A college text-book of geology

Front matter

Hmerican Science Series

Physics.

By A. L. Kimball, Amherst College.

Preface

This text-book on Geolog}^ is intended primarily for college students who are abeady in possession of the elements of physics, chemistry, and biolog}'.- It is intended to serve as a basis for a half-year's work; but by the judicious selection of material to be presented and omitted, the volume may be used for briefer courses, and supplemented by the numerous articles and treatises referred to in the text, it may be made the basis for more extended courses.

In the preparation of the volume it has been the purpose of the authors to present an outline of the saUent features of geolog};, as now developed, encumbered as little as possible by technicalities, and by details whose bearings on the general theme are unimportant. The attempt has been made to make the book readable, in the hope that many persons not in colleges or universities may be interested in following a connected account of the earth's history, and the means by which that history is recorded and read.

The general plan of the work ha-s been determined by the experience of the authors as instructors. Little emphasis is laid on the commonly recognized subdivisions of the science, such as dynamic geology, stratigraphic geology, physiographic geology, etc. The treatment proceeds rather from the point of view that the science is a unit, that its one theme is the history of the earth, and that the discussions of djmamic geolog}", physiographic geolog}^ etc., apart from their historical bearing, lose much of then* significance and interest. The effort has been, therefore, to emphasize the historical element, even in the discussion of special themes, such a5 the work of rivers, the work of snow and ice, and the origin and descent of rocks. This does not mean that phases of geology other than historical have been neglected, but it means that an effort has been made to give a historical cast to all phases of the subject, so far as practicable.

fix iv PREFACE

Throughout the work the central purpose has been not only to set forth the present status of knowledge, but to present it so that the student wHl be mtroduced to the methods and sphit of the science. To this end the worMng methods of the geologist have been implied as frequently as practicable. To this end also there has been frankness of statement relative to the hmitation of knowledge and the uncertamty of many tentative conclusions. The theoretical and mterpretative elements which enter mto the general conceptions of geology have been freely used, because they are regarded as an essential part of the evolution of the science, because they often help to clear the complete conceptions, and because they stimulate thought. The aim has been, however, to characterize h}i)othetical elements as such, and to avoid confusmg the mterpretations based on h}T)othesis, with the statements of fact and established doctrines. Especial care has been taken to recognize the uncertam nature of prevalent interpretations when they are dependent on unverified hypotheses, especially if this dependence is likely to be overlooked.

In many cases the topics discussed will be found to be presented in ways differing \videly from those which have become famHiar. In some cases, fundamentally new conceptions of familiar subjects are involved; in others, topics not usually discussed in text-books are stated with some fullness; and m still others, the emphasis is laid on points which have not commonly been brought into prominence. Whether the authors have been wise in departmg to this extent from beaten paths, the users of the volume must decide. Especial attention is dhected to the map work suggested at various points'in the text, as on pages 109, 194, 222, 288, 331, 366, 413, 475, 506, 659, 726, 771, and 845. The use of the topographic maps, folios, and other publications of the United States Geological Survey, somewhat as suggested, will be of great service in making the subject real. The reports of the several State Geological Surveys of the states where this book is used, will also be serviceable. It is suggested that instructors who use the maps and folios mentioned m the text wtII do well to plan for this work before reaching the ends of the chapters where mention is made of this work. The map work should be interwoven with the class-room

PREFACE V

wbrk, rather than added at its close. This adjustment must, of course, be left to the individual instructor.

In addition to the map work it is hardly necessary to emphasize the fact that field work is indispensable to the greatest efficiency. Specific directions for field work, however, even if they were needed,- are impracticable, since local fields vary so widely.

University of Chicago May 15, 1909.

ONTENTS

CHAPTER I PRELIMINARY OUTLINE p^^^

The Eakth in the Solar System 2

The Grand Divisions of the Earth 4

The atmosphere, 4. The hydrosphere, 6. The hthosphere, 7.

Chapter II The Materials Of The Earth And Their Arrangement

General H

Mantle rock, 11. Solid rock, 12. Stratified rock, 12. Igneous rock, 13. Metamorphic rock, 14.

Igneous Rocks 1^

Composition of, 23. Classification of, 27. Disruption of, 31.

Sedimentation and Sedimentary Rocks 34

Metamorphism and Metamorphic Rocks 42

Classifications and Nomenclatures 58

Structural Features Common to All Rocks 61

Ore-deposits ^ ^°

Reference List of Important Minerals 74

Reference List of the Commoner Rocks 81

CHAPTER HI

•THE GEOLOGICAL WORK OF THE ATMOSPHERE

Mechanical Work 8^

Dust, 89. Sand, 92. Dunes, 93.

Chemical Work 102

The Atmosphere a Conditioning Agency 103

Summary 109

vii vm

CHAPTER IV LAND WATERS — STREAMS The Work of Running Water 1^^

Erosive Work of Running Water 114

Beginning of a vaUey, 115. The permanent stream, 117. Modes of vaUey development, 119. Limits of growth, 121. The development of tributaries, 121. The struggle for existence among valleys and streams, 122. Pkacy, 123. Cycle of erosion 124. General characteristics of topographies developed by river erosion, 133. Special features resulting from special conditions of erosion, 133. Rate of degradation, 138.

Analysis of Erosion

Weathering, 140. Transportation, 144. Corrasion, 146. Corrosion (solution, etc.), 148.

Conditions Affecting the Rate of Erosion 149

Effects of Unequal H.^jidness 1^"^

Effect of Ch.^'ges of Level

The Aggradational Work of Running Water 177

Causes of deposition, 177. Location of aUuvial deposits and their topographic forms, 179.

Stre.oi Terraces

CHAPTER V GROUND (UNDERGROL^^D) WATER General Facts ^^"^

Work of Ground-water .= 20-

Chemical, 202. Quantitative importance of solution, 203. Deposition of mineral matter from solution, 205. Mechanical, 206.

Results of the Work of Grol^nd- water 206

Weathering, 206. Caverns, 206. Limestone sinks, 208. Creep, slumps, and landsHdes, 209. Summary, 210.

Springs "^^^

Mineral matter m solution, 213. Geysers, 213.

21Q Artesian Wells

CHAPTER VI Non-glacial Ice '^■^'^

General phenomena of, 236. Size, 237. Movement, 238. Likenesses and unhkenesses of glaciers and rivers, 239. Surface moraines, 242. Debris below surface, 245. Temperature, 245. Compression and friction as causes of heat, 248. Summary, 249.

CONTENTS ix

The Work of Glaciers 251

Erosion and transportation, 251. Varied nature of glacial debris, 255. Topographic effects of glacial erosion, 256. Positions in which debris is carried, 258. Wear of drift in transit, 262.

Deposition of Drift

Types of moraines, 266. Distinctive nature of glacial deposits,

267. Glaciated rock surfaces, 268.

Glacio-fluvial Work

Icebergs ^^

The Intimate Structure of Glacier Ice ^'o

Development of ice from snow, 275. Structure and arrangement of crystals in glacier ice, 278. Probable fundamental element in glacier motion, 280. Auxiliary elements, 282. Corroborative phenomena, 285. Other views of glacier motion, 286.

CHAPTER VII General Facts ^89

Processes in Operation in the Sea 292

Diastrophism, 292. Vulcanism, 295. Gradation, 295. Movements of the sea-water, 296. Waves, 297. Work of the waves, 299. Erosion, 299. Wave-erosion and horizontal configuration, 306. Transportation, 308. Deposition by waves, under- . tow, and shore-currents, 309. Effect of shore-deposition on coastal configuration, 314. The work of ocean-currents, 315.

Deposits on Ocean-bed • • ^^^

Shallow-water deposits, 317. Chemical and organic deposits in shallow water, 323. Limestone, 325. Deep-sea deposits, 326. Contrasted with shallow-water deposits, 326.

CHAPTER VIII General Facts '^'^'"

Changes taking place in lakes, 332. Lacustrine deposits, 333. Extinct lakes, 334. Salt lakes, 335. Origin of lake basins, 337.

Chapter IX Movements And Deformations Of Earth's Body (diastrophism)

Minute and Rapid Movements 338

Earthquakes, 338. The geologic effects of earthquakes, 344.

X CONTENTS

Secular Movements 351

Minor movements, 354. The great periodic movements, 354. Mountain-folding, 354. Plateau-forming movements, 355. Continent-forming movements, 356. Extent of the movements, 358. Causes of movements, 360. Original distribution of heat, 361.

CHAPTER X Intrusions • ^^"^

Extrusions '^^^

Fissure eruptions, 370. Volcanoes, 371. Distribution of volcanoes, 372. Relations of volcanoes, 376. Products of volcanoes, 377. Formation of cones, 379. Lavas, 383. Vol- • canic gases, 385. The causes of vulcanism, 388. Modes of reaching the surface, 391.

Chapter XIV ^ 436

General • •

Conceptions of the earth's composition, 436. Cxeneral characteristics of the Archean, 440.

Distribution and Local Development of Archean 443

Bearing of the Archean on theories of the origin of the earth, 446. Earher views of the Archean, 446. Life during the Archeozoic, 447. Duration of the Archeozoic, 447. CUmate, 447.

Chapter XV The Proterozotc Era

Stratigraphic relations, 448. Subdivisions, 450. Proterozoic sedimentation, 450. Extent of Proterozoic formations, 453. - The exposed formations, 453. Geographic relations of the exposed Proterozoic to the Archean, 455.

Proterozoic of the Lake Superior Region -436

The Huronian systems, 457. The Keweenawan system, 460. General considerations concerning the Lake Superior Proterozoic, 467. Sequence of events elsewhere, 470.

Proterozioc Rocks in Other Regions ^''^

Cordilleran region, 472. Eastern part of the United States, 472. Summary, 473. In other contments, 473.

Life During the Proterozoic Era "4' ^

„ 474

Climate

THE PALEOZOIC ERA

CHAPTER XVI THE CAIVIBRIAN PERIOD Formations .\nd Physical History '*' ^

The subdivisions of the Cambrian and their distribution, 476. Great submergence during the Cambrian, 478. Causes of submergence, 479. Basis for subdivision of the Cambrian, 481. Sedimentation in the Cambrian period, 484. Distribution and outcrops of the Cambrian system, 486. Cambrian in other continents, 490. Duration of the Cambrian period, 493.

xii CONTENTS

The Life of the Cambrian 494

Plant fossils, 495. Animal fossils, 495. Zoological provinces, 503. The succession of faunas, 504. The abrupt appearance of the Cambrian fauna, 505.

CHAPTER XVII THE ORDOVICIAN (LOWER SILURIAN) PERIOD Formations and Physical History

Sedimentation during the Ordovician period, 507. Sections of the Ordivician, 509. General conditions in the eastern part of the continent, 510. Igneous rocks, 513. General conditions and relations of the Ordovician system, 513. Close of period, 515. Economic products, 517. Foreign Ordovician, 517. Duration and climate, 520.

Life

Edaphic adaptations, 522. Provincial modifications, 522. Cosmopolitan development, 522. Marine Ufe, 523. Land life, 534.

Chapter XVIII The Silurian (upper Silurian) Period

Formations and Physical Hsitory 536

Silurian of the East, 537. Silurian of the West, 543. Summary, 544. Foreign Silurian, 546.

Silurian Life ^^"

The transition from the Ordovician, 547. The expansional stage of the Mid-Silurian fauna, 547. Foreign faunas and migrations, 554. The closing restrictional stage, 555.

Chapter XIX The Devonian Period

Devonian of the East, 561. Devonian of the West, 567. Areas where the Devonian cames to the surface, 569. Close of the Devonian, 571. Economic products, 570. Foreign Devonian, 571. CHmatic conditions, 574.

Devonian Life 574

The marine faunas, 574. Helderberg fauna, 575. Oriskany fauna, 577. Onondaga fauna, 578. Southern Hamilton fauna, 582. Northwestern Hamilton fauna, 585. Evidence of geographic connections, 586. Later Devonian (Chemung) fauna, 586. Devonian fauna in the Great Basin area, 587. Life of land waters, 588. Land life, 592.

CONTENTS xiii

Chapter XX The Mississippian (early Carboniferous) Period

East of the Great Plains, 597. In the Great Plains, 600. West of the Great Plains, 600. General considerations, 601. Lower Carboniferous of other continents, 603. Climate and duration,

The Lite of the Mississippian (Subcarboniferous) 606

Marine faunas, 606. Kinderhook fauna, 607. Osage fauna, 607. Waverly fauna, 61 L Great Basin fauna, 611. Genevieve (St. Louis-Kaskaskia) fauna, 613. Evolution of the fishes in the Mississippian period, 617. Land life of the Mississippian, 619.

Chapter XXI The Pennsylvanian (upper Carboniferous) Period

Pottsville conglomerate (Millstone grit), 620. Coal measures, 621. Productive coal-fields, 624. Coal, 630. Extent and relations of coal-beds, 637. Varieties of coal, 637. Iron ore, 638. Geographic conditions in the eastern interior, 639. Duration of the period, 640. Close of the period, 641. Carboniferous formations in other continents, 641.

Life of the Pennsylv.^jvtian Period

Plant hfe, 643. Climatic implications of the coal-plants, 651. Land animals, 652. Fresh-water life, 655. Marine hfe, 656.

i^

Chapter XXII The Permian Period

East of the Mississippi, 660. West of the Mississippi, 660. Correlation, 662. In Europe, 662. Other continents, 666. Permian glacial formations, 666. Clbse of the Paleozoic era, 668.

Life of the Permian ^^^

Impoverishment of life, 669. Plant life, 669. Land animals, 672. Fresh- water life, 674. Marine fauna, 674.

Problems of the Permian "^ '

xiv CONTENTS

THE MESOZOIC ERA

Chapter XXIII

Eastern Triassic — Newark series, 678. In the West, 684. On the Pacific slope, 685. Climatic conditions, 686. Close of the Trias, 687. Foreign Triassic, 687.

Life of the Triassic Period 690

Plant life, 691. Land animals, 691. Marine hfe, 695.

CHAPTER XXIV THE JURASSIC PERIOD Formations and Physical History • -• 7C2

Eastern part of the continent, 702. Western interior, 702. Pacific coast, 705. Thickness, 707. Surface distribution and position of beds, 707. Close of the Jurassic, 707. Orogenic movements, 707. Foreign Jurassic, 708. Jurassic coal, 709. CUmate, 710.

Life of the Period 710

Marine life, 711. Land hfe, 719.

CHAPTER XXV THE COMANCHEAN (LOWER CRETACEOUS) PERIOD Introductory 727

Atlantic and Gulf Border regions, 728. Texas-Mexico region, 731. Northern interior, 732. Pacific border, 733. Close of the Comanchean (Lower Cretaceous) period in North America, 735. Lower Cretaceous in other continents, 736.

Life "37

Terrestrial vegetation, 737. Land animals, 739. Fresh-water fauna, 739. Marine faunas, 740. .

. CHAPTER XXVI THE CRETACEOUS PERIOD

Atlantic border region, 742. Eastern Gulf border, 744. Western • Gulf region, 746. Western interior, 746. Pacific coast, 752. Close of the period, 752. Upper Cretaceous of other continents; 755. Climate, 759.

Life of the (Upper) Cretaceous 759

Land plants, 759. Land animals, 762. Sea life, 765.

CONTENTS XV

THE CENOZOIC ERA H

Chapter XXVII 1

The Eastern coast, 774. The Pacific coast, 774. The Western interior, 776. The Northwest, 778. General considerations. 778. Close of the Eocene in North America, 780. Eocene in other continents, 780.

Eocene Life 784

Transition from the Mesozoic, 784. Vegetation, 786. Land animals, 787. Marine life, 794. Climate, 797.

Oligocene Formations 797

Life of the Oligocene 801

CHAPTER XXVIII Formations and Physical History 805 Wl

Altantic coast, 805. GuK coast, 807. Pacific coast, 807. Non-marine deposits, 811. Igneous activity during the Miocene, 812. Close of the Miocene, 813. Miocene outside North America, 815.

Life of the Miocene, 818

Land plants, 818. Land animals, 819. Marine hfe, 824. Provincialism dominant, 824.

Chapter XXIX

Lafayette formation, 829. Marine Phocene, 835. The Atlantic coast, 835. The Pacific coast, 835. Crustal movements of the Phocene, 835. Foreign Pliocene, 840.

Life of the Pliocene 840

Land plants, 840. Land animals, 841. Marine hfe, 845.

CHAPTER XXX THE PLEISTOCENE OR GLACIAL PERIOD General 846

Distribution of glaciation, 846. Criteria of glaciation, 849. The development and thickness of ice-sheets, 861. Stages, in the history of an ice-sheet, 863.

XVI

CONTENTS

The Work of an Ice-sheet 864

Formations made by the ice-sheets, 864. Fluvio-glacial deposits, 869. Relations of stratified to unstratified drift, 872. Topographic distribution of stratified drift, 872. Changes in drainage effected by glaciation, 873. The succession of ice invasions, 874. The loess, 885. Duration of the glacial period, 890.

The Cause of the Glacial Period 894 •

Hypsometric hypotheses, 895. Astronomic hypotheses, 895. Atmospheric hypotheses, 898.

Formations Outside the Ice-sheets 899

On the Atlantic and Gulf coasts, 900. In the interior, 904. In the West, 905. Igneous rocks, 911.

Ch-\nges of Level During the Pleistocene 911

Life of the Pleistocene 9^^

Destructive effects of glaciation, 914. To-and-fro migration, 914. Climatic adaptations, 915. Land Ufe of the interglacial stages in glaciated areas, 916. Marine life, 917. The terrestrial Ufe of the non-glaciated regions, 918. Pleistocene life outside North America, 920. Man in the glacial period, 922.

CHAPTER XXXI The Present Period Compared With the Pleistocene 931

The end of the glacial period, 931. The end of the deformation period, 931. The channels on the continental borders, 933.

Life of the Hum.vn Period ^^^

The re-peopling of the glaciated areas, 935. The dynasty of man, 937. Man as a geological agency, 941. Prognostic geology, 942.

Reference Table of the Principal Groups of Plants 944

Reference Table pF the Principal Groups of Animals , 945

LIST OF PLATES

FACES PAGE

I. Dunes in contour 96

II. Streams disappearing in sand, gravel, etc., in an arid region . 120

III. YoutWul valleys, shore of Lake Michigan ...... 121

IV. Map illustrating piracy. Kaaterskill Creek, N. Y. . . 124 V. The widening of the river valley by the meandering of the

stream " 125

VI. Youthful valleys and topography in a region of slight rehef and

in a region of great relief 128

VII. Topographic maturity 132

VIII. Topographic old age 133

IX. Niagara Falls and its surroundings 156

X. Cushetunk and Round mountains, N. J 157

XI. Entrenched meanders. Conodoguinet Creek, Pa. . . . 172 XII. Section of the Cahfornia coast at Oceanside, showing changes

of level of the land 173

Xin. Alluvial fan at the base of mountains. Cucamonga, Cal. . 182

XIV. The Alluvial plain of the Missouri and Big Sioux; S. Dak. 183

XV. Limestone sinks due to ground-water 208

XVI. Glaciers of Glacier Peak, Wash. 236

X v^II. Glaciers and cirques of the Bighorn mountains .... 237

XVIIL An ill-drained plain of glacial drift ; Southern Wisconsin . . 270

XIX. Irregularities of coast developed by erosion and deposition . 303

XX. Shore-line of Marthas Vineyard, Mass 307

XXI, The upper end of Seneca Lake, N. Y 335

PART I

THE MATERIALS OF THE EARTH AND PROCESSES WHICH AFFECT THEM

Chapter I Preliminary Outline

Geology is essentially a history of the earth and its inhabitants. It treats of rocks and of the agencies and processes which have been involved in their formation, and from the rocks and their structures it attempts to make out the various stages through which the earth and the living things which have dwelt upon it have passed. It is one of the broadest of the sciences, and brings under consideration certain phases of other sciences, particularly astronomy, physics, chemistry, zoology, and botany.

Subdivisions. So broad a science has many subdivisions. That phase which treats of the outer relations of the earth is Cosmic or Astronomic Geology; that which treats of the constituent parts of the earth and its material is Geognosy, of which the most important branch is Petrology, the science of rocks. That phase which deals with the structural arrangement of the rocks is Geotectonic, or Structural Geology; that which deals with the forces involved in geologic processes is Dynamic Geology; that which treats of the face of the earth, or topographic form, is Physiographic Geology; that which concerns itself with the fossils that have been preserved in the rocks, and with the faunas and floras that have lived in the past, constitutes. Paleontologic Geology, or Paleontology. The treatment of the succession of events is Historical Geology, which is worked out chiefly from the succession of beds of rock formed in the progress of the ages. .

Besides these general subdivisions, there are special applications of geologic knowledge which give rise to other terms. Thus Fconomic Geology is concerned with the industrial applications of geologic knowledge, and Mining Geology, which is a sub-section of economic geology, with the application of geologic facts and pnnciples to mining. Other simHar subdivisions might be mentioned.

Dominant processes. Three sets of processes, now in operation on the surface of the earth, have given rise to most of its surface features. These processes have been designated Diastrophism, Vulcanism (or Volcanism), and Gradation. Diastrophism includes aU movements of the outer parts of the lithosphere, whether slow or rapid, gentle or violent, slight or extensive. Many parts of the land, especially along coasts, are known to be sinking slowly relative to the sea-level, while other parts are known to be rismg. The fact that sediments origmally deposited beneath the sea now exist, in some places, at great elevations, together with the fact that certain areas which were once land are now beneath the sea, proves that simHar changes have taken place in the past. Earthquakes are another illustration of diastrophism. Vulcanism includes all processes concerned with the movements of lava and other volcanic products, whether extruded at the surface or not. Vulcanism and diastrophism may be closely associated, for local movements are often associated with volcanic eruptions. Gradation includes aU those processes which tend to bring the surface of the lithosphere to a common level. Gradational processes belong to two categories — those which level down, degradation, and those which level up, aggradation. The transportation of material from the land, whether by ram, rivers, glaciers, waves, or winds, is degradation^ and the deposition of material, whether on the land or in the sea, is aggradation. Degradation affects primarily the higher parts of the lithosphere, while aggradation affects primarily the lower.

The Earth in the Solar System The earth as a planet. Though supremely important to us, the earth is but one of the minor planets which revolve about the rs COLLEGE UBRARY PRELIMINARY OUTLINE 3

sun. Of the eight planets, four, Jupiter, Saturn, Uranus, and Neptune, are much larger than the earth, while three, Mercury, Venus, and Mars, are smaller. There are a host of asteroids, but all together they do not equal the mass of the smallest planet. The average mass of the planets is more than fifty times that of the earth, and Jupiter, the largest, has more than three hundred times the mass of the earth. The earth's position is in no sense distinguished. It is neither the outer nor the inner, nor even the middle planet. Even in the inner group of four to which it belongs, it is neither the outermost nor the innermost member, though in this group it is the largest. Its average distance from the sun is about 92.9 million miles, and its period of revolution is 36534 days, a period longer than that of any of the inner planets, and shorter than that of any of the outer ones. Its period of rotation is not very different from that of Mars, but is much shorter than that of the larger planets whose periods of rotation are known. The plane of the earth's revolution approximates the planes of revolution of all the other planets. The orbit of the earth, like the orbits of the other planets, is an ellipse, and its eccentricity is about -q\. The inclination of the earth's axis, nearly 233^° (23° 27'), is less than that of some planets, and more than that of others.

Its satellite. The earth is peculiar in having one unusually large satellite, which has a mass -gV of its own. The larger planets have several satellites whose combined mass exceeds that of the moon, and perhaps in some few cases the individual satellites may be larger than the moon; but no other is g\- of the size of the planet about which it revolves. There is little doubt that the moon has played an important part in the history of the earth. It is the chief cause of oceanic tides, and the tides are efficient in the wear of the shores of the oceans and in the distribution of marine sediments. Tides have probably been of importance in the earth's history ever since the ocean came into existence.

Dependence on the sun. By far the most important external relation of the earth is its dependence on the sun, of which it is a mere satellite. Its mass is less than 3ToV"o o^ that of the sun, upon which it depends for nearly all its heat and light, and. through these, for nearly all of the activities that have given character to its history. A little heat and light are derived from other bodies, and an important source of energy is found in the interior of the earth itself; yet all of these are so far subordinate to the gTeat flood of energy which comes from the sun that they are quite inconsequential. The dependence of the earth on the sun has been intimate throughout its past history, and its future is locked up with the destiny of that gi'eat luminary. Geolog>^ in its broadest phases can therefore scarcely be separated from the study of the sun; but this falls in the province of the astronomer rather than the geologist. Meteorites. There are a multitude of smaU bodies passing through space in varying dkections and with varying velocities, and occasionaUy encountermg the earth to which they add then substance. Some of these meteorites revolve about the sun hke minute planets, but some of them come from such directions and with such velocities as to show that they do not belong to the sun's family. Some consist almost wholly of metal, chiefly hon aUoyed with a Httle nickel Qiolosiderites) ) some consist of metal and rock intimately mixed {syssiderites and sporadosiderites) ; and some consist whoUy of rock {asiderites) . It is now thought that the meteorites throw some light, perhaps important hght, on the early history of the earth. They are therefore of interest to the geologist. The amount of material added to the earth by then infaU is now relatively slight compared with the whole body of the earth; but their contributions m the cUstant past may perhaps have been greater.

The Graxd Divisions of the Earth

The constitution of the earth. The materials of the earth fall into three grand divisions: (1) The atmosphere, (2) i^^Q hydrosphere (water sphere), and (3) the lithosphere (rock sphere).

1. The Atmosphere Though the study of the atmosphere constitutes the science of meteorolog}^, the atmosphere is a part of the earth, and as a part of the earth, it falls withhi the provmce of geolog^^ It is an mtunate mixture of (1) all those substances that cannot take a liquid or solid state at the temperatures and pressures which prevail at the earth's

PRELIMINARY OUTLINE 5

surface, together with (2) such transient vapors as the various substances of the earth throw off. The first class form the permanent gases of the atmosphere, and consist of nitrogen about 79 parts, oxygen about 21 parts, carbon dioxide about .03 part, together with small quantities of argon, and several other rare constituents. Chief among the second Class is water vapor, which varies greatly in amount from time to time and from place to place. Here too, belong the gases which issue from volcanoes, and a great variety of volatile organic substances. Dust and other matter suspended in the air are usually regarded as impurities rather than constituents of the atmosphere; but they are of great importance because they affect its temperature and luminosity, and they facilitate the condensation of moisture.

Mass and extent. The total mass of the atmosphere is estimated to be TToi 0 0 0 of ^he mass of the earth. It exerts a pressure of about fifteen pounds per square inch at the sea-level. Its density decreases upward, but its actual height is not known. There is no direct evidence of its existence above a few hundred miles, but there are theoretical grounds for believing that it extends out very much farther.

Geologic activity. The atmosphere is the most mobile and active of the three great subdivisions of the earth. Its direct and indirect effects on water and rocks are so great that it must be regarded as one of the great agents of change. It acts chemically upon the rock substance of the earth, causing hardening of the rock in some cases, but more often causing disintegration, by means of which rock is reduced to soil-like material, and prepared for removal by winds and waters. When in motion, the atmosphere acts mechanically on the surface of the land, transporting dust and sand. Its greatest function, however, is in furnishing the conditions for water action. Rains, streams, glaciers, and all the various forms of moving water upon land, are dependent in one way or another on the atmosphere. On the ocean, too, wave action is dependent largely on the winds. Streams and waves, which are the most familiar agents of geologic change, are therefore to be credited as much to the atmosphere as to the hydrosphere.

A thermal blanket. The atmosphere is a thermal blanket to the rest of the earth. In its absence the heat of the sun would reach the surface with much greater intensity than now, and it would be radiated back from the surface almost as rapidly as received. During the night, there would be a degree of cold far greater than that now known to any part of the earth. In passing through the atmosphere, certain portions of the radiant energy of the sun are absorbed. Of the remainder which reaches the surface of the earth, a part is transformed into vibrations of lower intensity which are then more effectively retained by the atmosphere. The air thus distributes and equalizes the temperature. The two constituents of the atmosphere which are most efficient in this work are water vapor and carbon dioxide, and the climate of the earth is believed to have been very greatly affected by the varying amounts of these constituents in the atmosphere, as well as by variation. in the total mass of the atmosphere.

The function of the atmosphere in sustaining life and promoting all that depends on life is obvious.

2. The Hydrosphere

The water which lies upon the surface of the solid earth, is about -^^5^ part of the earth's mass. Were the solid part of the earth perfectly spheroidal, this amount of water would constitute a universal ocean a little less than two miles deep. Owing to the unevenness of the surface of the lithosphere, the water is chiefly gathered in great basins or troughs, occupying nearly three-fourths (72%) of the earth's surface. These basins are all connected, so that anything which changes the level of the water in one, changes the level in all.

Oceanic dimensions. The surface area of the ocean is estimated at 143,259,300 square miles. The area of the true oceanic basins is only about 133,000,000 square miles, but these basins are somewhat more than full, and the ocean water laps up on the continental shelves to the extent of more than 10,000,000 square miles. If about 600 feet of the upper part of the ocean were removed, the true ocean basins would be just full. Beneath about 20% of the ocean area, the bottom sinks to depths of between 6,000