ALBERT R. MANN LIBRARY
AT
CORNELL UNIVERSITY
Cornell University Library
GB 54.H7
i and their meaning; an intr
Cornell University Library
The original of tiiis book is in tine Cornell University Library.
There are no known copyright restrictions in the United States on the use of the text.
http://www.archive.org/details/cu31924013977560
EAETH FEATUEES 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
Mount Balfour and the Balfour Glacier in the Selkirks.
EARTH FEATURES
AND
THEIR MEANING
AN INTRODUCTION TO GEOLOGY
FOE THE STUDENT AND THE GENEEAL EEADEE
BY
WILLIAM HERBERT HOBBS
PKOFESSOK OF GEOLOGY IN THE UNIVERSITY OP MICHIGAN
AUTHOR OF "EARTHQUAKES, AN INTRODUCTION TO
SEISMIC GEOLOGY"; "CHARACTERISTICS OF
EXISTING GLACIERS " ; ETC.
Weto g0rfe
THE MACMILLAN COMPANY
1912
AU rights reserved
COPTEIOHT, 1912,
By the MACMILLAN COMPANY.
Set up and electrotyped. Published March, iqi2.
1(^S^^^/
liTotbraoti ^xas
J. S. Cushing Co. —Berwick & Smith Co.
Norwood, MaSB., U.S.A.
CM-*
TO THE MEMORY
OF
GEOEGE HUNTINGTON- WILLIAMS
PREFACE
The series of readings contained in the present volume give in somewhat expanded form the substance pf 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 land- scape 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 Cha,nnel coast, the gorge of the Ehine, 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 char- acter 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
vm PREFACE
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 his- torical division apart from the others. Again, a desire to treat the science of geology comprehensively has led some authors into in- cluding 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 illus- trate the readings by travels afield, which accounts for the promi- nence 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 glacia- tions.
Emphasis has also been placed upon the dependence of the domi- nant geological processes of any region upon existing climatic con- ditions, 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.
PREFACE ix
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 pro- fessional geologist to detect these new strands which have been
drawn into the web.
WILLIAM HERBERT HOBBS. Ann Abbob, Michigan, October 25, 1911.
CONTENTS
CHAPTER I The Compilation op Eabth Histoet
The sources of the history — Subdivisions of geology — The study of earth features and their significance — Tabular recapitulation — Geological processes not universal — Change, and not stability, the order of nature — Observational geology sersus speculative philosophy — The scientific attitude and temper — The value of the hypothesis — Beading refer-
CHAPTEK II
The Eiguke of the Earth
The lithosphere and its envelopes — The evolution of ideas concerning the earth's figure — The oblateness of the earth — The arrangement of oceans and continents — The figure toward vrhich the earth is tending
— Astronomical versus geodetic observations — Changes of figure dur- ing contraction of a spherical body — The earlier figures of the earth
— The continents and oceans at the close of the Paleozoic era — The flooded portions of the present continents — The floors of the hydro- sphere and 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
— The earth a magnet — The chemical constitution of the earth's sur- '] face shell — The essential nature of crystals — The lithosphere a com- plex of interlocking crystals — Some properties of natural crystals, minerals — The alterations of minerals — Reading references . . 20
CHAPTER IV /
The Bocks of the Earth's Surface Shell '
The processes by which rocks are formed — The marks of origin — The metamorphic rocks — Characteristic textures of the igneous rocks — The classification of rooks — Subdivisions of the sedimentary rocks
PAGE
30
Xii CONTENTS
— The different deposits of ocean, lake, and river — Special marks of littoral deposits — The order of deposition during a transgression of the sea — The hasins of deposition of earlier ages — The deposits of the deep sea — Reading references .
CHAPTER V
CONTOKTIONS OF THE StKATA WITHIN THE ZONE OP FlOW
The zones of fracture and flow — Experiments which illustrate the frac- ture and flow of solid bodies — The arches and troughs of the folded strata — The elements of folds — The shapes of rock folds — The over- thrust fold — Restoration of mutilated folds — The geological map and section — Measurement of the thickness of formations — The detection of plunging folds — The meaning of an unconformity — Reading refer- ences 40
CHAPTER VI
The Architectdee of the Fkactured SnPERSTRncxcRE
The system of the fractures — The space intervals of joints — The dis- placements upon joints: faults — Methods of detecting faults — The base of the geological map — The fleld map and the areal geological map — Laboratory models for study of geological maps — The method of preparing the map — Fold vs. fault topography — Reading references 55
CHAPTER VII
The Interrupted Character of Earth Movements : Earth- quakes AND Seaquakes
Nature of earthquake shocks — Seaquakes and seismic sea waves — The grander and the lesser earth movements — Changes in the earth's surface during earthquakes: faults and fissures — The measure of displacement — Contraction of the earth's surface during earthquakes
— The plan of an earthquake fault — The block movements of the disturbed district — The earth blocks adjusted during the Alaskan " earthquake of 1899 67
CHAPTER VIII
The Interrupted Character of Earth Movements : Earth- quakes A^fD Seaquakes {concluded)
Experimental demonstration of earth movements — Derangement of water flow by earth movement — Sand or mud cones and craterlets — The earth's zones of heavy earthquake — The special lines of heavy shock
— Seismotectonic lines — The heavy shocks above loose foundations
Construction in earthquake regions — Reading references ... 81
CONTENTS xiii
CHAPTER IX
The Rise of Molten Rock to the Earth's Sukpace ; Volcanic Mountains op Exudation
PAGE
Prevalent misconceptions about volcanoes — Early vievfs concerning vol- canic mountains — The birth of volcanoes — Active and extinct vol- canoes— The earth's volcano belts — Arrangement of volcanic vents along fissures, and especially at their intersections — The so-called fissure eruptions — The composition and the properties of lava — The three main types of volcanic mountain — The lava dome — The basaltic lava domes of Hawaii — Lava movements within the caldron of Kilauea — The draining of the lava caldrons — The outflow of the lava floods . 94
CHAPTER X
The Rise op Molten Rock to the Earth's Surface ; Volcanic Mountains of Ejected Materials
The mechanics of crater explosions — Grander volcanic eruptions of cinder cones — The eruption of Volcano in 1888 — The eruption of Taal volcano on January 30, 1911 — The materials and the structure of cin- der cones — The profile lines of cinder cones — The composite cone — The caldera of composite cones — The eruption of Vesuvius in 1906
— The sequence of events within the chimney — The spine of Pel6 — The aftermath of mud flows — The dissection of volcanoes — The formation of lava reservoirs — Character profiles — Reading references 115
CHAPTER XI
The Attack op the Weather
The two contrasted processes of weathering — The r61e of the percolating water — Mechanical results of decomposition: spheroidal weathering
— Exfoliation or scaling — Dome structure in granite masses — The prying work of frost — Talus — Soil flow in the continued presence of thaw water — The splitting wedges of roots and trees — The rock man- tle and its shield in the mat of vegetation — Reading references . . 149
CHAPTER XII
The Life Histories op Rivers
The intricate pattern of river etchings — The motive power of rivers — Old land and new land — The earlier aspects of rivers — The meshes of the river network — The upper and lower reaches of a river con- trasted— The balance between degradation and aggradation — The accordance of tributary valleys — The grading of the flood plain —
xiv CONTENTS
PAGE
The cycles of stream meanders — The cut-ofi of the meander — Meander scars — Elver terraces — The delta of the river— The levee — The sections of delta deposits ^^^
CHAPTER Xm
Earth Featokes shaped by Running Watek
The newly incised upland and its sharp salients — The stage of adolescence — The maturely dissected upland — The Hogarthian line of beauty — The final product of river sculpture : the peneplain — The river cross sections of successive stages — The entrenchment of meanders with renewed uplift — The valley of the rejuvenated river — The arrest of stream erosion by the more resistant rooks — The capture of one river by another — Water and wind gaps — Character profiles — Reading refer- ences
169
CHAPTER XIV
The Travels op the Underground Water
The descent within the unsaturated zone — The trunk channels of descend- ing water — The caverns of limestones — Swallow holes and limestone sinks — The sinter deposits — The growth of stalactites — Formation of stalagmites — The Karst and its features — A desert from the destruction of forests — The ponore and the polje — The return of the water to the surface — Artesian wells — Hot springs and geysers — The deposition 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 — Some characteristics of the desert waste — Dry weathering : the red and brown desert varnish — The mechanical breakdown of the desert rocks — The natural 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 the desert — The zone of the dwindling river — Erosion in and about the desert — Characteristic features of the arid lands — The war of dune and oasis — The origin of the high plains which front the Rocky Mountains — Character profiles — Reading references . . . . 209
CONTENTS XV
CHAPTER XVII Befeating Patterns in the Eabth Relief
PAGIi
The ■weathering processes under control of the fracture system — The fracture control of the drainage lines — The repeating pattern in drain- age networks — The dividing lines of the relief patterns : lineaments
— The composite repeating patterns of the higher orders — Reading references 223
CHAPTER XVIII
The Fokms cakved and molded by Waves
The motion of a water wave — Free waves and breakers — Effect of the breaking wave upon a steep, rocky shore : the notched cliff — Coves, sea arches, and stacks — The cut rock terrace — The cut and built terrace on a steep shore of loose materials — The work of the shore current — The sand beach — The shingle beach — Bar, spit, and bar- rier— The laud-tied island — A barrier series — Character profiles — Reading references 231
CHAPTER XIX
Coast Records op the Rise or Fall of the Land
The characters in which the record has been preserved — Even coast line the mark of uplift — A ragged coast line the mark of subsidence — Slow uplift of the coasts ; the coastal plain and ouesta — The sudden uplifts of the coast — The upraised cliff — The uplifted barrier beach — Coast terraces — The sunk or embayed coast — Submerged river channels — Records of an oscillation of movement — Simultaneous contrary move- ments upon a coast — The contrasted islands of San Clemente and Santa Catalina — The Blue Grotto of Capri — Character profiles — Reading references 245
CHAPTER XX
The Glaciers of Mountain and Continent
Conditions essential to glaciation — The snow-line — Importance of moun- tain barriers in initiating glaciers — Sensitiveness of glaciers to tem- perature changes — The cycle of glaciation — The advancing hemicycle
— Continental and mountain glaciers contrasted — The nourishment
of 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 — The marginal rook islands — Rock fragments which travel with the
XVI CONTENTS
ice — The grinding mill beneath the ice — The lifting of the grinding tools and their incorporation within the ice — Melting upon the glacier margins in Greenland — The marginal moraines — The outwash plain or apron — The continental glacier of Antarctica — Nourishment of continental glaciers — The glacier broom — Field and pack ice — The drift of the pack — The Antarctic shelf ice — Icebergs and snowbergs and the manner of their birth — Reading references .... 271
CHAPTER XXII
The Continental Glacieks op the " Ice Age "
Earlier cycles of glaoiation — Contrast of the glaciated and nonglaciated regions — The "driftless area" — Characteristics of the glaciated regions — The glacier gravings — Younger records over older : the glacier palimpsest — The dispersion of the drift — The diamonds of the drift — Tabulated comparison of the glaciated and nonglaciated regions — Unassorted and assorted drift — Features into which the drift is molded — Marginal or "kettle" moraines — Outwash plains — Pitted plains and interlobate moraines — Eskers — Drumlins — The shelf ice of the ice age — Character profiles 297
CHAPTER XXIII
Glacial Lakes which marked the Decline op the Last Ice Age
Interference of glaciers with drainage — Temporary lakes due to ice block- ing—The "parallel roads" of the Scottish glens— The glacial Lake Agassiz — Episodes of the glacial lake history within the St. Lawrence "Valley — The cresceutic lakes of the earlier stages — The early Lake Maumee — The later Lake Maumee — Lakes Arkona and Whittlesey
— Lake Warren — Lakes Iroquois and Algonquin — The Nipissing Great Lakes — Summary of lake stages — Permanent changes of drainage effected by the glacier— Glacial Lake Ojibway in the Hud- son's Bay drainage basin — Reading references 320
CHAPTER XXIV
The Uptilt op the Land at the Close op the Ice Age
The response of the earth's shell to its ice mantle — The abandoned strands as they appear to-day — The records of uplift about Mackinac Island
— The present inclinations of the uplifted strands — The hinge lines of uptilt — Future consequences of the continued uptilt within the lake region — Gilbert's prophecy of a future outlet of the Great Lakes to
the Mississippi — Geological evidences of continued uplift — Drowning *
of southwestern shores of Lakes Superior and Erie — Reading refer- ences •••••....,.. <J4o
CONTENTS xvii
CHAPTER XXV Niagara Falls a Clock op Eecent Geological Time
PAGE
Features in and about the Niagara gorge — The drilling of the gorge — . The present rate of recession — Future extinction of the American Fall.
— The captured Canadian Fall at Wintergreen Flats — The Whirlpool Basin excavated from the St. David's gorge — The shaping of the Lewiston Escarpment — Episodes of Niagara's history and their corre- lation with those of the glacial lakes — Time measures of the Niagara clock — The horologe of late glacial time in Scandinavia — Reading references 352
CHAPTER XXVI
Land Sculpture by Mountain Glaciers
Contrasted sculpturing of continental and mountain glaciers — Wind dis- tribution of the snow which falls in mountains — The niches which form on snowdrift sites — The augmented snowdrift moves down the valley: birth of the glacier — The excavation of the glacial amphi- theater or cirque — Life history of the cirque — Grooved and fretted uplands — The features carved above the glacier — The features shaped beneath the glacier — The cascade stairway in glacier-carved valleys — ■ The character profiles which result from sculpture by mountain glaciers
— The sculpture accomplished by ice caps — The Norwegian tind or beehive mountain — Reading references 367
CHAPTER XXVII Successive Glacier Types op a Waning Glaciation
Transition from the ice cap to the mountain glacier — The piedmont glacier — The expanded-foot glacier — The dendritic glacier — The radiating glacier — The horseshoe glacier — The inherited -basin glacier
— Summary of 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 s^racs — Bodies given up by the Glacier des Bossotis — The moraines — Selective melting upon the glacier surface — Glacier drainage — Deposits within the vacated valley — Marks of the earlier occupation of mountains by glaciers — Reading references . . . . ■ 390
xviii CONTENTS
CHAPTER XXIX A Study of Lake Basins
PAGE
Fresh water and saline lakes — Newland lakes — Basin-range lakes — Elft- ralley lakes — Earthquake lakes — Crater lakes — Coulee lakes — Morainal lakes — Pit lakes— Glint or colk lakes — Ice-dam lakes — Glacier-lobe lakes — Rock-basin lakes — Valley moraine lakes — Land- slide lakes — Border lakes — Ox-bow lakes — Saucer lakes — Crescentio levee lakes — Raft lakes — Side-delta lakes — Delta lakes — Barrier lakes — Dune lakes — Sink lakes — Karst lakes : poljen — Playa lakes — Salines — Alluvial-dam lakes — R&um6 — Reading references . 401
CHAPTER XXX
The Ephemeral Existence of Lakes
Lakes as settling basins — Drawing off of water by erosion of outlet — The pulling in of headlands and the cutting oS of bays — Lake extinction by peat growth — Extinction of lakes in desert regions — The role of lakes in the economy of nature — Ice ramparts on lake shores — Read- ing references 426
CHAPTER XXXI
The Origin and the Forms of Mountains
A mountain defined — The festoons of mountain arcs — Theories of origin of the mountain arcs — The Atlantic and Pacific coasts contrasted — The block type of mountain — Mountains of outflow or upheap — Domed mountains of uplift ; laccolites — Mountains carved from plateaus — The climatic conditions of the mountain sculpture — The eflect of the resistant stratum — The mark of the rift in the eroded mountains — 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
FACIHO PAGE
2. A. Layers compressed in experiments and showing the effect of a com-
petent 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 and
lateral displacements combined . .- . ... 72
B. Earthquake faults opened in Alaska in 1889 on which vertical
slices of the earth's shell have undergone individual adjustments 72
4. A. Experimental tank to illustrate the earth movements which are
manifested in earthquakes. The sections of the earth's shell are here 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 inter-
sected by master joints 82
6. A. Once wooded region in China now reduced to desert through de- forestation 156
B. " Bad Lands " in the Colorado Desert 156
6. A. Barren Karst landscape near the famous Adelsberg grottoes . . 188 B. Surface of a limestone ledge where joints have been widened through
solution 188
7. A. Ranges of dunes upon the margin of the Colorado Desert . . 210 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 South
Dakota 216
B. Castellated erosion chimneys in El Cobra Canon, 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 under
the control of joints 228
B. Composite pattern of the joint structures within recent alluvial
deposits of the Syrian Desert 228
11. A. Ripple markings within an ancient sandstone .... 232 B. Wave breaking as it approaches the shoye ..... 232
xix
XX LIST OF PLATES
FACING PAGE
PLATE
12. A. V-shaped caflon cut in an upland recently elevated from the sea,
San Clemente Island, California 256
B. A " hogback" at the base of the Bighorn Mountains, "Wyoming . 256
13. A. Precipitous front of the Bryant Glacier outlet of the Greenland
inland ice ^"^^
B. Lateral stream beside the Benedict Glacier outlet, Greenland . 272
14. View of the margin of the Antarctic continental glacier in Kaiser
WilhelmLand 282
15. A. An Antarctic ice foot with boat party landing . . . .290 B. A near view of the front of the Great Ross Barrier, Antarctica . 290
16. A. Incised topography -within the "driftless area" . . . .300 B. Built-up topography within the glaciated region . . . .300
17. A. Soled glacial bowlders which show differently directed striae upon
the same facet 306
B. Perched bowlder upon a striated ledge of different rock type, 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 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 B. Contour map of a fretted upland, Philipsbui-g Quadrangle, Mon- tana 372
20. Map of the surface modeled by mountain glaciers in the Sierra Nevadas
of California 376
21. A. View of the Harvard Glacier, Alaska, showing the characteristic
terraces 394:
B. The terminal moraine at the foot of a mountain glacier . . . 394
22. A. Model of the vicinity of Chicago, showing the position of the
outlet of the former Lake Chicago 400
B. Map of Yosemite Falls and its earlier site near Eagle Peak . . 400
23. A. View of the American Fall at Niagara, showing the accumulation
of blocks beneath 414
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 fqj comparison of coasts upon an upright and upon an in-
verted 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 trans-
gression 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 fiexures 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
xxi
xxii ILLUSTRATIONS IN THE TEXT
32. A plunging synoline
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 °2
35. Types of deceptive or erosional unconformities 53
36. A set of master joints in shale ^^
37. Diagram to show the manner of replacement of one set of joints by
♦ another ^^
38. Diagram to show the different combinations of joint series ... 56
39. View of the shore in West Greenland 67
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 Eica . . 68
61. Overturned bowlders from" Assam earthquake of 1897 .... 69
62. Post sunk into ground during Charleston earthquake .... 69 58. 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
66. 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
69. Fault with vertical and lateral displacements combined ... 72
60. Diagram to show how small faults may be masked at the earth's sur-
face 73
61. " Mole hiE " 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 .... 76
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 earth-
quake of 1872 .78
ILLUSTRATIONS IN THE TEXT xxiii
riG. PAGE
71. Marquetry of the rock floor in the Tonopah district, Nevada . . 79
72. Map of Alaskan coast to show adjustments of level during an earth-
quake 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 earth-
quake 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 Skaptd,r 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 shovring 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. LefQngwell 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
xxiv ILLUSTRATIONS IN THE TEXT
FIG.
104. Map to show the manner of outflow of the lava from Kilauea m 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 iEolian 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 mUes . 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 re-
spectively 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
180 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
ILLUSTRATIONS IN THE TEXT xxv
FIG. PAGE
134. The central cone of Vesuvius as it appeared after the eruption of 1906 132
135. A sunken road upon Vesuvius filled vrith 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 Boscotreoase .... 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 Boscotreoase 134
141. Three diagrams to illustrate the sequence of events during the cone-
building and crater-producing periods 185
142. The spine of Pel6 rising above the chimney of the volcano after the
eruption of 1902 136
143. Successive outlines of the Pel6 spine 137
144. Corrugated surface of the Vesuviau cone due to the mud flows which
followed the eruption of 1906 138
145. View of the Kammerbiihl 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 tufE in a partly dissected volcano of south-
western Colorado 140
148. Map and general view of St. Paul's rooks, 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 erup-
tion 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 arcting of a competent forma-
tion ... 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 rook 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 s^tions of upper portion of a river valley . . .161
xxvi ILLUSTRATIONS IN THE TEXT
ma.
PAOB
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-how 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 sec-
tions 168
175. Gorge of the Eiver Rhine near St. Gears 169
176. Valley with rounded shoulders characteristic of the stage of adoles-
cence • 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 descend-
ing 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 maimer of formation of stalactites and stalag-
mites ... 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. Pissure springs arranged at intersections of rock fractures . . . 190
201. Schematic diagrams to illustrate the difierent types of artesian wells . 191
202. Cross section of Geysir, Iceland 192
203. Apparatus for simulating geyser action I93
204. Cone of siliceous sinter about the Lone Star Geyser .... 194
ILLUSTRATIONS IN THE TEXT xxvii
TIG. PAGE
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 difBssion 204
212. " Mushroom Kock " 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 cafion in loess worn by trafSo 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 obstruc-
tion 210
220. Successive diagrams to illustrate the history of the town of Kunzeu
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
281. 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 Canon in Northern Lapland . 225
241. Map of the gorge of the Zambesi Eiver below Victoria Falls . . 225
XXviii ILLUSTRATIONS IN THE TEXT
riG. PAGE
242. Controlled drainage network of the Shepaug River in Connecticut . 226
243. A river netwrork 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 335
255. Ideal section cut by waves upon a steep rocky shore .... 236
256. Map showing the outlines of the island of Heligoland at diSerent
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
ILLUSTRATIONS IN THE TEXT xxix
ria. PAOE
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 pre-
serve 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 impor-
tant 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 JokuU, 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 tlie southern end of the conti-
nental 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
buUt 279
308. View of a drained lake bottom between the ice front and an aban-
doned moraine 280
309. Diagrams to show the manner of formation and the structure of an
outwash plain and fosse 280
XXX ILLUSTRATIONS IN THE TEXT
FIG. PAGE.
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. Tide-water 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 obliter-
ates 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 shovring 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 3q5
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 .... 308 336. Section in coarse till qiq
336. Sketch map of portions of Michigan, Ohio, and Indiana, showing the
distribution of moraines oio
337. Map of the vicinity of Devil's Lake, Wisconsin, partly covered by
the continental glacier 013
338. Moraine with outwash apron in front 013
339. Fosse between an outwash plain and a moraine .... 314
340. View along an esker in southern Maine 3^5
341. Outline map of moraines and eskers in Finland . . . . " 315
ILLUSTRATIONS IN THE TEXT xxxi
FIQ.
PAGE
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 relation-
ships 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 Eiver 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 Koy 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 Eiver 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 Con-
necticut river 338
368. The notched rook headland of Boyer Bluff on Lake Michigan . . 341
369. View of Mackinac Island from the direction of St. Ignaoe . . . 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. Ignaoe . . . 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
xxxii ILLUSTRATIONS IN THE TEXT
FIG.
PAGE
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 vrhere water
has been drained off °°^
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 . . . • ^^^
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 ouestas 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 Chamouix from the sfiraos 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 o^ Alaska . . . . . 386
ILLUSTRATIONS IN THE TEXT xxxiii
FIG. PAGE
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 lUecillewaet 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 sfiracs . . 391
418. View of the Qlader 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 Aletsoh Glacier .... 395
424. Effects of differential melting and subsequent re-freezing 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
482. 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 coulee 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 Mirjelen 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
xxxiv ILLUSTRATIONS IN THE TEXT
FIG.
PAGB
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. Diagramatic 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 th e 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 laocolite of the Carriso Mountain 441
480. Map of laccolitic mountains 441
481. Ideal sections of laccolite and bysmalite 442
482. The gabled fagade 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 464
487. Forms of crystals of a number of minerals 457
ILLUSTRATIONS IN THE TEXT xxxv
no. PAGE
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
wbich 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 Gfeologist.
Am. Jour. Sci. : American Journal of Science, New Haven.
Ann. de G6ogr. : Annales de G^ographie, Paris.
Ann. Rept. Geol. and Geogr. Surv. Ter. : Annual Keport of the Geological and Geographical Survey of the Territories (Hayden), Washington.
Ann. Kept. 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 Sur- vey, Lansing.
Ann. Rept. U. S. Geol. Surv. ; Annual Report of the United States Geological Survey, Washington.
Bull. Am. Geogr. Soe. : Bulletin of the American Geographical Society, New York.
Bull. Earthq. Inv. Com. Japan : Bulletin of the Earthquake Investigation Com- mittee of Japan, Tokyo.
Bull. Geogr. Soc. Philadelphia : Bulletin of the Geographical Society of Phila- delphia.
Bull. Geol. Soc. Am. : Bulletin of the Geological Society of America.
Bull. Mus. Comp. Zool. : Bulletin of the Museum of Comparative Zoology, Harvard College, Cambridge.
Bull. N. T. State Mus. -. Bulletin of the New York State Museum, Albany.
Bull. Soc. Beige d' Astronomic : Bulletin de la Soci6t6 Beige d'Astronomie, Brussels.
Bull. Soc. Beige G^ol. : Bulletin de la Socidt^ Beige de Geologic, Brussels.
BuU. Soc. So. Nat. Neuchatel : Bulletin de la Soci6t6 des Sciences Naturelles de Neuchatel.
Bull. Univ. Calif. Dept. Geol. : Bulletin of the University of California, Depart- ment 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. G^ol. Intern. : Comptes Rendus de la Congrfes G^ologique Inter- nationale.
Dept. of Mines, Geol. Surv. Branch, Canada : Department of Mines, Geological Survey Branch, Canada.
xxxvii
xxxviii EXPLANATORY LIST OF ABBREVIATIONS
Geogr. AVb. : 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. Sol. Imp. Univ. Tokyo : Journal of the College of Science of the Imperial University, Tokyo, Japan.
Jour. Geol. : Journal of Geology, Chicago.
Jour. Soh. Geogr. : Journal of School Geography.
Livret Guide Cong. G60I. Intern. ; Livret Guide Congrte G^ologique Inter- nationale.
Mem. Geol. Surv. India : Memoirs of the Geological Survey of India, Calcutta.
Mitt. Geogr. Ges. Hamb. : Mitteilungen der Geographische Gesellschaft, Ham- burg.
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 Com- pany, New York.
Naturw. Wochenschr. . Naturwissenschaftliche Wochenschrift.
Pet. Mitt. ; Petermanns Mittheilungen aus Justus Perthes' Geographischer Anstalt, Gotha.
Pet. Mitt., Erganzungsh. or Erg. : Petermanns Mittheilungen, Gotha (Ergan- zungsheft 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 -\cademy 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 Liunean 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 Geologi- cal 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.
EXPLANATORY LIST OP ABBREVIATIONS xxxix
Eept. Brit. Assoc. Adv. Sci. : Report of the British Association for the Advance- ment of Science.
Kept. Geol. Surv. Mich. ; Report of the Geological Survey of Michigan, Lansing.
Rept. Mich. Acad. Sci. : Report of the Michigan Academy of Science, Lansing.
Kept. Nat. Conserv. Com. : Report of the National Conservation Commission, Washington.
■Rept. Smithson. Inst. ; Report of the Smithsonian Institution, Wasliington.
Sci. Bull. Brooklyn Inst. Arts and Sci. ; Science Bulletin of the Brooklyn Insti- tute 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 Tech- nology, 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 Rooky Mountain Region (Powell), Wash- ington.
Zeit. d. Gesell. f . Erdk. z. Berlin : Zeitschrift der Gesellschraft fur Erdkunde zu Berlin.
Zeit. f. Gletscherk : Zeitschrift fur Gletscherkunde, Berlin.
EAKTH FEATUKES AND THEIE 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 writ- ings 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 opera- tion 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 chemis- try and physics, but its field overlaps that of many other im- portant 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 chap- ter 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 zoology and botany, though here considered from an essentially different viewpoint. This subdivision of our science is called paleontological geology or paleontology, which
B 1
2 EARTH FEATURES AND THEIR MEANING
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 com- piled 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 observa- tion — 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 sig- nificance, and if properly understood, a flood of Hght is thrown, not only upon present conditions, but upon many chapters of the earth's earlier history. Here the relation of our study to topog- raphy and geography is very close, so that the lines of separa- tion are but ill defined. The terms "physiographical geology," "physiography," and " geomorphology " are concerned with the configuraliion of the earth's surface — its physiognomy — and with the genesis of its individual surface features. It is this ge- netical 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
THE COMPILATION OF EARTH HISTORY 3
being laid upon the processes in the former and upon the result- ant features in the latter.
Under dynamical geology are included important subdivisions, .such as seismic geology, or the study of earthquakes, and vul- canology, 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 sub- division 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
JPetrographical Geology. Materials of the earth.
Geotectonic Geology. Architecture of the earth's outer
shell. Dynamical Geology. Earth processes.
Seismic Geology — earthquakes. Vulcanology^ volcanoes. Glacial Geology — 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.
jStatigraphic Geology. The pagination of earth records.
Historical Geology. The compiled record and its inter-
pretation. Paleonfological 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 " eco-
4 EARTH FEATURES AND THEIR MEANING
nomic 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 precipita- tion 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 dis- advantage 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 quak- ing 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.
THE COMPILATION OP EARTH HISTORY 5
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 faitoe 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 specu- lation.
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
6 EARTH FEATURES AND THEIR MEANING
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 in- dividual 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 eventu- ally 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" ia three volumes (Henry Holt, 190^1906), 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. Lon- don, 1902, pp. 1472.
W. B. Scott. An Introduction to Geology. 2d ed. MaemiUan, 1907, pp. 816.
J. D. Dana. Manual of Geology. New edition. American Book Com- pany, 1895, pp. 1087.
Joseph LbConte. Elements of Geology. (Revised by Fairchild.) Appleton, 1905, pp. 667.
A very valuable guide to the recent literature of dynamical and struc- tural geology is Branner's "Syllabus of a Course of Lectures on Elemen- tary 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.
THE COMPILATION OF EARTH HISTORY 7
John E. Mark. 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 Aveburt. 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. Soribners, New York, 1889, pp. 344.
G. DE La Noe et Emm. de Margehie. Les Formes du Terrain, Service GSographique de TArmee. Paris, 1888, pp. 205, pis. 48.
W. M. Davis. Practical Exercises in Physical Geography, with Accom- panying Atlas. Ginn and Co., Boston, 1908, pp. 148, pis. 45.
JohnMuir. 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. Salisbtjrt and W. W. Atwood. The Interpretation of Topo- graphic Maps, Prof. Pap., 60 U.S. Geol. Surv., pp. 84, pis. 170. D. W. Johnson and F. E. Matthes. The Relation of Geology to Topography, in Breed and Hosmer's Principles and Practice of Sur- veying, vol. 2. Wiley, New York, 1908. General Berthaut. Topologie, Etude du Terrain, Service Gfiogra- phique de I'Armee. Paris, 1909, 2 vols., pp. 330 and 674, pis. 265.
The United States Geological Survey issues free of charge a list of 100 topographic altas sheets which illustrate the more important physi- ographic types. In his " Traite de Gfiographie 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 modem 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, " AUgemeine Geologie." A recent French work to be recommended is Haug's "Traite de G6ologie" (Paris, 1907).
Some texts of physical geography may weU be consulted, especially Emm. de Martonne's " Traite de Geographie Physique.'.' Colin, Paris, 1909, pp. 910, pis. 48, and figs. 396.
Note. An explanatory list of abbreviations used in the reading refer- ences 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 un- known 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 ^j-gVir 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. Thotigh apparently not continuous, if we take into account the zone of underground water upon the continents, the hydro- sphere 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 atmos- phere, with a mass T^-^hnss tbat of the lithosphere. The atmos- phere 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
8
THE FIGURE OP THE EARTH 9
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 fioated 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 difiicult 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 circumnavi- gation of the globe. Circumnavigation, soon after again carried
10 EARTH FEATURES AND THEIR MEANING
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 fa- miliar 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 -^^ 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.
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 re- sponsive 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 weU 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 re- ferred to, and these departures, while not large, are believed to be full of significance. Lest the reader should gain a wrong impres- sion of their magnitude, it may be well to introduce a diagram drawn to scale and representing prominent elevations and depres- sions 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
THE FIGURE OF THE EARTH
11
DepT^ a^^ffSM._eqrr^''s_rvra'/fi<s_
a
Fig. 1. — Diagrams to afford ^ a correct impression of the measure of the inequalities upon the earth's surface com- pared to the earth's radius. The shell represented in 6 is liu of the earth's radius, and in a this zone is magnified for comparison with surface inequalities.
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 signifi- cant facts of distribution are disclosed.
One of the most signifi- cant facts involved in the distribution of land and sea, is a concentration of the land areas-within^ the iioTtfiern and- — the — seasi wrEMn the southern hemi- sphere. The noteworthy exception is the occurrence of the great and high Antarctic continent cen- tered 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 ex- istence 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 north- ward, the land masses are V-shaped and nar- row southward, hut this occurs mainly in the southern hemi- sphere. Lastly, there
rra.2.-Map on Mercator's projection to sW j^ ^^^^ indication of
reciprocal relation of tne land ana sea areas (alter
Gregory and Arldt). a belt of sea dividing
12
EARTH FEATURES AND THEIR MEANING
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.
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 dis- tribution of land and
Ansarf
$/)ieH water. There are.
however, depres- sions upon the sur- face of the land, in some cases extend- ing 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, Fio. 3. — The form toward which the figure of the earth which with its ex- 18 tending, a tetrahedron with symmetrically truncated -fcension divides cen-
°^ ^^' tral 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 more- over 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
THE FIGURE OF THE EARTH
13
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 ob- viously 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 measure- ment to be the case, and the result is further confirmed by pendulum studies upon the distribution of the earth's at- traction or gravity. If less of the mass of the earth is con- centrated in the southern hemisphere, its attraction as measured in vibrations of the pendulum should be cor- respondingly smaller.
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 meas- ured in an east-and-west direction across Europe revealed a differ- ent 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 soUdified at the sur- face to the temperature of the surrounding air, and the core still hot and continuing to lose heat, the core must continue to
FiQ. 4. — A truncated tetrahedron, showing how the depression upon one side of the cen- ter is balanced by the opposite projection.
14 EARTH FEATURES AND THEIR MEANING
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 ob- served 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 conti- nent 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 indi- cates a departure from it, and, as we shall see, a significant depar- ture 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 deforma- tion of the earth's shell. The first of these of which we have
THE FIGURE OF THE EARTH 15
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 adjust- ments 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 foxind 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
Fig. 5. — Approximations to earlier and present figures of the earth.
in somewhat similar grouping, namely, in South America, in South Africa, and in Australia (Figs. 3 and 5).
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
16 EARTH FEATURES AND THEIR MEANING
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 ex- tended 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 west- ward being somewhat farther south than the present northern coast of North America, since life forms migrated in the north- ern 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 tetra- hedrons 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 conti- nental 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 inter- rupted 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
THE FIGURE OF THE EARTH
17
7&rnafita/ro/ /^ce-a nr/rA /i/oeAr fO'Souf/r
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 tet- rahedron of the southern hemisphere, since this had its apex to the northward (Fig. 6).
Thus we see that the lithosphere can scarcely be regarded as a perfect spheroid, since in the course of geologic ages it has under- gone successive de- partures 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 bor- der, showed that this depression is characterized by profound depths, and so have afforded confirmation of the tetrahedral fig- ure. 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
Tbrra/taefra/ /^aces tvit^ft j^peje to A/orth ■/lcr<Ao/ //nes /n 'Soi//-/rerfi Alsm/s/a/ie/v.
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.
18
EARTH FEATURES AND THEIR MEANING
Fig. 7.
S S
-The continenta with submerged portions added (after Gilbert).
outline perpendicular to this face is imperfectly heart-shaped or like a flattened " peg top."
The flooded portions of the present continents. — We are accus- tomed to think of the continents as ending at the shores of the
oceans. If, however, we N N are to regard them as
platforms which rise from the ocean depres- sions, their margins should be considerably- extended, for a sub- merged shelf now prac- tically 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 por- tions 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.
The floors of the hydrosphere and atmosphere. — The highest altitudes upon the continents and the profoundest deeps of the
'/o,cxx> -/0.000
-2O,OO0
FiQ. 8. — Diagram to indicate the altitude of different parts of the lithosphere surface.
|
\ |
vJi P J |
|
r\ 1^ |
|
|
T~ — ■ ^ |
|
|
m |
^ M 1 * } |
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 -^ of the
THE FIGURE OF THE EARTH 19
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 Repebencbs for Chapteb 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, Lon- don, 1901), Chapters 1-2.
The departure of the spheroid toward the tetrahedron : —
W. LowTHiAN Green. Vestigesof 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. Joxir., vol. 13, 1899, pp. 225-251 (the best general statement of the argu- ments for a tetrahedral form).
W. Pbinz. L'echelle reduite des experiences gSologiques, Bull. Soc. Beige d'Astronomie, 1899.
B. K. Emerson. The Tetrahedral Earth and Zone of the Interconti- nental Seas, Bull. Geol. Soc. Am., vol. 11, 1911, pp. 61-106, pis. 9-14.
M. P. RuDSKi. Physik der Erde (Tauehnitz, 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. Abldt. Die Entwicklung der Kontinente und ihrer Lebewelt. Engel- mann, 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 sup- posed that the solid earth constituted a crust only which was floated upon a liquid interior. This notion was clearly an out- growth 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 sohd 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 litho- sphere 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 demon- stration. 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.
20
NATURE OF THE MATERIALS IN THE LITHOSPHERE 21
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 5§, 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 sur- rounding 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 arrange- ment, 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, oc- cur in the earth's crust that have not been found in meteorites,
22
EARTH FEATURES AND THEIR MEANING
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, how- ever, that the commonest of the minerals in the earth's shell are absent from meteorites, as the commoner constituents of meteor- ites are wanting in the earth's crust. This observation would go far to show that we may in the two cases be examining different Terresrr/a/ Rocks
Afereor/Tes am/rarer 7erresrr/a/ ffoc/fs. 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 ; 6, syenites and trachytes ; c, diorites and andesites ; d, gabbros and basalts ; e, ultra-basic rocks ; /, basic inclosures in basalt, etc. ; g, iron basalts of west Greenland ; h, iron masses of Ovif ak, west Greenland ; a'-d', meteorites in order of density (after Judd) .
portions of quite similar bodies ; and this view is strikingly con- firmed when the rocks of the two groups are arranged in the order of their densities (Fig. 9).
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.
NATURE OF THE MATERIALS IN THE LITHOSPHERE 23
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 pos- sible 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 argu- ment 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 (Mer- cury 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 role 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 accus- tomed 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
24
EARTH FEATURES AND THEIR MEANING
or soul of the crystal individual — it has its own structural make- up, its individuality. No more correct estimates of the compari- son 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. Just as the real nature of a
Crystal (gvartt)
AmorphouaSubsiiawe (Glaas)
Expansion by heat
person is first disclosed by his behavior under trying circum- stances, 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 outUnes of the crystal or employ a bat- tered.fragment — and then pre- pare a sphere of similar size and shape from a noncrystalline or amorphous substance like glass. If now these two spheres be in- troduced into a bath of oil and raised to a higher temperature,
the glass globe undergoes an
FiQ. 10.— Comparison of a crystalline enlargement without change of
with an amorphous substance when ex- its form ; but the Crystal ball
panded by heat and when attacked by reveals its individuality by ex- panding 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
Action of an acid
NATURE OF THE MATERIALS IN THE LITHOSPHERE 25
^;:-^iM
fire," allow each to be attacked by the powerful reagent, hydro- fluoric 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 Ht- tle etchings upon the surface display each the individuality of the sub- stance, and joining with their neigh- bors they send out a beautifully symmetrical and entirely character- istic picture (Fig. 11).
The lithosphere a complex of interlocking crystals. — To the lay- man the crystal is something rare Fig. ii. -"Light figure" seen upoa
and expensive, to be obtained from an etched surface of a crystal of
a jeweler or to be seen displayed in ^'^^'Jy ^^"^^ ^°'^"'^'"''^* ^"'^ the show cases of the great muse- ums. Yet the one most striking quality of the lithosphere which separates it from the hydrosphere and the atmosphere is its crys- talline 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 litho- sphere, 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 attri- tion under the rhythmic beating of the waves upon a shore or in the continuous rolling of pebbles on a stream bed, until as bat-
2G EARTH FEATURES AND THEIR MEANING
tered relics they are piled away together in a bed of sand. Yet no amount of such rough handling is sufficient to destroy the crys- tal 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 sand- stones 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 ; 6, 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 con- dition 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,
NATURE OF THE MATERIALS IN THE LITHOSPHBRE 27
in the vast number of instances it is the behavior, and not the appearance, of the mineral substance which is made use of for iden- tification.
When a mineral is broken under the blow of a hammer, in- stead 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 feld- spar, 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 jjolarized -light-, and such behavior is the basis of those branches of geological science known as optical mineralogy and as microscopical petrography. An out- line 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 con- solidation of molten rock material such as during a volcanic erup- tion 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 crys- tal — the Krystallkeller or " crystal cellar." It is from such pockets or veins within the rocks that the valuable ores are ob- tained, as are the crystals which are displayed in our mineral cabinets.
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 preexisting 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
28
EARTH FEATURES AND THEIR MEANING
solution, or solution and redeposition of mineral matter may go on together within the mass of the same rock. One new mineral Hiay have been produced from the dissolved materials-of-arnum- ber of earlier spejdea, or several new minerals may be thelresult of the alteration of a pj:fiexistiiig_min- gral with a more complex chemicaLsiructure. Where the newmineral has beenlormed " in place," it has sometimes been able to utilize the materials of all the minerals which before existed there, or it may Fig. 13.— Crys- have been obliged to inclose within itself those earlier ta o garnet constituents which it could not assimilate in its own
developed in
a schist with structure (Fig. 13).
grains of At Other times a crystal which is imbedded in ci" Y d b°" ^^'^^ ^^ been attacked upon its surface by the per- cause not as- colating solutions, and the dissolved eimiiated. 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 ^^°- 14.— A brought about has involved a nice adjustment of supply of the chemical constituents necessary to the formation of the new mineral or min- erals. In rocks which are aggregates of several min- FiQ. 15. — A new mineral eral species, a newly formed (hornblende) forming as an mineral may appear only at
intermediate reaction , •'
rim" between the mineral ^'^^ COmmon margin of Cer-
having irregular fractures tain of these species, thus showing that
St^ IL^I theysupplythosechemicalelementswhich
feldspar). 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.
crystal of aug- ite within the mass of a rock altered in part to form a rim of the min- erals horn- blende and magnetite. Note the orig- inal outline of the augite crystal.
NATURE OF THE MATERIALS IN THE LITHOSPHERE 29
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 Uthosphere 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 Uthosphere 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. Chambbblin. 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.
CHAPTER IV THE ROCKS OF THE EARTH'S SURFACE SHELL
The processes by which rocks are formed. — Rocks may be formed in any one of several Vays. When a portion of the molten lithosphere, so-called magma, cools and consolidates, the product is igneous rock. Either igneous or other rock may become dis- integrated at the earth's surface, and after more or less extended travel, either in the air, in water, or in ice, be laid down as a sedi- ment. Such sediments, whether cemented into a coherent mass or not, are described as sedimentary or clastic rocks. If the fluid from which they were deposited was the atmosphere, they are known as subaerial or eolian sediments ; but if water, they are known as subaqueous deposits. Still another class are ice-deposited and are known as glacial deposits.
But, as we have learned, rocks may undergo transformations through mineral alteration, in which case they are known as
metamorphic rocks. When these changes consist chiefly in the production of more soluble minerals at the surface, accom- panied by thorough disintegration, due to the direct attack of the atmosphere, the resulting rocks are called residual rocks.
The marks of ori- gin.— Each of the three great classes of rocks, the igneous, sedimentary, and meta- morphic, is characterized by both coarser and finer structures, in the examination of which they may be identified. The igneous
30
'^iSi*^i»ftjfc;{:i^:^S^
Fig. 16. — Laminated structure of sedimentary rock, Western Kansas (after a photograph by E. S. Tucker).
THE ROCKS OP THE EARTH'S. SURFACE SHELL 31
rocks having been produced from magmas, which are essentially- homogeneous, are usually without ilefin^i±e_direjctii)nal_structures__ due to an arrangement of their constituents, and are said to have a massive structure. Sedimentary rocks, upon the other hand, have been formed by an assorting process, the larger and heavier fragments having been laid down when there was high velocity of either wind or water current, and the smaller and lighter frag- ments during intermediate periods. They are therefore more or less banded, and are said to have a bedded or laminated structure (Fig. 16).
Again, igneous rocks, being due to a process of crystallization, are composed of mineral individuals which are bounded either by crystal planes or by irregular surfaces along which neighboring crystals have interfered with each other; but in either case the grains possess sharply angular boundaries. Quite different has been the result of the attrition between grains in the transpor- tation and deposition of sediments, for it is characteristic of the sedimentary rocks that their constituent grains are well rounded. EoUan sediments have usually more perfectly rounded grains than subaqueous deposits.
Glacial deposits, if laid down directly by the ice, are unstrati- fied, relatively coarse, and contain pebbles which are both faceted and striated. Such deposits are described as till or tillite. If glacier-derived material is taken up by the streams of thaw water and is by them redeposited, the sediments are assorted or stratified, and they are described as fluvio-gladal deposits.
The metamorphic rocks. — Both the coarser structures and the finer textures of the metamorphic rocks are intermediate between those of the igneous and the sedimentary classes. A metamorphosed sedimentary rock, in proportion to its alteration, loses the perfect lamination and the rounded grain which were Its distinguishing characters ; while an igneous rock takes on in the process an imperfect ..banding, and the sharp angles of its cohstituent grains become rounded_ off by _a.„sort _pf peripheral crushing - or granulation. Metamorphic rocks are therefore characterized by an imperfectly banded structure described as schistositii ojLmisissJbanding, and the constituent grains may be either angular or rounded. If the metamorphism has not been too intense or too long continued, it is generally possible to deter-
32 EARTH FEATURES AND THEIR MEANING
mine, particularly with the aid of the polarizing microscope, whether the original rock from which it was derived was of igneous or of sedimentary origin. There are, however, many examples which have defied a reliable verdict concerning their origin.
Characteristic textures of the igneous rocks. — In addition to the massiveness of their general aspect and the angular bound- aries of their constituents, there are many additional textures which are characteristic of the igneous rocks. While those that have consoUdated below the earth's surface, the intrusive rocks, are notably compact, the magmas which arrive at the surface of the lithosphere before their consolidation reveal special structures dependent either upon the expansion of steam and other gases within them, or upon the conditions of flow over the earth's sur- face. Magmas which thus reach the surface of the earth are de- scribed as lavas, and the rocks produced by their consolidation are extrusive or volcanic rocks. The steam included in the lava expands into bubbles or vesicles which may be large or small, few or many. According to the number and the size of these cavities, the rock is said to have a vesicular, scoriaceous, or pumi- ceous texture.
Most lavas, when they arrive at the earth's surface, contain crystals which are more or less disseminated throughout the molten mass. The tourist who visits Mount Vesuvius at the time of a light eruption may thrust his staff into the stream of lava and extract a portion of the viscous substance in which are seen beautiful white crystals of the mineral leucite, each bounded by twenty-four crystal faces. It is clear that these crystals must have developed by a slow growth within the magma while it was still below the surface, and when the inclosing lava has con- solidated, these earher crystals lie scattered within a groundmass of glassy or minutely crystalline material. This scattering of crystals belonging to an earlier generation within a groundmass due to later consolidation is thus an indication of interruption in the process of crystallization, and the texture which results is described as porphyritic (Fig. 17 b). . Should the lava arrive at the surface before any crystals have been generated and consoli- date rapidly as a rock glass, its texture is described as alassu (Fig. 17 c).
When the crystals of the earlier generation are numerous and
THE ROCKS OP THE EARTH'S SURFACE SHELL 33
needle-like in form, as is very often the case, they arrange them- selves " end on " during the rock flow, so that when consolida- tion has occurred, the rock has a kind of puckered lamination which is the characteristic of the fluxion or flow texture. This texture has sometimes been confused with the lamination of the sedi- mentary rocks, so that wrong conclusions have been reached
Fig. 17. — Characteristic textures of igneous rooks, a, granitic texture characteristic of the deep-seated intrusive rocks ; 6, porphyritio texture characteristic of the ex- trusive and of the near-surface intrusive rocks ; c, glassy texture of an extrusive rock.
regarding origin. At other times the same needle-like crystals within the lava have grouped themselves radially to form rounded nodules called spherulites. Such nodules give to the rock a spherulitic texture, which is nowhere better displayed than in the beautiful glassy lavas of Obsidian Cliff in the Yellowstone Na- tional Park.
Those intrusive rocks which consolidate deep below the earth's surface, part with their heat but slowly, and so the process of crystallization is continued without interruption. Starting from many centers, the crystals continue to grow until they mutually intersect in an interlocking complex known as the granitic tex- ture (Fig. 17 a).
Classification of rocks. — In tabular form rocks may thus be classified as follows : —
Igneous. Massive and with sharply angular grains.
Intrusive. Granitic or porphyritio texture.
Extrusive. Glassy or porphyritio texture; often also with vesicular, scoriaoeous, pumi- ceous, fluxion, or spherulitic textures.
34
EARTH FEATURES AND THEIR MEANING
Sedimentary. Laminated and with rounded grains.
(Subaerial. Sands and loess. Subaqueous. (See below.) Glacial. Coarse, unstratifled deposits with
faceted pebbles. Till and tillite. Fluvio-glacial. Stratified sands and gravels
with "worked over" glacial characters.
Metamorphic. Schistose tMetamorphic proper. Due to below surface and with grains either I changes, angular or rounded. [Residual. Disintegrated at or near surface.
Subdivisions of the sedimentary rocks. — While the eolian sediments are all the product of a purely mechanical process of lifting, transportation, and deposition of rock particles, this is not always the case with the subaqueous sediments, since water has the power of dissolving mineral substance, as it has also of furnishing a home for animal and vegetable life. Deposited materials which have been in solution in water are described as chemical deposits, and those which have played a part in the life process as organic deposits. The organic deposits from vege- table sources are peat and the coals, while limestones and marls are the chief depositories of the remains of the animal life of the water. The tabular classification of the sediments is as follows : —
Classification of Sediments.
Mechanical
Chemical
Organic
|
Subaqueous Deposited by water. Subaerial or Eolian Deposited by wind. Glacial Deposited by ice. Fluvio-glacial Glacier-'water deposit |
Conglomerate, sand- stone and shale. Sandstone and loess. Till and tillite. Sands and gravels. s. |
|
Calcareous tufa Oolitic limestone |
Deposited in springs and rivers. Deposited at the mouths of rivers between high and low tide. |
|
Formed of plant re- |
Peats and coals. |
|
mains. |
|
|
Formed of animal re- |
Limestones and |
|
mains. |
marls. |
THE ROCKS OP THE EARTH'S SURFACE SHELL 35
Winds are under favorable conditions capable of transporting both dust and sand, but not the larger rock fragments. The dust deposits are found accumulating outside the borders of des- serts as the so-called loess (Fig. 216), though the sand is never carried beyond the desert border, near which it collects in wide belts of ridges described as dunes. When this sand has been cemented into a coherent mass, it is known as eolian sandstone. A section of the appendix (B) is devoted to an outline description •of some of the commoner rock types.
The different deposits of ocean, lake, and river. — Of the sub- aqueous sediments, there are three distinct types resulting: (1) from sedimentation in rivers, the fluviatile deposits ; (2) from sedimentation in lakes, the lacustrine deposits ; and (3) from sed- imentation in the ocean, marine deposits. Again, the widest range of character is displayed by the deposits which are laid down in the different parts of the course of a stream. Near the source of a river, coarse river gravels may be found ; in the middle course the finer silts ; and in the mouth or delta region, where the ■deposits enter the sea or a lake, there is found an assortment of silts and clays. Except within the delta region, where the area of deposition begins to broaden, the deposits of rivers are stretched out in long and relatively narrow zones, and are so distinguished from the far more important lacustrine and marine deposits'.
Lakes and oceans have this in common that both are bodies of standing as contrasted with flowing water; and both are sub- ject to the periodical rhythmic motions and alongshore currents due to the waves raised by the wind. About their margins, the deposits of lake and ocean are thus in large part wrested by the waves from the neighboring land. Their distribution is always such that the coarsest materials are laid down nearest to the shore, and the deposits become ever finer in the direction of deeper water. Relatively far from shore may be found the finest sands and muds or calcareous deposits, while near the shore are sands, and, finally, along the beach, beds of beach pebbles or shingle. When cemented into coherent rocks, these deposits become shales or limestones, sandstones, and conglomerates, respectively.
As regards the limestones, their origin is involved in consid- erable uncertainty. Some, like the shell limestone or coquina of the Florida coast, are an aggregation of remains of moUusks
36 EARTH FEATURES AND THEIR MEANING
which live near the border of the sea. Other limestones are de- posited directly from carbonate of lime in solution in the water. A deposit of this nature is forming in southern Florida, both as a flocculent calcareous mud and as crystals of lime carbonate upon a limestone surface. Again, there is the reef limestone which is built up of the stony parts of the coral animal, and, lastly, the calcareous ooze of the deep-sea deposits.
The marine sediments which are derived from the conti- nents, the so-called terrigenous deposits, are found only upon the continental shelf and upon the continental slope just outside it. Of these terrigenous deposits, it is customary to distinguish: (1) littoral or alongshore deposits, which are laid down between high and low tide levels ; (2) shoal water deposits, which are found between low-water mark and the edge of the continental shelf ; and (3) aktian or offshore deposits, which are found upon the conti- nental slope. The littoral and shoal water deposits are mainly gravels and sands, while the offshore deposits are principally muds or lime deposits.
Special marks of littoral deposits. — The marks of ripples are often left in the sand of a beach, and may be preserved in the sand- stone which results from the cementation of such deposits (pi. 11 A). Very similar markings are, however, quite characteristic of the surface of wind-blown sand. For the reason that deposits are subject to many vicissitudes in their subsequent history, so that they sometimes stand at steep angles or are even overturned, it is important to observe the curves of sand ripples so as to dis- tinguish the upper from the lower surface.
In the finer sands and muds of sheltered tidal flats may be pre- served the impressions from raindrops or of the feet of animals which have wandered over the flat during an ebb tide. When the tide is at flood, new material is laid down upon the surface and the impressions are filled, but though hardened into rock, these surfaces are those upon which the rock is easily parted, , and so the impressions are preserved. In the sandstones of the Connecticut valley there has been preserved a quite remarkable record in the footprints of animals belonging to extinct species, which at the time these deposits were laid down must have been abundant upon the neighboring shores.
Between the tides muds may dry out and crack in intersecting
THE ROCKS OF THE EARTH'S SURFACE SHELL 37
lines like the walls of a honeycomb, and when the cracks have been filled at high tide, a structure is produced which may later be recognized and is usually referred to as " mud-crack " structure. This structure is of special service in distinguishing marine de- posits from the subaerial or continental deposits.
A variation in the direction of winds of successive storms may be responsible for the piling up of the beach sand in a pecul- iar " plunge and flow " or " cross-bedded " structure, a structure which is extremely common in littoral deposits, though simu- lated in rocks of eolian origin.
The order of deposition during a transgression of the sea. — Many shore lines of the continents are almost constantly migrat- ing either landward or seaward. When the shore line advances
FiQ. 18. — Diagram to show the order of the sediments laid down during a trans- gression of the sea.
over the land, the coast is sinking, and marine deposits will be formed directly above what was recently the " dry land." Such an invasion of the land by the sea, due to a subsidence of the coast, is called a transgression of the sea, or simply a transgression. Though at any moment the littoral, shoal water, and offshore deposits are each being laid down in a particular zone, it is evi- dent that each must advance in turn in the direction of the shore and so he deposited above the zones nearer shore. Thus there comes to be a definite series of continuous beds, one above the other, provided only that the process is continued (Fig. 18). At the very bottom of this series there will usually be found a thin bed of pebbly beach materials, which later will harden into the so- called basal conglomerate. If the size of the pebbles is such as to make possible an identification, it will generally be found that these represent the ruins of the rock over which the sea has advanced upon the land.
Next in order above the basal conglomerate, will follow the coarser and then the finer sands, upon which in turn will be laid down the offshore sediments — the muds and the lime deposits.
38 EARTH FEATURES AND THEIR MEANING
Later, when cemented together, these become in order, coarser and finer sandstones, shales, and limestones. The order of super- position, reading from the bottom to the top, thus gives the order of decreasing age of the formations.
A subsequent upHft of the coast will be accompanied by a recession of the sea, and when later dissected by nature for our inspection, the order of superposition and the individual character of each of the deposits may be studied at leisure. From such studies it has been found that along with the inorganic deposits there are often found the remains of life in the hard parts of such invertebrate animals as the mollusks and the Crustacea. These so-called fossils represent animals which were gradually developed from simpler to more and more complex forms; and they thus serve the purpose of successive page numbers in arranging the order of disturbed strata, at the same time that they supply the most secure foundation upon which rests the great doctrme of evolution.
The basins of earlier ages. — It was the great Viennese geolo- gist. Professor Suess, who first pointed out that in mountain regions there are found the thickest and the most complete series of the marine deposits; whereas outside these provinces the forma- tions are separated by wide gaps representing periods when no deposits were laid down because the sea had retired from the region. The completeness of the series of deposits in the moimtain districts can only be interpreted to mean that where these but lately formed mountains rise to-day, were for long preceding ages the basins for deposition of terrigenous sediments. It would seem that the lithosphere in its adjustment had selected these earlier sea basins with their heavy layers of sediment for zones of special upUft.
The deposits of the deep sea. — Outside the continental slope, whose base marks the limit of the terrigenous deposits, lies the deeper sea, for the most part a series of broad plains, but varied by more profound steep-walled basins, the so-called " deeps " of the ocean. As shown by the dredgings of the Challenger expedi- tion and others of more recent date, the deposits upon the ocean floor are of a wholly different character from those which are derived from the continents. Except in the great deeps, or between depths of five hundred and fifteen hundred fathoms,
THE ROCKS OF THE EARTH'S SURFACE SHELL 39
these deposits are the so-called " ooze," composed of the cal- careous or chitinous parts of algse and of minute animal organisms. The pelagic or surface waters of the ocean are, as it were, a great meadow of these plant forms, upon which the minute Crustacea, such as globigerina, foraminifera, and the pteropods, feed in count- less myriads. The hard parts of both plant and animal organisms descend to the bottom and there form the ooze in which are some- times found the ear bones of whales and the teeth of sharks.
In the deeps of the ocean, none of these vegetable or animal deposits are being laid down, but only the so-called " red clay," which is believed to represent decomposed volcanic material deposited by the winds as fine dust on the surface of the ocean, or the product of submarine volcanic eruption. From the absence of the ooze in these profound depths, the conclusion is forced upon us that the hard parts of the minute organisms are dissolved while falling through three or four miles of the ocean water.
Reading References for Chapter IV
J. S. DiLLBR. The Educational Series of Rook Specimens collected and
distributed by the United States Geological Survey, Bull. 150
U. S. Geol. Surv., 1898, pp. 1-400. L. V. PiHssoN. Rocks and Rook Minerals. Wiley, New York, 1908. Sir John Murrat. Deep-sea Deposits, Reports of the Challenger
expedition, Chapter iii. L. W. Collet. Les dep6ts marins. Doin, Paris, 1907 (Encyclopedie
Scientifique).
CHAPTER V
CONTORTIONS OF THE STRATA WITHIN THE ZONE OF
FLOW
The zones of fracture and fiow. — It is easy to think of the atmosphere and the hydrosphere as each sustaining at any point the load of the superincumbent material. At the sea level the weight of air upon each square inch of surface is about fifteen pounds, whereas upon the floor of the hydrosphere in the more profound deeps the load upon the square inch must be measured in tons. Near the lithosphere surface the rocks support by their strength the load of rock above them, but at greater depths they are unable to do this, for the load bears upon each portion of the rock with a pressure equivalent to the weight of a rock column which extends upward to the surface. The average specific gravity of rock is 2.7, and it is thus easy to calculate the length of the inch square column which has a weight equivalent to the crushing strength of any given rock. At the depth repre- sented by the length of such a column, rocks caimot yield to pres- sure by fracture, for the opening of a crack implies that the rock upon either side is strong enough to prevent the walls from clos- ing. At this depth, rock must therefore yield to pressure not by fracture, as it would at the surface, but by flow after the manner of a liquid ; and so the zone below this critical level is referred to as the zone of flow.
In contrast, the near-surface zone is called the zone of fracture. But different rocks possess different strengths, and these are subject to modifications from other conditions, such, for example, as the proximity of an uncooled magma. The zone of flow is therefore joined to the zone of fracture, not upon a definite surface, but in an intermediate zone described as the zone of fracture and flow.
Experiments which illustrate the fracture and flow of solid bodies. — A prismatic block prepared from stiff molders' wax, if crushed between the jaws of a testing machine, yields a system
40
CONTORTIONS OF THE STRATA
41
of intersecting fractures which are perpendicular to the free sur- faces of the block and take two directions each inclined by h^lf of a right angle to the direction of compression (Fig. 19). This experiment may illustrate the manner in which fractures are produced by the compression within the zone of fracture of the lithosphere, as its core continues to contract.
To reproduce the conditions within the zone of flow, it will be necessary to load the lateral surfaces of the block instead of leaving them unconstrained as in the above-described ex- periment. The experiment is best devised as in Fig. 20. Here a series of layers having varying degrees of rigidity is prepared from beeswax as a base, either stiffened by ad- mixture of varying proportions of plaster of Paris, or weakened by the use of Venice turpen- tine. Such a series of layers may represent rocks of as widely different characters as lime- stone and shale. The load which is to rep- resent superincumbent rock is supplied in the experiment by a deep layer of shot.
When compression is applied to the layers from the ends, these normally solid materials, instead of fracturing, are bent into a series of folds. The stiffer, or more competent, layers are found to be less contorted than are the weaker layers, particularly if the
Fig. 19. — Two inter- secting parallel series of fractures produced upon each free sur- face of a prismatic block of stiff molders' wax when broken by compression from the ends (after Daubrfie and Tresoa).
SKCiononUnc tU>
S&MononLwe cd
FiQ 20 Apparatus to illustrate the folding of strata within the zone of flow
(after Willis).
42
EARTH FEATURES AND THEIR MEANING
Fia. 21. — Diagrams representing a, an anticline ; b, a syncline ; and c, a mono- cline.
latter have been protected under an arch of the more competent layer (pi. 2 A).
The arches and troughs of the folded strata. — Every series of folds is made up of alternating arches and troughs. The arches of the strata the geologist calls anticlines or anticlinal folds, and the troughs he calls synclines or synclinal folds (Fig. 21). When a
stratum is merely dropped in a bend to a lower level without producing a complete arch or a complete trough, this half fold is termed a monocline.
Any flexuring of the strata implies a reduction, of their surface area, or, considering a single section, a shortening. If the arches and troughs are low and broad, the deformation of the strata is slight, the shorten- ing is comparatively small, and the folds are described as open (Fig. 22 b). If they be relatively both ^ high and narrow, the deformation is '
considerable, a larger amount of crustal shortening has gone on, and the folds are described as close (Fig. 22 c). This closing up of the folds may continue until their sides have practically the same slope, in which case they are said to be isoclinal (Fig. 22 d).
The elements of folds. — Folds must always be thought of as having ex- tension in each of the three dimensions of space (Fig. 23), and not as properly included within a single plane like the cross sections which we so often use in illustration. A fold may be conceived of as divided into equal parts by a plane which passes along the middle of either the arch or the trough, and is called the axial plane. The line in which this plane inter- sects the arch or the trough is the axis, which may be called the Crestline in an anticline, and the troughline in a syncline.
In the case of many open folds the axis is practically hori-
Fig. 22. — A comparison of folds to express increasing degrees of crustal shortening or progressive deformation within the zone of flow ; a, stratum before folding; b, open folds; c, close folds; d, isoclinal folds.
CONTORTIONS OF THE STRATA
43
zontal, but in more complexly folded regions this is seldom true. The departure of the axis from the horizontal is called the pitch, and folds of this type are described as pitching folds or plunging
^PJTCM.
Fig. 23. — Anticlinal and synclinal folds in strata (after Willis).
folds. The axis is in reality in these cases thrown into a series of undulations or " longitudinal folds," and hence pitch will vary along the axis.
The shapes of rock folds. • — By the axial plane each fold is divided into two parts which are called its limbs, which may have either the same or different average inclinations. To describe now the shapes of rock folds and not the degree of compression of the district, some additional terms are necessary. Anticlines or synclines whose limbs have about the same inclinations are known as upright or symmetrical folds. The axial plane of the symmetrical fold is vertical (Fig. 24). If this plane is inclined to the vertical, the folds are unsymmetrical. So soon as the steeper of the two limbs has passed the vertical position and inclines in the same direction as the flatter limb, the fold is said to be over- turned. The departure from symmetry may go so far that the axial plane of the fold lies at a very flat angle, and the fold is then said to be recumbent. The observant traveler by train along any of the routes which enter the Alps may from his car window find illustrations of most of these types of rock folds, as he may also,
44
EARTH FEATURES AND THEIR MEANING
Symn7a-/-/-/ca/
Unsymmetr/ca/ \
O^vrturhea/
though generally less easily, in passing through the Appalachian
Mountains.
In regions which have been closely folded the larger flexures of the strata may be found with folds of a smaller order of magnitude superimposed upon them, and these in turn may show crumplings of still lower orders. It has been found that the folds of the smaller orders of magnitude pos- sess the shapes of the larger flexures, and much is therefore to be learned from their careful study (Fig. 25). It is also quite generally discovered that parallel planes of ready parting, which are described as roch cleavage, take their course parallel to the axial plane within each minor fold. As was long ago shown by the pioneer British geologists, these planes of cleavage are essentially parallel and follow the fold axes throughout large areas.
The overthrust fold. — Whenever a stratum is bent, there is a tendency for its particles to be separated upon the convex side of the bend, at the same time that those upon the con- cave side are crowded closer together — there is tension in the former case and compression in the latter (Fig. 26). Within an unsymmet- rical or an overturned fold, the peculiar .dis- tortions in the different sections of the stratum are less simple and are r.„ oc a j j . .. ^
^ tia. Z5. — becondary and tertiary flexures supenm-
best illustrated by posed upon the primary ones.
Recumbent
Fig. 24. — Diagrams to illustrate the different shapes of rock folds.
Plate 2.
A. Layers compressed in experiments and showing the effect of a competent layer in the process of folding (after Willis).
B. Experimental production of a series of parallel thrusts within closely folded strata
(after Willis).
C. Apparatus to illustrate shearing action within the overturned limb of a fold.
CONTORTIONS OF THE STRATA 45
pi. 2 C. This apparatus shows two similar piles of paper sheets,. upon the edges of each of which a series of circles has been drawn.. When now one of the piles is bent into an unsymmetrical fold, it is seen that through an accommodation by the paper sheets sliding each over its neighbor large distortions of the circles have occurred. In that steeper limb which with closer folding will be overturned the circles have been drawn
out into long and narrow ^^^*^^^^^^^=^
ellipses, and this indicates ^^l^^^^^^^m'^-^/'^/ that those rock particles <^N^- •'^^^^^^^fcfr>>. ''.'^''^^ which before the bending ^ ^^f^^^^^^^^%. " w-"'
were included in the circle *' ^-J'
have been moved past each ■^'°- 26. —A bent stratum to illustrate tension „j.T „_ • J.1 J? j.i_ upon the convex and compression upon the
other m the manner of the „,„,^^, ^j^, (^j^,, y^^ gise). blades of a pair of shears.
Such extreme " shearing " action is thus locahzed in the under- turned limb of the fold, and a time must come with continuation of the compression when the fold will rupture at this critical place along a plane parallel to the longest axis of the ellipses or nearly parallel to the axial plane of the anticline. Such structures prob- ably occur in the zone of combined fracture and flow, up into which the beds are forced in cases of close compression. Relief thus being found upon this plane of fracture, the upper portion of the fold will now ride over the lower, and the displacement is described as a thrust or overthrust.
In the long series of experiments conducted by Mr. Bailey Willis of the United States Geological Survey, all the stages be- tween the overturned fold and the overthrust fold were reproduced. Where a series of folds was closely compressed, a parallel series of thrusts developed (pi. 2 B), so that a series of slices cutting across neighboring strata was slid in succession, each over the other, like the scales upon a fish or the shingles upon a roof. Quite remarkable structures of this kind have been discovered in rocks of such closely folded districts as the Northwest Highlands of Scot- land, where the overriding is measured in miles. Near the thrust planes the rocks show a crushing of the grains, and the planes them- selves are sometimes corrugated and polished by the movement.
Restoration of mutilated folds. — Since flexuring of the rocks takes place within the zone of flow at a distance of several miles
46 EARTH FEATURES AND THEIR MEANING
below the earth's surface, it is quite obvious that the results of the process can be studied only after some thousands of feet of super- incumbent strata have been removed. We are a little later to see by what processes this lowering of the surface is accomplished, but for the present it may be sufficient to accept the fact, realizing that before foldings in the strata can reach the surface, they must have passed through the upper zone of fracture.
It might perhaps be supposed that the anticlines would appear as the mountains upon the surface, and occasionally this is true ; as, for example, in the folded Jura Mountains of western Europe. More generally, the mountains have a synclinal structure and the valleys an anticlinal one ; but as no general rule can be applied, it is necessary to make a restoration of the truncated folds in each district before their character can be known.
The geological map and section. — The earth's surface is in most regions in large part covered with soil or with other inco- herent rock material, so that over considerable areas the hard rocks are hidden from view. Each locality at which the rock is found at the earth's surface "in place" is described as an outcropping or exposure. In a study of the region each such ftxposure must be examined to determine the nature of the rock,' especially for the purpose of correlation with neighboring exposures, and, in addition, both the probable direction in which it is continued along the surface — the strike — and the inclination of its beds — the dip. If the outcroppings are sufficiently numerous, and rock type, strike and dip, may all be determined, the folds of the dis- trict may be' restored with almost as much accuracy as though their curves were everywhere exposed to view. A cross section through the surface which represents the observed outcrops with their inclinations and the assumed intermediate strata in their probable attitudes is described as a geological section (Fig. 27). A map upon which the data have been entered in their correct loca- tions, either with or without assijmptions concerning the covered areas, is known as a geological map.
If the axes of folds are absolutely horizontal, and the surface of the earth be represented as a plain, the lines of intersection of the truncated strata with the ground, or with any horizontal sur- face, will give the directions of continuation of the individual strata. This strike direction is usually determined at each expo-
CONTORTIONS OP THE STRATA
47
sure by use of a compass provided with a spirit level. When that edge of the leveled compass which is parallel to the north-south line upon the dial is held against the sloping rock stratum, the
a ^ ^ (/ c
Fig. 27. — A geological section based upon observations at outcrops, but with the truncated arches restored.
angle of strike is measured in degrees by the compass needle. If the cardinal directions have been placed in their correct positions upon the compass dial, the needle will point to the northwest when the strike is northeast, and vice, versa (Fig. 28 a). Upon
Pig. 28. — Diagram to illustrate the manner of determining the strike of rock beds at an outcropping, a, a compass which has the cardinal directions in their natural positions ; 6, a compass with the east and west initials reversed upon the dial ; c, home-made clinometer in position to determine the dip.
the geologist's compass it is therefore customary to reverse the initials which represent the east and west directions, in order that the correct strike may be read directly from the dial (Fig. 28 b).
By the dip is meant the inclination of the stratum at any expo- sure, and this must obviously be measured in a vertical plane
48
EARTH FEATURES AND THEIR MEANING
along the steepest line in the bedding plane. The dip angle is always referred to a horizontal plane, and hence vertical beds have a dip of 90°. The device for measuring this angle of dip, the clinometer, is merely a simple pendulum which serves as an indi- cator and is centered at the corner of a graduated quadrant. A home-made variety is easily constructed from a square piece of board and an attached paper quadrant (Fig. 28 c), but the geolo- gist's compass is always provided with a clinometer attachment to the dial.
Since the strike is the intersection of the bedding plane with a horizontal surface, and the dip is the intersection with that partic- ular vertical plane which gives the steepest inclination, the strike and dip are perpendicular to each other. To represent them upon maps, it is more or less customary to use the so-called T symbols, the top of the T giving the direction of the strike and the shank that of the dip. If meridians are drawn upon the map, the direction or attitude of the T can be found by the use of a simple protractor; and when entered upon the map, the exact angle of the strike may be supplied by a figure near the top of the T, and the dip angle by a figure at the end of the shank. It is the custom, also, to make the length of the shank inversely proportional to the steepness of the dip, so that in a broad way the attitudes of the strata may be taken in at a glance (Fig. 29). It is further of
advantage to make the top of the T a double line, so that some symbol or color may show the correlations of the different expo- sures. To illustrate, in Fig. 29, the symbol marked a represents an outcrop of limestcne, the strike of which is 50° east of north (N. 50° E.), and the dip of which is 45° southeast. In the same figure b represents a shale outcrop in hori- zontal beds, which have in conse- quence a universal strike and a dip of 0°. An exposure of limestone in vertical beds which strike N. 60° E. is shown at c, etc.
Measurement of the thickness of formations. — When forma- tions still lie in horizontal beds, we may sometimes learn their
N
X
so
■fS
eo
IS
Fig. 29. — Diagram to show the use of T symbols to indicate the dip and strike of outcroppings.
CONTORTIONS OP THE STRATA
49
of OuTcrop
thickness directly either from the depth of borings to the under- Ijdng rock, or by measurements upon steep canon walls. If the beds stand vertically, the matter is exceedingly simple, for in this case the thickness is the width of the outcrops of the formation between the beds which bound it upon either side. In the general case, in which the beds are neither horizontal nor ver- tical, the thickness must be obtained indirectly from the width of the exposures and the angle of the dip. The factor by which the ex- posure width must be mul- tiplied is known as the sine of the dip angle (Fig. 30), which is given with sufficient accuracy for most purposes in the following table. It is obvious that in order to obtain the full thickness of a formation it is necessary to measure from the contact with the adjacent formation upon the one side to a similar contact with the nearest formation upon the other.
Fig. 30. — Diagram to show how the thickness of a formation may be obtained from the angle of the dip and the width of the ex- posures.
|
Natural Sines |
|||||
|
0° |
.00 |
35° |
.57 |
70° |
.94 |
|
5° |
.09 |
40° |
.64 |
75° |
.97 |
|
10° |
.17 |
45° |
.71 |
80° |
-.98 |
|
15° |
.26 |
50° |
.77 |
85° |
1.00 |
|
20° |
.34 |
55° |
.82 |
90° |
1.00 |
|
25° |
.42 |
60° |
.87 |
||
|
30° |
.50 |
65° |
.91 |
The detection of plunging folds. — When the axis of a fold is horizontal, its outcrops upon a plain will continue to have the same strike until the formation comes to an end. Upon a generally level surface, therefore, any regular progressive variation in the strike direction is an indication that the folds have a plunging or pitching character. Many serious mistakes of interpretation have been made because of a failure to recognize this evidence of plunging folds. The way in which the strikes are progressively modified will be made clear by the diagrams of Figs. 31 and 32,
50
EARTH FEATURES AND THEIR MEANING
the first representing a pitching anticline and the second a pitch- ing syncline. In both these reciprocal cases the strikes of the
FiG.31. — Combined surface and sectional views of a plunging anticline (after Willis).
beds undergo the same changes, and the dip directions serve to distinguish which of the two structures is present in a given case. There is, however, one further difference in that the hard layers
Fig. 32. — Combined surface and sectional views of a plunging syncline (after Willis).
CONTORTIONS OP THE STRATA
51
of the plunging anticline, where they disappear below the surface in the axis, will present a domed surface sloping forward like the back of a whale as it rises above the surface of the sea. Plunging folds in series will thus appear in the topography as a series of sharply zigzagging ranges at those localities where the harder layers intersect the surface. Such features are encountered in eastern Pennsylvania, where the hard formations of the Appala- chian Mountain system plunge northeastward under the later formations. The pitch of the larger fold is often disclosed by that of the minor puckerings superimposed upon it.
The meaning of an unconformity. — The rock beds, which are deposited one above the other during a transgression of the sea,
Fig. 33. — Unconformity between a lower and an upper series of beds upon the coast" of CaUfornia. Note how the hard layer stands in relief upon the connecting surface (after Fairbanks).
are usually parallel and thus represent a continuous process oX deEQsiticaU-_-Such beds are said to be conformable. Where, upon the other hand, two series of deposits which are not parallel to each other are separated by a break, they are said to form un- conformable series, and the break or surface of junction is an un- conformity (Fig. 33).
52
EARTH FEATURES AND THEIR MEANING
Here it is evident that the sediments which compose the lower series of beds have been folded in the zone of flow, though the upper series has evidently escaped this vicissitude. Furthermore, the surface which dehmits the lower series from the upper is some- what irregular and shows a hard layer standing in relief, as it would if it had opposed greater resistance to the attacks of the atmosphere upon it.
In reality, an unconformity between formations must be in- terpreted to mean that the lower series is not bnly older than the upper, as shown by the order of superposition, but that the time of its deposition was separated from that of the upper by a hiatus in which important changes took place in the lower series. The stages or episodes in the history of the beds represented in Fig. 33 may be read as follows (see Fig. 34 a-e) : —
(a) Deposition of the lower series during a transgres- sion of the sea.
(6) Continued subsidence and burial of the lower series beneath overlying sedi- ments, and'fleSur- ing in the zone of flow.
(c) Elevation of the combined de- posits to and far above sea level and
Fig. 34. -Series of diagrams to illustrate in succession the '"^^^^^^ by erosion
episodes involved in the historical development of an ^^ ^^^* thlckneSSeS
angular unconformity. The vertical arrows indicate of the upper Sedi-
the direction of movement of the land, and the horizontal rnpntd
arrows the direction of shore migration. iiieubb.
(d) A new sub- sidence of the truncated lower series and deposition of the upper series across its eroded surface.
(e) A new elevation of the double series to its present position above sea level.
CONTORTIONS OP THE STRATA
53
From this succession of episodes it is seen that a break of this kind between two series of deposits involves a double oscillation of subsidence followed by elevation — a large depression followed by a large elevation, a smaller subsidence followed by elevation. The time interval which must have been represented by these re- peated operations is so vast as at first to stagger the mind in con- templating it. When, as in this instance, the dips of the lower .series of beds differ from those of the upper, we have to do with an angular^wncmformity. It may be, however, that the lower iseries was not so far depressed as to enter the zone of flow, and that its beds meet those of the upper series with apparent con- formity. Such an unconformity is often extremely difficult to recognize, and it is described as a deceptive or erosional uncon- formity. ___ ""With a deceptive imconformity the clew to its real nature is usually some fact which indicates that the lower series of sedi- ments had been raised above the ._._^.^__^_ .-^='m- level of the sea before the upper series was deposited upon it. This may be apparent either in the irregularity of the surface on which the two series are joined, in some evidence of the action of waves such as would be fur- nished by a basal conglomerate in the upper series, or some in- dication of different resistance of different rocks of the lower series to attacks of the atmosphere upon them (Figs. 33 and 35 a-c). In most cases, at least,' the lowest member of the upper series will be a different type of rock from the uppermost mem- ber of the lower series, hence the frequent occurrence of the dis- cordant cross bedding in sand-
^. Bi//oin/y Surface.
X
S. Baso/ Con^/o/nerrrte.
C Depression of surface oyer a- ^\^eak roc/r.o/K/jora/ecffonoyer 6— strong rock.
FiQ. 35. — Types of deceptive or erosional unconformities.
stone should not deceive even the novice into the assumption of an unconformity.
54 EARTH FEATURES AND THEIR MEANING
Reading References to Chapter V
The zones of fracture and flow : — C. R. Van Hise. Principles of North American Precambrian Geology,
16th Ann. Rept. U.S. Geol. Surv., 1895, Ft. I, pp. 581-603. Bailey Willis. Mechanics of Appalachian Structure, 13th Ann. Rept.
U.S. Geol. Surv., 1893, Ft. II, pp. 217-253. A. Daubree. Etudes Synthetiques de Geologie Experimentale. Faris,
1879, pp. 306-328, pi. II. W. Fhinz. Quelques remarques generales a propos de I'essai de carte
tectonique de la belgique, etc., BuU. Soc. Beige Geol., vol. 18, 1904,
p. 143, pi. V.
Analysis of folds : — Van Hise and Willis as above ; de Margekie et Heim ; Les disloca- tions de I'ecorce terrestre (in French and German languages). Zurich, 1888.
Geological maps : — Wm. H. Hobbs. The Mapping of the Crystalline Schists, Jour. Geol, vol. 10, 1902, pp. 780-792, 858-890.
CHAPTER VI
THE ARCHITECTURE OF THE FRACTURED SUPER- STRUCTURE
The system of the fractures. — In referring to experiments made upon the fracture of solid blocks under compression (p. 41), it was shown that two series of parallel fractures develop perpendicular to each free surface of the block, and that these series are each of them inclined by half — _j2f_a_rightL,an^e-to the direction of compres- sion, and thus perpen- dicular to each other. The fragments into which a block with one free surface would thus tend to be divided should be square prisms perpendicular to the free surface. It would be interesting, if it were practicable, to learn from experiment how these prisms would be further fractured by a continuation of the com- pression. From me- chanical considerations involving the resolution of forces with refer- ence to the ready-formed fractures, it seems probable that the next series of fractures to form would bisect the angles of the first double series or set. Wherever rocks are found exposed in their original
55
.'■"'Utl',
"."•';. I
" vli"*i'*''*l-
Fig. 36. — A set of master joints developed in shale upon the shores of Casoiga Lake near Ithaca, New York (after U. S. G. S.).
56
EARTH FEATURES AND THEIR MEANING
Fia. 37. — Diagram to show how sets of master joints differing in direction by half a right angle may abruptly replace each other.
omnipresent double series or set of joints is the well-known set of master joints, and very often it is found developed practically alone (Fig. 36). Over large areas, the direction of the set of jnaster joints may remain practically con- stant, or this set may quite suddenly give place to a sim- ilar set which is, however, turned through half a right angle from the first (Fig. 37). Not infrequently two such sets of master joints are found together bisecting each other's angles within the same rocks, and to them
attitudes, they are, in fact, seen to be inter- sected by two parallel series of fractures which are perpendicu- lar to the earth's sur- face and to each other and are described as joints. In many cases more than two series of such fractures are found, yet even in these cases two more perfectly developed series are prominent and almost exactly perpendicular to each other as well as to the earth's surface. This
FiQ. 38. — Diagram to show the different combinations of the series composing two double sets of master joints, and in a, a, a additional disorderly fractures.
ARCHITECTURE OF FRACTURED SUPERSTRUCTURE 57
are sometimes added additional though less perfect series of joint planes.
Studied throughout a considerable district, the various series which make up these two sets of master joints may be seen locally
Fig. '69. — View on the shore at Holstensborg, West Greenland, to show the sub- equal spacing of the joints (after Kornerup).
developed in different combinations as well as in association with additional fissure planes which are not easily reduced to any simple law of arrangement
(Fig. 38 a, a, a). Only rarely are reg- ular joint series ob- served which do not stand perpendicular to the original atti- tude of the rock beds. In a few local- ities, however, rec- tangular joint sets have been discov- ered which divide the rock into prisms parallel to the earth's surface and with the joint series inclined to it each by half a right angle. Where the rock beds have been much disturbed, the complex of
Fig. 40. — View of an exposed hillside in Iceland upon which the snow collected in crannies along the joints brings out to advantage both the larger and the smaller intervals of the joint system (after Thoroddsen).
58 EARTH FEATURES AND THEIR MEANING
joints may be such as to defy all attempts at orderly arrange- ment.
The space intervals of joints. The same kind of subequal spac- ing which characterizes the fractures near the surface of the block in Daubrfe's experiment (Fig. 19, p. 41) is found simulated by the rock joints (Fig. 39). Such unit intervals between fractures may be grouped together into larger units which are separated by frac- tures of unusual perfection. We may think of such larger space units as having the smaller ones superimposed upon them (Fig. 40).
The displacements upon joints — faults. — In the vast majority of cases, the joint fractures when carefully examined betray no evidence of any appreciable movement of the two walls upon each other. Generally the rock layers are seen to cross the joints with- out apparent displacement. Joints are therefore planes of dis- junction only, and not planes of displacement.
Within many districts, however, a displacement may be seen to have occurred upon certain of the joint planes, and these are then described as faults. Such displacements of necessity imply
Fig. 41. — Faulted blocks of basalt divided by joints near Woodbury, Connecticut. To show the structure of the rock, some of the foliage has been removed in prepar- ing the sketch from a photograph.
a differential movement of sections or blocks of the earth's crust, the so-called orographic blocks, which are bounded by the joint planes and play individual r61es in the movement. A simple case of such displacements in rocks intersected by a single set of mas- ter joints is represented in the model of plate 4 C. The most promi- nent fault represented by this model runs lengthwise through the middle, and the displacement which is measured upon it not only varies between wide limits, but is marked by abrupt changes at the margins of the larger blocks. This vertical displacement upon
ARCHITECTURE OF FRACTURED SUPERSTRUCTURE 59
the fault is called its JhrojjL^ Though not illustrated by the model, horizontal displacements may likewise occur, and these will be more fully discussed when the subject of earthquakes is considered in the following chapter. An actual example of blocks displaced by vertical adjustment is represented in Fig. 41, a simple type of faulting which has taken place in rocks but slightly disturbed from their original attitude, but intersected by a relatively simple sys- tem of master joints. In those regions where the beds have been folded and perhaps overthrust before their elevation into the zone of fracture, and which are further intersected by disorderly fissure planes, the results are far more complex. In such cases the planes of individual displacement may not be vertical, though they are generally steeper than 45°. For their description it is necessary to make use of addi- tional technical terips (Fig. 42). The inclination of a sloping fault plane measured against the ver- tical is called thq^feo^e of the fault. The total displacement is measured along the plane of the fault from a point upon one limb to the point from which it was separated in the other. The additional terms are made sufficiently clear by the diagram.
Methods of detecting fatilts. — The first effect of a fault is usually to produce a crack at the surface of the earth ; and, provided there is a vertical displacement or throw, an^esfiaiEflaent^which rises upon the upthrown side of the fault. In general it may be said that escarpments which appear at the earth's surface as plane surfaces probably represent planes of fracture, though not neces- sarily planes of faulting. In many cases the actual displacements lie buried under loose rock d6bris near to and paralleling the es- carpment, and in some cases as a result of the erosional processes working upon alternately hard and soft layers of rock, the escarp- ment may later appear upon the downthrown side or limb of the fault (Fig. 43). As an illustration of a fault escarpment, the fagade of El Capitan and many other rock faces of the Ybsemite valley may be instanced.
FiQ. 42. — A fault in previously dis- turbed strata. AB, displacement; AC, throw ; BD, stratigraphic throw ; BC, heave ; angle CAB, hade.
60
EARTH FEATURES AND THEIR MEANING
Fig. 43. — Diagrams to show how an escarpment originally on the upthrown side of the iauli may, through erosion, appear upon the downthrown side.
When we have further studied the erosional processes at the earth's surface, it will be appreciated that faults tend to quickly
bury themselves from sight, where- as fold structures will long remain in evidence. Many faults will thus be overlooked, and too great weight is Ukely to be ascribed to the folds in accounting for the existing atti- tudes a,nd positions of the rock masses. Faults must therefore be sought out if mistakes of interpreta- tion are to be avoided.
The most satisfactory evidence of a fault is the discovery of a rock bed which may be easily identified, and which is actually seen displaced on a plane of fracture which intersects it (Fig. 42, p. 59). When such an easily recognizable layer is not to be found, the plane of displacement may perhaps be discovered as a narrow zone composed of angular fragments of the rock cemented together by minerals which form out of solution in water. Such a fractured rock zone which follows a plane of faulting is a" fault hreccia._ If the fault breccia,"or vein rock, is much stronger than the rock on either side, it may eventually stand in relief at the surface like a dike or wall. At other times the displacement pro- duces little fracture of the walls, but they slide over each other in such a manner as to yield either a smoothly cor- rugated or an evenly polished • surface which is described as
" Slickensides." It may be, f^o. 44. -A fault plane exhibiting "drag." however, that during the move- The opening is artificial (after Scott).
ARCHITECTURE OF FRACTURED SUPERSTRUCTURE 61
ment either one or both of the walls have " dragged," and so are curled back in the immediate neighborhood of the fault plane (Fig. 44).
When, as is quite generally the case, the actual plane of dis- placement of a fault is not open to inspection, the movement may be proven by the observation of abrupt, as contrasted with grad- ual, changes in the strikes and dips of neighboring exposures (Fig. 45) ; or by noting that some easily rec- ognized formation has been sharply offset in its outcrops (Fig. 46).
There are in addition many in- dications rather than proofs of the presence of faults, which must be taken account of in every general study of the geology of a district. Thus the outcrops of all neighbor- ing formations may terminate abruptly upon a straight Une which intersects all alike. Deep-seated fissure springs may be aligned in a striking manner, and so indicate
the course of a prominent fracture, though not necessarily of a fault. Much the same may be said of the dikes of cooled magma which have been injected along preexisting frac- tures.
The base of the geological map. — Modern topographic maps form an im- portant part of the library of the serious student of physiography; they are the gazetteer of this branch of science. Every civilized nation has to-day either completed a topographic atlas of its territory, or it is vigorously prosecuting a survey to furnish maps which represent the relief with some detail, and pub- lishing the results in the form of an atlas of quadrangles. Thus
Fig. 45. — Map to show how a fault may be indicated in abrupt changes of the strike and dip of neighboring exposures.
Fig. 46. — A series of parallel faults indicated by successive offsets in the course of an easily recognizable rock for- mation.
62 EARTH FEATURES AND THEIR MEANING
a relief map will erelong be obtainable of any part of the civilized world, and may be purchased in separate sections. Nowhere is this work being taken up with greater vigor than in the United States, where a vast domain representing every type of topographic pecul- iarity is being attacked from many centers. Here and elsewhere the relief of the land is being expressed by so-called contours or lines of equal altitude upon the earth's surface. It is as though a series of horizontal planes, separated by uniform intervals of 20 or 40 or 100 feet, had been made to intersect the surface, and the intersection curves, after consecutive numeration, had been dropped into a single plane for printing.
Where the slopes are steep, the contour lines in the topographic map will appear crowded together and so produce a deep shade upon the map ; whereas with relatively flat surfaces white patches will stand out prominently upon the map. More and more the topographic map is coming into use, and for the student of nature in particular it is important to acquire facility in interpreting the rehef from 'the topographic map. To further this end, a special model has been devised, and its use is described in appendix C. Usually before any satisfactory geological map can be prepared, a contoured topographic map of the district to be studied must be available. m»(
The field map and the areal geological map. — As the atlas of topographic maps is the physiographic gazetteer, so geological maps together constitute the reference dictionary of descriptive geology. Not only are topographic maps of many districts now generally available, but more and more it has become the policy of governments to supply geological maps in the same quadrangle form which is the unit of the topographic map. The geological map is, however, a complex of so many conventional symbols, that without some practical experience in the actual preparation of one, it is exceedingly difficult for the student to comprehend its significance. A modern geological map is usually a rectangular sheet printed in color, upon which are many irregular areas of in- dividual hue joined to each other like the parts of a child's pic- ture puzzle.
The colored areas upon the geological map are each supposed to indicate where a certain rock type or formation lies immediately below the surface, and this distribution represents the best judg-
ARCHITECTURE OF FRACTURED SUPERSTRUCTURE 63
ment of the geologist who, after a study of the district, has prepared the map. Unfortunately the conventions in use are such that his observation and his theory have been hopelessly intermingled in the finished product. Armed with the geological map, the student who visits the district finds spread out before him, it may be, a landscape of hill and valley, of green forest and brown farming land, which is as different as may be from the colored puzzle which he holds in his hand. Hidden under the farm vegetation or masked by the woods are scattered outcroppings of rock which have been the basis of the geologist's Judgment in preparing the map. Ex- perience shows that in order to bridge the wide gap between the geology in the landscape and the patches of color upon the map something more than mere examination of the colored sheet is necessary. We shall therefore describe, with the aid of laboratory models, the various stages necessary to the preparation of a geo- logical map, and every student should be advised to follow this by practical study of some small area where rocks are found in out- crop.
Though the published areal geological map represents both fact and theory, the map maker retains an unpublished field map or map of observations, upon which the final map has been based. This field map shows the location of each outcrop that has been studied, with a record of the kind of rock and of such observations as strike, dip, and pitch. Our task will therefore be to prepare : (1) a field map ; (2) an areal geological map ; and (3) some typical geological sections.
Laboratory models for the study of geological maps. — In order to represent in the laboratory the disposition of rock outcrops in the field, special laboratory tables are prepared with removable covers and with fixed tops, which are divided into squares num- bered like the township sections of the national domain (Fig. 47). To represent the rock outcrops, blocks are prepared which may be fixed in any desired position by fitting a pin into a small augur hole bored through the table. The outcrop blocks for the sedi- mentary rock types are so constructed as to show the strike and dip of the beds. (See Appendix D.)
The method of preparing the map. — To prepare the map, use is made of a geological compass with clinometer attachment, a protractor, and a map base divided into sections like the top of
64
EARTH FEATURES AND THEIR MEANING
the table, and on the scale of one inch to the foot. Each exposure represented upon the table is " visited " and then located upon the base map in its proper position and attitude. The result is the field map (Fig. 47), which thus represents the facts only, unless
t
-©r
o
o
"o
-r^^.
4=
o
M
J)
5^
■^r-f
f
L
i.
.e
^°i8%t M T^
^
o
i«J
I
1^
c
'Z
:^
tx-^
1-
j;
r
%
W-
'¥
fr
V'
K
^A
^t
^
Fig. 47. — Field map prepared from a laboratory table.
there have been uncertainties in the correlation of exposures or in determining the position of the bedding plane.
To prepare the areal geological map from the field map, it is first necessary to fix the boundaries which separate formations at
Fig. 48. — Areal geological map constructed from the field map of Fig. 47, with two selected geological sections.
the surface ; and now perhaps for the first time it is realized how large an element of uncertainty may enter if the exposures were widely separated. It is clear that no two persons will draw these lines in the same positions throughout, though certain portions
ARCHITECTURE OP FRACTURED SUPERSTRUCTURE 65
of them — where the facts are more nearly adequate — may cor- respond. In Fig. 48 is represented the areal geological map con- structed from the field map, with the doubtful area at one side left blank.
Some conclusions from this map may now be profitably con- sidered. The complexly folded sandstone formation at the left of the map appears as the oldest member represented, since its area has been cut through by the intrusive granite which does not intrude other formations, and is unconformably overlaid by the limestone and its basal layer of conglomerate. The limestone in turn is unconformably overlaid by the merely tilted sandstone beds at the right of the map. These three sedimentary forma- tions clearly represent decreasing amounts of close folding, from which it is clear that each earlier formation has passed through an episode not shared by that of next younger age. Of the other intrusive rocks, the dike of porphyry is younger than all the other formations, with the possible exception of the upper sandstone. Offsetting of the formations has disclosed the course of a fault, and from its relations to the dikes we may learn that of these the porphyry is younger and the basalt older than the date of the faulting.
The dashed lines upon the map (AB and CD) have been-selected as appropriate lines along which to construct geological sections (Fig. 48, below map), and from these sections the exposed thick- nesses of the different formations may be calculated. In one in- stance only, that of the conglomerate, can we be sure that this exposed thickness measures the entire formation.
Fold versus fault topography. — The more resistant or " stronger " rock beds, as regards attacks of the atmosphere, in the course of time come to stand in relief, separated by depressions which overlie the " weaker " formations. Simple open folds which are not plunging exercise an influence upon topography by producing "^nerallyTon^antl straight ^i3ges.™More complex fleXUres,'^nce "they"'generaily " plunge, make themselves apparent by features which in the map are represented by curves. Fracture structures, and especially block displacements, are differentiated from these curving features by the dominance of straight or nearly rectilinear lines upon the map. The effect of erosion is to reduce the asperity of features and to mold them with flowing curves. The frac-
66 EARTH FEATURES AND THEIR MEANING
ture structures are for this reason much more likely to be over- looked, and if they are not to elude the observer, they must be sought out with care. Fold and fracture structures may both be revealed upon the same map.
Reading References to Chapter VI
Joint systems : —
John Phillips. Observations made in the Neighborhood of Ferrybridge in the Years 1826-1828, Phil. Mag., 2d ser., vol. 4, 1828, pp. 401-409 ; Illustrations of the geology of Yorkshire, Pt. II, The Limestone Dis- trict. London, 1836, pp. 90-98.
Samuel HAtJGHTON. On the Physical Structure of the Old Red Sand- stone of the County of Waterford, considered with reference to cleav- age, joint surfaces, and faults. Trans. Roy. Soc. London, vol. 148, 1858, pp. 333-348.
W. C. Brogger. Spaltenverwerfungen in der Gegend Langesund-Skien, Nyt Magazin for Naturvidernskaberne, vol. 28, 1884, pp. 253-419.
Wm. H. Hobbs. The Newark System of the Pomperaug Valley, Con- necticut, 21st Ann. Rept. U. S. Geol. Surv., Pt. Ill, 1901, pp. 85-143.
Geological map : — Wm. H. Hobbs. The Interpretation of Geological Maps, School Science and Mathematics, vol. 9, 1909, pp. 644-653.
CHAPTER VII
THE INTERRUPTED CHARACTER OF EARTH MOVE- MENTS: EARTHQUAKES AND SEAQUAKES
Nature of earthquake shocks. — Man's belief in the stability of Mother Earth — the terra firma — is so inbred in his nature that even a light shock of earthquake brings a rude awakening. The terror which it inspires is no doubt largely to be explained by this
Fig. 49. — View of a portion of the ruins of Measina after the earthquake of December 28, 1908.
disillusionment from the most fundamental of his beliefs. Were he better advised, the long periods of quiet which separate earth- quakes, and not the lighter shocks which follow all grander dis- turbances, would occasion him concern.
67
68
EARTH FEATURES AND THEIR MEANING
Earthquakes are the sensible manifestations of changes in level or of lateral adjustments of portions of the continents, and the seismic disturbances upon the sea — seaquakes and seismic sea waves — relate to similar changes upon the floor of the ocean.
During the grander or catastrophic earthquakes, the changes are indeed terrifying, and have usually been accompanied by losses to hfe and property, which are only to be compared with those of great conflagrations or of inundations on thickly populated plains. The conflagration has all too frequently been an aftermath of the great historic earthquakes. The earthquake of December 28, 1908, in southern Italy, destroyed almost the entire population of a great city, and left of its massive buildings only a confused heap of rubble (Fig. 49). Two years later a heavy earthquake resulted in great damage to cities in Costa Rica (Fig. 50), while two years.
Fig. 50. — Ruins of the Carnegie Palace of Peace at Cartago, Costa Rica, de- stroyed when almost completed by the great earthquake of May 4, 1910 (after a photograph by Rear-Admiral Singer, XJ.S.N.).
earlier our own country was first really awakened to the danger in which it stands from these convulsive earth throes; though, as we shall see, these dangers can be largely met through proper methods of construction.
Earthquakes are usually preceded for a brief instant by sub- terranean rumblings whose intensity appears to bear no relation to the shocks which follow. The ground then rocks in wavelike
EARTHQUAKES AND SEAQUAKES
69
motions, which, if of large amplitude, may induce nausea, prevent animals from keeping upon their feet, and wreck all structures not specially adapted to withstand them. Heavy bodies are some- times thrown up from the ground (Fig. 51), and at other times
Fig. 51. — Bowlders thrown into the air and overturned during the Assam earthquake of 1897 (after R. D. Oldham).
similar heavy masses are, apparently because of their inertia, more deeply imbedded in the earth. Thus gravestones and heavy stone posts are often sunk more deeply in the ground and are surrounded by a hollow and perhaps by small open cracks in the surface (Fig. 52). When bodies are thrown upward, it would imply that a quick upward g- -^ movement of the ground had been fig. 52. suddenly arrested, while the burial of heavy bodies in the earth is prob- ably due to a movement which begins suddenly and is less abruptly terminated.
Seaquakes and seismic sea waves. — Upon the ocean the quakes which emanate from the sea floor are felt on shipboard as sudden joltings which produce the impression that the ship has struck upon a shoal, though in most instances there is no visible commotion in
1 • ; N,>,. — ~- ■
— Heavy post sunk deeper into the' ground during the Charleston earthquake of August 31, 1886 (after Dutton).
1 1
70
EARTH FEATURES AND THEIR MEANING
-K-.^
the water. The distribution of these shocks, as indicated either by the experiences of neighboring ships at the time of a particular shock, or by the records of vessels which at different times have sailed over an area of frequent seismic disturbance, appears to be
limited to narrow zones or lines (Fig. 53). The same tendency of under-sea disturbances to be localized upon defi- nite straight lines has been often illus- trated by the behavior of deep-sea cables which are laid in proximity to one another and which have been known to part simultaneously at points ranged upon a straight line.
Far grander disturbances upon the been reported at sea off Cape floor of the ocean have been revealed
Mendocino, California. ^^ ^^^ g^.^^^ g^^ ^^^^ _ ^j^^g so-Called
" tidal waves," properly referred to as tsunamis — which recur in those sea districts which adjoin the special earthquake zones upon the continents (p. 86) . The forerunner of such a sea wave approach-
FiG. 53. — Map showing the lo- calities at which shocks have
Fig. 54. — Effect of a seismic water wave at Kamaishi, Japan, in 1896 (after E. R.
Scidmore).
ing the shore is usually a sudden withdrawal of the water so as to lay bare a portion of the bottom, but this is well-recognized to be the premonition of a gigantic oncoming wave which sweeps all before it and is only halted when it has rolled over all the low-lying coun-
EARTHQUAKES AND SEAQUAKES
71
try and encountered a mountain wall. Such seismic waves have been especially common upon the Pacific shore of South America and upon the Japanese littoral (Fig. 54). These waves proceed from above the great deeps upon the ocean bottom, and clearly result from the grander earth movements to which these depres- sions owe their exceptional depth. The withdrawal of the water from neighboring shores may be presumed to be connected with a descent of