The Project Gutenberg eBook, Earth Features and Their Meaning, by William Herbert Hobbs

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EARTH FEATURES AND THEIR MEANING

THE MACMILLAN COMPANY

NEW YORK · BOSTON · CHICAGO DALLAS · SAN FRANCISCO

MACMILLAN & CO., Limited

LONDON · BOMBAY · CALCUTTA MELBOURNE

THE MACMILLAN CO. OF CANADA, Ltd.

TORONTO

Plate 1.

Mount Balfour and the Balfour Glacier in the Selkirks.

EARTH FEATURES AND
THEIR MEANING

AN INTRODUCTION TO GEOLOGY

FOR THE STUDENT AND THE GENERAL READER

WILLIAM HERBERT HOBBS

PROFESSOR OF GEOLOGY IN THE UNIVERSITY OF MICHIGAN
AUTHOR OF “EARTHQUAKES. AN INTRODUCTION TO
SEISMIC GEOLOGY”; “CHARACTERISTICS OF
EXISTING GLACIERS”; ETC.

New York THE MACMILLAN COMPANY 1921

Norwood Press J. S. Cushing Co.—Berwick & Smith Co. Norwood, Mass., U.S.A.

TO THE MEMORY

GEORGE HUNTINGTON WILLIAMS

PREFACE

The series of readings contained in the present volume give in somewhat expanded form the substance of a course of illustrated lectures which has now for several years been delivered each semester at the University of Michigan. The keynote of the course may be found in the dominant characteristics of the different earth features and the geological processes which have been betrayed in the shaping of them. Such a geological examination of landscape is replete with fascinating revelations, and it lends to the study of Nature a deep meaning which cannot but enhance the enjoyment of her varied aspects.

That there is a real place for such a cultural study of geology within the University is believed to be shown by the increasing number of students who have elected the work. Even more than in former years the American travels afar by car or steamship, and the earth’s surface features in all their manifold diversity are thus one after the other unrolled before him. The thousands who each year cross the Atlantic to roam over European countries may by historical, literary, or artistic studies prepare themselves to derive an exquisite pleasure as they visit places identified with past achievement of one form or another. Yet the Channel coast, the gorge of the Rhine, the glaciers of Switzerland, and the wild scenery of Norway or Scotland have each their fascinating story to tell of a history far more remote and varied. To read this history, the runic characters in which it is written must first of all be mastered; for in every landscape there are strong individual lines of character such as the pen artist would skillfully extract for an outline sketch. Such character profiles are often many times repeated in each landscape, and in them we have a key to the historical record.

An object of the present readings has thus been to enable the student to himself pick out in each landscape these more significant lines and so read directly from Nature. In the landscapes which have been represented, the aim has been to draw as far as possible upon localities well known to travelers and likely to be visited, either because of their historical interest or their purely scenic attractions. It should thus be possible for a tourist in America or Europe to pursue his landscape studies whenever he sets out upon his travels. The better to aid him in this endeavor, some suggestions concerning the itinerary of journeys have been supplied in an appendix.

Regarded as a textbook of geology, the present work offers some departures from existing examples. Though it has been customary to combine in a single text historical with dynamical and structural geology, a tendency has already become apparent to treat the historical division apart from the others. Again, a desire to treat the science of geology comprehensively has led some authors into including so many subjects as to render their texts unnecessarily encyclopedic and correspondingly uninteresting to the general reader. It is the author’s belief that there is a real need for a book which may be read intelligently by the general public, and it must be recognized that the beginner in the subject cannot cover the entire field by a single course of readings. The present work has, therefore, been prepared with a view to selecting for study those dominant geological processes which are best illustrated by features in northern North America and Europe. It is this desire to illustrate the readings by travels afield, which accounts for the prominence given to the subject of glaciation; for the larger number of colleges and universities in both America and Europe are surrounded by the heavy accumulations that have resulted from former glaciations.

Emphasis has also been placed upon the dependence of the dominant geological processes of any region upon existing climatic conditions, a fact to which too little attention has generally been given. This explains the rather full treatment of desert regions, of which, in our own country particularly, much may be illustrated upon the transcontinental railway journeys.

More than in most texts the attempt has here been made to teach directly through the eye with the efficient aid of apt illustrations intimately interwoven with the text. For such success as has been reached in this endeavor, the author is greatly indebted to two students of the University of Michigan,—Mr. James H. Meier, who has prepared the line drawings of landscapes, and Mr. Hugh M. Pierce, who has draughted the diagrams. Though credit has in most cases been given where illustrations have been made from another’s photographs, yet especial mention should here be made of the debt to Dr. H. W. Fairbanks of Berkeley, California, whose beautiful and instructive photographs are reproduced upon many a page.

As given at the University of Michigan, the lectures reflected in the present volume are supplemented by excursions and by so much laboratory practice as is necessary to become familiar with the more common minerals and rocks, and to read intelligently the usual topographical and geological maps. In the appendices the means for carrying out such studies, in part with newly devised apparatus, have been indicated.

The scope of the book precludes the possibility of furnishing the reader with the sources for the body of fact and theory which is presented, although much may be inferred from the names which appear beneath the illustrations, and more definite knowledge will be found in the references to literature supplied at the ends of chapters. A large amount of original and unpublished material is for a similar reason unlabeled, and it has been left for the professional geologist to detect these new strands which have been drawn into the web.

WILLIAM HERBERT HOBBS.

Ann Arbor, Michigan, October 25, 1911.

CHAPTER I
The Compilation of Earth History
	PAGE
The sources of the history—Subdivisions of geology—The study of earthfeatures and their significance—Tabular recapitulation—Geologicalprocesses not universal—Change, and not stability, the order of nature—Observationalgeology versus speculative philosophy—The scientificattitude and temper—The value of the hypothesis—Heading references	[1]
CHAPTER II
The Figure of the Earth
The lithosphere and its envelopes—The evolution of ideas concerning theearth’s figure—The oblateness of the earth—The arrangement ofoceans and continents—The figure toward which the earth is tending—Astronomicalversus geodetic observations—Changes of figure duringcontraction of a spherical body—The earlier figures of the earth—Thecontinents and oceans at the close of the Paleozoic era—Theflooded portions of the present continents—The floors of the hydrosphereand atmosphere—Reading references	[8]
CHAPTER III
The Nature of the Materials in the Lithosphere
The rigid quality of our planet—Probable composition of the earth’s core—Theearth a magnet—The chemical constitution of the earth’s surfaceshell—The essential nature of crystals—The lithosphere a complexof interlocking crystals—Some properties of natural crystals,minerals—The alterations of minerals—Reading references	[20]
CHAPTER IV
The Rocks of the Earth’s Surface Shell
The processes by which rocks are formed—The marks of origin—Themetamorphic rocks—Characteristic textures of the igneous rocks—Theclassification of rocks—Subdivisions of the sedimentary rocks—Thedifferent deposits of ocean, lake, and river—Special marks oflittoral deposits—The order of deposition during a transgression ofthe sea—The basins of deposition of earlier ages—The deposits of thedeep sea—Reading references	[30]
CHAPTER V
Contortions of the Strata within the Zone of Flow
The zones of fracture and flow—Experiments which illustrate the fractureand flow of solid bodies—The arches and troughs of the foldedstrata—The elements of folds—The shapes of rock folds—The overthrustfold—Restoration of mutilated folds—The geological map andsection—Measurement of the thickness of formations—The detectionof plunging folds—The meaning of an unconformity—Reading references	[40]
CHAPTER VI
The Architecture of the Fractured Superstructure
The system of the fractures—The space intervals of joints—The displacementsupon joints: faults—Methods of detecting faults—Thebase of the geological map—The field map and the areal geologicalmap—Laboratory models for study of geological maps—The methodof preparing the map—Fold vs. fault topography—Reading references	[55]
CHAPTER VII
The Interrupted Character of Earth Movements: Earthquakesand Seaquakes
Nature of earthquake shocks—Seaquakes and seismic sea waves—Thegrander and the lesser earth movements—Changes in the earth’ssurface during earthquakes: faults and fissures—The measure ofdisplacement—Contraction of the earth’s surface during earthquakes—Theplan of an earthquake fault—The block movements of thedisturbed district—The earth blocks adjusted during the Alaskanearthquake of 1899	[67]
CHAPTER VIII
The Interrupted Character of Earth Movements: Earthquakesand Seaquakes (concluded)
Experimental demonstration of earth movements—Derangement of waterflow by earth movement—Sand or mud cones and craterlets—Theearth’s zones of heavy earthquake—The special lines of heavy shock—Seismotectoniclines—The heavy shocks above loose foundations—Constructionin earthquake regions—Reading references	[81]
CHAPTER IX
The Rise of Molten Rock to the Earth’s Surface; VolcanicMountains of Exudation
Prevalent misconceptions about volcanoes—Early views concerning volcanicmountains—The birth of volcanoes—Active and extinct volcanoes—Theearth’s volcano belts—Arrangement of volcanic ventsalong fissures, and especially at their intersections—The so-calledfissure eruptions—The composition and the properties of lava—Thethree main types of volcanic mountain—The lava dome—The basalticlava domes of Hawaii—Lava movements within the caldron of Kilauea—Thedraining of the lava caldrons—The outflow of the lava floods	[94]
CHAPTER X
The Rise of Molten Rock to the Earth’s Surface; VolcanicMountains of Ejected Materials
The mechanics of crater explosions—Grander volcanic eruptions of cindercones—The eruption of Volcano in 1888—The eruption of Taalvolcano on January 30, 1911—The materials and the structure of cindercones—The profile lines of cinder cones—The composite cone—Thecaldera of composite cones—The eruption of Vesuvius in 1906—Thesequence of events within the chimney—The spine of Pelé—Theaftermath of mud flows—The dissection of volcanoes—Theformation of lava reservoirs—Character profiles—Reading references	[115]
CHAPTER XI
The Attack of the Weather
The two contrasted processes of weathering—The rôle of the percolatingwater—Mechanical results of decomposition: spheroidal weathering—Exfoliationor scaling—Dome structure in granite masses—Theprying work of frost—Talus—Soil flow in the continued presence ofthaw water—The splitting wedges of roots and trees—The rock mantleand its shield in the mat of vegetation—Reading references	[149]
CHAPTER XII
The Life Histories of Rivers
The intricate pattern of river etchings—The motive power of rivers—Oldland and new land—The earlier aspects of rivers—The meshesof the river network—The upper and lower reaches of a river contrasted—Thebalance between degradation and aggradation—Theaccordance of tributary valleys—The grading of the flood plain—Thecycles of stream meanders—The cut-off of the meander—Meanderscars—River terraces—The delta of the river—The levee—Thesections of delta deposits	[158]
CHAPTER XIII
Earth Features shaped by Running Water
The newly incised upland and its sharp salients—The stage of adolescence—Thematurely dissected upland—The Hogarthian line of beauty—Thefinal product of river sculpture: the peneplain—The river crosssections of successive stages—The entrenchment of meanders withrenewed uplift—The valley of the rejuvenated river—The arrest ofstream erosion by the more resistant rocks—The capture of one river byanother—Water and wind gaps—Character profiles—Reading references	[169]
CHAPTER XIV
The Travels of the Underground Water
The descent within the unsaturated zone—The trunk channels of descendingwater—The caverns of limestones—Swallow holes and limestonesinks—The sinter deposits—The growth of stalactites—Formationof stalagmites—The Karst and its features—A desert from thedestruction of forests—The ponore and the polje—The return of thewater to the surface—Artesian wells—Hot springs and geysers—Thedeposition of siliceous sinter by plant growth—Reading references	[180]
CHAPTER XV
Sun and Wind in the Lands of Infrequent Rains
The law of the desert—The self-registering gauge of past climates—Somecharacteristics of the desert waste—Dry weathering: the red andbrown desert varnish—The mechanical breakdown of the desert rocks—Thenatural sand blast—The dust carried out of the desert	[197]
CHAPTER XVI
The Features in Desert Landscapes
The wandering dunes—The forms of dunes—The cloudburst in thedesert—The zone of the dwindling river—Erosion in and about thedesert—Characteristic features of the arid lands—The war of duneand oasis—The origin of the high plains which front the RockyMountains—Character profiles—Reading references	[209]
CHAPTER XVII
Repeating Patterns in the Earth Relief
The weathering processes under control of the fracture system—Thefracture control of the drainage lines—The repeating pattern in drainagenetworks—The dividing lines of the relief patterns: lineaments—Thecomposite repeating patterns of the higher orders—Readingreferences	[223]
CHAPTER XVIII
The Forms carved and molded by Waves
The motion of a water wave—Free waves and breakers—Effect of thebreaking wave upon a steep, rocky shore: the notched cliff—Coves,sea arches, and stacks—The cut rock terrace—The cut and builtterrace on a steep shore of loose materials—The work of the shorecurrent—The sand beach—The shingle beach—Bar, spit, and barrier—Theland-tied island—A barrier series—Character profiles—Readingreferences	[231]
CHAPTER XIX
Coast Records of the Rise or Fall of the Land
The characters in which the record has been preserved—Even coast linethe mark of uplift—A ragged coast line the mark of subsidence—Slowuplift of the coasts; the coastal plain and cuesta—The sudden upliftsof the coast—The upraised cliff—The uplifted barrier beach—Coastterraces—The sunk or embayed coast—Submerged river channels—Recordsof an oscillation of movement—Simultaneous contrary movementsupon a coast—The contrasted islands of San Clemente andSanta Catalina—The Blue Grotto of Capri—Character profiles—Readingreferences	[245]
CHAPTER XX
The Glaciers of Mountain and Continent
Conditions essential to glaciation—The snow-line—Importance of mountainbarriers in initiating glaciers—Sensitiveness of glaciers to temperaturechanges—The cycle of glaciation—The advancing hemicycle—Continentaland mountain glaciers contrasted—The nourishmentof glaciers—The upper and lower cloud zones of the atmosphere	[261]
CHAPTER XXI
The Continental Glaciers of Polar Regions
The inland ice of Greenland—The mountain rampart and its portals—Themarginal rock islands—Rock fragments which travel with theice—The grinding mill beneath the ice—The lifting of the grindingtools and their incorporation within the ice—Melting upon the glaciermargins in Greenland—The marginal moraines—The outwash plainor apron—The continental glacier of Antarctica—Nourishment ofcontinental glaciers—The glacier broom—Field and pack ice—Thedrift of the pack—The Antarctic shelf ice—Icebergs and snowbergsand the manner of their birth—Reading references	[271]
CHAPTER XXII
The Continental Glaciers of the “Ice Age”
Earlier cycles of glaciation—Contrast of the glaciated and nonglaciatedregions—The “driftless area”—Characteristics of the glaciatedregions—The glacier gravings—Younger records over older: theglacier palimpsest—The dispersion of the drift—The diamonds ofthe drift—Tabulated comparison of the glaciated and nonglaciatedregions—Unassorted and assorted drift—Features into which thedrift is molded—Marginal or “kettle” moraines—Outwash plains—Pittedplains and interlobate moraines—Eskers—Drumlins—Theshelf ice of the ice age—Character profiles	[297]
CHAPTER XXIII
Glacial Lakes which marked the Decline of the Last Ice Age
Interference of glaciers with drainage—Temporary lakes due to ice blocking—The“parallel roads” of the Scottish glens—The glacial LakeAgassiz—Episodes of the glacial lake history within the St. LawrenceValley—The crescentic lakes of the earlier stages—The early LakeMaumee—The later Lake Maumee—Lakes Arkona and Whittlesey—LakeWarren—Lakes Iroquois and Algonquin—The NipissingGreat Lakes—Summary of lake stages—Permanent changes ofdrainage effected by the glacier—Glacial Lake Ojibway in the Hudson’sBay drainage basin—Reading references	[320]
CHAPTER XXIV
The Uptilt of the Land at the Close of the Ice Age
The response of the earth’s shell to its ice mantle—The abandoned strandsas they appear to-day—The records of uplift about Mackinac Island—Thepresent inclinations of the uplifted strands—The hinge lines ofuptilt—Future consequences of the continued uptilt within the lakeregion—Gilbert’s prophecy of a future outlet of the Great Lakes tothe Mississippi—Geological evidences of continued uplift—Drowningof southwestern shores of Lakes Superior and Erie—Reading references	[340]
CHAPTER XXV
Niagara Falls a Clock of Recent Geological Time
Features in and about the Niagara gorge—The drilling of the gorge—Thepresent rate of recession—Future extinction of the American Fall—Thecaptured Canadian Fall at Wintergreen Flats—The WhirlpoolBasin excavated from the St. David’s gorge—The shaping of theLewiston Escarpment—Episodes of Niagara’s history and their correlationwith those of the glacial lakes—Time measures of the Niagaraclock—The horologe of late glacial time in Scandinavia—Readingreferences	[352]
CHAPTER XXVI
Land Sculpture by Mountain Glaciers
Contrasted sculpturing of continental and mountain glaciers—Wind distributionof the snow which falls in mountains—The niches whichform on snowdrift sites—The augmented snowdrift moves down thevalley: birth of the glacier—The excavation of the glacial amphitheateror cirque—Life history of the cirque—Grooved and fretteduplands—The features carved above the glacier—The features shapedbeneath the glacier—The cascade stairway in glacier-carved valleys—Thecharacter profiles which result from sculpture by mountain glaciers—Thesculpture accomplished by ice caps—The Norwegian tind orbeehive mountain—Reading references	[367]
CHAPTER XXVII
Successive Glacier Types of a Waning Glaciation
Transition from the ice cap to the mountain glacier—The piedmontglacier—The expanded-foot glacier—The dendritic glacier—Theradiating glacier—The horseshoe glacier—The inherited-basin glacier—Summaryof types of mountain glacier—Reading references	[383]
CHAPTER XXVIII
The Glacier’s Surface Features and the Deposits upon its Bed
The glacier flow—Crevasses and séracs—Bodies given up by the Glacierdes Bossons—The moraines—Selective melting upon the glaciersurface—Glacier drainage—Deposits within the vacated valley—Marksof the earlier occupation of mountains by glaciers—Readingreferences	[390]
CHAPTER XXIX
A Study of Lake Basins
Fresh water and saline lakes—Newland lakes—Basin-range lakes—Rift-valleylakes—Earthquake lakes—Crater lakes—Coulée lakes—Morainallakes—Pit lakes—Glint or colk lakes—Ice-dam lakes—Glacier-lobelakes—Rock-basin lakes—Valley moraine lakes—Landslidelakes—Border lakes—Ox-bow lakes—Saucer lakes—Crescenticlevee lakes—Raft lakes—Side-delta lakes—Delta lakes—Barrierlakes—Dune lakes—Sink lakes—Karst lakes: poljen—Playa lakes—Salines—Alluvial-damlakes—Résumé—Reading references	[401]
CHAPTER XXX
The Ephemeral Existence of Lakes
Lakes as settling basins—Drawing off of water by erosion of outlet—Thepulling in of headlands and the cutting off of bays—Lake extinctionby peat growth—Extinction of lakes in desert regions—The rôle oflakes in the economy of nature—Ice ramparts on lake shores—Readingreferences	[426]
CHAPTER XXXI
The Origin and the Forms of Mountains
A mountain defined—The festoons of mountain arcs—Theories of originof the mountain arcs—The Atlantic and Pacific coasts contrasted—Theblock type of mountain—Mountains of outflow or upheap—Domedmountains of uplift; laccolites—Mountains carved fromplateaus—The climatic conditions of the mountain sculpture—Theeffect of the resistant stratum—The mark of the rift in the erodedmountains—Reading references	[435]
APPENDICES
A. The quick determination of the common minerals	[449]
B. Short descriptions of some common rocks	[462]
C. The preparation of topographical maps	[467]
D. Laboratory models for study in the interpretation of geological maps	[472]
E. Suggested itineraries for pilgrimages to study earth features	[475]
Index	[489]

LIST OF PLATES

PLATE
1.	Mount Balfour and the Balfour Glacier in the Selkirks		[Frontispiece]
FACING PAGE
2.	A.	Layers compressed in experiments and showing the effect of a competent layer in the process of folding	[44]
	B.	Experimental production of a series of parallel thrusts within closely folded strata	[44]
	C.	Apparatus to illustrate shearing action within the overturned limb of a fold	[44]
3.	A.	An earthquake fault opened in Formosa in 1906 with vertical andlateral displacements combined	[72]
3.	B.	Earthquake faults opened in Alaska in 1889 on which verticalslices of the earth’s shell have undergone individual adjustments	[72]
4.	A.	Experimental tank to illustrate the earth movements which aremanifested in earthquakes. The sections of the earth’s shell arehere represented before adjustment has taken place	[82]
	B.	The same apparatus after a sudden adjustment	[82]
	C.	Model to illustrate a block displacement in rocks which are intersectedby master joints	[82]
5.	A.	Once wooded region in China now reduced to desert through deforestation	[156]
5.	B.	“Bad Lands” in the Colorado Desert	[156]
6.	A.	Barren Karst landscape near the famous Adelsberg grottoes	[188]
6.	B.	Surface of a limestone ledge where joints have been widened throughsolution	[188]
7.	A.	Ranges of dunes upon the margin of the Colorado Desert	[210]
7.	B.	Sand dunes encroaching upon the oasis of Oued Souf, Algeria	[210]
8.	A.	The granite needles of Harney Peak in the Black Hills of SouthDakota	[216]
8.	B.	Castellated erosion chimneys in El Cobra Cañon, New Mexico	[216]
9.	Map of the High Plains at the eastern front of the Rocky Mountains		[220]
10.	A.	View in Spitzbergen to illustrate the disintegration of rock underthe control of joints	[228]
10.	B.	Composite pattern of the joint structures within recent alluvialdeposits of the Syrian Desert	[228]
11.	A.	Ripple markings within an ancient sandstone	[232]
11.	B.	Wave breaking as it approaches the shore	[232]
12.	A.	V-shaped cañon cut in an upland recently elevated from the sea,San Clemente Island, California	[256]
12.	B.	A “hogback” at the base of the Bighorn Mountains, Wyoming	[256]
13.	A.	Precipitous front of the Bryant Glacier outlet of the Greenlandinland ice	[272]
13.	B.	Lateral stream beside the Benedict Glacier outlet, Greenland	[272]
14.	View of the margin of the Antarctic continental glacier in KaiserWilhelm Land		[282]
15.	A.	An Antarctic ice foot with boat party landing	[290]
15.	B.	A near view of the front of the Great Ross Barrier, Antarctica	[290]
16.	A.	Incised topography within the “driftless area”	[300]
16.	B.	Built-up topography within the glaciated region	[300]
17.	A.	Soled glacial bowlders which show differently directed striæ uponthe same facet	[306]
	B.	Perched bowlder upon a striated ledge of different rocktype, Bronx Park, New York	[306]
	C.	Characteristic knob and basin surface of a moraine	[306]
18.	A.	Fretted upland of the Alps seen from the summit of Mount Blanc	[372]
18.	B.	Model of the Malaspina Glacier and the fretted upland above it	[372]
19.	A.	Contour map of a grooved upland, Bighorn Mountains, Wyoming	[372]
19.	B.	Contour map of a fretted upland, Philipsburg Quadrangle, Montana	[372]
20.	Map of the surface modeled by mountain glaciers in the Sierra Nevadasof California		[376]
21.	A.	View of the Harvard Glacier, Alaska, showing the characteristicterraces	[394]
21.	B.	The terminal moraine at the foot of a mountain glacier	[394]
22.	A.	Model of the vicinity of Chicago, showing the position of theoutlet of the former Lake Chicago	[400]
22.	B.	Map of Yosemite Falls and its earlier site near Eagle Peak	[400]
23.	A.	View of the American Fall at Niagara, showing the accumulationof blocks beneath	[414]
23.	B.	Crystal Lake, a landslide lake in Colorado	[414]
24.	A.	Apparatus for exercise in the preparation of topographic maps	[468]
	B.	The same apparatus in use for testing the contours of a map	[468]
	C.	Modeling apparatus in use	[468]

ILLUSTRATIONS IN THE TEXT

FIG.		PAGE
1.	Diagram to show the measure of the earth’s surface irregularities	[11]
2.	Map to show the reciprocal relation of areas of land and sea	[11]
3.	The tetrahedral form toward which the earth is tending	[12]
4.	A truncated tetrahedron to show the reciprocal relation of projection and depression upon the surface	[13]
5.	Approximations to earlier and present figures of the earth	[15]
6.	Diagrams for comparison of coasts upon an upright and upon an inverted tetrahedron	[17]
7.	The continents, including submerged portions	[18]
8.	Diagram to indicate the altitude of different parts of the lithosphere surface	[18]
9.	Diagram to show how the terrestrial rocks grade into the meteorites	[22]
10.	Comparison of a crystalline with an amorphous substance	[24]
11.	“Light figure” seen upon etched surface of calcite	[25]
12.	Battered sand grains which have developed crystal faces	[26]
13.	Unassimilated grains of quartz within a garnet crystal	[28]
14.	New minerals developed about the core of an augite crystal	[28]
15.	A common rim of new mineral developed by reaction where earlier minerals come into contact	[28]
16.	Laminated structure of a sedimentary rock	[30]
17.	Characteristic textures of igneous rocks	[33]
18.	Diagram to show the order of sediments laid down during a transgression of the sea	[37]
19.	Fractures produced by compression of a block of molder’s wax	[41]
20.	Apparatus to illustrate the folding of strata	[41]
21.	Diagrams of fold types	[42]
22.	Diagrams to illustrate crustal shortening	[42]
23.	Anticlinal and synclinal folds	[43]
24.	Diagrams to illustrate the shapes of rock folds	[44]
25.	Secondary and tertiary flexures superimposed upon the primary ones	[44]
26.	A bent stratum to illustrate tension and compression upon opposite sides	[45]
27.	A geological section with truncated arches restored	[47]
28.	Diagram to illustrate the nature of strike and dip	[47]
29.	Diagram to show the use of T symbols for strike and dip observation	[48]
30.	Diagram to show how the thickness of a formation is determined	[49]
31.	A plunging anticline	[50]
32.	A plunging syncline	[50]
33.	An unconformity upon the coast of California	[51]
34.	Series of diagrams to illustrate the episodes involved in the production of an angular unconformity	[52]
35.	Types of deceptive or erosional unconformities	[53]
36.	A set of master joints in shale	[55]
37.	Diagram to show the manner of replacement of one set of joints by another	[56]
38.	Diagram to show the different combinations of joint series	[56]
39.	View of the shore in West Greenland	[57]
40.	View in Iceland which shows joint intervals of more than one order	[57]
41.	Faulted blocks of basalt near Woodbury, Connecticut	[58]
42.	A fault in previously disturbed strata	[59]
43.	Diagram to show the effect of erosion upon a fault	[60]
44.	A fault plane exhibiting drag	[60]
45.	Map to show how a fault may be indicated by abrupt changes in strike and dip	[61]
46.	A series of parallel faults revealed by offsets	[61]
47.	Field map prepared from the laboratory table	[64]
48.	Areal geological map based upon the field map	[64]
49.	A portion of the ruins of Messina	[67]
50.	Ruins of the Carnegie Palace of Peace at Cartaga, Costa Rica	[68]
51.	Overturned bowlders from Assam earthquake of 1897	[69]
52.	Post sunk into ground during Charleston earthquake	[69]
53.	Map showing localities where shocks have been reported at sea off Cape Mendocino, California	[70]
54.	Effect of seismic water wave in Japan	[70]
55.	A fault of vertical displacement	[71]
56.	Escarpment produced by an earthquake fault in India	[72]
57.	A fault of lateral displacement	[72]
58.	Fence parted and displaced by lateral displacement on fault during California earthquake	[72]
59.	Fault with vertical and lateral displacements combined	[72]
60.	Diagram to show how small faults may be masked at the earth’s surface	[73]
61.	“Mole hill” effect above buried earthquake fault	[73]
62.	Post-glacial earthquake faults	[74]
63.	Earthquake cracks in Colorado desert	[74]
64.	Railway tracks broken or buckled at time of earthquake	[75]
65.	Railroad bridge in Japan damaged by earthquake	[75]
66.	Diagrams to show contraction of earth’s crust during an earthquake	[76]
67.	Map of the Chedrang fault of India	[76]
68.	Displacements along earthquake fault in Alaska	[77]
69.	Abrupt change in direction of throw upon an earthquake fault	[77]
70.	Map of faults in the Owens Valley, California, formed during earthquake of 1872	[78]
71.	Marquetry of the rock floor in the Tonopah district, Nevada	[79]
72.	Map of Alaskan coast to show adjustments of level during an earthquake	[79]
73.	An Alaskan shore elevated seventeen feet during the earthquake of 1899	[80]
74.	Partially submerged forest from depression of shore in Alaska during earthquake	[80]
75.	Effect of settlement of the shore at Port Royal during earthquake of 1907	[80]
76.	Diagrams to illustrate the draining of lakes during earthquakes	[83]
77.	Diagram to illustrate the derangements of water flow during an earthquake	[84]
78.	Mud cones aligned upon an earthquake fissure in Servia	[84]
79.	Craterlet formed near Charleston, South Carolina, during the earthquake of 1886	[85]
80.	Cross section of a craterlet	[85]
81.	Map of the island of Ischia to show the concentration of earthquake shocks	[87]
82.	A line of earth fracture revealed in the plan of the relief	[87]
83.	Seismotectonic lines of the West Indies	[88]
84.	Device to illustrate the different effects of earthquakes in firm rock and in loose materials	[88]
85.	House wrecked in San Francisco earthquake	[90]
86.	Building wrecked in California earthquake by roof and upper floor battering down the upper walls	[91]
87.	Breached volcanic cone in New Zealand showing the bending down of the strata near the vent	[96]
88.	View of the new Camiguin volcano formed in 1871 in the Philippines	[97]
89.	Map to show the belts of active volcanoes	[98]
90.	A portion of the “fire girdle” of the Pacific	[98]
91.	Volcanic cones formed in 1783 above the Skaptár fissure in Iceland	[99]
92.	Diagrams to illustrate the location of volcanic vents upon fissure lines	[100]
93.	Outline map showing the arrangement of volcanic vents upon the island of Java	[100]
94.	Map showing the migration of volcanoes along a fissure	[101]
95.	Basaltic plateau of the northwestern United States due to fissure eruptions of lava	[102]
96.	Lava plains about the Snake River in Idaho	[102]
97.	Characteristic profiles of lava volcanoes	[103]
98.	A driblet cone	[104]
99.	Leffingwell Crater, a cinder cone in the Owens Valley, California	[104]
100.	Map of Hawaii and its lava volcanoes	[106]
101.	Section through Mauna Loa and Kilauea	[106]
102.	Schematic diagram to illustrate the moving platform in the crater of Kilauea	[107]
103.	View of the open lava lake of Halemaumau	[108]
104.	Map to show the manner of outflow of the lava from Kilauea in the eruption of 1840	[109]
105.	Lava of Matavanu flowing down to the sea during the eruption of 1906	[110]
106.	Lava stream discharging into the sea from a lava tunnel	[111]
107.	Diagrammatic representation of the structure of lava volcanoes as a result of the draining of frozen lava streams	[112]
108.	Diagram to show the formation of mesas by outflow of lava in valleys and subsequent erosion	[112]
109.	Surface of lava of the Pahoehoe type	[113]
110.	Three successive views to show the growth of the island of Savaii, from lava outflow in 1906	[113]
111.	View of the volcano of Stromboli showing the excentric position of the crater	[116]
112.	Diagrams to illustrate the eruptions within the crater of Stromboli	[117]
113.	Map of Volcano in the Æolian Islands	[118]
114.	“Bread-crust” lava projectile from the eruption of Volcano in 1888	[119]
115.	“Cauliflower cloud” of steam and ash rising above the cinder cone of Volcano	[120]
116.	Eruption of Taal volcano in 1911 seen from a distance of six miles	[120]
117.	The thick mud veneer upon the island of Taal (after a photograph by Deniston)	[121]
118.	A pear-shaped lava projectile	[121]
119.	Artificial production of a cinder cone	[122]
120.	Diagram to show the contrast between a lava dome and a cinder cone	[123]
121.	Mayon volcano on the island of Luzon, Philippine Islands	[123]
122.	A series of breached cinder cones due to migration of the eruption along a fissure	[124]
123.	The mouth upon the inner cone of Mount Vesuvius from which flowed the lava of 1872	[124]
124.	A row of parasitic cones raised above a fissure opened on the flanks of Etna in 1892	[125]
125.	View of Etna, showing the parasitic cones upon its flanks	[125]
126.	Sketch map of Etna to show the areas covered by lava and tuff respectively	[126]
127.	Panum crater showing the caldera	[126]
128.	View of Mount Vesuvius before the eruption of 1906	[127]
129.	Sketches of the summit of the Vesuvian cone to bring out the changes in its outline	[128]
130.	Night view of Vesuvius from Naples before the outbreak of 1906, showing a small lava stream descending the central cone	[129]
131.	Scoriaceous lava encroaching upon the tracks of the Vesuvian railway	[130]
132.	Map of Vesuvius, showing the position of the lava mouths opened upon its flanks during the eruption of 1906	[131]
133.	The ash curtain over Vesuvius lifting and disclosing the outlines of the mountain	[132]
134.	The central cone of Vesuvius as it appeared after the eruption of 1906	[132]
135.	A sunken road upon Vesuvius filled with indrifted ash	[133]
136.	View of Vesuvius from the southwest during the waning stages of the eruption	[133]
137.	The main lava stream advancing upon Boscotrecase	[133]
138.	A pine snapped off by the lava and carried forward upon its surface	[133]
139.	Lava front pushing over and running around a wall in its path	[134]
140.	One of the ruined villas in Boscotrecase	[134]
141.	Three diagrams to illustrate the sequence of events during the cone-building and crater-producing periods	[135]
142.	The spine of Pelé rising above the chimney of the volcano after the eruption of 1902	[136]
143.	Successive outlines of the Pelé spine	[137]
144.	Corrugated surface of the Vesuvian cone due to the mud flows which followed the eruption of 1906	[138]
145.	View of the Kammerbühl near Eger in Bohemia	[139]
146.	Volcanic plug exposed by natural dissection of a volcanic cone in Colorado	[140]
147.	A dike cutting beds of tuff in a partly dissected volcano of southwestern Colorado	[140]
148.	Map and general view of St. Paul’s rocks, a volcanic cone dissected by waves	[141]
149.	Dissection by explosion of Little Bandai-san in 1888	[141]
150.	The half-submerged volcano of Krakatoa before and after the eruption of 1883	[142]
151.	The cicatrice of the Banat	[142]
152.	Diagram to illustrate a probable cause of formation of lava reservoirs and the connection with volcanoes upon the surface	[143]
153.	Effect of relief of load upon rocks by arching of a competent formation	[144]
154.	Character profiles connected with volcanoes	[146]
155.	Diagrams to show the effect of decomposition in producing spheroidal bowlders	[150]
156.	Spheroidal weathering of an igneous rock	[151]
157.	Dome structure in granite mass	[152]
158.	Talus slope beneath a cliff	[153]
159.	Striped ground from soil flow	[154]
160.	Pavement of horizontal surface due to soil flow	[154]
161.	Tree roots prying rock apart on fissure	[154]
162.	Bowlder split by a growing tree	[155]
163.	Rock mantle beneath soil and vegetable mat	[155]
164.	Diagram to show the varying thickness of mantle rock upon the different portions of a hill surface	[156]
165.	Gullies from earliest stage of a river’s life	[160]
166.	Partially dissected upland	[160]
167.	Longitudinal sections of upper portion of a river valley	[161]
168.	Map and sections of a stream meander	[163]
169.	Tree undermined on the outer bank of a meander	[164]
170.	Diagrams to show the successive positions of stream meanders	[164]
171.	An ox-bow lake in the flood plain of a river	[165]
172.	Schematic representation of a series of river terraces	[165]
173.	“Bird-foot” delta of the Mississippi River	[167]
174.	Diagrams to show the nature of delta deposits as exhibited in sections	[168]
175.	Gorge of the River Rhine near St. Goars	[169]
176.	Valley with rounded shoulders characteristic of the stage of adolescence	[170]
177.	View of a maturely dissected upland	[170]
178.	Hogarth’s line of beauty	[171]
179.	View of the oldland of New England, with Mount Monadnock rising in the distance	[171]
180.	Comparison of the cross sections of river valleys of different stages	[172]
181.	The Beavertail Bend of the Yakima River	[173]
182.	A rejuvenated river valley	[174]
183.	Plan of a river narrows	[174]
184.	Successive diagrams to illustrate the origin of “trellis drainage”	[175]
185.	Sketch maps to show the earlier and present drainage near Harper’s Ferry	[176]
186.	Section to illustrate the history of Snickers Gap	[177]
187.	Character profiles of landscapes shaped by stream erosion in humid climates	[177]
188.	Diagram to show the seasonal range in the position of the water table	[180]
189.	Diagram to show the effect of an impervious layer upon the descending water	[181]
190.	Sketch map to illustrate corrosion of limestone along two series of vertical joints	[181]
191.	Diagram to show the relation of limestone caverns to the river system of the district	[182]
192.	Plan of a portion of Mammoth Cave, Kentucky	[183]
193.	Trees and shrubs growing upon the bottoms of limestone sinks	[183]
194.	Diagrams to show the manner of formation of stalactites and stalagmites	[185]
195.	Sinter formations in the Luray caverns	[186]
196.	Map of the dolines of the Karst region	[187]
197.	Cross section of a doline formed by inbreak	[187]
198.	Sharp Karren of the Ifenplatte	[188]
199.	The Zirknitz seasonal lake	[189]
200.	Fissure springs arranged at intersections of rock fractures	[190]
201.	Schematic diagrams to illustrate the different types of artesian wells	[191]
202.	Cross section of Geysir, Iceland	[192]
203.	Apparatus for simulating geyser action	[193]
204.	Cone of siliceous sinter about the Lone Star Geyser	[194]
205.	Former shore lines in the Great Basin	[198]
206.	Map of the former Lake Bonneville	[199]
207.	Borax deposits in Death Valley, California	[201]
208.	Hollowed forms of weathered granite in a desert of Central Asia	[201]
209.	Hollow hewn blocks in a wall in the Wadi Guerraui	[202]
210.	Smooth granite domes shaped by exfoliation	[203]
211.	Granite blocks rent by diffission	[204]
212.	“Mushroom Rock” from a desert in Wyoming	[205]
213.	Windkanten shaped by sand blast in the desert	[205]
214.	The “stone lattice” of the desert	[206]
215.	Shadow erosion in the desert	[206]
216.	Cliffs in loess with characteristic vertical jointing	[207]
217.	A cañon in loess worn by traffic and wind	[207]
218.	Diagrams to illustrate the effects of obstructions in arresting wind-driven sand	[209]
219.	Sand accumulating on either side of a firm and impenetrable obstruction	[210]
220.	Successive diagrams to illustrate the history of the town of Kunzen upon the Kurische Nehrung	[210]
221.	View of desert barchans	[211]
222.	Diagrams to show the relationships of dunes to sand supply and wind direction	[211]
223.	Ideal section showing the rising mountain wall about a desert and the neighboring slope	[212]
224.	Dry delta at the foot of a range upon the borders of a desert	[213]
225.	Map of distributaries of streams which issue at the western base of the Sierra Nevadas	[213]
226.	A group of “demoiselles” in the “bad lands”	[214]
227.	Amphitheater at the head of the Wadi Beni Sur	[215]
228.	Mesa and outlier in the Leucite Hills of Wyoming	[216]
229.	Flat-bottomed basin separating dunes	[216]
230.	Billowy surface of the salt crust on the central sink of the desert of Lop	[217]
231.	Schematic diagram to show the zones of deposition in their order from the margin to the center of a desert	[217]
232.	Mounds upon the site of the buried city of Nippur	[218]
233.	Exhumed structures in the buried city of Nippur	[218]
234.	Section across the High Plains	[219]
235.	Section across the lenticular threads of alluvial deposits of the High Plains	[220]
236.	Distributaries of the foot hills superimposed upon an earlier series	[220]
237.	Character profiles in the landscapes of arid lands	[220]
238.	Rain sculpturing under control by joints	[224]
239.	Sagging of limestone above joints	[224]
240.	Map of the joint-controlled Abisko Cañon in Northern Lapland	[225]
241.	Map of the gorge of the Zambesi River below Victoria Falls	[225]
242.	Controlled drainage network of the Shepaug River in Connecticut	[226]
243.	A river network of repeating rectangular pattern	[226]
244.	Squared mountain masses which reveal a distribution of joints in block patterns of different orders	[228]
245.	Island groups of the Lofoten Archipelago	[229]
246.	Diagrams to illustrate the composite profiles of the islands on the Norwegian coast	[229]
247.	Diagram to show the nature of the motions within a free water wave	[231]
248.	Diagram to illustrate the transformation of a free wave into a breaker	[232]
249.	Notched rock cliff and fallen blocks	[233]
250.	A wave-cut chasm under control by joints	[233]
251.	Grand Arch upon one of the Apostle Islands in Lake Superior	[234]
252.	Stack near the shore of Lake Superior	[234]
253.	The Marble Islands, stacks in a lake of the southern Andes	[235]
254.	Squared stacks revealing the position of the joint planes on which they were carved	[235]
255.	Ideal section cut by waves upon a steep rocky shore	[236]
256.	Map showing the outlines of the island of Heligoland at different stages in its history	[236]
257.	Ideal section carved by waves upon a steep shore of loose materials	[237]
258.	Sloping cliff and boulder pavement at Scituate, Massachusetts	[237]
259.	Map to show the nature of the shore current and the forms which are molded by it	[238]
260.	Crescent-shaped beach in the lee of a headland	[239]
261.	Cross section of a beach pebble	[239]
262.	A storm beach on the northeast shore of Green Bay	[240]
263.	Spit of shingle on Au Train Island, Lake Superior	[240]
264.	Barrier beach in front of a lagoon	[241]
265.	Cross section of a barrier beach with lagoon in its rear	[242]
266.	Cross section of a series of barriers and an outer bar	[242]
267.	A barrier series and an outer bar on Lake Mendota at Madison, Wisconsin	[242]
268.	Series of barriers at the western end of Lake Superior	[243]
269.	Character profiles resulting from wave action upon shores	[243]
270.	The even shore line of a raised coast	[246]
271.	The ragged coast line produced by subsidence	[246]
272.	Portion of the Atlantic coastal plain at the base of the oldland	[246]
273.	Ideal form of cuestas and intermediate lowlands carved from a coastal plain	[247]
274.	Uplifted sea cave on the coast of California	[248]
275.	Double-notched cliff near Cape Tiro, Celebes	[248]
276.	Uplifted stacks on the coast of California	[249]
277.	Uplifted shingle beach across the entrance to a former bay upon the coast of California	[250]
278.	Raised beach terraces near Elie, Fife, Scotland	[250]
279.	Uplifted sea cliffs and terraces on the Alaskan coast	[250]
280.	Diagrams to show how excessive sinking upon the sea floor will cause the shore to migrate landward	[251]
281.	A drowned river mouth or estuary upon a coastal plain	[251]
282.	Archipelago of steep rocky islets due to submergence	[252]
283.	The submerged Hudsonian channel which continues the Hudson River across the continental shelf	[252]
284.	Marine clay deposits near the mouths of the Maine rivers which preserve a record of earlier subsidence and later elevation	[253]
285.	View of the three standing columns of the Temple of Jupiter Serapis, at Pozzuoli	[254]
286.	Three successive views to set forth the recent oscillations of level on the northern shore of the Bay of Naples	[255]
287.	Relief map of San Clemente Island, California	[256]
288.	Relief map of Santa Catalina Island, California	[257]
289.	Cross section of the Blue Grotto, on the island of Capri	[258]
290.	Character profiles of coast elevation and subsidence	[259]
291.	Map showing the distribution of existing glaciers and the two important wind poles of the earth	[263]
292.	An Alaskan glacier spreading out at the foot of the range which nourishes it	[264]
293.	Surface of a glacier whose upper layers spread with but slight restraint from retaining walls	[265]
294.	Section through a mountain glacier	[267]
295.	Profile across the largest of the Icelandic ice caps	[267]
296.	Ideal section across a continental glacier	[267]
297.	View of the Eyriks Jökull, an ice cap of Iceland	[268]
298.	The zones of the lower atmosphere as revealed by recent kite and balloon exploration	[269]
299.	Map of Greenland, showing the area of inland ice and the routes of explorers	[271]
300.	Profile in natural proportions across the southern end of the continental glacier of Greenland	[272]
301.	Map of a glacier tongue with dimple above	[273]
302.	Edge of the Greenland inland ice, showing the nunataks diminishing in size toward the interior	[274]
303.	Moat surrounding a nunatak in Victoria Land	[274]
304.	A glacier pavement of Permo-Carboniferous age in South Africa	[276]
305.	Diagrams to illustrate the manner of formation of scape colks	[277]
306.	Marginal moraine now forming at the edge of the continental glacier of Greenland	[279]
307.	Small lake between the ice front and a moraine which it has recently built	[279]
308.	View of a drained lake bottom between the ice front and an abandoned moraine	[280]
309.	Diagrams to show the manner of formation and the structure of an outwash plain and fosse	[280]
310.	Map of the ice masses of Victoria Land, Antarctica	[282]
311.	Sections across the inland ice and the shelf ice of Antarctica	[283]
312.	Diagram to show the nature of the fixed glacial anticyclone above continental glaciers	[284]
313.	Snow deltas about the margins of a glacier tongue in Greenland	[285]
314.	View of the sea ice of the Arctic region	[286]
315.	Map of the north polar regions, showing the area of drift ice and the tracks of the Jeannette and the Fram	[288]
316.	The shelf ice of Coats Land with surrounding pack ice	[290]
317.	Tidewater cliff on a glacier tongue from which icebergs are born	[290]
318.	A Greenlandic iceberg after a long journey in warm latitudes	[291]
319.	Diagram showing one way in which northern icebergs are born from the glacier tongue	[291]
320.	A northern iceberg surrounded by sea ice	[292]
321.	Tabular Antarctic iceberg separating from the shelf ice	[293]
322.	Map of the globe, showing the areas covered by continental glaciers during the “ice age”	[297]
323.	Glaciated granite bowlder weathered out of a moraine of Permo-Carboniferous age, South Australia	[298]
324.	Map to show the glaciated and nonglaciated regions of North America	[298]
325.	Map of the glaciated and nonglaciated areas of northern Europe	[299]
326.	An unstable erosion remnant characteristic of the “driftless area”	[300]
327.	Diagram showing the manner in which a continental glacier obliterates existing valleys	[301]
328.	Lake and marsh district in northern Wisconsin	[302]
329.	Cross section in natural proportion of the latest North American continental glacier	[303]
330.	Diagram showing the earlier and the later glacier records together upon the same limestone surface	[304]
331.	Map to show the outcroppings of peculiar rock types in the region of the Great Lakes, and some localities where “drift copper” has been collected	[305]
332.	Map of the “bowlder train” from Iron Hill, Rhode Island	[306]
333.	Shapes and approximate natural sizes of some of the diamonds from the Great Lakes region	[307]
334.	Glacial map of a portion of the Great Lakes region	[308]
335.	Section in coarse till	[310]
336.	Sketch map of portions of Michigan, Ohio, and Indiana, showing the distribution of moraines	[312]
337.	Map of the vicinity of Devil’s Lake, Wisconsin, partly covered by the continental glacier	[313]
338.	Moraine with outwash apron in front	[313]
339.	Fosse between an outwash plain and a moraine	[314]
340.	View along an esker in southern Maine	[315]
341.	Outline map of moraines and eskers in Finland	[315]
342.	Sketch maps showing the relationships of drumlins and eskers	[316]
343.	View of a drumlin, showing an opening in the till	[317]
344.	Outline map of the front of the Green Bay lobe to show the relationships of drumlins, moraines, outwash plains, and ground moraine	[317]
345.	Character profiles referable to continental glacier	[318]
346.	View of the flood plain of the ancient Illinois River near Peoria	[320]
347.	Broadly terraced valleys which mark the floods that once issued from the continental glacier of North America	[321]
348.	Border drainage about the retreating ice front south of Lake Erie	[321]
349.	The “parallel roads” of Glen Roy in the Scottish Highlands	[322]
350.	Map of Glen Roy and neighboring valleys of the Scottish Highlands	[322]
351.	Three successive diagrams to set forth the late glacial lake history of the Scottish glens	[324]
352.	Harvesting time on the fertile floor of the glacial Lake Agassiz	[325]
353.	Map of Lake Agassiz	[325]
354.	Map showing some of the beaches of Lake Agassiz and its outlet	[326]
355.	Narrows of the Warren River where it passed between jaws of granite and gneiss	[327]
356.	Map of the valley of the Warren River near Minneapolis	[327]
357.	Portion of the Herman beach on the shore of the former Lake Agassiz	[328]
358.	Map of the continental glacier of North America when it covered the entire St. Lawrence basin	[329]
359.	Outline map of the early Lake Maumee	[330]
360.	Map to show the first stages of the ice-dammed lakes within the St. Lawrence basin	[330]
361.	Outline map of the later Lake Maumee and its outlet	[332]
362.	Outline map of lakes Whittlesey and Saginaw	[333]
363.	Map of the glacial Lake Warren	[333]
364.	Map of the glacial Lake Algonquin	[334]
365.	Outline map of the Nipissing Great Lakes	[335]
366.	Probable preglacial drainage of the upper Ohio region	[337]
367.	Diagrams to illustrate the episodes in the recent history of a Connecticut river	[338]
368.	The notched rock headland of Boyer Bluff on Lake Michigan	[341]
369.	View of Mackinac Island from the direction of St. Ignace	[342]
370.	The “Sugar Loaf”, a stack of Lake Algonquin upon Mackinac Island	[342]
371.	Beach ridges in series on Mackinac Island	[343]
372.	Notched stack of the Nipissing Great Lakes at St. Ignace	[343]
373.	Series of diagrams to illustrate the evolution of ideas concerning the uplift of the lake region since the Ice Age	[344]
374.	Map of the Great Lakes region to show the isobases and hinge lines of uptilt	[345]
375.	Series of diagrams to indicate the nature of the recovery of the crust by uplift when unloaded of an ice mantle	[346]
376.	Portion of the Inner Sandusky Bay, for comparison of the shore line of 1820 with that of to-day	[350]
377.	Ideal cross section of the Niagara Gorge to show the marginal terrace	[353]
378.	View of the bed of the Niagara River above the cataract where water has been drained off	[353]
379.	View of the Falls of St. Anthony in 1851	[354]
380.	Ideal section to show the nature of the drilling process beneath the cataract	[355]
381.	Plan and section of the gorge, showing how the depth is proportional to the width	[355]
382.	Comparative views of the Canadian Falls in 1827 and 1895	[356]
383.	Map to show the recession of the Canadian Fall	[357]
384.	Comparison of the present with the future falls	[358]
385.	Bird’s-eye view of the captured Canadian Fall at Wintergreen Flats	[358]
386.	Map of the Whirlpool Basin	[360]
387.	Map of the cuestas which have played so important a part in fixing the boundaries of the lake basins	[361]
388.	Bird’s-eye view of the cuestas south of Lakes Ontario and Erie	[362]
389.	Sketch map of the greater portion of the Niagara Gorge to illustrate Niagara history	[363]
390.	Snowdrift hollowing its bed by nivation	[368]
391.	Amphitheater formed upon a drift site in northern Lapland	[369]
392.	The marginal crevasse on the highest margin of a glacier	[370]
393.	Niches and cirques in the Bighorn Mountains of Wyoming	[371]
394.	Subordinate cirques in the amphitheater on the west face of the Wannehorn	[371]
395.	“Biscuit cutting” effect of glacial sculpture in the Uinta Mountains of Wyoming	[372]
396.	Diagram to show the cause of the hyperbolic curve of cols	[372]
397.	A col in the Selkirks	[373]
398.	Diagrams to illustrate the formation of comb ridges, cols, and horns	[374]
399.	The U-shaped Kern Valley in the Sierra Nevadas of California	[375]
400.	Glaciated valley wall, showing the sharp line which separates the abraded from the undermined rock surface	[375]
401.	View of the Vale of Chamonix from the séracs of the Glacier des Bossons	[376]
402.	Map of an area near the continental divide in Colorado	[377]
403.	Gorge of the Albula River in the Engadine cut through a rock bar	[378]
404.	Idealistic sketch, showing glaciated and nonglaciated side valleys	[378]
405.	Character profiles sculptured by mountain glaciers	[379]
406.	Flat dome shaped under the margin of a Norwegian ice cap	[379]
407.	Two views which illustrate successive stages in the shaping of tinds	[380]
408.	Schematic diagram to bring out the relationships of the various types of mountain glaciers	[383]
409.	Map of the Malaspina Glacier of Alaska	[384]
410.	Map of the Baltoro Glacier of the Himalayas	[385]
411.	View of the Triest Glacier, a hanging glacieret	[385]
412.	Map of the Harriman Fjord Glacier of Alaska	[386]
413.	Map of the Rotmoos Glacier, a radiating glacier of Switzerland	[386]
414.	Outline map of the Asulkan Glacier in the Selkirks, a horseshoe glacier	[387]
415.	Outline map of the Illecillewaet Glacier of the Selkirks, an inherited-basin glacier	[388]
416.	Diagram to illustrate the surface flow of glaciers	[390]
417.	Diagram to show the transformation of crevasses into séracs	[391]
418.	View of the Glacier des Bossons, showing the position of accidents to Alpinists	[392]
419.	Lines of flow upon the surface of the Hintereisferner Glacier in the Alps	[393]
420.	Lateral and medial moraines of the Mer de Glace and its tributaries	[393]
421.	Ideal cross section of a mountain glacier	[394]
422.	Diagrams to illustrate the melting effects upon glacier ice of rock fragments of different sizes	[394]
423.	Small glacier table upon the Great Aletsch Glacier	[395]
424.	Effects of differential melting and subsequent refreezing upon a glacier surface	[396]
425.	Dirt cone with its casing in part removed	[396]
426.	Schematic diagram to show the manner of formation of glacier cornices	[397]
427.	Superglacial stream upon the Great Aletsch Glacier	[398]
428.	Ideal form of the surface left on the site of a piedmont glacier apron	[399]
429.	Map of the site of the earlier piedmont glacier of the Upper Rhine	[399]
430.	Diagram and map to bring out the characteristics of newland lakes	[402]
431.	View of the Warner Lakes, Oregon	[402]
432.	Schematic diagram to illustrate the characteristics of basin-range lakes	[403]
433.	Schematic diagram of rift-valley lakes and the valley of the Jordan	[403]
434.	Map of the rift-valley lakes of East Central Africa	[404]
435.	Earthquake lakes formed in 1811 in the flood plain of the Lower Mississippi	[404]
436.	View of a crater lake in Costa Rica	[405]
437.	Diagrams to illustrate the characteristics of crater lakes	[406]
438.	View of Snag Lake, a coulée lake in California	[406]
439.	Diagrams to illustrate the characteristics of morainal lakes	[407]
440.	Diagram to show the manner of formation of pit lakes	[408]
441.	Diagrams to illustrate the characteristics of pit lakes	[408]
442.	Diagram to show the manner of formation of glint lakes	[409]
443.	Map of a series of glint lakes on the boundary of Sweden and Norway	[409]
444.	Map of ice-dam lakes near the Norwegian boundary of Sweden	[410]
445.	Wave-cut terrace of a former ice-dam lake in Sweden	[410]
446.	View of the Márjelen Lake from the summit of the Eggishorn	[411]
447.	Diagrams to illustrate the arrangement and the characters of rock-basin lakes	[412]
448.	Convict Lake, a valley-moraine lake of California	[413]
449.	Lake basins produced by successive slides from the steep walls of a glaciated mountain valley	[414]
450.	Lake Garda, a border lake upon the site of a piedmont apron	[414]
451.	Diagrams to bring out the characteristics of ox-bow lakes	[415]
452.	Diagrammatic section to illustrate the formation of saucer-like basins between the levees of streams on a flood plain	[415]
453.	Saucer lakes upon the bed of the former river Warren	[416]
454.	Levee lakes developed in series within meanders in a delta plain	[417]
455.	Raft lakes along the banks of the Red River in Arkansas and Louisiana	[418]
456.	Map of the Swiss lakes Thun and Brienz	[419]
457.	Delta lakes formed at the mouth of the Mississippi	[419]
458.	Delta lakes at the margin of the Nile delta	[420]
459.	Diagrams to illustrate the characteristics of barrier lakes	[420]
460.	Dune lakes on the coast of France	[421]
461.	Sink lakes in Florida, with a schematic diagram to illustrate the manner of their formation	[421]
462.	Map of the Arve and the Upper Rhone	[426]
463.	View of the Arve and the Rhone at their junction	[427]
464.	A village in Switzerland built upon a strath at the head of Lake Poschiavo	[428]
465.	View of the floating bog and surrounding zones of vegetation in a small glacial lake	[429]
466.	Diagram to show how small lakes are transformed into peat bogs	[430]
467.	Map to show the anomalous position of the delta in Lake St. Clair	[431]
468.	A bowlder wall upon the shore of a small lake	[432]
469.	Diagrams to show the effect of ice shove in producing ice ramparts upon the shores of lakes	[433]
470.	Various forms of ice ramparts	[433]
471.	Map of Lake Mendota, showing the position of the ridge which forms from ice expansion and the ice ramparts upon the shores	[434]
472.	The great multiple mountain arc of Sewestan, British India	[436]
473.	Diagrams to illustrate the theories of origin of mountain arcs	[437]
474.	Festoons of mountain arcs about the borders of the Pacific Ocean	[438]
475.	The interrupted Armorican Mountains common to western Europe and eastern North America	[438]
476.	A zone of diverse displacement in the western United States	[439]
477.	Section of an East African block mountain	[439]
478.	Tilted crust blocks in the Queantoweap valley	[440]
479.	View of the laccolite of the Carriso Mountain	[441]
480.	Map of laccolitic mountains	[441]
481.	Ideal sections of laccolite and bysmalite	[442]
482.	The gabled façade largely developed in desert landscapes	[443]
483.	Balloon view of the Mythen in Switzerland	[444]
484.	The battlement type of erosion mountain	[445]
485.	Symmetrically formed low islands repeated in ranks upon Temagami Lake, Ontario	[445]
486.	Forms of crystals of a number of minerals	[454]
487.	Forms of crystals of a number of minerals	[457]
488.	A student’s contour map	[469]
489.	Models to represent outcrops of rock	[472]
490.	Special laboratory table set with a problem in geological mapping which is solved in [Figs. 47] and [48]	[472]
491.	Three field maps to be used as suggestions in arranging laboratory table for problems in the preparation of areal geological maps	[473]
492.	Sketch map of Western Scotland and the Inner Hebrides to show location of some points of special geological interest	[481]
493.	Outline map of a geological pilgrimage across the continent of Europe	[483]

EXPLANATORY LIST OF ABBREVIATIONS FOR JOURNAL NAMES IN READING REFERENCES

Am. Geol.: American Geologist.

Am. Jour. Sci.: American Journal of Science, New Haven.

Ann. de Géogr.: Annales de Géographie, Paris.

Ann. Rept. Geol. and Geogr. Surv. Ter.: Annual Report of the Geological and Geographical Survey of the Territories (Hayden), Washington.

Ann. Rept. Geol. and Nat. Hist. Surv. Minn.: Annual Report of the Geological and Natural History Survey of Minnesota, Minneapolis.

Ann. Rept. Mich. Geol. Surv.: Annual Report of the Michigan Geological Survey, Lansing.

Ann. Rept. U. S. Geol. Surv.: Annual Report of the United States Geological Survey, Washington.

Bull. Am. Geogr. Soc.: Bulletin of the American Geographical Society, New York.

Bull. Earthq. Inv. Com. Japan: Bulletin of the Earthquake Investigation Committee of Japan, Tokyo.

Bull. Geogr. Soc. Philadelphia: Bulletin of the Geographical Society of Philadelphia.

Bull. Geol. Soc. Am.: Bulletin of the Geological Society of America.

Bull. Mus. Comp. Zoöl.: Bulletin of the Museum of Comparative Zoölogy, Harvard College, Cambridge.

Bull. N. Y. State Mus.: Bulletin of the New York State Museum, Albany.

Bull. Soc. Belge d’Astronomie: Bulletin de la Société Belge d’Astronomie, Brussels.

Bull. Soc. Belge Géol.: Bulletin de la Société Belge de Géologie, Brussels.

Bull. Soc. Sc. Nat. Neuchâtel: Bulletin de la Société des Sciences Naturelles de Neuchâtel.

Bull. Univ. Calif. Dept. Geol.: Bulletin of the University of California, Department of Geology, Berkeley.

Bull. U. S. Geol. Surv.: Bulletin of the United States Geological Survey, Washington.

Bull. Wis. Geol. and Nat. Hist. Surv.: Bulletin of the Wisconsin Geological and Natural History Survey, Madison.

C. R. Cong. Géol. Intern.: Comptes Rendus de la Congrès Géologique Internationale.

Dept. of Mines, Geol. Surv. Branch, Canada: Department of Mines, Geological Survey Branch, Canada.

Geogr. Abh.: Geographische Abhandlungen.

Geogr. Jour.: Geographical Journal, London.

Geol. Folio U. S. Geol. Surv.: Geological Folio of the United States Geological Survey.

Geol. Mag.: Geological Magazine, London (sections designated by decades).

Jour. Am. Geogr. Soc.: Journal of the American Geographical Society, New York.

Jour. Coll. Sci. Imp. Univ. Tokyo: Journal of the College of Science of the Imperial University, Tokyo, Japan.

Jour. Geol.: Journal of Geology, Chicago.

Jour. Sch. Geogr.: Journal of School Geography.

Livret Guide Cong. Géol. Intern.: Livret Guide Congrès Géologique Internationale.

Mem. Geol. Surv. India: Memoirs of the Geological Survey of India, Calcutta.

Mitt. Geogr. Ges. Hamb.: Mitteilungen der Geographische Gesellschaft, Hamburg.

Mon. U. S. Geol. Surv.: Monograph of the United States Geological Survey, Washington.

Nat. Geogr. Mag.: National Geographic Magazine, Washington.

Nat. Geogr. Mon.: National Geographic Monographs, American Book Company, New York.

Naturw. Wochenschr.: Naturwissenschaftliche Wochenschrift.

Pet. Mitt.: Petermanns Mittheilungen aus Justus Perthes’ Geographischer Anstalt, Gotha.

Pet. Mitt., Ergänzungsh. or Erg.: Petermanns Mittheilungen, Gotha (Ergänzungsheft or Supplementary Paper).

Phil. Jour. Sci.: Philippine Journal of Science, Manila.

Phil. Trans.: Philosophical Transactions of the Royal Society, London.

Proc. Am. Acad. Arts and Sci.: Proceedings of the American Academy of Arts and Sciences.

Proc. Am. Assoc. Adv. Sci.: Proceedings of the American Association for the Advancement of Science.

Proc. Am. Phil. Soc.: Proceedings of the American Philosophical Society, Philadelphia.

Proc. Bost. Soc. Nat. Hist.: Proceedings of the Boston Society of Natural History, Boston.

Proc. Ind. Acad. Sci.: Proceedings of the Indiana Academy of Science.

Proc. Linn. Soc. New South Wales: Proceedings of the Linnean Society of New South Wales.

Proc. Ohio State Acad. Sci.: Proceedings of the Ohio State Academy of Science.

Prof. Pap. U. S. Geol. Surv.: Professional Paper of the United States Geological Survey, Washington.

Pub. Carneg. Inst.: Publication of the Carnegie Institution of Washington.

Pub. Mich. Geol. and Biol. Surv.: Publication of the Michigan Geological and Biological Survey, Lansing.

Quart. Jour. Geol. Soc. Lond.: Quarterly Journal of the Geological Society, London.

Rept. Brit. Assoc. Adv. Sci.: Report of the British Association for the Advancement of Science.

Rept. Geol. Surv. Mich.: Report of the Geological Survey of Michigan, Lansing.

Rept. Mich. Acad. Sci.: Report of the Michigan Academy of Science, Lansing.

Rept. Nat. Conserv. Com.: Report of the National Conservation Commission, Washington.

Rept. Smithson. Inst.: Report of the Smithsonian Institution, Washington.

Sci. Bull. Brooklyn Inst. Arts and Sci.: Science Bulletin of the Brooklyn Institute of Arts and Sciences.

Scot. Geogr. Mag.: Scottish Geographic Magazine, Edinburgh.

Smith. Cont. to Knowl.: Smithsonian Contributions to Knowledge, Washington.

Tech. Quart.: Technology Quarterly of the Massachusetts Institute of Technology, Boston.

Trans. Am. Inst. Min. Eng.: Transactions of the American Institute of Mining Engineers, New York.

Trans. Roy. Dublin Soc.: Transactions of the Royal Dublin Society.

Trans. Seis. Soc. Japan: Transactions of the Seismological Society of Japan, Tokyo.

Trans. Wis. Acad. Sci.: Transactions of the Wisconsin Academy of Sciences, Arts, and Letters, Madison.

U. S. Geogr. and Geol. Surv. Rocky Mt. Region: United States Geographical and Geological Survey of the Rocky Mountain Region (Powell), Washington.

Zeit. d. Gesell. f. Erdk. z. Berlin: Zeitschrift der Gesellschaft für Erdkunde zu Berlin.

Zeit. f. Gletscherk: Zeitschrift für Gletscherkunde, Berlin.

EARTH FEATURES AND THEIR MEANING

CHAPTER I

THE COMPILATION OF EARTH HISTORY

The sources of the history.—The science which deals with the chapters of earth history that antedate the earliest human writings is geology. The pages of the record are the layers of rock which make up the outer shell of our world. Here as in old manuscripts pages are sometimes found to be missing, and on others the writing is largely effaced so as to be indistinct or even illegible. An intelligent interpretation of this record requires a knowledge of the materials and the structure of the earth, as well as a proper conception of the agencies which have caused change and so developed the history. These agencies in operation are physical and chemical processes, and so the sciences of physics and chemistry are fundamental in any extended study of geology. Not only is geology, so to speak, founded upon chemistry and physics, but its field overlaps that of many other important sciences. The earliest earth history has to do with the form, size, and physical condition of a minor planet in the solar system. The earliest portion of the story belongs therefore to astronomy, and no sharp line can be drawn to separate this chapter from those later ones which are more clearly within the domain of geology.

Subdivisions of geology.—The terms “cosmic geology” and “astronomic geology” have sometimes been used to cover the astronomy of the earth planet. The later earth history develops, among other things, the varied forms of animal and vegetable life which have had a definite order of appearance. Their study is to a large extent zoölogy and botany, though here considered from an essentially different viewpoint. This subdivision of our science is called paleontological geology or paleontology, which in common usage includes the plant as well as the animal world, or what is sometimes called paleobotany. In order to fix the order of events in geological history, these biological studies are necessary, for the pages of the record have many of them been misplaced as a result of the vicissitudes of earth history, and the remains of life in the rock layers supply a pagination from which it is possible to correctly rearrange the misplaced pages. As compiled into a consecutive history of the earth since life appeared upon it, we have the division of historical geology; though this differs but little from stratigraphical geology, the emphasis in the case of the former being placed on the history itself and in the latter upon the arrangement of events—the pagination of the record.

So far as they are known to us, the materials of which the earth is composed are minerals grouped into various characteristic aggregates known as rocks. Here the science is founded upon mineralogy as well as chemistry, and a study of the rock materials of the earth is designated petrographical geology or petrography. The various rocks which enter into the composition of the earth’s outer shell—the only portion known to us from direct observation—are built into it in an architecture which, when carefully studied, discloses important events in the earth’s history. The division of the science which is concerned with earth architecture is geotectonic or structural geology.

The study of earth features and their significance.—The features upon the surface of the earth have all their deep significance, and if properly understood, a flood of light is thrown, not only upon present conditions, but upon many chapters of the earth’s earlier history. Here the relation of our study to topography and geography is very close, so that the lines of separation are but ill defined. The terms “physiographical geology”, “physiography”, and “geomorphology” are concerned with the configuration of the earth’s surface—its physiognomy—and with the genesis of its individual surface features. It is this genetical side of physiography which separates it from topography and lends it an absorbing interest, though it causes it to largely overlap the division of dynamical geology or the study of geological processes. In fact, the difference between dynamical geology and physiography is largely one of emphasis, the stress being laid upon the processes in the former and upon the resultant features in the latter.

Under dynamical geology are included important subdivisions, such as seismic geology, or the study of earthquakes, and vulcanology, or the study of volcanoes. Another large subject, glacial geology, belongs within the broad frontier common to both dynamical geology and physiography. A relatively new subdivision of geological science is orientational geology, which is concerned with the trend of earth features, and is closely related both to physiography and to dynamical and structural geology.

Tabular recapitulation.—In a slightly different arrangement from the above order of mention, the subdivisions of geology are as follows:—

Subdivisions of Geology

Petrographical Geology.		Materials of the earth.
Geotectonic Geology.		Architecture of the earth’s outer shell.
Dynamical Geology.		Earth processes.
Seismic Geology—earthquakes.Vulcanology—volcanoes. GlacialGeology—glaciers, etc.
Physiographical Geology.		Earth physiognomy and its genesis.
Orientational Geology.		The arrangement and the trend of earth features.

In one way or another all of the above subdivisions of geology are in some way concerned in the genesis of earth physiognomy, and they must therefore be given consideration in a work which is devoted to a study of the meaning of earth features. The compiled record of the rocks is, however, something quite apart and without pertinence to the present work. As already indicated its subdivisions are:—

Astronomic Geology.	Planetary history of the earth.
Statigraphic Geology.	The pagination of earth records.
Historical Geology.	The compiled record and its interpretation.
Paleontological Geology.	The evolution of life upon the earth.

In every attempt at systematic arrangement difficulties are encountered, usually because no one consideration can be used throughout as the basis of classification. Such terms as “economic geology” and “mining geology” have either a pedagogical or a commercial significance, and so would hardly fit into the system which we have outlined.

Geological processes not universal.—It is inevitable that the geology of regions which are easily accessible for study should have absorbed the larger measure of attention; but it should not be forgotten that geology is concerned with the history of the entire world, and that perspective will be lost and erroneous conclusions drawn if local conditions are kept too often before the eyes. To illustrate by a single instance, the best studied regions of the globe are those in which fairly abundant precipitation in the form of rain has fitted the land for easy conditions of life, and has thus permitted the development of a high civilization. In degree, and to some extent also in kind, geologic processes are markedly different within those widely extended regions which, because either arid or cold, have been but ill fitted for human habitation. Yet in the historical development of the earth, those geologic processes which obtain in desert or polar regions are none the less important because less often and less carefully observed.

Change, and not stability, the order of nature.—Man is ever prone to emphasize the importance of apparent facts to the disadvantage of those less clearly revealed though equally potent. The ancient notion of the terra firma, the safe and solid ground, arose because of its contrast with the far more mobile bodies of water; but this illusion is quickly dispelled with the sudden quaking of the ground. Experience has clearly shown that, both upon and beneath the earth’s surface, chemical and physical changes are going on, subject to but little interruption. “The hills rock-ribbed and ancient as the sun” is a poetical metaphor; for the Himalayas, the loftiest mountains upon the globe, were, to speak in geological terms, raised from the sea but yesterday. Even to-day they are pushing up their heads, only to be relentlessly planed down through the action of the atmosphere, of ice, and of running water. Even more than has generally been supposed, the earth suffers change. Often within the space of a few seconds, to the accompaniment of a heavy earthquake, many square miles of territory are bodily uplifted, while neighboring areas may be relatively depressed. Thus change, and not stability, is the order of nature.

Observational geology versus speculative philosophy.—There appears to be a more or less prevalent notion that the views which are held by scientists in one generation are abandoned by those of the next; and this is apt to lead to the belief that little is really known and that much is largely guessed. Some ground there undoubtedly is for such skepticism, though much of it may be accounted for by a general failure among scientists, as well as others, to clearly differentiate that which is essentially speculative from what is based broadly upon observed facts. Even with extended observation, the possibility of explaining the facts in more than one way is not excluded; but the line is nevertheless a broad one which separates this entire field of observation from what is essentially speculative philosophy. To illustrate: the mechanics of the action which goes on within volcanic craters is now fairly well understood as a result of many and extended observations, and it is little likely that future generations of geologists will discredit the main conclusions which have been reached. The cause of the rise of the lava to the earth’s surface is, on the other hand, much less clearly demonstrated, and the views which are held express rather the differing opinions than any clear deductions from observation. Again, and similarly, the physical history of the great continental glaciers of the so-called “ice age” is far more thoroughly known than that of any existing glacier of the same type; but the cause of the climatic changes which brought on the glaciation is still largely a matter for speculation.

In the present work, the attempt will be, so far as possible, to give an exposition of geologic processes and the earth features which result from them, with hints only at those ultimate causes which lie hidden in the background.

The scientific attitude and temper.—The student of science should make it his aim, not only clearly to separate in his studies the proximate from the ultimate causes of observed phenomena, but he should keep his mind always open for reaching individual conclusions. No doctrines should be accepted finally upon faith merely, but subject rather to his own reasoning processes. This should not be interpreted to mean that concerning matters of which he knows little or nothing he should not pay respect to the recognized authorities; but his acceptance of any theory should be subject to review so soon as his own horizon has been sufficiently enlarged. False theories could hardly have endured so long in the past, had not too great respect been given to authorities, and individual reasoning processes been held too long in subjection.

The value of the hypothesis.—Because all the facts necessary for a full interpretation of observed phenomena are not at one’s hand, this should not be made to stand in the way of provisional explanations. If science is to advance, the use of hypothesis is absolutely essential; but the particular hypothesis adopted should be regarded as temporary and as indicating a line of observation or of experimentation which is to be followed in testing it. Thus regarded with an open mind, inadequate hypotheses are eventually found to be untenable, whereas correct explanations of the facts by the same process are confirmed. Most hypotheses of science are but partially correct, for we now “see through a glass darkly”; but even so, if properly tested, the false elements in the hypothesis are one after the other eliminated as the embodied truth is confirmed and enlarged. Thus “working hypothesis” passes into theory and becomes an integral part of science.

Reading References for Chapter I

The most comprehensive of general geological texts written in English is Chamberlin and Salisbury’s “Geology” in three volumes (Henry Holt, 1904-1906), the first volume of which is devoted exclusively to geological processes and their results. An abridged one-volume edition of the work intended for use as a college text was issued in 1906 (College Geology, Henry Holt). Other standard texts are:—

Sir Archibald Geikie. Text-book of Geology, 4th ed. 2 vols. London, 1902, pp. 1472.

W. B. Scott. An Introduction to Geology. 2d ed. Macmillan, 1907, pp. 816.

J. D. Dana. Manual of Geology. New edition. American Book Company, 1895, pp. 1087.

Joseph LeConte. Elements of Geology. (Revised by Fairchild.) Appleton, 1905, pp. 667.

A very valuable guide to the recent literature of dynamical and structural geology is Branner’s “Syllabus of a Course of Lectures on Elementary Geology” (Stanford University, 1908).

On the relation of geology to landscape, a number of interesting books have been written:—

James Geikie. Earth Sculpture or the Origin of Land-Forms. New York and London, 1896, pp. 397.

John E. Marr. The Scientific Study of Scenery. Methuen, London, 1900, pp. 368.

Sir A. Geikie. The Scenery of Scotland. 3d ed. Macmillan, London, 1901, pp. 540.

Sir John Lubbock. The Scenery of Switzerland and the Causes to which it is Due. Macmillan, London, 1896, pp. 480.

Lord Avebury. The Scenery of England. Macmillan, London, 1902, pp. 534.

Sir A. Geikie. Landscape in History, and Other Essays. Macmillan, London, 1905, pp. 352.

N. S. Shaler. Aspects of the Earth. Scribners, New York, 1889, pp. 344.

G. de La Noe et Emm. de Margerie. Les Formes du Terrain, Service Géographique de l’Armée. Paris, 1888, pp. 205, pls. 48.

W. M. Davis. Practical Exercises in Physical Geography, with Accompanying Atlas. Ginn and Co., Boston, 1908, pp. 148, pls. 45.

John Muir. The Mountains of California. Unwin, London, 1894, pp. 381.

Upon the use and interpretation of topographic maps in illustration of characteristic earth features, the following are recommended:—

R. D. Salisbury and W. W. Atwood. The Interpretation of Topographic Maps, Prof. Pap., 60 U.S. Geol. Surv., pp. 84, pls. 170.

D. W. Johnson and F. E. Matthes. The Relation of Geology to Topography, in Breed and Hosmer’s Principles and Practice of Surveying, vol. 2. Wiley, New York, 1908.

Général Berthaut. Topologie, Étude du Terrain, Service Géographique de l’Armée. Paris, 1909, 2 vols., pp. 330 and 674, pls. 265.

The United States Geological Survey issues free of charge a list of 100 topographic atlas sheets which illustrate the more important physiographic types. In his “Traité de Géographie Physique”, Professor E. de Martonne has given at the end of each chapter the important foreign maps which illustrate the physiographic types there described.

“The Principles of Geology”, by Sir Charles Lyell, published first in three volumes, appeared in the years 1830-1833, and may be said to mark the beginning of modern geology. Later reduced to two volumes, an eleventh edition of the work was issued in 1872 (Appleton) and may be profitably read and studied to-day by all students of geology. Those familiar with the German language will derive both pleasure and profit from a perusal of Neumayr’s “Erdgeschichte” (2d ed. revised by Uhlig. Leipzig and Vienna, 2 vols., 1895-1897), and especially the first volume, “Allgemeine Geologie.” A recent French work to be recommended is Haug’s “Traité de Géologie” (Paris, 1907).

Some texts of physical geography may well be consulted, especially Emm. de Martonne’s “Traité de Géographie Physique.” Colin, Paris, 1909, pp. 910, pls. 48, and figs. 396.

Note. An explanatory list of abbreviations used in the reading references follows the List of Illustrations.

CHAPTER II

THE FIGURE OF THE EARTH

The lithosphere and its envelopes.—The stony part of the earth is known as the lithosphere, of which only a thin surface shell is known to us from direct observation. The relatively unknown central portion, or “core”, is sometimes referred to as the centrosphere. Inclosing the lithosphere is a water envelope, the hydrosphere, which comprises the oceans and inland bodies of water, and has a mass ¹/₄₅₄₀ that of the lithosphere. If uniformly distributed, the hydrosphere would cover the lithosphere to the depth of about two miles, instead of being collected in basins as it now is. Though apparently not continuous, if we take into account the zone of underground water upon the continents, the hydrosphere may properly be considered as a continuous film about the lithosphere. It is a fact of much significance that all the ocean basins are connected, so that the levels are adjusted to furnish a common record of deposits over the entire surface that is sea-covered.

Enveloping the hydrosphere is the gaseous envelope, the atmosphere, with a mass ¹/_1200000 that of the lithosphere. The atmosphere is a mixture of the gases oxygen and nitrogen in parts by volume of one of the former to four of the latter, with a relatively small percentage of carbon dioxide. Locally, and at special seasons, the atmosphere may be charged with relatively large percentages of water vapor; and we shall see that both the carbon dioxide and the vapor contents are of the utmost importance in geological processes and in the influence upon climate. Unlike the water which composes the hydrosphere, the gases of the atmosphere are compressible. Forced down by the weight of superincumbent gas, the layers of the atmosphere at the level of the sea sustain a pressure of about fifteen pounds to the square inch; but this pressure steadily decreases in ascending to higher levels. From direct instrumental observation, the air has now been investigated to a height of more than twelve miles from the earth’s surface.

The evolution of ideas concerning the earth’s figure.—The ideas which in all ages have been promulgated concerning the figure of the earth have been many and varied. Though among them are not wanting the purely speculative and fantastic, it will be interesting to pass in review such theories as have grown directly out of observation.

The ancient Hebrews and the Babylonians were dwellers of the desert, and in the mountains which bounded their horizon they saw the confines of the earth. Pushing at last westward beyond the mountains, they found the Mediterranean, and thus arrived at the view that the earth was a disk with a rim of mountains which was floated upon water. The rare but violent rainfalls to which they were accustomed—the desert cloudburst—further led them to the belief that the mountain rim was continued upward in a dome or firmament of transparent crystal upon which the heavenly bodies were hung and from which out of “windows of heaven” the water “which is above the earth” was poured out upon the earth’s surface. Fantastic as this theory may seem to-day, it was founded upon observation, and it well illustrates the dangers of reasoning from observation within too limited a field.

As soon as men began to sail the sea, it was noticed that the water surface is convex, for the masts of ships were found to remain visible long after their hulls had disappeared below the horizon. It is difficult to say how soon the idea of the earth’s rotundity was acquired, but it is certainly of great antiquity. The Dominican monk Vincentius of Beauvais, in a work completed in 1244, declared that the surfaces of the earth and the sea were both spherical. The poet Dante made it clear that these surfaces were one, and in his famous address upon “The Water and the Land”, which was delivered in Verona on the 20th of January, 1320, he added a statement that the continents rise higher than the ocean. His explanation of this was that the continents are pulled up by the attraction of the fixed stars after the manner of attraction of magnets, thus giving an early hint of the force of gravitation.

The earth’s rotundity may be said to have been first proven when Magellan’s ships in 1521 had accomplished the circumnavigation of the globe. Circumnavigation, soon after again carried out by Sir Francis Drake, proved that the earth is a closed body bounded by curving surfaces in part enveloped by the oceans and everywhere by the atmosphere. The great discovery of Copernicus in 1530 that the earth, like Venus, Mars, and the other planets, revolves about the sun as a part of a system, left little room for doubt that the figure of the earth was essentially that of a sphere.

The oblateness of the earth.—Every schoolboy is to-day familiar with the fact that the earth departs from a perfect spherical figure by being flattened at the ends of its axis of rotation. The polar diameter is usually given as ¹/₂₉₉ shorter than the equatorial one. This oblateness of the spheroid was proven by geodesists when they came to compare the lengths of measured degrees of arc upon meridians in high and in low latitudes.

Fig. 1.—Diagrams to afford a correct impression of the measure of the inequalities upon the earth’s surface compared to the earth’s radius. The shell represented in b is ¹/₁₀₀ of the earth’s radius, and in a this zone is magnified for comparison with surface inequalities.

The oblateness of the geoid is well understood from accepted hypotheses to be the result of the once more rapid rotation of the planet when its materials were more plastic, and hence more responsive to deformation. An elastic hoop rotating rapidly about an axis in its plane appears to the eye as a solid, and becomes flattened at the ends of its axis in proportion as the velocity of rotation is increased. Like the earth, the other planets in the solar system are similarly oblate and by amounts dependent on the relative velocities of rotation.

The departure of the geoid from the spherical surface, owing to its oblateness, is so small that in the figures which we shall use for illustration it would be less than the thickness of a line. Since it is well recognized and not important in our present consideration, we shall for the time being speak of the figure of the earth in terms of departures from a standard spherical surface.

The arrangement of oceans and continents.—There are other departures from a spherical surface than the oblateness just referred to, and these departures, while not large, are believed to be full of significance. Lest the reader should gain a wrong impression of their magnitude, it may be well to introduce a diagram drawn to scale and representing prominent elevations and depressions of the earth ([Fig. 1]).

Wrong impressions concerning the figure of the lithosphere are sometimes gained because its depressions are obliterated by the oceans. The oceans are, indeed, useful to us in showing where the depressions are located, but the figure of the earth which we are considering is the naked surface of the rock. In a broad way, the earth’s shape will be given by the arrangement of the oceans and the continents. As soon as we take up the study of this arrangement, we find that quite significant facts of distribution are disclosed.

Fig. 2.—Map on Mercator’s projection to show the reciprocal relation of the land and sea areas (after Gregory and Arldt).

One of the most significant facts involved in the distribution of land and sea, is a concentration of the land areas within the northern and the seas within the southern hemisphere. The noteworthy exception is the occurrence of the great and high Antarctic continent centered near the earth’s south pole; and there are extensions of the northern continent as narrowing land masses to the southward of the equator. Hardly less significant than the existence of land and water hemispheres is the reciprocal or antipodal distribution of land and sea ([Fig. 2]). A third fact of significance is a dovetailing together of sea and land along an east-and-west direction. While the seas are generally A-shaped and narrow northward, the land masses are V-shaped and narrow southward, but this occurs mainly in the southern hemisphere. Lastly, there is some indication of a belt of sea dividing the land masses into northern and southern portions along the course of a great circle which makes a small angle with the earth’s equator. Thus the western continent is nearly divided by a mediterranean sea,—the Caribbean,—and the eastern is in part so divided by the separation of Europe from Africa.

Fig. 3.—The form toward which the figure of the earth is tending, a tetrahedron with symmetrically truncated angles.

The figure toward which the earth is tending.—Thus far in our discussion of the earth’s figure we have been guided entirely by the present distribution of land and water. There are, however, depressions upon the surface of the land, in some cases extending below the level of the sea, which are not to-day occupied by water. By far the most notable of these is the great Caspian Depression, which with its extension divides central and eastern Asia upon the east from Africa and Europe upon the west. This depression was quite recently occupied by the sea, and when added to the present ocean basins to indicate depressions of the lithosphere, it shows that the earth’s figure departs from the standard spheroid in the direction of the form represented in [Fig. 3]. This form approximates to a tetrahedron, a figure bounded by four equal triangular faces, here with symmetrically truncated angles. Of all regular figures with plane surfaces the tetrahedron has the smallest volume for a given surface, and it presents moreover a reciprocal relation of projection to depression. Every line passing through its center thus finds the surface nearer than the average distance upon one side and correspondingly farther upon the other ([Fig. 4]).

Astronomical versus geodetic observations.—Confirmation of the conclusions arrived at from the arrangement of oceans and continents has been secured in other fields. It was pointed out that the earth’s oblateness was proven by comparison of the measured degrees of latitude upon the earth’s surface in lower and higher latitudes, the degree being found longer as the pole is approached. Any variation from the spherical surface must obviously increase the size of the measured degree of latitude in proportion to the departure from the standard form, and so the tetrahedral figure with one of its angles at the south pole will require that the degrees of latitude be longer in the southern than they are in the northern hemisphere. This has been found by measurement to be the case, and the result is further confirmed by pendulum studies upon the distribution of the earth’s attraction or gravity. If less of the mass of the earth is concentrated in the southern hemisphere, its attraction as measured in vibrations of the pendulum should be correspondingly smaller.

Fig. 4.—A truncated tetrahedron, showing how the depression upon one side of the center is balanced by the opposite projection.

Other confirmations of the tetrahedral figure of the earth have been derived from a comparison of astronomical data, which assume the earth to be a perfect spheroid, with geodetic measurements, which are based upon direct measurements. Thus the arc measured in an east-and-west direction across Europe revealed a different curvature near the angle of the tetrahedral figure from what was found farther to the eastward.

Changes of figure during contraction of a spherical body.—If we inquire why the earth in cooling should tend to approach the tetrahedral figure, an answer is easily found. When formed, the earth appears to have been a but slightly oblate spheroid, or practically a sphere—the shape which of all incloses the most space for a given surface. Cooled and solidified at the surface to the temperature of the surrounding air, and the core still hot and continuing to lose heat, the core must continue to contract though the outer shell is no longer able to do so. The superficial area being thus maintained constant while the volume continues to diminish, the figure must change from the initial one of greatest bulk to others of smaller volume, and ultimately, if the process should continue indefinitely, to the tetrahedron, which of all regular figures has the minimum volume for a given surface.

That a contracting sphere does indeed pass through such a series of changes has been shown by the behavior of contracting soap bubbles and of rubber balloons, as well as by experiments upon the exhaustion of air contained in hollow metal spheres of only moderate strength. In all these instances, the ultimate form produced indicates an indenting of four sides of the sphere which have the positions of the faces of a tetrahedron. The late Professor Prinz of Brussels secured some extremely interesting results in which he obtained intermediate forms with six angles, but unfortunately these studies were not prepared for publication at the time of his death.

The earth’s departure from the spheroid in the direction of the modified tetrahedron is, as we have seen, no hypothesis, but observed fact revealed in (1) the concentration of the land about a central ocean in the northern hemisphere; in (2) the antipodal relation of the land to the water areas, and in (3) the threefold subdivision of the surface into north and south belts by the two greater oceans and the Caspian Depression.

The earlier figures of the earth.—The manner in which continent and ocean are dovetailed into each other in an east-and-west direction has been generally adduced as additional evidence for the tetrahedral figure as above described. Closer examination shows that instead of being in harmony with this figure, it indicates a departure from it, and, as we shall see, a significant departure which undoubtedly has its origin in the earlier history of the planet. The mediterranean seas of both the eastern and the western hemispheres likewise interfere with the perfection of the tetrahedral figure and require an explanation.

Let us then examine in outline the past history of the world with reference especially to the evolution of the continents and to the times and the manners of surface change. It is now well known that there have been three major periods of great deformation of the earth’s shell. The first of these of which we have record came at the end of the first great era of geologic history, the so-called Eozoic era; a second great transformation came at the close of the second or Paleozoic era; and a third began at the end of the next or Mesozoic era, an adjustment which is apparently continuing to-day. Each of these great surface deformations was accompanied by great volcanic eruptions of which we have the evidence in the lavas remaining for our inspection, and each was followed by the formation of great glaciers which spread over large areas of the existing continents.

Before the earliest of these great changes, the earth appears to have approximated in its figure somewhat closely to the ideal spheroid, for it was everywhere enveloped in the hydrosphere as a universal ocean. Toward the close of this period came the adjustments which brought the lithosphere to protrude through the hydrosphere in shield-like continents whose distribution, as shown by the rocks of this period, is of great significance. Within the northern hemisphere rose three land shields spaced at nearly equal intervals and at nearly equal distances from the northern pole. One of these was centered where now is Hudson Bay, another about the present Baltic Sea, and the relics of the third are found in northeastern Siberia. These earliest continents have been referred to as the Laurentian, Baltic, and Angara shields. Within the southern hemisphere shields appear to have developed in somewhat similar grouping, namely, in South America, in South Africa, and in Australia ([Figs. 3] and [5]).

Fig. 5.—Approximations to earlier and present figures of the earth.

These coigns or angles of a form into which the earlier spheroid of the earth was being transformed have persisted through the greater part of subsequent geologic time, and have been enlarged by the growth of sediments about them as well as by the later elevation and wrinkling of these deposits into marginal mountain ranges.

The continents and oceans which arose at the close of the Paleozoic era.—At the close of the second great era in the recorded history of the earth, the now somewhat enlarged continents were profoundly altered during a series of convulsive movements within the surface shell of the lithosphere. When these convulsions were over, there was a new disposition of land and sea, but one quite different from the present arrangement. Instead of being extended in north-south belts, as they are at present, the continents stretched out in broad east-west zones, one in the northern and the other in the southern hemisphere. To the broad southern continent of which so little now remains, the name “Gondwana Land” has been given, and to the sea which divided the northern from the southern continent the name “Ocean of Tethys.” The northern continent stretched across the site of the present Atlantic Ocean as the “North Atlantis”, its northern shore to the westward being somewhat farther south than the present northern coast of North America, since life forms migrated in the northern ocean from the site of Behring Sea to that of the present North Atlantic.

This arrangement of land and water during the middle period of the earth’s recorded history, when considered with reference both to its earlier and to its later evolution, may perhaps be best accounted for by the assumption that the lithosphere was then shaped like [Fig. 5] (middle). In this figure two truncated tetrahedrons are joined in a common plane of contact which may be described as the twin plane. This medial depression upon the lithosphere was occupied by the intercontinental sea, the Ocean of Tethys.

Near the close of this second great era of the earth’s continental history, crustal convulsions, which were perhaps the most remarkable in the entire record, resulted in the almost complete disappearance of the southern continent and a concentration of the land within the northern hemisphere as a somewhat interrupted belt surrounding a central polar ocean ([Figs. 3] and [5]).

Upon the assumption of twin tetrahedrons in the intermediate era of continental evolution, both the Ocean of Tethys of that time and its present remnants, the Caribbean and Mediterranean seas, are accounted for. The V-shaped continent extensions and the A-shaped oceans of the southern hemisphere ([Fig. 2]) may likewise be considered as relics of the now largely submerged tetrahedron of the southern hemisphere, since this had its apex to the northward ([Fig. 6]).

Fig. 6.—Diagrams for comparison of shore lines upon tetrahedrons which have an angle, the first at the south and the second at the north.

Thus we see that the lithosphere can scarcely be regarded as a perfect spheroid, since in the course of geologic ages it has undergone successive departures from this original form. In its present state it has been described as tetrahedral, though we must keep in mind that the sharp angles of that figure are deeply truncated. The soundings first by Nansen and more recently by Peary in the Arctic basin, far to the north of the continental border, showed that this depression is characterized by profound depths, and so have afforded confirmation of the tetrahedral figure. To match this depression at the northern extremity of the earth’s axis, a high continent reaching to elevations in excess of 10,000 feet has been penetrated by Sir Ernest Shackleton at the opposite extremity of this polar diameter. Considering its size and its elevation, the Antarctic continent with its glacier mantle is the largest protuberance upon the surface of the lithosphere.

In our study of the departures of the earth from the standard spheroidal surface, we might even go a step farther and show how the tetrahedron, which best represents the symmetry of the present figure, is somewhat deformed by a flattening perpendicular to the Pacific Ocean. To draw attention to this flattening of the earth, it has sometimes been described as “potato-shaped”, since the outline perpendicular to this face is imperfectly heart-shaped or like a flattened “peg top.”

Fig. 7.—The continents with submerged portions added (after Gilbert).

The flooded portions of the present continents.—We are accustomed to think of the continents as ending at the shores of the oceans. If, however, we are to regard them as platforms which rise from the ocean depressions, their margins should be considerably extended, for a submerged shelf now practically surrounds all the continents to a nearly uniform depth of 100 fathoms or 600 feet. The oceans thus more than fill their basins and may be thought of as spilling over upon the continents. In [Fig. 7], the submerged portions of the continents have been joined to those usually represented, thus adding about 10,000,000 square miles to their area, and giving them one third, instead of one fourth, of the lithosphere surface.

Fig. 8.—Diagram to indicate the altitude of different parts of the lithosphere surface.

The floors of the hydrosphere and atmosphere.—The highest altitudes upon the continents and the profoundest deeps of the ocean are each removed about 30,000 feet, or nearly 6 miles, from the level of the sea. In comparison with the entire surface of the lithosphere, these extremes of elevation represent such small areas as to be almost inappreciable. Only about ¹/₈₀ of the lithosphere surface rises more than 6000 feet above sea level, and about the same proportion lies deeper than 18,000 feet below the same datum plane ([Fig. 8]). Almost the entire area of the lithosphere is included either in the so-called continental plateau or platform, in the oceanic platform, or in the slope which separates the two. The continental platform includes the continental shelf above referred to, and represents about one third of the entire area of the planet. This platform has a range of elevation from 6000 feet above to 600 feet below sea level and has an average altitude of about 2300 feet. The oceanic platform slopes more steeply, ranges in depth from 12,000 to 18,000 feet below sea level, and comprises about one half the lithosphere surface. The remaining portion of the surface, something less than one eighth of all, is included in the steep slopes between the two platforms, between 600 and 12,000 feet below sea. The two platforms and the slope between them must not, however, be thought of as continuous features upon the surface, but merely as representing the average elevations of portions of the lithosphere.

Reading References for Chapter II

On the evolution of ideas concerning the earth’s figure:—

Suess. The Face of the Earth (Clarendon Press, 1906), vol. 2, Chapter 1.

v. Zittel. History of Geology and Paleontology (Walter Scott, London, 1901), Chapters 1-2.

The departure of the spheroid toward the tetrahedron:—

W. Lowthian Green. Vestiges of the Molten Globe, Part 1. London, 1875. (Now a rare work, but it contains the original statement of the idea.)

J. W. Gregory. The Plan of the Earth and Its Causes, Geogr. Jour., vol. 13, 1899, pp. 225-251 (the best general statement of the arguments for a tetrahedral form).

W. Prinz. L’échelle reduite des expériences géologiques, Bull. Soc. Belge d’Astronomie, 1899.

B. K. Emerson. The Tetrahedral Earth and Zone of the Intercontinental Seas, Bull. Geol. Soc. Am., vol. 11, 1911, pp. 61-106, pls. 9-14.

M. P. Rudski. Physik der Erde (Tauchnitz, Leipzig, 1911), Chapters 1-3 (the best discussion of the geoid from the purely mathematical standpoint, so far as the spheroid is concerned).

The earlier figures of the earth:—

Th. Arldt. Die Entwicklung der Kontinente und ihrer Lebewelt. Engelmann, Leipzig, 1907. (Contains a valuable series of map plates, showing the probable boundaries of the continents in the different geological periods).

CHAPTER III

THE NATURE OF THE MATERIALS IN THE LITHOSPHERE

The rigid quality of our planet.—For a long time it was supposed that the solid earth constituted a crust only which was floated upon a liquid interior. This notion was clearly an outgrowth of the then generally accepted Laplacian hypothesis of the origin of the universe, which assumed fluid interiors for the planets, the crust being suggested by the winter crust of frozen water upon the surface of our inland lakes. To-day the nebular hypothesis in the Laplacian form is fast giving place to quite different conceptions, in which solid particles, and not gaseous ones, are conceived to have built up the lithosphere. The analogy with frozen water has likewise been abandoned with the discovery that frozen rock, instead of floating, sinks in its molten equivalent.

Yet even more cogent arguments have been brought forward to show that whatever may be the state of aggregation within the earth’s core—and it may be different from any now known to us—it nevertheless has many of the properties recognized as belonging to solid and rigid bodies. Provisionally, therefore, we may regard the earth’s core as rigid and essentially solid. It was long ago pointed out by the late Lord Kelvin that if our lithosphere were not more rigid than a ball of glass of the same size, it would be constantly passing through periodic six-hourly distortions of great amplitude in response to the varying attractions of the moon. An equally striking argument emanating from the same high authority is furnished by the well-known egg-spinning demonstration. For illustration, Kelvin was accustomed to take two eggs, one boiled and the other raw, and attempt to spin them upon their ends. For the boiled, and essentially solid, egg this is easily accomplished, but internal friction of the liquid contents of the raw egg quickly stops any rotary motion which is imparted to it. Upon the same grounds it is argued that had the earth’s interior possessed the properties of a liquid, rotation must long since have ceased.

A stronger proof of earth rigidity than either of these has been lately furnished by the instrumental study of earthquakes. With the delicate apparatus which is now installed for the purpose, heavy earthquakes may be sensed which have occurred anywhere upon the earth’s surface, the earth movement sending its own message by the shortest route through the core of the earth to the observing station. A heavy shock which occurs in New Zealand is recorded in England, almost diametrically opposite, in about twenty-one minutes after its occurrence. The laws of wave propagation and their relation to the properties of the transmitting medium are well known, and in order to explain such extraordinary velocity it is necessary to assume that for such impulses the earth’s interior is much more rigid than the finest tool steel.

Probable composition of the earth’s core.—In deriving views concerning the nature of the earth’s interior we are greatly aided by astronomical studies. The common origin long ago indicated for the planets of the solar system and the sun has been confirmed by the analysis of light with the aid of the spectroscope. It has thus been found that the same chemical elements which we find in the earth are present also in the sun and in the other stellar bodies. Again, the group of planets of the solar system which are nearest to the sun—Mercury, Venus, the Earth, and Mars—have each a high density, all except Mars, the most distant, having specific gravities very closely 5½, that of Mars being about 4. This average specific gravity is also that of the solid bodies, the so-called meteorites, which reach the surface of our planet from the surrounding space. Yet though the earth as a whole is thus found to have a specific gravity five and a half times that of water, its surface shell has an average density of less than half this value, or 2.7.

The study of meteorites has given us a possible clew to the nature of the earth’s interior; for when both terrestrial and celestial rock types are classified and placed in orderly arrangement, it is found that the chemical elements which compose the two groups are identical, and that these are united according to the same physical and chemical laws. No new element has been discovered in the one group that has not been found in the other, and though some compounds of these elements, the minerals, occur in the earth’s crust that have not been found in meteorites, and though some occur in meteorites which are not known from the earth, yet of those which are common to both bodies there is agreement, even to the minor details ([Fig. 9]). It is found, however, that the commonest of the minerals in the earth’s shell are absent from meteorites, as the commoner constituents of meteorites are wanting in the earth’s crust. This observation would go far to show that we may in the two cases be examining different portions of quite similar bodies; and this view is strikingly confirmed when the rocks of the two groups are arranged in the order of their densities ([Fig. 9]).

Fig. 9.—Diagram to show how terrestrial rocks grade into those of the meteorites. 1, oxygen; 2, silicon; 3, aluminium; 4, alkali metals; 5, alkaline earth metals; 6, iron, nickel, cobalt, etc.; a, granites and rhyolites; b, syenites and trachytes; c, diorites and andesites; d, gabbros and basalts; e, ultra-basic rocks; f, basic inclosures in basalt, etc.; g, iron basalts of west Greenland; h, iron masses of Ovifak, west Greenland; a’-d’, meteorites in order of density (after Judd).

In a broad way, density, structure, and chemical composition are all similarly involved in the gradations illustrated by the diagram; and it is significant that while there are terrestrial rocks not represented by meteorites, the densest and the most unusual of the terrestrial rocks are chemically almost identical with the less dense of the celestial bodies.

The earth a magnet.—The denser, and likewise the more common, of the meteorite rocks—the so-called meteoric irons—are composed almost entirely of the elements iron, nickel, and cobalt. Such aggregates are not known as yet from terrestrial sources, although transitional types appear to exist upon the island of Disco off the west coast of Greenland. If it were possible to explore the earth’s interior, would such combinations of the iron minerals be encountered? Apart from the surprising velocity of transmission of earthquake waves, the strongest argument for an iron core to the lithosphere is found in the magnetic property of the earth as a whole. The only magnetic elements known to us are those of the heavy meteorites—iron, nickel, and cobalt,—and the earth is, as we know, a great magnet whose northern pole in British America and whose southern pole in Antarctica have at last been visited by Amundsen and David, respectively. The specific gravity of iron is 7.15, and those of nickel and cobalt, which in the meteorites are present in relatively small amounts, are 7.8 and 7.5, respectively. Considering that the surface shell of the earth has a specific gravity of 2.7, these values must be regarded as agreeing well with the determined density of the earth (5.6) and the other planets of its group (Mercury 5.7, Venus 5.4, Mars 4).

The chemical constitution of the earth’s surface shell.—The number of the so-called chemical elements which enter into the earth’s composition is more than eighty, but few of these figure as important constituents of the portion known to us. Nearly one half of the mass of this shell is oxygen, and more than a quarter is silicon. The remaining quarter is largely made up of aluminium, iron, calcium, magnesium, and the alkalies sodium and potassium, in the order named. These eight constituent elements are thus the only ones which play any important rôle in the composition of the earth’s surface shell. They are not found there in the free condition, but combined in the definite proportions characteristic of chemical compounds, and as such they are known as minerals.

The essential nature of crystals.—A crystal we are accustomed to think of as something transparent bounded by sharp edges and angles, our ideas having been obtained largely from the gem minerals. This outward symmetry of form is, however, but an expression of a power which resides, so to speak, in the heart or soul of the crystal individual—it has its own structural make-up, its individuality. No more correct estimates of the comparison of crystal individualities would be obtained by the study of outward forms alone of two minerals than would be gained by a judgment of persons from the cut of their clothing. Too often this outward dress tells only of the conditions by which both men and crystals have been surrounded, and but little of the power inherent in the individual. Many a battered mineral fragment with little beauty to recommend it, when placed under suitable conditions for its development, has grown into a marvel of beauty. Few minerals are so mean that they have not within them this inherent power of individuality which lifts them above the world of the amorphous and shapeless.

Fig. 10.—Comparison of a crystalline with an amorphous substance when expanded by heat and when attacked by acid.

Just as the real nature of a person is first disclosed by his behavior under trying circumstances, so of a crystal it is its conduct under stress of one sort or another which brings out its real character. By way of illustration let us prepare a sphere from the mineral quartz—it matters not whether we destroy the beautiful outlines of the crystal or employ a battered fragment—and then prepare a sphere of similar size and shape from a noncrystalline or amorphous substance like glass. If now these two spheres be introduced into a bath of oil and raised to a higher temperature, the glass globe undergoes an enlargement without change of its form; but the crystal ball reveals its individuality by expanding into a spheroid in which each new dimension is nicely adjusted to this more complex figure ([Fig. 10]).

We may, instead of submitting the two balls to the “trial by fire”, allow each to be attacked by the powerful reagent, hydrofluoric acid. The common glass under the attack of the acid remains as it was before, a sphere, but with shrunken dimensions. The crystal, on the other hand, is able to control the action of the solvent, and in so doing its individuality is again revealed in a beautifully etched figure having many curving outlines—it is as though the crystal had possessed a soul which under this trial has been revealed. This glimpse into the nature of the crystal, so as to reveal its structural beauty, is still more surprising when the crystal is taken from the acid in the early stages of the action and held close beneath the eye. Now the little etchings upon the surface display each the individuality of the substance, and joining with their neighbors they send out a beautifully symmetrical and entirely characteristic picture ([Fig. 11]).

Fig. 11.—“Light figure” seen upon an etched surface of a crystal of calcite (after Goldschmidt and Wright).

The lithosphere a complex of interlocking crystals.—To the layman the crystal is something rare and expensive, to be obtained from a jeweler or to be seen displayed in the show cases of the great museums. Yet the one most striking quality of the lithosphere which separates it from the hydrosphere and the atmosphere is its crystalline structure,—a structure belonging also to the meteorite, and with little doubt to all the planets of the earth group. A snowflake caught during its fall from the sky reveals all the delicate tracery of crystal boundary; collected from a thick layer lying upon the ground, it appears as an intricate aggregate of broken fragments more or less firmly cemented together. And so it is of the lithosphere, for the myriads of individuals are either the ruins of former crystals, or they are grown together in such a manner that crystal facets had no opportunity to develop.

Such mineral individuals as once possessed the crystal form may have been broken and their surfaces ground away by mutual attrition under the rhythmic beating of the waves upon a shore or in the continuous rolling of pebbles on a stream bed, until as battered relics they are piled away together in a bed of sand. Yet no amount of such rough handling is sufficient to destroy the crystal individuality, and if they are now surrounded with conditions which are suitable for their growth, their individual nature again becomes revealed in new crystal outlines. Many of our sandstones when turned in the bright sunlight send out flashes of light to rival a bank of snow in early spring. These bright flashes proceed from the facets of minute crystals formed about each rounded grain of the sand, and if we examine them under a lens, we may note the beauty of line formed with such exactness that the most delicate instruments can detect no difference between the similar angles of neighboring crystals ([Fig. 12]).

Fig. 12.—Battered sand grains which have taken on a new lease of life and have developed a crystal form. a, a single grain grown into an individual crystal; b, a parallel growth about a single grain; c, growth of neighboring grains until they have mutually interfered and so destroyed the crystal facets—the common condition within the mass of a rock (after Irving and Van Hise).

This individual nature of the crystal is believed to reside in a symmetrical grouping of the chemical molecules of the substance into larger and so-called “crystal molecules.” The crystal quality belongs to the chemical elements and to their compounds in the solid condition, but not to ordinary mixtures of them.

Some properties of natural crystals, minerals.—No two mineral species appear in crystals of the same appearance, any more than two animal species have been given the same form; and so minerals may be recognized by the individual peculiarities of their crystals. Yet for the reason that crystals have so generally been prevented from developing or retaining their characteristic faces, in the vast number of instances it is the behavior, and not the appearance, of the mineral substance which is made use of for identification.

When a mineral is broken under the blow of a hammer, instead of yielding an irregular fracture, like that of glass, it generally tends to part along one or more directions so as to leave plane surfaces. This property of cleavage is strikingly illustrated for a single direction in the mineral mica, for two directions in feldspar, and for three directions in calcite or Iceland spar. Other properties of minerals, such as hardness, specific gravity, luster, color, fusibility, etc., are all made use of in rough determinations of the minerals. Far more delicate methods depend upon the behavior of minerals when observed in polarized light, and such behavior is the basis of those branches of geological science known as optical mineralogy and as microscopical petrography. An outline description of some of the common minerals and the means for identifying them will be found in appendix A.

The alterations of minerals.—By far the larger number of minerals have been formed in the cooling and consequent consolidation of molten rock material such as during a volcanic eruption reaches the earth’s surface as lava. Beginning their growth at many points within the viscous mass, the individual crystals eventually may grow together and so prevent a development of their crystal faces.

Another class of minerals are deposited from solution in water within the cavities and fissures of the rocks; and if this process ceases before the cavities have been completely closed, the minerals are found projecting from the walls in a beautiful lining of crystal—the Krystallkeller or “crystal cellar.” It is from such pockets or veins within the rocks that the valuable ores are obtained, as are the crystals which are displayed in our mineral cabinets.

Fig. 13.—Crystal of garnet developed in a schist with grains of quartz included because not assimilated.

There is, however, a third process by which minerals are formed, and minerals of this class are produced within the solid rock as a product of the alteration of preëxisting minerals. Under the enormous pressures of the rocks deep below the earth’s surface, they are as permeable to the percolating waters as is a sponge at the surface. Under these conditions certain minerals are dissolved and their material redeposited after traveling in the solution, or solution and redeposition of mineral matter may go on together within the mass of the same rock. One new mineral may have been produced from the dissolved materials of a number of earlier species, or several new minerals may be the result of the alteration of a preëxisting mineral with a more complex chemical structure. Where the new mineral has been formed “in place”, it has sometimes been able to utilize the materials of all the minerals which before existed there, or it may have been obliged to inclose within itself those earlier constituents which it could not assimilate in its own structure ([Fig. 13]).

Fig. 14.—A crystal of augite within the mass of a rock altered in part to form a rim of the minerals hornblende and magnetite. Note the original outline of the augite crystal.

At other times a crystal which is imbedded in rock has been attacked upon its surface by the percolating solutions, and the dissolved materials have been deposited in place as a crown of new minerals which steadily widens its zone until the center is reached and the original crystal has been entirely transformed ([Fig. 14]). It is sometimes possible to say that the action by which these changes have been brought about has involved a nice adjustment of supply of the chemical constituents necessary to the formation of the new mineral or minerals. In rocks which are aggregates of several mineral species, a newly formed mineral may appear only at the common margin of certain of these species, thus showing that they supply those chemical elements which were necessary to the formation of the new substance ([Fig. 15]). Thus it is seen that below the earth’s surface chemical reactions are constantly going on, and the earlier rocks are thus locally being transformed into others of a different mineral constitution.

Fig. 15.—A new mineral (hornblende) forming as an intermediate “reaction rim” between the mineral having irregular fractures (olivine) and the dusty white mineral (lime-soda feldspar).

Near the earth’s surface the carbon dioxide and the moisture which are present in the atmosphere are constantly changing the exposed portions of the lithosphere into carbonates, hydrates, and oxides. These compounds are more soluble than are the minerals out of which they were formed, and they are also more bulky and so tend to crack off from the parent mass on which they were formed. As we are to see, for both of these reasons the surface rocks of the lithosphere succumb to this attack from the atmosphere.

In connection with those wrinklings of the surface shell of the lithosphere from which mountains result, the underlying rocks are subjected to great strains, and even where no visible partings are produced, the rocks are deformed so that individual minerals may be bent into crescent-shaped or S-shaped forms, or they are parted into one or more fragments which remain imbedded within the rock.

Reading References for Chapter III

Theories of origin of the earth:—

Thomson and Tait. Natural Philosophy. 2d ed. Cambridge, 1883, pp. 422.

T. C. Chamberlin. Chamberlin and Salisbury’s Geology, vol. 2, pp. 1-81.

Rigidity of the earth:—

Lord Kelvin. The Internal Condition of the Earth as to Temperature, Fluidity, and Rigidity, Popular Lectures and Addresses, vol. 2, pp. 299-318; Review of evidence regarding the physical condition of the earth, ibid., pp. 238-272.

Hobbs. Earthquakes (Appleton, New York, 1907), Chapters xvi and xvii.

Composition of the earth’s core and shell:—

O. C. Farrington. The Preterrestrial History of Meteorites, Jour. Geol., vol. 9, 1901, pp. 623-236.

E. S. Dana. Minerals and How to Study Them (a book for beginners in mineralogy). Wiley, New York, 1895.

On the nature of crystals:—

Victor Goldschmidt. Ueber das Wesen der Krystalle, Ostwalds Annalen der Naturphilosophie, vol. 9, 1909-1910, pp. 120-139, 368-419.

EARTH FEATURES ANDTHEIR MEANING