epochs and periods of the history of the earth in the inverse order of their oc- currence. Commencing with a thorough discussion of " causes now in operation" i. e., geological history of the present time, as that which is best known, they make this the basis for the study of the epoch immediately preceding, and which, therefore, is most like it. Having acquired a knowledge of this, the studsnt passes to the preceding, and so on. This has the great advantage of passing ever from the better known to the less known, which is the order of induction. Other GENERAL PRINCIPLES. 2T1 geologists prefer to follow the natural order of events. This has the great advantage of bringing out the philosophy of the history the law of evolution. The first EK ' method is the best method of in- ' sy vestigation ; the second method is the best method of presenta- tion. In common with most 4 cenozoie. geologists, I shall, therefore, fol- low the order of history. As in human history, so in the geological history, the recorded events of the earliest times are very few and meagre, but become 3 ; M esozoic..., more and more numerous and in- teresting as we approach the present time. Our account of the Archaean era will, therefore, be quite general, and will not en- ter into any subdivisions, al- though this era was very long. In the next era we will go into the description of the several ages, in the next into the pe- riods, and in the next even into 2. Palaeozoic., the epochs. Prehistoric Eras. Previous to even the dimmest and most imperfect records of the history of the earth there is, as already said (p. 265), an infinite abyss of the unrecorded. This, however, hardly belongs strictly to geol- ogy, but rather to cosmic philos- ophy. We approach it not by i- Archrean... written records, but by means of more or less probable general scientific reasoning. We pass on, therefore, without pause to the lowest system of rocks contain- FIG. 246. Ideal General Section of the Whole Series of J Strata, showing the Principal Divisions and Subdi- ing the record of the earliest era. visions. Carboniferous. Silurian. 272 LAURENTIAN SYSTEM OF ROCKS CHAPTER II. LAURENTIAN SYSTEM OF ROCKS AND ARCHAEAN ERA. IT is one of the chief glories of American geology to have estab- lished this as a distinct system of rocks and a distinct era. It had been long known that beneath the lowest Palaeozoic rocks there still existed strata of unknown thickness, highly metamorphic and apparently destitute of fossils.' These had been usually regarded as lowermost Palaeozoic as the earliest defaced leaves of the Palgeo- zoic volume. But the study of the Canadian rocks, by Sir William Logan, revealed the existence of an enormous thickness of highly-con- torted, metamorphic strata, everywhere unconformable with the over- lying Potsdam or lowest Silurian. More recent observations show this relation not only in Canada, but also in New York, on Lake Superior, in Nebraska, Montana, Idaho, Wyoming, Colorado, Utah, Nevada, Texas, New Mexico, and Arizona. Nor is it confined to our own coun- try, for the same unconformable relation has been found by Murchison on the west coast of Scotland, between the lowest Silurian (Cambrian) and an underlying gneiss, evidently corresponding to the Laurentian of Canada. Similar rocks, and in similar unconformable relation, have been found underlying the lowest Silurian in Bohemia, and also in Swe- den and Bavaria. Such general unconformity shows great and wide- spread changes of physical geography at this time. There seems no longer any doubt, therefore, that it should be regarded as a distinct system. The following figures give the relation between the Palaeozoic and the Laurentian in New Mexico, in Canada, and in Scotland. FIG. 247. Section across Santarita Mountain, New Mexico: c. Carboniferous; S, Silurian; A, Archscan ; m, metalliferous vein (after Gilbert). These, then, are the oldest known rocks. They form the first vol- ume of the recorded history of the earth. Yet they evidently are not the absolute oldest ; evidently they do not constitute any part of the primitive crust. For they are themselves stratified or fragmental AND ARCILEAN ERA. 273 rocks, and therefore formed from the debris of other rocks still older than themselves ; and these last possibly from still older rocks. Thus, FIG. '248. Section showing Primordial unconformable on the Archaean : 1, Archaean or Laurentian ; Primordial or Lowest Silurian (after Logan). FIG. 249. Diagram Section, showing the Structure of the North dial ; c, Lower Silurian (Jukes). : a, Laurentian ; 6, Primor- we search in vain for the so-called primary rocks of the original crust. Thus is it with all history. No history is able to write its own beginning. Rocks. There is nothing very characteristic in the rocks of the Laurentian system. They do not differ very conspicuously from those of other periods ; consisting, in fact, only of altered sandstones, lime- stones, and clays. They are all, however, very much contorted and very highly metamorphic. In Canada they consist mainly of the schist series, passing on the one hand into gneiss and granite, and on the other into hornblehdic gneiss, syenites, and diorites; of sandstones, passing into qnartzites; and of limestones, passing into marbles, which are sometimes even intrusive. These together, in Canada, form a series of rocks at least 40,000 feet thick. Interstratified with these are found immense beds of iron-ore 100 or more feet thick, and great quantities of graphite, sometimes impreg- nating the rocks, and sometimes in pure seams. In rocks of this age occur the great iron-beds of Missouri, of New Jersey, of Lake Superior, and of Sweden. The quantity of iron found in these strata is far greater than in any other. It may be well called the Age of Iron. The following figures show the contortion of the strata (Fig. 250), and the mode of occurrence of the iron (Figs. 251, 252). FiQ. 252. 74 LAURENTIAN SYSTEM OF ROCKS Area in North America. 1. These strata cover the greater portion of Labrador and Canada, and then, turning northwestward, extend to an unknown distance, but probably to the Arctic Ocean. The area forms a broad V, within the arms of which is inclosed Hudson's Bay. It may be seen on map, p. 278. This is the only extensive area known on the continent. 2. On the eastern slopes of the Appalachian chain un- doubted patches are found as far south as Virginia, and a considerable area in this region is referred with much probability to the same. This is shown on map, p. 278, as the area left blank. Its further extension southward along the chain is still doubtful, though probable. 3. In the Rocky Mountain region extensive lines and areas of outcrop are known, trending in the general direction of the chain, especially a large area in the Basin region. 4. Several small patches are also found scat- tered about in the basin of the Mississipi, apparently exposed by erosion. Doubtless the Laurentian rocks are far more extended, but covered and concealed by other and later rocks. The area mentioned is the area of surface-exposure. It represents so much of Archaean sea-bot- tom as was subsequently raised into land, and not afterward again covered by sediments; or, if so covered, again exposed by erosion. Physical Geography of Archaean Times. As these are the oldest known rocks, we know nothing of the land from which they were formed. But since, during the rest of the geological history, the con- tinent has developed from the north toward the south, it seems most probable that this earliest land lay still farther north, and disappeared when the Laurentian area was elevated into land. Time represented. The enormous thickness of these rocks (40,000 feet in Canada, and still greater in Bohemia and Bavaria) certainly in- dicates a very great lapse of time. It is probable that the Archaean era is longer than all the rest of the recorded history of the earth put together ; and yet, precisely as in the beginnings of human history, the record is almost a blank. The events are few, and imperfectly re- corded. Evidences of Life. We have already explained (p. 136) how iron- ore is ^present accumulated. We have there shown that all accumu- lations or this kind now going on are formed by the agency of organic matter. It is almost certain that the same is true for all times, and therefore that iron-ore accumulations are the sign of the existence of organic matter, and quantity of the ore accumulated is a measure of the amount of organic matter consumed in doing the work. The immense beds of iron-ore found in the Laurentian rocks are, therefore, evidence of the existence of organisms in great abundance. That these organ- isms were chiefly vegetable, we have the further evidence derived from the great beds of graphite ; for graphite, as we shall see hereafter is only the extreme term of the metamorphism of coal. AND ARCHAEAN ERA. 275 Of the existence of animal organisms the evidence is not yet com- plete, although it is probable that the lowest forms of Protozoa, such as Rhizopods, were abundant. We have seen that limestones are abun- dant among the Laurentian rocks. Now, the limestones of subsequent FIG. 253. Fragment of Eozoon, of the Natural Size, showing Alternate Lamince of Loganite and Dolomite (after Dawson.) geological epochs are almost wholly composed of the accumulated shelly remains of lower organisms, especially nullipores and coccolites among plants, and rhizopods among animals. The existence of rhizopods is believed by many to have been demon- strated. There have been found abundantly, in the Laurentian lime- stones of Canada, of Bohemia, of Bavaria, and elsewhere, large, irreg- ular, cellular masses, which are fwmur-/'-'-' "> believed by the best authori- iglliii i ,...,,, ( ties to be the remains of a gi- gantic foraminiferous rhizopod. The supposed species has been called Eozoon l Canadense. Fig. 253 is a section of an Eozoonal mass, natural size, in which the white is the cal- careous matter secreted by the rhizopod, and the dark corre- sponds to the animal matter of ,. , . ., Fro. 254,-Diagram of a Portion of Eozoon cut vertically: the rhizopod itself ; and Flff. A, B, - much discussion as to the organic or mineral nature of these curious structures. If these irregular masses be indeed of animal origin, as 1 Dawn animal. 276 PALAEOZOIC SYSTEM OF ROCKS. seems most likely, it is evident that they belong to the lowest forms of compound protozoa lower far than the symmetrically-formed forami- nifera of later times. It is precisely in such almost amorphous masses of protoplasmic matter that, according to the evolution hypothesis, the animal kingdom might be expected to originate. Some very obscure tracings, suggesting the possible existence of marine worms, have been found both in Canada and in Bohemia ; but as yet we have no reliable evidence of any animals higher than the pro- tozoa. It is impossible to say that other animals of low form did not exist ; yet the absence of any reliable trace in rocks not more metamor- phic than some of the next era, which are crowded with fossils of many kinds, seems to indicate a paucity, if not an entire absence, at this time, of such animals. CHAPTER III. PRIMARY OR PALAEOZOIC SYSTEM OF ROCKS AND PALAEOZOIC ERA. General Description. THIS is a distinct system of rocks, revealing a distinct time-world a distinct rock-system, containing the record of a distinct life-system. The rock-system is distinct, being everywhere unconformed to the Laurentian below and the Secondary above a bound volume volume second of the Book of Time. The life-system is also equally distinct, being conspicuously different from that which precedes and that which follows. Whatever of life existed before, its record is too imperfect to give us a clear conception of its character. But in the Palaeozoic the evidences of abundant and very varied life are clear ; about 20,000 spe- cies having been described. It stands out the most distinct era in the whole history of the earth. The Archaean must be regarded as the mythical period. Here, with the Palaeozoic, commences the true dawn of history. Rocks Thickness, etc. The rocks of this system, although less powerful than the preceding, are also of enormous thickness compared with those of later geological times, being in the Appalachian region about 40,000 feet. It is believed that we are safe in saying that the time represented by them is equal to all subsequent time to the present. There is nothing very characteristic in the rocks composing Palaeo- zoic strata, though the practised eye may often distinguish them by their lithological character. Though strongly folded and highly meta- GENERAL DESCRIPTION. 277 morphic in some regions, these characters are not so universal as in the Laurentian. In the United States the rocks of the whole system are conformable. In Europe, on the contrary, the principal divisions are usually uncon- formable. In this country, therefore, the subdivisions are founded almost wholly on change in the life-system ; while in Europe the same subdivisions are founded on unconformity of the rock-system, as well as change in the life-system. Further, in this country, in passing from Pennsylvania, through New York, into Canada, we pass over the out- cropping edges of the whole system, from the highest to the lowest ; and finally into the Laurentian (Fig. 255). This, taken in connection with the conformity of the rocks, shows that during the Palaeozoic the continent in this part was successively developed, from the north toward the south, by bodily upheaval of the Laurentian area and successive ex- posure of contiguous sea-bottom. In Europe the oscillations seem to have been more frequent and violent. Fig. 255 is a section from Pennsylvania to Canada, showing the JJS ' li S I A. FIG. 255. Ideal Section north and south from Canada to Pennsylvania : A, Archsean ; L S and US, Silurian ; Z>, Devonian ; C, Carboniferous. relation of the subdivisions to each other, and the manner in which they lie on the Archaean. This will be better understood if the map on page 284 be at the same time carefully examined. Area in the United States. The area in the Eastern United States in which the country rock belongs to this system is seen in the map given on next page (Fig. 256). It may be stated roughly to embrace all that part included between the Great Lakes on the north, the Blue Ridge of the Appalachian chain on the east, the Prairies on the west, and Middle Alabama and Southern Arkansas on the south. It includes the richest portion of our country. Besides this great continuous area there are also areas of imperfectly known size and shape in the Rocky Mountain region, and on either side of the Sierra Nevada. If we compare the Palaeozoic rocks of the Appalachian region with the same in the central portion of the Mississippi basin, we ob- serve the following changes as we go westward : (a.) The rocks in the Appalachian region are highly metamorphic ; as we go westward, they become less and less so, until in the region about the Mississippi River they are wholly unchanged, (b.) In the Appalachian region they are strongly and complexly folded; as we go west, these folds pass into 278 PALAEOZOIC SYSTEM OF ROCKS. gentle undulations, which die away into horizontally (see section on p. 244). (c.) In the Appalachian region they are about 40,000 feet thick ; as we go west, they thin out until the whole series is only 4,000 feet at the Mississippi, (d.) In the Appalachian region grits and sand- GENERAL DESCRIPTION. 279 stones and shales predominate greatly over limestones ; as we go west, the proportion of limestone increases, until these are the predominating rocks. These four changes are closely connected with each other, and all with the formation of the Appalachian chain, as we have already explained in the chapter on Mountain-Formation (p. 252, et seq.). Physical Geography of the American Continent. At the beginning of the Palaeozoic era the land was the Laurentian area, already described, excepting Archaean areas exposed by erosion. From this nucleus, dur- ing Palaeozoic times, the continent was developed southward, until, at the end, it included also the Palceozoic area just described. The accompanying map (Fig. 257) ' gives approximately the area of land at FIG. 257. Existing Seas. Lakes, etc.. shaded black ; Portions of Continent then covered, lighter; Land of that Time left white ; where Outline known, surrounded with Full Line ; when doubtful, by Dotted Line. the beginning. The map of the physical geography of Cretaceous times (p. 452) gives somewhat less approximately its area at the end. It will be seen that the continent was already sketched out at the begin- ning, and steadily developed throughout its continuance. There is much reason to believe that a considerable body of land existed at this 1 A map similar to the above, but containing also small scattered patches of Archaean exposures, is sometimes spoken of as an Archaean map of North America, or map of Archaean land. It must be borne in mind, however, that it represents indeed land of Archaean strata, but, for that very reason, not of Archaean time, but of Silurian time. 280 PALAEOZOIC SYSTEM OF ROCKS. time to the east of the Appalachian region, much of which afterward disappeared by subsidence. It is only thus that we can explain the thick strata of this region. Subdivisions. The Palaeozoic era is divided into three ages, which are embodied in three distinct subordinate rock-systems. These ages are each characterized by the dominance of a great class of organisms. They are : 1. The Silurian System, or Age of Invertebrates, or some- times called Age of Mollusks ; 2. The Devonian System, or Age of Fishes; and, 3. The Carboniferous System, or Age of Acrogens and Amphibians. These are three chapters in the Palaeozoic volume. These three systems are generally conformable with each other in the Palaeozoics of the United States, as we have already shown, but elsewhere they are often unconformable. Before taking up the first in the order of time, viz., the Silurian, it is necessary to say something of the interval which in our record separates the Archaean from the Palaeo- zoic era. The Interval. We have already seen that the lowest Silurian lies unconformably on the upturned and eroded edges of the crumpled strata of the Lauren tian. We have also shown (p. 179) that unconform- ability indicates always an oscillation of the earth's crust at the ob- served place. More definitely it indicates an upheaval, by which the lower series of rocks became land-surface, and were at the same time, perhaps, crumpled ; then a long period unrecorded at that place, during which the land was eroded and the edges of the crumpled rocks were ex- posed; then a subsidence, and the deposit of the upper series of rocks on these exposed edges. Now, oscillation necessitates increase and decrease of land-surface. Evidently, therefore, such increase and decrease of land- surface took place in the unrecorded interval between the Archaean and Palaeozoic eras ; and the length of this unrecorded interval is measured by the amount of erosion which the Laurentian underlying the lowest Palaeozoic has suffered. We have stated that the land at the beginning of the Silurian age was approximately the Laurentian area. The shore- line of the earliest Palaeozoic sea was the line of junction between the Silurian and Laurentian (see map, p. 278). But this was not the shore- line at the end of the Archaean time. , Evidently this shore-line was much farther south ; evidently the land-area was much greater at the end of the Archaean than at the beginning of the Silurian. The Archae- an era was closed by the upheaval into land-surface and the crumpling of the strata of the whole Laurentian area, and much more. Then fol- lowed an interval of which we know nothing, except that it was of long duration, during which the crumpled Laurentian strata forming the then land-surface were deeply eroded. Then, at the end of this interval came a subsidence down to the shore-line already indicated as the Silurian shore-line, and the Silurian age commenced, its first sediments being GENERAL DESCRIPTION. 281 of course deposited on the exposed edges of the submerged Laurentian rocks. I have attempted to illustrate these facts by the following diagrams (Fig. 258), in which c represents a section north and south through the Laurentian and Palaeozoic rocks. The crumpled Laurentian strata, with their outcropping eroded edges, are seen to underlie the lowest Silurian to some distance. This is the actual condition of things. The manner FIG. 258. Ideal Sections, showing how Unconformity is produced. in which this condition was brought about is shown in a and b. In a we have represented the supposed condition of things during the inter- val, s I being the sea-level, and s the shore line ; in b the condition of things at the end of the interval or beginning of the Silurian, when by subsidence the shore-line had been shifted northward to s', and on the exposed edges of the strata of the previous land-surface, from s to s', Silurian sediments had begun to deposit. We have spoken thus far only of the unconformity of the New York rocks on the Canadian rocks. This phenomenon may be explained, as we have seen, by local oscillations, with increase and decrease of land- area during the lost interval. But, when we remember that the same unconformity is found in the most widely-separated localities, over the whole area of the United States, we are forced to the conclusion that the lost interval, as compared with the Silurian, was probably a continental period a period of widely-extended land composed of 282 PALAEOZOIC SYSTEM OF ROCKS. Laurentian rocks. The whole of this land disappeared by submergence at the beginning of the Silurian, except the Canadian area, and prob- ably a considerable area in the Basin region, and perhaps a few islands or larger areas in Silurian seas between. In all speculations on the origin of the animal kingdom by evolu- tion, it is very necessary to bear in mind this lost interval, for it was evidently of great duration. SECTION 1. SILURIAN SYSTEM : AGE OF INVERTEBRATES. The Rock-System. The rocks of this age have been carefully studied in England, by Sedgwick and Murchison ; in Russia and Sweden, by Murchison ; in Bohemia, by Barrande ; and in New York, by Hall. The divisions and subdivisions established by these geologists have become the standard of comparison elsewhere. The system was first clearly defined by Murchison in Wales. The name Silurian (from Silures, the Roman name for the inhabitants of Wales) was given by him, and is now universally adopted. But the most perfect examples are, perhaps, those found in Bohemia and in New York. We have already given (Fig. 255) a section of the Palaeozoics of New York, including, of Course, the Silurian. Some geologists call the lower portion Cambrian a name given bv Sedgwick. Subdivisions. The following table gives the divisions and subdi- visions of the Silurian system and the corresponding periods of this age in this country : rOriskany Period. TT r,., . I Lower Helderbere Upper Silurian. j Sa]ina [Niagara ( Trenton Lower Silurian. -I Canada [Primordial The larger divisions, viz., Lower and Upper Silurian, are generally recognized ; also, the Primordial is generally recognized ; by some as a subdivision of the Silurian, by others as more distinct than the other periods and as synonymous with Cambrian. The subdivisions are, with this exception, local, each country having its own ; but they are synchronized, as far as possible, by comparison. Character of the Rocks. The Silurian, like nearly all rocks, are greatly disturbed and metamorphosed in mountain-regions, though less so than the Laurentian ; but in Sweden and Russia, and in the valley of the Mississippi, Silurian rocks are found in their original horizon- tal position, and not greatly changed from their original sedimentary condition. Area in America. By turning to the map (p. 278) it will be seen : 1. That the Silurian is attached to the Laurentian nucleus as an irregu- SILURIAN SYSTEM: AGE OF INVERTEBRATES. 283 lar border on the outer side of the V-shaped area ; 2. Again, the Ap- palachian Laurentian region is also bordered on the west side by Silurian ; 3. Also we observe large patches in the interior one about Cincinnati, another occupying the southern portion of Missouri and northeastern portion of Arkansas, and one in Middle Tennessee ; 4. Also, large areas are known to occur in the Rocky Mountain region and in the Basin region between the Wahsatch Mountain and the Sierra ; but their outlines are yet too little known to describe them accurately. Physical Geography. At the beginning of the Silurian, as already said, the land was approximately the Laurentian area (Fig. 257). The Silurian, which embraces the great V-shaped Laurentian area on the southeast, south, and southwest, was then the sea-bottom border of the coast of that Silurian continent. The Silurian bordering the Appalachian Laurentian was also then a sea-bottom bordering the Silurian continent in that region. It is probable, also, that the Silurian of the Rocky Mountain region also borders Laurentian areas, and these areas repre- sent Silurian continents, and the Silurian border the marginal sea-bot- tom of that time. The other patches mentioned in the interior were probably bottoms of open seas. Now, the Silurian area represents so much of Silurian sea-bottoms as were raised into land-surfaces during or at the end of Silurian times, and not subsequently covered by sea. 1 Therefore, at the beginning of Silurian times the land was the Laurentian area ; while at the end of the Silurian times the land was increased by the addition of the Silurian area. This addition was not all made at once, but very gradually. The steps of this increase have been carefully studied in New York. The following map (Fig. 259) shows the principal successive steps, as does also the section (Fig. 255) with which it should be compared. Inspec- tion of these figures shows not only the Silurian bordering the Laurentian, but the rocks of the several periods bordering each other successively ; so that in walking from Pennsylvania to Canada, or to the Adirondack Mountains of New York, we successively walk over the Carboniferous, the Devonian, the Silurian, and the Laurentian ; and in the Silurian over rocks of the successive periods, from the highest to the lowest. This plainly shows that during Silurian times the continent (Laurentian area) was slowly upheaved, and contiguous sea-bottoms successively added to the land, and the shore-line gradually pushed southward from the Canadian region, and probably westward from the land-mass along the Appalachian. Of course, therefore, the oldest Silurian shore-line was the most northern and eastern. This is the primordial beach. Primordial Beach and its Fossils. As already stated, the element- ary character of this treatise renders it impossible to take the several 1 This is true as a broad, general fact ; but patches of Silurian may also be exposed by removal of later deposits by erosion. 284 PALEOZOIC SYSTEM OF ROCKS. periods of this age. We must confine ourselves to a general descrip- tion of the age only. But there is so peculiar and special an interest connected with the dawn of life on the earth, that, before taking up the life-system of the whole age, it seems necessary to say something of the earliest fauna. We have seen that at the beginning of Silurian times a large V- shaped mass of land occupied the region now embraced by Canada and Labrador, and stretched northwestward to an unknown distance, the two arms of the V being nearly parallel to the two present shores of the American Continent ; further, that a land-mass of extent unknown occupied the position of the eastern slope of the Appalachian chain ; also, that land of unknown extent occupied the position of the Rocky Mountains ; and the continent was thus early sketched out. Now, southward of the first-mentioned land-area and betw r een the other two FIG. 259. Oeolojrical Map of New York : o, Archaean ; PS, Primordial ; LS, Lower Silurian ; US, Upper Silurian ; d, Devonian ; SC, Subcarboniferous ; C, Coal-measures. there was a great interior sea, which we will call the Interior Palceozoic Sea. The shores of that sea beat upon the continental masses north, east, and west, and accumulated, on exposed places, a beach. Patches of that earliest beach still remain. They are found, of course, closely bordering the Laurentian rocks, Canadian and Appalachian, and lying un- conformably upon them. They are the primordial sandstones and slates of Canada, New York, Pennsylvania, Virginia, and probably Tennes- SILURIAN SYSTEM: AGE OF INVERTEBRATES. 285 FIGS. 260-69. AMEBICAX PRIMORDIAL FOSSILS: 260. Plant: Scolithus linearis (after Hall). 261. Brachiopod : a and 6, Linsrula acmninata (after Lo^an). 262. Lineula antiqua (after Hall). _ 263. Gasteropod : Ophileta compacta. 264. Cephalopod : Orthoceras. 265. Pteropod : Hyahthis primor- dialis (after White). 266. Tracks: Crustacean (after White). 267. Trail of Marine Worm (after Logan). 263. Conocoryphe Kingii (after White). 269. Agnostus interstrictus (after White). 286 PALEOZOIC SYSTEM OF ROCKS. see, and possibly Georgia. The fact that these are indeed remnants of a beach is proved by the existence, in almost every part, of shore-marks of all kinds such as ripple-marks, sun-cracks, worm-tracks, worm-bor- ings, broken shells, etc. This, then, is the old primordial beach. It is of the extremest inter- est to the geologist because it marks the outline of the earliest Silurian sea, and contains the remains of the earliest Silurian fauna. Indeed, we may say it contains the remains of the earliest known fauna. It is true, the lowest Rhizopods probably existed in Archaean times, but these cannot be said to constitute a fauna. With the very commence- ment of Silurian times, however, we find at once a considerable variety of animal forms. What, then, was the character of this earliest fauna and flora ? If we could have walked along that beach when it was washed by pri- mordial seas, what would we have found cast ashore ? We would have found the representatives of all the great types of animals except the vertebrata. The Protozoa were then represented by sponges and Rhizopods ; the Radiates by Hydrozoa (graptolites) and Cystidean Crinoids ; the mollusks by Brachiopods, Gasteropods (Pleurotomaria), Pteropods, and even Cephalopoda (orthoceras) ; and the Articulates by Crustaceans (trilobites, etc.) and Worms. Plants are represented by Fucoids. These widely-distinct classes are already clearly differentiated and somewhat highly organized. Nor is the fauna a meagre one in number of species. In the United States and Canada alone about 200 species are already known, of which nearly 100 are trilobites. About a dozen species of plants are also known. When we recollect the great FIGS. 270-278FoRKiGN PRIMORDIAL FOSSILS: 270. Oldhamia antiqua. probably a plant. 271 Arenico- 272. Linpulclla ferrufrinea. 273. Theca Davidii. 274. Modiolopsis lites didymus. Worm-tubes, solvensis. 275. Orthis Hicksii. Olenus macrurus. 270. Obolella sagittalis. 277. Hymenocaris vermicauda. 278. SILURIAN SYSTEM: AGE OF INVERTEBRATES. 287 age of these rocks and their usual metamorphism, and the fragmentary character of all fossil fauna, it seems certain that great abundance and variety of life existed already in these early seas. Of this life the tri- lobites, by their size, their abundance, their variety, and their high organization, must be regarded as the dominant type. Among the largest trilobites known at all are some from this period. The Para- doxides, represented in Figs. 279 and 280, attained a length of twenty FIG. 279. Paradoxides Bohemicus, Foreign. FIG. 280. Paradoxides Hsrlani, x J (after Kogers), American. inches. English beds of the same age furnish specimens of the same genus two feet long. We give in the above figures a few of the more remarkable primor- dial forms taken from the rocks of this country, and of foreign coun- tries. They are intended only to give a general idea of the fullness and variety of the primordial life ; the affinities of these fossils will be discussed hereafter. General Remarks on First Distinct Fauna. There are several points of great philosophic interest suggested by the nature of these first organisms : 1. Plants in this, and in all other geological periods, are far less numerously represented in a fossil state than animals. This cannot be because animals were more abundant than plants, for since the animal kingdom subsists on the vegetable kingdom, and since every animal consumes many times its own weight of food, plants must have been always more abundant than animals. The true reason of the greater abundance of animal remains is to be found in the fact that the hard parts of animals are far more indestructible than any portion of vege- table tissue. . 2. At the end of the Archa3an times when the Archaean volume 288 PALAEOZOIC SYSTEM OF ROCKS. closed we find only the lowest Protozoan life. But with the opening of the next era, apparently with the first pages of the next volume, we find already all the great types of structure except the vertebrata. And these not the lowest of each type, as might have been expected, but al- ready trilobites among Articulata, and Cephalopods among Mollusca animals which can hardly be regarded as lower than the middle of the animal scale. We must not hastily conclude, however, that these widely-divergent and highly-organized types originated together at once. We must re- member that between the Archaean and Palaeozoic there is a lost interval of enormous duration. Evidently, therefore, the Primordial fauna is not the actual first fauna. Evidently we have not yet recovered the leaves in which is recorded the gradual differentiation of these widely-distinct types. All this must have taken place during the lost interval. But if, on the other hand, we suppose, as many do, that evolution proceeds always " with equal steps," then we are forced to the very im- probable conclusion that the lost interval is equal to all geological times which followed to the present ; for the differentiation of types which occurred during that interval is equal in value to all that has taken place since. Therefore, we are compelled to admit that there have been in the history of the earth periods of rapid change in physical geography, and periods of comparative quiet in this respect ; that, corresponding with these, there have been also periods of rapid evolution of the organic kingdom, developing new forms, and periods in which forms are more stationary. The periods of rapid change are marked by unconformity, and are therefore unfortunately often lost. As we proceed, we will probably find many examples of rapid change which must be accounted for in a similar manner. General Life- System of the Silurian Age. There were evidently extraordinary abundance and variety of life in the Silurian. These early seas literally swarmed with living beings. The quantity and variety of life the number of individuals and of species were probably not less than at the present time ; though orders, classes, and departments, were less diversified. Over 10,000 species have been described from the Silurian alone (Barrande) ; and these must be regarded as only a small fragment of the actual fauna of the age. In cer- tain favored localities, the number of species found in a given area of a single stratum will compare favorably with the number now existing in an equal area of our present sea-bottoms. Yet, in all this teeming life there is not a single species similar to any found in any other geological time. And not only are the species peculiar, but even the genera, the families, and the orders, are different from those now existing. GENERAL LIFE-SYSTEM OF THE SILURIAN AGE. 89 FIG. 2S4. r , G . 28 5. FIGS. 281-286. SILURIAN PLANTS: 2S1. Sphenothallus an, back-view; c, front-view, showing opening (after Nicholson). PALEOZOIC SYSTEM OF ROCKS. FIGS. 305. 306. GBAPTOLITES : 305. Dendrograptus Ilallianus (after Hall). 306. Graptolites Clintonensis (after Hall). Polyzoa. There are many kinds of compound coralline animals, probably allied to the Bryozoa (sea-mats) of our present seas, found in the Silurian. The doubtful affinities of these Palseozoic forms, and the difficulty of separating them sharply from certain forms of true corals on ffl FIG. 807. Living Polyzoa : Flustra truncata : a, natural size ; &, enlarged to show the cells. the one hand, and from certain forms of graptolites on the other, seem to require their notice in this connection, although their affinities are probably molluscoid. Two of the Silurian forms are represented on page 297, Figs. 308 and 309. EcMnoderms. During Silurian times the class of Echinoderms was represented principally by Crinoids, A Crinoid is a stemmed Echino- derm, usually with branching arms. The animal consists of a long jointed stalk, rooted to the sea-bottom, and bearing atop a rounded or ANIMALS. 297 pear-shaped body, covered with calcareous plates (calyx), from the mar- gin of which spring the arms, which may be long and profusely branched, FIGS. 80S and 309. SIIXRIAN POLYZOA : 308. Fenestella elegans (after Hall). 809. Alecto auloporoides (after Hall). or short and simple, or absent altogether. In the middle of the calyx, between the bases of the arms, is placed the mouth. Their general structure and appearance will be better understood by examination of the following figures of living Crinoids. FIG. 810. FIG. 811. FIGS. 310 and 311. LIVING CRIXOIDS: 310. Rhizocrimis Lofotensis (after Thompson). 811. Pentacrinus Caput-Medus.i?. . At present, leaving out the Holothurians, or sea-cucumbers, which, having no shell, are little apt to be preserved as fossils, the class of 298 PALEOZOIC SYSTEM OF ROCKS. Echinoderms may be conveniently divided into three orders, viz. : the Echinoids, or sea-urchins ; the Asteroids, or starfishes ; and the Cri- noids. The members of the first and second orders are free moving, while those of the third are stemmed. Of these orders the Crinoids are the lowest, as proved not only by their simpler organization, but also by the fact that a living Crinoid, the Comatula, is attached when young, but free when mature. FIG. 812. A Living Free Crinoid Comatula rosacea. the Feather-Star : , free adult ; b, fixed young (after Forbes). Now, in Silurian times, the stemmed Echinoderms are very abun- dant, while the free are very rare : at the present time, on the contrary, the reverse is the case. Thus, in the course of time, the former de- creased until they are now almost extinct, while the latter increased until they are now very abundant. If we take the abundance of Echino- derms during geological times as constant, and represent the course of FIG. 813. Diagram showing the General Distribution in Time of Stemmed and Free Echinoderms. time by the absciss A B (Fig. 313), and the abundance by distance from A JB to C D, then the parallelogram would represent this fact. If, now, we draw the diagonal, C J3, then the shaded triangle would ANIMALS. 299 represent the stemmed, and the unshaded the free, and the diagonal the line of decrease of the one and increase of the other; and the whole figure the general relations of the two sub-classes throughout time. In the Palaeozoic the stemmed predominate ; in the Mesozoic the two are equally represented ; in modern times the free predominate. FIG. 318 a. FIG. 319. FIGS. 314-819. SILURIAN CRINOIDS: 314. Caryocrinus ornatus. 315. Pleurocystitis squamosus. 816. Pseudocrinus bifasciatus. 317. Lepadocrinus Gebhardii. 318. Glyptocrinus deeadactylus (after Hall) : a, specimen with arms ; 6, larger specimens without the arms. 319. Ichtbyocrinus sublaevis (after Hall). Stemmed Echinoderms, or Crinoids, may be divided into three fami- lies, viz.: 1. Crinids ; 2. Cystids ; 3. Blastids. Crinids are the typi- cal Crinoids, with branching arms, already illustrated from living exam- 300 PALAEOZOIC SYSTEM OF ROCKS. FIGS. 320-824 SiLtmrAN CRINOFDS ANT> ASTEROIDS: 820. Mnriacrinus noWlissiaius (after Hall). 321. Ilomocrimis sroparius (after Hall). 322. Heterocrinus simplex (after Meek). 323. Protaster Scdg- wickii. S24. 1'aliuaster Sba'fferi (after Hall). ANIMALS. 301 pies (Figs. 310-312). Cystids are of a bladder-like form (hence the name), and are either without arms, or else have few, short, simple arms springing from near the centre of the upper part of the body, the mouth being probably on one side. The radiated structure in these is imperfect. Blastids (Gr. 0/la<7TOC, a bud} had a bud-shaped body, with five petalloid spaces (ambulacra) radiating from the top and reaching half-way down the body (see Figs. 510-513, p. 382). If Crinids are com- parable to inverted Starfishes with many arms and set upon a stalk, the Cystids and Blastids may be compared to Sea-urchins similarly set. All these families are found in the Silurian. The Cystids pass away with the Silurian, and are therefore characteristic of that age. The Blastids pass away before the end of the Carboniferous age, and are therefore characteristic of the Palaeozoic era. The Crinids continue, though in diminished numbers, to the present day. Figures of Blastids are given under the Carboniferous, where they were far more abundant. Mollusks AcepTials or Bivalves. Bivalves may be divided into two great sub-classes, viz., Lamellibranchs (leaf-gills) and Brachiopods (arm-feet). The valves of Lamelli- branchs are right and left ; those of Brachiopods are upper and lower, or dorsal and ventral. Brachiopods 'are much less highly organized than the other sub-class, and differ so essentially in their organization that some of the best naturalists remove them not only from the class of Acephals, but from the department of Mollusca, and ally them rather with the Worms. Their general resemblance in external form to bivalves makes it more convenient to treat them under that head, until the question of their affinity is more definitely settled. General Description of a Brachiopod. A Brachiopod shell consists of two valves, a dorsal and a ventral. The ventral is the larger, and usually projects beyond the dorsal, at the hinge, as a prominent beak. This pro- jecting portion is often perforated to give passage to a muscular peduncle, by which the shell is attached in the living animal. The following figures (Figs. 325, 326) of Brachiopods, living and extinct, will make these points clear. The viscera of a Brachiopod fill but a small space in the shell, this cavity being occupied principally by two long spiral arms (hence the name), which probably subserve the functions of respiration and alimentation. These arms are attached to a curious bony apparatus, sometimes itself spiral in form. Figs. 327-329 show the internal structure described above. In the present seas the Lamellibranchs are extremely abundant, anatina, showing the muscular ped- uncle bv which the shell is attached. 302 PALAEOZOIC SYSTEM OF ROCKS. FIG. 326. Rhynchonella sulcata: side-view, dorsal view, and showing suture FIG. 329. FIGS. 827-329. SHOWING THE STBrcrrRE OF BKACHTOPODS : 827. Spirifer striatus (Carboniferous) : a, dorsal surface ; b, interior, showing the bony spirals. 828. Tercbratula llavrsrcns diving species): . exterior surface ; 6, showing bony structure for attachment of spiral arms. 329. Spirifer hysterica (Carboniferous) : a, exterior; 6, showing bony spires. while the Brachiopods are nearly extinct, being represented by very few species. In Silurian times, on the contrary, the very reverse is the case, bivalve shells being represented mostly by Brachiopods. Taking the number of bivalve species throughout geological times as constant, then the general relation of these two sub-classes to each in time may be roughly represented by the following diagram, in which the lower ANIMALS. 303 triangle represents Brachiopods, the upper Lamellibranchs, and the com- mon diagonal the line of decrease of one and increase of the other. FIG. 330. Diagram showing the General Alteration of Brachiopods to Lamellibranchs. The abundance of individuals and the number of species of this order in Silurian times are almost incredible. The following figures represent some of the common and characteristic forms. FIGS. 331-334.-Sii.cRiAX BRACIUOPODS : 331. Orthis Davidsonii. 332. Orthis porcata. 833. Spirifer Cumberlandiaa: a, ventral valve; 6, dorsal valve; c, suture. 334. Pentamerus Knightii. It is very difficult to give any general distinctive mark of Silurian Brachiopods, although, of course, the species and even the genera are peculiar, and may be recognized by the paleontologist. It may be said, however, that the straight-hinged or square-shouldered Brachiopods, including the Spirifer family, the Strophomena or Leptena family, and 304 PALAEOZOIC SYSTEM OF ROCKS. the Productus family, are characteristic of the Palaeozoic, though not of the Silurian. Lamellibranchs. We have said that Lamellibranchs are also found in the Silurian, but not so abundantly as the Brachiopods. Lamelli- branchs are divided into Siphonates and Asiphonates, i. e., those with FIG. 336. FIG. 337. FIG. 338. FIG. 389. FIGS. 335-339. SILURIAN LAMELLIBRANCHS: 335. Orthonota parallela. 336. Cardiola interrupta (after Hall). 837. Avicula Trentonensis (after Ilall). 338. Ambonychia belli striatus (after Hall). 3S9. Tellenomya curta (after Hall). and those without breathing-siphons behind. The Siphonates are the higher. At present the Siphonates are the more abundant in Palaso- zoic times the Asiphonates. We give seme figures above. FIG. 340. FIG. 841. FIG. 342. FIGS 340-342 SILURIAN GASTEROPODS: 840. Pleurotomaria dryope. 341. Pleurotoiuaria agave. 842. Murchisoni gracilis. ANIMALS. 305 Gasteropods Univalves. Land and fresh-water Gasteropods have not been found in the Silurian. If we divide marine Gasteropods or univalves into those having beaked shells and those having smooth- mouthed or beakless shells, the former being carnivorous and the latter herbivorous, then only the smooth-mouthed or beakless shells have FIG. 345. FIG. 344. FIG. 346. FIGS. 343-346. SILURIAN GASTEROPODS AND PTEROPODS : 843 Cyrtolites compressus (after Hall). 844. Cyrtolites Trentonensis (.after Hall). 345. Cyrtolites Dyeri (after Meek). 346. Conularia Trentonensis (after Hall), a Pteropod. been found in the Silurian. The beaked-shelled are usually regarded as the more highly-organized class. The affinities of Conularia (Fig. 346) and Tentaculites are little understood. They are usually placed among Pteropods. Cephalopods Chambered Shells. These are by far the most high- ly organized of Mollusks, and the most powerful among Invertebrates. They are represented in the present seas by the Nautilus, the Squids, and the Cut- tle-fishes. If we divide all known Cepha- lopods into Dibranchs (two-gilled) and f ... Tetrabranchs (four-gilled), the former be- ing naked and the latter shelled, then, at the present time, the Dibranchs, or naked, vastly predominate, there being only a single genus of shelled or Tetrabranchs known, viz., the Nautilus, and of this genus only three or four species. In the Silurian age, and. for many ages after- ward, only the shelled existed. The naked or Dibranchs are decidedly the higher in organization. 20 FIG. 347. P pilius) : c c, hood ; nel. arly Nautilus (Nautilus pom- mantle ; b, its dorsal fold ; 9, eye ; t, tentacles ; /, fun- 306 PALAEOZOIC SYSTEM OF ROCKS. Again, if we divide chambered shells into those having simple septa and central or subcentral tube or siphon (Nautilus tribe], and those having septa plaited at their junction with the shell (plaited suture) and dorsal tube (ammonite tribe), then in the Silurian age the former only were represented. Again, if we divide the Nautilus tribe into straight-shelled and coiled-shelled, then the straight-chambered shells greatly predominated. Straight-chambered shells are called Orthoceratites (opflo^, straight ; wepaf, horn). The Orthoceratites, therefore, are a very striking feat- ure of the Silurian age. They may be defined as straight-chambered a, Onnoceras. b, Actinoceras. c, Huronto. d, Section of Siphuncle of Huronia. FIG. 848. , ft, c, d, Showing Structure of Orthoceratite. shells, with simple partitions and a central or subcentral siphon-tube (siphuncle). The siphuncle of the family was large in proportion to the shell, and had often a beaded structure (Fig. 348, a, , c, d). The genera are founded largely on the form of this part. They existed in great numbers, and attained very great size. Speci- mens have been found fifteen feet long, and eight to ten inches in diam- eter. They were, without doubt, the most powerful animals of that ANIMALS. 307 time, the tyrants and scavengers of these early seas. We give, in Fig. 357, a restoration of the creature. They are entirely characteristic of FIG. 850. FIG. 851. ' JK^niignM i^MMnBHBMBMHBBHMWfflSflffl^Bffil FIG. 853. FIGS. 849-353. SILTJRIAN CEPIIALOPODS : $49. Orthoceras medullare (after Meekl. 850. Ormoeeras tenufilum, showing chambers and siphimele (after Hall). 351. Orthoceras vertebrale (after Hall). 852. Orthoceras multicarneratum (after Hall). 353. Orthoceras Duseri (after Hall). 308 PALAEOZOIC SYSTEM OF ROCKS. the Palaeozoic ; commencing in the Primordial, extending through into the Carboniferous, and passing out there. They attained their maxi- mum of development in size and number in the Silurian. Although straight-chambered shells (Orthoceratites) are most abun- dant and characteristic, coiled shells of the same tribe are also found, and some of them of considerable size. Some of these are close-coiled FIG. 354. FIG. 855. FIG. 856. FIGS. 854-856. SILURIAN CKMIALOPODS : 854. Trocholites Ammonias (after Hall): a, exterior;/;, cast, showing septa, 855. Lituites Graftonensis (Meek and Worthen). 356. Lituites cornu-anetis. shells, true Nautilus family ; others open-coiled, and more nearly allied to the straight. Barrande gives 1,622 species of Cephalopods in the Silurian. Articulates Worms. These are fleshy animals without skeletons, and are therefore not preserved. They are known only by their tracks, their borings, and their tubes. Nevertheless, some 185 species, accord- ing to Barrande, have been described from the Silurian of different ANIMALS. 309 countries. Fig. 358 represents worm-tubes, and Fig. 359 worm-tracks, from Upper Silurian. Crustacea Trilobites. The principal representatives of the articu- late department in Silurian times were Crustaceans, but mostly of a very characteristic order of that class, now long extinct, viz., Trilobites. General Description. The carapace or shell of these curious creat- ures was convex and usually smooth above, and flat or concave below, and divided transversely, like most Crustacea, into a number of movable FIG. 3n7. Restoration of Orthoeeras, the shell bein