Manual of Geology: treating of the principles of the science with special reference to American geological history ... Revised edition

Front matter

MANUAL OF GEOLOGY TREATING OF THE PRINCIPLES OF THE SCIENCE WITH SPECIAL REFERENCE TO AMERICAN GEOLOGICAL HISTORY BY JAMES D. DANA ., PROFESSOR EMERITUS OF GEOLOGY AND MINERALOGY IN YALE UNIVERSITY: AUTHOR OF A SYSTEM OF MINERALOGY ; CORALS AND CORAL ISLANDS ; VOLCANOES J REPORTS OF WILKES'S EXPLORING EXPEDITION, ON GEOLOGY, ON ZOOPHYTES, AND ON CRUSTACEA, ETC " Speak to the Earth and it shall teach thee " ILLUSTRATED BY OVER FIFTEEN HUNDRED AND SEVENTY-FIVE FIGURES IN THE TEXT, AND TWO DOUBLE-PAGE MAPS FOURTH EDITION NEW YORK •:• CINCINNATI •:• CHICAGO AMERICAN BOOK COMPANY COPYRIGHT, 1894, BT JAMES D. DANA W. P.

Preface

. In the preparation of the new edition of this Manual, the work has been wholly rewritten. North American Geological History is still, how ever, its chief subject. The time divisions in this history, based on the ascertained subdivisions of the formations, were first brought out in my Address before the meeting of the American Association at Providence in 1855 ; and in 1863, the " continuous history " appeared in the first edition of this Manual, written up from the State reports and other geological publications. The idea, long before recognized, that all observations on the rocks, however local, bore directly on the stages in the growth of the Con tinent derives universal importance from the recognition of North America as the world's type-continent — the only continent that gives, in a full and simple way, the fundamental principles of continental development.

Since 1863, when the first edition of this work was published, investigation, through the geological workers of the United States, Canada, and Mexico, has been extended over nearly all parts of the continent, so that its history admits of being written out with much fullness. The Government Expeditions over the Rocky Mountain region, under F. V. Hayden, Clar ence King, Captain Wheeler and others, and earlier, those especially of the Pacific Railroad Explorations, and the Mexican Boundary Commission, were large contributors to this result ; and also, since 1879, the able corps of the United States Geological Survey. As the rewritten book shows, new principles, new theories, and widely diverse opinions on various subjects are among the later contributions, along with a profusion of new facts relating to all departments of the science.

The Cambrian formation has been traced through a large part of the continent, and the number of its fossils has been increased, chiefly by C. D. Walcott, from a few to hundreds. The Appalachian Mountain structure has been shown by Clarence King, Dr. G. M. Dawson, and R. G. McCon nell to have been repeated in the great post-Cretaceous mountain-making of the Rocky Mountain region. The Reptiles, Birds, and Mammals of the Mesozoic and Tertiary have continued coming from the rocks until the species recognized much outnumber those of any other continent. The canons and other results of erosion in the west have thrown new light, through their investigators, on the work of the waters. Besides, the science of

PREFACE.

. petrology has elucidated much of the obscure in the constitution, relations, and origin of rocks. Moreover, America, from early in the century, has been receiving instruction through the development and parallel progress of the Science in Europe and other lands. The first edition of this Manual owed much to the advice of the able paleontologist, F. B. Meek, and also to his skill as a draftsman; and the work still bears prominent evidence of his knowledge, judgment, and scru pulous exactness, traits which give a permanent value to all the results of his too soon ended labors. In this new edition, the Paleozoic paleontology is largely indebted to Professor C. E. Beecher and Professor H. S. Williams ; the Jurassic, of western America, to Professor A. Hyatt ; the Cretaceous, to Professor Hyatt, Mr. T. W. Stanton, Mr. R. P. Whitfield, and Professor R. T. Hill ; and the Tertiary, as regards the Invertebrates, to Professor G. D. Harris.

With respect to the Vertebrates of the Jurassic, Cretaceous, and Tertiary, very valuable aid has been received from Professor Marsh, and also in the part on Tertiary Mammals from Professor W. B. Scott. The account of the arrangement and distribution of the Jurassic and Cretaceous rocks of western America was prepared with the assistance of Mr. J. S. Diller ; and that with regard to the marine Tertiary of the country was chiefly written for its place by Professor Harris. I am further indebted to Professor A. E. Verrill for his revision of the pages on the Animal Kingdom. Moreover, the replies to requests for information have placed me under obligation to almost all the geologists of the Continent, those of Canada as well as the United States, — and especially to Sir William Dawson, Mr. A. R. C. Selwyn, Dr. G. M. Dawson, Mr. Clarence King, Mr. C. D. Walcott, Professor N. S. Shaler, Professor S. H. Scudder, Mr. Frank Leverett, Professor R. T. Hill, Professor W. Upham, Pro fessor G. F. Wright, Professor J. J. Stevenson, Mr. Wm. H. Dall, Dr. C. A. White, and Professor J. P. Iddings.

Throughout this volume, the dates of papers containing cited facts or views are often stated. If a condensed bibliography, containing in brief form the titles of the most important geological and paleontological works and papers, arranged under the year of publication, were accessible to the student, these dates would be a sufficient means of reference. Without such a Bibliography they may serve as a help in consulting, besides Reports of Geological Surveys, the serial scientific publications. It is best to com mence the search with the periodical containing the most geological papers, notes, and book notices, and follow on with the others. The American Journal of Science commenced in 1818 ; the American Naturalist, in 1868 ; the American Geologist, in 1888 ; the Bulletin of the American Geological Society, in 1890; the Journal of Geology, Chicago, in 1893. Then refer to the Proceedings and Memoirs of American Scientific Soci-

PREFACE.

. eties or Academies, in the following order: Academy of Natural Sciences of Philadelphia ; American Philosophical Society, Philadelphia ; Society of Natural History, Boston ; American Academy, Boston ; Lyceum of Natural History, and later, Academy of Sciences, New York; and so on, not over looking the Reports of the American Association for the Advancement of Science. The foreign serial works of most importance to the geologist are the Journal of the Geological Society of London ; the Geological Magazine, London ; Bulletin of the Geological Society of France ; " Comptes Rendus " of the Academy of Sciences, Paris ; Jahrbuch f iir Mineralogie, Geologie und Palaeontologie, Stuttgart; Zeitschrift der deutschen geologischen Gesell schaft, Berlin ; Jahrbuch der k.-k. geologischen Reichsanstalt, Vienna.

For foreign facts and views I am largely indebted to the able English works of Sir Archibald Geikie, Professor Prestwich, and Professors Etheridge and Seeley, the very full Traite de Geologie of Professor A. de Lapparent, and the Elemente der Geologie of Dr. Credner. As the volume is necessarily larger than that of the edition of 1880, — partly through more text, but also through a greater profusion of illustra tions, — the instructor may find it convenient, in his use of the Historical part, to take up successively its two great subjects, the geological and physical history of the continents, and the history of its life. JAMES D. DANA. New Haven, Conn., January, 1895. TABLE OF CONTENTS.

Introduction

. PAGE 5. Heat 253 PAGE Relatiens of the Science of Geology . 9 1. Sources of Heat 253 Subdivisions of Geology 13 2. Expansion and Contraction . . 259 3. Igneous Action and its Results 265

Part I

— Physiographic Geology. Volcanoes 267 Non-volcanic Igneous Eruptions 297 1. The Earth's General Contour and Surface Subdivisions .... 15 Thermal Waters, Geysers . . 305 4. Metamorphism 309 2. System in the Reliefs of the Land . 30 5. Mineral Veins, Lodes, Local Ore Deposits 327 3. System in the Courses of the Earth's Feature Lines 35 6. Hypogeic Work : Earth-shaping, Mountain-making .... 345 4. Oceanic and Atmospheric Movements and Temperature ... 42 1. Characteristics of Disturbed 5. Geographical Distribution of Plants and Animals 52 Regions and Mountains . . 351 2. Subordinate Effects attending Orographic Movements : Ef fects from Pressure, Earth-

Part II

— Structural Geology. 1. Rocks: theirConstituents and Kinds 61 quakes 369 2. Terranes : their Constitution, Characteristics, Positions, and Arrangement 89 3. Originof theEarth'sForm and Features : Orogenic Work, Epeirogenic Work . . . 376

Part IV

— Historical Geology. Agencies and General Subdivisions . 117 Subdivisions in Geological History . 397 1. Chemical Work 118 Review of the System of Life . . . 413 2. Life : its Mechanical Work and Rock Contributions 140 1. Animal Kingdom 414 2. Vegetable Kingdom 434 General Remarks on Rock-making 141 Protective and Other Beneficial Effects 155 I. ARCHÆAN TIME 440 1. Subdivisions: Rocks .... 445 Transporting Effects 156 2. Life 453 Destructive Effects 157 3. The Atmosphere as a Mechanical Agent 158 II. PALEOZOIC TIME .... 460 4. Water as a Mechanical Agent . . 166 Fresh Waters : Rivers and Lakes . 171 The Ocean 209 I. Cambrian Era 462 1. North American .... 464 1. Lower Cambrian Period 470 Freezing and Frozen Water: Glaciers, Icebergs 230 2. Middle Cambrian Period 474 3. Upper Cambrian Period 476 TABLE OF CONTENTS.

PAGE PAO« 2. Foreign 480 Topographic Changes in the Indian Ocean : Gondwana Land . . . 737 3. Geographical and Physical Conditions and Progress . 483 II. Lower Silurian Era .... 489 III. MESOZOIC TIME 738 1. North American .... 489 1. American Triassic and Jurassic 2. European 517 Period 739 3. General Observations : 2. Foreign Triassic and Jurassic . 768 Rocks ; Climate ; Biologi- 3. General Observations : Contical Progress 524 nental Comparisons ; Cli mate ; Biological Changes ; Upturnings and Mountain- 4. Upturning at the close of the Lower Silurian .... 526 Hi. Upper Silurian Era .... 535 making 791 1. North American .... 535 4. American Cretaceous Period . 812 1. Niagara Period . .« . 538 5. Foreign 856 2. Onondaga Period . . . 552 6. General Observations: Geologi- 3. Lower Helderberg Period 658 2.

Foreign 563 cal ; Geographical ; Biological ; Gondwana Land . . 867 3. General Observations : Geo- Post-Mesozoic Revolution . . . 874 logical ; Geographical ; Biological 570 IV. CENOZOIC TIME 879 IV. Devonian Era 575 1. North American .... 575 I. Tertiary Era 879 1. Oriskany Period . . . 577 1. North American .... 880 2. Corniferous Period . . 579 2. Foreign 919 3. Hamilton Period ... 592 3. General Observations : Bio- 4. Chemung Period . . . 602 logical ; Orogenic and 2. Foreign 622 Epeirogenic ; Climate . 928 3. General Observations : Geo- II. Quaternary Era 940 logical ; Geographical . . 628 4. Upturning at the close of 1. Glacial Period 943 1. American 943 the Devonian .... 630 2. Foreign 975 V. Carbonic Era 631 3. Cause of Glacial Climate 978 1. North American .... 633 2. Champlain Period ... 981 1.

Subcarboniferous Period 636 2. Carboniferous Period . 647 1. American 981 2. Foreign 995 3. Permian Period ... 660 3. Pleistocene Life .... 997 4. Recent Period 1012 2. Foreign 693 General Observations : Biolog- 3. General Observations : Geological and Geographical ; Meteorological ; Formaical ; Antarctic Continent ; Epeirogenic 1016 tion of Coal .... 708 General Observations on Geological History 1023 General Observations on Paleozoic Time 714 Geological Time 1023 Post-Paleozoic, or Appalachian, Revolution 728 Climate ; the Earth's Development . 1026 Progress in the Earth's Life .... ABBREVIATIONS. Gein. — Geinitz. Gld. — Gould. Gm. — Gmelin. N. & W. — Newberry & Worthen. Ag. — L. Agassiz. B. — E. Billings. Newb. — J. S. Newberry. Barr. — J. Barrande. Beyr. — E. Beyrich. Gopp. — H. R. Goppert.

Golds. — Golds uss. O. & N. — Owen & Norwood. Ow. — R. Owen (London),. Blum. — J. F. Blumenbach. Blv. — D. de Blainville. H.— J. Hall. Br. — H. G. Bronn. Brngt. — Brongniart. Brod. — Broderip. Bu. — L. von Buch. H. & M. — Hall & Meek. Hald. — S. S. Haldeman. Hising. — W. Hisinger. Hk. — E. Hitchcock. Hux.— T. H. Huxley. Pack. — A. S. Packard. Phill. — J. Phillips. Plien. — T. Plieninger. Portl. — J. E. Portlock. Con. — T. A. Conrad. R. — F. Romer. Rem. — A. Re'mond. Kon. — L. de Koninck. D.— J. D. Dana. Dalm. — J. W. Dalman. Dav. — T. Davidson. Defr. — Defrance. L. — J. Leidy. S. — J. W. Salter. L. & H. — Lindley & Hutton. Lam . — Lamarck . Saff.— J. M. Safford. Sc — S. H. Scudder. Schafh. — Schafhautl. Desh. — G. P. Deshayes. Linn. — Linnæus. Lmx. — Lamouroux. Dawson, Dn. — Sir Wm. Daw- Schlot. — E.F. von Schlotheim, Schp. — W. P. Sehimper.

Lsqx., Lx. — L. Lesquereux. Lye. — Lycett. son. D'Orb. — Alcide d'Orbigny. Sedg. — A. Sedgwick. Shum. — B. F. Shumard. Sow. — Sowerby. E. & H. — Edwards & Haime. Ehr.— Ch. G. Ehrenberg. M. — F. B. Meek. Mant. — G. Mantell. Mey. — H. von Meyer. Mh. — O. C. Marsh. St. — Stokes. Eich. — E. Eichwald. Sternb. — K. von Sternberg. Suck. — Suckow. Emmr. — H. F. Emmrich. Montf. — Denys de Montfort. Morr. — Morris. Fabr. — Fabricius. Falc. — H. Falconer. Flem. — J. Fleming. Fer. — Ferussac. Ung. — Unger. Mort. — S. G.Morton. Mull. — Miiller. Van. — Vanuxem. Murch. — R. I. Murchison. Vern. — E. de Verneuil. G. & H. — Gabb & Horn. N. & P. — Norwood & Pratten. Woodw. — J. Woodward.

INTRODUCTION.

. Kingdoms of nature. — Science, in her survey of the earth, has recog nized three kingdoms of nature, — the animal, the vegetable, and the inorganic ; or, naming them from the forms characteristic of each, the ANIMAL KINGDOM, the PLANT KINGDOM, and the CRYSTAL KINGDOM. An individual in either kingdom has its systematic mode of formation or growth. The plant or animal, (1) endowed with life, (2) commences from a germ, (3) grows by means of imbibed nutriment, and (4) passes through a series of changes and gradual development to the adult state, when (5) it evolves new seeds or germs, and (6) afterward continues on to death and dissolution. It has, hence, its cycle of growth and reproduction, and cycle follows cycle in indefinite continuance. The crystal is (1) a lifeless object, and has a simpler history ; it (2) begins in a nucleal molecule or particle ; (3) it enlarges by external addition or accretion alone ; and (4) there is, hence, no proper development, as the crystal is perfect, however minute ; (5) it ends in simply existing, and not in reproducing ; and, (6) being lifeless, there is no proper death or necessary dissolution.

Such are the individualities in the great kingdoms of nature displayed upon the earth. But the earth also, according to Geology, has been brought to its present condition through a series of changes or progressive formations, and from a state as utterly featureless as a germ. Moreover, like any plant or animal, it has its special systems of interior and exterior structure, and of interior and exterior conditions, movements, and changes ; and, although Infinite Mind has guided all events toward the great end, — a world for mind, — the earth has, under this guidance and appointed law, passed through a regular course of history or growth. Having, therefore, as a sphere, its comprehen sive system of growth, it is a unit or individuality, not, indeed, in either of the three kingdoms of nature which have been mentioned, but in a wider, — a World Kingdom. Every sphere in space must have had a related system of growth, and all are, in fact, individualities in this Kingdom of Worlds.

INTRODUCTION.

. Geology treats of the earth in this grand relation. It is as much removed from Mineralogy as from Botany and Zoology. It uses all these depart ments ; for the species under them are the objects which make up the earth and enter into geological history. The science of minerals is more immediately important to the geologist, because aggregations of minerals constitute rocks, or the plastic material in which the records of the past were made. The earth, regarded as such an individuality in a world kingdom, has not only its comprehensive system of growth, in which strata have been added to strata, continents and seas defined, mountains reared, and valleys, rivers, and plains formed, all in orderly plan, but also a system of currents in its oceans and atmosphere, — the earth's circulating-system ; its equally world wide system in the distribution of heat, light, moisture, and magnetism, and of plants and animals; its system of secular variations (daily, annual, etc.) in its climate and all meteorological phenomena.

In these characteristics the sphere before us is an individual, as much as a dog, or a tree ; and, to arrive at any correct views on these subjects, the world must be .regarded in this capacity. The distribution of man and nations, and of all productions that pertain to man's welfare, comes in under the same grand relation ; for, in helping to carry forward man's progress as a race, the sphere is working out its final purpose. There are, therefore, Three departments of science, arising out of this individual capacity of the earth. I. Geology, which treats of (1) the earth's structure, and (2) its system of development, — the latter including its progress in rocks, lands, seas, mountains, etc. ; its progress in all physical conditions, as heat, moisture, etc. ; its progress in life, or its vegetable and animal tribes.

II. Physiography, which begins where Geology ends, — that is, with the adult or finished earth, — and treats (1) of the earth's final surface arrangements (as to its features, climates, magnetism, life, etc.); and (2) of its system of physical movements or changes (as atmospheric and oceanic currents, and other secular variations in heat, moisture, magnetism, etc.). III. The earth with reference to man (including ordinary Geog raphy): (1) the distribution of races or nations, and of all productions or conditions bearing on the welfare of man or nations ; and (2) the progressive changes of races and nations. The first of these departments considers the structure and growth of the earth ; the second, its features and world-wide activities in its finished state ; the third, the fulfillment of its purpose in man.

Relation of the earth to the universe. — While recognizing the earth as a sphere in a world kingdom, it is also important to observe that it holds a very subordinate position in the system of the heavens. It is one of the smaller satellites of the sun, — its size about 120^000 that of the sun. And the planetary system to which it belongs, although 3,000,000,000 of miles in radius, is but one among myriads, the nearest star being 7000 times

INTRODUCTION.

. farther off than Neptune. Thus it appears that the earth is a very small object in the universe. Hence we naturally conclude that it is a dependent part of the solar system ; that, as a satellite of the sun, in conjunction with other planets, it could no more have existed before the sun, or our planetary system before the universe of which it is a part, than the hand before the body which it obediently attends. Although thus diminutive, the laws of the earth are the laws of the universe. One of the fundamental laws of matter is gravitation; and this we trace not only through our planetary system, but among the fixed stars, and thus know that one law pervades the universe. The rays of light which come in from the remote limits of space are a visible declaration of unity; for this light depends on molecular vibrations, — that is, the ultimate constitution and mode of action of matter ; and, by the identity of its principles or laws, whatever its source, it proves the essential identity of the molecules of matter.

Meteoric stones are specimens of celestial bodies occasionally reaching us from the heavens. They exemplify the same chemical and crystal lographic laws as the rocks of the earth, and have afforded no new element or principle of any kind. The moon presents to the telescope a surface covered with the craters of volcanoes, having forms that are well illustrated by some of the earth's volcanoes, although of immense size. The principles exemplified on the earth are but repeated in her satellite. Thus, from gravitation, light, meteorites, and the earth's satellite, we learn that there is oneness of law through space. The elements may differ in different systems, but it is a difference such as exists among known elements, and even if exemplifying new laws, such laws cannot be at variance with those illustrated by nature within reach of terrestrial investi gation.

The universe, if open throughout to our explorations, would vastly expand our knowledge, and science might have a more beautiful superstruc ture, but its basement-laws would be the same. A treatise on Celestial Mechanics printed in our printing-offices would serve for the universe. The earth, therefore, although but an atom in immensity, is immensity itself in its revelations of truth; and science, though gathered from one small sphere, is the deciphered law of all spheres. It is well to have the mind deeply imbued with this thought, before entering upon the study of the earth. It gives grandeur to science and dignity to man, and will help the geologist to apprehend the loftier charac teristics of the last of the geological ages. Special aim of geology, and method of geological reasoning. — Geology is sometimes defined as the science of the structure of the earth.

But the ideas of structure and origin of structure are inseparably connected, and in all geological investigations they go together. Geology had its very begin ning and essence in the idea that rocks were made through secondary causes ; and its great aim has ever been to study structure in order to com-

INTRODUCTION.

. prehend the earth's history. The science, therefore, is a historical science .- It finds strata of sandstone, clayey rocks, and limestone, lying above one another in many successions; and, observing them in their order, it assumes, not only that the sandstones were made of sand by some slow process, clayey rocks of clay, and so on, but that the strata were successively formed; that, therefore, they belong to successive periods in the earth's past ; that, con sequently, the loivest beds in a series were the earliest beds. It hence infers, further, that each rock indicates some facts respecting the condition of the sea or land at the time when it was formed, one condition originating sand deposits, another clay deposits, another lime, — and, if the beds extend over thousands of square miles, that the several conditions prevailed uniformly to at least this same extent.

The rocks are thus regarded as records of successive events in the history, — indeed, as actual historical records ; and every new fact ascertained by a close study of their structure, be it but the occurrence of a pebble, or a seam of coal, or a bed of ore, or a crack, or any marking whatever, is an addition to the records, to be interpreted by careful study. Thus every rock marks an epoch in the history ; and groups of rocks, periods ; and still larger groups, eras or ages ; and so the eras which reach through geological time are represented in order by the rocks that extend from the lowest to the uppermost of the series. If, now, the great beds of rock, instead of lying in even horizontal layers, are much folded up, or lie inclined at various angles, or are broken and dislocated through hundreds or thousands of feet in depth, or are uplifted into mountains, they bear record of still other events in the great history ; and should the geologist, by careful study, learn how the great disturbance or uplifting was produced, and succeed in locating its time of occurrence among the epochs registered in the rocks, he would have inter preted the record, and added not only a fact to the history, but also its explanation.

The history is, hence, a history of the upturnings of the earth's crust, as well as of its more quiet rock-making. If, in addition, a fossil shell, or coral, or bone, or leaf, is found in one of the beds, it is a relic of some species that lived when that rock was forming ; it belongs to that epoch in the world represented by the particular rock containing it, and tells of the life of that epoch ; and, if numbers of such organic remains occur together, they enable us to people the seas or land, ta our imagination, with some of the kinds of life that belonged to the ancient epoch. Moreover, as such fossils are common in a large number of the strata, from the lowest containing signs of life to the top, — that is, from the oldest beds to the most recent, — by studying out the characters of these remains in each, we are enabled to restore to our minds, to some extent, the popula tion of the epochs, as they follow one another in the long series.

The strata are thus not simply records of moving seas, sands, clays, and pebbles, and disturbed or uplifted strata, but also of the living beings that have in>

INTRODUCTION.

. succession occupied the land and waters. The history is a history as com plete as can be learned from the fossils of the life of the globe, as well as of its rock-formations ; and the life-history, imperfect though it be, is the great topic of Geology : it adds tenfold interest to the other records of the rocks. These examples are sufficient to explain the basis and general bearing of geological history. The method of interpreting the records rests upon the simple principle that rocks were made as they are now made, and life lived in olden time as it now lives ; and, further, the mind is forced into receiving the conclusions arrived at by its own laws of action. We observe that many of the common rock-strata consist of the same materials that make up the deposits of sand and gravel of sea-beaches or sand-flats, or of the clays or muds of the bottoms of estuaries or the borders of rivers, and that they are arranged in beds like the modern deposits, even have, at times, ripple-marks aud other evidences of the action of water or wind ; and further remark that these hard rocks differ from the loose sand, clay, or pebbly deposits simply in being consolidated into a rock; and, in other places, discover these sand-deposits in all states of consolidation, from the soft, movable sand, through a half-compacted •condition, to the gritty sandstone.

By such steps as these, the miud is borne along irresistibly to the conclusion that rocks were slowly made through common-place operations. These few examples elucidate the mode of reasoning upon which geo logical deductions are based. In using the present in order to reveal the past, we assume that the forces in the world are essentially the same through all time; for these forces are based on the very nature of matter, and could not have changed. The ocean has always had its waves, and those waves have ever acted in the same manner. Running water on the land has ever had the same power -of wear and transportation and mathematical value to its force. The laws of chemistry, heat, electricity, and mechanics have been the same throughout time. The plan of living structures is fundamentally one, for the whole series belongs to one system, as much almost as the parts of an animal to one body ; and the relations of life to light and heat, and to the atmosphere, have ever been the same as now.

The laws of the existing world, if perfectly known, are consequently a key to past history. SUBDIVISIONS OF GEOLOGY. (1) Like a plant or animal, the earth has its systematic external form und features, which should be reviewed. (2) Next, there are the constituents of the structure to be considered: first, their nature; second, their general arrangement. (3) Next, the successive stages in the formation of the structure, and the concurrent steps in the progress of life, through past time.

INTRODUCTION.

. (4) Next, the general plan or laws of progress in the earth and its life. (5) Finally, there are the active forces and mechanical agencies which were the means of physical progress, — spreading out and consolidating strata, raising mountains, ejecting lavas, wearing out valleys, bearing the material of the heights to the plains and oceans, enlarging the oceans, destroying life, and performing an efficient part in evolving the earth's structure and features. These topics lead to the following subdivisions of the science : — I. Physiographic Geology, — a general survey of the earth's surface features. II. Structural Geology, — a description of the rock-materials in the structure of the globe, — that is, of its kinds of rocks, and of their arrange ment or positions. III. Dynamical Geology, — an account of the agencies or forces that have produced geological changes, and of the laws, methods, and results of their action.

IV. Historical Geology, — an account of the earth's geological his tory, or the successive events or steps in the making of the rock-strata, and of the continents, seas, mountains, and valleys, in the progress of the earth's living species, and in all changes that have gone forward in the earth's development. In the study of the science, a previous knowledge of the methods of change taught in the Dynamical section is desirable in order fully to comprehend Historical geology ; andl a knowledge of the actual facts and their succession given in the Historical section is desirable to understand the causes of events and methods of change. There is reason, therefore, for studying Dynamical geology before Historical as well as after it. It is here made to precede. But the last topic under it — -that of the formation of mountains — will be best appreciated after the student is familiar with the facts presented in the Historical section.